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A commonly assumed ship-to-ship collision scenario is a rigid ship bow hitting the side of a ship perpendicularly. A bulbous bow has a slender shape overall but a rounded end, while a stem has a sh...

Marine structures are subjected to slamming loads characterised by high hydrodynamic pressure within short time durations. Such loads can cause local and global damages to structures. This paper, w...

Explosion Risk Analysis (ERA) is typically performed to evaluate the explosion Design Accidental Loads (DALs) for the topside structures and facilities on offshore platforms. The total number of Vapor Cloud Explosion (VCE) scenarios in the real world is innumerable. To deal with the entire range of possible scenarios, the ERA is typically performed in a probabilistic manner, where the actual scenarios are replaced by a certain number of representative scenarios. A representative scenario is defined by several independent variables such as gas cloud volume, position and shape, and each of them should be probabilistically determined by using the concept of a flammable gas cloud frequency distribution. However, since an actual gas cloud is an object that extends into the 3D space, it is difficult to determine the shape of the actual gas cloud (or the fuel concentration distribution) with a particular scalar variable. In the existing ERA approach, therefore, the shape cannot be determined by the flammable gas cloud frequency distribution but is conservatively assumed to be a rectangular or other type of simple 3D geometry. Beyond the accuracy of this assumption, consistently determining the aspect ratio or other geometric parameters of the applied 3D geometry is a big issue in practice. Currently, for a given gas cloud volume, there is little guidance on how to determine the geometric parameters of the applied 3D geometry, and the relevant data is eventually left to the engineering judgment and experience. Therefore, in most cases, ERA results may vary from engineer to engineer, rather than being uniquely evaluated. In view of this, the current study aims to develop a method to take into account the shape of the gas cloud. The proposed method is designed to determine an equivalent gas cloud that can represent a specific number of actual clouds. The actual clouds refer to a group of gas clouds that are subdivided into the same category in the flammable gas cloud frequency distribution. Unlike the existing equivalent gas clouds, the proposed gas cloud has an inhomogeneous fuel concentration distribution whereby the shape can be implicitly considered.

This paper presents a finite element approach to calculate the full resistance profile of a spudcan deeply penetrating in dense sand overlying clay, in which a potential for an installing spudcan to experience a sudden uncontrolled punch-through failure exists. A modified Mohr-Coulomb model characterized by incorporating a four-phase variation of the mobilized strength and dilation parameters with an equivalent accumulated plastic strain is developed and tested for the overlying dense silica sand. An extended Tresca model is used for the strain softening of the underlying clay. A series of large deformation finite-element (LDFE) analyses are carried out, varying the strength and dilation parameters as well as the spudcan geometries. A fairly good performance of the present approach is verified by validating against groups of centrifuge tests data, allowing the numerical study to be extended parametrically. The four-phase variation of the mobilized strength and dilation parameters involved in the progressive failure of the upper dense sand is parametrically studied and extended to cover the range of sand relative densities that are of practical interest. Additionally, comparisons with the typical existing LDFE analyses using both simple and sophisticated constitutive models are carried out. It shows that the present approach performs fairly well to calculate the full resistance profile of a spudcan deeply penetration in both thin and thick dense sand overlying clay, especially the peak and post-peak resistance, within around 5% of the corresponding centrifuge tests results.

This paper presents a benchmark study on the ultimate limit state analysis of a stiffened plate structure subjected to uniaxial compressive loads, initiated and coordinated by the ISSC 2022 technical committee III.1-Ultimate Strength. The overall objective of the benchmark is to establish predictions of the buckling collapse and ultimate strength of stiffened plate structures subjected to compressive loads. Participants were asked to perform ultimate strength predictions for a full-scale reference experiment on a stiffened steel plate structure utilizing any combination of class rules, guidelines, numerical approaches and simulation procedures as they saw fit. The benchmark study was carried out blind and divided into three phases. In the first phase, only descriptions of the experimental setup, the geometry of the reference structure, and the nominal material specifications were distributed. In the second phase, the actual properties of the reference structure were included. In the third and final phase, all available information on the reference structure and measured properties were distributed, including the material properties and laser-scanned geometry. This paper presents the results obtained from seventeen submitted FE simulations as well as details on the experiment. It also presents comparisons of the force versus the displacement curve, failure modes and locations for each phase, among others, and a discussion on the participants’ ability to predict the characteristics of the reference experiment with the information that is available for the phase. The outcome of the study is a discussion and recommendations regarding the design of finite element models for the ultimate state analysis of stiffened plate structures, with emphasis on the prediction of the ultimate capacity, force-displacement curve, and failure mode and location related to access to data, uncertainties and modeling of the material properties, geometric imperfections and distortions, and residual stresses.

A dynamic analysis of flexible risers in large-scale domain remains the offshore industry’s most challenging problems. The cross section of flexible risers, consisting of multiple metallic and plastic layers, experiences inter-layer slippage during cyclic bending. This results in nonlinear and hysteretic bending behavior with rapid dissipation of energy. For the dynamic analysis for flexible risers in the large scale domain, this study replaces all layers of risers with simple beam elements with a nonlinear beam model. This simplified beam model traces the curvature to bending moment relation and captures the variation of stiffness due to the onset of slippage. The suggested methods formulate both the change of configuration due to large deflection and contact between risers and seabed, therefore more realistic and effective analysis can be achieved. In the current study, the details of the analysis method are described. Main formulation methods of the total Lagrangian framework and the interaction between risers and seabed are described as well as the nonlinear bending model for flexible risers. The validity of the developed method is examined through some case studies, and the results show how the bending nonlinearity of flexible risers affects the overall behavior of riser systems during its operation.

The slamming phenomenon commonly exists in many practical engineering areas. When slamming occurs, structures are likely subjected to slamming load characterized by high pressure within a short time duration. This load can cause damages to structures. In this study, slamming loads on flat stiffened aluminum plates of high-speed vessels and its dynamic response are investigated numerically. The numerical method is validated against the relevant experimental data from the open literature. The explicit finite element with Multi-Material Arbitrary Lagrangian-Eulerian solver is adopted to simulate the slamming impact of flat stiffened plates. Bilinear strain hardening with no strain-rate hardening was considered for the numerical simulation of the aluminum plate models. The effect of the heat-affected zone (HAZ) on the response of flat stiffened aluminum plate is also numerically evaluated and found to be considerable. The effect of structural flexibility, impact velocity, and air cushion are investigated through parametric studies. The results showed that the impact pressure and deflection of the flat stiffened aluminum plate increased with the impact velocity, while higher rigidity of the plate resulted in a less deflection and higher pressure as well as shorter impact pressure duration. In addition, the air cushion effect was found to be significant which reduced peak impact pressure, deflection of the aluminum plate model and lengthened the impact pressure duration. The discussion of predicted results of slamming pressure and deflection of the stiffened plate model are presented in detail.

A flexible pipe connects offshore platforms to the flowlines and transport gas and oil. It may experience axial compression due to the reversed end cap effect during installation. This can trigger radial buckling or lateral buckling of tensile armor layers. The ultimate strength assessment of the flexible pipe is complicated and time-consuming due to material nonlinearity, large deformation, and nonlinear contact mechanism. These difficulties make the nonlinear analyses difficult to converge. This paper proposes a simplified 5-layered model which can improve the convergence without deteriorating the accuracy. Analytical methods are suggested to determine an equivalent layer to replace inner four layers. In addition, the factor of penetration tolerance (FTOL) of shell element layers needs to reflect the thinning of polymer layers, which makes the axial stiffness equal to the solid element model. Analytical methods are used to determine the factor and a stepwise increasing approach is applied in a numerical analysis. The 5-layered model with the stepwise FTOL application is verified by comparing with 8-layered model, analytical model and experiment results with respect to axial and bending stiffness. The model is used for an ultimate strength assessment, the failure mechanism and the interaction between layers are investigated in detail with incremental loading.

There have been many attempts to widely utilize built-up H sections to secure flexibility in structural design. Much research into the structural strengths and limit states of built-up H sections, therefore, has been carried out. However, a practical redesign methodology taking advantage of built-up H beams has yet to be introduced into the offshore industry. In this study, a comprehensive investigation into built-up H sections is carried out, based on which, a new redesign procedure for weight reduction herein is suggested. First of all, on the basis of the ANSI/AISC 360-10, the differences between the rolled H and built-up H sections are investigated in terms of their various strengths. Then, a secondary-member redesign procedure is established as a means of reducing structural weight by replacing rolled H sections with built-up H sections. In that procedure, the built-up H section cross-section is modified according to the failure modes of reference rolled H sections. The redesign procedure is verified by a nonlinear finite element analysis and four-point bending test. Through the nonlinear finite element analysis and experiment on the reference rolled H section and built-up H section obtained by the redesign procedure, it is observed that the weight of the built-up H section is reduced by about 15% while a flexural strength similar to that of the reference rolled H section is maintained. The suggested redesign procedure is then applied to three floating production storage offloading (FPSO) topside modules for demonstration purposes. In the results, the total structural weights of the reference topside modules are reduced by approximately 3%–5% by employing built-up H sections in secondary members in lieu of rolled H sections. The results indicate that, in many cases, built-up H sections can be used as secondary members to reduce the structural weight of topside modules.

For a recent explosion risk assessment, a CFD (computational fluid dynamics)-based simulation has been widely employed using a 3D geometry model for a gas dispersion and an explosion simulation. For an accurate determination of explosion design loads, extensive scenarios covering various leak locations, leak directions, leak rate, wind directions and so on are treated in the simulation. In order to reflect the results of the risk assessment into detailed topside design, the time-consuming simulations should be begun from the early stage of the detailed design. However, the congestion level of 3D geometry modeling available at the early stage is considerably different from the as-built model due to the lack of modeling of small-sized piping, pipe supports, electrical supports, detailed structural outfitting and so on. This study proposes a method to generate 3D pipe modeling to compensate for the low congestion level. A random approach is made for the generation by randomly determining a starting point of a pipe spool, pipe diameter, pipe direction, a straight pipe length, number of pipes in a pipe spool and so on. For a pipe running through a given box range, it is cut out on the box boundary and a pipe type connecting with a neighboring module is also realized. For a longitudinally long pipe rack, the inside pipe spools have different features. The generation algorithm is adjusted to realize these features. The generated method doesn't include any algorithm to avoid existing other geometric entities such as equipment or structural members. Thus, the generated pipe length is generally reduced after combining with a module containing equipment and structural members. A study is also done about how much the pipe length of the as-built model needs to be increased considering the overlap. The proposed method is verified by combining the generated pipe model with a module containing only structure & equipment, and comparing explosion analysis results with the corresponding as-built model. A dispersion simulation is also performed for the validity of the proposed model in the dispersion simulation as well.

As the demand of environmental restriction and cost reduction increases, a fin-typed structure attached around a propeller has been developed to reduce fuel consumption rate of a vessel by improving the thrust force. Some cracks have been reported in the vicinity of the root of the fin, i.e. the welded line between ESD and hull structure, which is caused by the fluctuating hydrodynamic force induced by the ship motion. As a method of fatigue strength assessment for ship structure, the spectral approach has been widely utilized based on the hydrodynamic loads calculated using the potential theory. However, the nonlinear vertical lift force caused by the vertical motion of the ESD cannot be properly considered in the spectral approach. To take into account the nonlinear effect, computational fluid dynamics (CFD) analysis was adopted for the assessment of hydrodynamic force and the results were approximated by an artificial neural network (Lee et al., 2016). Then, a time series of vertical velocity of energy saving device under irregular wave is converted to time series of lift force and lift moment using the trained neural networks. However, in the previous research, ship speed, heading angle and ESD shape were fixed and their variations could not be reflected in the safety assessment. The proposed neural network is a useful method, but it cannot be directly applied to an actual project due to the absence of information about the artificial neural network itself. This research aims at developing simplified formulas to predict lift force and lift moment instead of the neural network and covering heading angle, ESD shape and ship speed additionally. To investigate the effects of additional variables, a series of CFD analysis is performed and polynomial regression models are employed to examine their relationship with the life force and moment. The established simplified formulas are used for the generation of a time series of lift force and lift moment required in ultimate and fatigue strength assessments. The rainflow counting method is used for the calculation of fatigue damage and the resultant long term distribution is compared with a simplified long term distribution method. The proposed formulas are expected to be used for the same type of ESD with different configuration and ship speed without any further time consuming CFD analysis.

Flammable gas cloud frequency distribution forms a key part of the CFD-based probabilistic explosion risk analysis (ERA). As part of this series work, a new type of gas cloud frequency distribution, namely, ignited gas cloud frequency distribution has been proposed in the previous study (Part I). On that basis, an advanced gas cloud frequency distribution is to be introduced continuously, which is the ultimate goal of this series work. The current study contributes to proposing a multivariate frequency distribution (MVFD), which can provide both gas cloud size and position to investigate explosion scenarios. As a matter of fact, the consequences of explosion accidents may vary greatly, depending on variables such as ignition position, gas cloud size, position and shape etc. So far, except for the gas cloud size that can be provided by the gas cloud frequency distribution, the remaining variables are more likely to be determined by engineering judgement and experience. Though this process follows standard guidelines or recommended practices, the determined variables can vary widely depending on engineers. In the present study, however, the gas cloud position is determined probabilistically as well. In order to input the gas cloud position into the proposed MVFD, the volumetric center of the equivalent gas cloud (ESC) is used to quantify the cloud position. To this end, the methodology introduced in Part I is consistently adopted in this paper, but the volumetric center is additionally monitored in the whole transient process of cloud propagation. Using the proposed distribution, the explosion scenarios can be investigated without the subjective engineering judgment of the gas cloud position, and therefore one can expect that the investigated explosion scenarios become more accurate than before. This paper provides a detailed procedure on how to obtain the proposed distribution and then presents case studies to demonstrate how the ERA results can change with the choice of the gas cloud position. Through the case studies, it is proven that the proposed MVFD is more reasonable for evaluating correct explosion design accidental loads.

Explosion risk analysis (ERA) is known as one of the dedicated safety studies for offshore installations and its purpose is to evaluate the explosion design accidental loads (DALs) on offshore topside structures and facilities. In general, ERA is more likely to be implemented in a probabilistic manner because it has a problem that needs to deal with a large number of explosion scenarios. In the probabilistic ERA, flammable gas cloud frequency distribution is a kind of intermediate result, which can be obtained by integrating the results of frequency analysis and gas dispersion modeling. In general, this distribution is applied to investigate a certain number of appropriate explosion scenarios to evaluate exceedance curves, which are commonly used to determine the DALs. The use of Computational Fluid Dynamics (CFD) to perform the gas dispersion and associated ignition probability modeling has become a trend in recent offshore projects. In most cases, however, the gas cloud frequency distribution has not yet fully benefited from the CFD models due to the high computational cost. Generally, the distribution is derived only using some particular values rather than reflecting the overall results of the CFD simulations. These particular values are likely to be steady-state results calculated using constant leak rates, which reduces the benefits of the CFD simulations and time dependent ignition probability model (TDIIM). In addition, when using these values, the effect of time-varying leak rates has only been considered with some simplified methods and the accuracy of these methods is still controversial. In the current study, the main effort is focused on proposing a new type of gas cloud frequency distribution that can reflect the overall results of the CFD simulations performed with the time-varying leak rates. One aspect to keep in mind is that every instantaneous gas cloud can pose a hazard of explosion, regardless of whether the leak rate varies with time or not. Corresponding to that, a specific method is further proposed which takes into account the entire transient process of gas cloud propagation, i.e. the time history of gas cloud size and ignition probability. This paper also presents case studies in which the gas clouds are both partially and fully selected in CFD simulations, and the final results are investigated in terms of overpressure exceedance curves. The case studies have shown that the transient process of gas cloud propagation is important for obtaining an accurate ERA, and that the proposed distribution dose not results in a significant increase in total computational cost.

During their lifetime, marine structures may be exposed to accidental loadings such as from collisions or explosions, as well as environmental loadings such as from slamming, sloshing and green water. Such loadings can cause damage to structures. Therefore, to minimize such damage, advanced and robust design guidelines should be formulated. Among those loads, in this study, explosions imparting an impulsive pressure loading containing a rapid increase in pressure and a short duration that can cause serious casualties, property losses, and marine pollution were considered. In this paper, a practical and robust method for damage assessment of marine structures exposed to explosion loads based on a single degree of freedom (SDOF) system and numerical simulations is proposed. The SDOF method was improved by introduction of new and better idealization resistance for the system and consideration of the effect of strain-rate, and subsequently was verified by a numerical method developed using the commercial ABAQUS software package. The numerical method was itself validated by comparison with relevant pulse pressure test data available in the open literature (good correlation was shown). Based on the validated numerical models, a rigorous parametric study of the structural response of stiffened plates having actual scantlings of offshore structures was performed. The numerically obtained maximum deformations were compared with the results from the improved SDOF method in a parametric study, and the variation of both methods was verified. Finally, simple yet accurate and reliable formulations for prediction of structural response were empirically derived. These formulations are expected to be usefully employed as a first-hand tool for prediction of damage extent of marine structures (including offshore structures) due to explosion loads.

As the boundary condition of jack-up systems, the foundation model of offshore structures affects the vibration mode and structural behavior of leg structure. Typical simple foundation models such as pinned and linear spring do not reflect either plastic failure or the resulting plastic behavior of the soil. Alternatives, for example Model B for clay, derived for consideration of the nonlinear behavior of soil, have been studied continuously improved until recently. Additionally, research on the foundation models has been continued in multi-layered soils, especially stiff-over-soft layer soils that present a risk of punch-through. In the present study, a yield envelope analysis is performed for the situation wherein the jack-up leg penetrates deeply through the soft clay and meets the stiff clay. In such soft-over-stiff clays, both the upper soft clay and the lower stiff clay affect the spudcan, thereby complicating the use of a foundation model of single clay. The numerical analysis used in this study is based on the Coupled Eulerian-Lagrangian (CEL) analysis with strain-dependent nonlinear soil properties. Installation effect and corresponding soil disturbance are considered in the following yield envelope analysis. Undrained bearing capacities of the single clay are calculated at the deep embedment for checking the suitability of the analysis, and continuous results have shown to the existing theories of single clay. In the soft-over-stiff clay case, the lower stiff clay has a significant effect on the capacities in the vertical, horizontal and rotational directions. The squeezing effect does not affect the vertical tensile capacity but increases the compressive vertical, horizontal, and rotational capacities. The stronger the underlying clay and the deeper the embedment, the asymmetrically larger the yield envelope becomes. The calculated yield envelopes are normalized to the increased capacities, and a yield envelope expression for the soft-over-stiff clay is suggested for the normalized shape. The effect of the maximum capacity on the yield envelope equation is examined, and it is found that the largely calculated capacity can make the yield envelope close to the paraboloid.

In recent years, the dynamic responses of flexible risers have been the focus of many researchers. Most flexible risers undergo a substantial level of irregular motion from environmental loadings, which involves a continuous slip of helical wires. The slip of helical wires especially leads to a hysteretic effect by reducing the bending stiffness, making it hard to predict the dynamic responses of flexible risers. The current study, as an extension to Part I, presents a new large-scale dynamic analysis method for flexible risers. The suggested method creates a large-scale model for the dynamic analysis that considers a geometric and bending nonlinearity of flexible risers. The kinematics of each beam element is formulated based on a Green-Lagrangian strain and the interaction with the seabed, providing a realistic analysis of flexible risers. In particular, the current study introduces a direct moment correction method that modifies the internal force vector using an improved analytical model. The improved analytical model is assigned at each node of the large-scale model and estimates an accurate bending hysteresis curve considering the effect of shear deformation and varying tension. The suggested method corrects the bending moment and shear force of all beam elements based on the bending hysteresis curves obtained from the improved analytical model, by which a complex bending behavior of flexible risers is reflected in a large-scale domain. As a result, this study achieves a more accurate prediction of the dynamic responses and fatigue damage of flexible risers. A new dynamic analysis program, called OPFLEX, is developed herein based on the suggested analysis method. Using the developed program, the current study conducts several numerical investigations to identify the effect of the shear deformation and varying tension. Consequently, it is confirmed that the shear deformation of internal layers reduces the fatigue damage of helical wires by delaying the increase of internal stress. It is also identified that the effect of varying tension deteriorates the fatigue life of flexible risers through a continuous change of contact pressure during bending.

This paper presents a benchmark study on the slamming responses of offshore structures’ flat-stiffened plates. The objective was to compare the fluid-structure interaction (FSI) simulation methodologies, modeling techniques, and established researchers' experiences in predicting slamming pressure. Three research groups employing the most common commercial software packages for numerical FSI simulations (i.e. LS-Dyna ALE, LS-Dyna ICFD, ANSYS CFX, and Star-CCM+/ABAQUS) participated in this study. Wet drop test data on flat-stiffened aluminum plates of light-ship-like bottom structures available in the open literature was utilized for validation of the FSI modeling. A summary of the experimental conditions including the geometry model and material properties, was distributed to the participants prior to their simulations. A parametric study on flat-stiffened steel plates having actual scantlings used in marine installations was performed to investigate the effect of impact velocity and plate rigidity on slamming response. The FE simulation results for the total vertical forces acting on the stiffened plates and their structural responses to those forces, as obtained from the participants, were analyzed and compared. The reliable and accurate predictions of slamming loads using the aforementioned commercial FSI software packages were evaluated. Additionally, equivalent static slamming pressures resulting in the same permanent deflections, as observed from the FSI simulations, were reported and compared with analytical models proposed by the Classification Standards DNV and existing experimental data for calculation of the slamming pressure. The study results showed that the equivalent load model depends on the water impact velocity and plate rigidity; that is, the equivalent static pressure coefficient decreases with an increase in impact velocity, and increases when impacting structures become stiffer.

Flexible risers have been widely utilized for the transfer of oil and gas products from a well to production units. The components of flexible risers, unlike steel risers, experience complex contact phenomena during bending. The contact between helical wires and adjacent layers especially causes a significant level of bending nonlinearity, making it hard to estimate the structural responses. Accordingly, a large-scale dynamic analysis of flexible risers usually involves an analytical model that predicts the bending moment and axial stress of helical wires based on theoretical approaches. The analytical model consists of an axis-symmetrical model and a bending model. Among them, the bending model plays a critical role in the prediction of the bending responses of flexible risers. The conventional bending models usually neglect the shear deformation of internal layers and continuity of sliding force, which leads to a significant error of analysis. Furthermore, the previous bending models assume that the contact pressure on helical wires is constant during bending. In real operating conditions, however, most flexible risers experience a considerable change of tension that governs the slip of helical wires. Hence, the current study presents a new dynamic analysis method for flexible risers. The suggested analytical model improves the bending model based on an accurate estimation of the internal strain field considering the shear deformation and continuous sliding force. Also, this study proposes a stiffness update method to reflect the effect of varying tension in the dynamic analysis. The presented method updates the bending property of flexible risers considering the continuous change of the contact pressure from varying tension. For the validation of suggested method, the current study carries out numerical simulations with a pure bending and varying tension for the internal diameter 7 inches flexible risers. It is identified that the suggested analytical model provides accurate analysis results. Moreover, it is found that the effect of varying tension gives a significant impact on the bending behavior of flexible risers by changing the slip condition of helical wires. Part I of this series of papers describes the detailed formulation method for the analytical model and with some verification examples. The suggested analytical model is expanded to the large-scale dynamic analysis in Part II for the investigation of the effect of shear deformation and varying tension.

The evaluation of impact induced slamming loads experienced by ships and offshore structures using advanced fluid structure interaction methods may be a challenging task involving complex and time consuming engineering solutions. This is the reason why to date the application of well understood and validated quasi-response approaches remains the most rational alternative used by experts for preliminary design assessment. Based on a benchmark study carried out by the International Ship and Offshore Structures Congress Technical Committee II.1 on Quasi-Static Response this paper demonstrates the practical use of quasi response prediction methods for the assessment of impact loads on modern free fall lifeboats. The case study presented is considered relevant in terms of technical background and ship design for safety. Following a brief review rationalising the practical relevance of the engineering solutions examined, the influence of high speed impact is evaluated using linear-elastic and nonlinear beam models, as well as a nonlinear transient dynamic finite element analysis idealisation. Comparisons of the methods presented against experimental results led to the conclusion that the nonlinear quasi-dynamic beam approach accounts for the influence of the dynamic effects of strain by suitably idealising the effects of nonlinear geometric stiffness. It may therefore be more appropriate to employ this approach at preliminary stage, especially when conducting comparisons against simplistic linear methods used for rigid structures (e.g. stiffened steel and aluminium panels), or advanced nonlinear finite element analysis, and other multi-physics methods that may be computationally complex and time consuming.

Designs of flexible pipe utilized in offshore dynamic riser applications need to be subjected to strength assessment over the entire length in relation to the dynamic motion of the connected floater. Such assessment includes many uncertain factors including soil properties, internal pressure, ocean metadata and others. Failure of flexible pipe, moreover, leads to serious financial loss and environmental degradation. Thus, flexible pipe failure modes and safety need to be evaluated in terms of ultimate strength under various loads. Ultimate strength assessment of flexible pipe is quite complicated and time-consuming compared with that of a steel catenary riser, due to the composite materials, geometric complexity and the contact mechanism between layers The material nonlinearity and large deformation, moreover, render stable convergence of nonlinear analysis to a solution problematic. This paper proposes practical and stable methods of ultimate-strength assessment using 8-layered and 5-layered 3D FE models subjected to axial tensile and compressive loads, respectively. In the 5-layered model, four inner layers (carcass, pressure sheath, pressure armour, and anti-friction tape) are replaced by one equivalent pressure layer which plays a role of withstanding pressure applied together with the axial compression. It aims to improve the convergence of nonlinear analysis by simplifying the interactions between layers. For each analysis, the failure mechanisms and the interactions between layers are investigated in detail with incremental loading. The effect of initial imperfection, i.e. ovalizations are also examined. In the compressive strength analysis, the influence of various external pressure are additionally studied.

A cylindrical type hull appurtenance attached to the side hull like a seawater caisson or a riser tube is subject to the drag force and inertia force defined by the Morison equation, as well as global hull girder loads. When performing fatigue analysis for a structure subject to Morison load, the nonlinearity of drag term in the Morison equation should be taken into account for an exact structural safety assessment in an appropriate way. This research proposes a time domain approach (Level II method) in order to take into account the nonlinearity, as well as a proper combination of local hotspot stress induced by the Morison load and global hull girder stress. It is based on a representation of irregular wave as a combination of a large number of regular waves. It enables for reflecting phase angle differences of particle velocities along the vertical tube. The phase difference with global stress is also taken into account. The method also includes the vertical motion of the caisson and the wave elevation above the mean water line. The nonlinearity of drag term can be treated by linearizing the drag term in the frequency domain analysis. For the frequency domain approach, Level I method using a linearization coefficient is proposed to yield a fatigue damage equivalent to the time domain analysis. It incorporates the different contributions of horizontal velocities along the vertical tube on the hotspot stress at a connecting bracket on the side hull. The proposed methods are verified through a caisson in FPSO example and various comparative studies. Various comparative studies are performed for a verification of spectral code, an evaluation of rain flow counting effect and the combination of motion and stretching effect, a verification of rain flow counting(RFC) code and linearization coefficient. Through these case studies, it is found that the proposed spectral fatigue analysis with linearized Morison’s force results in a good agreement with time domain analysis with a drastic reduction of computational time. In this paper, a FPSO model with a seawater caisson is used as a verification example. The proposed methods are expected to be applied to many similar tubular structures on the side of floating offshore structure, such as, riser guide tube, protection frame for riser guide tube, collision protector and so on. In addition, jacket or jack-up rig consisting of tubular members could be good applications.

The crashworthiness of the striking vessel and the struck vessel should be evaluated when ship collision occurs because it can cause severe consequences. In this study, a simplified method based on analytical approach is suggested for small scale ship collision event and compare it with the solution obtained from the finite element method. The shape of the striking vessel and struck vessel are selected as a bow (bulbous bow and stem) and a double hulled structure, respectively. From the finite element results, it is confirmed that the stem case is more vulnerable than the bulbous bow case, because its sharp shape can cause the local failure of struck vessel much earlier. The way to consider the geometry effect of striking vessel called contact based method is introduced in this paper. It is verified that the finite element results and the simplified method results are found to have an appropriate correlation.

As environmental regulations have become more onerous, the demand for LNG and LNG carriers has increased. The LNG cargo containment system (LNG CCS) is one of the most important facilities in LNG carriers, and many membrane types of LNG CCS have been developed so far. Traditionally, sloshing model tests are performed and a series of statistical approaches are used to obtain design sloshing pressures. Then, these design loads are utilized to analyze the structural safety of LNG CCS. In the sloshing model test, the flat and almost rigid plate is used as the LNG CCS instead of a real model, and thus, the hydroelasticity effect cannot be considered. In the present research, the effect of hydroelasticity on sloshing pressure was investigated in a fluid-structure interaction simulation. A wet drop simulation was conducted, and its results subsequently were compared with wet drop experimental test results to ensure their validity. Then, two types of structure model, namely a flat-rigid plate model and a flat-flexible CCS model, were generated to investigate the effect of hydroelasticity. Also, a fluid hitting analysis model was devised to realize the sloshing phenomenon, and the two generated structure models were applied as the structure domain in the analysis. In the fluid hitting simulations, it was found that the hydroelasticity effect on sloshing pressure is significant. Thus, correction factors, which are quantitative values of the effect of hydroelasticity, were derived (and are proposed herein). Finally, a structural safety assessment procedure for consideration of the hydroelasticity effect was derived (and is suggested herein).

In stratified deposits, where an interbedded strong layer overlays a weak layer, the punch-through failure may occur and a soil plug from the overlying stronger layer usually exists beneath the advancing spudcan. Due to the limited understanding of the soil plug mechanism which really exists in both the strong-over-soft and soft-over-strong strata, no consideration of the soil plug effects is included in the present methods recommended in the current industry guidelines. This paper reports numerical simulations of spudcan deep penetration in multi-layer sediments with an interbedded sand layer, and aims at providing a better understanding of the soil plug mechanism. An extended Tresca model incorporating the clay strain softening together with an extended Mohr–Coulomb model with incorporation of the progressive failure of sand is used in this study. By comparing with the existing large deformation finite-element (FE) results and the centrifuge tests results, the numerical analyses conducted in thi...

There have been many attempts worldwide to reduce ships׳ fuel consumption. With fuel-efficiency improvement as their designed purpose, a variety of energy saving devices (ESD) have been developed and deployed. Much research with respect to ESD performance already has been carried out; however, the issue of ESD structural safety has received relatively little attention. According to the current approach to ESD structural safety assessment, the Morison equation, which is calculated for a certain probability-level velocity, is applied, or alternatively, a spectral method based on the assumption of a linear system between wave and response is utilized. Unfortunately, this methodology does not take into account the nonlinearity of hydrodynamic loads. Therefore, a new ESD structural safety assessment procedure that utilizes the neural network and time-domain simulation using the Gumbel fitting method is herein proposed. The procedure entails four main steps: sea-keeping analysis, hydrodynamic load analysis and neural network, ultimate strength analysis, and fatigue strength analysis. The important features of the proposed procedure are, in order, as follows. First, to consider the nonlinearity of hydrodynamic force acting on the ESD, computational fluid dynamics (CFD) analysis is carried out on samples consisting of various wave heights and periods. The neural network is then trained based on the CFD analysis results for the prediction of hydrodynamic loads. Second, to take into account the randomness of the peak hydrodynamic force, a three-hour time-domain simulation is repeated 20 times for each sea state of a wave-scatter diagram, and Gumbel parameters are calculated for long-term analysis. Third, approximate long-term analyses using a contribution coefficient and short-term analysis are performed for an efficient long-term analysis.

The tension leg platform (TLP) is one of the conventional offshore oil- and gas-production structures. With the TLP, relatively small motion responses can be maintained, as most vertical wave loads are resisted by a tendon system consisting of multiple tendons, the high vertical stiffness of which makes possible the achievement of a quite low heave natural frequency. Notwithstanding this advantage, the TLP requires a complicated and time-consuming design process. In this study, a multi-objective optimization system for both the hull form and tendon system was developed. The maximum heave response and the total weight of the hull and tendons are formulated as objective functions. Five modules were developed to automate TLP modeling and performance assessment as the preliminary steps to optimization. In the first module, the panel and Morison models are automatically generated to model the hull form and tendon system based on predefined design parameters. Light weight and displacement at several stages are estimated in the mass estimation module based on surface area and hull volume. After generation of the hydrodynamic model, the motion response of a TLP is calculated by the commercial program DNV. WADAM (DNV software, 2013). The dynamic tension loads of the tendons are added to the restoring force matrix of motion equation, under the assumption that the tendons are linear springs. In the post process module, ultimate limit state (ULS) and fatigue limit state (FLS) assessment procedures are carried out. In the optimization module, multi-objective optimization by a simulated annealing (SA) method is performed. Thereby, eight different optimum solutions forming a Pareto set are obtained and compared with each other to determine their validity. Based on those solutions, the present study determined that the pontoon volume and the sectional area of the tendons dominantly affect the following objective values: the maximum heave response and the total weight of the hull and tendons.

Ships and offshore structures are exposed to various cyclic loads such as winds, waves and currents in their lifetime. These loads can result in fatigue failures at hot spots, and to prevent such failures, accurate estimates of the fatigue life of hot spots should be performed in the design stage. The cycle distribution indicative of the probability distribution of cycles is an important factor in estimation of fatigue damage to mechanical systems in frequency-domain methods. It depends on not only the counting method but also the statistical properties of random processes. Many studies deriving both theoretical and approximate models of rainflow-counted cycle distributions in wide-banded non-Gaussian processes have been conducted. However, some of the existing models require multi-dimensional integrations with much computational time or yield inaccurate representations of rainflow-cycle distributions. Therefore, a new model providing accurate cycle distributions is needed for fatigue analysis of offshore structures under wide-banded non-Gaussian random loadings. This paper consists of two parts: Part I is devoted to the development of a new approximation model for the joint probability distribution of mean and amplitude of rainflow-counted cycles in wide-band Gaussian random processes. This probabilistic model derived in Gaussian random processes could be transformed to the corresponding non-Gaussian processes through non-linear transformation technique. Part II discusses the detailed procedure and accuracy of the proposed model in fatigue damage assessment of offshore structures under non-Gaussian random loading. Rainflow cycle distribution in wide-band non-Gaussian processes is derived using the Hermite transformation function. The accuracy of the proposed model is verified through two numerical examples, and compared to other existing models.

This is Part II in a series of papers. Part I [1] investigated the slamming responses of flexible flat stiffened steel and aluminum plates using the nonlinear explicit finite element code LS-Dyna with the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) solver. Subsequently, a simplified finite element FSI model of water hitting structures that is realistically close to the slamming phenomenon occurring on the bottom part of offshore structures was proposed. The proposed FSI methodology presented in Part I was verified by comparison with the relevant test data. It was evident that the use of the proposed numerical method presented in Part I was very effective for a benchmarking investigation of slamming load considering the hydroelastic effect. However, the method required much effort in terms of computation time and power analysis resources. The present study, Part II, aimed, as an alternative to the FSI analysis approach, to develop empirical formulae for prediction of slamming loads acting on deformable flat stiffened plates used in marine applications. This paper begins by describing the limitations of the existing approaches based on theoretical, experimental and even numerical studies conducted in the past for estimation of slamming loads. Next, it presents, based on the simulation methodology developed in Part I, rigorous parametric studies that had been performed on actual scantlings of marine-seagoing structures. The effects of structural geometry and water impact velocity on slamming pressure are then investigated in detail. Subsequently, the parametric results are analyzed and utilized to derive empirical formulations for the prediction of slamming loads acting on flat stiffened plates of marine structures. The accuracy and reliability of the proposed formulations are established by comparison with the results of the test and other existing formulations. The proposed formulations are expected to be used for the purposes of the design without any time-consuming FSI analysis of advanced and optimal structures that are robust to slamming.

A slug flow (a series of liquid slug separated by a gas pocket) is a flow pattern that often occurs in a multiphase oil and gas riser system. Dynamic and structural responses of a riser can be increased by time varying forces of the slug flow. In this paper, a methodology has been developed for the evaluation of the effects of slug flow on the dynamic and structural responses of different types of risers. A global dynamic analysis is carried out to calculate the dynamic responses of a riser induced by the slug flow and environmental loads. Idealized regular slug flow are considered as a time dependent variation of internal fluid flow according to results of multiphase flow simulations. Local stress and fatigue damage of risers are estimated using an in-house program, which considered the hysteresis effects of a flexible riser. The verifications of the numerical model and slug modeling method are carried out prior to the parametric study. Parametric studies are performed in order to quantitatively identify the design conditions that are most affected by the slug flow. The riser system connected to a moored vessel in deep-water is considered for the parametric study. Different riser materials, riser configurations, production stages (flow rates) and environmental conditions are considered. Study results demonstrate that the dynamic effects due to slug flow are significant for a flexible lazy wave riser, middle production stage and mild sea states. While the maximum value of dynamic loads is slightly changed even with considerable slug flow, range of fluctuation (standard deviation) of bending curvature is substantially increased under mild sea states with high probability of occurrence. This means that a main impact of the slug flow is the increased fatigue damage of a riser structure. The fatigue damage with and without slug flow has been estimated in order to investigate the fatigue contribution of the slug flow.

An un-bonded, multi-layered assembly of heterogeneous materials in the cross-section of flexible risers enables riser systems to withstand a large radius of curvature while preventing structural damage or buckling of many critical areas such as a touch down zone. The contact surfaces between layers exhibit a continuous sliding of metallic components after a certain level of bending, which lowers the bending stiffness of flexible risers. Such interactions are beneficial in order to reduce the stress of tensile wires, but complicate prediction of the limit state and fatigue damage of riser systems in that the sliding of metal layers involves a complex contact phenomenon that is difficult to solve numerically. This series of papers deals with the development of an improved analysis method for flexible risers using theoretical approaches. Part I, as presented in these pages, develops an analytical model that is capable of estimating the cross-sectional stiffness of flexible risers with consideration of external loads and inter-layer interaction. In Part II, the proposed analytical model is applied to a large-scale riser model to achieve an efficient global dynamic analysis of flexible risers. This paper, the first part of a two-paper series, proposes an analytical model that formulates the stress of tensile armor layers through equilibrium equations that take into account the shear and radial deformation of polymer layers. The equilibrium equations are derived under a linear differential equation for each component of flexible risers, so that not only the shear interaction forces but also the contact pressure originating from the bending of layers is considered in the analytical model. The shear interaction forces between two tensile armor layers are formulated through an equivalent shear stiffness by which the shear stiffness of layers is linearly modeled as a series of springs. The proposed model is verified by comparison with the nonlinear bending stiffness from existing bending models and finite element (FE) analysis. For additional verification, the axial stresses of inner and outer tensile armor also are compared. And for further, more in-depth understanding, a case study of various combined load cases is carried out.

Ships and offshore structures are exposed to various cyclic loads such as winds, waves and currents in their lifetime. These loads can result in fatigue failures at hot spots, and so to prevent such failures, accurate estimates of the fatigue life of hot spots should be performed in the design stage. The cycle distribution indicative of the probability distribution of cycles is an important factor in estimation of fatigue damage to mechanical systems in frequency-domain methods. It depends on not only the counting method but also the statistical properties of random processes. Many studies deriving both theoretical and approximate models of rainflow-counted cycle distributions in wide-banded non-Gaussian processes have been conducted. However, some of the existing models require multi-dimensional integrations that require much computational time or yield inaccurate representations of rainflow-cycle distributions. Therefore, a new model providing accurate cycle distributions is needed for fatigue analysis of offshore structures under wide-banded non-Gaussian random loadings. This paper consists of two parts: Part I is devoted to the development of a new approximate model of the joint probability distribution (JPD) of mean and amplitude of rainflow-counted cycles in wide-banded Gaussian random processes. Nonlinear regression analysis is performed to approximate the conditional probability density function (PDF) of the cycle mean value. The accuracy of the proposed model was verified through two numerical examples. This probabilistic model derived in Gaussian random processes could be transformed to the corresponding non-Gaussian processes through a non-linear transformation technique. In the forthcoming Part II, the accuracy of the proposed model for fatigue-damage assessment of offshore structures under non-Gaussian random loading will be discussed.

Due to the spatial complexity and fabrication characteristics of offshore platforms, it is inevitable to encounter overlaps or proximity of weld lines in tubular joints. Several international standards such as American Petroleum Institute (API), American Welding Society (AWS), and American Institute of Steel Construction (AISC) regulate the minimum distance between primary weld beads; however, any logical and detailed background of this limitation has not been presented. For a non-compliant weld joint where the regulation is not met, fracture toughness calculation is a typical index to verify the structural integrity. This research consists of two parts. First, weld residual stress distributions are calculated by a 3D thermo-mechanical nonlinear Finite Element Analysis. Two crossing welds, a T-weld crossing on a butt weld, are simulated in one model. A separate tee and a butt welding simulations are also performed for a comparative purpose. Second, SIFs and J-integral values are calculated at the surface and deepest crack tip locations for four different types of semi-elliptical surface cracks. Four cracks are embedded into the weld model and the residual stress distribution from the 3D thermo-mechanical FEA are mapped to a 3D FE crack model as initial conditions. An additional axial tensile load is also imposed. SIF values are compared with those using the weighting function method for the butt weld model subject to three load cases, i.e., tensile stress only, weld residual stress only, and both of them. From the simulation, a tubular joint containing a chord girth weld intersected with weld beads of brace is found to show lower the SIF values than that having only a girth weld on chord.

In strong-over-soft strata, there is a potential for an installing spudcan to experience a punch-through failure, where the spudcan would penetrate a certain distance at a short time after the peak penetration resistance. Such a sudden punch-through failure would seriously threaten safety of the offshore structures, even causing casualties. This paper reports results from numerical modeling of spudcan penetration in sediments with interbedded medium-loose sand overlying a clay layer. Three typical stratifications commonly encountered in the field are considered: (1) sand-uniform clay, (2) sand-nonuniform clay and (3) uniform soft clay-sand-uniform soft clay-uniform stiff clay. Further, a field case encountered in the South China Sea was provided and analyzed. The analyses are carried out using a large deformation finite-element (LDFE) approach of the Coupled Eulerian–Lagrangian (CEL) in ABAQUS/Explicit. An extended Tresca model with incorporation of strain softening through two additional parameters of δ rem and ξ 95 following Einav and Randolph [1] is used for the clay, whereas the Mohr–Coulomb model is adopted for the sand. In particular, a simplified method, in which no additional parameters are required as input data, is presented to predict the punch-through resistance profile of a spudcan foundation on sediments with interbedded medium-loose sand layer. By comparison with the previously published LDFE results, recent centrifuge tests data as well as measures from one field case in the South China Sea, the numerical analyses conducted in this study are verified and discussed, and better performance of the simplified method is shown for prediction of the penetration resistance profile of a spudcan foundation on sediments with interbedded medium-loose sand layer.

An offshore platform normally is exposed to flammable oil and gas in the course of its operation. Uncontrolled hydrocarbon leaks or accumulated flammable gas clouds present very dire threats, due to the probability of resultant fire and explosion. Fire accident represents a major part of the total risk to offshore platforms. Nevertheless, existing procedures of fire risk analysis (FRA) are still not comprehensive enough to systematically evaluate the risks to topside structure safety. As reported in this paper, a new FRA procedure is established to overcome the deficiencies of existing ones. Prior to introducing the new FRA procedure, this paper reviews two types of existing FRA procedures and points out a common problem, namely the difficult application of design accidental loads (DALs) to structural consequence analysis. In this regard, the new procedure includes an extended structural consequence analysis rather than follow the conventional means of determining specific DALs. Moreover, the new procedure offers a new concept of cumulative failure frequency for assessment of topside module structure safety. The role of cumulative failure frequency is to probabilistically identify failed members of objective structures. The key advantages of the new FRA procedure are twofold: first, effective structural consequence analysis without the need for DALs, and second, probabilistic safety assessment of topside structure subjected to a certain number of prescribed fire-accident scenarios. The cumulative failure frequency, in fact, becomes useful information for determination of risk mitigation measures (e.g. passive fire protection (PFP), deluge systems, etc.). This paper not only presents a detailed description of the new FRA procedure but also demonstrates an example using a real FPSO separation module.

Minimization of the motion response of a floating offshore structure is a critical issue, since it is directly related to operation time. In this paper, a fully automated procedure for hull-form optimization of a semi-submersible floating production unit (FPU) is introduced. As a preliminary step to optimization, three modules are developed: panel generation, mass estimation, and condition setting. In the first module, 10 geometric parameters are defined to represent the hull form. Once the values of those parameters are determined, the panel model for the motion analysis is automatically generated. The second module for mass estimation determines weight and center of gravity. Mass is divided into several components, which are estimated based on the surface area and volume of the hull. By applying the simulated annealing (SA) method, multi-objective optimization is performed. Objectives for minimization are the 3-h heave most probable extreme value (MPEV) and the structural weight. Each objective is weighted. Then, according to the different values of the two weighting factors, four optimal solutions are obtained. Based on those solutions, it is found that the total hull height is proportional to the structural weight and inversely proportional to the heave motion.

When certain mechanisms, such as plastic deformations and temperature gradients, occur and are released in a structure, stresses remain because of the shape of the structure and external constraints. These stresses are referred to as residual stresses. The base material locally expands during heating in the welding process. When the welding is completed and cooled to room temperature, the residual stresses are left at nearly the yield strength level. In the case of friction stir welding, the maximum temperature is 80% to 90% of the melting point of the materials. Thus, the residual stresses in the welding process are smaller than those in other fusion welding processes; these stresses have not been considered previously. However, friction stir welding residual stresses are sometimes measured at approximately 70% or above. These residual stresses significantly affect fatigue behavior and lifetime. The present study investigates the residual stress distributions in various welding conditions and shapes of friction stir welding. In addition, the asymmetric feature is considered in temperature and residual stress distribution. Heat transfer analysis is conducted using the commercial computational fluid dynamics program Fluent, and results are used in the finite element structural analysis with the ANSYS Multiphysics software. The calculated residual stresses are compared with experimental values using the X-ray diffraction method.