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Structures, Safety and Reliability

2010;():1-12. doi:10.1115/OMAE2010-20003.

In the current study, the effect of spud-can-soil interaction modeling as well as wave load modeling and sea-state on the reliability index of a three-leg North-sea drilling type jack-up platform is studied. The platform has operated in depths of 95–105m and its three main legs modeled as pipe elements and the main deck is also modeled using general beam and also shell/plate elements. The spud-can foundation is modeled using elasto-plastic, hyper-elastic springs as well as spud-model of usfos using a general non-associative plasticity model. The wave-in-deck load is considered using API-RP2A [1], Shell and Statoil models. The SH and SV-seismic waves are considered separately at spud-can base as scaled. The preliminary results of this study showed that the annual probability of failure for the Jack-up platform is significantly affected by the random-sea and wave-in-deck modeling as well as spud-can-soil interaction modeling and soil parameters. Study of importance factors indicated that more refined spud-can-soil interaction modeling as well as wave-in-deck load modeling might improve the calculation models and hence reduce their associated uncertainty in reliability analysis of jack-up type platform.

Commentary by Dr. Valentin Fuster
2010;():13-19. doi:10.1115/OMAE2010-20010.

The configuration of braces has a considerable influence in the lateral behavior of pile supported steel jacket type offshore platforms. In this paper nonlinear static push over analyses and hysteresis curves are used to investigate lateral behavior of a jacket type offshore platform considering different configurations for vertical bracings of the jacket. One sample platform, constructed in Persian Gulf, is precisely modeled using the finite element program Opensees. Since the lateral response of offshore platforms is completely dependent on their foundations’ behavior, an accurate model is used for modeling the foundation of this structure. Soil-pile-structure interaction is considered using Beam on Nonlinear Winkler Foundation model (BNWF). Lateral and vertical soil stiffness and end bearing were considered using p-y, t-z and q-z nonlinear models, respectively. Moreover, the buckling behavior of the braces is considered in the simulation of the platform to consider the behavior of the platform after buckling, and redistribution of the shear forces in the structure due to the bracings failures. The structure was modeled using nonlinear beam-column elements which have the ability to consider the spread of plasticity along the elements. The sections used for elements are fiber sections which are suitable for considering composite section of pile-grout-leg. Displacement controlled nonlinear static pushover analysis and cyclic loading analysis are conducted applying lateral load which its pattern is according to the predominant vibration modes. Three different configurations for lateral bracings are assumed and the behavior of the platform using these three bracing forms is investigated. The first configuration considered is the original bracing of the platform which is a combination of X and chevron braces; the second one is a case in that X braces were used in all of the bays of the jacket; and in the third form chevron braces are used for all of the bays. According to the push over and hysteresis curves, it is concluded that in the jacket modeled using X vertical braces, lateral load capacity, ductility, residual strength ratio and the absorbed energy in cyclic loadings are considerably more than the jackets which were modeled using a combination of chevron and X braces or just chevron braces. In comparison with the jacket modeled using only chevron braces, the jacket constructed using a combination of X and chevron bracings presents better lateral behavior and capacity.

Commentary by Dr. Valentin Fuster
2010;():21-30. doi:10.1115/OMAE2010-20033.

In the dynamic analysis of a floating structure, coupled analysis refers to a procedure in which the vessel, moorings and risers are modeled as a whole system, thus allowing for the interactions between the various system components. Because coupled analysis in the time domain is impractical owing to prohibitive computational costs, a highly efficient frequency domain approach was developed in a previous work, wherein the drag forces are linearized. The study showed that provided the geometric nonlinearity of the moorings/risers is insignificant, which often holds for ultra-deepwater systems, the mean-squared responses yielded by the time and frequency domain methods are in close agreement. Practical design is concerned with the extreme response, for which the mean upcrossing rate is a key parameter. Crossing rate analysis based on statistical techniques is complicated as the total response occurs at two timescales, with the low frequency contribution being notably non-Gaussian. Many studies have been devoted to this problem, mainly relying on a technique originating from Kac and Siegert; however, these studies have mostly been confined to a single-degree-of-freedom system. The aim of this work is to apply statistical techniques in conjunction with frequency domain analysis to predict the extreme responses of the coupled system, in particular the modes with a prominent low frequency component. It is found that the crossing rates for surge, sway and yaw thus obtained agree well with those extracted from time domain simulation, whereas the result for roll is less favorable, and the reasons are discussed.

Commentary by Dr. Valentin Fuster
2010;():31-39. doi:10.1115/OMAE2010-20045.

The most challenging aspect of a deepwater development is the riser system, and a cost-effective choice is the Steel Catenary Riser (SCR). Fatigue is often a governing design consideration, and it is usually most critical at the touchdown point (TDP) where static and dynamic bending stresses are highest. Unfortunately, it is also at this region that uncertainty is the maximum. The increased uncertainty casts doubt on the applicability of generic safety factors recommended by design codes, and the most consistent way of ensuring the structural safety of the SCR is to employ a reliability-based approach, which has so far not received attention in SCR design. As the number of basic random variables affects the complexity of a reliability analysis, these variables should be selected with caution. To this end, the aim of this paper is to draw up a comprehensive list of design parameters that may contribute meaningfully to the uncertainty of the fatigue damage. From this list, several parameters are selected for sensitivity studies using the commercial package Orcaflex. It is found that variations in seabed parameters such as soil stiffness, soil suction and seabed trench can have a pronounced influence on the uncertainty of the fatigue damage at the touchdown point.

Commentary by Dr. Valentin Fuster
2010;():41-49. doi:10.1115/OMAE2010-20046.

Flexible marine risers are compliant to external forces from waves, current and platform motions, and clashing between risers is an important concern. In deepwater developments where the number of connected risers is large, it is not economical to space them too far apart. In this regard, it is necessary to establish the probability of riser clashing throughout the service life; however, at present there appears to be no systematic procedure for assessing this risk. This paper presents a novel procedure for estimating the probability of riser clashing based on post-processing results obtained from time domain simulations of flexible risers subjected to random wave loads. First, an efficient technique is employed to sieve out critical pairs among riser elements. From these element pairs, the time history of a normalized clearance parameter is derived from the nodal displacements of the elements. Subsequently, the mean up-crossing rate of this parameter is extracted and extrapolated to the threshold of clashing using extreme value theory. As this method is still in its early developmental stage, critical effects such as vortex-induced vibrations and wake interference will not be considered in the present work.

Commentary by Dr. Valentin Fuster
2010;():51-58. doi:10.1115/OMAE2010-20052.

The conditions for damage stability and survivability of a ship struck in a collision in arbitrary sea-state are, from a structural point of view, determined by the size and shape of the damage opening in its side shell. In the current investigation, explicit finite element analyses (FEA) are presented of a ship-to-ship collision scenario in which the damage opening of a struck ship is calculated for a selection of damage degradation models and realistic material properties, here referred to as model and material properties uncertainties. The model uncertainty is considered as a possible (user-related) insecurity in the selection of the most appropriate damage criterion for the analysis; the shear failure and the forming limit diagram (FLD) criteria were compared in the current investigation. The uncertainty in material properties is accounted for in the constitutive material model description and the material parameters used in the two criteria. The size and shape of the damage openings predicted by the FEA are used in damage stability analyses in which the struck ship is subjected to wave motions in an arbitrary sea-state and flooding into the damage opening. The survivability of the struck ship is estimated for all of the damage opening cases. One of the main conclusions is that the high degree of accuracy that a researcher on structure analysis strives for has to be considered together with the natural variation of the sea-state that defines the characteristics in the following damage stability analysis. Consequently, by adoption of a holistic approach, in which structural integrity and damage stability research are combined using a systematic parameter (sensitivity) and collision-scenario-based analysis, simplified models and criteria can be developed more efficiently and with higher precision. It will also be clearer which variables are the most important to focus on when analyzing the survivability or risk for capsizing.

Commentary by Dr. Valentin Fuster
2010;():59-64. doi:10.1115/OMAE2010-20054.

There are some components subjected to compressive and bending loading in ship and offshore structures and fatigue cracks were fond in compressive side of these components caused by fluctuation loadings, during their service. For better understanding the fatigue behavior of these components subjected to compressive to compressive loading, plate specimens with center crack (CCP) and plate specimens with double edge crack (DECP) have been designed for the experiment for examining the fatigue crack growth under axial compressive fluctuation loading. In this paper, a high strength steel plate was used as the test material. Fatigue test has been performed using MTS810 material testing system. Experimental results show that cracks can be propagated under compressive to compressive loading. It also shows that the cracks propagated to a certain length and then arrested completely. The experimental procedure and the phenomena are described. The stress-strain and the residual stress during a cycle were simulated by FEA. The stress intensity factor of the crack by residual stress and its propagation life were estimated and compared with the test data. The residual stress plays a very important role in crack growth under compression to compression fluctuation load.

Topics: Fatigue cracks
Commentary by Dr. Valentin Fuster
2010;():65-73. doi:10.1115/OMAE2010-20069.

The random decrement technique is an averaging technique that can be used to extract the free decay response of a structure from its stationary random vibratory response. The free decay response can then be used to identify the vibratory characteristics of the structure. The main advantage of the technique is that the identification of the parameters of the structure is achieved without previous knowledge of the excitation forces. This paper extends the random decrement technique to obtain the mode shapes of the structure using the concept of a multichannel random decrement technique (MCRD). This technique is based on extracting simultaneous random decrement records from measurements made at several points on the structure. The change in mode shapes as a result of occurrence of damage to the structure is then used to detect the presence and location of damage. The method is illustrated and validated using two case studies and finite element modeling. Excellent agreement between mode shapes obtained using the MCRD and those obtained from classical modal analysis techniques was found. The use of changes in the mode shapes of the structure to detect the presence and location of damage was successful.

Topics: Shapes
Commentary by Dr. Valentin Fuster
2010;():75-91. doi:10.1115/OMAE2010-20076.

This paper presents a literature survey on time-dependent statistical modelling of extreme waves. The focus is twofold: on statistical modelling of extreme waves and time-dependent statistical modelling. The first part will consist of a thorough literature review of statistical modelling of extreme waves and wave parameters. The second part will focus on statistical modelling of time- and space-dependent variables in a more general sense, and will focus on the methodology and models used also in other relevant application areas. It was found that limited effort has been put on developing statistical models for waves incorporating spatial and long-term temporal variability and it is suggested that model improvements could be achieved by adopting approaches from other application areas. Finally, a review of projections of future extreme wave climate is presented.

Topics: Waves
Commentary by Dr. Valentin Fuster
2010;():93-99. doi:10.1115/OMAE2010-20087.

The strict regulations of the actual SOLAS regarding damage stability may sometimes interfere with the design wishes of the ship owners. A reason may be that the approach stated in the SOLAS regulation is merely a probabilistic method. In this study the application of an alternative approval procedure to the pure probabilistic SOLAS damage stability calculations is presented. The procedure differs from probabilistic SOLAS regulations in the form that it considers the structural strength characteristics of the vessel design within damage stability assessment. A multi purpose vessel (MPV) with one large cargo hold is analyzed. A reference design complying with SOLAS regulations with a double hull width of 2.2m is optimized based on the alternative approval procedure and it is shown that the optimized design with ice class reinforcement and a double hull width of 1.5m has an equivalent level of safety. Thus, the double hull width could be reduced by 40% and the cargo hold breadth could be enlarged by 10%. This latter fact may be of particular interest for ship owners and designers as the flexibility of utilization of the vessel could lead to an improved cost benefit.

Commentary by Dr. Valentin Fuster
2010;():101-107. doi:10.1115/OMAE2010-20111.

Rainflow counting is widely accepted as the method that is most suited to analysis of fatigue damage of materials submitted to irregular loading. Formulas such as the Wirsching-Light and the Dirlik one allow to take into account spectral shape and bandwidth in an empirical or semi-empirical manner to obtain a best estimate damage reduction of the rainflow counting with respect to the narrow-band approximation. However, if one considers parametric shape families of common use for the spectra, a more straightforward way is to make damage depend on the shape parameter of the family rather than on the spectral moments. We provide here such semi-empirical parametric formulas for the Jonswap, Wallops, Triangle and Power-tail families. In addition, the ICA formula allows us to extend the above formulas to the well-known bimodal spectral shape proposed by Ochi-Hubble.

Commentary by Dr. Valentin Fuster
2010;():109-121. doi:10.1115/OMAE2010-20114.

Crossing intensity constitute an important response characteristic for randomly vibrating structures, especially if one is interested in estimating the risk against failures. This paper focusses on developing approximations by which estimates of the crossing intensities for response of marine structures can be obtained in a computationally efficient manner, when the loads are modeled as a special class of non-Gaussian processes, namely as LMA processes. Ocean waves exhibit considerable non-Gaussianity as marked by their skewed marginal distributions and heavy tails. Here, a new class of processes-the Laplace driven Moving Average (LMA) processes are used to model the ocean waves. LMA processes are non-Gaussian, strictly stationary, can model in principle any spectrum and have the additional flexibility to model the skewness and the kurtosis of the marginal distribution. The structure behavior assumed is limited to quadratic systems characterized by second order kernels, which is common for marine structures. Thus, an estimation of the crossing intensities of the response involves studying the crossing characteristics of a LMA process passing through a second order filter. A new computationally efficient hybrid method, which uses the saddle point approximations along with limited Monte Carlo simulations, is developed to compute crossing intensity of the response. The proposed method is illustrated through numerical examples.

Topics: Approximation
Commentary by Dr. Valentin Fuster
2010;():123-131. doi:10.1115/OMAE2010-20124.

Wave-induced vibrations, also known as whipping and springing, are defined as the high frequency response of ship structures. In this paper, the fatigue damage caused by whipping and springing is presented by investigating the amidships section of a 2800 TEU container ship that operates in the North Atlantic Ocean. A simplified fatigue model, originally from the generalized narrow-band approximation for Gaussian load, is employed to include the damage contribution from wave-induced vibrations. In this model, the significant response range hs and the mean stress up-crossing frequency fz are simplified using only the wave-induced loading and encountered wave frequency, respectively. The capacity and accuracy of the model is illustrated by application on the measurements of the 2800 TEU container ship for different voyages during 2008. The whipping-induced contribution to the extreme response is investigated by means of the level crossing approach. It shows that the level crossing model for Gaussian load cannot be used for the prediction of extreme responses, such as the 100-year stress, based on a half-year full-scale measurement. It is found that a more complicated non-Gaussian model is required to consider the contribution from whipping.

Topics: Fatigue damage , Ships
Commentary by Dr. Valentin Fuster
2010;():133-141. doi:10.1115/OMAE2010-20125.

The hydrodynamic analysis of a 4400 TEU container ship with constant forward speed is carried out by the nonlinear numerical code WASIM in the time domain under severe sea states. Straightforward fatigue estimation is performed using the rainflow counting approach based on simulated time series of stresses. The narrow-band approximation, which has been validated in previous work with good accuracy by full-scale measurement of a 2800 TEU container ship, is implemented to estimate the fatigue damage based on the same responses. It is concluded that a slight deviation from the Gaussian process does not influence the fatigue estimation by narrow-band approximation. In addition, extreme response is defined by the level up-crossing approach. The Gaussian crossing model using Rice’s formula is employed to predict the extreme response based on the responses from above numerical analysis. It shows that the Gaussian model is not suitable for this prediction. A more complicated level crossing model is proposed which is based on the Laplace Moving Average method. Its accuracy in prediction of extreme responses is analyzed and presented with good agreement by means of numerical simulations.

Topics: Fatigue , Containers , Waves , Ships , Seas
Commentary by Dr. Valentin Fuster
2010;():143-150. doi:10.1115/OMAE2010-20130.

In this paper, we establish a mechanical model of the structural dynamic analysis for the legs of fixed pile leg marine structures, where the hull doesn’t float out of the water. We make a calculation and analysis of the dynamic model by using the finite element analysis software “ABAQUS”. Also the cushioning mechanism of different materials is analyzed. Accordingly we establish a dynamic analysis model of a cushioning system. An elastic modulus linear model is used to simulate rubber materials, in order to carry out a qualitative analysis of the cushioning effect of a cushioning device in a reinforcing area. Taking rubber materials’ different elastic modulus and different load impact frequencies as variables, a series of calculation results and analysis are presented. Through a comparative analysis of the different structural dynamic response between the buffered and the non-buffered, we draw the conclusion that the dynamic load of the legs has been effectively alleviated with a cushioning device which is helpful in leg-protection.

Commentary by Dr. Valentin Fuster
2010;():151-158. doi:10.1115/OMAE2010-20140.

The traditional method to assess fatigue damage of ship structures assumes moderate wave amplitudes and linear responses. This method can be questioned when applied on container ships that are characterized by large deck openings, because the low torsion rigidity of this type of ship makes it sensitive to oblique waves. In this paper, the 3D hydrodynamic code WASIM is used to simulate a 4400 TEU container ship operating in the North Atlantic Ocean. Nonlinear wave loads are utilized for direct calculation of the stress histories under severe sea states. The warping stress from wave-induced torsion is separated from the stress components from vertical and horizontal bending. The contribution to fatigue damage accumulation from warping stresses is evaluated. For comparison, the results from the numerical simulations and fatigue calculations are verified with full-scale measurements made on a similar type of container vessel.

Commentary by Dr. Valentin Fuster
2010;():159-170. doi:10.1115/OMAE2010-20142.

To ensure survival of floating structures in rough seas, a precise knowledge of global and local loads is an inevitable integral part for safe design. One of the key parameters is the vertical bending moment. Not only vertical forces but — as previous investigations revealed — also longitudinal forces significantly contribute to the vertical wave bending moment. Three segmented ships, equipped with force transducers, are investigated systematically in high and steep regular waves and in harsh wave environments at several cruising speeds to identify the structural loads. The model tests are carried out in the seakeeping basin of the Technical University Berlin at a scale of 1:70. To cover possible influences of the bow geometry, three different types of vessels are chosen, a bulk carrier with a full bow, a Ro/Ro vessel and a container vessel with a V-shaped frame design. For identifying the influence of the wave height and steepness on the vertical bending moment, model tests in regular waves with different crest/trough asymmetries are performed with the Ro/Ro vessel and the bulk carrier. The program can be subdivided into three parts, each characterized by the same wave lengths with varying wave steepness. The first test series includes regular waves with small amplitudes, thus linear wave theory can be applied. In the second part the same (regular) wave lengths have been generated with increased wave heights, i.e. increasing crest/trough asymmetries and wave profiles within Stokes II domain. During the last part of the experimental program the wave heights are further increased to reach wave profiles within Stokes III domain. For the evaluation of the test results in regular waves — in particular in high steep waves — the results are compared to the respective Response Amplitude Operator determined by the transient wave package technique. Here the focus lies on the asymmetry of the hogging and sagging loads with respect to the wave steepness and the bow geometry of the investigated ship models. Furthermore, the influence of the freeboard height on the vertical bending moment is analysed. For this purpose a container vessel is investigated with two different freeboard configurations in a harsh wave environment.

Topics: Stress , Waves , Shapes
Commentary by Dr. Valentin Fuster
2010;():171-178. doi:10.1115/OMAE2010-20174.

Transmission pipelines, which are usually subjected to a variety of loads ranging from internal pressure to external pressure ground loading, transport gases and liquids at high pressures. Furthermore, the pipelines exhibit various forms of damage from environmentally induced cracks to manufacture and operation induced cracks. Therefore, it is of great engineering significance to establish acceptable levels for the flaws revealed. The acceptance levels for flaws are based on the concept of fitness for purpose. By this principle, a particular fabrication is considered to be adequate, provided that the conditions to cause failure are not reached. Less severe flaws are accepted without further consideration. Moreover, if flaws are more severe, rejection is not necessarily automatic. Based on fitness for purpose, decisions on whether rejection or repair might be justified, either in the light of previously documented experience with similar material, stress and environmental combinations or on the basis of an engineering critical assessment (ECA). The present paper focuses on two objectives. Firstly, the remaining strength and fracture toughness of the pipeline with defects along the longitudinal direction of the pipeline have been obtained. Secondly, the utilizing reliability of the pipeline, which has been tested to have inner flaws and to what extent this kind of defects may exist in the pipeline purchased from the same manufacturer, has been evaluated. Based upon the description of the defects found in the pipeline, the remaining strength assessment is performed with the application of different codes such as B31G and DNV. Also, an ECA sample for the embedded flaw in plates will be performed according to the existing BS 7910 methodology. The initial crack depth is 2a and the width is 2c. According to the BS7910 standard, the fracture and fatigue assessment is applied to obtain the fracture ratio Kr and the load ratio Lr. A 4th Runge-Kutta method is used to calculate crack growth rate da/dN and the final size of the crack af and cf . Also the Failure Assessment Diagram (FAD) can be changed according to the material stress-strain curve. This work might provide useful reference to safety design and application of pipelines.

Commentary by Dr. Valentin Fuster
2010;():179-188. doi:10.1115/OMAE2010-20187.

We present a stochastic model of sea storms for describing long-term statistics of extreme wave events. The formulation generalizes Boccotti’s equivalent triangular storm model (Boccotti 2000) by describing an actual storm history in the form of a generic power law. The latter permits the derivation of analytical solutions for the return periods of extreme wave events and associated statistical properties. Finally, we assess the relative validity of the new model and its predictions by analyzing wave measurements retrieved from two NOAA-NODC buoys in the Atlantic and Pacific Oceans.

Topics: Waves , Storms , Seas
Commentary by Dr. Valentin Fuster
2010;():189-198. doi:10.1115/OMAE2010-20188.

Third-order quasi-resonant interactions among free waves and associated modulational instabilities can significantly affect the statistics of various surface features in narrowband waves. In particular, modulational instabilities tend to induce intermittent amplifications on the surface displacements, causing their statistics to deviate from the linear Gaussian and second order models. Herein, we investigate the nature of such instabilities on the statistical and spectral characteristics of deep-water waves generated in a large wave basin. We analyze the spectral changes that occur as waves propagate along the basin, develop bounds on the spectrum bandwidth, and interpret various statistics based on third-order Gram-Charlier distributions.

Commentary by Dr. Valentin Fuster
2010;():199-207. doi:10.1115/OMAE2010-20197.

At present, it is immature to precisely evaluate the ice-induced fatigue life of platform. One of the key problems for accurate estimation of the ice-induced fatigue life is to establish the rational ice load model. A new method of establishing the environment load model to evaluate the ice-induced fatigue life is put forward based on the two basic vibration forms that are structure resonance and non-resonance existing in the ice-induced platform. Compared with the traditional classification method for ice-induced conditions, not only the ice thickness and the ice velocity of corresponding ice zone will be thought as the important influencing factors, but also the platform natural vibration characteristics will be considered in this method. Finally, based on this method, the environment load model of ice zone No.4 in China’s Bohai Sea is established to do the simulation test of node’s fatigue stress of one platform in this sea area. The results show that the bending stress can’t be ignored in the vulnerability analysis of the dynamic response of the ice-induced platform; the relation between the natural vibration frequency and the period of the ice broken must be considered to classify the work condition in the estimation of the platform ice-induced fatigue; the result can’t be guaranteed determinately more secure when the ice-induced fatigue analysis is conducted by simple amplifying the ice thickness or the ice velocity under the condition that ignores the nature frequency of the platform.

Topics: Fatigue , Stress , Ice , Vibration
Commentary by Dr. Valentin Fuster
2010;():209-217. doi:10.1115/OMAE2010-20215.

A numerical method of stress concentration factors (SCFs) in tubular K-joints under combined loadings was proposed, and the loading effects on SCFs were investigated by using this method. Since it couldn’t present the state of stress concentration very well if the displacements were used as the boundary conditions, the load boundary conditions were selected to analyze the stress distribution of the tubular K-joints. In view of the disadvantages of the superposition of stress distributions from each uniaxial loading mode, an alternative method for the calculation of the hot spot stress under combined loadings was provided. In this method, three basic types of basic loadings namely, axial loading, in-plane and out-of-plane bending moments were applied to the tubular joints simultaneously by virtue of the submodeling technique in ANSYS system. Then the stress distributions along the intersection of tubular K-joints under combined loadings were developed directly, and the hot spot stresses were obtained through extrapolation. Finally, according to parametric study, the loading effects of ice thickness and drift direction on SCF distributions and values were studied through this method. The investigation provides the theoretical basis for determining SCF in tubular K-joints under different load cases in evaluating the ice-induced fatigue of offshore platforms, and has very important engineering application value.

Commentary by Dr. Valentin Fuster
2010;():219-225. doi:10.1115/OMAE2010-20233.

Based on up-crossing rate analysis of the combined still water and wave loads and the first order reliability method, a simplified analytical combination method is developed, by which a general Weillbull characteristic of wave loads can be incorporated in the prediction of extreme values of the combined still water and wave loads acting on ocean-going ships. The results predicted by the developed analytical method agree very well with those by the existing numerical methods. Further, based on type II extreme value distribution and developed load combination method, a probabilistic model for the extreme value of the combined loads is established, which would be very convenient to use in reliability analysis of a ship hull girder. Finally, numerical analysis is carried out to demonstrate the established combination method and probabilistic model.

Topics: Stress , Waves , Water
Commentary by Dr. Valentin Fuster
2010;():227-237. doi:10.1115/OMAE2010-20241.

A method to evaluate ship response in heavy seas is presented. A ship maneuvering in a stochastic environment is difficult to model because of the rarity and significant nonlinearity of the large motion responses. In the proposed method, critical wave groups are defined and used to separate the complexity of the nonlinear dynamics of ship response from the complexities of a probabilistic description for the response. In this formulation, wave groups may be considered as a possible method to solve the problem of rarity. With the characteristic information about the wave groups, the problem of rarity can be solved in a deterministic manner. Characteristics, including the distributions for the number of waves in a group and wave amplitude, period, and steepness for the waves in a group are presented.

Topics: Waves , Ships , Seas
Commentary by Dr. Valentin Fuster
2010;():239-244. doi:10.1115/OMAE2010-20246.

More recently, there is a clear tendency to consider the improvement of safety as a vital factor in the marine industry. In this paper, general statistics of ship accidents in South Korea are considered and the main objective of this research is to investigate the causes and circumstances of all maritime accidents in the country from 1990 to 2007. The research carries out the most common causes of accidents on marine operation finally. This study result shows that safety problem really exists in the whole ocean industry and prevention of fatal accidents should focus on the research for the risk assessment and prediction methodology.

Commentary by Dr. Valentin Fuster
2010;():245-250. doi:10.1115/OMAE2010-20247.

Summarize the definition of design return period in existent codes and industry criteria. Joint return periods of different ocean environmental conditions are determined from the view of project life and risk probability. Based on equivalent bivariate maximum entropy distribution, joint design parameters of wave height and wind speed are estimated occurred in some place of Bohai Sea. The calculating results show that even if the return value of each environmental factor, such as wave height or wind speed, is small, their combinations usually lead to larger joint return periods. This will make the investment of ocean platform lower.

Topics: Wind velocity , Waves
Commentary by Dr. Valentin Fuster
2010;():251-259. doi:10.1115/OMAE2010-20253.

The characters of the collision scenario when a semi-submersible is struck by a containership are studied in this paper, through the model test, simplified analytical method and numerical simulation. The model test is conducted in the Deepwater Offshore Basin in Shanghai Jiao Tong University. Two special devices are designed to fulfill the model test. One is Ship Launching Device, simplified as SLD, who can launch the striking ship with controllable velocity and in any horizontal direction. The other is Energy Absorbing Device, simplified as EAD, who can simulate the buffer effect of the column structure and collect the collision force as well. A numerical simulation is completed to get the approximate stiffness of the column structure, which is used to adjust the property of EAD. The motions of semi-submersible are obtained, and the collision force and the tension forces of mooring lines are also got. Collision scenario characters for semi-motion and tension force are summarized by the analysis of the model test results. The second collision phenomenon is observed. The collision force dominates the collision moment and the tension force of the mooring lines lags behind. A NTNU in-house program developed by analytical simplified method is also verified by the model test result. The comparison proves the feasibility of the program.

Commentary by Dr. Valentin Fuster
2010;():261-268. doi:10.1115/OMAE2010-20263.

This paper proposes an in-situ estimation of crack shape from crack opening displacement (COD) by using of a visualized database system consisting of numerical calculation data of normalized crack opening displacement (NCOD) for some kinds of crack types. The relation between crack depth and corresponding NCOD is discussed based on FE analysis results, from which a crack shape estimation principle is deduced. Visualized software named NCOD Database System was developed to facilitate convenient in-situ estimation of crack shape. Shapes of three kinds of surface crack, partial circle crack in plate, fatigue cracks in gusset weld joint and in large-scale member, are successfully estimated by this system. The paper is supported by the Programme of Introducing Talents of Discipline to Universities (B08031).

Commentary by Dr. Valentin Fuster
2010;():269-278. doi:10.1115/OMAE2010-20287.

The main purposes of investigating the generation of the rogue waves in offshore engineering include: 1) prediction of its occurrence to protect the offshore structure from attacking; 2) the experimental investigation of rogue waves/structure interaction for the structure design. The latter one calls high requirement of wave generation and calculation. In this paper, we establish a spatial domain model of fourth order nonlinear Schrödinger (NLS) equation for describing deep-water wave trains in moving coordinate system. For the first purpose mentioned above, this paper presents the evolution of random wave trains in real sea state described by the Joint North Sea Wave Project (JONSWAP) power spectrum numerically, which is governed by the NLS equation. The parameters of the spectrum are evaluated to discuss their effect on the occurrence of rogue waves. For the second purpose to generate rogue waves in experimental tank efficiently, the transient wave is focused for its allowance of precise determination of concentration place/time. First we simulate the three-dimensional transient waves in the numerical tank modeling the deepwater basin with double-side multi-segmented wave-maker in Shanghai Jiao Tong University (SJTU) with linear superposing theory. To discuss its nonlinearity for the guidance of experiment, the transient wave is set as the initial condition of the NLS equation and the difference from the linear simulation is presented, which could be given as the suggestion to the preparation of experiment.

Commentary by Dr. Valentin Fuster
2010;():279-285. doi:10.1115/OMAE2010-20301.

Mast is defined as Grade A equipment which is essential for the safety and continued combat capability of the ship. Therefore, it is very necessary to evaluate the mast shock resistance under air explosion. However, it is very difficult to get the shock loading on the structure aroused by air explosion since it is varied with time and space. Flux-Corrected-Transport (FCT) algorithm is a high-order, monotone, preserving positive finite-difference algorithm. In this paper, a program is presented for air explosion load calculation based on FCT algorithm. Taking the coupling of air and mast into consideration, the pressure time history of each element on the surface of the mast is computed according to the program. A data file about the pressure load is written to integrate the commercial finite element software of LSDYNA. Furthermore, the dynamic responses of the mast are calculated based on LSDYNA. The capability of shock resistance under air explosion of the mast can be evaluated according to the calculation results, which can provide references for mast anti-explosion design.

Commentary by Dr. Valentin Fuster
2010;():287-296. doi:10.1115/OMAE2010-20324.

The plastic collapse load play a critical role in the entire procedure of constructing Failure Assessment Diagrams (FADs) of cracked tubular structures. In this study, the models of square-to-square hollow section (SHS) T-, Y- and K-joints with the actual 3D surface cracks obtained from previous fatigue tests are generated, and their general plastic collapse behavior has been studied carefully using the elastic-plastic finite element analysis. Based on the extensive FE analyses, the fully plastic collapse load solutions for the cracked SHS joints under brace end axial loads are proposed and derived. The load-displacements curves are plotted for predicting the plastic collapse loads based on the twice-elastic compliance criterion. Compared with formulae proposed in BS7910 [1], it was found that a conservative plastic collapse load for the SHS joints under brace end axial loads is produced using the proposed formulae. Future works are recommended for validation of BS7910 [1] FADs for damaged SHS T-, Y- and K-joints based on the numerically calculated plastic collapse load.

Topics: Safety , Stress , Collapse
Commentary by Dr. Valentin Fuster
2010;():297-304. doi:10.1115/OMAE2010-20329.

The safety of occupants in free-fall lifeboats (FFL) launched from a skid is addressed, and the focus is on numerical evaluation of acceleration loads during water impact. This paper investigates the required level of detail when modeling the physics of a lifeboat launch in waves. The first part emphasizes the importance of the non-linearity of the wave surface. Severity of impacts in linear (Airy) waves is compared to impacts in regular Stokes waves of the 5th order. Correspondingly, severity of impacts in irregular waves of the 2nd order is statistically compared to impacts in linear irregular waves. Theory of the two wave models are also briefly presented. The second part discusses the importance of a more detailed modeling of the launching system. This concerns especially cases for which damage to the mother vessel induces major lifeboat heel angles. A three-dimensional skid model is presented, along with validation against experimental measurements. In addition, the wave induced motion of the mother vessel is included. Consequences on the severity of the impact of the lifeboat in regular waves are discussed. This study is based on MARINTEK’s impact simulator for free-fall lifeboats, in which slamming loads are evaluated based on momentum conservation, a long wave approximation, and a von Karman type of approach. It is coupled here to the SIMO software, also developed at MARINTEK. Performance of this coupling is discussed.

Commentary by Dr. Valentin Fuster
2010;():305-314. doi:10.1115/OMAE2010-20343.

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valves. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions and repair and monitoring become more difficult. Gas lift valves and oil well systems have been modeled but system failure modes are not well understood. In this paper a quasi-steady-state fluid-mechanical model is constructed to study failure modes and sensitivities of a gas-lifted well system including the reservoir, two-phase flow within the tubing, and gas lift valve geometry. A set of three differential algebraic equations of the system is solved to determine the system state. Gas lift valve, two-phase flow, and reservoir models are validated with well and experimental data. Sensitivity analysis is performed on the model and sensitive parameters are identified. Failure modes of the system and parameter values that lead to failure modes are identified using Monte Carlo simulation. In particular, we find that the failure mode of backflow through the gas lift valve with a leaky check valve is sensitive to small variations in several design parameters.

Commentary by Dr. Valentin Fuster
2010;():315-321. doi:10.1115/OMAE2010-20346.

According to the Qiantang River Bridge project, the pile structures are adopted as protection devices for bridge piers. Because the pile structures are not contact with bridge piers, the traditional design method of impact force control is no longer applicable. Therefore, this paper studies on the large deformation characteristics of piles, takes the interaction between the piles and soil into account by “m” method, puts forward a new anti-collision design method, and at last determined the form and dimension of pile structures. LS-DYNA, a nonlinear explicit dynamic finite-element code, was used for analysis. The protection devices are used to Qiantang River Bridge, and then some useful conclusions were achieved.

Topics: Design , Rivers
Commentary by Dr. Valentin Fuster
2010;():323-328. doi:10.1115/OMAE2010-20351.

This paper recites the non-linear dynamic finite element simulation to the whole collision process of the collision between the anti-collision equipment and the bridge pile cap in the rough waves and surge with an example, basing on the solving technique of explicit non-linear finite element method and the mechanism in bridge-anti-collision equipment collision, including the process of building a reasonable FEA model and the numerical calculation and analysis of the collision process in considering the hydrodynamics influence by non-linear finite element analysis code MSC.Dytran / Patran. Described in detail attached to the collision analysis needs to consider such as determination of element size control, the selection of material models and failure criteria, the definition of contact and friction and so on. It is pointed that local strength of the anti-collision equipments and the bridge pile caps, as well as the collision impact force in the rough seas and surge. On the basis of numerical simulation, the general laws and characteristics of the impulse response of collision are achieved, which can be extended to the design of the bridge anti-collision devices and reinforcement for pier caps with providing a range of meaningful conclusions.

Commentary by Dr. Valentin Fuster
2010;():329-337. doi:10.1115/OMAE2010-20363.

During the last century there has been an increase in frequency of ships colliding with waterway embankments of inland waterways. The increasing occurrence of shipping accidents threatens the safety of freights and the serviceability of existing structures, such as tunnel and bridge abutments which are located near embankments. It is necessary to develop a method enabling prediction of the collision force and the stopping distance of a collision ship. In the related project “Hochdonn” of BAW (Federal Waterways Engineering and Research Institute, Germany) collision experiments were performed to analyze the impact mechanisms in grounding problems. A 3D FE model using a Coupled Eulerian Lagrangian approach is used to recalculate the collision experiment of BAW with the test ship “Gerda”. The stopping distance and the reaction force that are indicated by the numerical simulation match the field test results very closely. In addition two parametric studies are undertaken to quantify the influences on the collision process from the initial velocity as well as the bow type of the ship.

Commentary by Dr. Valentin Fuster
2010;():339-344. doi:10.1115/OMAE2010-20384.

This work describes the application of the Bootstrap technique to assess relevant information about the structural damage due to the action of a random loading time domain simulation. The Bootstrap methodology allows the estimation of the standard deviation confident interval calculated over a single time domain analysis. Two numerical applications are presented to exemplify the using of the confident intervals to obtain information on the cumulative damage of a structure subject to these random loadings.

Commentary by Dr. Valentin Fuster
2010;():345-352. doi:10.1115/OMAE2010-20464.

Marine riser is an important component of oil and gas drilling and production system. It is essentially a slender pipe conveying fluid between well-head and floating production unit. They are formed out of three basic types of configuration namely, free hanging, “lazy-wave (SWLR)” riser. Risers are subjected to varied static, quasi-static and dynamic forces. For the safety of design, the behavior of the riser under these forces is thoroughly investigated. 3D Nonlinear dynamic analysis of riser is obtained in the time domain using finite element software package ABAQUS/Aqua. The response histories so obtained are employed for the study of fatigue reliability analyses of riser. It is based on a bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. Uncertainty modeling, especially of fatigue crack growth parameters, is undertaken using bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using Monte Carlo Simulation. The bi-linear crack growth models are found to lead to higher fatigue life estimation. Sensitivity behavior pertinent to limit state adopted has been thoroughly investigated. These findings implicate inspection schemes for components of the marine structures to ensure minimization of the surprises due to wide scatter of the fatigue phenomenon in marine environment.

Commentary by Dr. Valentin Fuster
2010;():353-361. doi:10.1115/OMAE2010-20500.

Lifeboats are used as an evacuation system on a wide variety of offshore structures and marine vehicles. Currently, International Maritime Organization (IMO) Lifesaving Appliances (LSA) Code does not specify thermal protection and ventilation criteria for lifeboats. A test program was conducted to assess the system thermal protection and microclimate of SOLAS approved lifeboats for the Arctic environment. Some of the research findings of the first phase experiments are reported in this paper. In conducting experiments with a 72-person SOLAS approved lifeboat, the study found that the lifeboat only had a ventilation rate of 2 litres per second with vents open only, which may not be adequate. Inadequate ventilation will result in high concentration of carbon dioxide, causing headache, dizziness, restlessness, breathing difficulty, sweating, and increased heat rate, cardiac output and blood pressure. All of these may adversely affect lifeboat occupants in performing survival tasks. Using a thermal manikin, only slight decrease in thermal resistance (less than 10%) was observed in many test cases, when active ventilation was implemented (ventilation rate of 31 and 42 litres per second) and when side hatches were opened (ventilation rate of 95 litres per second). This suggests that reasonable increase in ventilation rate may be implemented without trading off much in thermal protection. However, a more noticeable decreases in thermal resistance (15% to over 30%) were observed when clothing was wet. This suggests it is critical to stay dry. A mathematical model was also developed to assess heat and cold stress of lifeboat occupants under different environment, lifeboat, occupant and ventilation conditions.

Topics: Lifeboats
Commentary by Dr. Valentin Fuster
2010;():363-368. doi:10.1115/OMAE2010-20505.

FPSOs, moored permanently at deep seas, are more vulnerable to fatigue failures due to cyclic loads from environmental conditions and operating cycles. In welded structures, the fatigue crack initiation strongly depends on the structural geometry and the stress concentration. In this work, a parent conventional hull is taken and an equivalent hemispherical hull, which could be a refined shape hull structure with smooth transition joints, is created with equal volumes. In both the hulls, the common critical location selected for comparison is a stiffener on central girder of the hull structures and the fatigue damage at full load and ballast load conditions are calculated, for design life of 25 years, by using simplified fatigue assessment procedure and S-N curve. It is observed that the hemispherical hull structure components have higher fatigue capacity than the conventional hull structure.

Topics: Fatigue damage , Hull
Commentary by Dr. Valentin Fuster
2010;():369-375. doi:10.1115/OMAE2010-20506.

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated response extreme values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, a more efficient version of the time simulation technique (ETS) is introduced to derive the probability distribution of response extreme values from a much smaller sample of simulated extreme values.

Commentary by Dr. Valentin Fuster
2010;():377-384. doi:10.1115/OMAE2010-20516.

Based on the differential equation of beam section, two methods for beam on Winkler foundation in complex conditions are presented with displacement, angular rotation, moment and shear compatibility conditions of the two adjacent beam elements utilized. Together with the two moment boundary conditions at beam ends, (4n+2) equations can be got. In order to obtain analytic solution, two additional equations are required. The first method to obtain additional equations is depended on the principles of minimum potential energy and virtual work, and the second one makes advantage of boundary conditions of shear at two beam ends. The proposed methods in this paper can be used to compute beam on Winkler foundation in complex conditions which include complex beam conditions, such as variable Young’s modules and section, complex foundation conditions, namely variable coefficient of subgrade reaction, and complex loads consisting of concentrated force, moment and random distributed load. The displacement equations both for beam elements and for the whole beam, which have same structure as the classic Hetenyi solution, can be derived from the proposed method. Therefore, the equations for angular rotation, moment and shear of the beam can also be obtained from their differential relationship with displacement equation. The computation of an example shows that the results by the proposed method are consistent with that by the finite element method when the elements are small enough. Furthermore, element partition in the proposed method is thoroughly different from that in the classic finite element method even the former is belong to the later in the overall meaning.

Commentary by Dr. Valentin Fuster
2010;():385-398. doi:10.1115/OMAE2010-20518.

The paper considers buckling of unstiffened truncated conical shells under simultaneously acting quasi-static axial compression and an independent external hydrostatic pressure. This is both numerical and experimental study. Domains of combined stability were obtained using the finite element method for a range of geometrical parameters. Cones are clamped at one end and free to move axially at the other end, where all the other degrees of freedom remain constrained. Shells are assumed to be from mild steel and the material is modeled as elastic perfectly plastic. The FE results indicate that the static stability domains remain convex. The failure mechanisms, i.e., asymmetric bifurcation and axisymmetric collapse are discussed together with the spread of plastic strains through the wall thickness. Also, the combined stability domains are examined for regions of purely elastic behavior and for regions where plastic straining exists. The latter is not convex and repercussions of that are discussed. The spread of the latter is computed for a range of the (radius-to-wall-thickness)-ratios. Experimental results are based on laboratory scale models. Here, a single geometry was chosen for validation of numerically predicted static stability domain. Parameters of this geometry were assumed as follows: the ratio of bigger radius, r2 , to smaller radius, r1 , was taken as (r2 /r1 ) = 2.02; the ratio of radius-to-wall-thickness, (r2 /t), was 33.0, and the cone semi-angle was 26.56°, whilst the axial length-to-radius ratio was, (h/r2 ) = 1.01. Shells were CNC-machined from 252mm diameter solid steel billet. They had heavy integral flanges at both ends and models were not stress relieved prior to testing. Details about the test arrangements are provided in the paper. In particular, the development details and experience of the test rig for independent/combined loading of cones are given. The current contribution complements Ref. [1].

Commentary by Dr. Valentin Fuster
2010;():399-405. doi:10.1115/OMAE2010-20521.

The real-time algorithm of higher order spectral analysis is proposed in order to investigate the non-linear and non-stationary ship motions. The algorithm is based on the time-varying vector auto-regressive (TVVAR) modeling and can be considered as an extension of the instantaneous cross spectral analysis. The TVVAR model is fitted to the data using the Kalman filter algorithm and the instantaneous higher order spectra can be evaluated by using the TVVAR coefficients. In order to examine the validity of the proposed algorithm, numerical simulations of the real-time estimation of higher order spectra was carried out based on the full scale ship motion data. The accuracy of the proposed algorithm was investigated by comparing with the stationary analysis based on the FFT. Furthermore, the algorithm was extended to the trispectral analysis and a result was shown to demonstrate the usefulness of the proposed algorithm.

Commentary by Dr. Valentin Fuster
2010;():407-416. doi:10.1115/OMAE2010-20546.

This paper focuses on a reliability analysis of an offshore wind turbine support structure which is part of an assessment and monitoring framework for wind turbines in operation. The reliability analysis builds upon structural, loading, limit state and uncertainty models comprising design, production and erection data. This model basis facilitates the reliability analysis of the ultimate, the fatigue and the serviceability limit states utilizing stochastic finite elements. The complexity of the individual models dictates an efficient solution scheme for the reliability analysis. Such an algorithm is developed in the present paper consisting of an adaptive response surface algorithm and an importance sampling Monte-Carlo algorithm. The response surface algorithm is based on predetermined experimental designs and facilitates the adjustment of design parameters for an optimized prediction variance in the design point region. Approaches for the consideration of multiple design points and the augmentation of the design for reduction of the prediction variance are introduced. In this paper, a reliability analysis for a tripod support structure of a Multibrid M5000 wind turbine is performed. A comparison with the target reliabilities specified in DIN EN 1990 (2002) shows that the requirements are fully met. However, the consideration of system reliability leads to the conclusion that at the end of the service life there is a significant probability of fatigue damages. The quantification of the reliability for the individual structural components for all limit states facilitates an identification of sensitive components. The results of this study can support the targeted application of monitoring systems, the optimization of the support structures and additionally highlight the need for criteria to the systems reliability.

Commentary by Dr. Valentin Fuster
2010;():417-425. doi:10.1115/OMAE2010-20610.

Learning is often presented as the major goal for an accident investigation process. But it often remains unclear how learning is directed or discussed in the text of investigations reports. If learning is emphasized in the report, how is the relationship between the learning processes proposed in the investigation and theories of effective learning amongst individuals, in organizations and across organizations and societal sectors? This paper presents the results of using a suggested tool to analyze the premises for learning from accident investigations. The paper compares accident investigation reports provided by different actors (operator and authority) in the Norwegian oil and gas industry covering same events.

Commentary by Dr. Valentin Fuster
2010;():427-431. doi:10.1115/OMAE2010-20612.

Structural design of ships against collision requires prediction of the extent of damage to stiffened plates subjected to impact. Structural components such as stiffened plates, bulkheads are our concern. In ship structures stiffened plates are furnished with vertical or horizontal stiffeners to sustain conventional acting loads such as shearing, bending and local buckling. The consideration of collision in ship structural design is necessary for tankers where accidents may cause serious environmental pollution. In predicting the extent of collision damage, finite elements (FE) modeling of stiffened plates using ABAQUS software is applied to demonstrate collision scenario. Typical stiffened plates of tanker in service with different configurations of stiffeners are selected to examine absorbed energy for each one. The aim of this paper is to select the proper stiffener shape absorbing more deformed energy. These analyses of stiffened plates will guide ship designers to properly select effective stiffener absorbing higher deformed energy when simulate full scale ship against collision.

Commentary by Dr. Valentin Fuster
2010;():433-440. doi:10.1115/OMAE2010-20619.

Laboratory experiments have been carried out in the directional wave tank at Marintek (Norway) to study the nonlinear dynamics of surface gravity waves and the occurrence of extreme events, when the wave field traverses obliquely an ambient current. A condition of partial opposition has been considered. Tests on regular waves have shown that the current can trigger the formation of large amplitude waves. In random wave fields, however, this only results in a weak deviation from the statistical properties observed in absence of a current.

Topics: Waves , Seas
Commentary by Dr. Valentin Fuster
2010;():441-449. doi:10.1115/OMAE2010-20620.

The ultimate hull girder strength of a Capesize bulk carrier under combined global and local loads in hogging and alternate loading condition (AHL) is evaluated using nonlinear finite element (FE) analysis with ABAQUS software. A three-cargo-hold FE model with fine mesh in the middle cargo hold is developed for the nonlinear FE analysis. The initial geometrical imperfections are introduced in the double bottom of the middle cargo hold. Both material and geometrical nonlinearities are taken into account in the FE model. The most critical situation for the longitudinal strength assessment of bulk carriers in hogging is the AHL condition with middle cargo hold empty under combined global and local loads. The local loads, i.e. the external sea pressure and internal cargo pressure are adopted according to Common Structural Rules for bulk carriers (CSR-BC). The ultimate hull girder strength with various local pressure load levels is investigated for the heavy cargo AHL in hogging condition. It is found that the ultimate strength of the hull girder can be significantly reduced due to the action of the local pressure loads compared with that obtained under pure hogging bending.

Commentary by Dr. Valentin Fuster
2010;():451-459. doi:10.1115/OMAE2010-20656.

The purpose of this paper is to describe the conceptual TLP designs for the pre-salt area in Santos Basin, which is one of the alternative Dry Tree solutions evaluated for this area. The TLPs were evaluated for 2,200 m of water depth for Santos Basin environmental conditions, where extreme values for wave and wind are significantly higher than for Campos Basin. Three different TLP scenarios have been studied for specific process plants and number of dry completion wells (TTRs). Both wellhead platforms and full production, drilling and quarters platforms have been considered. Characteristics of hull dimensions are shown for the three scenarios with comparisons to existing TLP installations from other worldwide locations. Possible Brazilian construction sites, transportation, integration and installation are discussed. TLP motions, tendon tensions and other performance results are summarized and compared to design criteria. Composite tendon material was investigated and compared with steel tendons in terms of hull size and installation procedure. Main results of TTR analyses are presented, including strength, stroke, fatigue and interference between risers during operation and installation.

Commentary by Dr. Valentin Fuster
2010;():461-467. doi:10.1115/OMAE2010-20665.

Collision is one of the common accidents during the lifetime of deep sea submersibles. Now scholars have paid more and more attention to research on collision strength. The FEM is adopted to simulate the collision between two submersibles in this paper. Based on the research on some key collision indices, plastic energy absorption, collision force and critical collision speed, the approach of assessing collision strength was obtained. Besides, we discuss the motion lag of the submersibles and consider the impact of some collision parameters, such as water pressure, impact position, angle, and radius of indenter. Finally, some significant conclusions were made.

Commentary by Dr. Valentin Fuster
2010;():469-474. doi:10.1115/OMAE2010-20670.

The risk and reliability based fitness-for-services (FFS) assessment addressed in this paper is a quantitative risk assessment (QRA) based FFS study on subsea oil or gas pipelines. The main purpose of QRA is to determine the target reliabilities for different pipeline segments. And then, a structure reliability analysis (SRA) method is used to calculate the maximum safe operating pressure, which indicates the pipeline retaining pressure capacity. It has been revealed that the target reliability or target safety level should reflect the consequences of failure, inspection and monitoring scheme etc. But, in the traditional FFS assessment (e.g. B31G and DNV RP F101), the identification of pipeline target reliability is merely based on qualitative judgments and experience. In this paper, however, a quantitative risk assessment will be performed based on abundant data from pipeline inspection and monitoring records, pipeline environmental impact assessment (EIA) study report, pipeline corrosion study report, pipeline emergency response scheme and anything else which can affect the pipeline failure consequences. An example of pipeline FFS assessment performed at four levels has revealed that the risk and reliability based FFS results have much better consistency to the reality.

Commentary by Dr. Valentin Fuster
2010;():475-484. doi:10.1115/OMAE2010-20678.

Three different possible choices of statistical models for multivariate Weibull distributions are considered and compared. The concept of “a correlation field” is introduced and is subsequently applied for the purpose of comparing the different models. Linear combinations of Weibull distributed random variables are considered, and expressions for the corresponding probability density functions are established. Furthermore, a simplified procedure for approximating the resulting density function is described. Comparison is made between the statistical moments of increasing order for the specific case of two Weibull components. This example of application arises e.g. in connection with mechanical design of a column which is subjected to a bi-axial bending moment.

Commentary by Dr. Valentin Fuster
2010;():485-492. doi:10.1115/OMAE2010-20680.

Despite the efforts to reduce ship accidents, the collision between supply vessels and offshore platforms continue to happen. The aim of the present paper is to evaluate the ultimate buckling residual strength of a typical semi-submersible platform column damaged by a supply vessel collision. DNV recommendations are adopted for the sideways collision and either bow or stern collision, corresponding to a supply vessel of 5,000 ton displacement with impact velocity of 2 m/s. Additionally, some cases considering the tendency in the world market for supply vessels with larger dimensions and more powerful propulsion are considered. This brings additional questions about the structural integrity of the floating unit represented by the column residual strength able to resist the deck loading up to the repair work to be performed. In this paper the program ABAQUS was used to carry out the dynamic collision simulation. The buckling analysis was validated using a finite element model considering geometric and material nonlinearities as in previous correlation studies with small scale experiments. Residual strength results from the damaged column are compared with the ultimate strength of equivalent intact column to estimate the safety margin associated with the column structural capability after supply vessel collision.

Commentary by Dr. Valentin Fuster
2010;():493-504. doi:10.1115/OMAE2010-20682.

In several practical applications joint environmental description is required. In the last years increasing attention has been given to importance of inclusion of wind sea and swell components in a joint environmental model. Presence of wind sea and swell will affect design and operability of fixed and floating offshore structures as well as LNG terminals. In the present study joint environmental description has been established for three locations: Southern North Sea, Northwest Shelf of Australia and off coast of Nigeria. Hindcast data have been used in the analysis. Uncertainties related to the proposed fits are discussed focusing particularly at wind sea and swell modelling. Attention is given to location specific features of the wave climate and adopted partitioning procedure for the wave components. Implication of the uncertainties for design of marine structures is shown. An approach for joint environmental description for marine operations is proposed.

Topics: Modeling , Wind , Seas
Commentary by Dr. Valentin Fuster
2010;():505-511. doi:10.1115/OMAE2010-20689.

Each and every quantitative risk assessment comprises some basic activities that have to be developed to allow the quantification of the risks involved in the operation of a system or process. Basically, it must be estimated the likelihood of the identified undesired events as well as the magnitude of their consequences. When analyzing the risks in the operation of a ship, the same process has to be followed. For each specific phase of the ship mission, all undesired events must be correctly determined and evaluated. Many different types of undesired events must be investigated, such as: collision, grounding, fire, and explosion. There are many techniques and much work involved in the estimation of the likelihood of the events. The same occurs for the evaluation of their consequences. The purpose of this paper is to study the consequences of a liquefied natural gas (LNG) leakage from the rupture of a pipeline from an LNG vessel during cargo loading at a terminal in the Brazilian coast. Once defined the total leaked quantity, a software will be used to calculate the consequences. The main idea is to identify all possible consequences and verify their magnitudes on the coast. This is not a simple task because it requires the comprehension of the development of the phenomena involved after the leakage. There is also the need of the identification of many variables, including the atmospheric conditions and the ignition sources, in order to verify the extent of possible cascading events. It will be taken special attention to consequences to human beings as a result of exposure to heat radiation effects from possible fires and explosions. As a preliminary study, where the likelihood of the events will not be estimated, the final objective is to use the obtained results to predict preventive and mitigating measures, in order to reduce the consequences to people on the coast.

Commentary by Dr. Valentin Fuster
2010;():513-519. doi:10.1115/OMAE2010-20690.

In the present paper, the bow of the striking ship is assumed to collide the midship part of the struck ship. In the previous model, the striking ship is approximated by a lumped mass system, and the struck ship by one mass and one spring system. In the present model, the struck ship is replaced by a system composed of rigid bars and elasto-plastic hinges. This model not only can express the response of the struck ship more reasonably, but also dose not require much time for numerical simulation. Some numerical results are shown to show the effectiveness of braking panels.

Commentary by Dr. Valentin Fuster
2010;():521-526. doi:10.1115/OMAE2010-20700.

A vibration and fatigue simulation system[1]–[3] based on the dynamic analysis in time domain of the ship structures when subjected to a certain sea state is presented. The development of the system is based on VC++ and OpenGL. The system covers the simulation of 3D waves, ship hull, free vibration and forced vibration of the ship, as well as the fatigue damage prediction of the ship structures. Irregular waves are simulated by linear superposition of a series of regular cosine waves for a certain sea state. On the basis of this expression of the irregular waves, the wave loads can also be obtained in a similar way. The stress history of the ship structures may be acquired by a dynamic response analysis. The free vibration of the ship hull can give the natural frequencies and the relevant vibration modes. The forced vibration can show the time variant displacement, velocity and acceleration caused by the exciting force of the propeller, main engine and wave slamming. The fatigue prediction is performed by means of rain flow counting and Miner rule[16] . The numerical example shows the system is applicable in the vibration response and fatigue perdition of the ship structures and the visual interface is user-friendly.

Commentary by Dr. Valentin Fuster
2010;():527-534. doi:10.1115/OMAE2010-20732.

The fleet of subsea systems, such as pipelines, trees and manifolds etc is at present rapidly increasing. This paper presents a risk based inspection planning approach that is new for subsea equipments. Very few publications on RBI of subsea equipments exist and have been actually applied due to the complexity of subsea components. In this paper, failure modes are described, offshore reliability database is used and risk analysis is performed taking into account the consequences of failure of equipments mainly in terms of Asset (Loss of production, Unavailability, Costs of repair). Risk level and corresponding inspection effort are estimated. An overall inspection plan is then provided for all structural components of subsea equipments: pipelines, trees and manifolds. Such a methodology has the benefit to rank the components and results in optimization of the inspection and maintenance effort.

Commentary by Dr. Valentin Fuster
2010;():535-540. doi:10.1115/OMAE2010-20738.

The laying of subsea pipelines is usually carried out by means of S-lay technology. As stingers become longer and longer, wave-induced fatigue damage problem for the stinger main hinges connecting both the stinger and the stinger adjustment frame fore leg to the vessel stern via a common shaft may have to be taken into consideration. Usually, there is a structural health monitoring system (SHMS) to be installed on the stinger for performance assessment. A procedure for assessing long-term wave-induced fatigue damage to a stinger is thus presented in this paper with continuum damage mechanics (CDM)-based fatigue damage assessment method. By taking the stinger of DPV7500 being built for Chinese Offshore Oil Engineering Corporation (COOEC) as an example, a structural health monitoring-oriented finite element model of the stinger is established. Water tank experiments were conducted to measure the roll, pitch and heave motion responses of the vessel model for different wave height, wave period and directions. The measurement data are then used to carry out the stress analyzes of the stinger to identify stress characteristics at hot spots of the stinger. The accumulative fatigue damage at hot spots during the stinger design life is finally evaluated using a CDM-based fatigue damage evolution model.

Commentary by Dr. Valentin Fuster
2010;():541-547. doi:10.1115/OMAE2010-20750.

Offshore wind energy offers a huge potential for expansion of renewable energy in the world. However, placement of wind turbines in harsh offshore environments is an engineering challenge, which requires development of suitable foundation designs. This paper shows different foundation types which are decided by different cases of the seabed. The mono pile foundation for 1.5MW offshore wind turbine is designed for Bohai Bay in China considering the soil and seabed condition. A three dimensional (3D) finite element model of mono pile is established to analyze horizontal ultimate bearing capacity, stress and deformation under lateral loads. Through the modal analysis, the natural frequencies of the mono-pile are given. Results show that the designed mono pile can effectively preclude resonance with the wind turbine. Dynamic responses under wave and seismic loads are analyzed by transient analysis method. And at same time, dynamic amplification factors of stress and deformation are studied. The paper reaches the conclusion that the dynamic effect must be attached importance in the design. Moreover, based on the theory of stability, the simplified method to assess the ultimate buckling moment of the mono pile is put forward. Comparisons are also made with results calculated by codified rules of DNV and ABS and Finite Element Method. Besides, fatigue analysis is carried out according the codified rules of API. The numerical results demonstrate that the design of mono pile is reasonable and reliable. The research results can provide the reference for practical engineering in China.

Commentary by Dr. Valentin Fuster
2010;():549-559. doi:10.1115/OMAE2010-20766.

The objective of this work is to assess the uncertainties introduced by different approaches for fatigue damage of ship structural details including discrete, closed-form and spectral approaches. The fatigue life is predicted by S-N approach based on the long-term notch stress range. The influence of ship main characteristics, operational profile based on a mission or loading condition, wave climatic data including the use of different scatter diagrams, wave spectra, heading distributions and consider the random origin of the mean stress effect, imperfection, weld shape improvement, hot spot calculation and resulting notch stress concentration factors to the fatigue life of a but welded ship structural component are accounted for.

Topics: Fatigue damage , Ships
Commentary by Dr. Valentin Fuster
2010;():561-567. doi:10.1115/OMAE2010-20771.

Research on vibration based damage identification using changes of modal parameters has been expanding rapidly over the last decades. And modal parameter identification methods are essentially important since these parameters directly affect the damage detection results. Many modal identification methods have been proposed and further verified based on numerical and laboratory data. However, few papers are reported on comparative investigation of several output-only modal identification methods, especially based on measured signals from offshore platform in service. The main objective of the present paper is to investigate the effectiveness and applicability of several output-only modal identification methods, including Ibrahim Time Domain(ITD), Eigensystem Realization Algorithm(ERA), Auto-Regressive Moving Average method(ARMA) and Stochastic Subspace Identification(SSI) methods. First the four modal identification methods are briefly reviewed. Field measurements are conducted and vibration signals are recorded for modal identification. The effectiveness is compared for the four identification methods. Useful results could be obtained.

Commentary by Dr. Valentin Fuster
2010;():569-574. doi:10.1115/OMAE2010-20778.

The procedure of reliability-based fatigue analysis of LNG carrier of membrane type under wave loads is presented. The stress responses of the hot-spots in regular waves with different wave heading angles and wave length are evaluated by global ship FEM. Based on the probabilistic distribution function of hot-spots’ short-term stress-range using spectral-based analysis, Weibull distribution is adopted and discussed for fitting the long-term probabilistic distribution of stress-range. Based on linear cumulative damage theory, fatigue damage is characterized by an SN relationship, and limit state function is established. Structural fatigue damage behavior of several typical hot spots of LNG middle ship section is clarified and reliability analysis is performed. It is believed that the presented results and conclusions can be of use in calibration for practical design and initial fatigue safety evaluation for membrane type LNG carrier.

Commentary by Dr. Valentin Fuster
2010;():575-583. doi:10.1115/OMAE2010-20788.

Reliable prediction of ductile fracture is essential in analysis of accidental response of ships and offshore structures. The consequences of fracture are significant. It may imply a significant reduction in structural capacity. It may also pose a potential hazard for human safety, as well as lead to an environmental and economical loss, e.g. caused by tanker collision or grounding. A series of five steel-plate indentation tests were conducted at the Norwegian University of Science and Technology (NTNU), Department of Mariner Technology, during late fall 2007. These are performed quasi-statically on various configurations of stiffened panels. The tests represent hull or deck plates in ships or platform structures subjected to accidental actions from ship-ship collisions, ship grounding or dropped object impacts. Various configurations of stiffened panels are tested, all laterally by a cone shaped indenter until fracture occurred. The specimen dimensions represent a 1:3 scale of the dimensions found in medium sized tankers, i.e. plate thickness of 5 mm. Naturally, because damaged hull and cargo tanks may cause severe environmental consequences, focus is on the plastic deformation and fracture resistance of the panels. The panel tests are primarily intended to serve as verification for advanced finite element simulations using a failure criterion based on instability mechanisms, i.e. local necking. This is implemented into the non linear explicit finite element code LS-DYNA and is referred to as the BWH instability criterion. In addition, the influence of the element size with respect to onset of failure is studied using three different element sizes for the various test cases. Although, attention is primarily placed on accidental scenarios, such as ship collision and grounding, the experimental results are of considerable relevance for other types of abnormal actions, e.g. dropped objects on deck and subsea structures, and stiffened panels subjected to explosion or ice actions.

Commentary by Dr. Valentin Fuster
2010;():585-594. doi:10.1115/OMAE2010-20796.

Spectral fatigue analysis approach is highly recommended for fixed offshore platform design and reassessment by API. This method is a computationally efficient method, being able to handle the random nature of environmental ocean wave conditions during calculating wave loads on the offshore platforms and subsequent structural responses. However, its fundamental theory is based on the assumption of linearity of both structural system and wave loading mechanism. Although this method is critically appropriate to be applied in offshore platform design and fatigue assessment for deep water scenarios where wave and force nonlinearities are not very severe, it has still been widely utilized for the design and assessment of shallow water platforms in offshore industry without carefully considering possible errors caused by strong nonlinear factors between ocean waves and forces. The source giving rise to the errors is because of the difficulties in choosing suitably correct wave heights for a series of wave periods required for producing transfer functions between sea state spectra and structural response spectra. Therefore, the studies to justify the possible errors of the spectral fatigue analysis method for shallow water platforms have been provoked. This paper presents the results of the studies of investigating the errors from currently existing spectral fatigue analysis method. A new technical approach that can reduce the errors in the spectral fatigue analysis of shallow water platforms is introduced. The proposed technical approach is mainly focused on producing realistic transfer functions between sea state spectra and structural response spectra, which can reasonably reflect the individually local sea state data by using wave height-period joint probability density function. Hence the fatigue damage and life at the tubular joints of offshore platforms can be more precisely predicted. The spectral fatigue analysis of a practical shallow water jacket platform in the recent platform design project has been performed using the proposed approach and the results are discussed.

Commentary by Dr. Valentin Fuster
2010;():595-603. doi:10.1115/OMAE2010-20811.

The average wave steepness may be defined as the ratio between the significant wave height Hs and the wavelength associated to either the zero-up-crossing mean period Lm or the peak period Lp (Bitner Gregersen et al., 1998). This parameter may be calculated from wave data at fixed locations, as well as by starting from theoretical spectra. In this paper the average wave steepness is firstly analyzed by considering a JONSWAP spectrum. It is shown that for this spectrum the ratio Sm = Hs /Lm , as well as Sp = Hs /Lp , depends upon the values of the spectrum parameters. The theoretical values are then compared with wave data in the Mediterranean Sea, Pacific Ocean, Atlantic Ocean and North Sea. The values of Sm and Sp are deeply investigated for severe sea states, with large values of Hs . It is obtained that in severe sea state the observed values of wave steepness, defined as either Sm or Sp , are always in the range defined by theoretical spectra; therefore, the extreme values of Sm and Sp , which are of interest for naval architecture, may be obtained from theoretical analysis, as a function of extreme values of Hs .

Commentary by Dr. Valentin Fuster
2010;():605-613. doi:10.1115/OMAE2010-20822.

It has been well recognized the importance of dynamic soil-structure interaction for several structures founded on soft soils. In order to examine the effects of soil-structure interaction, the substructure method is applied to the dynamic response evaluations of offshore platform. Since the offshore platform is generally subjected to severe dynamic forces such as wave, current and seismic forces, it is very important to clarify the dynamic response characteristics for the reliable design of the platform. For the idealized three-dimensional offshore platform subjected to random waves and seismic forces with the pile-soil foundation system, the dynamic response evaluations were carried out through the modal analysis. On the other hand, the uncertainty effects of dynamic forces and structural properties play very important roles on the reliability evaluations of offshore platform. If the limit state function is given by the most critical situations of dynamic responses, the reliability evaluations of the platform can be effectively calculated by the reliability index with the results obtained from Monte Carlo Simulation (MCS) method. Since the uncertainty of the random wave and seismic forces is critical for dynamic response evaluations, it is necessary to clarify the effects of uncertainties for the reliable design of the offshore platform.

Commentary by Dr. Valentin Fuster
2010;():615-619. doi:10.1115/OMAE2010-20830.

Riser has been and will be an important part of the offshore production system. Due to the complexity of environment, reliability evaluation is needed for the riser system to ensure the safety and durability. The analysis of the failure modes of riser system was studied in this article, firstly, the main factors of which may lead to failure of riser system were analyzed and classified; secondly, a time-variant reliability evaluation method was proposed to calculate fatigue reliability of riser system; finally, riser fatigue reliability of a certain FPSO is calculated by the proposed method, the results showed that time-variant reliability can better externalize safety of riser during structural service. The proposed method of this paper was significance in safety and integrity assessment of riser system.

Commentary by Dr. Valentin Fuster
2010;():621-627. doi:10.1115/OMAE2010-20842.

Circumferential welds, also called splice welds, on steel piles installed in marine and offshore structures are potential fatigue hot spots, given harsh and cyclic loadings. This paper is devoted to linear elastic fracture mechanics (LEFM)-based fatigue assessment for pile splice welds under bending moments. Specifically, this paper studies two important factors for the LEFM analysis of surface crack at the circumferential welds, stress intensity factor (SIF) and stress concentration factor (SCF). So far, LEFM-based fatigue assessment has not been widely conducted for steel tubular piles. Particularly, the two factors have not been properly developed with regard to typical features of marine steel piles, i.e., small aspect ratio of crack shape, thin-walled condition, misalignments at splice, thickness transition, complicated loading effects including bending moments, etc. The determination of SIF heavily relies on a geometry function for the growth of a fatigue crack. Based on a literature review, this paper has identified a database of geometry functions that may be suitable in practice for the circumferential welds on tubular piles with a thickness ratio around 10 under bending moments. SCF is used to accommodate stress concentration effects due to assorted geometric misalignments at a pile splice, including diameter difference (or concentricity), center eccentricity, thickness difference, and out of roundness. Currently, SCFs for tubular connections are produced for axial forces. In this paper, a practical approximation to the SCF under bending moments is mathematically formulated on the basis of a flat-plate configuration. This paper presents practical information for LEFM-based fatigue assessment of steel piles installed in marine and offshore works.

Topics: Steel , Welded joints
Commentary by Dr. Valentin Fuster
2010;():629-637. doi:10.1115/OMAE2010-20874.

Several sour gas leakage accidents have occurred in the offshore platform during the past decades, such as the Kab 121 platform in 2007, which caused serious consequences mainly resulting from the lethal toxicity of hydrogen sulfide (H2 S). Under the threat of H2 S, it is a challenge to exploit resource in the sour gas filed. Especially during the drilling operation, an abrupt blowout or kick could bring huge amount of H2 S, envelop the platform and disperse in the cabins. The present paper is aimed at introducing our analysis of H2 S dispersion both in the outer deck and inner mud treatment cabin so as to fully assess the potential poisoning during well blowout. The method we chosen was computational fluid dynamics according to the spatial environment characteristics of the offshore platform. First, we drew a comparison between accident consequences deriving from the wellhead configurations of an opened bell nipple and a sealed rotary blowout preventer (BOP) in the outer deck under various wind directions and speeds. The instantaneous concentrations and hazard zone distributions show that the second configuration is much better from the view of accident control. And the accident severity is much lower when the wind blows from the larboard, not from the prow for both configurations. As a result, the potential hazard zone would not envelop the entire platform with suitable platform position and arrangements of the mud return ditch, accommodation, helicopter deck et al. Then, the simulations of H2 S dispersion in the mud treatment cabin were conducted in case of the closed outlet doors, opened outlet doors and sealed cabin with air ventilator working. An immediately dangerous to life level may come up in a short break with the door closed. In such a dangerous situation, H2 S can only be made to disperse to other areas through the opened door or effectively ventilated away by means of a ventilator. It is a good practice to isolate the cabins with the potential H2 S leakage and install ventilator. And a simple model was proposed to calculate the working time for the ventilator.

Commentary by Dr. Valentin Fuster
2010;():639-645. doi:10.1115/OMAE2010-20914.

In this paper it is presented a brief description of a method that consists in using Bayesian Belief Network (BBN) created by converting Fault Trees (FT) to determine the most probable causes of a failure in a system given some evidence through observation. In addition, it is presented an example based on the steering gear system of a containership focusing on the cases in which the vessel is operating in restricted waters or performing the procedure for mooring/unmooring. The steering gear system was chosen due to its importance to restricted water navigation and to the mooring operation. The system must be completely available for these situations; otherwise, the ship security and the crew safety are implicated.

Topics: Gears , Networks
Commentary by Dr. Valentin Fuster
2010;():647-657. doi:10.1115/OMAE2010-20941.

Offshore platform resistance to cyclic storm actions is addressed. In order to achieve the best economy of the structure especially when assessing existing structures, the ultimate capacity of the structure is utilized. This means that parts of the structure may be loaded into the non-linear range and consequently the load-carrying resistance of the structure against future load cycles may be reduced. In such cases it is required to carry out a check of the cyclic capacity of the structure. Such checks are required in the ISO 19902 code for Fixed Steel Offshore Structures. The paper presents a proposal for how a load history for cyclic checks can be established. The method is in line with what is included in the NORSOK N-006 standard on “Assessment of structural integrity for existing load-bearing structures”. The load-history for the waves in the design storm may be expressed as ratio of the dimensioning wave. The ratio will be different for check of failure modes where the entire storm will be relevant such as crack growth, compared to failure modes like buckling where only the remaining waves after the dimensioning wave need to be accounted for. Using simple order statistics and simulation, the statistics for the ith (Hi ), i = 1, 2, 3, 4 etc. highest wave in the storm is studied in some detail, assuming that the maximum wave (H1 ) is equal to an extreme wave obtained by a code requirement. Environmental contours for the pair (H1 ,H2 ) are established by Inverse FORM for design conditions. Further, the long term statistics for load effects that are expressed as a function of H1 , .., H4 , i.e. L = f(H1 , .., H4 ), are determined. The R-year value LR for the load effect L is determined by structural reliability techniques, and the most probable combination (design point) (H1 *, .., H4 *) for L = LR is determined. The design point values Hi *, as well as the design point value for the significant wave height, are determined for different load effects, and their characteristics for different types of load effects are discussed. The paper gives advice also on how to establish the magnitude for the remaining waves in the storm.

Commentary by Dr. Valentin Fuster
2010;():659-669. doi:10.1115/OMAE2010-20971.

Jacket type structures in offshore structural industry consist of a large number of tubular members with various dimensions, which are joined to each other by welding that makes connections to be rigid. Diagonal members have relatively small dimensions, legs or chords have larger dimensions in general. Although the connections at joints are made rigidly, the actual joint behaviors under wave loadings are not fully rigid in the vicinity of connections due to local deformations of members having large diameters. In the short term, due to ultimate wave and earthquake loadings, some plastic deformations can also occur in members at some critical joints so that related members cannot be behave as rigidly connected and some releases of member forces occur. In the long term, fatigue damages can be observed at some joints that damaged members loose their functionality partly or fully as depending on damage rates. All these phenomena can be considered as member deterioration. A special treatment of deteriorated members can be used in the structural analysis by using a computation model that allows flexibility of damaged members at joints. The solution of this problem can be achieved by introducing a fictitious member concept, which can be derived as depending on actual member dimensions and joint configurations. The technique of using fictitious members introduces additional degrees of freedom that are not desirable in the analysis. A procedure which uses modified stiffness and mass matrices for flexibly connected members are more practical and attractive since a) no additional degrees of freedom are introduced, b) member-release and fixed-connection conditions can be directly obtained, c) a general member-end condition in any direction can be easily specified, d) a failure mechanism can be easily determined, e) in the fatigue damage calculation the load carrying capacity of the member can be used until the whole member cross-section is damaged and f) natural frequencies and mode shapes of damaged structural system can be estimated in terms of the natural frequencies and mode shapes of the undamaged structural system. The paper introduces a general formulation of a partly connected member to be used in structural analysis. For this purpose, a spring-beam element is defined using massless spring systems at member ends. An algorithmic procedure is presented to update member stiffness and mass matrices as well as member consistent load vector.

Commentary by Dr. Valentin Fuster
2010;():671-675. doi:10.1115/OMAE2010-20973.

Fatigue damage is one of the main reasons of the failure of Semi-Submersible platform. As the complex of random loading, it is difficult to analyze fatigue life accurately and determine the sensitivity of parameters. In this paper, the fatigue life on key-components of semi-submersible platform is analyzed with Spectral-based analysis method. Firstly, the stress responses of whole model platform under the random wave loads are calculated. The calculation results of whole model platform for cut-boundary interpolation are used in local model to calculate the key-component stress responses of local model. Generating the fatigue stress energy spectrum by scaling the wave energy spectrum and the complex fatigue stress transfer function in detail local model is described next. The stress response of short-term sea-state is assumed to obey Rayleigh distribution, and the spectral moments are calculated. Finally, the fatigue life of key components is analyzed according to S-N curve and Palmgren-Miner’s rule. The results show that the fatigue life of the connection meets the specification requirements, and the key components are the fatigue sensitive areas of semi-submersible platform.

Commentary by Dr. Valentin Fuster
2010;():677-687. doi:10.1115/OMAE2010-20979.

The material strength of aluminum alloys can be seriously deteriorated by the heat-affected zones (HAZ) caused by welding. The reduction of material strength in HAZ may lead to a serious reduction of the aluminium structural strength. The present paper is mainly concerned with the investigation on the ultimate capacity and the collapse behaviour of three types of aluminium stiffened panels (i.e., closed-stiffened panels, T-stiffened panels and flatbar-stiffened panels) considering the effects of various types of welds, the residual stresses induced by the related welds, and the combination of welds and the induced residual stresses, under axial compression and transverse compression, respectively. The investigations are carried out by nonlinear finite element (FE) analysis using ABAQUS. Based on the FE results, the material softening effects on structural capacity with respect to the parameters (i.e.: plate slenderness β and column reduced slenderness λ) are addressed. Finally, conclusions based on the investigation are made.

Commentary by Dr. Valentin Fuster
2010;():689-694. doi:10.1115/OMAE2010-21027.

In this paper, it starts by providing a description of the scenarios of damages that can be found in the various accidental events. Based on a traditional standardized approach for calculating longitudinal strength of ships, an improved method for longitudinal strength of damaged ships, which can be applied to any traditional type of ships, is presented in next section. The variation of loads in damaged ships is related to the effects of the ingress of water with the consequences of changing the ship displacement, a non-symmetric floating body, and the load case of compartments before damage, etc. The computing methods used in this paper allow for research on corresponding applications. Using this computer program, a few case studies are carried out and the results are discussed.

Commentary by Dr. Valentin Fuster
2010;():695-702. doi:10.1115/OMAE2010-21030.

Dolphins are essential structures used in gas and oil terminals for berthing and mooring purposes. In essence, design of these structures is mostly based on linear analysis, so that the effect of nonlinearity on the actual responses is not considered. In this paper, the effect of nonlinearity caused by the gap modeling between parts of the structure is investigated and the modeling is carried out using accurate and efficient finite element software. As a case study, a mooring dolphin located in the Persian Gulf is simulated in order to demonstrate the nonlinear analysis effects. In contrary to the most common mooring dolphins, the dolphin is designed to have flexible structural behavior. As a result, it is comprised of three short piles connected by a triangle-shape box girder from the top. The box girder is composed of two triangle plates as flanges, which are connected by means of vertical stiffeners acting as web. Besides, three torsion bars are used to change the direct shear to a torsion shear to get a more efficient force transmission. In the piles-box girder connection, where piles pass through the holes prepared in the flanges, there is a considerable difference between the diameter of piles and holes as a tolerance (gap) for construction and codes provisions (BS 6349). The nonlinear effects are mainly caused by the gap in this connection besides the vertical supports of the box girder and the torsion bars. This paper contains the results of a nonlinear static analysis in which the gap between different parts is modeled using nonlinear spring elements to reach a convergence in solving the equations. Results are then compared with the linear static analysis of a similar dolphin, which gaps are not modeled in it and nonlinear effects are presented.

Commentary by Dr. Valentin Fuster
2010;():703-712. doi:10.1115/OMAE2010-21032.

This paper describes deck design and mating operation of the semisubmersible Floating Production Unit (FPU) OPTI-EX. It also addresses engineering challenges, and how these were met by a novel design and proper analysis. OPTI-EX is the first production semi-submersible mated by a combination of guide plates and shoe bracket at 8 contact points. The challenges of mating include dimensional control of the mating, hull deflections control by guide plates, dimensional tolerance criteria, and welding connections. Because of the flexibility of OPTI-EX pontoon in the water, the horizontal deflection of four columns is significant, which makes the adjustment of column position important. The use of half “Y” shape guide plates installed on column top helps the deck bring the column back into desired location under deck self weight. The deck self weight is shed at 8 contact points. The dimensional tolerance criteria are critical, since fatigue is sensitive in the deck column connection area. By selecting the placement of guide plates, the tolerance can be achieved. The mating operation went very well in terms of time and final tolerance. Welding work after the landing of truss deck on column is easy to access and of simple design. All the work is done in open space rather than confined in a small space. A sensitivity study was done to check the contact forces during the mating process. The results show that there will be significant changes in contact forces assuming different initial gaps present between deck and column top guides during the mating. However, all these forces can be accommodated by a proper local reinforcement design. While mating the deck and column, a built-in stress due to the loads in mating is normally involved. The support conditions change between mating stage and in-place condition. This means modeling the in-place condition only will lead to an unrealistic result. In this paper, it is shown that a prestress approach has to be utilized to predict the final in-place strength condition with the consideration of built-in stress in deck at mating stage.

Topics: Design
Commentary by Dr. Valentin Fuster
2010;():713-718. doi:10.1115/OMAE2010-21050.

Pitting corrosion has been one of main corrosion type of immersed ship hulls, which can bring heavy damage and even accidents to in-service ships, particularly to aged ships. To investigate the effects of pits on the ultimate compressive strength of mild steel plates, a series of non-linear FEM analysis on plates with partial depth corrosion pits are undertaken in the paper, changing the size, intensity and location of pits and the slenderness of plates. It is shown that the eccentricity induced by single side distributed pits has considerable degrading effects on the ultimate compressive capacity of plates. Although the degree of pit corrosion (DOP) does reflect the effect of pits to a large extent, it is not enough to rely on DOP exclusively to represent the extent of damage caused by pits, as “volume effect” should be considered.

Commentary by Dr. Valentin Fuster
2010;():719-723. doi:10.1115/OMAE2010-21052.

This paper describes how a structural vibration monitoring procedure was used in combination with structural analysis modeling to determine the foundation capacity of an existing offshore platform. The project was initiated due to concern that the foundation capacity was marginal for a needed expansion of the topsides process system. The field work consisted of conducting a sequence of vibration measurement for a range of increasing deck loads. Since the platform’s resonant frequencies depend on the structural stiffness and mass, each deck loading (mass) produced a slightly different resonance condition. The set of resonant frequencies obtained for the different deck loads allows one to deduce the stiffness of the structure-foundation system. The stiffness of the steel structure (jacket and deck) was established with structural analysis. The remaining unknown is the axial and lateral stiffness of the pile foundation. Using the torsional resonant frequencies, the lateral stiffness of the pile array and individual piles was established. Then using the translational resonance, the axial pile stiffness was found. Based on these analyses, the platform’s foundation capacity was estimated to be significantly greater than that predicted by the standard pile capacity design recipe. The structure was deemed adequate for the planned topside modification.

Commentary by Dr. Valentin Fuster
2010;():725-730. doi:10.1115/OMAE2010-21059.

Protection structures for FPSO caissons are required to be strong enough not to contact caisson pipes even when the protection structure is damaged by impact by attendant vessels. In the present structural design process, non-linear commercial packages are employed for the collision analyses. However, non-linear collision analyses using commercial packages are still time-consuming and expensive to operate especially at the initial design stage. In this study, validation of the adopted commercial package was firstly performed using collision test results on unstiffened tubulars. Then, a rigorous parametric study was conducted on simple protection structures by changing the collision velocity and the scantling of the protection structure. A simple analytical expression was derived assuming that the kinetic energy of the striking vessel is dissipated by plastic elongation of tubulars and rotation of plastic hinges. Using the parametric study results, an equation for the modification factor was obtained by which the effects of local denting and dynamic behavior can be considered. The developed procedure was also substantiated using numerically predicted extent of damages of an actual protection structure.

Commentary by Dr. Valentin Fuster
2010;():731-736. doi:10.1115/OMAE2010-21070.

A new semi-active control strategy is developed to adjust the voltage/current of magnetorheological (MR) dampers to track the optimal/desired damping force by the Linear Quadratic Regulator (LQR) method. Integrated with the new strategy, a modified inverse dynamic model is proposed to evaluate the value of evolutionary variable based on the Bouc-Wen model of MR dampers. This new strategy is firstly applied to a one-story building under El-Centro earthquake, and it is further applied to the dynamic response control of a fixed jacket offshore platform. The results show that this new semi-active control strategy can achieve efficient vibration control.

Commentary by Dr. Valentin Fuster
2010;():737-746. doi:10.1115/OMAE2010-21078.

One of the standardized procedures used in the design of floating systems and their mooring and production lines is the so-called short-term design approach where the system is analyzed for some specific extreme environmental conditions. Along with this procedure, a nonlinear time-domain coupled dynamic analysis, considering the floater and its risers and mooring lines, is nowadays feasible to be employed in the design practice. One important and challenging aspect of this process is concerned with the estimation of the characteristic short-term extreme values of the system response parameters based on the sampled time-series. In this paper a common procedure used to establish these extreme values for floater system response parameters, which is based on a Weibull distribution model for the time-series peaks, is reviewed in the light of a recently proposed approach based on a general parametric model for the average conditional exceedance rate of peaks. It is shown that the former model corresponds to a particular case of the latter one. Numerical results are presented for the response parameters of a turret-moored FPSO considering a short-term coupled analysis of the whole system under an extreme environmental condition of wind, wave and current. Specifically, the extreme response of surge motion, top tension of the most loaded mooring line and DnV’s utilization factor for the most critical section of a 8″ SLWR (Steel Lazy Wave Riser) are investigated.

Topics: FPSO , Mooring
Commentary by Dr. Valentin Fuster
2010;():747-757. doi:10.1115/OMAE2010-21079.

In the past, there were very serious casualties under the actions of extreme waves including loss of precious lives. There are cases like loss of M V Derbyshire (Faulkner, 2001) due to hatch cover failure in extreme weather conditions. Use of composite materials in marine fields as major or minor components off floating platforms is discussed in this paper. Application of composites on board ships reduces the self weight and lowers the position of vertical centre of gravity of the floating vessel. There are advantages in using composite structures in marine environment. A link-span fitted with a composite deck and the feasibilities of using composite for hatch covers of bulk carrier ship is described in this paper. In the case of bulk carriers, failure of hatch cover especially in the forward part of the vessel leads to flooding of the forward cargo compartment and occasionally results in fatal casualty. The foremost hatch cover and the next one within 25% length of the vessel.

Commentary by Dr. Valentin Fuster
2010;():759-765. doi:10.1115/OMAE2010-21088.

The toroidal shell is a special type of shells of revolution, which is hardly solved by analytical method. To show the nonlinear structural characteristics of a circular toroidal shell with ring-stiffened ribs due to external pressure, both material nonlinear and geometric nonlinear Finite Element Analyses (FEA) have been presented in this paper, especially for the stability to the type of pressure hull. In the presented Finite Element Method (FEM), the elastic-plastic stress-strain relations have been adopted, and the initial deflection of toroidal shell created by manufacture was also taken into account. The analytic results eventually indicate that by nonlinear FEA such a new type of ring-stiffened circular toroidal shell could be used to a main pressure hull as the traditional ring-stiffened circular cylindrical shell, which could obtain kinds of performance in underwater engineering, such as better stability and more reserve buoyancy to the classical ring-stiffened cylindrical shell.

Commentary by Dr. Valentin Fuster
2010;():767-772. doi:10.1115/OMAE2010-21089.

The paper utilizes numerical method with performance experiment to investigate the key factors affecting the performance of the Self-Contained Underwater Breathing Apparatus (SCUBA) system. A SCUBA system consists of a high pressure compressed air tank and a pressure regulator. The development of scuba diving is based on the invention of the regulator. The high pressure air carried by diver must be reduced to the pressure in the ambient environment by the regulator before the diver can breathe it. The life of the diver thus depends on the performance and stability of the regulator. Results show that the index performance of regulator can be predicted and used to improve the performance of the regulator. The numerical method can thus effectively reduce the develop time for new regulators and related products, as described in this article.

Commentary by Dr. Valentin Fuster
2010;():773-785. doi:10.1115/OMAE2010-21101.

We present an analysis of airborne wave observations collected in the Gulf of Tehuantepec. The data includes LIDAR measurements of the surface displacement as a function of two horizontal dimensions and time in fetch-limited conditions, with fetches between 50 and 300 km and winds between 10 and 20 m/s. The spatio-temporal data have an advantage over the commonly used single point time-series measurements allowing direct estimates of the wavelength and wave slope, including spatial information such as the lengths of crests exceeding threshold wave heights and slopes. The statistics of these wave parameters are particularly important for risk assessment of off-shore structures and in other ocean engineering applications. We present an analysis of several statistical wind-wave parameters, including the joint probability distribution function (pdf) of wave amplitudes and wavelengths, the pdf of wave heights, wavenumber vectors, and wave slopes, including the statistics of crests lengths exceeding threshold wave heights or slopes. The empirical findings from the LIDAR data are related to the analytical work by Longuet-Higgins (1957) [1] for a linear spectrum, including the average length of contours surrounding large wave heights. The effect of second-order nonlinearities on the distribution of crest lengths is investigated with numerical stochastic simulations from computed directional wavenumber spectra. The results show that second-order nonlinearities can increase the crest length density of large waves by about a factor of two or more. The results are discussed in the context of predicting wave statistics for ocean engineering applications.

Topics: Waves , Modeling
Commentary by Dr. Valentin Fuster
2010;():787-794. doi:10.1115/OMAE2010-21136.

In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. The complete Gurson model has been applied to simulate the ductile fracture, which is characterized by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f0 and hardening exponent n.

Commentary by Dr. Valentin Fuster
2010;():795-808. doi:10.1115/OMAE2010-21163.

The Fatigue Design Assessment (FDA) procedure is a part of ShipRight design, construction and lifetime ship care procedures developed by Lloyds Register. The work presented in this paper is on the application of a Stress Concentration Factor (SCF) approach to the FDA procedure to develop an effective and user friendly tool for the assessment of fatigue performance of cut-outs in transverse web frames for longitudinal stiffeners. Investigation of hull damage statistics has identified that fatigue cracking in way of cut-outs and end brackets of longitudinal stiffeners affects a reasonable proportion of ships in service. This can become an ongoing ‘through life’ maintenance issue as defects can re-occur if the design of the structural detail has insufficient fatigue performance. A spectral fatigue assessment of this structural detail has traditionally been based on localised mesh refinement applied to a global finite element (FE) model and hydrodynamic calculated using first principle methods. These traditional methods are time, resource and labour intensive processes and therefore limited in its application. The SCF approach developed offers a faster and simpler method for the determination of hotspot stresses for standardised cut-out details.

Commentary by Dr. Valentin Fuster
2010;():809-814. doi:10.1115/OMAE2010-21179.

In principle, the reliability of complicated structural systems can be accurately predicted by standard Monte Carlo simulation methods, but the computational burden may be prohibitive. A new Monte Carlo based method for estimating system reliability that aims at reducing the computational cost is therefore proposed. It exploits the regularity of tail probabilities to set up an approximation procedure for the prediction of the far tail failure probabilities based on the estimates of the failure probabilities obtained by Monte Carlo simulation at more moderate levels. In the paper the usefulness and accuracy of the estimation method is illustrated by application to some particular examples of structures with several thousand potentially critical limit state functions. The effect of varying the correlation of the load components is also investigated.

Commentary by Dr. Valentin Fuster
2010;():815-821. doi:10.1115/OMAE2010-21185.

A geostationary FPSO does not weathervane like a turret-moored ship-shaped FPSO. The offshore industry has limited experience for direct offloading from such geostationary installations in the North Sea. In recent years direct offloading operations from geostationary FPSOs by DP shuttle tankers, have been developed, and key operational principles for such a system are presented in this paper. Probabilistic modeling of the collision risk between shuttle tanker and geostationary FPSO in direct offloading is presented, followed by a case study. It is found that the collision risk level is much lower in the direct offloading than in the tandem offloading. The results demonstrate that the direct offloading approach is a promising concept with inherent safety designed in the operations, i.e. the positioning philosophy of the shuttle tanker and a large separation distance.

Commentary by Dr. Valentin Fuster
2010;():823-829. doi:10.1115/OMAE2010-21186.

Risk of riser operations on dynamic positioning mobile offshore drilling units is addressed in this paper. Primary focus is placed on the blowout risk caused by failure of Emergency Quick Disconnection in the situation of DP failure, i.e. position loss. The risk model is established by an event tree linked with fault trees using RiskSpectrum® software where various equipment failures and operational errors that may lead to failure of DP system and well barriers are identified and modeled. Both overbalanced operation with standard marine drilling risers and underbalanced operations with a set of high-pressure completion/workover risers are analyzed in this paper. And the blowout frequency is presented for a given yearly riser operational activity level. Sensitivity studies are performed for several key variables and recommendations are proposed in order to further reduce the blowout risk of riser operation on DP drilling units.

Commentary by Dr. Valentin Fuster

Alaa E. Mansour Symposium on Ship and Offshore Structural Design

2010;():831-836. doi:10.1115/OMAE2010-20053.

It is well known from linear analyses in stochastic seaway that the mean out-crossing rate of a level r is given through the reliability index, defined as r divided by the standard deviation. Hence, the reliability index becomes inversely proportional to the significant wave height. For non-linear processes the mean out-crossing rate depends non-linearly on the response level r and a good estimate can be found using the First Order Reliability Method (FORM), see e.g. Jensen and Capul (2006). The FORM analysis also shows that the reliability index is strictly inversely proportional to the significant wave height irrespectively of the non-linearity in the system. However, the FORM analysis only gives an approximation to the mean out-crossing rate. A more exact result can be obtained by Monte Carlo simulations, but the necessary length of the time domain simulations for very low out-crossing rates might be prohibitive long. In such cases the property mentioned above for the FORM reliability index can be assumed valid in the Monte Carlo simulations making it possible to increase the out-crossing rates and thus reduced the necessary length of the time domain simulations by applying a larger significant wave height than relevant from a design point-of-view. The mean out-crossing rate thus obtained can then afterwards be scaled down to the actual significant wave height. Some previous results using this property have been presented by Tonguc and Söding (1986), albeit in a more empirical way. In the present paper the usefulness of this property to estimate extreme wave loads will be evaluated considering the overturning of a jack-up rig.

Commentary by Dr. Valentin Fuster
2010;():837-847. doi:10.1115/OMAE2010-20133.

This paper proposes a design concept in the area of overtopping ocean wave energy devices. Current models of overtopping devices are limited to locations which experience large wave activity. The high energy associated with large waves enables the ocean water waves to overtop the device ramp into the collection reservoir which discharges through a power generating turbine. The ramp height is necessarily large to establish a sufficient collection reservoir head. The proposed design concept can utilize small waves by implementing a gearing system which can transfer water to the collection reservoir. This allows for greater access to the wave energy resource at more locations than currently possible. A simplified model of this design concept is presented and applied to varying wave conditions off the coast of California.

Topics: Waves , Wave energy
Commentary by Dr. Valentin Fuster
2010;():849-855. doi:10.1115/OMAE2010-20272.

The study on slamming of high speed trimaran is a very complicated and significant topic. Ship slamming is a nonlinear unsteady problem in which dynamic boundary and coupling among structure, air and water should be considered. It is a key issue in the ship research field. Unlike the slamming of ordinary monohull, trimaran slamming include not only main hull bottom slamming, but also slamming of trimaran cross structure, which makes trimaran slamming more complicated. In this paper, a reduced scale model of trimaran was constructed and an experiment was carried out in order to study the slamming pressure distribution on main hull and cross structure. By using nonlinear simulation software Autodyn, a finite element model was established to simulate the process that trimaran model enters into water. The accuracy and reliability of numerical simulation is validated by comparing experimental data and numerical simulation results. Furthermore, some valuable conclusions on trimaran slamming are obtained.

Commentary by Dr. Valentin Fuster
2010;():857-863. doi:10.1115/OMAE2010-20276.

For the warship cabin under explosive loading, the detail structure in cabin corner can easily be torn by the high-strength shock wave converging at the structure corner. In order to avoid that the crevasse occurs at the corner firstly, three strengthening structure forms were designed for the cabin corner: thickening connection, circular connection and inclined plate connection. Failure process of the joint in the two-cabin structure under the explosive loading was simulated by the nonlinear dynamic software DYTRAN. Comparing the response of the corner strengthening structure to that of the conventional structure, it was concluded that three strengthening structure forms changed failure mode of the cabin structure effectively and the crevasse initiated at the explosion pressure release hole on the transverse bulkhead, which reduced the tearing of the cabin corner. To seek more reasonable corner strengthening structure, the pressure and the stress on the bulkhead under the explosive loading of the three corner strengthening structures were compared. The results showed the inclined plate connection can prevent the shock wave from concentrating at the corner, decrease the stress on the longitudinal bulkhead, and resist the shock wave spreading into the inner cabins most effectively in the three strengthening forms.

Commentary by Dr. Valentin Fuster
2010;():865-874. doi:10.1115/OMAE2010-20334.

This paper deals with the structural reliability analysis (SRA) of ultimate strength of a Capesize bulk carrier in hogging and alternate hold loading condition (AHL). The ultimate strength in hogging and AHL condition is very important for the safety of bulk carriers since the local loads due to internal cargo loads and external sea pressure can reduce the ultimate bending capacity. In the present paper, the characteristic ultimate bending capacity of the subject bulk carrier is investigated by nonlinear finite element (FE) analysis and the characteristic value of the global and local loads are determined in accordance with the Common Structural Rules for bulk carriers (CSR-BC). The uncertainties associated with the loading capacity and load effects are appropriately modelled. The First Order Reliability Method (FORM) is adopted to calculate the annual probability of failure of this bulk carrier in hogging and AHL condition. The effect of heavy weather avoidance on the global and local loads is also evaluated in the SRA. The results show that the local loads have a significant impact on the failure probability of such vessels in the hogging and AHL condition.

Commentary by Dr. Valentin Fuster
2010;():875-885. doi:10.1115/OMAE2010-20483.

The process of decommissioning ships, usually followed by ship scraping, is associated not only with significant costs but also with many environmental and toxic hazards. Thus, re usability of such ships will offer more advantages than recycling its steel. This paper proposes reusing a post-service life vessel by converting it into an Ocean Wave Energy Device. A systematic study on the preliminary estimation of energy extraction levels from ocean waves is presented, with some discussion on the energy of a ship-shaped body in motion in irregular seas. Heave motion was found to be more favorable for such applications. Thereafter concept designs have been proposed for the device, and the designs have been analyzed in terms of the extent of relative motion and power that could be obtained from each of them. Numerical simulation techniques have been used to generate the results. Through this analysis, it has been established that the idea of constructing such a device is feasible. The relatively slower but high-force systems can use a hydraulic take-off unit instead of electrical inductance generators. The mooring system has also been identified as a separate design problem. Further design issues including optimizing the different parameters, cost analysis and fabrication issues have not been considered — this work is aimed to provide the starting point of the design exercise.

Topics: Design , Ocean energy , Ships
Commentary by Dr. Valentin Fuster
2010;():887-897. doi:10.1115/OMAE2010-20637.

This paper proposes a simulation method for obtaining the estimate of the long term correlation coefficients between different low-frequency wave-induced loads acting on a ship hull. They are essential part of the load combination procedures in design and strength evaluations. Existing theory is limited to linear time-invariant systems with weakly stationary stochastic inputs such as waves during a single sea state (short-term). The simulation treats the non-stationary wave elevations during the ship’s entire life (long-term) as a sequence of different stationary Gaussian stochastic processes. Different sea states (HS , T0 , Wave Direction) are sampled, using rejection sampling, from the joint probability density functions fitted to every Marsden zone on the ship’s route. The time series of the loads are simulated from the load spectra for each sea state, including the effects of loading condition, heading, speed, seasonality and voluntary as well as involuntary speed reduction. The estimates of the correlation coefficients are then calculated from these time series. The simulation time can be significantly reduced (to the order of seconds rather than hours and days) by introducing the seasonal variations into a single voyage. It is proven that the estimate of the correlation coefficient, obtained by simulating only a single voyage, approaches the true correlation coefficient in probability as the number of simulated load values increases. The simulation method can also be used for finding the long-term exceedance probabilities of the peak values of individual loads as well as for analyzing various load combinations (linear and nonlinear). Related concepts and limitations of this method are demonstrated by an example of a containership operating between Boston, MA and Southampton, UK.

Topics: Simulation
Commentary by Dr. Valentin Fuster
2010;():899-910. doi:10.1115/OMAE2010-20712.

Multicriterial design methodology with safety as one of the design objectives is presented. The aim of the paper is to analyze the influence of safety based design objectives on generated nondominated designs on the Pareto frontier. Possible improvements in nondominated designs are investigated by comparison to ones obtained with the standard design procedure when safety criteria are used as design constraints only. It is assumed that safety based objectives and targets act as attractors, driving nondominated designs along the constant cost/weight contours in design space towards its safer regions. Global safety objectives (for hogging/sagging modes), are based on the maximization of ultimate longitudinal strength in vertical bending calculated via the extended IACS incremental-iterative method. Applied compound safety measures for gross-panel (stiffened panel with associated girders) are based upon 34 failure modes, belonging to serviceability/collapse subsets. Objectives based on the maximization of safety measures are applied together with standard design objectives such as minimization of initial cost and weight. The following problems were solved with different sets of objectives: (a) minimize cost and weight objectives subject to safety constraints (used for reference), (b) only the maximization of local safety measures is added to (a) as additional objectives, (c) only the maximization of global safety measures is added to (a) as additional objectives, (d) maximization of safety measures ad (b) and (c) are added to (a) as additional objectives. For each of the problems (a–d) the developed design procedure is executed. It contains two basic tasks for structural design of realistic (non-academic) problems: (1) multicriterial optimization with topology / geometry design variables; (2) multicriterial optimization of gross-panels with scantling / material design variables. Design procedure steps are executed using a fast and balanced collection of analysis and synthesis modules/methods of the OCTOPUS design system: • Determination of design load sets; • MOGA / MOPSO based generation of nondominated designs for the selected ship structure; • For each design the following analysis blocks are executed: – calculation of ship’s primary and racking response fields, – calculation of ship’s ultimate longitudinal strength, – calculation of serviceability and collapse safety criteria on the gross-panel level. Comparisons of results, based on generated Pareto hyper-surfaces and on subset of preferred designs, are given for problems (a–d). Insights into the results of optimization process, using 5-D graphics for design and attribute spaces, are also presented. Design problems of modern RoPax and SWATH structures are used in case studies.

Commentary by Dr. Valentin Fuster
2010;():911-920. doi:10.1115/OMAE2010-20754.

The aim of this study was to evaluate the load characteristics of steel and concrete tubular members under jet fire, with the motivation to investigate the jet fire load characteristics in FPSO topsides. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) [1]. To obtain reliable load values, jet fire tests were carried out in parallel with a numerical study. Computational fluid dynamics (CFD) simulation was used to set up an adiabatic wall boundary condition for the jet fire to model the heat transfer mechanism. A concrete tubular member was tested under the assumption that there is no conduction effect from jet fire. A steel tubular member was tested and considered to transfer heat through conduction, convection, and radiation. The temperature distribution, or heat load, was analyzed at specific locations on each type of member. ANSYS CFX [2] and Kameleon FireEx [3] codes were used to obtain similar fire action in the numerical and experimental methods. The results of this study will provide a useful database to determine design values related to jet fire.

Topics: Steel , Concretes , Stress , Fire
Commentary by Dr. Valentin Fuster
2010;():921-928. doi:10.1115/OMAE2010-20757.

The edge condition of the plating in a continuous stiffened-plate structure is neither simply supported nor clamped because the torsional rigidity of the support members at the plate edges is neither zero nor infinite. In a robust ship structural design, it is necessary to accurately take into account the effect of the edge condition in analyses of plate behavior. The aim of this study is to investigate the elastic large deflection behavior of plates with partially rotation-restrained edges in association with the torsional rigidity of the support members and under biaxial compression. An analytical method was developed to solve this problem using the nonlinear governing differential equations of plates. The validity of the developed method was confirmed by the comparison with nonlinear finite element method solutions with varying values for the torsional rigidity of the support members, plate aspect ratio, and biaxial loading ratio. The developed method was found to give very accurate results for the large deflection analysis of plates with partially rotation-restrained edges, and was proved very useful for the robust design of ship structures.

Commentary by Dr. Valentin Fuster
2010;():929-934. doi:10.1115/OMAE2010-20759.

The present paper is part of the study to develop the advanced computer aided manufacture (CAM) system called the changeable die system (CDS) that applies the cold-forming technique to produce curved metal plates with complex, three-dimensional geometry. This paper focuses on the algorithm of predicting the spring-back behavior using nonlinear finite element method, which is a key element within the framework of the CDS process. The validity of the algorithm is confirmed by comparison with experimental results obtained by the CDS machine in the cold-forming process of curved steel plates.

Commentary by Dr. Valentin Fuster
2010;():935-948. doi:10.1115/OMAE2010-20760.

The aim of this study is to investigate the crashworthiness characteristics of steel-plated structures subject to low temperatures that are equivalent to the Arctic environment. Structural crashworthiness with regard to crushing and fracture is a key element in the strength performance assessment of ship collisions in the Arctic, which provides the primary motivation for the study. This article is a sequel to the authors’ previous paper [1]. In contrast to the previous paper, which dealt with test structures made of American Society for Testing and Materials (ASTM) A500-type carbon steel with the wall slenderness coefficient (b/t) of 37.5, the present paper considers grade A steel with the wall slenderness coefficient (b/t) of 25. Crushing tests are undertaken on square tubes subject to a quasi-static crushing load at both room and low temperatures. The effect of low temperatures on the material properties is examined on the basis of tensile coupon test results. The crushing behavior of the square tubes in this test is compared with LS-DYNA computations. It is found that low temperatures have a significant effect on the crashworthiness of steel-plated structures in terms of mean crushing loads and brittle fracture. The use of grade A steel for ships and offshore structures in an Arctic environment is not relevant. The modeling techniques for the structural crashworthiness analysis presented in this paper are found to be pertinent by comparison with experimental results and nonlinear finite element method computations. It is suggested that the collision-accidental limit state design of ships intended to operate in the Arctic region be carried out by taking the effect of low temperatures into account.

Commentary by Dr. Valentin Fuster
2010;():949-958. doi:10.1115/OMAE2010-20761.

The aim of this paper is to examine the effect of wind on the thermal diffusion characteristics of floating production storage and offloading (FSPO) topside models subject to fire. It is motivated by the need to identify the fire loads on FPSO topsides, taking into account the effects of wind speed and direction. The results of an experimental and numerical study undertaken for these purposes are reported here. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) [1]. An experiment was performed on a 1/14-scale FPSO topside model using a wind tunnel test facility. The locations of the heat source of the fire were varied, as were the speed and direction of the wind, and the temperature distribution was measured. Computational fluid dynamics (CFD) simulations using the ANSYS CFX program were performed on the test model, with the results obtained compared with the experimental results. It is concluded that wind has a significant effect on the thermal diffusion characteristics of the test model and that the CFD simulations are in good agreement with the experimental results. The insights developed in this study will be very useful for the fire engineering of FPSO topsides.

Topics: Flow (Dynamics) , Heat , Fire , FPSO , Wind
Commentary by Dr. Valentin Fuster
2010;():959-966. doi:10.1115/OMAE2010-20917.

This paper presents a computational modeling of accidental fire actions on the topside structures of a floating, production, storage and offloading (FPSO) unit. According to the assumed scenario, the accident results in a jet fire, which loads the structure by temperature increments and pressures generation on their exposed surfaces. Temperature distributions were obtained by computational fluid dynamics (CFD) simulations, using the ANSYS CFX commercial code. The temperature versus time histories computed were first approximated (idealized) by smoother curves, based on fewer time-points, while retaining the maximum and minimum values. A similar procedure was also followed for the pressure variations. For the consequence (action effect) analysis the LSDYNA nonlinear finite element program was employed and the structures were modeled using shell finite elements with nonlinear (elastic-thermal plastic) constitutive relations. On the structure surfaces non coinciding grids were used for the two kinds of analyses (i.e., the CFD and FEM), in order to accommodate the diverse requirements of the different problems. The procedure of assignment the pressure and temperature loadings directly from the CFD results to the FEM model is described and representative results are given through the application of the methodology to a sample problem.

Commentary by Dr. Valentin Fuster
2010;():967-979. doi:10.1115/OMAE2010-20996.

This paper presents a structural failure assessment of the engine room region for a post-Panamax containership, outside 0.4L amidships, subject to an unexpected excessive hull girder bending moment and shearing force, with reference to the hull failure accident of the post-Panamax containership MSC Napoli. The ultimate strength of both the hull girder and stiffened panels in an as-built post-Panamax containership is estimated using ANSYS nonlinear finite element (FE) analysis and ALPS/HULL and ALPS/ULSAP programs to clarify the safety of the structures concerned. Finally, this paper proposes an assessment procedure for preventing an unexpected catastrophe such as that experienced by the MSC Napoli. Such accidents, although rare, result in significant economic losses and marine pollution.

Commentary by Dr. Valentin Fuster
2010;():981-989. doi:10.1115/OMAE2010-21007.

Marine structures such as ships and offshore platforms are mostly made of metals and designed with damage tolerance. This design philosophy requires accurate prediction of fatigue crack growth under service conditions. Now more and more people have realized that only a fatigue life prediction method based on fatigue crack propagation (FCP) theory has the potential to explain various fatigue phenomena observed. However, it is not the case that any type of FCP theory can work. As a matter of fact, from the very fundamental question of fatigue crack driving forces to the more complicated fatigue crack growth rate expressions all are needed critical examination. In the past several years, the group led by the authors have made some efforts in developing a unified fatigue life prediction (UFLP) method for marine structures. By unified method the authors mean that the method should be able to explain all the observed fatigue phenomena. In this paper, an overview of these researches is carried out and our main research results are presented.

Commentary by Dr. Valentin Fuster
2010;():991-999. doi:10.1115/OMAE2010-21044.

The dynamic response of one-way stiffened plates with clamped edges subjected to uniformly distributed blast-induced shock loading is theoretically investigated using a singly symmetric beam model. The beam model is based on the rigid-perfectly plastic assumption. The bending moment-axial force capacity interaction relation or yield curve for singly symmetric cross-section is derived and explicitly presented. The deflection condition that a plastic string response must satisfy is determined by the linearized interaction curve and associated plastic flow rule. Moreover, the possible motion mechanisms of the beam are discussed under different load intensity. Finally the dynamic response of a one-way stiffened plate is calculated theoretically and numerically. Good agreements are obtained between the presented theoretical results and those from numerical calculations of the FEM software ANSYS and ABAQUS/Explicit. It is concluded that the basic assumptions and approximations for simplifying calculations are reasonable and the beam model in theoretical analysis is adoptable. The example also shows that an arbitrary blast load can be replaced equivalently by a rectangular type pulse.

Commentary by Dr. Valentin Fuster
2010;():1001-1013. doi:10.1115/OMAE2010-21053.

Strength of offshore structures including FPSOs consists broadly of three aspects which are global intact and damaged strength, and local strength. Any of these strength aspects can be assessed by either prescriptive rule or finite element analysis (FEA). While many considerations relate to behavior in the linear elastic regime, the buckling and ultimate strength of both structural components (plate and stiffened panels) and structural systems can involve material and geometric nonlinearity behavior beyond the elastic region. With the development of computers and robust methods for nonlinear FEA, there has been a tremendous increase in the number of studies of structures under plastic or elasto-plastic behavior. However, even with today’s computers and software, nonlinear FEA of offshore structures remains complex and is not routinely applied in design analysis. Considerable effort therefore continues to be devoted to the development of simplified methods for rapid structural assessment and design analysis, instead of lengthy and complex nonlinear FEA. In this paper, various bucking and ultimate strength methodologies for plate and stiffener panels are first introduced. Each method is then compared with collected test data for buckling and ultimate strength of plate panels and stiffeners. Finally, conclusions are summarized based on the comparison study.

Commentary by Dr. Valentin Fuster
2010;():1015-1020. doi:10.1115/OMAE2010-21093.

The weight of glass fiber reinforced polymer composite (GFRP) plate is about one fourth of the steel plate and can be used in ship and offshore structures, so that the payload can be increased. However comparative studies on the behaviour of steel and GFRP composite plates with square opening have not been studied in detail. The experimental studies on steel and GFRP plates with and without openings are carried out for the combined loading of axial compression and out-of-plane loads. The in-plane and out-of-plane deflections are measured. The reduction in the axial load carrying capacity of the plates due to out-of-plane load is quantified. The effect of column slenderness ratio and plate slenderness ratio on the collapse load of simply supported stiffened plates is presented. Two sets of interaction equations are developed, one for the steel plate and another for the GFRP composite plate.

Commentary by Dr. Valentin Fuster
2010;():1021-1030. doi:10.1115/OMAE2010-21178.

To achieve an economic and reliable ship design, the ship structure has to be designed with adequate safety margins. This can be accomplished by a reliability-based limit states design approach, in which probabilistic methods are used to guide the development of the design criteria. A tool is developed to probabilistically assess the capacity distribution of the stiffened panels. A Monte Carlo simulation scheme, which samples a number of probability distributions, has been applied using PROBAN (DNV) which interactively utilizes PULS (DNV) as an efficient ultimate strength prediction tool for plated panels. It is demonstrated that this tool can successfully link the two internationally recognized programs, i.e. PROBAN and PULS for probabilistic assessment of stiffened panel’s ultimate strength. The results demonstrate that the suitability of the assumed distribution for the strength can be assessed with relatively little computational time, where the yield stress and imperfection sizes are treated as random variables. Such results are very informative and useful for further development of existing safety format ensuring a safe, economic and reliable ship design.

Commentary by Dr. Valentin Fuster

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