ASME Conference Presenter Attendance Policy and Archival Proceedings

2013;():V001T00A001. doi:10.1115/OMAE2013-NS1.

This online compilation of papers from the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2013) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Offshore Technology

2013;():V001T01A001. doi:10.1115/OMAE2013-10002.

The WindFloat is a semi-submersible floating foundation supporting multi-megawatt wind turbines. A full-scale 2MW WindFloat demonstration unit was installed off the coast of Portugal in October 2011. Many instruments are installed on this prototype to measure the environmental conditions and the response of the platform at the site. The first section of the paper focuses on the validation of the wave measurements obtained from two radar-based wave probes onboard the platform. The wave elevation at the site is reconstructed and typical wave statistics are computed. The results are compared and validated with independent buoy measurements close to site. The second section of the paper presents estimates of prevailing wave direction and directional wave spectra based on platform motions. These results are also benchmarked with onsite buoy measurements.

Topics: Waves
Commentary by Dr. Valentin Fuster
2013;():V001T01A002. doi:10.1115/OMAE2013-10017.

The Murphy Kikeh Spar in Malaysia was the first Spar to employ a catamaran floatover method to install the topsides onto the Spar hull at the platform offshore location. The single column hull of the Spar dictates that the most practical way to float a topsides onto the Spar hull is by use of a catamaran system where the topsides structure forms a connection between the twin barges. For the Kikeh Spar the topside was first loaded out onto a single barge before being transferred to the catamaran system. The transfer operation was performed in sheltered waters. After the transfer and installation of the seafastening, the topside was towed to the installation site. The transfer of the topside weight was accomplished by deballasting the Spar and no quick release mechanism was used in the separation of the topside from the catamaran barges.

While the operations described were successfully implemented for the 4,000 MT Kikeh topside in a relative mild environment, improvements are recommended to perform this operation for heaver topsides in harsher environments such as the Gulf of Mexico.

This paper summarizes an internal study by Technip to extend and improve the floatover installation of Spar topsides to the Gulf of Mexico. This paper presents a step-by-step overview of an improved process for the installation of topsides having transportation weights of up to 25,000 ST. Similarities and improvements compared with Kikeh floatover installation will be discussed with particular focus on the following areas:

1. A new method of loading out of the topsides from the fabrication yard directly to the catamaran barges, requiring only one operation and eliminating the need for a transfer barge.

2. Specific Catamaran Ocean tow design and analysis considerations for the Gulf of Mexico. This addresses the effects of the harsher environment on the barges, grillage and topsides structure. A novel method of preloading the catamaran system is presented that reduces or, in some instances, eliminates the requirement for additional topside steel weight to accommodate additional motion-induced dynamic loads on the catamaran system during the ocean tow. The preloading also eliminates the risk associated with the operation of cutting tied-down braces.

3. A quick load release system is described which enables the rapid separation of the barges from the topside following the appropriate level of topside load transfer to the Spar hull.

Topics: Spar platforms , Hull
Commentary by Dr. Valentin Fuster
2013;():V001T01A003. doi:10.1115/OMAE2013-10033.

Tension leg platform is an important compliant offshore structure used for oil exploration and drilling operations in deep water. This paper deals with the choice of four typical structural forms of the tension leg platform in harsh environmental conditions in the Liwan gas field of South China Sea. These platforms are designed based on a new principle of similarity in order to have the same similarity criterion. The three-dimensional radiation/diffraction theory is used for hydrodynamic analysis, and the far-field solution is used to get mean wave drift forces. The time-domain nonlinear dynamic responses are studied considering the couple between the mooring and the platform. The motion of the platform considers the coupled effect of TTRs, SCRs and the tendons. Wind, wave and current are assumed to be collinear. Considering the platform motion and the utilization ratio of tendon, the most suitable structure type is advised for the Liwan gas field.

Commentary by Dr. Valentin Fuster
2013;():V001T01A004. doi:10.1115/OMAE2013-10034.

The dynamics of a turret-moored FPSO with 12 catenary chain-steel wire-chain mooring lines are studied to investigate the influence of the hull/mooring coupled system from both linear and nonlinear stiffness of steel wire. The two simulated results are compared with experimental statistics, it indicated that the results using nonlinear stiffness are slightly larger than using linear stiffness and closer when compared with the experimental statistics. Then it illustrates that the polynomial approximation of nonlinearity for steel wire is applicable. The difference between linear and nonlinear results of steel wire is not apparent due to steel wire doesn’t belong to the viscoelastic material. This paper also researches the motion of FPSO with taut chain-polyester-chain mooring lines because polyester rope has viscoelastic properties. The results are unstable when using lower and upper linear stiffness in the time-domain simulation. But the dynamic response of FPSO is good by nonlinear stiffness. Thus, the nonlinear elastic method should be used in engineering works to decrease the damaged risk of the structure.

Commentary by Dr. Valentin Fuster
2013;():V001T01A005. doi:10.1115/OMAE2013-10041.

The paper describes available information about eight accidents worldwide related to the stability of petroleum units since 2000. A further seven stability incidents in Norway during the same period are also described. These accidents and incidents are used as cases for an evaluation of the existing Norwegian regulations on the stability of floating offshore petroleum units.

The accidents and incidents are grouped in categories based on the causes, and examples are given for each group.

The paper describes which Norwegian requirements could have prevented the accidents if they are complied with. Most of the accidents have involved deviations from the Norwegian regulations. Compliance with Norwegian regulations and (high-level) standards would cover most of the accident scenarios. However, some changes in the regulations have been found necessary. The paper describes these amendments, and the reasoning behind them.

Commentary by Dr. Valentin Fuster
2013;():V001T01A006. doi:10.1115/OMAE2013-10085.

Non-linear time domain irregular wave simulations have been performed for the Kvitebjørn jacket platform located in the North Sea with the aim to quantify the dynamic amplification. The jacket is a slender structure installed in about 190m water depth.

For each of the selected extreme sea states both quasi-static and dynamic response simulations have been carried out for several wave realizations using different seeds. Based on the quasi-static response and dynamic response, equivalent dynamic amplification factors (EDAFs) were calculated for different response measures in the jacket. EDAF is the factor one has to multiply the q-probability quasi-static response with in order to obtain an adequate estimate of the q-probability dynamic response. The EDAFs are to be used in ultimate limit state (ULS) and accidental limit state (ALS) analyses of the platform.

Simulations were performed applying a Gaussian wave with Wheeler stretching and a second-order wave model.

This paper focuses on the selection of wave kinematics method and on the establishment of the EDAF analysis procedure.

Commentary by Dr. Valentin Fuster
2013;():V001T01A007. doi:10.1115/OMAE2013-10103.

J-tube pull-in is a conventional riser installation method used for connecting pipelines to offshore platforms. In general the J-tube pull-in analysis is performed either using simplified analytical models or finite element method. Finite element (FE) tools become more important for the scenarios with complicated J-tube design, because of its flexibility in adapting material properties, 3D geometry, and multiple bends.

Conventional FE models employ tube-to-tube contact elements (ITT elements in Abaqus) to model the contact between J-tube and riser, where both J-tube and riser are all modeled as beams. In this paper, a new FE model is developed that modeled the J-tube as a surface. The new model was benchmarked with the conventional ITT element model and shows a better performance in terms of convergence speed using a simple geometry. The new model was further applied to a 3D multiple bends J-tube application. The impacts from J-tube size, J-tube bend radius, number of bends, and material yield strength were also studied. The outcome of this analysis provides guidance for effective and reliable J-tube pull-in studies.

Commentary by Dr. Valentin Fuster
2013;():V001T01A008. doi:10.1115/OMAE2013-10109.

Current velocity, profile, direction, and duration may affect hydrodynamic loads and VIM of offshore structure. It is often recommended that physical experiments are carried out in sheared current, in multiple directions and for sufficiently long period of time to investigate the hydrodynamic characteristics of deep draft offshore structures to obtain better correlation to the field measurements. This necessitates generating sheared current with acceptable turbulence level. This paper presents a recent advancement in generating sheared current in a flume tank facility. In this process, the test specimen remains moored and the water flows past with its velocity varied with depth as long as necessary.

A combination of synthetic and wire meshes are used to provide the required amount of blockage onto the circulating channel flow of the flume tank to obtain specified current distribution across the cross-section and at the longitudinal center of the tank. The final set-up of the current screen provided a sheared flow distribution within 10% of the targets. Also, the measured turbulence level was below 10% in all the locations measured.

VIM studies of a model spar were successfully carried out in the generated sheared current in the flume tank facility. The ability to accurately model the sheared flow essentially improves the accuracy of the measured VIM type response measurements. The generated sheared current can also be applied for other hydrodynamic experiments where sheared current is relevant.

Topics: Flumes
Commentary by Dr. Valentin Fuster
2013;():V001T01A009. doi:10.1115/OMAE2013-10112.

In seismically active areas of the world an offshore jacket structure has to be designed for seismic loads. Since the structure must meet both strength and ductility requirements, a two stage design for Strength Level Earthquake (SLE) and Ductility Level Earthquake (DLE) is generally used. Normal procedure for designing such a structure for SLE condition is to use Response Spectrum method of analysis (RSA). The main advantage of RSA is that it is computationally very efficient. Time Domain Analysis (TDA) is used mostly to analyze DLE condition.

A response spectrum depicts the maximum response to a ground motion of a single degree of freedom system having different natural periods but the same degree of damping. A design response spectrum is a smoothened average of several earthquake motions. It is a property of the ground motion with a given recurrence interval at the particular region of interest.

RSA is a frequency domain analysis technique based on mode superposition approach. API RP 2A specifies that the modal responses be combined using a Complete Quadratic Combination (CQC) of modal responses. For the directional response combination, API RP 2A recommends applying 100% of the spectral acceleration for the two orthogonal lateral directions and 50% for the vertical and using the Square Root of Sum of Squares (SRSS) combination to obtain the maximum response. With this approach it is possible to conduct only one analysis, with any reference system, and the resulting structure will have all members that are designed to equally resist earthquake motions from all possible directions.

RSA based on mode superposition is valid strictly for a linear system. A jacket structure with its pile-soil system is not truly a linear system due to soil nonlinearity. Therefore, linearization of the pile-soil system is necessary. The stiffness of a pile is dependent on the pile head loads. Thus the response from the RSA will be very much load or deformation dependent for the pile-soil stiffness. Software used here for the analyses has an iterative analysis option for obtaining the appropriate linearized stiffness.

TDA is a step by step time integration procedure for the entire system including the piles and there is no linearization involved for the foundation stiffness as the pile-soil stiffness at discrete points of the pile are calculated at each time instant within the program. The TDA is more precise for the given time history but more time consuming as a series of ground acceleration time histories are normally required for the TDA approach.

The results from RSA are expected to be conservative especially for the design of piles. However, this can only be confirmed from a series of TDA performed using ground acceleration time histories.

This paper demonstrates that more accurate and less conservative results can be obtained by using a combination of RSA and TDA even for SLE condition. However, several simulations for TDA are required for confidence in the design to ensure that all structural elements have achieved the maximum conditions. Essentially, RSA can be used for jacket member design and TDA can be used specifically for pile design. Thus the authors believe the design of an entire jacket could be more economical if this combined approach is judiciously used.

Commentary by Dr. Valentin Fuster
2013;():V001T01A010. doi:10.1115/OMAE2013-10136.

This paper describes a direct time domain modeling procedure for suction pile lowering through the splash zone. The paper specifically addresses pile lowering from a relatively small installation vessel with the pile oriented with its long axis nearly parallel to the water surface. The model consists of time domain simulation of the crane/wire/sling/pile system. This system is subjected to vessel motions at the crane tip, hydrodynamic loading on the pile, and pile flooding.

The paper also describes the sensitivity of winch wire tension to seastate, lowering velocity and hydrodynamic model coefficients, and suction pile characteristics.

Finally, a brief comparison is made between this model and others that have been proposed within the industry, some of which have been incorporated in design standards and guidelines such as DNV-RP-H103.

Topics: Suction , Modeling
Commentary by Dr. Valentin Fuster
2013;():V001T01A011. doi:10.1115/OMAE2013-10175.

Oil and gas installations both onshore as well as offshore are often built in modules and components on a different location than where those facilities are commissioned. Although stress and fatigue damage have proven to be significant on these structures during ocean transport, ocean transportation is often not adequately accounted for during design. Prior to arrival on its final destination, constructions are exposed to severe motions when carried on a modules carrier, a general purpose vessel or towed by 1 or more ocean tugs. The authors argue that calculations on the significance and effect of these motions should be based on proper motion response calculations instead of currently used ‘rules of thumb’. Especially regarding the continuing growth of the size of these carriers and the weight of the structures which both increase the negative effect of sea behavior and affects the loads on the constructions. This article aims to explain the importance of design for transport during the design-phase of these onshore and offshore structures. A distinction is made between structures transported aboard a barge, semi-submersible or general purpose vessel and floating structures transported through a ‘wet-tow’ operation.

Commentary by Dr. Valentin Fuster
2013;():V001T01A012. doi:10.1115/OMAE2013-10189.

It has been known that bottom hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves (Whittaker et al. 2007). OWSCs are in general large buoyant flaps, hinged at the bottom of the ocean and oscillating back and forth under the action of incoming incident waves (Schmitt et al. 2012, Renzi and Dias 2012). The oscillating motion is then converted into energy by pumping high-pressure water to drive a hydro–electric turbine.

This paper deals with numerical studies of wave loading on an OWSC using the FLUENT software. In numerical simulation of wave loading on an OWSC using mesh-based methods, the mesh around the flap is required to be updated frequently. This is due to the large amplitude rotation of the OWSC around the hinge. In this work, the remeshing was achieved by using the so-called dynamic mesh approach built in FLUENT. Furthermore, the motion of the OWSC is updated in time using a fourth order multi point time integration scheme coupled with the flow solver. The results for the flap motion and the excited torque on the hinged position were compared with experimental data obtained in a wave tank at Queen’s University of Belfast. The results showed the capability of the numerical model with a dynamic mesh approach in modeling large amplitude motions of the flap. In addition, the pressures at various locations on the flap were compared with the experimental measurements in order to demonstrate the accuracy of the proposed model in capturing local features of the flow as well as the global features.

Commentary by Dr. Valentin Fuster
2013;():V001T01A013. doi:10.1115/OMAE2013-10190.

This paper is a continuation of a series of investigation for the dual stiffness approach for polyester mooring lines. Tahar et. al. (2012) has presented the global performance comparison between the dual stiffness method and the traditional method for the Spar platform. As shown in that study, there are appreciable differences between the former and the later methods especially in lateral motions, which, however, result in little difference in SCR strength response. Is it because the Spar has better motion characteristics than other wet tree floating platforms such as the semisubmersible and FPSO? This paper will investigate the effect of the dual stiffness method and the traditional method to SCR response for a Semisubmersible platform. The fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the dual stiffness approach. Two axial stiffnesses (EA) of polyester line, post installation (static) stiffness and storm (dynamic) stiffness have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. The SCR will be modeled and analyzed using ABAQUS software.

Commentary by Dr. Valentin Fuster
2013;():V001T01A014. doi:10.1115/OMAE2013-10201.

This paper presents the method and results from a fatigue study of conductors at the conductor guides’ levels as well as a static and dynamic hydrodynamic analysis of a 20″ conductor installed on a North Sea jacket platform using the OrcaFlex 9.5 computer program. This study presents only the results of the analyses and does not comment on the program’s efficiency or ability to model the various physical effects. Due to the conductor guides’ situation with gaps between the conductor and its guides, which was reported by the offshore survey, there was great demand for a fatigue life study of the conductors. The hydrodynamic analysis was performed with Finite Element Software, the OrcaFlex 9.5, for regular waves for 1-, 10- and 100-year return period sea states. Afterwards, fatigue calculations were carried out according to the output from the hydrodynamic analysis. The main achievements from this study are:

• Some elements have been identified as key factors, which have a great influence on the fatigue life of the conductors at the guides’ levels: bending moment, wall tension of the conductor, hydrodynamic & structural damping factors (which are seen to be main sources of uncertainty) as well as the clearances (gaps) between the conductor and its guides at the different guide decks.

• The bending moment could be reduced by decreasing the gap for the conductor in the guides.

• Selecting a DFF (Dynamic Fatigue Factor) of 2 means that the fatigue life of the conductors at the guide levels will not match the requirement for conductor fatigue service life, according to our analysis; and the conductors need inspection and modification.

Topics: Fatigue , North Sea
Commentary by Dr. Valentin Fuster
2013;():V001T01A015. doi:10.1115/OMAE2013-10235.

Because LNG terminals are located increasingly close to shore, the importance of shallow-water effects associated with low-frequency (LF) waves increases as well. The LF wave spectrum in these areas is generally complex, with multiple frequency peaks and/or directional peaks due to LF wave interaction with the shore. Both free and bound LF waves at the same frequency can be present. Since LF waves are potentially very significant for moored vessel motions, it is important to include their effect in an early stage of the terminal design. This requires an efficient and relatively simple tool able to estimate the LF wave spectrum in nearshore areas. The benefit of such a procedure with respect to state-of-the-art response methods is the ability to include the LF free wave distribution in a local wave field in the vessel response calculation.

The objectives of the present study are to identify such a tool, and to evaluate the use of its output as input for a vessel motion calculation. Three methods, designed for the determination of wave spectra of free wave-frequency (WF) waves, were applied to artificial LF wave fields for comparison of their performance. Two stochastic methods, EMEP (Hashimoto et al., 1994) and BDM (Hashimoto et al., 1987) and one deterministic method, r-DPRA (De Jong and Borsboom, 2012) were selected for this comparison. The foreseen application is beyond the formal capabilities for which these three methods were intended. However, in this study we have investigated how far we can take these existing methods for the determination of directional LF wave spectra.

Sensitivity analyses showed that the EMEP method is the most suitable method of the three for a range of LF wave fields. The reconstructed LF wave spectra using EMEP resembled the input spectra most closely over the whole range of water depths and frequencies, although its performance deteriorated with increasing water depth and wave frequency. Subsequently, a first effort was made to use the information in the reconstructed EMEP LF wave spectrum of a representative shallow-water wave field for a first estimate of the motions of a moored LNG carrier. The results were acceptable. This is a first indication that EMEP output might be used to calculate the motions of an LNG carrier moored in shallow water.

Topics: Waves
Commentary by Dr. Valentin Fuster
2013;():V001T01A016. doi:10.1115/OMAE2013-10313.

Ships and most offshore structures are commonly launched into water from dry docks. But, sometimes due to practical and economical restrictions, launching of some offshore structures takes place at floating launching platforms, usually barges. The latter type of launching is quite “usual” for jacket rigs. To assess the success of the launch operation usually numerical simulations are enough and model tests are usually not required. However, more recently, as part of the construction project of the new Brazilian jack-up oil platforms, an innovative launching operation was required: launching a jack-up using a jacket launching barge. The marked differences between the two types of structures involve hydrodynamic and structural complexities. From the hydrodynamical point of view, due to its greater volume compared to jackets, the jack-up may be subjected to greater loads as enters in water during launching, and then suffers great motions that may affect the barge motions and these, affect back, the jack-up motions. From the structural point of view, the heavier jack-up structure may cause greater stresses on the barge deck and rocker arms that should be properly addressed in order to determine the necessity for reinforcements in the jack-up and/or the barge. To assess the feasibility of this innovative launching operation, model tests were required to measure the motions of the barge and the jack-up and to evaluate the loads on the rocker arms. The present paper describes the experimental approach developed for simulating this unique launching operation. The calibration procedures are outlined and the main results of the systematic set of tests carried out are presented. Furthermore, the influence of some launching parameters such as friction coefficient, position of the center of gravity of the jack-up and initial launching angle are discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T01A017. doi:10.1115/OMAE2013-10340.

Diffraction calculations overpredict motion RAO’s and force RAO’s in cases where a small layer of water is present on top of a submerged body. This was observed after conducting model tests on a free floating SSCV Thialf and a captive submerged cylinder. A parameter study is done to get a better understanding of why diffraction calculations overpredict the forces in heave direction. From this study it was observed that unrealistically high water elevations existed on top of the cylinder causing the heave forces to be overestimated. A damping lid is therefore implemented to decrease this water elevation. On top of that, a new method is developed to be able to capture the dependency of the force RAO on the wave height. This method uses the instantaneous submergence height (the height of water on top of the submerged body) to determine the time averaged force RAO for a given wave height and wave frequency.

Topics: Waves
Commentary by Dr. Valentin Fuster
2013;():V001T01A018. doi:10.1115/OMAE2013-10342.

The semi-submersible heavy-lift vessel Mighty Servant 3 sank off the port of Luanda, Angola in the morning of December 6th, 2006 during a ballast operation to offload the drilling platform Aleutian Key. The official investigations carried out after the accident identified an error in the control of the submerging ballast operations as the direct cause of the sinking. However, the detailed phenomenons and reasons for the sudden excessive trim development has not been investigated further. This paper intends to identify the most likely sceneario which lead to the hydrostatic stability failure during the discharge operation by computing the flooding process during the ballast operation in the time domain. A numerical progressive flooding simulation method is presented for applications like accident investigations or damage stability assessments. This method is modified to fit the special requirements of simulating the operational procedures of semi-submersible vessels in the time domain. Extensions like the inclusion of pump elements but also the multi-body interaction of the cargo and the vessel with regard to the hydrostatics is presented. The direct flooding simulation computes the flux between the compartments based on the Bernoulli equation and the current pressure heads at each intermediate step. Large and partly flooded holes are taken into account as well as optional air compression and flooding through completely filled rooms. Pressure losses due to viscous effects are taken into account by applying semi-empirical discharge coefficients to each opening. The flooding paths are modeled by directed graphs.

A detailed investigation of the Mighty Servant 3 accident and an identification of the possible failure modes leading to the sinking of the vessel is presented. This will help to better understand the phenomenons leading to critical situations during the submerging procedure of semi-submersible heavy-lift vessels and to avoid such accidents in the future. Applying time domain flooding simulations allows to predict the ship behavior during ballast operations to identify critical situations and to better schedule the different steps of such an operation in advance.

Topics: Accidents , Floods
Commentary by Dr. Valentin Fuster
2013;():V001T01A019. doi:10.1115/OMAE2013-10368.

Traditional offshore wind turbines are normally supported by circular monopiles which are fabricated by rolling thick plates and welding them longitudinally. Due to the significant capital cost associated with the fabrication of such large circular cylinders, a new recommended innovative design to overcome such problem is introduced by replacing the circular cylinder with a vertical pile of octagonal cross-sectional shape. An efficient and very accurate semi-analytical/numerical solution based on the Scaled Boundary Finite Element Method (SBFEM) is developed to calculate the wave diffraction forces acting on the octagonal cylinders where no fundamental solutions known exist. Compared to the traditional Boundary Element Method (BEM), the SBFEM is free from the irregular frequency difficulty which means that it does not suffer from computational stability problems at sharp corners. The SBFEM solution also exhibits an enormous reduction of elements used to calculate the wave diffraction compared to the Finite Element Method (FEM), hence, a significant reduction in computational time. The SBFEM computation of the diffraction force demonstrates highly accurate results with a small number of surface elements. The presented method shows significant advantages, and is suitable for engineering applications especially the wave-structure interaction in the practical design.

Commentary by Dr. Valentin Fuster
2013;():V001T01A020. doi:10.1115/OMAE2013-10372.

This work presents the application of Artificial Intelligence Technology to a database that stores valuable information about structural analysis results of flexible pipes used in Oil and Gas Industry. The main objective is to create a low-cost computational tool capable to infer structural analysis results that can be used as a preliminary design for flexible pipes. This application uses Data Mining concepts for database preprocessing techniques, learning and prediction, multidimensional interpolation algorithms for knowledge gain. The usefulness and the reliability of this methodology are illustrated by means of numerical examples.

Commentary by Dr. Valentin Fuster
2013;():V001T01A021. doi:10.1115/OMAE2013-10376.

In April 2012 Heerema Marine Contractors (HMC) successfully performed the float-over of the North Rankin B topsides. With a weight of 24,250mT it is the heaviest deck installed in a single piece in the harsh offshore environment of Australia’s North West Shelf to date. During years of preparations extensive hydrodynamic analyses have been undertaken to achieve design loads, optimize the float-over equipment and ultimately determine operability limits for the installation.

As the float-over analysis contains various components with non-linear characteristics (e.g. fenders, leg mating units, mooring lines), a non-linear problem solving approach was used. The selected analysis tools use an efficient time step integration enabling a large number of simulations to be run. The strong non-linearity of the system requires a Monte Carlo simulation to obtain statistically reliable results. The numerical model included purpose built modules for the float-over equipment (such as LMUs) taking into account relevant details and project specific geometries. Available data from model tests was used in calibration and validation of the model.

Towards the offshore execution phase, the limiting combinations of wind sea and swell waves were identified. Forthese combinations the system response remained (just) within its ‘Not To Exceed’ values. During the offshore operation, prior to the actual float-over, the spectral wave conditions, wind and current were constantly monitored to validate the forecasts. Also the barge motions were measured and compared against results from the numerical models. This was used in support of the final decision to proceed with the float-over operation.

Commentary by Dr. Valentin Fuster
2013;():V001T01A022. doi:10.1115/OMAE2013-10402.

Validation of the HVS (Heave and VIM Suppressed) semi-submersible’s global performance is carried out through wave basin model tests and correlation with numerical analysis. As an application for a wet-tree floater, Technip designed the HVS semisubmersible to have reduced heave and VIM (Vortex Induced Motion) response. The HVS semisubmersible has blisters attached to the columns. The blister breaks the coherence of the vortex shedding along the length of the column and as a result reduces the VIM response. The redistribution of pontoon volume because of the blister reduces the heave excitation loading. The blister also provides adequate buoyancy for quayside integration.

To validate the improved hull global performance, the wave basin model tests were performed. In the tests, collinear hurricane environments of the Gulf of Mexico were considered for two different headings. Truncated mooring was applied to simulate prototype mooring system. Wind loading was applied by means of a dynamic controlled wind winch. Current was generated by a wire attached to a system of pulleys. The measured hull responses were correlated with MLTSIM, a Technip in-house time-domain nonlinear motion analysis program. Through the correlation with model test results, the improved HVS semisubmersible global motion in hurricane environments is validated.

Commentary by Dr. Valentin Fuster
2013;():V001T01A023. doi:10.1115/OMAE2013-10405.

Oil offloading from Spread Mooring System (SMS) FPSO is usually done by means of a dynamically positioned shuttle tanker (DPST) in tandem configuration. The ST receives the oil pumped by the FPSO from a bow or stern offloading station, and the operation may take up to 3 days. In order to minimize the risks associated with the operation, the shuttle tanker (ST) should be kept within a safety zone with respect to the FPSO, which is usually given as a minimum distance between the two ships and an aperture angle from the FPSO centerline. In order to guarantee the tanker position during the whole operation, the operation must be performed with tankers provided with DP (dynamic positioning) systems. Since SMS FPSOs may be not aligned to the environmental forces, keeping the shuttle tanker in position may be a hard task for the DP system, depending on the environmental conditions. There are non-rare situations in which the ST must be disconnected and the operation interrupted.

The present paper applies a methodology based on static calculation of DP capacity for evaluating the downtime of such offloading operation. The three generations of DP tankers applied in Brazilian waters are considered. Santos and Campos Basin long-term (8-year) environmental conditions (current, wind, local-sea and swell) are used in the downtime calculation. The main objective is to provide a quantitative tool to analyze important parameters of the operation, in order to support some redefinitions in the operational procedure adopted by Petrobras. The main parameters are: the angle of the safe green-zone defined from the FPSO centerline, the installed DP power, the necessity of bow and stern offloading stations in the FPSO, among others.

The results indicated that due to the large variation of wave-wind conditions along the year, both offloading stations are indeed necessary, since the ST can avoid the conditions in which it is pushed towards the FPSO. The results also indicated that incrementing the angle that defines the green-zone substantially decreases the offloading downtime. However, such decision also depends on a comprehensive risk analysis, since in that case the ST may be kept in a perpendicular position related to the FPSO. The risk analysis is beyond the scope of the present work. The DP power specified for the ST generations 2 and 3 are shown to be quite adequate, since it is demonstrated that increasing this power will not lead to a substantial reduction in the downtime.

Topics: Downtime , Mooring , Water
Commentary by Dr. Valentin Fuster
2013;():V001T01A024. doi:10.1115/OMAE2013-10411.

In recent years, the design procedures of risers and mooring system for floating production systems (FPS) have had more feedback. In this way mooring and risers designers can identify, even in an early stage, the constraints imposed by one system over the other.

This work presents an evaluation of the crossing of the information obtained from the analyses of risers and mooring system. Different riser and mooring analysis procedures are applied to a typical FPS for deep water applications. First, failure zones of the riser system are identified, so a safe operating limit zone can be defined. Then, the excursions of the platform are calculated taking into account the global response of the coupled system (hull, mooring lines and risers). Finally, the results are crossed in order to verify if the excursions of the platform are within the safe operating area.

The evaluation presented here shows the important of correctly defining the safe operational zones and how the crossing of information can be conservative or not within the design process of mooring lines and risers.

Commentary by Dr. Valentin Fuster
2013;():V001T01A025. doi:10.1115/OMAE2013-10426.

Subsea umbilical systems developed for deep offshore applications become more and more demanding regarding injection capacity, number of functionalities and water depth. Some applications, such as subsea boosting, subsea separation or gas lift are even more severe, leading to tube temperature, which can exceed, in some cases 70°C.

These operating conditions and requirements are significantly impacting the performance of the main umbilical. The most common solution, to avoid such issues, is to design thicker tubes to improve the strength of the umbilical cross section. The positive effect of the wall thickness increase has to be opposed to major drawbacks, such as weight increase and fatigue performance degradation generating more issues than providing solutions.

To face these challenges, Vallourec Umbilicals, with the technical support of TOTAL SA headquarter Technology Division, has developed a new manufacturing process for seam welded stainless steel tubes (SAF 2507), with higher mechanical properties and tighter wall thickness tolerances. The benefit of this innovation is to provide for a given application (i. e. pressure, water depth and temperature) thinner tubes able to meet severe operating conditions without impacting performances of the umbilical structure.

This paper, after a description of the manufacturing process and product qualification protocol (that led to a Type Approval Certificate from Bureau Veritas in October 2012), presents the technical advantages brought by seam welded solution, compared to seamless super duplex tubes.

Topics: Design
Commentary by Dr. Valentin Fuster
2013;():V001T01A026. doi:10.1115/OMAE2013-10445.

Dynamic analysis of thin rectangular elastically supported stiffened plates with axial loads is presented. A floating airport is modeled as a horizontal Kirchhoff’s plate, which is elastically supported at the ends; and is subjected to the impact of aircrafts landing and deceleration over its length. This sets the free-free-free-free plate into high-frequency vibration, causing flexural stress waves to travel over the plate. First, the beam natural frequencies and modeshapes in either direction are generated with these complexities. The Eigen value analysis of the governing differential equation is done, using the weighted summation of the product of the beam modes. The accuracy of the frequencies is compared with those from FEA studies. The radiation pressure on the bottom side of the plate is included to reduce the frequencies by the added-mass effect. The plate is then subjected to decelerating shock loads. The vibratory response is analyzed by the computationally efficient normal mode analysis. The amplification factor vs. the taxiing time of the moving load is generated.

Commentary by Dr. Valentin Fuster
2013;():V001T01A027. doi:10.1115/OMAE2013-10467.

The system is aimed at establishing the information support system for small craft operators. This system can also be used for playback and simulation purpose. The combination of 3D map service of Google earth and GPS can supply real time position and 3D geographic virtual circumstance. In this study, authors make the small craft assistant information system based on Microsoft.Net technology, GPS and Google Earth.

Topics: Simulation
Commentary by Dr. Valentin Fuster
2013;():V001T01A028. doi:10.1115/OMAE2013-10496.

The control algorithm normally used in Dynamic Positioning (DP) Systems is based on linear control theory (proportional-derivative or linear quadratic MIMO controller), coupled to an Extended Kalman Filter (EKF) to estimate the environmental forces and wave filtering. Such controllers and estimators have problems of performance and stability related to large variations of loading (for tankers for example) or environmental conditions. The adjustment of controller gains and parameters of EKF is a complex process. Therefore, other techniques are being applied.

An investigation into the area of control of mechanical systems was made, carrying out theoretical and experimental studies involving nonlinear robust control techniques applied to dynamic positioning of floating vessels. Two robust control techniques were applied and compared: first order sliding mode control (SMC) and higher order sliding mode control (HOSM). It is known that the main drawback of SMC is the presence of high-frequency oscillations called chattering. This undesirable effect can be eliminated by using HOSM. In order to ascertain the performance of the controller under the DP system, time-domain simulations were done. Furthermore, the technique of sliding mode requires higher order derivatives of the vessel’s position signal. Therefore was developed an exact real-time differentiator, a mathematical technique used to obtain the signal derived from the position signal in real time. To validate the simulated controller, experimental tests were performed considering a small-scale model of a DP tanker.

The results confirmed the robustness of the HOSM controller, the good performance of the differentiator and the elimination of the chattering problem.

Commentary by Dr. Valentin Fuster
2013;():V001T01A029. doi:10.1115/OMAE2013-10522.

DP offshore operations require that the vessel reference point is controlled in relation to a fixed reference frame. In some cases, the heading is allowed to change, searching for an adequate angle in which the environmental conditions induce small loads in the hull and minimum DP power is required to keep the position. This DP mode is usually referred as “weathervane control”.

There is no exact method to determinate the minimum energy heading of a vessel in real-time with only the sensors available in a DP system. Therefore, some control strategies were developed in order to control the vessel heading, and estimate the optimal value.

This paper addresses the analysis of the final equilibrium heading of several published weathervane control strategies for two different DP vessels: a typical DP tanker and an asymmetrical DP crane-barge. A static procedure is used to calculate the final heading and the DP power and thrust demand for each controller.

Several environmental combinations of wind, current, local sea-waves and swell are considered, with a systematic variation of the intensities and the direction of the environmental agents.

For non-aligned environmental agents, differences in the final heading are verified for the zero yaw and zero sway strategies. These headings are also compared to the exact minimum power heading, obtained by an optimization procedure considering the DP thrust allocation.

Commentary by Dr. Valentin Fuster
2013;():V001T01A030. doi:10.1115/OMAE2013-10526.

Offshore operations involving several floating units are becoming more frequent nowadays. Such operations are used for sub-sea equipment installation and undersea structures launching, for example. This kind of operations requires a high level of coordination between the vessels, which today is made without the ship’s information exchange, being each ship individually commanded. Therefore, in those cases a cooperative control could be applied, ensuring that the relative distance between the ships are maintained in limited range, controlling operational parameters such as the lifting line traction. The benefits of this control are shown when compared to the non cooperative control by means of a experimental setup with two DP vessels.

Topics: Vessels
Commentary by Dr. Valentin Fuster
2013;():V001T01A031. doi:10.1115/OMAE2013-10550.

Studies of wave climate, extreme ocean events, turbulence, and the energy dissipation of breaking and non-breaking waves are closely related to the measurements of the ocean surface. To gauge and analyze ocean waves on a computer, we reconstruct their 3-D model by utilizing the concepts of stereoscopic reconstruction and variational optimization. This technique requires a pair of calibrated cameras — cameras whose parameters are estimated for the mathematical projection model from space to an image plane — to take videos of the ocean surface as input. However, the accuracy of camera parameters, including the orientations and the positions of cameras as well as the internal specifications of optics elements, are subject to environmental factors and manual calibration errors. Because the errors of camera parameters magnify the errors of the 3-D reconstruction after projection, we propose a novel algorithm that refines camera parameters, thereby improving the accuracy of variational 3-D reconstruction. We design a multivariate error function that represents discrepancies between captured images and the reprojection of the reconstruction onto the images. As a result of the iteratively diminished error function, the camera parameters and the reconstruction of ocean waves evolve to optimal values. We demonstrate the success of our algorithm by comparing the reconstruction results with the refinement procedure to those without it and show improvements in the statistics and spectrum of the wave reconstruction after the refinement procedure.

Topics: Calibration , Seas
Commentary by Dr. Valentin Fuster
2013;():V001T01A032. doi:10.1115/OMAE2013-10569.

The free-running model ship test is an important measure to investigate into ship performance and response in various situations. Its basis is the Froude’s similarity law. Since Reynolds number remains quite different between model ships and full-scale ones, phenomena concerning viscosity cannot be similar.

One of the most specific features related to the viscosity is the friction resistance and thus propeller load that differs to a large extent between model ships and full-scale ones. This difference affects torque and thrust responses in waves and wind, and also rudder effectiveness or manoeuvrability.

The authors have developed a prototype of auxiliary thruster that assists free-running model ships’ propeller. The auxiliary thruster can control its forward force and adjusts the model ship propeller load to arbitrarily time varying target values. The prototype consists of a duct fan, a load cell mounting the duct fan on it, an amplifier for the duct fan, and a PC. The PC controls the auxiliary thrust, or forward force generated by the duct fan, using the model ship speed and the force data measured by the load cell.

This report presents the prototype of auxiliary thruster and its trial test applied to a free-running model ship to study the effect of propeller load on manoeuvrability. The trial test clarified how the auxiliary thrust or the propeller load affects the model ship responses to steering. Theoretical calculations simulating the effect of propeller load corresponding to the trial test confirmed these effects. Applicability of the auxiliary thruster to other free-running model tests is also discussed.

Topics: Ships
Commentary by Dr. Valentin Fuster
2013;():V001T01A033. doi:10.1115/OMAE2013-10651.

Hyundai Heavy Industries Co., Ltd. has developed a LNG-FPSO with production capacity of 2.5 MTPA to meet the demand of the offshore LNG well development in near future. This paper presents the experimental and the numerical study on the mooring and the offloading performances of the developed LNG-FPSO.

In the FEED design stage, the turret mooring system and the offloading mooring system with side-by-side arrangement were designed using the quasi-dynamic method and verified through the model tests using the truncation method in the basin. It is known that the quasi-dynamic method is very efficient to calculate many environmental combinations for the evaluation of the turret mooring system and the offloading availability. And the numerical parameters for the truncated system are calibrated with the model test results with the same truncated set-up and it is expanded into the full depth model.

In this study, it was verified that the safety of the station keeping system under the extreme environmental condition and the offloading performance satisfied the design requirement. Especially, the hydrodynamic and mechanical couplings between LNG-FPSO and LNG carrier were considered as the important parameters in the offloading availability with ship-to-ship arrangement. Finally, the design parameters such as the relative motion, line tension and fender load of offloading system were evaluated with considering the dynamic positioning system.

Commentary by Dr. Valentin Fuster
2013;():V001T01A034. doi:10.1115/OMAE2013-10675.

Floating unit, mooring lines and risers comprise an integrated dynamic system that respond to environmental loading due to wind, waves and currents in a complex way. Correct consideration of wave energy spreading may be extremely significant for the floating unit response and the mooring and riser system integrity assessment as the directional spectrum induce vessel coupled responses. The riser and system response of a spread-moored Floating, Production, Storage and Offloading unit (FPSO) is assessed employing a cos2s function for the wind-generated wave spreading acting together with a unidirectional swell system. A coupled dynamic analysis is performed considering simultaneously the dynamics of the vessel and of the slender structural system. A case study is presented and the results assessed considering typical offshore Brazilian conditions.

Commentary by Dr. Valentin Fuster
2013;():V001T01A035. doi:10.1115/OMAE2013-10682.

Dynamic analyses of mooring line systems are computationally expensive. Over the last decades an extensive variety of methods to reduce this computational cost have been suggested. One method that has shown promising preliminary results is a hybrid method which combines finite element analysis and artificial neural networks (ANN). The present study presents a novel strategy for selecting, arranging and normalizing training data for an ANN. With this approach one ANN can be trained to perform high speed dynamic response prediction for all fatigue relevant sea states and cover both wave frequency motion and slow drift motion. The method is tested on a mooring line system of a floating offshore platform. After training a full fatigue analysis is carried out. The results show that the ANN with high precision provides top tension force histories two orders of magnitude faster than a full dynamic analysis.

Commentary by Dr. Valentin Fuster
2013;():V001T01A036. doi:10.1115/OMAE2013-10684.

Several codes are currently available for design and analysis of pressure vessels. Two of the main contributors are the American Society of Mechanical Engineers providing the ASME VIII code, Ref /4/ and the Technical Committee for standardization in Brussels providing the European Standard, Ref /2/.

Methods written in bold letters will be considered in the discussion presented in this paper.

The ASME VIII code, Ref /4/, contains three divisions covering different pressure ranges:

Division 1: up to 200 bar (3000 psi)

Division 2: in general

Division 3: for pressure above 690 bar (10000 psi)

In this paper the ASME division 2, Part 5, “design by analysis” will be considered. This part is also referred to in the DNV-OS-F101, Ref /3/, for offshore pressure containing components.

Here different analysis methods are described, such as:

Elastic Stress Analysis

Limit Load Analysis

Elastic Plastic Analysis

The Elastic Stress Analysis method with stress categorization has been introduced to the industry for many years and has been widely used in design of pressure vessels. However, in the latest issue (2007/2010) of ASME VIII div. 2, this method is not recommended for heavy wall constructions as it might generate non-conservative analysis results.

Heavy wall constructions are defined by: (R/t ≤ 4) with dimensions as illustrated in Figure 1.

In the case of heavy wall constructions the Limit Load Analysis or the Elastic-plastic method shall be used. In this paper focus will be on the Elastic-plastic method while the Limit Load Analysis will not be considered.

Experience from recent projects at IKM Ocean Design indicates that the industry has not been fully aware of the new analysis philosophy mentioned in the 2007 issue of ASME VIII div.2. The Elastic Stress Analysis method is still (2012) being used for heavy wall constructions.

The NS-EN 13445-3; 2009, Ref /2/, provides two different methodologies for design by analysis:

Direct Route

Method based on stress categories.

The method based on stress categories is similar to the Elastic Stress Analysis method from ASME VIII div. 2 and it will therefore not be considered in this paper.

Commentary by Dr. Valentin Fuster
2013;():V001T01A037. doi:10.1115/OMAE2013-10707.

The Leg Mating Unit (LMU) is a critical component in ensuring safe method of installing topsides of offshore oil and gas platforms by the float-over method. Traditionally, topsides are lifted onto the substructure (e.g. jacket) using heavy lift crane vessels. However, the ‘lift’ method of installation is constrained by the availability of a limited number of heavy lift vessels in the region, with high day rates. As an alternative to modular installation with light crane vessels, float-over installation enables installation of a single pre-commissioned integrated deck, minimizing offshore hook-up time and cost. Further, float-over method is particularly suited to shallow water depth locations, remote locations (with no access to crane vessels). In a float-over installation, the deck is transported on a cargo barge to the pre-installed substructure location. The barge is guided into the jacket slot and positioned so that the stabbing cone on each leg is aligned with the corresponding jacket leg. The barge is then ballasted down (aided by the falling tide) so that the topside load is transferred from the barge to the jacket. Once the load is transferred and sufficient clearance is achieved between the deck structure and barge support structure, the barge is withdrawn from the slot. The transfer of load is the crucial step of a float-over installation and should occur in a controlled manner under the dynamic influence of environmental forces. This smooth load transfer is achieved using LMU’s. LMU’s are customized leg and deck mating units, used to dampen the impact loads generated during the mating process. They consist of steel structures with elastomer elements and are designed to perform three primary functions, absorb shocks, limit hammering onto the structures and provide defined stiffness between deck and sub-structure. The objective of this paper is to outline the design philosophy of a LMU and address the behavior of the LMU under the combination of vertical and horizontal loads during the mating process. The paper also recommends guidelines on the selection of elastomer stiffness based on load-displacement relationship. The LMU is analysed in ABAQUS, a commercially available finite element (FE) analysis package considering a non-linear time-domain analysis. The results from the FE analysis are compared with the qualification tests for the elastomer and LMU assembly performed on-site to establish correlation.

Topics: Design
Commentary by Dr. Valentin Fuster
2013;():V001T01A038. doi:10.1115/OMAE2013-10709.

One of the possible uses of a Mono Column Platform is its application as a Hub Platform for people transportation to very far offshore production platforms. Considering a trip from shore to the Hub by a High Speed Vessel (HSV) and by helicopter from the hub to the production platforms, the use of this kind of unit can provide small motions for helicopter operations and a sheltered area for people landing. The use of the moonpool area as an internal dock for the HSV docking was studied in this paper considering an opening in the Mono Column hull to allow the access of the HSV to the moonpool area. The hydrodynamics aspects of the moonpool behavior were assessed considering both experimental and numerical tools and a comparison between these approaches had been also performed.

Commentary by Dr. Valentin Fuster
2013;():V001T01A039. doi:10.1115/OMAE2013-10721.

Nowadays, the development of Arctic oil/gas fields is still more challenging. Designing of various-type drilling unit solutions operable in rigorous ice conditions is underway to meet that challenge.

The present report includes results of the development of a promising design of an ice-resistant turret-based floating drilling unit with the hull of wedge shape similar to ice-going ship’s fore in terms of angle of bow rake, entrance angle of water line, flare of frames. This architectonic type was selected based on the survey of world experience and comparison with similar Arctic solutions.

Testing in the seaworthiness basin and ice test basin proved high seaworthiness performances of this ship. Particularly demonstrative are good controllability in ice conditions achieved by the specific hull shape and use of turret arrangement, as well as relatively lower ice loads compared with other solutions.

Topics: Drilling , Ice , Ships , Wedges , Hull
Commentary by Dr. Valentin Fuster
2013;():V001T01A040. doi:10.1115/OMAE2013-10732.

Currently, the Jacket platform plays pivotal role in offshore oil exploitation. However, since many fixed platforms become ageing, the periodic detection and safety assessment is necessary for the platform structures during the service. In this paper, a sensitivity diagnose method is proposed, based on the fact that some members of jacket platform are very sensitive to the damage of the structure. At first, we numerically calculated the dynamic response of the platform with failure members in extreme sea environment, where the pile-soil interaction was considered. Then the sensitive members and the sensitivity distribution were concluded. The statistics show that the group of sensitive members is entirely different in different destructive cases. In other words, the change of stress on the members, caused by the member failure, is unique. Therefore, the sensitivity diagnose method is feasible and we can find the failure members based on the change of stress.

Commentary by Dr. Valentin Fuster
2013;():V001T01A041. doi:10.1115/OMAE2013-10737.

Floatover is a new method for installing integrated topside of a spar platform. It has several obvious advantages such as less time and cost compared with derrick lifting. In general, the floatover installation consists of three procedures: firstly a single barge is used for long-distance transportation of the topside in order to get good stability; secondly two barges take place of the single barge for floatover installation near the operating site; finally the topside is transferred from the two barges to the spar hull and the installation is completed. Between the first and second procedures, the case occurs that the single transportation barge is sided left and right by two floatover barges in the second procedure with close proximity. This case is concerned by many designers and operators for the security problem brought by possible large relative motions and forces of the three barges in side by side configuration.

The hydrodynamics of side-by-side barges are much more complex than that of a single barge in waves. In numerical simulation, it is a challenge to consider all effects including the hydrodynamic interactions, the shielding effects, the viscous effects and the wave resonance effect which has been observed in the gaps between the barges and has a significant impact on wave drift forces. In this paper, motion responses and wave drift forces were calculated in frequency domain for both the multi-body system and the single body. Far-field, middle-field and near-field method were all carried out to calculate wave drift forces. Numerical analysis was executed using potential flow code WAMIT. Corresponding model tests were also performed in the Deepwater Offshore Basin in Shanghai Jiao Tong University. Comparison between numerical and experimental results shows that numerical results agree well with the experiment and that middle-field method has better convergence than near-field method. The comparison between the multi-body system and single body shows that the hydrodynamic interactions (including wave shielding effect and Helmholtz resonance of water in the gaps) are remarkable and motion responses in the multi-body system are larger than single barge at some frequencies.

Commentary by Dr. Valentin Fuster
2013;():V001T01A042. doi:10.1115/OMAE2013-10749.

The current demand of liquefied natural gas (LNG) from remote marine locations pushes the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred between shuttle carrier (LNGC) and terminal. Even if the tandem configuration is the primary choice for LNG transfer at rough offshore locations, side-by-side configurations would be the preferred option because of existing midship coupling manifolds on the present carrier fleet (no need for manifold modifications) as well as standard mooring systems and transfer-process-chains similar to oil-transfer. Therefore, the operation conditions at rough seas have to be improved to allow side-by-side LNG-transfer and to reduce offloading downtime.

Within the SOTLL-project, side-by-side LNG transfer up to HS = 3 m is reached as a transfer limit using a new flexible pipe design, the advantages of sheltered areas at the leeside of the terminal barge and an optimized ship transfer position due to a flexible longitudinal offloading position. In addition to the evaluation of the hydrodynamic characteristics of this multibody system, one key aspect is the analysis of the exciting forces and motions due to wave amplification between the ships. In the gap between the hulls, the incoming wave field is amplified and changes dramatically. Depending on gap width, longitudinal offset, wave heading and length, large wave amplifications, standing waves and other resonance phenomena are observed which may result in high relative motions and increased forces of the entire mooring system. In this paper, the gap effects are investigated in detail with numerical approaches in frequency domain, validated by model tests at TU Berlin. A typical offloading scenario with barge and carrier is investigated for different gap sizes to identify suitable transfer configurations and ensure safe LNG offshore transfer up to HS = 3 m.

Commentary by Dr. Valentin Fuster
2013;():V001T01A043. doi:10.1115/OMAE2013-10763.

Resonance behavior and collision could possibly occur in the process of material replenishment and/or rescue procedures between multiple vessels in ocean environment. In this paper, a numerical model consisting of a free-floating barge and a cargo ship in parallel is built up to study the collisions between them in waves. Hydrodynamic interactions and wave-exciting forces on the vessels are calculated based on 3-D linear potential theory, and near field approach is adopted to deal with the second-order drift forces, which are found to be critically distance-dependent and wave direction-dependent. By synthesizing both the first- and second-order response, the relative motion trajectory of two vessels is obtained, which is applied to calculate the collision time in regular waves with different spacing distances and wave angles. Second-order drift forces are found dominant with respect to collision time. Results indicate distance between vessels and wave direction play the sensitive roles in prediction of collision time while the influence of various collision points can be ignored.

Commentary by Dr. Valentin Fuster
2013;():V001T01A044. doi:10.1115/OMAE2013-10838.

The ComFLOW wave model has been employed to study the impact of nonlinear wave groups on cylindrical monopiles. Four nonlinear wave groups are selected from fully nonlinear waves generated by a 2D ComFLOW model, representing wave groups with the largest or the second largest crest heights, the largest wave height and a wave group consisting of consecutive large waves. These four wave groups are used to investigate the wave loads on the foundation and the platform in a 3D ComFLOW model. Model results show that the maximum wave loads on the foundation and the platform by nonlinear wave groups are determined by their individual wave crest height. This study presents a relationship between platform level and wave impact on the platform, as the vertical force on the platform is the combination of buoyancy force (if inundated) and wave impact force due to wave run-up. Results also show that wave loads on the foundation and wave impact on the platform decrease as the wave period increases from 13s to 16s (typical wave period at German Bight). A wave group can cause a larger wave load on the foundation and the platform than regular waves, considering a regular wave height equal to the maximum wave height, regardless of the associated wave period (period of individual wave or peak period).

Topics: Nonlinear waves
Commentary by Dr. Valentin Fuster
2013;():V001T01A045. doi:10.1115/OMAE2013-10860.

Offshore platforms are equipped with wave instrumentation at deck extremities to measure incoming wave elevations. When those instruments are close to large structural members they record the diffracted wave as well as the incident wave. This paper studies the effect of the diffracted wave on the measured wave height. First and second order diffracted wave elevations are computed for a model Tension Leg Platform (TLP) that was tested in the Offshore Basin of the Maritime Research Institute Netherlands (MARIN) offshore basin as part of the CresT Joint Industry Project (JIP). Their respective contributions to the wave spectrum are compared at locations near the structure. These calculations are useful for identifying the best locations for wave probes. The diffraction solution is used in forward calculations to compute the wave height and wave crest at locations under the deck from the undisturbed wave. These calculations can be used to set the air gap under the deck. Conversely, this paper introduces an inverse method to retrieve the undisturbed wave height and crest from the measured data by inverting the diffracted wave coefficients. The calculations are verified using measurements of undisturbed and diffracted waves under the TLP model. This work was sponsored by the Climatology and Simulation of Eddies (CASE) JIP.

Commentary by Dr. Valentin Fuster
2013;():V001T01A046. doi:10.1115/OMAE2013-10893.

Complex flows involving waves and free-surfaces occur in several problems in hydrodynamics, such as fuel or water sloshing in tanks, waves breaking in ships, offshore platforms motions, wave action on harbors and coastal areas. The computation of such highly nonlinear flows is challenging since waves and free-surfaces commonly present merging, fragmentation and cusps, leading to the use of interface capturing Arbitrary Lagrangian-Eulerian (ALE) approaches. In such methods the interface between the two fluids is captured by the use of a marking function that is transported in a flow field. In this work we simulate these problems with a 3D incompressible SUPG/PSPG parallel edge-based finite element flow solver associated to the Volume-of-Fluid (VOF) method. The hyperbolic equation for the transport of the marking function is also solved by a fully implicit parallel edge-based SUPG finite element formulation. Global mass conservation is enforced adding or removing mass proportionally to the absolute value of the normal velocity at the interface. All those techniques were successfully implemented in a computational code, which has been suitably used to carry out several studies. The performance and accuracy of the proposed solution method is tested in the simulation waves and in the interaction between waves and a semisubmersible structure. Results count on the establishment of a relaxation zone close to the domain outflow, which partially absorbs incoming waves, avoiding their reflection.

Commentary by Dr. Valentin Fuster
2013;():V001T01A047. doi:10.1115/OMAE2013-10899.

Increased interest in deepwater and ultra-deepwater field development is leading the industry to revisit the standard design and testing methods of production risers. The main challenges involved in a deepwater project are the riser sizing, management of external pressure at maximum water depth with acceptable weight to keep low stress at riser-vessel interface, the riser/vessel dynamic interaction and riser installation. The most critical areas on the riser system are the top section, where most of the damage occurs, and the touch-down zone. While the second area can be addressed by effective riser design, the connection to the production facility is always required and it must be sufficiently strong to provide an appropriate fluid containment. A connector failure can lead to highly flammable fluid leakage, putting in danger the entire production vessel and the surrounding environment. Technological advancements are leading to new equipment layouts with more efficient materials selection. The top section and connector can be subjected to different kinds of loads and environment induced degradation.

Applying the actual standards is sometimes difficult due to the fact that they generally are too conservative. With the objective of closing this gap, this paper provides an overview of the issues regarding different types of connectors in different working environments. The criteria for achieving an optimum connection selection depends on the type of riser utilized, riser configuration, metocean conditions, support vessel type, properties of fluid/chemicals transported inside the riser, life expectancy, maintenance and accessibility. Three main kinds of connection seals were analyzed, namely pure metallurgical (i.e. welding), pure mechanical (i.e. flange, threaded connections) and a combination thereof for line-end interface (i.e. metal-to-composite). All of the seals must provide retention of internal fluids and exclusion of external fluids and impurities. In an offshore environment, both of these characteristics must be achieved through the use of at least one metal-to-metal contact between mating interfaces and one static seal (i.e. metal rings).

Topics: Pipeline risers
Commentary by Dr. Valentin Fuster
2013;():V001T01A048. doi:10.1115/OMAE2013-10920.

This study investigates new methods to improve deepwater monitoring and addresses installation of advanced sensors on “already deployed” risers, flowlines, trees, and other deepwater devices. A major shortcoming of post installed monitoring systems in subsea is poor coupling between the sensor and structure. This study provided methods to overcome this problem. Both field testing in subsea environments and laboratory testing were performed. Test articles included actual flowline pipe and steel catenary risers up to twenty-four inches in diameter. A monitoring device resulting from this study can be installed in-situ on underwater structures and could enhance productivity and improve safety of offshore operations. This paper details the test results to determine coupling methods for attaching fiber optic sensor systems to deepwater structures that have already been deployed. Subsea attachment methods were evaluated in a forty foot deep pool by divers. Afterword, structural testing was conducted on the systems at the NASA Johnson Space Center. Additionally a 7,000 foot deep sensor station was attached to a flowline with the aid of a remote operated vehicle. Various sensor to pipe coupling methods were tested to measure tensile load, shear strength and coupling capability. Several adhesive bonding methods in a subsea environment were investigated and subsea testing yielded exceptionally good results. Tensile and shear properties of subsea application were approximately 80 percent of those values obtained in dry conditions. Additionally, a carbide alloy coating was found to increase the shear strength of metal to metal clamping interface by up to 46 percent. This study provides valuable data for assessing the feasibility of developing the next generation fiber optic sensor system that could be retrofitted onto existing subsea pipeline structures.

Commentary by Dr. Valentin Fuster
2013;():V001T01A049. doi:10.1115/OMAE2013-10922.

The Free Hanging Solid Ballast Semisubmersible (FHSB) Semi is an innovative high performance submersible design that is suitable for the dry tree application because of its extremely low heave motion response. In this paper, the FHSB Semi has been designed to support dry tree application in the Gulf of Mexico (GoM) for similar conditions; payload, environment, water depth and number of risers such as that applied to an existing Spar platform in the GoM. Holstein Spar has been considered for this purpose. Numerical results for the semisubmersible motion response in extreme hurricane and loop current are presented in this paper. The paper also compares the FHSB Semi designs that support certain payload/number of risers in the GoM, in the North West Shelf Australia and in Brazil. Key figures from this comparison are presented in the paper. The paper includes discussions on the proposed project execution plan covering the fabrication, transportation and installation as well as the added risks and risk mitigations associated with the new semisubmersible design and execution.

Commentary by Dr. Valentin Fuster
2013;():V001T01A050. doi:10.1115/OMAE2013-10932.

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calculated by SRA and calibrated to a target failure probability.

The current analysis methods for wellhead fatigue are associated with high sensitivity to variations in some input parameters. Some of these input parameters are difficult to assess, and sensitivity screening is often needed and the worst case is then typically used as a basis for the analysis. The degree of conservatism becomes difficult to quantify, and it is therefore equally difficult to find justification to avoid worst case assumptions.

By applying SRA to the problem of wellhead fatigue, the input parameters are accounted for with their associated uncertainty given by probability distributions. In performing SRA all uncertainties are considered simultaneously, and the probability of fatigue failure is estimated and the conservatism is thereby quantified. In addition SRA also provides so-called uncertainty importance factors. These represent a relative quantification of which input parameter uncertainties contribute the most to the overall failure probability, and may serve well as guidance on where possible effort to reduce the uncertainty preferably should be made. For instance, instrumentation may be used to measure the actual structural response and thus eliminate the uncertainty that is associated with response calculations. Clearly measurements obtained from an instrumented system will have its own uncertainty. Other options could be to perform specific fatigue capacity testing or pay increased attention to logging of critical operational parameters such as the cement level in the annulus between the conductor and surface casing.

This article deals with the use of measurements for fatigue life estimation. Continuous measurements of the BOP motion during the drilling operations have been obtained for a subsea well in the North Sea. These measurements are used both in conventional (deterministic) analysis and in SRA (probabilistic analysis) for fatigue in the wellhead system. From the deterministic analysis improved fatigue life results are obtained if the measured response replaces the response obtained by analysis. Furthermore, SRA is used to evaluate the appropriate magnitude of the design fatigue factor when fatigue analysis is based on measured response. It is believed that the benefit from measurements and SRA serve as an improved input to the decision making process in the event of life extension of existing subsea wells.

Commentary by Dr. Valentin Fuster
2013;():V001T01A051. doi:10.1115/OMAE2013-10973.

The tensioner stroke range for a Dry Tree Semisubmersible (DTS) in a severe environment application is long and must be well defined for the concept to work. The range needs to be properly calculated and optimized. In this paper, the authors elaborate the design procedures for determining the tensioner stroke on a DTS, and demonstrate the design optimization of a conventional semisubmersible floater in the Center of Gulf of Mexico (GOM) 8000 feet water depth. The primary focus is on the design of the platform configuration to reduce the tensioner stroke by investigating each stroke component, while keeping the robustness of the design. A 4-Column ring pontoon conventional deep draft semisubmersible is re-configured considering the balance between the floater hull size and maximum tensioner stroke range. It is identified that the deck vertical layout, quayside /in-place stability and hull dynamic motions are the key parameters for DTS configuration design iterations. The hull principal particulars are first determined using a frequency domain screening approach to minimize dynamic motions for various loading combinations. Top Tensioned Riser (TTR) and export risers with Steel Catenary Configuration (SCRs) are modeled in the integrated screening process, and mooring line setups are optimized for the global motion analyses. For the governing design cases, riser pipe and tensioner details are modeled for various target tensioner characteristics in a dedicated riser design tool in time-domain. Sensitivity analyses including the tensioner stiffness curve, platform pitch and keel guide effects on the global performance are studied and presented. It can be concluded from the design example that when properly configured, a reasonable tensioner stroke is achievable for typical DTS applications in GOM environment.

Commentary by Dr. Valentin Fuster
2013;():V001T01A052. doi:10.1115/OMAE2013-10988.

Brazilian offshore scenario requires innovative solutions in order to increase the efficiency of operations, while still keeping the safety and the economical constraints. The global market and naval industry cannot meet the demand of the growing exploration activities, related to the recent discovery of new oil fields. Furthermore, they are located at a large distance from the shore (approximately 300km) and the environmental conditions are severe. These facts impose more logistics problems for materials and persons transportations.

The retrofitting of vessels is an alternative for increasing their operability and this option has been studied by Transpetro, subsidiary of Petrobras responsible for the Ducts, Terminals, Maritime Transport and Natural Gas segments. This paper presents two real cases of retrofitting related to the application of new technologies for positioning, including Dynamic Positioning (DP) Systems or manually operated thrusters.

In the first case, a conventional tanker will be converted to a DP2 tanker, in order to operate close to drill ships and to transfer fluids during the drilling operation. The second case is related to an offshore delivering vessel used for bunkering operation. This vessel is moored at a monobuoy, and up to 2 receiving vessels may be connected to it. An aft-azimuth thruster will be installed in the vessel, increasing its capacity to change heading in harsh environmental conditions and to keep a safe distance from the monobuoy during fast variations of environmental conditions.

Both cases require the application of a comprehensive design methodology for thruster dimensioning. The retrofitting imposes tight constraints to the layout and positioning of the thrusters. Also, the proximity to others vessels and the harsh environmental conditions in the offshore fields requires a combination of static and dynamic calculation in order to evaluate power consumption and drift motions of the vessels.

Commentary by Dr. Valentin Fuster
2013;():V001T01A053. doi:10.1115/OMAE2013-11004.

The use of FLNG units for gas exploration and production offshore is a subject in study by some oil companies. More complex and sophisticated than a FPSO production plant, a gas production plant has strict motion criteria in order to have an optimal operational performance. Due to this, designers have been trying hull concepts with small initial stability and higher roll motion periods in order to reduce the unit motions and improve the plant performance. Indeed, the increase of roll natural period dramatically reduces the first order roll motions. However, the unit still responds at its resonance due to second order excitation. These kinds of loads are also more complex and require a great computational power to be evaluated. Due to its complexity, which would involve the solution of a non-homogeneous free surface boundary condition, some approximations are used in order to assess the second order loads and motions. In this paper, the different formulations for the first part of QTF, contributed by first order quantities, are revisited and the differences are highlighted. Furthermore the approximations for the computation of the second part of the QTF, contributed by the second order potential, are benchmarked for the case of a FLNG operating in deep water depth.

Commentary by Dr. Valentin Fuster
2013;():V001T01A054. doi:10.1115/OMAE2013-11026.

The present study examines a new concept of wave energy conversion inspired by geological tidal-bowls (i.e. toilet bowls) and blowholes in nature, capitalizing on the Helmholtz resonance phenomenon. Tidal-bowls are of interest because they concentrate ocean wave energy in a basin while in resonance with incoming waves. Tidal-bowls are formed as sea channels grow landwards into a water basin, which can result in a high pulsating current of water inside the channel in and out of the basin. The resonance of water inside the basin produced by asymmetry of its narrow water channel allows for the capture of Helmholtz mode, which is the most energetic mode of the ocean waves. Thus, the objective of this project is to experimentally investigate the geometry of tidal-bowls in a wave tank including the size of the basin and the channel in order to obtain Helmholtz resonance. The model in the wave tank is scaled using the Froude number. Preliminary experiments were carried out measuring the water surface, demonstrating a strong correlation of the model to the theoretical Helmholtz mode’s model, σH2 = gHB/A0L. Where a basin with maximum water depth, H, and horizontal area, A0, is connected to the sea by a narrow strait of width, B, and the strait length, L. The proposed geometry can be used to harvest wave energy through either pulsating current of the channel using a water-turbine or using an air-turbine on the top of the basin. This study aims to catalyze future works in effective applications of this model towards wave energy conversion device development. Thus, we investigated the effects of the device’s length, and the device’s winglet’s angle at the inlet of the channel on wave amplification inside the basin. In addition, we experimentally demonstrated that the flow dampening inside the channel has no effect on basin’s resonance frequency.

Commentary by Dr. Valentin Fuster
2013;():V001T01A055. doi:10.1115/OMAE2013-11096.

The offshore oil production industry is currently expanding its activities in even deeper waters, using moored floating platforms submitted to extreme environmental conditions. Therefore, the design of mooring systems to keep the position of such platforms is of vital importance to assure safety and economical feasibility for offshore oil production.

In this context, this work presents optimization procedures to find the minimum offsets for floating platforms under environmental loads, taking radius, azimuth, pre-tension and material of the mooring lines as design variables.

Considering that such optimization procedures require high computational costs, due to the need of nonlinear static and dynamic analyses with Finite Element models for each candidate solution, among the various meta-heuristic algorithms the Particle Swarm Optimization method (PSO) was chosen due to its simplicity, efficiency and parallel capability.

Results of typical systems are presented, which indicate that the method is effective.

Commentary by Dr. Valentin Fuster
2013;():V001T01A056. doi:10.1115/OMAE2013-11100.

During the stage of designing or re-assessment of a jacket-type offshore structure, one of the most important phases is the re-evaluation of environmental loads, which are exerted by the waves, the wind and the currents, and updating the new data (e.g. meteocean, new regulation, structural damaging etc.). The former is mainly caused by the randomness or uncertainty nature of the marine environment as well as the presence of marine growth that makes the modeling of environmental loading more complicated. The general species of marine growth may be divided into two main categories: vegetable (e.g. algae) and animal (e.g. mussel, anemones, and corals). Indeed, the structures can be covered by many marine organisms quickly (in the first weeks). The influence of bio-colonization on an offshore structure can be measured at several levels: obstruct or prohibits a visual inspection of the subjacent support, cost of procedures of cleaning for oil industries and increasing the hydrodynamic forces on the structure. Considering the latter, loading change due screen effect and added mass are shown to be the most relevant. This study aims to provide a probabilistic modeling of marine growth colonization in the Gulf of Guinea. A physical matrix response surface is used in view to provide a probabilistic modeling of the environmental loading on Jacket type offshore structures for quasi-static behavior in the presence of marine growth. The paper focuses on uncertainty and sensitivity studies respecting to the effects of wave, wind-sea and currents.

Commentary by Dr. Valentin Fuster
2013;():V001T01A057. doi:10.1115/OMAE2013-11101.

Nowadays, challenge for requalification of existing offshore platforms through the reassessment process leads to consider the importance of updating new information (e.g. environmental data, new regulations, etc.). Regarding to this information and depending on offshore fields, data of marine growth colonization is shown to have a dominant effect. This is a real challenge in the Gulf of Guinea that this study focuses on. Marine growth is known to cause adverse effects on the performance of offshore structures. Its presence can change the roughness and the diameter of structural members and hence change the level of hydrodynamic coefficients. Moreover, modifying the added mass can change the natural period and hence dynamic responses of structures. Even platforms with the best protection schemes against marine organisms will after few weeks at least few months start to be covered by various types of marine growth. Generally, it was also recognized that the most important source of loading exerted on offshore structures comes from hydrodynamic actions which are influenced by hydrodynamic coefficient values. The colonization process is very complex and results are in a large diversity of marine growth type (animal, vegetal — hard, soft) and species. This study therefore proposes a stochastic modeling of marine growth and the roughness of hard species based on Response Surface Methodology. A geometrical description of nth order of Stokes model, formed by a random linear combination of deterministic vectors is employed. Finally, the complexity level of roughness modeling is analyzed and the results are discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T01A058. doi:10.1115/OMAE2013-11126.

The low-frequency quadratic transfer function (QTF) is defined as the second-order wave loads occurring at the frequency equal to the difference frequency (ω1ω2) of two wave frequencies (ω1, ω2) in bi-chromatic waves of unit amplitude. The exact formulation of the QTF which is recalled here is difficult to implement due to numerical convergence problems mainly related to the evaluation of an arduous free surface integral. This is why several approximations have been used for practical engineering studies. They have been the subject of a detailed review in [5]. Following this work, two closely-related formulations are investigated in this paper.

In [2], the classical formulations of QTF are examined by an analysis based on the Taylor development with respect to Δω for Δω ≪ 1 and an expansion of QTF in power of Δω is then obtained. It is shown that the zeroth-order term is a pure real function equal to the drift loads and that the term of order O(Δω) is a pure imaginary function.

The second-order low-frequency wave loading of order Oω) contains a free-surface integral representing the second-order corrective forcing on the free surface. Since the integrand is of order O(1/R4) with R as the radial coordinate, the free-surface integral converges rapidly with the radial distance. Unlike what has been assumed in previous studies of particular cases, this free-surface contribution is, in general, not negligible for high Δω compared to other components and the complete QTF.

Depending whether we use the Oω) approximation for the whole QTF or only for this free surface integral, it leads to two different approximations. The first one is called original Oω) approximation, because it is on this form that the Oω) approximation was first described in [2]. If we use the Oω) approximation only for the free surface integral, we call this approximation the practical Oω) approximation. It is shown in this paper that the original formulation fails to predict the behaviour of the QTF even for small Oω). Comparison for the Oω) approximation of the free surface integral is performed against the analytical solution and the exact numerical formulation. The results are improved compared to when we neglect this free surface integral for the range of Δω of interest, but still the agreement with the exact solution is not ideal. A path for further improvement is finally proposed.

Topics: Stress , Waves , Approximation
Commentary by Dr. Valentin Fuster
2013;():V001T01A059. doi:10.1115/OMAE2013-11128.

Water-driven debris generated during tsunamis and hurricanes can impose substantial impact forces on structures that are often not designed for such loads. This paper presents the design and results of an experimental and theoretical program to quantify these potential impact forces. Two types of prototypical debris are considered: a wood log and a shipping container.

Full-scale impact tests at Lehigh University were carried out with a wooden utility pole and a shipping container. The tests were carried out in-air, and were designed to provide baseline, full-scale results. A 1:5 scale shipping container model was used for in-water tests in the Oregon State University large wave flume. These tests were used to quantify the effect of the fluid on the impact forces.

Results from both experimental programs are presented and compared with theoretical predictions. The analytical predictions are found to be in sufficient agreement such that they can be used for design. A fundamental takeaway is that the impact forces are dominated by the structural impact, with a secondary affect provided by the fluid. Both forces are quantified in the paper.

Topics: Water
Commentary by Dr. Valentin Fuster
2013;():V001T01A060. doi:10.1115/OMAE2013-11138.

Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were performed of the wake flow behind the azimuthing thrusters on two different DP vessels, a Semi-submersible and a Drill Ship. The flow velocities were measured in a large number of cross sections at different distances from the thrusters. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the transverse and vertical velocity components.

First, measurements were carried out on a DP Semi-submersible (scale 1:40), which was equipped with 8 azimuthing thrusters. The results of the PIV measurements show the wake flow, interacting with nearby thrusters and the opposite pontoon of the semi-submersible. An example is shown in Figure 1 below. Deflection of the thruster wake, caused by the Coanda effect, was observed. The results for thrusters with a 7 deg downward tilt were compared with the results for thrusters with a horizontal propeller axis. Furthermore, the effect of ambient current was investigated.

Second, measurements were carried out on a DP Drill Ship (scale 1:40), which had 6 azimuthing thrusters. The results of these PIV measurements also gave insight in the wake flow behind the azimuthing thrusters and the interactions between neighbouring thrusters. An example is shown in Figure 2 below. In this case, special attention was paid to the development of the thruster wake along the vessel length, up to a distance of more than 40D downstream.

The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are foreseen in the near future.

Commentary by Dr. Valentin Fuster
2013;():V001T01A061. doi:10.1115/OMAE2013-11248.

The possibility of incorporating a wave-energy extractor into a current design of the WindFloat platform is examined. First, to absorb wave energy, a rolling cam shape, with rotary power take-off, is attached to a tubular truss member of the WindFloat located above the calm-waterline. Based on the assumption that the extractor is operating in beam seas, numerical predictions for the coupled 3-DOF system (surge, heave and pitch motions) were completed for an ideal-fluid situation. The degradation of the performance of the wave energy extractor because of viscous effects was discussed in [1]. Second, a design of a versatile bi-directional rotary system, named the UC Berkeley Double-Ratchet Mechanism (UCB-DRM) was made. This mechanism can produce a unidirectional rotational motion, thus facilitating the power take off by a generator. A physical unit was constructed. The efficiency and performance of this mechanical system is assessed by introducing a known, bi-directional torque input and measuring the torque output.

Topics: Wave energy
Commentary by Dr. Valentin Fuster
2013;():V001T01A062. doi:10.1115/OMAE2013-11260.

Traditionally, the design practice of floating production systems (FPS) employed uncoupled numerical tools where firstly the hydrodynamic analysis of the hull is performed with the lines represented by scalar models (leading to the hull motions); subsequently, these motions are prescribed at Finite Element (FE) models of the lines. Nowadays, it is widely acknowledged that coupled analysis tools should be employed for deep-water applications, considering that the overall behavior is dictated by the interaction between the hydrodynamic behavior of the hull and the structural behavior of the lines.

In this context, considering that in some situations the use of coupled formulations can lead to excessive computing times, this work presents a formulation for the analysis of FPS, referred here as the semi-coupled (S-C) strategy. Its goal is to attain faster simulations than a coupled formulation, with better accuracy than usually provided by the classical uncoupled scheme. In this strategy, for each load case a coupled static simulation is performed. From this simulation a global 6-DOF stiffness matrix that represents the array of lines is automatically calculated and added to the global matrix for the subsequent dynamic analysis to solve the equations of motion of the hull. Therefore, this dynamic analysis will adequately consider the nonlinear stiffness contribution of the lines, as well as the effect of the current profile acting on them, all evaluated at the static mean position for each load case. Case studies are presented to compare the computational costs and accuracy of this S-C strategy with coupled formulations.

Commentary by Dr. Valentin Fuster
2013;():V001T01A063. doi:10.1115/OMAE2013-11319.

This paper identifies and evaluates various offshore platforms which could be feasible for the deep sea operations in the Turkish Black Sea. The aim of this study is to propose a fuzzy based decision making model for selecting an optimum production system by using effective and major criteria at the same time. The model incorporates fuzzy linguistic variables in place of numbers. The conventional methods for production system selection are inadequate for dealing with the imprecise or vague nature of linguistic assessment. To overcome this difficulty, a fuzzy based multi-criteria decision-making method is proposed. Proposed approach is based on the combination of fuzzy analytic hierarchy process (FAHP) and the fuzzy TOPSIS methods. The application could be interpreted as demonstrating the effectiveness and feasibility of the proposed model.

Commentary by Dr. Valentin Fuster
2013;():V001T01A064. doi:10.1115/OMAE2013-11332.

This paper describes work performed within a Joint Industry Project aiming to evaluate the lifetime of deep sea handling ropes. Various HMPE (High Modulus Polyethylene) fiber ropes, with and without coatings, have been studied under both tensile and cyclic bend over sheave (CBOS) loading. A large test program has enabled both tension-cycle to failure relationships and empirical expressions for residual strength after cycling to be determined. A special device was then developed to apply a known couple to the sheave, allowing both dynamic friction measurements to be made and the influence of applied couple on cycles to failure to be measured. These experimental data were used in the development of a numerical model which can be used to study the influence of rope and sheave parameters.

Topics: Fibers , Durability , Pulleys , Ropes , Seas
Commentary by Dr. Valentin Fuster
2013;():V001T01A065. doi:10.1115/OMAE2013-11340.

An alternative to lifted installation of topsides by a derrick barge is installation of single, integrated offshore platform topsides by floatover method. Floatover installation reduces hook-up and commissioning, which results in overall schedule and cost savings. Numerous papers were written recently to describe many aspects of the floatover operations. Nature of the floatover is such that it requires detailed engineering analyses, numerical simulations, model testing, and planning to evaluate all phases of the operation [Ref 5], [Ref 6]. Proper analysis of floatover requires numerical simulations using time-domain methodology to evaluate the system non-linearities inherent in the floatover hardware, fendering, mooring lines. Normally, weight transfer stages are given a high profile however it is found that the docking and undocking stages are equally as important. These sensitive stages of the floatover operation occur when the barge is entering the jacket slot prior to the floatover and exiting the jacket slot afterwards. The operation is sensitive to the prevailing weather and the number of simulations to make sure the operations can be performed safely is significant. Results of the docking and undocking analyses usually determine the weather standby and thus workability. This paper will address the docking and undocking stages of floatover for a barge that does not have its own propulsion. The paper shall include a concurrent investigation on effects of weather criteria. Stiffness of the hardware, mating lines/cross lines, mooring lines and the effect they have on the system will be discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T01A066. doi:10.1115/OMAE2013-11366.

In this paper we review hydrodynamic and nautical studies for offshore LNG operations. Based on full mission bridge simulations, model tests campaigns, time domain simulations, fast time maneuvering simulations and downtime assessments, we address the major findings in terms of weather limitations, tugboat requirements and other critical aspects for the berthing and offloading operation.

Commentary by Dr. Valentin Fuster
2013;():V001T01A067. doi:10.1115/OMAE2013-11381.

A common procedure of installing a Tension Leg Platform is to float it over the tendons and lower it over the tendons. A key phase of this procedure is lock-off, i.e. the instant when the tendons become firmly attached to the TLP hull. The lock-off position of the tendons on the hull should be within specified tolerances. Lock-off on the tendons higher than targeted results in a reduced draft which implies reduced displacement and therefore typically reduced payload capacity or reduced payload reserve. Lock-off on the tendons lower than targeted results in increase in draft and brings the deck closer to mean water which adversely affects the airgap requirements. Hence lock-off at the correct draft is a key aspect of a successful installation procedure. The general geometric properties of several TLPs are such that there is a significantly more heave motion under swell conditions than under sea states with much smaller peak periods (for the same significant wave height). These larger motions under swell conditions can make it harder to achieve the desired lock-off elevation. Such swell conditions are known to occur of the West Coast of Africa. Waiting for the swell to subside may not be an attractive alternative for TLP installation as these swells may be present for extended periods of time. Two engineering solutions are provided which reduce the heave motion of a free floating TLP. Both of these solutions are temporary changes to the TLP itself and should be implemented as a part of the TLP design spiral. One of the solutions implies an increase of TLP mass; the second solution reduces the vertical stiffness of the free floating TLP. The heave natural period is significantly increased in both methods, and heave motions are reduced. Both methods increase the heave response amplitude operator (RAO) for periods below 8 to 12 seconds and reduce the heave RAO magnitudes above 12 seconds. Implementing one of these methods in the installation procedure may be most applicable to situations where the installation window may contain prolonged periods of persistent swell.

Commentary by Dr. Valentin Fuster
2013;():V001T01A068. doi:10.1115/OMAE2013-11387.

The deck height of a tension leg platform or semi-submersible depends in large part on the expected crest height. This expected crest height is the result of the sea state, i.e. the incoming wave train, and local enhancement due to the vessels diffraction of the wave train. These local enhancements are usually determined by a combination of numerical computations and model tests. Quite often a crest enhancement factor is defined which takes into consideration these local amplification effects. Extrapolating the enhancement factor from extreme conditions to survival conditions may lead to significantly large crests and result in a very high deck elevation.

Many studies, including the CresT JIP address the characteristics of the crests within a given sea state and in the absence of a vessel. This paper addresses the effect of the presence of a vessel on the crest heights, and in particular the high crests which will ultimately determine deck height. The paper is based on experimental measurements of wave elevations underneath and around various tension leg platforms and semi submersibles. The investigated sea states comprise of a series of long crested irregular waves, generated in a model basin, which describe extreme and survival conditions in the Gulf of Mexico. The crest heights underneath the vessel are measured and compared with crests which occur without the presence of the vessel. Numerical predictions of the local amplification are also made, based on 1st order diffraction analysis and the as-measured incident wave train. A narrative is provided on the differences in crest height and observed phenomena.

Topics: Waves , Seas
Commentary by Dr. Valentin Fuster
2013;():V001T01A069. doi:10.1115/OMAE2013-11391.

This work shows an experimental-numerical procedure used to determine added mass of ships. A numerical Finite Element ship hull model is constructed according to the weight distribution and the drafts over its length during the full scale vibration measurement. The rate breadth/draft for each frame of the hull is considered as the independent variable of a quadratic function representing the added mass distribution, which coefficients are determined in order to minimize the sum of squared differences between the natural frequencies obtained numerically in respect with those obtained in the correspondent full scale measurement. The added mass coefficients obtained from the experimental data of a first ship are then used to predict numerically the natural frequencies of a second ship of the same type.

Topics: Vibration , Ships , Hull
Commentary by Dr. Valentin Fuster
2013;():V001T01A070. doi:10.1115/OMAE2013-11392.

This paper presents an overview of the model testing of a new turret-type CALM buoy concept developed by WISON for shallow water (20m–80m) applications. In the WISON design the outer body of the buoy is hexagonal, a geometry that allows for ease of fabrication while retaining hydrodynamic efficiency. The overall objective of the model tests was to demonstrate the performance of this new design for a typical shallow water environment under both operating and survival conditions. Additionally, the model tests were intended to provide data to calibrate the numerical models for buoy motions and line tensions used in the design, and to give guidance regarding the suitability of the buoy freeboard and deckhouse arrangement.

Topics: Design , Water , Buoys
Commentary by Dr. Valentin Fuster
2013;():V001T01A071. doi:10.1115/OMAE2013-11407.

The influence from a current on wave drift forces and resulting slowly varying vessel responses can be quite significant. In this paper the effect is reviewed and further investigated. Several works have been published on this complex topic during the last 20–25 years, while it is only to a little extent taken consistently into account in standard industry tools. Simplified methods are often used, if any, and /or empirical correction from model test data. Thus there is a need to improve standard tools in this respect.

The effects on slowly varying vessel motions and resulting extreme mooring line loads are demonstrated through time series sequences from selected, previous model tests with FPSO’s and semisubmersibles in steep irregular waves. Wave-current interaction effects that can be larger than the effects from current and wind alone are identified. It is also confirmed from these examples that extreme mooring forces usually occur due to extreme slow-drift motions.

An overview description is given of a new, general numerical potential theory code for industry use, MULDIF-2, where wave-current-structure interaction is consistently included as a basic element in the formulation. Main items in the approach are addressed and referred to previous works in the literature. Results from an initial comparison against previous results on drift forces on a vertical column are given, and a good agreement is found. Further verification and validation work is in progress.

Commentary by Dr. Valentin Fuster
2013;():V001T01A072. doi:10.1115/OMAE2013-11409.

Many deck structures are located at elevations low enough to be impacted by large waves. However, due to the highly complex and impulsive nature of the interactions between wave crests and intricate deck structures, establishing reliable estimates of extreme pressures and forces for use in design remains challenging. In this paper, results from an extensive set of three-dimensional scale model tests conducted to support the design of a large pile-supported pier (or jetty) are presented and discussed. Relationships between maximum wave-in-deck loads and the deck clearance (air gap) are presented and discussed. Results from numerical simulations of the wave-structure interaction process obtained using the three-dimensional CFD software FLOW-3D® are also presented and discussed. Finally, some initial comparisons between the numerical and physical modelling are also included. This paper provides new insights concerning the character and magnitude of the hydrodynamic pressures and loads exerted on intricate pile-supported deck structures due to impact by non-linear shallow-water waves, and the relationships between the hydrodynamic forcing and the deck clearance or air gap.

Commentary by Dr. Valentin Fuster
2013;():V001T01A073. doi:10.1115/OMAE2013-11412.

Bechtel has been contracted for and is in the process of executing multiple onshore Liquefied Natural Gas (LNG) Engineering, Procurement, and Construction (EPC) projects utilizing the modular construction strategy. Modules and associated pieces of equipment have to be shipped to the job site from various manufacturing and fabrication facilities across open oceans.

Naval Architects play a key role to assure safe and effective module ocean transportation. Primary naval architectural work consists of a routing study, module design criteria definition, ballasting and stability analysis, grillage and seafastening design, transportation vessel selection and support for module load out and load in.

The main challenge is to make the modules, which are originally designed for onshore assembly, sound for ocean transportation. Therefore, module design criteria related to ocean transportation become crucial. Among these criteria, the wave induced inertia loads and vessel deflection have great impact on designed module structure integrity. In order to define inertia loads and deflection appropriately, the interface between vessel and module becomes a main concern. It raises the question of whether the transport vessel and module should be fully integrated. It also increases complexity of the hydrodynamic interaction, which has been demonstrated by widely divergent methods used to address the interface issue in offshore industry. More importantly, whether or not the interface is thoroughly taken into account is critical to successful module design and fully meeting the Marine Warranty requirements. In order to achieve safe and economic module design, a direct method of integrating vessel and module is considered preferable in analysis and design when the inertia effects and structure hydrodynamic response are significant.

This paper will provide an overview of integrated vessel and module interaction analysis for the module ocean transportation and focus on the method and procedure of how Bechtel performs analyses: i) spectral motion analysis with a fully coupled constitutive model and ii) vessel and module interaction analysis utilizing an integrated 3D model with fully hydrodynamic loads transferred. In order to determine realistic extreme load case, the equivalent design wave selection will be addressed as well.

Commentary by Dr. Valentin Fuster
2013;():V001T01A074. doi:10.1115/OMAE2013-11433.

This paper addresses some preliminary discussion concerning the performance of different dynamic observer-controller-based positioning system for a shuttle tanker by considering her operational condition changes. The observer-controller models are nonlinear passivity based observer and backstepping controller, Extended Kalman Filter and Proportional-Derivative like controller, and notch filter and Proportional-Integral-Derivative controller. The variation of the operational condition of the vessel is obtained by changing the vessel draft and the environmental condition, such as relative incidence of the current, waves and wind, and the peak period of the sea wave spectrum. The performance of the dynamics of the vessel is investigated through numerical simulation in which a saturation model for the actuators is included. Preliminary results have indicated that the nonlinear observer-backstepping controller is the easiest approach to select the observer and control parameter in the case of variation of the draft.

Commentary by Dr. Valentin Fuster
2013;():V001T01A075. doi:10.1115/OMAE2013-11441.

The floating storage unit Navion Saga at the Volve field in the North Sea suffered from two mooring line breaks in steel wire ropes in 2011. Investigations of the broken ropes indicated that a possible failure mechanism could be high stresses near the wire socket induced by large bending moments in leeward mooring lines. The scope of the current study has been to make a numerical model capable of capturing such behaviour of the steel wire rope and to check if the minimum bending radius could be as low as the rope’s specified minimum bending radius.

The numerical model has revealed a possible failure mechanism. The connecting link plate between the upper chain segment and the upper wire segment lies initially on the seabed. When lifted off the seabed, the link plate and the wire socket will fall to the seabed at a higher speed than the upper wire segment. A transverse wave in mooring line plane propagating towards fair-lead is generated when the wire socket hits the seabed. The wave leads to large curvature in the wire near the socket. Sensitivity studies of the governing parameters have been performed to assess the uncertainties of the numerical model. A modified system is presented and it is shown that the phenomenon which is likely to have caused failure in the original system will not occur for the modified one.

Topics: Failure , Mooring , Storage
Commentary by Dr. Valentin Fuster
2013;():V001T01A076. doi:10.1115/OMAE2013-11450.

Advanced hydrodynamic analyses of floating LNG terminals are presented in the paper. They consist of the complex interaction of multiple bodies and the coupling effect of seakeeping (wave diffraction and radiation around bodies) and sloshing (liquid motions in tanks). Based on the recent development to introduce the dissipation in potential flows and new formulations of boundary element method, the seakeeping analysis is enhanced to be able to make accurate predictions of gap resonances and major dynamic effect of liquid motion in tanks.

Commentary by Dr. Valentin Fuster
2013;():V001T01A077. doi:10.1115/OMAE2013-11460.

This paper introduces new hybrid wire rope concepts by combining steel wire and UHMWPE fibers. These novel High Modulus Hybrid Ropes (HMHR) result in enhanced strength and significant reduction of linear weight compared to steel wire rope (SWR) of equivalent constructions. Compared to fiber ropes (FR) this new family of ropes shows increased protection against crushing, fretting and wear.

In-house and independent laboratory tests showed increased breaking load / weight advantages compared to SWR, while also revealing improved bending fatigue resistance. Additionally, test results show similar axial stiffness and torque performance to SWR, which can be modeled by using analytical algorithms. Furthermore creep rate of these ropes is comparable to that of SWR.

Finally, the authors discuss the expected performance of these ropes working in deep waters and under high load conditions in offshore and subsea systems. Additional work required to validate and introduce these ropes in the industry is addressed as well.

Commentary by Dr. Valentin Fuster
2013;():V001T01A078. doi:10.1115/OMAE2013-11467.

Development of Turret Mooring Systems (TMS) for harsh environment and large number of risers has led to a drastic increase of the size of the chaintable and consequently of the turret cylinder diameter. Furthermore, harsh environments usually require relatively deep drafts. As a consequence, the volume of entrapped water in large turrets increases to levels never designed for before. In some cases, the mass of the entrapped water can be comparable to the turret mass. Whilst this entrapped water does not exert any weight on the weathervaning system, its acceleration due to ship motions induces inertia loads which could affect the balance of loads. Estimation of these inertia loads is easily carried out assuming the entrapped water as frozen. However, to what extent is this assumption valid in view of the large amount of entrapped water involved and of the extreme ship motions expected in harsh environments? Should we expect sloshing of the entrapped water? In this paper, insights will be drawn from numerical techniques of diverse complexity. This will be preceded by a brief literature review on sloshing in moonpools. Practical analysis and design recommendations will be proposed. Operational aspects related to installation will be covered as well.

Commentary by Dr. Valentin Fuster
2013;():V001T01A079. doi:10.1115/OMAE2013-11468.

In this paper the design robustness of Tension Leg Platform (TLP) tendon and tendon foundation systems of a TLP that is located in offshore Western Australia is investigated.

A case study of a TLP that is self-stable (without tendons) has been considered. The study involves the numerical simulation of progressive failure of tendons in cyclonic events. The TLP response during the transition from a restrained TLP with all tendons to the free-floating condition has been numerically simulated. The numerical results from this simulation have been verified against physical model test measurements. The numerical simulation is repeated for a TLP with an optimized hull design that does not maintain stability when all tendons fail. Cost versus benefit in these two cases is quantified and compared.

The progressive failure of the TLP Gravity Base Foundation (GBF) system has also been investigated in this paper. One of the potential failure modes for this type of foundation is the loss of suction underneath the foundation. Increasing the amount of solid ballast in the GBF increases the net downward load on the soil and reduces the reliance on the soil suction. Numerical simulations of the progressive loss of suction are performed for two cases; 1) slightly over designed foundation to include extra ballast and 2) optimized foundation design that is highly rely on the soil suction. Again, cost versus benefit in these two cases is presented.

The paper provides clear insights supported by calculations and model tests for proposed design robustness that could be built in a TLP design at a relatively small additional cost to address uncertainties associated with designing TLP in offshore Western Australian harsh environment region.

Commentary by Dr. Valentin Fuster
2013;():V001T01A080. doi:10.1115/OMAE2013-11471.

The paper presents a theoretical study on an active hybrid decomposed mooring system for model testing of offshore platform in wave basin. The basic concept and the working principles are described. Important issues for achieving a correct simulation will be discussed. The feasibility of the approach is demonstrated based on numerical investigations. Plans for potential implementation in an ocean basin are also discussed.

Commentary by Dr. Valentin Fuster
2013;():V001T01A081. doi:10.1115/OMAE2013-11585.

Designing large, innovative ships is a complex assignment and an interesting one for every company: Ship designers are capable of supporting the client’s requests and deliver a design quicker than any other related industry involved in the design of complex objects. However, at the same time it is felt that the industry could improve their innovative output by applying a more focused method to design these innovative and complex ships. Existing methods either mitigate the complex relations between systems of a ship, or concentrate on the design of a single object.

The design models evaluated in this paper, amongst others, the design spiral and system engineering, do not describe these important aspects of the ship-design cycle: To evolve beyond the current innovations, current design models do not suffice. The model presented in this paper takes the complex relations through different levels of decomposition into account and presents the possibility to track and support change during the design process. In future research, the model will be applied and developed further towards approaches and tools to support innovative design.

Commentary by Dr. Valentin Fuster
2013;():V001T01A082. doi:10.1115/OMAE2013-11628.

An Offshore Platform Global Weight Management Database Program (OPGWMDP) has been developed and used to manage and monitor the platform system weights in design, fabrication, installation and operation. It is the first weight management database specialized to support the Spar system engineering design, construction, installation and offshore operation. This database program, which was developed in the Microsoft Access environment, is portable, cost friendly and integrated with graphic user interfaces (GUI). This program not only has the data storage and operational functions like a common database program, but also provides calculation functions specifically developed for offshore platform design and operation, such as horizontal force and moment calculation for mooring or other positioning systems, riser systems, fixed ballast and variable ballast automatic balance calculation, adjustment of displacement and topsides payloads eccentricity and variation. The program can also provide fast data for the platform stability verification under various operational conditions. These functions usually are not included in other similar weight control database programs. This system includes 4 main modules to provide the data input, edit and output functions and several sub-modules to manage different group data. This architecture gives the system a flexible structure which can be easily supplemented with new functions in the future. The specifically designed GUI provides a friendly interface for the user to interact with this system. This weight control database program has been verified with as-built platform data, and is applicable for a variety of offshore platforms, including Spar, semisubmersible, TLP and FPSO.

Commentary by Dr. Valentin Fuster
2013;():V001T01A083. doi:10.1115/OMAE2013-11640.

A numerical simulation and system identification approach to the dynamic equilibrium of a catenary riser has been developed. A finite DOF representation of the dominant dynamics is constructed using frequency domain identification by applying nonlinear signal theory techniques on response data series when exciting the structure with sinusoidal motions at the top. Data series are obtained through numerical integration of a finite differences simulation model on the basis of the six nonlinear partial differential equations describing the riser dynamics. Dynamic equilibrium is mathematically formulated by the very same equations that implicate both geometric and hydrodynamic nonlinearities; the latter are depicted by Morison’s formula. Thus, spatio-temporal series are generated for riser bending moments induced by sinusoidal heave motions of various amplitudes and frequencies. These data are consequently transformed to the frequency domain where complex Singular Value Decomposition is applied in order to derive the full nonlinear spectrum. The significant harmonics of the riser’s spectrum for the bending moment on the 2D plane of reference are demonstrated to be the three lower odd harmonics and a set of orthogonal modes for these three significant harmonics is derived. The methodology proposed is finally applied to a typical test case for validation.

Commentary by Dr. Valentin Fuster

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