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IN THIS VOLUME


Offshore Technology

2005;():3-13. doi:10.1115/OMAE2005-67003.

This paper presents analysis procedures of a prototype semi-submersible which consists of rectangular pontoons and square columns. The proposed semi-submersible is based on standard cubic boxes constructed by stiffened flat panel line assembly techniques. The prediction of global loads and responses is of great importance to engineers for assessing the safety of marine structures. In this paper analysis methods to determine the global structural loads and to assess the viability of the multi-purpose semi-submersible are described. Static loads in the still water condition and wave induced dynamic loads of cylindrical members in different design conditions were determined to investigate the viability of the proposed semi-submersible. Further the assessment of the safety against failure due to excessive maximum loads was accomplished in ultimate limit states.

Commentary by Dr. Valentin Fuster
2005;():15-24. doi:10.1115/OMAE2005-67008.

In this paper we show how a drillstring can be modeled in terms of limited numbers of masses and springs for the purpose of building a test rig. The model should represent the drillstring dynamics to a reasonable degree of accuracy. We will use the real component of the Frequency Response Function and the stability diagram as measures of dynamic similarity between the model and drillstring. We will also show how the chosen modes can be decoupled and used in obtaining the bit displacement. The decoupled modes will be used in a proposed test rig configuration that would increase flexibility in adding or removing modes from the system. Obtaining a representative model of the test rig is critical to our ability to extrapolate laboratory data into field applications.

Commentary by Dr. Valentin Fuster
2005;():25-36. doi:10.1115/OMAE2005-67013.

The present paper discusses the mathematical modeling of risers and riser-like structures applied in a positioning context for deep-water floating vessels. The main purpose of the paper is to show that an estimate for the optimal vessel position, sufficient for most practical applications, is obtained from measurements of the riser inclinations or related parameters at lower end, and optionally upper end, through a solution based on the variably tensioned beam differential equation. Due to the ease of implementation this solution is well suited for direct application in on-line riser monitoring systems. The method is an attractive alternative to on-line FE-analyses, application of pre-computed regression curves based on idealized loading or black-box neural networks, which has been proposed by others to be applied as basis for interpretation of the measured riser responses. The basic idea behind the method is based on the observation that the riser inclinations or stress-joint moments at upper and lower end have mainly two causes. Firstly an effect caused by the position of the riser top end relative to the wellhead due to permanent vessel offset and slow drift vessel motions, and secondly the effects of transverse current down the riser. The general theory behind the method will be outlined. It will then be shown how the method adapts to drilling-risers with flex-joints, risers with stress-joints and also to the special case of well intervention with coiled tubing in open sea without applying a work-over or marine riser. The performance of the method is illustrated using simulated vessel and riser dynamic response data. The simulations are performed for selected vessel types both for deep-water and shallower waters applying state-of-the-art software for simulation of the riser and vessel dynamic response in random sea states.

Commentary by Dr. Valentin Fuster
2005;():37-43. doi:10.1115/OMAE2005-67016.

The cycle time to first production is a primary determinant of the net present value (NPV) of an oil and gas asset. The cost, complexity and risk inherent in deepwater field developments, combined with the relative lack of experience in their execution, often encourages engineers to proceed cautiously in field development. However, a successful fast-track development schedule from discovery to first oil may bring significantly better economic returns. This paper investigates the key parameters influencing cycle time for different facility types, and outlines a wide range of measures that may be adopted to accelerate the time to first production.

Topics: Cycles
Commentary by Dr. Valentin Fuster
2005;():45-56. doi:10.1115/OMAE2005-67020.

The accurate prediction of SPM vessel yaw motion is important to its mooring system design. Inconsistencies have been observed between the numerical and model test predictions of offloading responses. In some cases, the numerical simulation predicted unstable yaw behavior of the vessel (fishtailing) while the model tests did not show such instability. This discrepancy between experiment and theory casts doubt as to whether the numerical simulation predicts correctly the vessel yaw motion. The work presented in this paper investigates the following two hypotheses to possibly explain the non-expected fishtailing in the numerical simulations: The mooring software may not accurately integrate non-linear differential equations that describe the yaw motion of the SPM vessel. Some damping terms may be under-estimated in the software (user input issue). To validate the integration scheme of the system of non-linear differential equations as implemented in the mooring software, a stability analysis has been conducted on a shuttle tanker moored to a West Africa deep water buoy. Variations of parameters like the hawser length, its axial stiffness and the vessel’s drag coefficients have been studied to explore their impacts on the vessel yaw stability. The approach is to identify without performing any time domain simulations, the domains of stability by linearizing the differential equations of SPM vessel’s yaw motion around its equilibrium point. The validity of the developed approach is then confirmed by performing time domain simulations of the same case. The second conjecture which may explain the non-expected fishtailing in numerical simulations was that some damping terms may be under-estimated. A semi empirical formula for the drag moment can be derived from rotation tests and comparisons were performed with the engineering model implemented in the mooring analysis software. The results show that by calibrating this damping term with the one derived from the experiments, the numerical simulations would match the stable yaw motion behavior as predicted during model tests. Following the above findings, a tool has been developed to fit the yaw drag moment engineering model based on experimental measurements, for any case of mooring analysis.

Topics: Mooring , Vessels , Yaw
Commentary by Dr. Valentin Fuster
2005;():57-63. doi:10.1115/OMAE2005-67021.

Fatigue is often the governing criterion for deepwater riser design. Fatigue assessment methods based on SN curves typically use fatigue safety factors and there is a lack on consensus about the appropriate Design Fatigue Factor (DFF) for VIV that should be applied. A DFF of 20 is commonly used in project specifications to account for a larger uncertainty in prediction of fatigue damage due to VIV, i.e. fatigue life due to VIV should be at least 20 times more than the service life of the riser. Recent case studies and benchmarking assessments, have confirmed that the DFF of 20 for VIV is not always an optimal choice. Owing to the complexity of the VIV problem, often semi-empirical solutions are adopted in the riser VIV analysis. The implicit uncertainty and bias associated with the VIV analysis model, needs to be considered along with the VIV fatigue safety factor, in order to establish the adequate acceptance criterion for VIV induced fatigue. The stochastic variables governing the VIV and their associated uncertainties can vary widely from case to case and this in turn will influence the VIV safety factor. Different stochastic variables may be relevant based on the floater type, riser type, environmental modeling, analysis methodology etc. In addition to these stochastic variables, the uncertainty in the VIV specific variables, such as Strouhal number, bandwidth parameters, damping effects, lift coefficient, response models (Amplitude /Diameter ratio), etc also need to be considered in the evaluation of the VIV safety factor. In this paper, guidance is provided by means of case studies for establishing case specific VIV safety factors. By performing a set of dedicated analyses for the prevailing uncertain input parameters, the resulting uncertainty in the VIV fatigue damage is estimated. The enhanced risk based criterion presented in DNV Recommended Practice RP-F204 [7], developed as part of the Norwegian Deepwater Programme (NDP) is applied for these case studies to calculate the required VIV fatigue safety factors. This project was sponsored by NDP, whose contribution and support is gratefully acknowledged.

Commentary by Dr. Valentin Fuster
2005;():65-77. doi:10.1115/OMAE2005-67024.

The application of a method for fatigue damage assessment applied to the relative effects of mooring, wave, and Vortex Induced Vibration (VIV) is presented. It is a simpler procedure than rainflow counting and uses a bandwidth parameter with standard deviation applied to modify narrow band fatigue estimation. This bandwidth parameter has been successfully applied by the authors to investigate in detail the curvature (bending stress) response of the Foinaven lazy-wave umbilical for investigations into VIV. This parameter is valuable since as a single value it may be used to track the variation of bandwidth response with time and against other measures such as varying current speed. The method is conveniently based on a peak counting approach. Low values close to zero indicate a narrow band process whereas values near unity indicate a broad-band process. It is a much better measure than kurtosis for determining bandwidth. Curvature and environmental data were gathered by the Foinaven Umbilical Monitoring System installed on the Foinaven Petrojarl IV floating production unit located in the Altantic margin, West of Shetland. The VIV frequency range owing to current excitation considered is that which is considered as being above consequential wave frequencies (0.2 to 2Hz). This paper presents results of fatigue damage assessment for the periods where a strong variation of bandwidth was observed. 64 records (each lasting about 10 minutes) representing a daily acquisition of about 8 hours were processed showing the correlation between the bandwidth of curvature and the amount of fatigue experienced by the umbilical. Strong VIV regimes are often characterized by an intense activity around a particular frequency and the impact of such narrow-band events on the fatigue life of the structure is clearly demonstrated. This paper also presents the relative contributions of mooring, and waves/vessel motions, and VIV to fatigue damage. Although wave excitation remains the main source of fatigue, VIV appears to potentially contribute to a significant part of the overall fatigue damage.

Commentary by Dr. Valentin Fuster
2005;():79-89. doi:10.1115/OMAE2005-67033.

Problems and issues arising in the design and operation of mono-hull FPSOs have been apparent for many years. This has resulted in shortcomings of current design philosophy being highlighted. Environmental downtime of 10 to 14 days per year is not uncommon in both new-built and converted FPSOs. The financial loss resulting from this downtime during the life of a field, is a significant factor in the overall field economics. The importance of selecting the principal dimensions of the hull to optimise the natural periods of the FPSO is discussed in this paper. Converted FPSOs are therefore at a disadvantage as the natural periods are already set by the principal dimensions and mass properties, which may not suit the environmental conditions at site, potentially resulting in high downtime. However, it is also noted that preliminary design of new built-FPSOs has also tended to neglect this aspect of design and opportunities to optimise motion performance have often been lost by following standard ship building practices in selecting initial principal hull dimensions. This paper explores a new rational approach to the design of the FPSOs, relative to the field environmental conditions, particularly for deep water locations. The main objective is to achieve minimum downtime to avoid financial penalties caused by the associated loss of production. Preliminary hull sizing software for optimising the hull principal dimensions, relative to the mooring and riser requirements and the field environmental conditions for a given storage capacity and topside load is presented. Benefits for the riser and mooring systems and for hull material and fabrication costs are also outlined.

Topics: Design , Hull
Commentary by Dr. Valentin Fuster
2005;():91-98. doi:10.1115/OMAE2005-67035.

This paper presents results of a study into calibrating beam elements properties for use in non-linear finite element analyses, such that the elements simulate the behaviour of real members. Following the API approach, it is recommended that the material strength is de-rated to achieve a well-behaved element. A large number of typical “imperfect” offshore tubular members were modelled using shell elements and analysed to determine the full range of their load-displacement behaviour. The complete practical range of combination of the slenderness and D/t (diameter to thickness) ratios were considered. Then the beam elements’ material was de-rated to achieve the same results as obtained from the shell model. The result of the study is a two-way table which gives the required de-rating as a function of the slenderness ratio and the D/t ratio.

Commentary by Dr. Valentin Fuster
2005;():99-106. doi:10.1115/OMAE2005-67038.

Fatigue verification is an important issue in steel risers design, demanding a good representation of the loading conditions that will occur during the riser entire lifetime. PETROBRAS has carried out a series of measurement and acquisition programs over the past decade, including the Campos Basin simultaneous acquisition of waves, current and wind data. The campaigns are called the PROCAP1 in Marlim Field and PROCAP2 in Barracuda Field. Those programs provided simultaneous environmental data (wave, wind and current) containing multimodal / multidirectional sea-states that occur in Campos Basin, with two main peaks dominating the total energy content [20,21]. As fatigue damage calculation depends on the stresses variations during the lifetime of the structure, the set of loads used in the analysis should be complete enough to represent all possible situations. The high number of loading conditions used in riser fatigue verification associated with the random time-domain analysis that demands a high computer time for processing the analysis, impact the design schedule. The frequency domain approach, based on linearization techniques, is an alternative tool for riser analysis and has been studied mainly for structural fatigue verification applications. For this particular application, due to the low intensity of loadings, the geometric nonlinearity is considered by means of a previous nonlinear static analysis, followed by a dynamic frequency domain analysis on the deformed model. The nonlinearity of the drag part of Morison’s formula has to be conveniently treated by linearization techniques. This work presents a comparative study where the results using a frequency domain analysis are compared to the results of a time domain analysis. Both approaches were used in the analysis of a steel lazy-wave riser (SLWR) model connected to a spread-moored FPSO, submitted to fatigue environmental loadings considering the bimodal/bidirectional characteristic of Campos Basin sea-states. The analyses were performed using the PETROBRAS’s in-house computer codes ANFLEX, ALFREQ and POSFAL developed and implemented as part of projects from CENPES/PETROBRAS with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”.

Commentary by Dr. Valentin Fuster
2005;():107-113. doi:10.1115/OMAE2005-67040.

Several feasibility studies on the use of large diameters steel catenary’s risers for higher water depths have been made by PETROBRAS. It should be mentioned that the structural design criteria, traditionally employed for such risers under extreme and operational conditions, were based on the API-RP-2RD. This design criterion makes use of single safety factor on von Mises stress. Recently, a new design standard DnV-OS-F201 for metallic risers has been proposed. This standard, based on limit state design principles, are presented on the LRFD (Load and Resistance Factor Design) format, allowing different riser design alternatives to take into account the environmental conditions. This paper presents the main feasibility study outlines of an 18” Steel Lazy Wave Riser (SLWR), attached to the border of a spread-moored FPSO at 1800 water depth, for an extreme design condition, considering both the API and DnV criteria. All numerical analysis were accomplished by the PETROBRAS’s in-house computer code ANFLEX.

Commentary by Dr. Valentin Fuster
2005;():115-122. doi:10.1115/OMAE2005-67046.

Global configuration design of subsea umbilical risers in deep water is a major challenge due to extreme environmental and operational requirements. The critical issues considered in design are the interference between umbilicals in the presence of strong loop and submerged current, and the on-bottom stability along with the strength and fatigue requirements. The vessel motion primarily controls the selection of the configuration, catenary or lazy wave, and the latter is an obvious choice in the presence of significant heave motion. The length and routing on the seabed ensures the on-bottom umbilical stability by dissipating the axial load through soil friction. However, the interference with other subsea components as well as the space availability can also be a governing criterion in the routing. Finally, all these design requirements must be satisfied economically from the perspective of overall cost of the project without compromising quality of the product and safety of design. This paper presents a brief outline of the global configuration design of umbilicals accounting for various design considerations. The host is a semi submersible in a water depth of 6050-ft in the Gulf of Mexico. The lazy wave configurations of the chemical injection and control umbilicals are considered for the study presented herein. The methodology of design for the global configuration is discussed considering different environmental loadings such as the 100-yr and 10-yr loop current, and 100-yr submerged current. The stability of the umbilical on the seabed is discussed on the basis of analysis results for the environmental loadings with dominant vessel motions. The phenomenon of “walking” under the influence of dynamic loading is investigated and the necessary considerations in design to prevent the umbilicals from “walking” are also discussed.

Topics: Design
Commentary by Dr. Valentin Fuster
2005;():123-134. doi:10.1115/OMAE2005-67051.

Pemex Exploration Producción owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of these fixed offshore platforms were built during the 70s and 80s and have already exceeded their design service life. In order to meet the growing demand for oil and natural gas it is necessary to extend the service life of these platforms by at least another 15 to 30 years. To meet this extended service life, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness and possible locations of fatigue problems. To extend the fatigue life of the welded joints, inspections are required to be performed according to a risk based inspection planning procedure. In the present paper, an overview of the reassessment study procedure is outlined and pertinent results are presented for more than twenty platforms which were studied in a recent project. The most important engineering considerations and various analyses involved in the study are discussed in detail. The platforms cover the categories of Drilling, Production, Gathering and Habitation. Depending on the category, Pemex specifications assign different levels of acceptable reliabilities and reserve strength ratios. The ultimate strength of the platforms is determined using a detailed finite element model of jacket, piles and deck structures and a state-of-the-art non-linear progressive collapse analysis technique commonly known as ‘pushover’ analysis. The analytical structural models include local joint flexibility of the jacket joints, soil-pile interaction, geometric non-linearity and material plasticity. They also include the information of damages and deterioration obtained from inspection reports, such as dents, bents, cracks etc. The joint strength modeling is performed using the latest available procedures that use non-linear load-deformation curves. Fatigue analyses are based on spectral analysis technique and include the dynamic response of the structure to wave loads. The structural models for fatigue analysis include the effects of local joint flexibility (LJF) and soil-pile interaction. Results for one typical platform are presented in complete detail to facilitate the understanding of the reassessment study procedure. A typical risk based inspection planning for extending fatigue service life is also presented. Finally, the summary of results for 28 platforms is presented to appreciate the importance of the various analytical parameters. It is hoped that this very wide database of results for platforms of different configurations can serve as a useful resource for the offshore industry in general.

Commentary by Dr. Valentin Fuster
2005;():135-145. doi:10.1115/OMAE2005-67052.

Pemex Exploration y Producción owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of these fixed offshore platforms were built during the 70s and 80s and have already exceeded their design service life. To meet the growing demand for oil and natural gas it was found economic to extend the service life for these platforms by at least another 15 to 30 years. To meet this extended service life, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness and possible locations of fatigue problems. To extend the fatigue life of the welded joints, inspections are required to be performed according to a risk based inspection planning procedure. As a part of the reassessment study non-linear pushover and spectral fatigue analyses are conducted. The effect of joint behavior, viz. the local joint flexibility and strength, on the structural ultimate capacity and fatigue life is discussed in this paper. In conventional analysis the tubular joints are assumed to be rigid and the flexibility effects due to shell deformations are ignored. In this present paper, the effect of the joint behavior is included in the analysis and its implications on the results are discussed. For the ultimate strength analysis both API and MSL formulations for the load-deformation behavior of the joint are studied and compared. For the fatigue analyses, local joint flexibility modeling using Buitrago’s formulation is used. Results including and excluding these effects are compared. Effect of grouting of the joint is also studied. Comprehensive results of the study for a number of platforms, which cover the categories of Drilling, Production, Gathering and Habitation, are presented. The effects of local joint flexibility and joint strength on structural behavior have been recognized to be important in the recent publications of the recommended practices and the codes such as the API RP 2A. However, comprehensive discussions and the results of application of these aspects are rare in the published literature. This paper addresses these issues and presents the results of a large number of platforms of different configurations, indicates some noticeable trends and suggests some general conclusions.

Commentary by Dr. Valentin Fuster
2005;():147-156. doi:10.1115/OMAE2005-67062.

As the exploitation of hydrocarbon moves towards deeper waters, the global dynamic response of a floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to prohibitive computational time. This has spurred the development of more efficient methods that account for the various couplings, including frequency domain approaches. It is paramount for the complex coupling mechanisms to be well understood before appropriate simplifications and assumptions can be made. In this paper, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is examined which captures the important underlying physics. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are stochastically linearized. The model allows key coupling effects to be identified: among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain spectral analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of certain common approximations, and recommendations are made regarding the development of efficient modeling techniques.

Commentary by Dr. Valentin Fuster
2005;():157-164. doi:10.1115/OMAE2005-67065.

A study on the loading of an oblique surface wave and a surface current field on a fixed vertical slender cylinder in a 3D flow frame is illustrated in the present paper. The three dimensional expressions describing the characteristics of the combined wave-current field in terms of mass, momentum and energy flux conservation equations are formulated. The parameters before the interaction of the oblique wave-free uniform current and current-free wave are used to formulate the kinematics of the flow field. These expressions are also employed to formulate and calculate the loads imparted by the wave-current fluid flow on a bottom mounted slender vertical cylinder. The surface current considered in this report, is assumed uniform and acting over a layer of fluid that extends from the free surface to a specified finite depth. Prior work assumes that uniform currents existed over the total depth of the fluid domain. In this paper we extend the approach considered in Zaman and Baddour (2004) for the wave-current analysis. Morison et al equation is deployed for the load computations in all cases. The above model is utilized to compute the loads on a slender cylinder for a wave with varying range of incidence current field. Computations of the moments are also done for the case when current is existed over the whole water depth of the domain.

Topics: Waves , Cylinders
Commentary by Dr. Valentin Fuster
2005;():165-174. doi:10.1115/OMAE2005-67066.

Mooring design for F(P)SOs in West of Africa offshore environment is in many cases governed by the squall driven condition. In the past, the squall condition was typically analyzed by using the peak wind speed with associated wind direction. However, due to its inherent transient nature, the squall event formulated in the time history with varying wind speed and direction is more appropriate and could be potentially more critical for the mooring system design. This approach has been adopted in the design and analysis of recent F(P)SO mooring systems. The F(P)SOs are turret-moored in various water depths in offshore West of Africa. A series of squall time histories have been applied to predict the global responses of the F(P)SO in the time domain. Each squall time history, which provides a unique combination of wind speed and direction variations, is analyzed in five nominal directions covering a sector of 90 degrees from East to West. Squall time histories are also applied to analyze the tandem offloading operation. The results are compared with those of the conventional constant wind speed approach and a few interesting observations are made. The paper also provides some insights into the F(P)SO yaw motions, as well as their relations to the changing wind direction. Analysis results demonstrate that using the squall time series with changing wind speed and direction is more critical than the conventional constant wind speed approach in the tandem offloading scenario. It is therefore recommended that mooring analysis using squall time series should at least be used for the tandem offloading simulations.

Commentary by Dr. Valentin Fuster
2005;():175-184. doi:10.1115/OMAE2005-67073.

Author’s previous work Wanderley [1] presented an efficient numerical method to investigate VIV phenomenon on circular cylinders. The numerical model solves the unsteady Reynolds Average Navier–Stokes equations for slightly compressible flows using the Beam–Warming implicit factored scheme. In the present work, the effect of the turbulence model on the results is evaluated for both Baldwin Lomax and k-ε models. To demonstrate the quality of the numerical method, results for the transversal oscillation of a cylinder laterally supported by spring and damper are compared with experimental data. The application of the turbulence models showed the much better agreement of the k-ε model with the experimental results.

Commentary by Dr. Valentin Fuster
2005;():185-192. doi:10.1115/OMAE2005-67092.

The idea of using float-over installations for minimal facilities platforms was shown to offer significant advantages, especially when coupled with a substructure installed by a jack-up rig. Recently, float-over installations of minimal facilities have been conducted by the cantilevered method by ICON Engineering Pty Ltd (ICON). The operation involves the platform topsides being loaded and transported to site on a barge, skidded over the barge bow, and lowered onto the jacket. The paper presents results of a research project undertaken by the Australian Maritime College (AMC) in conjunction with ICON, with the objective to investigate motions of a barge and loads exerted on the jacket when the two are docked together for a smooth load transfer operation. The model of an installation barge has been tested in the AMC wave basin and response amplitude operators of the barge motions have been determined for both the free floating and docked conditions. A range of wave periods and heights has been investigated. Model test results have been used to verify numerical predictions used in the design, and to get insight into uncertainties, which may otherwise be difficult to assess using standard software.

Commentary by Dr. Valentin Fuster
2005;():193-201. doi:10.1115/OMAE2005-67095.

We study the non-linear decay motion of a 2D plate experimentally and analytically. The plate was hinged to the bottom of a wave flume and was positioned at a certain initial angle. The restoring force on the plate was derived from two horizontal pre-tensioned springs. To maintain the system characteristics linear, the springs were selected to allow a maximum 18 degrees of rotation for the plate. The position, velocity and the acceleration of the plate were retrieved from the load cells attached to the springs. The plate was released from its initial position at t = 0 and allowed to oscillate. The free-surface elevation was captured using a high frame per second (200 fps) digital camera. In addition, two wave probes on either side of the plate were installed. It was observed that the high stiffness of the springs produced a mild impact to the water that caused a relatively large water run-up and water jet. This event, consequently, made the decay motion very non-linear. A formulation based on the linear theory was developed to help with the understanding and interpreting the physics of the problem. The presented experiment aims to benchmark various numerical techniques such as Smoothed Particle Hydrodynamics (SPH) that intend to simulate free-surface and water impact problems. Although the setup did not model a green water incident, most of the features in the problem, like initial water impact, run up and water jet resemble the physics of green water. In the designed experiment, not only body 3D effects were minimum, but also the system characteristics were linear. Moreover, in contrast to the dam break experiments, perfect initial conditions were achieved. Therefore, the effects of the flow nonlinearities such as the plate impact to the water, water run up-down and water jet were studied without interference of the body nonlinearities. The impact of these effects on the damping and the added mass were highlighted.

Topics: Motion
Commentary by Dr. Valentin Fuster
2005;():203-211. doi:10.1115/OMAE2005-67097.

A simple method for solving water impact loads underneath decks of offshore structures is developed. In the present paper the emphasis is on the vertical loads, but in principle the suggested method can also be applied to horizontal loading. The suggested method is three-dimensional and valid for general deck geometries and arbitrary incoming wave direction. First and second order wave amplification due to the large-volume structure is included in the analysis. An important feature of the present approach is that added mass of the instantaneous wetted deck area is approximated by the added mass of thin rectangular or elliptical plates. A numerical tool for solving the impact loads is implemented. This tool uses the results from an a priori second order diffraction analysis of the platform hull. In particular the wave-in-deck simulation program applies linear and quadratic transfer functions from the diffraction analysis as input. Since pre-computed hydrodynamic quantities are used in the simulations, very fast computations can be performed. The method is validated against experiments. Results from scaled model tests of the Statfjord A gravity-based structure (GBS) have been compared to numerical results. The comparisons are limited to regular waves. Satisfactory results are obtained from the numerical simulations. The theoretical results compare well with the experiments for the most severe cases. The vertical loads on the deck are well reproduced both during the water entry phase and the water exit phase. Moreover, the duration of the wave-in-deck event is satisfactorily predicted.

Commentary by Dr. Valentin Fuster
2005;():213-219. doi:10.1115/OMAE2005-67098.

With increasing attention to global warming, expectations for Floating Production Storage and Offloading (FPSO) System to develop a gas field of which reserve is smaller, or Floating Storage and Regasification Unit (FSRU) as LNG terminal are growing. Offloading transfer of LNG requires rigid loading arm instead of flexible hose because of low temperature of LNG. It involves the relative motion estimation between LNG FPSO/FSRU and a shuttle tanker to design loading arm and to estimate operability of offloading work. The operability in environmental conditions of the sites affects on the decision of the storage capacity and the necessary number of LNG carriers for transportation. For this task of motion study, the motion estimation programs for alongside and tandem alignment have been developed. This paper presents outline of the programs and some validation results. An example of application of the calculation results to design FSRU is also shown.

Topics: Motion , FPSO , Mooring , Tankers
Commentary by Dr. Valentin Fuster
2005;():221-229. doi:10.1115/OMAE2005-67100.

Hydrodynamic interaction and clashing of two long flexible cylinders in uniform steady current have been studied. Model tests have been performed in the towing tank at Marintek in Trondheim. The riser models were free to move in vortex induced vibrations (VIV) and wake induced vibrations (WIO). The main objectives of the experimental campaign were to acquire high quality data that can be used to enhance the understanding of the mechanisms that drives riser clashing and to provide benchmark data for riser clashing code validation. Model tests were performed both for straked and naked risers, and for risers with and without bumper elements attached. Riser spacing, inflow angle, riser pretension and current velocity were systematically varied. The analysis of the recorded data has emphasized on assessing the spatial distribution of clashing, the relative impact velocities, WIO and VIV.

Topics: Pipeline risers
Commentary by Dr. Valentin Fuster
2005;():231-238. doi:10.1115/OMAE2005-67104.

The second-order diffraction potential around a truncated cylinder is considered. The solution method is based on a semi-analytical formulation for the double frequency diffraction potential. The later is properly decomposed into three components in order to satisfy all boundary conditions involved in the problem. The solution process results in a Sturm-Liouville problem for the ring-shaped outer fluid region, which is defined by the geometry of the structure. The matching of the potentials along the boundaries of neighborhood fluid regions is established with the aid of the ‘free’ wave component. The calculation of integral of the pressure distribution on the free surface is carried out using an appropriate Gauss-Legendre numerical technique. The efficiency of the method described in the present is validated through comparative numerical results.

Commentary by Dr. Valentin Fuster
2005;():239-244. doi:10.1115/OMAE2005-67111.

The DNV offshore standard for position mooring, DNV-OS-E301 [1], was issued June 2001 based on the result from a joint industry project. A new revision was issued in October 2004. The consequences for mobile units when applying the new standard, compared to the old class rules, have been a major concern for operators of mobile units. A comparison study has therefore been initiated, where four relevant units are considered. We have applied the mooring design for existing mobile units that operate in Norwegian waters. Two different water depths are covered. The new standard, DNV-OS-E301, applies a partial safety factor format. However, the main difference when applying DNV-OS-E301 compared to the old POSMOOR [2] rules is that low frequency (LF) motions will have to be taken into account when calculating the line tensions. The results for the four mobile units are compared also to other relevant codes, i.e. the Draft International Standard ISO 19901-7 [4] and the present Norwegian regulations for offshore structures, NMD [5 & 6]. It should be noted that the present NMD regulations still do not require that LF motions are taken into account. Due to the partial safety factor format in DNV-OS-E301, more or less all of the units fulfill the requirements even though LF motion is accounted for. However, when comparing the results to the NMD regulations, the introduction of LF motion is crucial for almost all of the mobile units studied, as they have problems in fulfilling the requirements when this response is accounted for. Simply including LF motion in design would tend to increase the required strength of the resulting mooring line designs, and thereby raise the safety level if nothing else is done with the NMD regulations. Thus, provided that the present safety level for mobile units is sound, the present NMD safety factors could either be reduced or the partial factor format in DNV-OS-E301 could be adopted in order to maintain the safety level for mobile units when LF motion is taken into account.

Topics: Safety , Design , Mooring
Commentary by Dr. Valentin Fuster
2005;():245-254. doi:10.1115/OMAE2005-67117.

This paper presents a newly developed Floating Regasification Unit (FRU) concept for LNG import in the US Golf of Mexico (GOM) region. The FRU can be converted from a Suezmax tanker and has a turret mooring system. One of the key technologies of the FRU concept is the Side-By-Side (SBS) mooring between the FRU and the LNG Carrier (LNGC) for offloading. The mooring lines and pneumatic fenders between the two vessels should be able to restrain the relative motions to ensure the safe transfer of LNG. Advanced hydrodynamics modeling techniques have been applied, which include full hydrodynamic interactions between the FRU and the LNGC with full QTFs considering the shallow water effect. The anchor legs are modeled as dynamically, fully coupled to the turret and the FRU. The feasibility of the FRU concept is established for LNG import in the GOM, with a variety of workable water depths ranging from 40 meters to more than 100 meters with an uptime exceeding 98%. Analysis results demonstrate the significance of hydrodynamic interactions between the two vessels in close proximity arrangement.

Commentary by Dr. Valentin Fuster
2005;():255-263. doi:10.1115/OMAE2005-67118.

Pipelines are the most convenient and safe medium of oil and gas transportation from offshore fields. Such pipelines will often have free spans when passing through areas with uneven sea floor. These free spans may experience vibrations due to vortex shedding when subjected to ocean current, which may lead to unacceptable accumulation of fatigue damage. The bottom topography may result in multi-span pipelines with short contact area between adjacent spans. Such neighbor spans may interact dynamically, meaning that the response of one span can not be found unless its neighbors are taken into account. The purpose of the present paper is to illustrate possible consequences of interaction and present a consistent method for classification of interaction. Two aspects of interaction are studied, namely the possible energy transfer due to hydrodynamic forces from one span to its neighbor of different length, and how the stiffness properties and length of the contact area between spans influence the interaction. The first aspect is studied by using an empirical model for the analysis of vortex induced vibrations (VIV) of multi-span pipelines. This model applies a frequency domain solution procedure that must describe the interaction between the pipe and the seafloor by linear theory. It was observed from the results that the spans interact dynamically, and the distribution of lift forces along the length of pipe shows that energy is transferred from the span with smallest amplitude to the span with largest amplitude. The second aspect is studied by use of an analysis model where the contact between pipe and seafloor has a more realistic representation. This model is based on a non-linear finite element method, and the dynamic analysis is carried out in time domain. By using this model it is possible to characterize the mechanical coupling between the spans. A systematic study illustrates how seafloor stiffness and local profile of the seafloor will influence the interaction.

Commentary by Dr. Valentin Fuster
2005;():265-272. doi:10.1115/OMAE2005-67119.

The technology for transferring LNG between two floating vessels offshore is a crucial component in floating LNG facilities now in the planning stage. A duplex yoke mooring system and conventional boom-to-tanker LNG offloading arms were recently developed to carry out tandem offloading of LNG in open sea areas with significant wave heights up to 5.5 meters. This paper will present a methodology to evaluate the operability of the LNG offloading in severe environments. The relative motions between the FPSO and the carrier were predicted by a frequency-domain method. A series of model tests have been carried out. A good agreement between the predictions and measurements has verified and validated the analysis method and procedures. Berthing operations in seastates with a significant wave height of 3.5 meters were also conducted successfully. Therefore, it can be concluded that the technology of the duplex yoke mooring and boom-to-tanker loading arms is ready to be implemented for a safe and reliable transfer of LNG in harsh open sea environments for LNG facilities.

Commentary by Dr. Valentin Fuster
2005;():273-277. doi:10.1115/OMAE2005-67121.

This paper presents experimental results concerning flow-induced oscillations of rigid-circular cylinders in tandem. Preliminary results are presented: new measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders in tandem are shown. The oscillations are due to vortex-induced vibrations (VIV). Models are mounted on an elastic base fitted with flexor blades and instrumented with strain gages. The base is fixed on the test section of a water channel facility. The flexor blades possess a low damping characteristic [ζ ≈ 0.008 and less] and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3,000 to 13,000 and reduced velocities, based on natural frequency in still water, range up to 12. The interference phenomenon on flow-induced vibrations can be investigated by conducting experiments in two ways: first, the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base; subsequently, an investigation will be carried out letting both cylinders oscillate transversally. The results for an isolated cylinder are in accordance with other measurements in the literature for m* ≈ 2 and m* ≈ 8. For the tandem arrangement (m* ≈ 2), the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 3·0D to 5·6D. These initial results validate the experimental set up and lead the way for future work; including tandem, staggered and side-by-side arrangements with the two cylinders free to move.

Commentary by Dr. Valentin Fuster
2005;():279-288. doi:10.1115/OMAE2005-67122.

BP Exploration has developed a new concept in providing safety cover for personnel working on offshore installations. The concept involves the use of a new design of rigid inflatable boat, which can be deployed in emergencies to recover casualties from the water, sustain their life and then return them to a surgical facility for primary care. This paper describes the innovative trials run by QinetiQ in order to simulate elements of the vessel and to build a more complete picture of its usability and operational capability. The team has taken a total systems multi-disciplinary approach to the solution, encompassing offshore engineering, surgical expertise and ergonomics. The trials encompass the retrieval of casualties from a wave tank onto an afterdeck mock-up, their treatment under conditions of motion in a large displacement motion simulator and the flow of casualties within a mock-up of the vessel. The paper describes how the data gathered from these trials was then integrated to allow an appreciation of likely casualty handling capability of the vessel.

Commentary by Dr. Valentin Fuster
2005;():289-295. doi:10.1115/OMAE2005-67128.

Lazy-wave steel risers appear as a possible solution for ultra deepwater oil fields in Campos Basin. The design of such a solution, however, is a very time consuming task as several configurations must be studied, including static, dynamic and fatigue analysis. In the first cycle of design, simplified models can be used to speed up the selection process of feasible configurations. This paper presents a parametric analysis that was implemented on a computer tool, aiming to select the feasible geometric configurations for a lazy-wave steel riser in the first cycle of design.

Commentary by Dr. Valentin Fuster
2005;():297-307. doi:10.1115/OMAE2005-67130.

Numerical integration of the governing differential equations of three-dimensional riser statics may experiment large stability problems due to the inclusion of bending stiffness, as the leading order term becomes usually very small. These numerical problems may be avoided by working with a perfectly flexible cable model. However, such a model cannot deal with all the boundary conditions, as for an ideal cable there is no continuity of curvature at the touchdown point, at the top and at the points where there is change in the submerged weight. At the touchdown region, for instance, the perfectly flexible cable model overestimates the maximum curvature. To overcome these difficulties, one approach that can be adopted is to use firstly a perfectly flexible cable model and correct later the results with analytical expressions obtained from a boundary layer method. This approach is based on the fact that bending stiffness is relevant only at small boundary layers around the points where the perfectly flexible cable model cannot represent the curvature continuity. For a two-dimensional formulation it was already shown that this approach is very good. For a three-dimensional formulation, however, the analytical expressions become too cumbersome, the authors could not so far find an exact analytical solution and, therefore, the problem must be solved numerically. Another possible approach is to use finite elements method, as many full nonlinear commercial softwares do. However, it is not difficult to face convergence problems. This work presents a numerical method to solve the set of differential equations of the three-dimensional riser statics, including the bending stiffness. The results obtained are compared to a full nonlinear well-known commercial computer code.

Commentary by Dr. Valentin Fuster
2005;():309-317. doi:10.1115/OMAE2005-67131.

In deep water oil production, Dynamic Positioning Systems (DPS) strategy has shown to be an effective alternative to tugboats, in order to control the position of the shuttle tanker during offloading operations from a FPSO (Floating Production, Storage and Offloading System). DPS reduces time, cost and risks. Commercial DPS systems are usually based on control algorithms which associate Kalman filtering techniques with Proportional-Derivative (PD) or Optimal Linear Quadratic (LQ) controllers. Since those algorithms are, in general, based on constant gain controllers, performance degradation may be encountered in some situations, as those related to mass variation during the loading operation of the shuttle tanker. The positioning performance of the shuttle changes significantly, as the displacement of the vessel increases by a factor of three. The control parameters are adjusted for one specific draught, making the controller performance to vary. In order to avoid such variability, a human-based periodic adjustment procedure might be cogitated. Instead and much safer, the present work addresses the problem of designing an invariant-performance control algorithm through the use of a robust model-reference adaptive scheme, cascaded with a Kalman filter. Such a strategy has the advantage of preserving the simple structure of the usual PD and LQ controllers, the adaptive algorithm itself being responsible for the on-line correction of the controller gains, thus insuring a steady performance during the whole operation. As the standard formulation of adaptive controllers does not guarantee robustness regarding modeling errors, an extra term was included in the controller to cope with strong environmental disturbances that could affect the overall performance. The controller was developed and tested in a complete mathematical simulator, considering a shuttle tanker operating in Brazilian waters subjected to waves, wind and current. The proposed strategy is shown to be rather practical and effective, compared with the performance of constant gain controllers.

Topics: Tankers
Commentary by Dr. Valentin Fuster
2005;():319-326. doi:10.1115/OMAE2005-67133.

A set of experimental results on the dynamics of taut-leg mooring-lines was generated by means of towing-tank tests. These results will be employed by Petrobras as an experimental paradigm for the calibration and validation of numerical codes based on finite-elements method (FEM). The setup allowed combining first and second order motions on the top of the line with different amplitudes and frequencies. The first order motions were emulated by means of circular harmonic motions while alternate horizontal translations represented the drift motions. It was also possible to emulate a uniform in-plane current profile along the suspended length of the model. The model was composed by three different segments. An intermediate rubber hawser connected the top and anchor chain segments and allowed considerable elongation of the line during the tests. Tension at the top of the model was measured by means of a load cell and the second-order motion was registered optically. This paper presents the procedure adopted for the tests and also some preliminary comparisons between experimental results and numerical simulations.

Commentary by Dr. Valentin Fuster
2005;():327-335. doi:10.1115/OMAE2005-67134.

The response of a combined Spar/ risers/mooring lines system is conventionally determined by conducting nonlinear time domain analysis. The system nonlinearity is introduced by the mooring nonlinear force, the friction between the buoyancy-can and the preloaded compliant guide, and the quadratic model of the fluid related damping. Obviously, during the design process, it is important to understand the sensitivity of the Spar responses to various parameters. To a great extent, these objectives cannot be readily achieved by using time domain analysis since, in this context, elements with frequency dependent representation such as the added masses and supplementary damping must be incorporated in the analysis; this may require the use of elaborate convolution techniques. This attribute of the time domain solution combined with the necessity of running a significant number of simulations makes it desirable to develop alternative methods of analysis. In the present paper, a frequency domain approach based on the method of the statistical linearization is used for conducting readily a parametric study of the combined Spar system. This method allows one to account by an equivalent linear damping and an equivalent linear stiffness for the mooring nonlinearity, friction nonlinearity, and the damping nonlinearity of the system. Further, frequency dependent inertia and radiation damping terms in the equations of motion are accommodated. This formulation leads to a mathematical model for the combined system, which involves five-by-five mass, damping and stiffness matrices. In the solution procedure, the equivalent parameters of the linear system are refined in an iterative manner, and by relying on an optimization criterion. This procedure is used to assess the sensitivity of representative Spar system responses to various design parameters. Further, the effect of various design parameters on the combined system response is examined. The environmental loadings considered are of the JONSWAP format of a 100-yr hurricane in the Gulf of Mexico.

Commentary by Dr. Valentin Fuster
2005;():337-347. doi:10.1115/OMAE2005-67137.

Dynamic positioning systems (DPS) comprise the utilization of active propulsion to maintain the position and heading of a vessel. Sensors are used to measure the actual position of the floating body, and a control algorithm is responsible for the calculation of forces to be applied to each propeller, in order to counteract all environmental forces, including wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous power to be attenuated, possibly causing damage to the propeller system. A filtering algorithm is then used to separate high frequency components from the low frequency ones, which are indeed controlled. Usual commercial systems apply Kalman filtering technique to perform such task, which includes a full model of the system. Furthermore, an adaptive on-line estimation algorithm is also used to evaluate the wave peak frequency, since the model in Kalman Filter depends on such parameter. The controller itself is based on a simple proportional-derivative (PD) actions. This paper presents all the mathematical formulation of the Kalman Filter, adaptive algorithm and the controller used in commercial DPS and performs a critical analysis of those models. Some illustrative results of a dynamic positioned shuttle vessel are presented, considering the incidence of waves, current and winds.

Commentary by Dr. Valentin Fuster
2005;():349-356. doi:10.1115/OMAE2005-67138.

This paper describes a component approach of coupled motions for design of deepwater CALM offloading system in West Africa environment. Confident offloading buoy motions coupled with mooring and offloading line dynamics is identified as one of the key design challenges. In deepwater systems, components from the wave forces (exciting forces and radiated wave forces), viscous damping forces and mooring forces follow different scaling laws. We can not properly scale up the measured global responses of the coupled system to full-scale to verify the design. Component approaches overcome many of the test engineering and scale up weaknesses associated with truncated physical modeling. An effective application of a component approach develops a model test strategy for the purpose of validating the design analysis tools. In this paper we present a strategy for model testing, design tool validation and full-scale analyses. Differences between the component approach and the current industry practice are highlighted.

Topics: Motion , Buoys
Commentary by Dr. Valentin Fuster
2005;():357-365. doi:10.1115/OMAE2005-67139.

Pipelines located on the decks of FPSO systems are exposed to damage due to sea waves induced random loading. In this context, a methodology for estimating the fatigue life of conveying-fluid pipelines is presented. The pipeline is subjected to a random support motion which simulates the effect of the FPSO heaving. The equation of motion of the fluid-carrying pipeline is derived by assuming small amplitude displacements, modeling the empty pipeline as a Bernoulli-Euler beam, and adopting the so-called “plug-flow” approximation for the fluid (Païdoussis, 1998). Random vibration analysis is carried out by the Galerkin method selecting as basis functions the natural modes of a beam with the same boundary conditions as the pipeline. The discretized equations of motion are used in conjunction with linear random vibration theory to compute the stress spectrum for a generic section of the pipeline. For this purpose, the power spectrum of the acceleration at the deck level is determined by using the Response Amplitude Operator of the FPSO hull. Finally, the computed stress spectrum is used to estimate the pipeline fatigue life employing an appropriate S-N fatigue curve of the material. An illustrative example concerning a pipeline simply-supported at both ends is included in the paper.

Commentary by Dr. Valentin Fuster
2005;():367-375. doi:10.1115/OMAE2005-67140.

This paper describes an analytical implementation of the component approach for motion predictions of a deepwater CALM buoy as described in the companion paper “Component Approach for Confident Predictions of Deepwater CALM Buoy Coupled Motions — Part 1: Philosophy”. The implementation of the approach starts with a “model-of-the-model” validation of the analytical tool. Emphasis is given to making an accurate analytical characterization of the model as tested. To capture the strong coupling between the buoy motions and line dynamics the analyses described herein were carried out in the time-domain. This allows a rigorous treatment of the hydrodynamic forces on the buoy as well as the non-linear mooring loads when analyzing the buoy responses in waves. Since the validation analysis is a model-of-the-model practice at model scale, the proper application of the validated tool to the full-scale system is discussed. This involves modeling of the exact full-scale system and the proper selection of the hydrodynamic coefficients for the buoy and lines. In this paper we will present the numerical modeling procedures and the results from validation work to confirm that the analytical tool is validated correctly. Detailed results from validation analysis versus model test data will be shown for system components including buoy hydrodynamics from the forced oscillation test, line tension from line oscillation test, and the motions and tensions of integrated buoy/mooring/riser system. We point out that the hydrodynamic coefficients at model scale can not be directly applied to the full-scale system analysis even though they are from model test measurements. We will present the difference between the results of the model-scale system using model scale hydrodynamic coefficients and those based on a proper range of the coefficients at full-scale. This will highlight the need to design component tests to determine appropriate full scale coefficients in order to improve the accuracy of full-scale design predictions. These results will show the advantages of adopting a component approach over the common industry practices in the areas of correct use of model test data, validation analysis and the analysis of the coupled CALM buoy system responses in waves.

Topics: Motion , Buoys
Commentary by Dr. Valentin Fuster
2005;():377-382. doi:10.1115/OMAE2005-67143.

Buoyancy Can Riser Tensioner (BCRT) systems provide tension to the Top Tensioned Riser (TTR) systems. The main benefit of employing BCRT is that it can minimize the interaction between the floating platform and riser system. The possible lateral impact and dynamic loads which may occur between the SPAR hull and BCRT are minimized by placing the so-called Compliant Guides (CGs) at the sliding interfaces. The CGs are designed to allow relatively free sliding of the BCRT in the riser axial direction, while preventing/minimizing lateral impact loads by providing lateral compliance. CGs are made of high stiffness and abrasion resistant material, such as elastomer materials. Objective of this study is to develop a numerical capability and analysis procedure to evaluate the spar-riser interaction mechanism and their effects on spar motion and riser response. The newly developed frictional interface element is an essential component of an FE model for the time simulation of the coupled motions of SPAR and riser systems. The algorithm can be used to investigate the dynamic and frictional interaction between the SPAR and BCRTs, such as the friction’s impact on the SPAR motion and riser stresses, and the dynamic load/wear requirements on the CGs.

Commentary by Dr. Valentin Fuster
2005;():383-389. doi:10.1115/OMAE2005-67144.

A multi-strip numerical method combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a finite-element structural dynamics response has been developed previously to analyze the flow-structure interaction of a flexible riser subjected to fixed and non-uniform, two-dimensional shear currents. In this paper, we expand on the previous work using the tool to numerically compute the VIV loads and motions of a vertically tensioned riser in a stepped current. The flow conditions for this stepped current configuration were chosen to match a set of laboratory experiments carried out in the Delta Flume at Delft Hydraulics. In addition to the stepped current, the multi-strip method was extended to accommodate a three-dimensional skew current exposed to a vertically tensioned riser. Note that in this case, the skew current is non-uniform in both direction and magnitude, and the flow conditions and riser configuration were chosen to match a set of rotating-rig experiments made by Marintek. For both configurations (stepped and skewed currents), comparisons of in-line and transverse VIV displacements are presented between numerical and experimental results.

Commentary by Dr. Valentin Fuster
2005;():391-398. doi:10.1115/OMAE2005-67148.

This paper investigates the hydrodynamic interaction between several vertical cylinders freely floating in each other’s close vicinity. The physical aspect of hydrodynamic interaction is rather complicated and numerically accurate scheme is highly recommended to study this complex phenomenon. In the present study, 3D source-sink method has been adopted to determine the hydrodynamic coefficients and wave exciting forces by taking into account accurately the effect of hydrodynamic interaction among the different floating bodies. In order to check the convergence of numerical results, mean wetted surface of the floating cylinders are divided into different numbers of rectangular panels. A computer code is developed using 3D source-sink formulations and the validation of the computer code has been justified by comparing the present results with that of the published ones. In order to study the interaction effect, numerical results for a single cylinder have been compared with those of the multi-cylinder ones. The separation distance between the cylinders has been varied to examine the interaction effect for head sea condition. Finally some conclusions have been drawn on the basis of the present analysis.

Topics: Waves , Cylinders
Commentary by Dr. Valentin Fuster
2005;():399-403. doi:10.1115/OMAE2005-67153.

Optimization techniques for shipbuilding have been developed to design large-scaled commercial vessels based on classification rules and analytical data, but most of the target model in the past were restricted to the mid-ship section design in the preliminary design stage. By contrast, preceding researches about the design optimization of detailed structures are relatively rare since narrow information has been provided as the design guide for such structures. In this study, the minimum weight design of a ship hull block was carried out based on the finite element analysis. The target model for design optimization is a stiffened deck block of LNG carrier for upholding a main boiler. The weight of a ship hull block and the main dimensions of structural members are chosen as an objective function and design variables, respectively. An optimization algorithm was adopted to escape the local minima and reduce CPU time in analysis procedure, and finite element analysis was performed to determine the constraint parameters at each iteration step in optimization loop. Optimization results were compared with an existing ship, and the effects of optimum design were examined from various points of view.

Commentary by Dr. Valentin Fuster
2005;():405-412. doi:10.1115/OMAE2005-67155.

In this paper a nonlinear fiber elements is used for modeling of pile soil intraction. In the model, both of steel pile and surronding soil nonlinear behavior is considered using fiber element. In this paper the model is developed using DRAIN-3DX software. The method used in this paper, however, allows pile and surronding soil inelastic behavior to be modeled accurately using a single elements. The model is used to simulate nonlinear behavior of pile -soil system and the results are compared with the other analytical and available experimental results. The lateral capacities of offshore piles can be calculated using methods presented in this paper. The analysis results using method presented in this paper in terms of pile head load deformation, pile lateral capacity and pile internal forces are in a good agreement with the other available analytical or experimental results. The model can be used for the pile soil structure interaction analysis of jacket type offshore structures.

Commentary by Dr. Valentin Fuster
2005;():413-420. doi:10.1115/OMAE2005-67156.

A method for predicting interaction between risers is presented. The method is efficient and handles different riser systems exposed to complex environmental data. To achieve this, the method utilizes pre-established data for forces on risers in close proximity. Interaction effects concerning mean values as well as the dynamic forces at vortex shedding frequencies are stored. The interaction effects cause large wake induced oscillations (WIO), and the vortex induced vibrations (VIV) are influenced as well. The method uses the database of forces in a strip theory manner to obtain excitation forces on risers and other slender bodies exposed to current. The method is implemented in a non-linear dynamic finite element tool, HYBER. During the simulation, the clearance between the risers is followed up. If collisions occur, relevant data are recorded, and the simulation continues. Fatigue and possible single event damage are assessed. In the present paper, results from the method are compared with comprehensive measurements in model scale, and the computed results show good agreement with the measurements. Databases exist for bare cylinders with equal and unequal diameters as well as for two geometries of strakes.

Commentary by Dr. Valentin Fuster
2005;():421-429. doi:10.1115/OMAE2005-67171.

The objective of this work is to describe and discuss a design process organization approach named ‘Solution-Focused Design’. This method has been created and used successfully for AMV, Ship and Offshore Platform Design over the last ten years with reasonable advantages. It was first created in a context were the decision of what concept to be adopted supersedes the use of the classical design spiral, suggesting a combination of the spiral with morphological charts, since the design processes of the concurring concepts are completely different from each other. The advantages are in terms of allowing for the introduction of creative ideas into the Conceptual Design process, eventually leading to an innovative product or design solution, establishing a sound design sequence and rationalizing the search for design tools and knowledge, parameter and task organization and classification (free variables, restrictions, performance specifications) and the design process itself. In this work the Solution-Focused Design process is described and compared to some usual ones, which normally rely on ‘problem-focused’ strategies for problem solving. It is shown how the process evolves from a pre-established scenario and a design briefing or specification to a Design Methodology. This is done in brainstorming sessions, using sketches and interactive design flowcharts (similar to design spirals), adapted for this approach. The objective is to provide the designer with means to define quickly and efficiently the optimal configuration of the design, while incorporating novelties into it. In order to show some features of the approach, three different examples of designs previously developed are presented: one for a SPAR Buoy Platform, other for an FPSO, and another for the feasibility and concept of a ‘fully submersible semi-displacement fast boat’, with morphological comparisons of different concepts of means of support in the later.

Commentary by Dr. Valentin Fuster
2005;():431-436. doi:10.1115/OMAE2005-67174.

Brazil Campos Basin is under large offshore oil production activity. Many new giant oil fields are under development. Brazil has been using tankers ship converted as FPSO platform vessel. At this moment three FPSO are in final construction phase to be installed in Campos Basin. Many oil cargo tankers are transformed to work as FPSO. Although they are showing good characteristics, some problems are having been emerging. The turret design approach was used in the primary FPSO’s but recently the spread mooring system with rigid risers is also under installation in Campos Basin. New ideas about Floating Production Storage and Offloading (FPSO) system have been in focus since some projects started working in an appropriately FPSO design direction. This paper spotlights one of these ideas: the FPSO tankers arrangement as a possible optimization procedure. MARPOL rule is followed but an alternative procedure is suggested to consider vessel control under wave’s action and at the same time looking for stress minimization. The aim in this procedure is increase the operator capability. The idea is allow the FPSO operator change the load condition using ballast in segregated tanks assuring a more suitable vessel condition in waves. Stability, wave response, stresses and operation ability should be also merit functions in any optimization process. A box shape FPSO-BR is used to highlight the procedure application. Results are presented and conclusion pointed out.

Topics: Design , Optimization , FPSO
Commentary by Dr. Valentin Fuster
2005;():437-447. doi:10.1115/OMAE2005-67178.

Petrobras is considering the single-line FSHR (Free Standing Hybrid Riser) design as an option for 18-in export risers in 1800 m water depth. This paper provides the background on the studies carried out to achieve confidence that the concept is feasible for such application, and explains the main features of the design that take best advantage of local practices and Petrobras capabilities. The installation studies consider a Mobile Offshore Drilling Unit (MODU) for deployment of the FSHR system. The analyses assess the allowable sea states for such operation. Another design driver is the fatigue damage due to VIV (Vortex Induced Vibrations), which will also be addressed in the paper. Model tests were performed in order to support the conclusions of the numerical analyses that established the operation window for the FSHR system deployment.

Topics: Water , Hybrid risers
Commentary by Dr. Valentin Fuster
2005;():449-464. doi:10.1115/OMAE2005-67185.

The proposed paper shall list and present the issues from a mooring and hydrodynamic point of view relative to the design of a floating structure connected to another floating structure in surface and to the seabed with traditional anchoring legs, such as a Flotel with an F(P)SO. The paper will propose tentative ways to properly design the connections between the two floating structures which will limit their relative excursions in order to allow the setting of a gangway between them. In addition, the design should forbid at any time any potential contact between the two structures in intact and damage conditions, and should trigger at any time acceptable tensions in the various anchoring and mooring systems. The paper will address the appropriate mooring and hydrodynamic analysis to be used for such exercise. Safety issues such as quick disconnection in case of fire are investigated. Operational issues are to be discussed as well.

Commentary by Dr. Valentin Fuster
2005;():465-476. doi:10.1115/OMAE2005-67191.

This paper deals with the parametric study of relative wave heights of ships in short crested irregular seas. Here linear potential theory has been used to describe the fluid motion and 3-D sink-source technique with forward speed has been used to determine hydrodynamic forces for surface ship advancing in waves at constant speed. The time domain simulations of relative wave heights of typical container ship, bulk carrier, pure car carrier (PCC) and general cargo ships in short crested irregular waves have been carried out for three different sea states and for the parametric study of these vessels, different sizes of ships have been considered. Empirical roll damping has been taken into account in time domain analyses of motion responses of sea going ships in rough seas and for ensuring longer time simulation of the random sea waves, unequal frequency spacing has been used. Finally, the numerical results of the maximum and the significant values of irregular relative wave heights are discussed by comparing with some requirements by a classification society of shipping for the different sizes of ships which might provide some important information for the designer of ocean going vessel to assess possible deck load or bottom impact force on future regulation of designing ship.

Topics: Waves , Ships , Seas
Commentary by Dr. Valentin Fuster
2005;():477-483. doi:10.1115/OMAE2005-67192.

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex induced vibrations (VIV). This reduction will increase fatigue life, and also reduce drag magnification from cross-flow vibrations. But sections with strakes will also have a larger drag coefficient than the bare riser. Hence, the extension of a section with strakes along a riser should be large enough to reduce oscillations, but not too long in order to limit drag forces from current and waves. The optimum length and position for a given riser will therefore vary with current profile. Dynamic response from waves should also be taken into account. The purpose of the present paper is to illustrate the influence from strakes on VIV, as well as on static and dynamic response for a drilling riser. Hydrodynamic coefficients for a cylinder with helical strakes are found from experiments and applied in an empirical model for the analysis of VIV. The result from the VIV analysis is used for a second calculation of drag forces that are applied in an updated static analysis. Dynamic stresses from regular waves are also presented, but VIV are not considered for these cases. A simple study of length and position of the section with strakes is carried out for some standard current profiles. Results are presented in terms of oscillation amplitudes, fatigue damage, bending stresses and riser angles at ends. The study is based on test data for one particular strake geometry, but the analysis method as such is general, and the computer programs used in the study can easily apply other test data.

Commentary by Dr. Valentin Fuster
2005;():485-493. doi:10.1115/OMAE2005-67201.

The torpedo anchor is a novel kind of device to moor floating offshore structures. It has been proved in practice that this kind of anchoring may be used for both drilling and production offshore activities. For drilling, it is indeed easily recoverable and for large production, it has enough holding power even for large production platforms. There are a lot of soil-interaction aspects to be considered and the installation is one of them. The installation procedure is to release the torpedo from a high enough position from the sea bottom to allow the device to reach the terminal velocity: A correct amount of kinetic energy at the bottom is essential for the penetration. Besides this, the anchor has to reach the bottom in a vertically up right in order to maximize the final holding power in all directions. Therefore, the work addresses two hydrodynamic aspects for the installation design and analysis. The first is the drag minimization and the second is the directional stability. If the drag is be kept to a minimum (without compromising, later on, the soil interaction) then the terminal velocity is higher. The work shows that parameters like the mass and the shape are essential for this. On the other hand, the shape and mass distribution have a strong influence on the directional stability. One important parameter is the rear line length connected to the anchor, which is necessary for further connection with the final mooring line: this parameter influences both the terminal velocity and the directional stability. The presence of the rear line and its role is a novel problem and it seems to have no parallel in other filed applications. The work addresses all this aspects under the light of a novel model testing performed in a model basin that is 15 m deep. It is important to say that this model testing procedure has been conceived to attend specifically the torpedo anchor evaluation. For that matter, the work presents an extrapolating mathematical model. Besides that, an analytical model is shown for the directional stability, together with time domain numerical evaluation. Different model have been used in the tests performed with and without the rear line. Finally, the work presents the model testing design including the use of imaging processing to get the anchor tracking during the launching.

Commentary by Dr. Valentin Fuster
2005;():495-500. doi:10.1115/OMAE2005-67203.

The Exploration and Production Department of Petrobras asked the R&D Center the development of a production system for 1800 m water depth, in Campos Basin, which would allow the installation of Steel Caterany Risers (SCR) in the starboard side of a production platform. The subsurface buoy concept was chosen as one of the alternatives. This concept has being developed since the preliminary studies conducted in the first phase of JIP Deepstar. This concept has an advantage of uncoupling the movements of the platform from the risers, reducing the loads due to the risers in the platform and allowing the installation of this system before the installation of the production vessel, anticipating the production of the field. This article shows the main characteristics of the buoy, its sizes, results of structural analyses and installation procedures for a buoy which sustains 14 SCR and 5 umbilicals in one side and 14 flexible jumpers and the same 5 umblicals in the opposite side. This concept was tested in two model test tanks, where it was verified that this concept is feasible. Also, information related to a field trial with a prototype installed in Brazilian waters will be presented.

Commentary by Dr. Valentin Fuster
2005;():501-510. doi:10.1115/OMAE2005-67225.

The results of a numerical study of the viscous oscillating flow around a pair of circular cylinders are presented herein, for a constant frequency parameter, β, equal to 50, and Keulegan-Carpenter numbers, KC, ranging between 0.2 and 10. The cylinders were placed side-by-side to the oncoming flow, for a pitch to diameter ratio, P/D, equal to 2. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables. The vorticity contours generated from the solution were used mainly for the flow visualization, whereas the stream-lines and isobars are shown in some cases. At low values of the Keulegan-Carpenter number the flow remains symmetrical with respect to the horizontal axis of symmetry of the solution domain. As the Keulegan-Carpenter number is increased asymmetries appear in the flow, which are eventually amplified and lead finally to more complicated vortex-shedding patterns. These asymmetries generate an aperiodic flow configuration at consecutive cycles, which becomes almost chaotic as KC grows larger. For the various Keulegan-Carpenter numbers examined the time-histories of the hydrodynamic forces are presented, and the r.m.s. values of the hydrodynamic forces and the coefficients of the in-line force were evaluated.

Commentary by Dr. Valentin Fuster
2005;():511-519. doi:10.1115/OMAE2005-67229.

The present paper reports on the application of Risk Based Inspection (RBI) planning as conducted on a recent project for Pemex Exploración y Producción. The project considers the engineering assessments required for a possible service life extension of 23 fixed steel jacket structures located in the Bay of Campeche. In the project RBI plays an important role as this quantifies the relation between the overall performances of the platforms in their present condition with their future deterioration and facilitates the identification of strategies for cost efficient condition control by means of targeted inspection and maintenance activities and if necessary repair and strengthening. For the purpose of facilitating efficient RBI for larger number of structures of more or less similar characteristics, a generic scheme for RBI is pursued. The specific aspects of the generic RBI for the considered group of structures in regard to risk acceptance criteria, modeling of inspection qualities and the probabilistic fatigue and fracture mechanical modeling are outlined. Finally also a study is reported whereby the benefit of the RBI approach is assessed as a means of risk control in comparison to standard code based requirement for inspection planning.

Commentary by Dr. Valentin Fuster
2005;():521-528. doi:10.1115/OMAE2005-67237.

Cylindrical Shells are widely used in many structural designs, such as offshore structures, liquid storage tanks, submarine hulls, and airplane hulls. Most of these structures are required to operate in a dynamic environment. The acoustic signature of submarines is very critical in such high performance structure. Submarines are not only required to sustain very high dynamic loadings at all time, but also being able maneuver and perform their functions under sea without being detected by sonar systems. Reduction of sound radiation is most efficiently achieved at the design stage, and the acoustic signatures may be determined by considering operational scenarios, and modal characteristics. The acoustic signature of submarines is generally of two categories; broadband which has a continuous spectrum; and a tonal noise which has discrete frequencies. Therefore, investigating the dynamic characteristics of cylindrical shells is very critical first step in developing a strategy for modal vibration control for specific operating conditions. Unlike those of beam structure, the lowest natural frequency does not necessarily correspond to the lowest wave index. In fact, the natural frequencies do not fall in ascending order of the wave index in cylindrical shells. Mode shapes associated with each natural frequency are combination of Radial, Longitudinal, and Circumferential modes. In this paper, a scaled model of submarine hull segment under shear diaphragm boundary conditions is analyzed analytically and numerically. Then experimental modal analysis of the scaled model utilizing a fixed response approach was performed to obtain the modal characteristics of the cylindrical shell between 0 and 800 Hz. The cylinder was excited at predetermined points with an impact hammer, while the response was measured using an accelerometer at specified fixed point. Designing a boundary condition that simulate a shear diaphragm is very challenging task by itself. A total of ten natural frequencies were found within that range with their corresponding mode shapes. The experimental data were correlated with those results obtained analytically and numerically using the finite element methods using MSC.NASTRAN software. The results were found to be in excellent agreement.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2005;():529-541. doi:10.1115/OMAE2005-67245.

Despite designer’s best efforts, Vortex-Induced Motion (VIM) of spars remains a significant problem. While strakes have proven highly effective at suppressing riser VIM, all three straked classical spars in the Gulf of Mexico continue to experience occasional VIM events. Since VIM can cause riser damage, mooring line fatigue, and even work stoppage, its suppression is a major priority. But designers will only successfully eliminate VIM once reliable predictions tools become available. VIM is both roughness and Reynolds number dependent, and spars lie outside the validated range of current computational tools in both these areas. This paper presents the development of Reynolds-Averaged Navier-Stokes (RANS) methods to predict real-world spar VIM behavior. It includes the ability to address rough surfaces and high super-critical Reynolds numbers. Validation is provided against existing experimental data where available. The resulting algorithms were used to assess effectiveness of two alternative control strategies for suppressing spar VIM. The first strategy consists of actively injecting fluid in a direction tangential to the spar. The second strategy is passive, and relies on adding an external sleeve to the spar. Active injection was found highly effective, and even capable of totally eliminating VIM. The passive alternative was also found to reduce VIM, but is not as effective as active injection.

Commentary by Dr. Valentin Fuster
2005;():543-547. doi:10.1115/OMAE2005-67246.

Conventional optimal control theories usually deal with optimal control force that involves only structural information. In order to control the excessive vibrations of offshore platforms more effectively, this paper presents a control strategy, which employs both information of the controlled system and external loading, namely feedforward-feedback (FF-FB) control strategy. FF-FB control strategy requires that the input signal be a process of white noise for obtaining the optimal control result. So, an analog filter is designed in this paper to approximately represent the random wave force by a process of white noise. In order to investigate the feasibility and effectiveness of the proposed method, a numerical example applied to an offshore platform is presented in this paper. The numerical results demonstrate that the proposed method offers advantage in performance over the control method employing only state feedback.

Commentary by Dr. Valentin Fuster
2005;():549-555. doi:10.1115/OMAE2005-67257.

In deep and ultra-deep water petroleum exploitation activities, floating production systems such as semi submersible platforms and FPSO (Floating Production, Storage and Offloading) units have been commonly employed. However, the utilization of flexible risers in ultra-deep waters has been hindered by technical and economical reasons. On the other hand, first order motions from the floating unit due to environmental loads are not favorable to the use of Steel Catenary Risers (SCR) in a free-hanging configuration. This fact has motivated several studies on hybrid riser systems, including the system studied in this work, which is based on a sub-surface buoy with large dimensions, moored to the seabed by tethers. This system employs flexible lines connecting the floating unit to the buoy, in the region where dynamic effects are more relevant due to the floating unit motions, and also SCRs that extend from the buoy to the seabed, in the region where dynamic motions are not so significant. The objective of this work is to describe a solution procedure for the analysis of such a hybrid riser system. This procedure is based on an analytical formulation that is solved numerically. One of the main features of this procedure is the fact that it takes into account the effects of current loads acting on the lines. Current profiles can be considered, with direction and velocities varying with depth, therefore configuring a full three-dimensional solution. This procedure can be employed either as a preliminary static analysis tool, to be used in parametric studies in order to assess the feasibility of candidate configurations of hybrid riser systems, or else for the generation of finite-element meshes for a full time-domain nonlinear dynamic simulation. It is important to start the dynamic simulation from a statically balanced configuration, since the transient effects can be dramatically shortened and the total simulation time can be reduced. The results obtained from this procedure are compared with a discrete solution obtained using a nonlinear finite-element based solver. The strategy considered here is intended to be an approach that will speed up the tasks involved in the design of hybrid risers systems based on the subsurface buoy concept.

Commentary by Dr. Valentin Fuster
2005;():557-566. doi:10.1115/OMAE2005-67279.

The paper presents two different approaches to construct subsets of current profiles from a large set of long term current profiles for the purpose of performing calculations for riser fatigue damage from vortex induced vibrations (VIV). The subsets are intended to reproduce the fatigue damage from the full set of current profiles. In the first approach, the full set of profiles is first sorted into bins based on current magnitude, direction and shear in the profile. The profiles within each bin are then reduced to a single constructed profile through one of many possible current averaging schemes. The present study includes two types of constructed profiles; one profile is generated by the average value of the currents for each bin and the other by the average value plus one standard deviation. The second approach is based on first performing a simplified and computationally efficient VIV analysis of the full set of profiles. The profiles are then sorted into bins by the dominant excitation mode, and then a single profile is chosen to represent all the profiles that excite the mode of interest. The chosen profile for the mode of interest has VIV power-in which is close to the average power-in for all the profiles that excite the mode. The number of profiles in the subset is equal to the number of modes that are excited by the full set of profiles. The VIV power-in in this paper is estimated through a simplified procedure that is consistent with the SHEAR7 methodology. Other available codes can also be used for the simplified VIV calculations.

Commentary by Dr. Valentin Fuster
2005;():567-572. doi:10.1115/OMAE2005-67283.

A protective cushion layer sandwiched between a gravity type caisson and the surrounding soil can improve the seismic stability of the caisson. In this paper, an interaction model was developed for analyzing such structure under earthquake loading. Interfaces on either side of the cushion were modeled as elements of finite thickness having different stiffness and constitutive properties. Participation from each element of the interaction system was taken into the account by incorporating appropriate factors at the respective interfaces. Seismic analyses were performed on a gravity-type caisson subjected to an actual earthquake motion. Comparison of the analysis results with a similar caisson without any protective cushion showed that the use of cushion yields a significant reduction of seismic load on the structure.

Commentary by Dr. Valentin Fuster
2005;():573-577. doi:10.1115/OMAE2005-67284.

Cylindrical offshore storage tanks are mainly used to store critical liquids such as petroleum and LNG. Failure of these tanks after an earthquake not only results in high financial losses but can also create an environmental disaster as well. Seismic isolation technique has been utilized in this study to mitigate the damaging effects of lateral earthquake forces. In this study, seismic isolation is used in cylindrical offshore storage tanks. Seismic isolators are protected from the marine environment by a water-proof flexible membrane that allows the horizontal movement of the tank. The mathematical model takes the following into account: mass of the stored liquid, added mass of the water surrounding the tank, horizontal stiffness and damping of the isolation system. Internal forces due to the hydrostatic and hydrodynamic forces acting on the system during an earthquake have been calculated by using the shell-moment theory. The results of the analyses are compared for both the isolated and fixed-base tanks.

Commentary by Dr. Valentin Fuster
2005;():579-584. doi:10.1115/OMAE2005-67287.

The spectral analysis of nonlinear random wave loadings on circular cylinders is performed in this paper by means of nonlinear spectral analysis. The study is carried out by expressing the wave profile and velocities of water particles as a nonlinear composition of the first order wave profile. Under the assumption of the first order wave profile being a zero-mean Gaussian process, the random wave spectra of finite amplitude waves are given. In order to solve the loading spectra of the finite amplitude random waves, the drag force is extended into power series of velocity. The loadings of the finite amplitude random waves are then expressed as nonlinear compositions of the first order wave profile and its derivatives. These techniques made it easier to compute the spectral densities of the finite amplitude random wave loadings.

Commentary by Dr. Valentin Fuster
2005;():585-594. doi:10.1115/OMAE2005-67289.

The effects of slowly-varying wave drift forces on the nonlinear dynamics of mooring systems have been studied extensively in the past 30 years. It has been concluded that slowly-varying wave drift may resonate with mooring system natural frequencies. In recent work, we have shown that this resonance phenomenon is only one of several possible nonlinear dynamic interactions between slowly-varying wave drift and mooring systems. We were able to reveal new phenomena based on the design methodology developed at the University of Michigan for autonomous mooring systems and treating slowly-varying wave drift as an external time-varying force in systematic simulations. This methodology involves exhaustive search regarding the nonautonomous excitation, however, and approximations in defining response bifurcations. In this paper, a new approach is developed based on the harmonic balance method, where the response to the slowly-varying wave drift spectrum is modeled by limit cycles of frequency estimated from a limited number of simulations. Thus, it becomes possible to rewrite the nonautonomous system as autonomous and reveal stability properties of the nonautonomous response. Catastrophe sets of the symmetric principal equilibrium, serving as design charts, define regions in the design space where the trajectories of the mooring system are asymptotically stable, limit cycles, or non-periodic. This methodology reveals and proves that mooring systems subjected to slowly-varying wave drift exhibit many nonlinear phenomena, which lead to motions with amplitudes 2–3 orders of magnitude larger than those resulting from linear resonance. A turret mooring system (TMS) is used to demonstrate the harmonic balance methodology developed. The produced catastrophe sets are then compared with numerical results obtained from systematic simulations of the TMS dynamics.

Commentary by Dr. Valentin Fuster
2005;():595-604. doi:10.1115/OMAE2005-67290.

Mooring systems for floating production systems, including the mooring lines and anchors, are currently designed on the basis of individual components. The most heavily loaded line and anchor are checked under extreme loading conditions with the system of lines intact and with one line removed. However, the performance of the floating production system depends more directly on the performance of the system of lines and anchors rather than on the performance of a single line or anchor. The on-going study conducts the reliability analyses using realistic probabilistic descriptions of the extreme met-ocean conditions (hurricanes and loop currents) for the Gulf of Mexico. Presented in this paper is the methodology of calculating the probabilities of failure of a classical Spar during a 20-year design life for individual components.

Commentary by Dr. Valentin Fuster
2005;():605-610. doi:10.1115/OMAE2005-67294.

In the last years, the most of offshore oil and gas reserves discoveries in Brazil are placed in ultra-deep water depths. Petroleum production from these offshore fields needs developments with novel solutions in terms of necessary technologies and economical viability. The use of vertical rigid risers such as top tensioned risers (TTR) and others like combined systems as self standing hybrid risers and steel catenary risers for ultra-deep waters have shown viable from both, technical and economical aspects. However, there are needs for detailed studies on their dynamic behavior in order to improve, particularly, the understanding of influence of the environment as wave and current, and floating platform oscillations at the riser top. The present work presents studies on vertical top tensioned riser dynamic behavior through time domain simulations of its displacements and respective, bending moments and stresses. Influences of the vortex induced vibrations (VIV) and waves on the riser service life reduction are analyzed. Maximum and minimum envelops for displacements and stresses along riser length are shown.

Commentary by Dr. Valentin Fuster
2005;():611-617. doi:10.1115/OMAE2005-67306.

In this paper vertical piles have been studied with a view to identifying the range of sea-states suitable for the safe pile driving operation. Pile configuration, including the non-linear foundation and the gap between the pile and the pile sleeve shims have been modeled using the finite elements (FE) analysis facilities within ABAQUS. Dynamic analyses of the system for various sea-states characterized by significant wave heights and mean zero upcrossing periods and modeled as a combination of several wave components, have been performed. Repeating the above procedure can generate a table of safe and unsafe sea-states. If the prediction is repeated N times from which n times proved to be safe, then it can be said that the predicted sea-state is safe with a probability of 100(n/N)%. The significant wave height (Hs) and mean zero upcrossing period (Tz) of a future sea-state of a location in NE Pacific (near 46° N 131° W) were generated using the artificial neural networks (ANNs) already trained for this purpose — the location of US National Oceanographic Data Center (NODC) Buoy 46005 is used in this study. The Hs and Tz of some future sea-states were generated from their corresponding conditional 7-parameter probability density functions (pdf’s) given some information including a number of previously measured Hs’s and Tz’s. The parameters of the pdf’s have been estimated from the outputs of 2 different 7-network sets of trained ANNs. This gives a predicted sea-state for a specific time in future. The methodology explained in this paper can identify all control parameters and offer possible solution strategy. The finding reveals that how slight changes in the design configuration can be beneficially exploited to limit the pile response.

Topics: Seas
Commentary by Dr. Valentin Fuster
2005;():619-627. doi:10.1115/OMAE2005-67307.

DORIS Engineering and Stolt Offshore have jointly designed the three Girassol hybrid riser towers installed offshore Angola. They were the first riser towers designed to provide very efficient insulation properties in very deepwater conditions. Some towers are under development or construction for other fields but in similar design conditions. Based on the experience of Girassol, DORIS engineering and Stolt Offshore have developed the concept to adapt to much deeper waters (around 2500 m) and more severe environments such as the Gulf of Mexico loop current or hurricane waves. This paper will present the key drivers for design of riser towers in these environments addressing for example the impact on the bundle cross section, buoyancy requirements, top tank size, flexible jumper lengths. Limitations will be defined and explained both in terms of design, fabrication and installation. As FPSO units are likely to be part of the development of the Gulf of Mexico deepwater areas in the not-so-distant future, the benefit of riser towers will become obvious, given the existing facilities for fabrication along the coast and spreads for installation. Other areas such as the Mediterranean sea, or even the Northern North Sea are potential candidates for this type of riser arrangement which provides multiple benefits over steel catenary risers and flexible pipes.

Topics: Design , Hybrid risers
Commentary by Dr. Valentin Fuster
2005;():629-636. doi:10.1115/OMAE2005-67309.

In this paper, we performed a lot of parametric analysis of cast T-joints. The geometry model was established using Solidworks. Non-linear analysis was carried out using the commercial finite element programme Ansys. Parametric equations of ultimate loading capacity derived from the results of finite element analysis are presented for the usual range of basic shapes of T-joints under axial loading, in-plane and out-of-plane bending. The sensitivity of the ultimate loading capacity in cast tubular joints to variation in the geometric parameters has been assessed. Besides the parameters which governing the stresses in welded joints, an additional parameter ρ that is defined by C. D. Edwards has been introduced to describe the size of fillet. In this paper, the sensitivity of ultimate loading capacity of cast tubular joints to the parameter ρ is presented.

Commentary by Dr. Valentin Fuster
2005;():637-646. doi:10.1115/OMAE2005-67310.

A new delivery scheme based on an Extendable Draft Platform (EDP), designed for large fields in ultra-deepwater, offers potential benefits that minimize start-up costs and enhance overall economics. This paper describes an EDP design intended for deployment in water depth of 8,500 ft in a Gulf of Mexico environment. This deck design is one of the largest to date, using the EDP delivery method. The paper discusses design philosophy in detail, explaining the unique design features of the topsides for dockside commissioning, the deck connection system, and the riser systems and their integration. The topsides has process capacity of 200,000 bopd and about 475 mmcfd of gas. The deck also supports a drill rig with a hook capacity of 1,500 kips. The 40,000-ton topsides are 316 ft by 316 ft, supported by four 76-ft diameter columns. In addition to supporting the equipment, the topsides design supports 12,500 tons of riser tension. The complete process and drilling systems are assembled and commissioned dockside in less than 40 ft water depth. The entire system is floated on the deck barge, with the columns lowered through the deck and locked into position on site. Then the columns are de-ballasted, and the platform is raised to the operating draft. The top tensioned risers (TTRs) are supported by hydraulic tensioners. The mooring system, a major component of the lowering mechanism that deploys the EDP, comprises 12 lines composed of chain and polyester rope. These lines are pre-set and connected to the hull prior to raising the deck.

Commentary by Dr. Valentin Fuster
2005;():647-658. doi:10.1115/OMAE2005-67311.

This paper describes the fully nonlinear free-surface deformations of initially calm water caused by water-entry and water-exit of a horizontal circular cylinder with both forced and free vertical motions. This has relevance for marine operations as well as for the ability to predict large amplitude motions of floating sea structures. A new numerical method called the CIP (Constrained Interpolation Profile) method is used to solve the problem. In this paper, the circular cylinder and free surface interaction is treated as a multiphase problem, which has liquid (water), gas (air) and solid (circular cylinder) phases. The flow is represented by one set of governing equations, which are solved numerically on a non-uniform, staggered Cartesian grid by a finite difference method. The free surface as well as the body boundary is immersed in the computational domain. The numerical results of the water entry and exit force, the free surface deformation and the vertical motion of the cylinder are compared with experimental results, and favorable agreement is obtained.

Commentary by Dr. Valentin Fuster
2005;():659-663. doi:10.1115/OMAE2005-67323.

The successful use of mega-frame theory for big buildings on the ground to control the vibration reminds us to investigate a new kind of platform using the theory. The concept platform is made up of major columns and major beams. The major columns stand vertical and horizontal load. The major beams endure the horizontal load and averagely transfer the weight of the platform to the columns. The static analysis, mode analysis and simulation of vibration control are performed for the concept platform and other three types of platforms for comparisons. It is concluded that the concept platform is feasible in functionality since the platform has high holistic intensity and can avoid the partial breakage because of the columns of the MFPs being more equal burden under static loadings as compared with other kinds of platforms. It can also avoid the big distortion under the severe external loadings because of the MFPs having the equal mass and stiffness distribution. And the displacement of MFPs is very small under the randomexternal loadings. Furthermore, the MFPs will have similar structures and suitable for blocking manufacturing.

Commentary by Dr. Valentin Fuster
2005;():665-671. doi:10.1115/OMAE2005-67332.

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to have easy installation process and protect drilling equipment from environmental forces, and it is desirable the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies, refs. /7,10,11,12,13/, have been carried out to predict the water dynamics inside the moonpool using analytical and experimental tools, this last due to the high viscous influence in the responses, and the study of coupled dynamics of floating units with moonpools have been presented in previous works (refs. /1,2,3,8,9,17/), with especial focus on FPSO’s and Monocolumn type units. In the present study, the objective is to verify the reliability of numerical approach in heave dynamic evaluation of monocolumns. With this approach the objective is to tune the natural period the floater and moonpool. Two monocolumn example units were numerically calculated using WAMIT software, making possible to verify the 6 degree of freedom of the unit and the 3 of the moonpool. As the software does not evaluate viscous forces, decay tests were performed in order to verify the external viscous damping to be considered in the numerical calculations. The calibrated numerical model transfer functions (RAO) were compared with regular and transient wave tests, where heave and moonpool heave amplitudes could be compared.

Topics: Damping
Commentary by Dr. Valentin Fuster
2005;():673-680. doi:10.1115/OMAE2005-67333.

After the boom of converted floating, production, storage and offloading systems, based on the old VLCC design, many engineering institutes started thinking about optimum dimension for new units. However, these new FPSOs designs carried out worldwide concerns about good seakeeping behavior when considering general weather conditions, in order to apply their project to different locations around the globe. Analyzing the Brazilian specific conditions, it was verified that, considering waves, current and wind characteristics, the dimension proportions found in the projected units were not the best options, mostly because of the swell waves influence. Thus, in a cooperative project between University of Sao Paulo and PETROBRAS, the best dimensions for a specific case were studied, based on real premises from Campos Basin. During the study, the roll motion, which usually creates operational limits during hard environmental conditions, was focused. It was possible not only to evaluate the best breath and draught relations, but also the inclusion of a structured skirt in the ship bilge. The influence of different shapes in the ship’s bow and stern were also evaluated, showing interesting results regarding the forces applied on the vessel. All the analyses were conducted considering numerical analysis, and the final dimensions were applied to a scaled model, which allowed to verify the real behavior of the projected unit in a test basin. As a conclusion, it was possible to define an optimized hull for the PETROBRAS premises, giving them a real design to be used in future explorations.

Topics: FPSO
Commentary by Dr. Valentin Fuster
2005;():681-688. doi:10.1115/OMAE2005-67344.

Nonlinear response of piles is the most important source of potentially nonlinear behavior of pile-supported offshore platforms due to earthquake excitations. Earthquake design of offshore platforms is one of the main parts in offshore platforms design on which seismic soil pile structure interaction could be the main concern. In this paper, seismic response of offshore piles embedded in layered soil deposits is determined using a BNWF model (Beam on Nonlinear Winkler Foundation). Results of the free field ground motion analyses using equivalent-linear or nonlinear approaches are applied as input seismic excitations to the support nodes of BNWF model. Sensitivity of the results to input ground motions are discussed and addressed in this paper. Computed responses are also compared with centrifuge test results.

Commentary by Dr. Valentin Fuster
2005;():689-698. doi:10.1115/OMAE2005-67357.

The LargE Admissible Perturbation (LEAP) methodology is developed further to solve static stress redesign problems for shell elements. The static stress general perturbation equation, which expresses the unknown stresses of the objective structure in terms of the baseline structure stresses, is derived first. This equation depends on the redesign variables for each element or group of elements; namely the plate thickness. LEAP enables the designer to redesign a structure to achieve specifications on modal properties, static displacements, forced response amplitudes, and static stresses. LEAP is implemented in code RESTRUCT which post-processes the FEA results of the baseline structure. Changes on the order of 100% in the above performance particulars and in redesign variables can be achieved without repetitive FEA’s. Several numerical applications on a simple plate and an offshore tower are used to verify the LEAP algorithm for stress redesign.

Commentary by Dr. Valentin Fuster
2005;():699-705. doi:10.1115/OMAE2005-67361.

A new ‘universal’ structural joint with multiple applications was devised. Its application as a ‘beam-to-column joint’ was already investigated experimentally, and proven to be promising. The results were reported in previous OMAE conferences and elsewhere. Another fruitful application conceived for this joint is in the context of joining (diagonal) braces to the frame members of a structure. In particular, in chevron (inverted V) bracing systems, where the dominating exchanged forces between the combined diagonal braces and the horizontal brace (beam) are ‘shear ,’ the use of the devised joint as a joining member can substantially improve the overall behaviour of the structure under dynamic loading, cyclic or else. This is due to the high ‘shear flexibility’ of this joint, emanating from 1) its high ‘shear deformation capacity,’ and 2) the ‘restored ductility’ of its material, both substantiated through experimental studies. However, numerical push-over study of a typical one-storey frame proved the effectiveness of using such notion in chevron-braced frames in every respect—the flexibility of the frame increased substantially. The elastic stiffness and the elastic limit decreased to more reasonable values, which guarantee the stable, non-violent failure of the frame if subjected to overload. Moreover, since this joint works in a ‘sacrificial capacity’ and is in the form of a ‘self-contained’ separate entity, connected to the joined-diagonals and the horizontal braces through bolts, it can be ‘replaced,’ if it receives excess damage, leaving the non-replaceable elements, braces, intact. Finally, since it is capable of joining structural elements of different materials to each other, it is expected to have a potential for being used in every structure, offshore or onshore, prone to be subjected to dynamic, cyclic or else, in its service life.

Commentary by Dr. Valentin Fuster
2005;():707-713. doi:10.1115/OMAE2005-67362.

This paper details an experimental study on bilge keels of up to 20% of half beam of a FPSO with rectangular geometry. Both free decay and forced oscillation tests were conducted on a range of bilge keel widths at different amplitudes and frequencies. The results show that, for a given amplitude of roll motion, the damping coefficient increases with increasing bilge keel size up to a certain point and then declines. Numerical simulations using the free surface random vortex method were performed on rectangular cross-sections with bilge keels which show good agreement with the experimental results. Simulations of the effect of even larger keels indicates that the same trend for damping coefficient as a function of keel size is found. An examination of the simulation results suggests a likely explanation for this behaviour.

Topics: Modeling , Keel
Commentary by Dr. Valentin Fuster
2005;():715-722. doi:10.1115/OMAE2005-67363.

The problem of a circular cylinder array slowly oscillating in both diffraction and radiation wave fields is considered in the present work. As a result of the interaction between the wave fields and the low-frequency motion, nonlinear wave loads may be separated into the so-called wave-drift added mass and damping. They are force components proportional to the square of the wave amplitude but in phase of the acceleration and velocity of the low-frequency motion respectively. The frequency of the slow oscillation is assumed to be much smaller than the wave frequency. Perturbation expansion based on two time scales and two small parameters is performed to the order to include the effects of the acceleration of the low-frequency motion. Solutions to these higher order potentials are suggested in the present work. Wave loads including the wave drift added mass and damping are evaluated by the integration of the hydrodynamic pressure over the instantaneous wetted body surface.

Commentary by Dr. Valentin Fuster
2005;():723-728. doi:10.1115/OMAE2005-67364.

The conversion of ageing tankers is a tempting alternative to newbuildings when FPSO units are planned. The low cost of an already classified hull and an existing pumping facility compensate for the repair and updating costs a conversion may demand. Newbuildings on the other hand allow for improved production plant and storage tank lay-out. The trade offs between conversions and newbuildings are analyzed from the point of view of strength and general arrangement. Converted tankers carry a serious strength drawback since the emptied engine space enhances the high buoyancy region at the stern which significantly increases bending moment amidships. In general one or more cargo tanks must be kept at partial loading to compensate for the increased bending moment. A new building lay-out is analyzed in which the tanker engine space is completely eliminated. Utility systems are resized and placed on the main deck aft of the production plant. Accommodation spaces are also resized and rearranged in order to fit into the reduced deck space available. The gains in strength are quantified and the lay-out implications are discussed in detail. A comparative study is presented between a conversion of a hypothetical tanker and an equivalent new building.

Topics: Design , FPSO
Commentary by Dr. Valentin Fuster
2005;():729-738. doi:10.1115/OMAE2005-67367.

Experience has shown that regional factors can contribute to significant variability in decommissioning costs for offshore platforms. Nevertheless, previous studies aimed at estimating decommissioning costs have been valuable inputs for operators, regulators and decommissioning contractors as they develop abandonment and decommissioning strategies and public policy. Estimated costs associated with decommissioning offshore oil and gas platforms in Australia have not been reported widely in the open literature. This paper summarises the physical and regulatory environment in Australia, and develops a cost model which reflects the decommissioning challenges for fixed platforms in this remote region. This cost model could be readily applied to other remote petroleum provinces provided that mobilization times and the nature of the local ocean environment are known. The total cost of decommissioning all 39 current fixed platforms in Australian waters is estimated to be between US$845 and US$1044 million.

Commentary by Dr. Valentin Fuster
2005;():739-746. doi:10.1115/OMAE2005-67370.

The analysis procedure for heavy marine transports is based on spectral approaches and on the use of 3D hydrodynamic and structural models. The analyses are aimed at designing the transport supports and at guaranteeing the structural safety during the voyage. The analyses are typically conducted concurrently with the design of the spar structures. Brief description is given of the analysis procedure. Its application is illustrated with examples from recently completed projects. Main effort in most projects is in local strength and fatigue issues caused by localized support loads between the spar and ship hulls. The spar transports have had relatively large overhanging sections, which are subject to wave slamming and impact loads under rough seas.

Commentary by Dr. Valentin Fuster
2005;():747-754. doi:10.1115/OMAE2005-67376.

A new innovative ‘universal’ structural joint with multiple applications was devised. The two major conceived contexts for the use of this joint are ‘joining beams to columns,’ and ‘joining diagonal braces to horizontal ones.’ The main features of this joint are its high rotational capacity, its high shear deformation capacity, its high energy-dissipation capacity, its ability to contain damage, and its repalceability. Due to its geometry, it can well lend itself to protection measures against fire, normally practiced by the involving industries. This makes it a good candidate for being used in structures related to oil and gas industry, offshore or onshore. Through numerical modelling of the joint, also using mechanical properties of ‘mild steel,’ one of the best potential materials for the fabrication of the joint, at elevated temperatures, the ‘bending behaviour’ of the joint at various temperatures was studied. Additionally, the effects of using various thermal insulating materials, used for covering the joint, in reducing the temperature of various parts of the joint were investigated. Though not supported by any experiments, all these numerical analyses showed the potential of this joint for presenting improved behaviour during a fire scenario, as a result of using some insulating agents.

Topics: Joints , Fire
Commentary by Dr. Valentin Fuster
2005;():755-765. doi:10.1115/OMAE2005-67377.

A deep-sea riser drilling vessel is now under construction in Japan for the purpose of scientific drilling. The target water depth of the vessel is 2500m for the time being and will be extended to 4000m in future. In the field of offshore oil drilling, the riser drilling has been practiced for many years and in recent years it is confirmed that the target water depth of 3000m with using steel riser pipes will be possible. However, the drilling up to the water depth of 4000m has not been experienced at all and will have a lot of technical issues. One of the most important issues is to make the weight of the riser pipe lighter and its axial rigidity higher. The purpose of this research is to make clear the design concept of the CFRP riser pipe that has multiple traplock structure and develop a CFRP riser pipe capable of drilling in 4000m deep waters. Here, referring to a series of research results on the leg fixing device for jackup rigs where similar fitting structure is adopted, a prototype of CFRP riser pipe which has multiple traplock structure is designed and five experimental models are fabricated and tested. Resulting from the comparison of the experimental results with those of finite element analysis, the findings have been obtained and it has been confirmed that it will be possible to develop an actual CFRP riser pipe that has enough strength for 4000 m water depth.

Commentary by Dr. Valentin Fuster
2005;():767-773. doi:10.1115/OMAE2005-67384.

An extensive model test program was conducted to explore the effectiveness of alternate strake designs to reduce Truss Spar VIV response. Different strake configurations were tested to minimize VIV response. The paper presents results of the model tests. The model test set-up is described, important parameters that are modeled (including hull and truss geometry, strake configuration, mass and mooring properties) and considerations of instrumentation and test methodology are discussed. The paper also describes the analysis of the test results and shows the effectiveness of new strake design. The present results are compared with VIV response of existing Truss Spars with conventional strake design.

Commentary by Dr. Valentin Fuster
2005;():775-782. doi:10.1115/OMAE2005-67385.

Time domain simulations are required when analyzing nonlinear vessel behaviour. The usual approach conducting time domain simulations is to transform a complex valued function of frequency dependent damping and added mass to a convolution integral in the time domain. Evaluating the integrals during time domain simulations is computational expensive and the accuracy of the calculation of the limit value of added mass in diffraction calculations is dependent on the panel size of the model. In this paper, an alternative approach based on a polynomial model for damping proposed by K.E. Kaasen et al is extended from a single degree of freedom to a 6 degrees of freedom model of a heavy lift barge. Polynomials for contributions of velocity to the damping force are constructed generically using a least square curve fitting method. The polynomials then are transformed to the time domain counterpart using a state space representation. The quality of the fits of the damping function has a large influence on the resulting damping force in time domain. Furthermore, the higher the order of the differential equation, the larger the number of variables to integrate during a time domain simulation. Consequently, the presented method is not necessarily more efficient in simulations than the traditional retardation functions.

Commentary by Dr. Valentin Fuster
2005;():783-791. doi:10.1115/OMAE2005-67393.

For subsea pipelines installed in areas with uneven seabed free spans may occur and fatigue failure due to vortex induced vibrations (VIV) is one of the main concerns related to these spans. In order to install pipelines in such areas the safety against fatigue failure from in-line (IL) and cross-flow (CF) VIV must be documented. Although maximum oscillation amplitudes in the IL direction are considerably smaller than the maximum amplitudes in the CF direction, the IL fatigue damage normally prevails and may limit the allowable span length. The reason for this is that the IL vibrations initiate at a lower current velocity (i.e., reduced velocity) than the CF vibrations and would hence be excited for a longer period of time. Prediction tools for VIV may be split into parametric Response Models such as described in DNV-RP-F105 and methods based on empirical coefficients such as SHEAR7 and VIVANA. Methods based on force coefficient have until recently been limited to CF VIV due to lack of hydrodynamic coefficients for IL response. This paper presents results from forced IL oscillation experiments of a smooth, rigid cylinder in uniform flow. The results are presented as dynamic in-line coefficients for the pure IL regime, i.e. reduced velocity between 1 and 4, at Reynolds number 24.000. The results are compared with IL results from free oscillation experiments found in the literature.

Commentary by Dr. Valentin Fuster
2005;():793-802. doi:10.1115/OMAE2005-67396.

With the move to the development of remote, deepwater fields, increasing use is being made of floating production, storage and offloading (FPSO) facilities from which oil is intermittently offloaded to a shuttle tanker via offloading lines and an anchor leg mooring buoy. The response of the individual components of these systems is significantly influenced by hydrodynamic and mechanical coupling between adjacent components, precluding the use of traditional analysis techniques such as displacement RAOs derived from tank model tests or diffraction/radiation analyses of the independent components. Consequently, the reliable and accurate design of these complex systems requires an analysis tool capable of determining the fully coupled response of each of the individual components of the system. A recently-developed time domain coupled analysis tool has been extended to incorporate a frequency domain coupled analysis capability. This tool combines radiation/diffraction theory with a non-linear finite element (FE) structural analysis technique used for the analysis of slender offshore structures. This paper describes the application of frequency domain analysis to the coupled FE/floating structure problem, with particular consideration given to the linearisation of viscous drag loads on floating structures and the treatment of low-frequency second-order loads in the frequency domain. Results from frequency domain and time domain coupled analyses of a typical West of Africa type offloading system are compared, highlighting areas of application where frequency domain coupled analysis can offer significant benefits when used in conjunction with time domain analysis. Based on this, recommendations are made for the appropriate use of frequency and time domain coupled analysis for this type of system.

Commentary by Dr. Valentin Fuster
2005;():803-809. doi:10.1115/OMAE2005-67399.

The air gap response and potential deck impact of ocean structures under waves is the main topic of this research. In this paper, an analytical prediction of the air gap for floating offshore structures using direct Boundary Element Method (BEM) is presented. The main advantage of direct boundary element method is the fact that one can determine the total velocity potential directly. Direct BEM is more versatile and computationally more efficient than indirect BEM. Besides, the direct BEM can easily be coupled with other numerical methods, e.g. finite element method (FEM) in order to carry out structural analysis of the platform’s deck due to possible impact. Firstly, the direct boundary element method will be reviewed. Secondly, the boundary value problem of interaction between regular sea waves and a semi-submersible and air gap responses due to the motion of the platform and the local wave elevations (including both radiation and diffraction waves) will be described. Then, the direct boundary element method will be applied to predict of the air gap at different field points of ALBORZ semi-submersible drilling unit, which is the largest semi-submersible drilling platform under construction for a location in the Caspian Sea, North of Iran. In addition, the results obtained from the direct BEM will be compared with those obtained by the designers of the ALBORZ semi-submersible. To determine the influence of the structure’s motions on the air gap, the results for both fixed and free-floating structure cases will be compared. Physical simulations using model will be carried out in the future in order to compare the results of the experiments with predictions.

Commentary by Dr. Valentin Fuster
2005;():811-819. doi:10.1115/OMAE2005-67403.

The Snorre TLP has been operating in the Northern North Sea since 1992. During the last year, an extensive reanalysis of tether tension, TLP motion and airgap has been carried out in order to ensure that the TLP can withstand updated environmental conditions and design requirements. It is the object of the present paper to describe the analysis methods which have been employed to determine extreme tether loading and to compare analysis results with model test results. The model tests were carried out in the facilities at MARIN in the Netherlands and considered more than 200 three hour realizations of irregular seastates with and without wind and current. Typical fatigue seastates as well as extreme 100 year and 10 0000 year seastates were investigated for both head and quartering seas. The extensive database of model test results gives an opportunity to study the accuracy of the analysis methods. The analysis of the TLP response was carried out in the time domain using the SIMO software developed by Marintek. In the work described here, the results from a purely linear radiation-diffraction analysis is used as the main load model in SIMO together with slender element additions based on the incident wave kinematics. The slender elements give loads on tethers and risers, viscous loads on the TLP hull and also provide a simple third order load model for the excitation of high frequent response in the surface zone. SIMO has also been implemented with second order quadratic force transfer functions, but it is concluded that the second order loading does not give significant contributions to extreme tether tension. It is concluded that, at least for the Snorre TLP, the relatively simple analysis model gives good agreement with measured extreme tether tension provided that the TLP is modeled carefully. By filtering the tether tension time series, the contributions due to wave frequent and high frequent (resonant) tether tension may be separated. It is shown that the main discrepancy between model test and analysis results of tether tension lies not in the magnitude but in the simultaneity of wave frequent and high frequent tether response. If this is due to the tendency for large waves to propagate towards the front of the nonlinear wave envelope, this may be extremely difficult to capture in analysis models.

Commentary by Dr. Valentin Fuster
2005;():821-829. doi:10.1115/OMAE2005-67404.

The experience in the Brazilian offshore production systems is to adopt the traditional riser solution composed of unbonded flexible pipes at a free-hanging catenary configuration. In deep waters, the tendency has been to use different pipe length sections (normally two), each of them designed to resist typical loadings. At the bottom, pipe structure is dimensioned against external pressure, axial compression, bending and torsion, for example. The theoretical prediction of riser responses under the crescent combined loading conditions is a key issue at the TDP region. The potential failure modes are buckling of the armour tendons and also rupture of the high resistance tapes. Much effort has been done in order to have available, from the market, larger envelopes of certified methodologies and qualified products, applicable to the Brazilian ultra-deep scenarios. Since 2002, an extensive R&D Program has been conducted (i) to improve current design evaluation tools & criteria and (ii) to establish representative test procedures and scope, for prototype qualification against the potential failure modes associated with combined axial compression, bending and torsion, at the TDP regions of bottom riser sections in ultra-deep water depths. This paper describes the main steps of the R&D Program, as below: I. Improvement of computational tools to better represent the behavior of the tendons, II. Consolidation of a new strategy for structural analysis, under more realistic conditions, III. Issue of a more adequate set of pipe technical specifications, and IV. Review of both theoretical and experimental results obtained from Feasibility Technical Studies and offshore field tests, respectively. Some examples and results are showed to illustrate, step by step, the whole process covered by the cited Program. Finally, the authors document their main conclusions for further discussion.

Commentary by Dr. Valentin Fuster
2005;():831-840. doi:10.1115/OMAE2005-67407.

This paper reviews the current practice for the in-place design of Spar hulls. Both the commonly-used approach and the state-of-the-art procedure for the maximum strength and fatigue conditions will be presented. Key assumptions for various design approaches will be discussed along with advantages and disadvantages of each approach. The review will focus on how each approach generates hydrodynamic loadings, performs global motions analysis, and maps design loads from motion analyses to structural finite-element model. Important aspects relating to Spar design will be addressed. In particular, effect of vortex induced hull motions (VIM) will be discussed, and an approach for including the VIM effect in the design of moorings and risers will also be described. Impact on the maximum strength and fatigue capacity of critical structural components due to the assumptions employed in the commonly-used design approach will be evaluated and quantified as compared to the results from the more rigorous state-of-the-art approach.

Topics: Design , Spar platforms , Hull
Commentary by Dr. Valentin Fuster
2005;():841-844. doi:10.1115/OMAE2005-67409.

As part of the effort to achieve an economical development of offshore fields, focus is directed towards reducing construction cost while ensuring that the quality of construction details is maintained. This construction quality is particularly important for deepwater developments because of the increased demand for higher structural integrity to minimize in-service inspection, maintenance, and repairs. High structural integrity requires that particular attention is given to improved welding details. Achieving high integrity, while maintaining adequate control on cost, requires a thorough evaluation of the constructability factors that have major influence on a project CAPEX. These factors include project organization, contracting plans, materials and welding specifications, fabrication schemes and erection and load-out procedures. While significant benefits may be achieved through knowledge base transfer and lessons learned, these factors are typically design specific and can vary on a project by project basis. I this paper, constructability parameters will be discussed along with yard selection criteria and impact of design requirements on construction cost.

Commentary by Dr. Valentin Fuster
2005;():845-851. doi:10.1115/OMAE2005-67413.

Field development in deep water often require floaters to be moored in close proximity, e.g. tender or offset drilling from spars, Dry Tree Units operating in conjunction with FPSOs. Mooring line interference is a key issue which needs to be considered in the design of the systems. This paper provides a study of this for a cell spar and semi-submersible operating in close proximity. Several nominal separation distances between the vessels were considered in the study. This paper presents an example for an initial separation distance of 530 ft (10% of the water depth). A time domain analysis of multi-vessel coupled responses and mooring line motion was performed to determine the potential of mooring line interference for a 100 year Hurricane and Loop Current condition. Results show that the 100 year Hurricane case would dominate for the mooring line interference for this case, and the interference for very close coupling might be avoided by pre-planning. In the worst case buoyancy might be added to the semi-submersible mooring to eliminate interference.

Commentary by Dr. Valentin Fuster
2005;():853-860. doi:10.1115/OMAE2005-67415.

Arctic deepwater regions present special criteria for platform designs. These criteria include the ability to resist local and global loads from ice in addition to the loads conveyed by waves, wind and current. In some cases, a platform has to be moved to avoid contact with ice. This paper addresses the technical feasibility of a Spar design that can be disconnected from its mooring and riser system and towed out of the path of an encroaching iceberg. The paper also examines motions of the hull and load in the mooring lines in extreme loading conditions. The hull and mooring are sized for 100-year hurricane conditions in the North Atlantic region. Estimates of pack ice loading are compared to the responses and results show that the maximum mooring loads are governed by 100-year hurricane conditions and not the ice conditions. The paper presents conceptual design for quick release mooring and riser systems and preliminary dimension analyses of various components are provided.

Commentary by Dr. Valentin Fuster
2005;():861-868. doi:10.1115/OMAE2005-67418.

There has been substantial development in codes for three-dimensional linear hydroelasticity, but to the authors’ knowledge there does not seem to be any problems put forth in the open literature to serve as an appropriate benchmark problem for a floating structure. We present in this paper a detailed description of such a problem for three-dimensional hydroelastic computer codes. The structure is a rectangular, box-shaped ‘barge’ with dimensions 100 m × 10 m × 2 m. Its deformational response primarily involves vertical and horizontal bending. Detailed descriptions of the structure, the finite element structural shell model, and the hydrodynamic fluid model, based on the constant panel Green function method, are provided. Fine meshes are used to minimize discretization errors. Detailed results, including dry natural periods and modes, wet natural periods, and wave-induced displacements and stresses are presented. The detail is sufficient for others to use the problem as a benchmark for other codes.

Commentary by Dr. Valentin Fuster
2005;():869-873. doi:10.1115/OMAE2005-67425.

Corrosion is the key factor responsible for the degradation of ship structures and in no place is this truer than water ballast tanks. Coating protection system is a continual research interest for classification societies and coating industries. Up to now, most coating performance analyses are qualitative not quantitative. Coating life prediction is almost always based on experience and various assumptions, due to unavailability of practical data support systems. This paper describes a preliminary impartial investigation of coating life carried out with interviewees from the Australian Defence Science and Technology Organization (DSTO), shipyards, coating supplier and an independent expert. Plate surface, edges and welds in ballast tank were considered and the influences of dry film thickness (DFT) and surface preparation (SP) are addressed. The investigation gives some insight into the life of practical coating systems for water ballast tanks. Coating life is proposed to be representable by a normal distribution when corrosion breakdown is below 10% of plate thickness, which is of practical implication.

Commentary by Dr. Valentin Fuster
2005;():875-879. doi:10.1115/OMAE2005-67436.

This paper describes an analytical model to calculate the setdown in waves with and without current. This model is based on a second order quadratic transfer function and a Lagrangian transformation for the effect of current. The method is validated against basin experiments with a large number of wave probes through the basin. The wave signals are separated in incoming and reflected bound and free waves based on the phase differences between the wave probes. The optimum probe spacing and transient phenomena in the restricted sized basin are discussed. The resulting separated bound wave shows good comparison to the setdown calculated with the analytical model.

Topics: Waves
Commentary by Dr. Valentin Fuster
2005;():881-890. doi:10.1115/OMAE2005-67437.

Wave run-up on deepwater offshore structures may contribute to wave overtopping of the platform deck. It may also cause undesirable loads including impact loads on the underside of the deck when combined with other hydrodynamic phenomena beneath the platform deck. In this study a 1:40 scale model of an unmanned mini-TLP design was subjected to a series of design sea environments for the Gulf of Mexico. Complimentary testing of compliant and fixed hull configurations was performed and the wave run-up was measured at several locations around the hull in both head and quartering sea orientations of the platform. The wave elevation data reported here was obtained at three critical points on the hull and a reference wave gage well upstream of the model. Analysis of the data was performed using an orthogonal third order Volterra series system analysis technique. The wave run-up measurements were compared and correlated to the incident wave elevation measurements. The results indicate that the run-up is more pronounced in the rigid hull model configuration than for the compliant model configuration and this was the case for both headings. Further, the analysis revealed a strong linear relationship between wave run-up and the incident wave field over the high wave energy frequency band. This result adds credence to the previously proposed simplified computational procedures for estimating the second order wave run-up on large cylinders in random waves.

Commentary by Dr. Valentin Fuster
2005;():891-899. doi:10.1115/OMAE2005-67439.

The Horn Mountain Production Spar was installed in 5,400 feet of water in June 2002. This was the deepest floating production unit at that time. A comprehensive instrumentation program was initiated to measure spar and riser responses (Edwards et al, DOT 2003), while motion comparisons were presented on previous publication (Halkyard et al, OMAE 2004). The present paper discusses the results of these measurements and compares with analytical predictions of spar mooring tension during hurricane Isidore in September 2002. Particular attention has been placed on the importance of Coulomb friction between wire chain and the fairlead bearing to the dynamic tension of mooring lines. Mooring tensions were measured at chain jack location (inboard tension), while analytical models computed those tensions at the fairlead location (outboard tension). Our conclusion is that there is excellent agreement between field measurements and computed tensions at the chain jacks when fairlead friction is included, and when the vessel motions are accurately predicted. Ignoring fairlead friction results in a slightly conservative estimate for the tension at the chain jack. This has been the standard practice in all spar designs to date.

Commentary by Dr. Valentin Fuster
2005;():901-907. doi:10.1115/OMAE2005-67447.

Spudcan foundation, penetrating into stratified soils is studied using H-adaptive finite element method (FE) together with Remeshing and Interpolation Technique with Small Strain model (RITSS). This is to investigate the potential hazard of punch-through failure in layered soil profiles. There are two series of analysis conducted. Firstly, for a pre-embedded spudcan foundation, a series of parametric studies has been undertaken in order to find the critical distance between the spudcan and the layer boundary, within which a punch-through failure is likely to occur. Soil profile is a uniform stiff clay layer overlaying a uniform soft clay layer with soil strength ratio (upper layer soil strength to lower layer strength) varying from 1 to 10. Secondly, continuous analysis has been undertaken for 2 cases with shear strength ratio of 2 and 3 and thickness of the upper stiff layer of one spudcan diameter. It is found that, in small strain analysis, the critical distance to the layer boundary is increasing with increasing the strength ratio. It reaches at a limit of 1.0 ∼ 1.25 D (D is the spudcan diameter) when the strength ratio is 5 or larger. In large deformation analysis, the critical distance is much lower than the one from small strain analysis. This is due to the trapped top layer soil underneath the spudcan, which cannot be simulated in small strain analysis.

Topics: Soil
Commentary by Dr. Valentin Fuster
2005;():909-917. doi:10.1115/OMAE2005-67450.

This paper investigates the dynamic behaviour of a hanging chain for excitation frequencies that lie in the vicinity of natural frequencies. The results are validated using two different solution methodologies. The study is conducted for both the undamped and the damping included cases. The calculations concern mainly the free end of the chain as this is the point that performs the larger motions.

Topics: Resonance , Chain , Damping
Commentary by Dr. Valentin Fuster
2005;():919-924. doi:10.1115/OMAE2005-67455.

Modification of deep-water floaters often involves module installation using a floating crane vessel. The impact forces caused by relative motions between the floating vessels represent a major challenge during set down on the floater deck due to the large inherent variability of these forces. Traditionally the difficulties in predicting impact forces during module installation have been overcome by the use of experienced based rules of thumb rather than accurate simulations and calculations. One has to some degree relied on the indeed present but un-quantifiable effect of human intelligence of the operation supervisor. Traditionally the impact forces are taken either by elastic deformation of the module itself and/or the installation guides or by permanent deformation of intermediate structural elements through e.g. plastic yielding of ductile metal members or crushing of wood members. Designing the module and the guides to be able to take the entire probable range of impact forces is difficult due to the inherent contradiction between wanted flexibility and required strength. The large uncertainties of the impact energy imply that it is difficult to design these intermediate elements to cover all possible impact energy levels. Furthermore, these elements cannot be applied in cases where repeated impacts may occur. An attractive alternative to the traditional solutions is application of industrial shock absorbers. The performance of these is predictable and they can be designed to cover the estimated range of impact energy. This paper will present a more precise and consistent design and analyses methodology that gives a more accurate measure on the reliability of the operation in accordance with code requirements. The paper will show application of industrial shock absorbers as an alternative to traditional solutions for impact handling during offshore module installation to floating vessels, illustrated with experience gained by the installation of two modules on the Visund Semi. Results from multi-body simulations and model tests comparing traditional methods with the proposed solution will be given. The significant benefits obtained with respect to increased operational performance, reduced acceleration loads on the installed equipment, the increased predictability of the operation, and the consistent safety level in accordance with code requirements, will be highlighted. The possibility to apply designed damping for other offshore applications like dropped object protection etc, is also discussed.

Topics: Damping
Commentary by Dr. Valentin Fuster
2005;():925-932. doi:10.1115/OMAE2005-67456.

For weight optimised deep-water structures as well as subsea equipment and piping, designing for dynamic loading from e.g. ocean waves, rotating and pulsating equipment is a challenge. A special case is the acoustic vibrations experienced in the steel piping in either end of flexible risers. Excessive vibrations have been experienced both on topside and on subsea equipment. To the authors knowledge, fatigue failure in gas containing pipes has been the result in at least two known cases, one due to acoustic vibrations, and another caused by a traditional piston compressor. During the design process it is generally a problem to predict the inherent level of damping in the structures or the equipment in order to estimate the response as accurately as possible. Much effort has been spent trying to predict the inherent damping. However, little has been done to deliberately increase structural damping in order to reduce the dynamic response significantly. Controlled application of structural damping is an alternative to changing stiffness or inertia characteristics of the structure to avoid resonance, and is often the only solution for broadband loading where resonance cannot be avoided. This paper describes solutions for two types of frequently occurring resonance problems on offshore installations and discusses general possibilities for the use of designed damping. One solution, applicable for high frequency acoustically induced vibrations in piping, is successfully applied on a full-scale mock-up of a pipe segment with a blind flange and a flange with valve, representing two real world problem details. The applied damping solution is a tailored design. Another example shows use of standard industrial dampers for vibration control of a piston compressor skid. The latter is implemented offshore and by visual control vibrations were significantly reduced. On site measurements will be conducted later. The paper will cover design and construction of the actual vibration dampers including selection of damping material. Selection of damping material depends on the occurring frequency and temperature range. The dampers should be designed to obtain maximum damping effect given the stiffness, inertia, excitation and response amplitudes of the structure. Avoiding resonance by designing natural frequencies away from excitation frequencies is sometimes close to impossible. Therefore, the deliberate addition of damping to substructures at which high stress is expected at high frequency and localized vibration is probable, can be a fatigue and risk reducing design measure that by far exceeds anything that can be achieved through other means.

Commentary by Dr. Valentin Fuster
2005;():933-941. doi:10.1115/OMAE2005-67457.

A numerical simulation system has been developed for safety evaluation of berthing and side-by-side transfer offshore operations between two vessels. Water-tank experiment has been conducted on behaviors of connected two vessels in waves, providing accuracy of the system. Also behavior of two vessels during ship-to-ship transfer offshore operation of crude oil has been simulated by the present system to demonstrate usefulness of the system.

Topics: Simulation , Mooring , Vessels
Commentary by Dr. Valentin Fuster
2005;():943-948. doi:10.1115/OMAE2005-67459.

For truss SPAR platforms, damping plates are employed to increase the added mass and viscous drag in the heave direction. The paper aims to investigate experimentally the hydrodynamic forces on heave plates with different opening sizes. Model scale experiments were carried out on three 40 by 40cm flat plates to investigate the effects of oscillation amplitude and opening size on the hydrodynamic forces acting on the plates whilst undergoing forced heave motion. Three different methods of data processing were investigated, namely, directly processing unfiltered data to obtain the hydrodynamic coefficients, filtering the data before evaluating the coefficients and fitting the filtered data to a sine curve before evaluating the coefficients. The most accurate method was then selected and used to evaluate the experiment results. The coefficient values obtained were compared with the published data available. The effects of various variables such as opening size, KC number, frequency of oscillation on the hydrodynamic coefficients were also studied. Relative magnitudes of the drag and inertia components of the overall hydrodynamic load are compared.

Commentary by Dr. Valentin Fuster
2005;():949-953. doi:10.1115/OMAE2005-67473.

The K2 North project located in Green Canyon Block 518 (GC518) in the Gulf of Mexico ties-back four subsea oil wells to a host facility (Marco Polo TLP) in the GC 608, in approximately 4300 feet water depth. The plan is to tie-back the subsea wells via dual pipe in pipe insulated flowlines through an insulated production manifold. Each well will connect to the manifold via insulated flowlines or insulated flowline jumpers. The flowlines connect to the host facility via Steel Catenary Risers (SCRs). These SCRs consist of heavy wall 6.625-inch pipe with wet buoyant insulation necessary to minimize the impact of top tension loads on the host facility. This paper discusses the key aspects of the design of the SCRs, and highlights the importance of a prudent engineering approach to deliver the optimal riser solution. These include the early identification of the potential for interference between the two project risers, and with the third party risers and adjacent TLP tendon. This design aspect was exacerbated by the weight limit imposed on the tieback risers, and by the use of VIV suppression strakes suitable for the selected installation method. The paper describes the solutions developed to avoid interference relating to riser layout and configurations and selection of wet insulation coating system. The paper also presents the qualification tests planned and conducted to prove the suitability of the wet insulation coating system for the S-lay installation method. K2 North (GC518) risers will be the first application of this coating on SCRs. The paper also demonstrates the importance of allowing sufficient margin in early stage of design to accommodate future changes as design matures, especially for fast track projects like this one. The K2 North SCRs represent typical subsea tieback scenarios in the Gulf of Mexico. The experience gained during the engineering design of these risers is therefore of significant value with regard to assisting in safe and economic design of such future developments.

Commentary by Dr. Valentin Fuster
2005;():955-971. doi:10.1115/OMAE2005-67475.

It is well known that hydrodynamic forces can have significant effect on the dynamic stability and performance of moving offshore structures that are deployed for the exploration and exploitation of seabed geo-resources. This paper presents an integral transform approach for investigating the effects of non-stationary behavior of the seabed on the Morison hydrodynamic force associated with vertically moving submerged and partially buried offshore structures located in the neighborhood of an exploration zone. For this, the fluid-structure-soil dynamic interaction boundary value problem is modeled as a one-degree-of-freedom system, and the geo-mechanical behavior of the seabed is idealized as a spring and visco-elastic damper. In particular, the corresponding fluid velocity potential and acceleration kernels in the Morison force empirical relation as modified by the seabed poro-mechanics are computed and their implications for the design of such offshore structures analyzed.

Commentary by Dr. Valentin Fuster
2005;():973-980. doi:10.1115/OMAE2005-67486.

In the present study, non-linear wave loads such as the wave drift force, wave drift damping and wave drift added mass, acting on a moored body is evaluated based on the potential theory. The body is oscillating at a low frequency under the non-linear excitation of waves. The problem of interaction between the low-frequency oscillation of the body and ambient wave fields is considered. A moving coordinate frame following the low frequency motion is adopted. Two small parameters, which measure the wave slope and the frequency of slow oscillations (compared with the wave frequency) respectively, are used in the perturbation analysis. So obtained boundary value problems for each order of potentials are solved by means of the hybrid method. The fluid domain is divided into two regions by an virtual circular cylinder surrounding the body. Different approaches, i.e. boundary element method and eigen-function expansion, are applied to these two regions. Calculated nonlinear wave loads are compared to the semi-analytical results to validate the present method.

Commentary by Dr. Valentin Fuster
2005;():981-990. doi:10.1115/OMAE2005-67496.

The most closely related attribute to the actual building cost of a new ship is its structural arrangement. Other particulars related to that cost, such as the pay load capacity, the propulsion installation, the endurance of the ship, etc., are more or less defined in the specification document. On the other hand, the structural integrity of the vessel is not in any case directly considered by the ship owner. It is responsibility of the designer to analyse it in depth and within the limits of a given budget. The latter feature directly implies that the strength and production cost of a new vessel are closely related and require special attention in the design stage since this is the time to make substantial changes with the least cost penalty. Considering the above, a method is proposed for the estimation of structural integrity of the hull in terms of production cost. The potential of this method lies in the fact that strength and cost virtues are expressed as functions of the same fundamental variables. As a result the method is particularly useful in the early design stages. Despite the simplistic approach, the foundation is set for a more rational approach of cost analysis in ship design.

Commentary by Dr. Valentin Fuster
2005;():991-996. doi:10.1115/OMAE2005-67500.

Flow interaction between thrusters is important, as many offshore vessels are equipped with rotating (azimuthing) thrusters for ease of manoeuvring and dynamic positioning. Accurate prediction of available thrust to resist horizontal environmental forces is essential allowing the realistic station-keeping capability of these vessels to be established. Additionally such thrusters are in many cases used, possibly in conjunction with tunnel thrusters and main propeller to provide propulsive power in transit. This paper presents the influence of two thrusters in close proximity, azimuthing so that flow into one thruster is influenced by the flow through the second thruster. The work extends that previously presented in OMAE 2002, based on a comprehensive experimental study to establish the influence on thruster performance of variations in current inflow velocity, thruster power, and thruster position relative to the hull.

Topics: Vessels
Commentary by Dr. Valentin Fuster
2005;():997-1006. doi:10.1115/OMAE2005-67507.

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilised as a cost effective riser solution. Furthermore, with the advent of issues such as non-pristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armour wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterisation of the wave environment in the global fatigue analysis of flexible risers.

Commentary by Dr. Valentin Fuster
2005;():1007-1020. doi:10.1115/OMAE2005-67522.

Hydrodynamic added mass and damping are old and popular research topics in the field of offshore structures engineering. The concept of added mass has been used very intelligently in the design of modern deepwater floating vessels. The turning point of the conventional Spar to a Truss-Spar is a typical example in which the added mass created by the heave plates in a Truss Spar efficiently reduced the steel weight and the subsequent cost of the Spar hull. However, the damping is not utilized as efficiently as the added mass in the design of the floating offshore platforms. It should be noted that at resonance damping plays an important role in controlling the response amplitude. This resonance is called damping controlled response. An offshore platform efficiently designed to reduce the wave excitation forces and increase the separated-flow damping could qualify as a platform to operate even near resonance. Such design could make this concept cost effective, as well as operationally more productive with minimum downtime. The principal purpose of this paper is to describe an offshore platform design that could face the resonance efficiently. The paper applies the concept of both hydrodynamic added mass and separated-flow damping intelligently in the design of a large floating vessel on column-stabilized principle. The platform is designed to face resonance due to extreme waves and utilizes the damping to control its motion, thereby qualifying its field application. The design is verified and justified with the help of a scaled-model study in a large wave tank. The results are presented herein.

Commentary by Dr. Valentin Fuster
2005;():1021-1040. doi:10.1115/OMAE2005-67541.

In this paper a new Infinite Element is presented to solve the problem of two-dimensional linear water wave diffraction by arrays of solid bodies. The method is valid for any number of arrays of arbitrary individual geometries, number and configuration of bodies. Example problems are solved. The results are compared with the analytical solutions for the special case of wave diffraction by arrays of bottom-mounted vertical circular cylinders. The comparison shows a good agreement, thus verifying the accuracy of the infinite elements for water wave diffraction problems.

Topics: Diffraction , Waves , Modeling
Commentary by Dr. Valentin Fuster
2005;():1041-1053. doi:10.1115/OMAE2005-67551.

The action of environmental loads such as wind and waves on offshore steel structures is locally emphasized by complex tubular connections, giving place to high stress concentration in correspondence of welds between pipe elements. This phenomenon, which heavily influences the fatigue life of the joint and, as a consequence, the operability of the whole platform, can be quantified by the Stress Concentration Factor. SCF can be determined either by experimental approach, numerical analysis and parametrical formulas developed mainly in the seventies-eighties by specialized authors like Kuang, Wordsworth, Smedley and Efthymiou. Even though these formulas, quoted as reference in the main world recognized rules, represented for a long time a useful tool for the designers of most projects, some discrepancies have been found to exist among them. A research has then been jointly promoted by University of Genoa and RINA Industry in order to compare the results of the parametric formulas with those coming from a finite element analysis performed on very refined numerical models made of brick elements. In this investigation attention has been focused on Y/T type joints; a wide range of configurations has been analysed by varying the most important parameters defining the geometry of the joint. Each configuration has been loaded by axial force and in/out of plane bending moment.

Commentary by Dr. Valentin Fuster
2005;():1055-1064. doi:10.1115/OMAE2005-67552.

In the development of design methods for single-pile flexible dolphins used for the berthing of bulk carriers, the design stress has been gradually increased to the yield stress to employ the elastic energy absorption capacity of the pile more optimally. In recent guidelines, e.g. EAU 1996 [3] and PIANC 2002 [5], also the plastic yielding capacity is implicitly or explicitly included in the ultimate load-bearing capacity. This movement towards plastic design is not accompanied by the development of calculation models and design criteria to assess the plastic load-bearing capacity and relevant failure modes such as local buckling and ovalisation of the cross-section. To investigate the nonlinear load-deflection behaviour of a dolphin, a numerical model was developed based on the Beam on Nonlinear Winkler Foundation (BNWF) method [7] and extended for the plastic range of the steel pile using plasticity theory, which has been verified and confirmed by FEM analysis. The resulting Bruijn model enables the assessment of the elastic and plastic load-bearing capacity in terms of energy absorption including all significant failure modes, resulting in an improved assessment of the safety against failure and allowing more optimal designs. Case study calculations with the Bruijn model show that buckling, and to a lesser degree ovalisation, are significant failure modes for a diameter-wall thickness (D/t) ratio of 62 and 83, and should therefore be included in the design guidelines. The use of the plastic yielding capacity is limited to a low diameter-wall thickness ratio (about 40), where an increase in the elastic energy absorption capacity of up to 1,37 times the original elastic energy absorption capacity can be obtained after some plastic yielding. At a larger D/t ratio the buckling sensitivity reduces the advantages of plastic design.

Topics: Design
Commentary by Dr. Valentin Fuster
2005;():1065-1071. doi:10.1115/OMAE2005-67553.

In this paper, the float-off operation of a land-built crude oil tank (COT) loaded out and towed on launching dual-submersible barges is numerically simulated by a time-domain vessel-mooring-riser coupled dynamic analysis program with multiple floating bodies. The study is particularly focused on the maximum load prediction on connectors and the minimum gap prediction between barges and the COT. In case of simpler modeling, the time-domain simulation results are compared with frequency-domain results. Then, the time-domain model is run for more realistic situation with hawsers and mooring line. In hydrodynamic analysis, the interactions among the 3 floating bodies are fully taken into consideration. In the frequency-domain analysis, the connectors between barges are modeled by equivalent translational and rotational springs, the stiffness of which is estimated using Euler’s beam theory. In order to assess the possible occurrence of contact between COT and barges, the relative motions between barges and the COT at several points of interest were investigated.

Commentary by Dr. Valentin Fuster
2005;():1073-1079. doi:10.1115/OMAE2005-67558.

The present paper introduces a concept of a hybrid riser, Self Standing Hybrid Riser (SSHR), particularly in development for ultra deepwater around 3000 meters. Main problems faced in the initial definitions for design of such risers in the described scenario are discussed. Unique laboratory test with reduced model carried out in waves, current and riser top oscillation experiments in a very deepwater wave tank is shown. And, experimental results are shown and discussed, especially considering difficulties faced to carry out this experiment. Discussions are addressed in terms of vortex induced vibration (VIV) around buoyancy can, riser line pipe and flexible jumper.

Commentary by Dr. Valentin Fuster
2005;():1081-1085. doi:10.1115/OMAE2005-67578.

Traditional naval architecture analysis of ship survival-ability involves analyzing the vessel righting arm curve in isolation or as compared to a steady wind heeling moment. It is well known that ships do not capsize in calm water and infinite time but do so in rough seas and do so dynamically in short time. Understanding the shortcomings of the traditional approach especially for advanced vessels numerous researchers in the ship dynamics field have proposed an alternative view of ship stability analysis, which involves dynamical analysis. Unfortunately, most ship motions analysis is linear and uses ideal flow theory both of which are inadequate descriptions of large amplitude ship rolling motion. Moreover, much of engineering nonlinear vibrations analysis involves perturbation methods where the nonlinearity is assumed small. Numerical simulation and physical model testing to find this critical behavior is time consuming and expensive. An alternative approach involves directly calculating critical behavior. This approach was originally developed for periodic forcing at a single frequency possibly the linear natural frequency. However, nonlinear resonance is much more complicated than linear resonance and more than a single frequency, multi-valued behavior can occur over a range of frequencies. Moreover, the phenomena which are possible for a nonlinear system are much more varied and diverse. In this paper we will analyze a traditional displacement hull form’s critical response in harmonic waves at the system’s linear natural frequency and compare this to the system’s critical response in random waves. We will compare these responses for different values of damping using these different analysis techniques.

Commentary by Dr. Valentin Fuster
2005;():1087-1095. doi:10.1115/OMAE2005-67582.

A new floater concept based on proven technology but combining this technology in a novel way has been designed for Deepwater operation in the Gulf of Mexico and other deepwater locations. The Octabuoy SDM (Shallow Draught, Mooring assisted) is a semi-submersible with a design and a tuned mooring system that ensure improved motion characteristics compared to other semi-submersibles. This makes the floater very well suited for accommodating Steel Catenary Risers (SCRs) in the Gulf of Mexico and dry tree solutions for West Africa. Motion results calculated for a typical Gulf of Mexico wave environment, using a coupled time domain approach are confirmed by extensive model testing of the concept, which proves the ability of the proposed concept to accommodate SCRs specifically for the Gulf of Mexico. Because of its relative shallow draught (23m) the platform is less susceptible to vortex induced motions (VIM) in loop currents compared to SPAR and other deep draught floaters. Theoretical considerations and model tests have proved the favorable platform behavior with respect to VIM. The Octabuoy SDM has a favorable topside load-to-hull weight ratio, which makes it a highly competitive solution compared to other semi-submersibles. Another advantage is that the platform may be outfitted and commissioned quayside.

Commentary by Dr. Valentin Fuster
2005;():1097-1104. doi:10.1115/OMAE2005-67583.

For mooring chains of offshore floating production units, API (American Petroleum Institute) recommends the use of its TxN fatigue curve considering the MBL (Minimum Breaking Load) of an ORQ (Oil Rig Quality) chain even if the chain has a higher grade. This curve has been used in mooring system design of offshore floating production units since the draft edition of API Recommended Practice for Design, Analysis and Maintenance of Catenary Mooring for Floating Production Systems in May 89 and several fatigue tests have been done by petroleum industries, chain manufacturers and research centers. Those fatigue tests show that the use of the MBL of an ORQ chain for higher grades is a conservative assumption. This paper will present an overview of the fatigue curves of materials for mooring lines: stud and studless chains, steel wire ropes and polyester fiber ropes. This overview is based on recent tests, rules and published papers.

Topics: Fatigue , Mooring
Commentary by Dr. Valentin Fuster
2005;():1105-1113. doi:10.1115/OMAE2005-67586.

A model of an oil production floater is experimentally tested in regular and irregular waves for both fixed and free-moving structure. Measurements mainly concern the rigid body motions and the free surface elevation at 24 locations: 6 inside a moon-pool and 18 along the walls and above an immersed pontoon. Two natural periods of oscillation are experimentally identified inside the moon-pool and compared to linear diffraction and radiation computation and analytical formulas. Outside the moon-pool the wave run-up can be significant and strongly nonlinear, some modal periods are identified and compared to simplified formulation.

Commentary by Dr. Valentin Fuster
2005;():1115-1121. doi:10.1115/OMAE2005-67587.

The paper presents the development of a methodology for the definition of the operational limits for Self-Elevating Units, during the “jack-up” and “jack-down” operations. The methodology is based on the structural analysis of the unit during the impact between the spud can and the soil. The mechanics of the impact is based on the first principle of Momentum Theory and the Virtual Work Theory. The study includes the development of a motion analysis in order to obtain the allowable maximum wave heights that guarantee the integrity of the unit.

Commentary by Dr. Valentin Fuster

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