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Engineering applications of topology optimization (TO) geared towards creating concept structures with superior properties are notoriously challenging. Multi-performance and uncertainty risks are essential considerations in the design of offshore structures. In this paper, a reliability-based multi-performance concept-detailed co-design (RBMPCD) framework is developed for the two-phase optimization of offshore structures. In the 1st phase, a multi-performance optimization (MPO) system consisting of the extended TO technique and the combination weighting strategy is utilized to perform the multi-objective TO study to capture the creative concept layout. The polynomial penalty model using the continuation method serves as a theory support for the 1st phase. In the 2nd phase, a decoupled reliability-based size optimization (RBSO) strategy is suggested to industrialize the concept pattern. The deterministic size optimization (DSO) cycles and inverse reliability analysis (RA) cycles are alternately run to conduct the reliability-based detailed design of offshore structures. A modified conjugate Fletcher-Reeves (MCFR) algorithm incorporating the classical function type criterion is presented to perform inverse RA under complex implicit probability constraints. A numerical example including a 3D T-beam, and an engineering application of a pillar deck are exploited to show the effectiveness and state-of-the-art of the RBMPCD framework. • A reliability-based multi-performance concept-detailed co-design framework is first developed. • New pillar deck is uncertainty-resistant, spatially rationalized, high-performance. • A modified conjugate Fletcher-Reeves (MCFR) algorithm is presented. • MCFR possesses advanced stability, robustness, accuracy, and efficiency. [ABSTRACT FROM AUTHOR]

Control room operators on offshore platforms play an important role in realizing the safety instrumentation functions of the high integrity pressure protection system (HIPPS) during overpressure emergencies. Human reliability analysis is critical to the overpressure operation of offshore HIPPS, yet there are few studies. To remedy this gap, the paper proposes an integrated human reliability analysis method to analyse the impact of human intervention on offshore HIPPS safety. Considering the effects of context and cognitive performance on human reliability comprehensively, the improved cognitive reliability and error analysis model (CREAM) is established; The correction effect of external environmental factors on response failure probability is quantified, and the improved human cognitive reliability (HCR) model is developed; The two improved models are organically combined to form an integrated methodology. Based on the behaviour characteristics of personnel emergency response, human error probability is calculated in stages to analyse human reliability. The findings indicate that the overall human error probability for HIPPS overpressure operations is 0.124 and particular attention needs to be paid to observation and decision-making operations. Besides the theoretical background, the paper provides practical contributions to personnel training, mission planning, and risk management of HIPPS to minimize the possibility of human error. • An integrated human reliability analysis method for offshore HIPPS is proposed. • The cognitive performance correction coefficient is introduced to improve CREAM. • The modification effect of external environmental factors is quantified to improve the HCR model. • Human error probabilities are calculated in stages based on personnel task characteristics. [ABSTRACT FROM AUTHOR]

Corrosion defects can significantly reduce the ultimate bearing capacity of thick-walled pipelines in deep water. Uneven depth defects pose a serious threat to the integrity of pipeline structures. However, in the study of uneven depth defects, the influence of chord-cut defects is often overlooked. To study the impact of chord-cut corrosion defect on pipeline collapse response, a dynamic model is established. Two sets of full-scale pipeline experiments are conducted to verify the model. The validated model is used to obtain the pipeline collapse responses under different defect parameters, which include defect length, defect depth, defects' location, and pipeline ovality. The changes in pipeline collapse pressure and collapse mode under different defect parameters are analyzed. The research shows the increase in defect length and the decrease in diameter-to-thickness ratio (D/t) will enhance the influence of defect angle θ on pipeline collapse configuration. And the change of collapse mode will lead to abnormal changes in collapse pressure. With the increase of internal defect length, the collapse mode variation rule is "∞" - "∩" - "P" - "U". A collapse pressure empirical formula for pipeline with chord-cut corrosion defects is obtained. The empirical formula fits well with the numerical simulation results. • A 3D dynamic fluid cavity model for pipelines with chord-cut corrosion defects is proposed. • Two sets of full-scale experiments are conducted on pipelines with chord-cut corrosion defects. • The variation law of pipeline collapse mode under different external chord-cut defect angle θ are analyzed. • The influence of the size of the chord-cut defects on the mode and pressure of pipeline collapse is analyzed. • A collapse pressure empirical formula for pipeline with chord-cut corrosion defects is obtained. [ABSTRACT FROM AUTHOR]

Bulkhead structure is an important support form widely used in ship components, which is not only affected by loads and random factors but also has various failure forms. In this work, a novel reliability-based topology-topography optimization (RBTTO) method is presented to address the bulkhead lightweight and reliability design problems with multi-failure modes. Within the framework of reliability-based design optimization (RBDO), an innovative topology-topography optimization (TTO) method based on material penalization and node activation (MPNA) model and a new reliability assessment strategy considering joint influences of multi-failure modes are proposed. In the TTO approach, the topography deformation nodes are activated by the MPNA matrices according to the element density of SIMP (Solid Isotropic Microstructures with Penalization) to realize the integrated design of topology and topography optimization. The multi-failure reliability assessment strategy is developed, which establishes the most probable point based uncertainty analysis through the probabilistic criterion of system safety and the expert elicitation theory. This RBTTO method is applied to the RBDO of a typical ship bulkhead. The findings show that the bulkhead obtained by RBTTO satisfies the reliability considering uncertain multi-failure modes and exhibits superior overall lightweight and mechanical properties than those of other methods. • Reliability-based optimization for ship bulkheads with multi-failure modes. • Topology-topography optimization using material penalization and node activation model. • Multi-failure mode assessment strategy for reliability evaluations. • Optimized bulkhead satisfies reliability requirements and achieves better lightweight and mechanical properties. [ABSTRACT FROM AUTHOR]

The mooring line failure can cause significant drift, continuous mooring line failures, riser collision and production shutdown, but mooring line repairs and replacements are time consuming, and there is a lack of associated short-term emergency measures. To solve the above problems, a short-term recovery strategy is proposed in this paper to suppress low-frequency motions. The strategy contains a control process consisting of five controllers and a thrust distribution optimization algorithm and a prediction process consisting of the LSTM and Dense neural network. The simulation results show that all five controllers can suppress the low-frequency motion after mooring line failure, and the control efficiency of the controller can be improved and the control force and power consumption can be reduced with the help of the predictive correction algorithm. The combination of the predictor-corrector algorithm and the model-free adaptive controller provides the best comprehensive performance. The direction of the control force and the average propulsion angle of the thrusters are independent of the wave direction and are related to the direction of the failed mooring line. • A short-term recovery strategy is proposed for the adverse consequences after mooring line failure. • The advantages and disadvantages of the five controllers are compared in terms of motion control effect, control force magnitude respectively. • The differences in thruster stability and power consumption between the five controllers are compared. • The change laws of thruster are studied under different directions of wave and mooring line failure. [ABSTRACT FROM AUTHOR]

This study focused on buckling propagation and its arrest in buried offshore pipelines crossing strike-slip faults. A buried offshore pipeline with integral arrestors was analyzed using nonlinear vector-form-intrinsic-finite-element-method-based shell elements and modified soil springs. The effects of different external pressures on structural response were investigated. Integral arrestors with different design parameters were tested to prevent flattening and flipping propagation. The results showed seven or five fluctuations in the flattening parameters with alternating orthogonal directions along the pipeline. These fluctuations led to multi-failure modes, including S-shaped and Z-shaped flexures, two local collapses followed by flattening propagations, and a single local collapse followed by flipping propagation. Integral arrestors should be designed specifically to provide sufficient local bending rigidity in the circumferential direction. The arrest mode of flattening propagation is stopping the earliest fluctuations whilst that of flipping propagation is stopping the earliest fluctuations and depressing the downstream reverse ovalities. For the same arrestor thickness, the smallest effective arrestor length to stop flattening propagation was significantly less than that for flipping propagation. The arrest is temporary because of gross deformation of cross-sections at integral arrestors or the downstream pipes, and secondary failure accidents are likely to occur with any external perturbation. • Buckling and collapse analysis of buried offshore pipeline crossing strike-slip fault using nonlinear VFIFE method. • Multi-failure modes determined by seven or five fluctuations of flattening parameters along the deformed pipeline. • Different arrest modes and effective arrestor parameters for flattening propagation and flipping propagation. [ABSTRACT FROM AUTHOR]

The uncertainty and extremity of ocean conditions make the design of marine structures a challenging process. Reliability-based optimization is a powerful approach to enhance material utilization and ensure system survivability. In this paper, a novel reliability-based design optimization (RBDO) method that combines topology, shape, and size optimization is proposed to achieve the design of marine structures. Within the framework of decoupled sequence optimization and reliability assessment, the developed method employs an innovative multi-technology combined optimization Scheme and a new reliability assessment algorithm. The combined optimization Scheme is established by simultaneously incorporating topology, shape, and size optimization technologies to further improve the lightweighting effect. The advanced arc interpolation algorithm is developed, which dynamically selects the advanced mean value method or arc interpolation method based on the trend judgment criterion to accelerate the calculation for the most probable point in reliability assessments. The RBDO examples of the wind turbine jacket and platform stiffened plate are utilized to verify the proposed method's applicability and validity, and the results are compared with those of other approaches. The findings prove that this RBDO method can effectively address reliability-based design problems of marine structures and is superior in providing lightweighting structures compared with others. • Combination of reliability-based topology, shape, and size optimization. • Advanced arc interpolation algorithm for reliability assessments. • Reliability-based optimization of the wind turbine jacket and platform stiffened plate. • The optimized results ensure reliability while achieving lighter weight. [ABSTRACT FROM AUTHOR]

Abstract The characteristics of the flow-induced vibration (FIV) of two tandem flexible cylinders with or without helical strakes were experimentally investigated in a towing tank. The two cylinder models with a center-to-center spacing of 8.0 D (where D is the smooth cylinder diameter) were allowed to oscillate in both the cross-flow (CF) and in-line (IL) directions. The uniform flow was simulated by towing the cylinder models along the tank. The corresponding Reynolds numbers, defined by the towing carriage velocity and the smooth cylinder diameter, ranged approximately from 800 to 16000. The strake with a pitch of 17.5 D and a height of 0.25 D , which is generally believed to be the most effective configuration for reducing vortex-induced vibration (VIV) of an isolated flexible cylinder in water, was adopted in the experiment. The experimental results, including the displacement responses, frequency responses and mean drag force coefficients, were analyzed by comparing with those of the isolated smooth and straked cylinders. The FIV of the upstream cylinder with helical strakes is in reasonable agreement with that of the isolated straked cylinder, which means that the performance of the helical strakes for suppressing the FIV of the upstream cylinder is almost independent of the downstream cylinder at the selected spacing T = 8.0 D in the experiment. The FIV suppression efficiency of the helical strakes of the downstream cylinder was observed to decrease in both the CF and IL directions due to the unsteady wake from the upstream cylinder. Under certain conditions, the displacement amplitude of the downstream cylinder can even be enhanced by the helical strakes. Moreover, the dynamic responses of a smooth cylinder placed upstream or downstream of a straked cylinder were also investigated for improving the understanding of the FIV behaviors of a two-tandem-cylinder system. Highlights • An experiment was conducted to investigate the FIV of two tandem flexible cylinders fitted with or without helical strakes in a towing tank. • The performance of helical strakes for suppressing the FIV of the upstream cylinder is almost independent of the downstream cylinder. • The FIV suppression efficiency of helical strakes of the downstream cylinder was observed to decrease in both the CF and IL directions. [ABSTRACT FROM AUTHOR]

Real-time prediction of deterministic platform motions in the coming future seconds was essential. The long-short-term memory networks (LSTM) and the fully connected neural networks were employed to establish the neural networks for motion predictions. The predicted results were compared with those calculated by AQWA. The results demonstrated that the predictive model could offer a fast and accurate prediction of the motion response of the semi-submersible platform. The sliding window model was built to predict motion without real input and monitor mooring line failure. The results showed that the predictive model based on the sliding window model can also achieve accurate prediction of platform motion. With the help of the predictive model, a control module written in Python was activated on the basis of the identification of mooring line failure. The control results show that the proportional-integral-derivative (PID) controller was very good at recovering the kinematic performance after mooring line failure and suppressing the continuous failure of the remaining mooring lines. • Predictive models were developed and their accuracy verified. • Motion response prediction without real motion input was achieved by applying sliding window techniques and data reorganisation. • The range of the predictive model used was studied by varying environment parameters. • Diagnosis of mooring line failure was achieved by the fit between the true and predicted response. • The control module was developed in Python and integrated into AQWA to study the recovery of motion after mooring line failure. [ABSTRACT FROM AUTHOR]

Although the lateral oscillation of steel catenary risers (SCRs) contributes to trench production, traditional riser-seabed models cannot simulate the process. This study proposes a modified lateral riser-seabed interaction model and integrates it into the SCR model based on the vector form intrinsic finite element (VFIFE) method to simulate the trench formation process. The effects of the loading history, extra soil berm resistance, and cyclic vertical riser-soil interactions were included in the soil model. A 3D trench could be developed under the cyclic out-of-plane movement of the SCR. The trench formation process, seabed resistance distribution, axial tension, bending moment, and stress range distribution of the SCR were discussed in detail to illustrate the effects of the soil model. Different loading amplitudes and nonlinear soil parameters were chosen to determine their influence on the trench shape and distribution of the horizontal seabed resistance. The developed trenches were then inserted into the fresh SCR model before executing the dynamic analysis, in which only the cyclic heave motion was applied to the hang-off point. The simulation results showed that the trenches produced by cyclic sway motions beneficially influenced the fatigue life of the SCR. However, the effect would differ with the profiles. • A modified lateral riser-seabed interaction model was proposed and integrated into the VFIFE model. • A 3D trench can be developed under the cyclic out-of-plane movement of the SCR. • The trench formation process and the mechanical behavior of SCR were analyzed. • Sway motion amplitude and soil parameters can affect the trench shape. • Sway-motion-induced trenches beneficially influenced the fatigue performance of the SCR. [ABSTRACT FROM AUTHOR]

Mooring disconnections occur frequently in a floating production system (FPS), probably resulting in the instability of structures or even offshore accidents. Therefore, this paper investigates the influence of mooring disconnections on the response of a SEMI system under regular waves and irregular waves. Two failure modes were performed and compared according to the disconnected positions of mooring lines: the top breakage and bottom unlatch. A numerical model of a fully coupled hull/mooring/riser system was established through a self-developed program to obtain the detailed behavior of each component. The results indicated that the platform motion became a 4-period nonlinear response containing a super-harmonic motion when three moorings unlatched. The disconnected moorings produced a large deformation due to the sliding of anchor points. The riser tension exhibited robustness under mooring failure because of its low tension level and the robustness of platform heave. Compared with bottom unlatch conditions, the platform under top breakage conditions showed a larger-amplitude fluctuation and a slightly farther offset particularly three moorings all disconnected. The total mooring force under top breakage conditions would lose more fairlead tension at the failure time, thus inducing a lower mooring force for the platform. • The dynamic behavior of a coupled platform/mooring/riser model was described through a self-developed program. • The response of SEMI system under bottom unlatch conditions was calculated in details. • The motion response and mechanical characteristics of lazy-wave risers in the SEMI system were described. • The dynamic characteristics of the unlatched mooring lines before and after the disconnection moment were reported. [ABSTRACT FROM AUTHOR]

Floating production system (FPS) may face various risks during their service life, among which mooring failure is one of the most disastrous events. In order to assess and quantify the resilience of FPS under mooring failure, it is essential to use reliable restoration fragility models. But for now, there are no available specific recovery models. The main barrier is uncertainty of nature and complexity of recovery actions. This paper aims to take uncertainty of recovery schedule into account and propose recovery models for mooring failure accident. Based on literature review and a detailed questionnaire that elicits knowledge from experts, the restoration schedule and uncertainty are defined and quantified. Through correlated schedule uncertainty analysis model (CSUAM), uncertainty influence on recovery model is measured. And after Monte Carlo Simulation (MCS), Gaussian mixture model (GMM) and normal distribution model are introduced to establish probabilistic recovery model of different damage level. Recovery models are then presented and validated for a certain FPS in South China Sea, including restoration task scheduling, seasonal influence, spare availability and two-mobilizations for different damage levels. • Probabilistic fragility recovery models for FPS under mooring failure is proposed. • Correlated schedule uncertainty analysis model is generated. • Standardization of damage level based on progressive failure modes of FPS under mooring failure. • The effect of physical reality is considered. [ABSTRACT FROM AUTHOR]

As a frequently occurring and widely distributed environmental load, an internal solitary wave (ISW) always appears with a large amplitude and may result in the instability of deepwater structures and offshore accidents. In this study, a self-developed program was established to describe the dynamic behavior of a coupled platform–mooring–riser system. The response of the platform is based on the rigid body dynamics theory, that of the risers/mooring lines is obtained by the lump mass method, and the coupled effect between them is achieved by a semi-coupled analysis. Two types of riser configurations, steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWRs), in the coupled system were compared under only ISWs and combined excitation of ISWs and random waves. The numerical results were proven to be reliable through validation against previous literature. The results show that different riser configurations have little influence on the platform response or mooring top tension. Both riser configurations exhibit a second-order deformation mode along the riser length and a half-wave shape with time. Additionally, a mutation in the horizontal tension, shear force, and bending moment occurs near the ISW interface. Compared with SCRs, LWRs always produce larger motion characteristics and responses with time differences owing to their complex configuration. • A coupled platform-mooring-riser numerical model has been described through a self-developed program. • The responses under only ISW and combined excitation of ISW and RW are calculated. • Comparisons have been made between two kinds of riser configurations in the coupled system: SCR and LWR. • The mechanism of tension, shear force and bending moment in both riser configurations have been discussed in detail. [ABSTRACT FROM AUTHOR]

Pipe bend is a common junction in pipeline system. In this study, the post-buckling behavior of deep-sea thick-walled pipe bend under external pressure is studied by numerical simulation. A 4-node tetrahedron solid element based on vector form intrinsic finite element method (VFIFE) is introduced to simulate the whole buckling process. First, the VFIFE models of the thick-walled pipes are built and the results of the VFIFE method, traditional finite element method, and the experiments from other publications are compared to verify the proposed method. Then, collapse modes and collapse mechanism of the pipe bend when buckling propagates to the elbow under dynamic and quasi-static conditions are analyzed. Finally, the influence of different geometric parameters on the crossover pressure of the pipe bend is studied. These results can be used to support the buckling arrest design of pipe bends. • Analysis of post-buckling behavior of pipe bend based on VFIFE tetrahedral solid element. • A self-contact algorithm for the collision of the inner wall of pipe based on VFIFE tetrahedral solid element. • The collapse mode and collapse mechanism of pipe bend is analyzed. • The influence of transition zone on post-buckling critical pressure is discussed. • Effect analysis of the relative bend radius, the diameter–thickness ratios, and the bend angle. [ABSTRACT FROM AUTHOR]

Complex marine environment causes significant variations in the dynamic loads and soil parameters that affect the design of a monopile. Despite these uncertainties, a multi-objective optimization technique is developed to produce the optimal designs. This optimization is based on the robust design concept, in which the design robustness , cost , and safety are considered as the optimal objectives. The responses of a monopile to arbitrary dynamic loads are initially determined using a transient finite element technique, which considers unknown soil parameters. These responses include the maximum rotation and displacement at the midline, maximum axial stress, and fatigue damage. Next, the variations (regarding standard deviations) in these responses, which are determined by the Monte–Carlo simulations, are adopted to define the design robustness. Finally, the cost and safety requirements of the monopile are evaluated, and robust design optimization is performed. The aforementioned robust design protocols are demonstrated with a 5 MW offshore wind turbine, which produces the optimal design. Therefore, the optimal designs for the monopile are investigated based on various safety requirements. Moreover, the design parameters that promote the design robustness with optimum efficiency are studied and proposed. • The uncertainties in dynamic loads and soil parameters are considered simultaneously in the design of a monopile. • The robust design considers design robustness in different limit states, safety, and costs simultaneously in optimization. • Transient finite element analyses and MCS are incorporated to simulate the dynamic responses of the monopile. • The most efficient design parameters for improving design robustness are explored based on the outcome of a case study. [ABSTRACT FROM AUTHOR]

Long-distance offshore pipelines often inevitably pass through seismic faults that may threaten pipeline safety. A buckling analysis was conducted on offshore buried pipelines crossing active strike-slip faults using nonlinear VFIFE shell elements and modified soil springs. With respect to practical pipeline performance criteria, deformation responses under three scenarios are examined, including the seismic response with different initial pressure conditions, post-earthquake recondition for pressurized pipelines, and post-earthquake resumption for unpressurized pipelines. The results have showed multi-failure modes, including over bending with excessive tensile strain, over bending with obvious depression deformation on the compressive side of the bends, and sequential local collapses accompanied by buckling propagation along the pipeline. For pressurized pipelines, the post-earthquake recondition has a compression effect and leads to greater sectional distortions, even local collapse accompanied by buckling propagation. For pipelines with the maximum flattening parameter smaller than 15.00% at zero-pressure, critical points of the first local collapse have a linear distribution. For unpressurized pipelines, the post-earthquake resumption improves the sectional deformation but intensifies the axial tensile strain. An unpressurized pipeline with a local collapse gains a V-shape dent during the final pressurization. The results are useful for developing performance-based seismic design methodologies for offshore pipelines. • Buckling analysis of offshore buried pipeline cross strike-slip faults using VFIFE shell elements and modified soil springs. • Performance-based design methodology with emphasis on different loading histories of strike-slip fault and pressure loading. • Multi-failure modes including over bending and sequential local collapses accompanied by buckling propagation. • Useful distribution characteristic of the first local collapse critical points for pipelines across strike-slip faults. [ABSTRACT FROM AUTHOR]

The dynamic collapse of pipelines in deep water can create pressure pulses that affect the performance of surrounding structures. In this study, the pipe implosion process was analysed using the Lagrangian–Eulerian method. The numerical model was calibrated using the experimental data, and the responses of the pipelines and surrounding fluid during pipeline collapse and buckle propagation were obtained. The fluid–structure interaction mechanism of the composite arrestors attenuating the pressure pulse was investigated. The simulation results showed that the implosion of a pipeline was a transient high-energy dynamic event. The fluid surrounding the pipe rushed towards the pipe surface at a high velocity during implosion and dynamic buckle propagation, which resulted in a spatial change in pressure distribution. Notably, the unidirectional fibre reinforced polymer (FRP) arrestor had a positive effect on reducing the peak pressure of the pulse wave. Furthermore, the effects of different arrestor spacings and materials on pipe buckling and implosion pressure were discussed, and the design of FRP arrestors was optimised. • The collapse and implosion process is analysed via the Lagrangian–Eulerian method. • The pressure pulse characteristics with and without the FRP arrestors are analysed. • The effect of arrestors on the pressure pulse depends on materials and spacing. • The design of the FRP arrestors was optimised. [ABSTRACT FROM AUTHOR]

As a primary catastrophic factor, an internal solitary wave (ISW) is typically characterized by a large amplitude and a long duration, which can cause severe damage to deep-sea platforms due to large drift or the rupture of mooring lines. In this paper, a secondary development of ANSYS AQWA was performed to study the dynamic response of the platform system under the combined action of surface wave load and ISW load. Then, mooring line failure under the ISW was analysed, and the differences between the simultaneous and progressive failure of the two mooring lines were compared. Numerical simulation results showed that under the action of the ISW, the movements of the platform and the tensions of mooring lines markedly increased. The low-frequency slow-drift motion under the combined action of the ISW and a regular wave was less than that under the action of the ISW. Mooring line failure worsened the movement of the platform and the mooring line section tension under the ISW. However, the transient responses of the platform caused by the progressive failure of two mooring lines were less than those caused by the simultaneous failure. • The effects of the ISW vertical pressure-difference force and viscous force on the platform are calculated. • The motion under the combined action of the regular wave and ISW and under the action of the ISW alone is compared. • The influences of the ISW force of the mooring system on the platform motion and mooring line tension are analysed. • The mooring line failure process is calculated under the ISW environment and it is compared with the intact condition of the mooring system. • Comparisons are made between the simultaneous failure and progressive failure of the two mooring lines. [ABSTRACT FROM AUTHOR]

The oscillation of steel catenary riser (SCR) would develop a trench in a few months, while the trench could affect the structural behavior of the riser in turn. However, the influence of the trenches on the fatigue damage of SCR is still controversial. In this study, the SCR model is established using the vector form intrinsic finite element (VFIFE) method, integrated with the nonlinear seabed model to develop the trenches. Different static platform offset distributions, low frequency (LF) surge motions, and the soil parameter and heave motion amplitude combinations are selected to investigate the influences on the trench formation. Dynamic analyses are performed to determine the effects of the trenches on the mechanical behavior of SCR based on the VFIFE model, with the predefined trenches inputted previously through the switching method and the inserting method. Simulation results prove that the trenches developed only by heave motion or including the LF surge motion have a positive effect in reducing the fatigue damage, while the trenches considering static platform offset have little influence. Furthermore, the effects of shear strength distribution of trench surface, trench depth, and trench position on the dynamic performance of SCR are discussed. • Performance of SCR is analyzed via the vector form intrinsic finite element method. • Seabed trenches considering static offset and surge motion of platform are developed. • Trenches would help to reduce the fatigue damage of SCR using the inserting method. • Effect of trench depth and position on the behavior of SCR depend on trench profile. [ABSTRACT FROM AUTHOR]

The dynamics of a steel catenary riser (SCR) are highly nonlinear and strongly affected by the interaction between riser and seabed. Possessing the advantages of robustness and high efficiency, the vector form intrinsic finite element (VFIFE) method is applied to solve this problem. The nonlinear hysteretic riser-seabed interaction is integrated into the model. Simulation results are compared to the commercial software Ocraflex, and high similarities prove the capability of VFIFE to simulate the dynamics of SCR on the nonlinear seabed. In dynamic analysis, a larger mass damping coefficient should be applied in the touchdown zone (TDZ) and flowline part to increase the stabilization. In contrast, the coefficient in the catenary part has only minor influences on the behavior of SCR. To improve the computation efficiency, the effect of using coarse meshes with different densities in the catenary part has been investigated, while the fine meshes are applied in the TDZ and flowline part. In the dynamic analysis of TDZ, the distribution of seabed reaction force and stress range are present and discussed. Furthermore, the effects of different loading parameters and hanging-off positions on the trench profiles and fatigue damage of SCR have been studied in detail. • Vector form intrinsic finite element method is used to model the steel catenary riser. • Nonlinear hysteretic riser-seabed interaction is integrated into the model. • The dynamic performances of the riser within the touchdown zone are analyzed. • Effect of platform location on the trench profile and fatigue damage is studied. [ABSTRACT FROM AUTHOR]

Tension-leg platform (TLP) is a very complex rigid-flexible coupling multi-body system. The failure of some tendons will induce continuous failure and lead to the platform capping. In this paper, a mechanistic study of a typical TLP under tendon breakage is addressed. The time-domain nonlinear results for the floating hull coupled with mooring lines were obtained to analyse the equilibrium position and amplitude of the steady-state response. The change laws of the balance positions of the motions and the reasons for the changes in the movement amplitudes were explored. By changing the wave parameters, the change rules of nonlinear motions and the damage caused by the change in the natural period were investigated. • Studied the change law of the stiffness of the platform system under five different tendon(s) fracture cases. • The nonlinear motion and nonlinear load of the platform are analysed qualitatively and quantitatively. • The influence of frequency change on platform movement after single tendon failure is analysed. [ABSTRACT FROM AUTHOR]

Slip distributions have been proved to have significant influence on the flow-past-cylinder problems. However, rare studies focus on the square cylinders. To obtain better understanding of the effect of slip distributions on the flow-past-square-cylinder problems that widely appear in the engineering fields, the flows past a single square cylinder, and two tandem square cylinders under different slip distributions and gap ratios are numerically investigated using the lattice Boltzmann method at low-Reynolds-numbers (25 ∼ 200 , mainly at 100). The flow structures and drag properties (including the friction drag and the pressure drag) are analyzed. Various slip distributions are found to have minor impacts on the drag reduction of a single cylinder (3.5% in maximum) but larger impacts on that of the tandem cylinders, especially the downstream cylinder (100% in maximum). In addition, flank side slip is found to have important contributions to the drag reduction. • The effect of slip distributions on the flow past a square cylinder is investigated. • The effect of slip distributions on two tandem square cylinders is explored. • The total drags (friction & pressure drag) and the lift drags are detailed examined. • Drag reduction between different slip distributions are analyzed in details. [ABSTRACT FROM AUTHOR]

Reliability assessment (RA) is pivotal to enhancing the efficiency and robustness of reliability-based optimization. Classical RA algorithms suffer from inefficiency and non-convergence problems such as bifurcation, periodic oscillations, and chaos. In this paper, a bi-directional adaptive conjugate gradient (BACG) algorithm incorporating a newly developed concave vs. convex decision criterion (CCDC) is suggested. The BACG technique is harmoniously fused with the sequential optimization and reliability assessment (SORA) to expedite design timelines while simultaneously enhancing precision. Within the BACG, an adaptive acceleration factor is presented to dynamically activate the positive and negative acceleration of the most probable point (MPP) search, specifically tailored for convex and concave performance functions. The concavity/convexity of the performance function is adjudicated via the CCDC using two adjoining MPPs. The validation of the convergence of BACG is established through rigorous mathematical formulation. A comprehensive study is conducted to assess the robustness, stability, and efficiency of BACG via matching with eight prominent inverse RA methods. The study encompasses six inverse reliability problems, one reliability-based design optimization (RBDO), and two reliability-based topology optimization (RBTO) projects involving a simply supported beam and a submarine pressure hull. Lastly, the BACG is utilized to perform the RBTO design for a jacket offshore platform. • A novel bi-directional adaptive conjugate gradient (BACG) method is first suggested. • BACG features advanced robustness, stability, efficiency, and convergence. • A new concave vs. convex decision criterion is first developed. • First RBTO of the submarine pressure hull using SORA with BACG. [ABSTRACT FROM AUTHOR]

The Revolving FloatingCrane (RFC) is a vital engineering vessel for offshore lifting operations, with its operational safety and stability being of utmost importance. Nevertheless, potential mismatches in the lifting trajectory and ballast water allocation during manual operations may result in low efficiency, instability, and energy dissipation. Thus, a novel collaborative matching design (CMD) method of the lifting trajectory and ballast water allocation is proposed for the RFC intelligence. By establishing one-to-one correspondences between the lifting trajectory and the ballast water allocation, the CMD method is first developed based on the point to point (PTP) theory. Then, the dynamics model of lifting operations and CMD optimization model are established. To explore the superiority, the proposed CMD method is compared with the conventional method, finding that the CMD method surpasses the conventional sequential design method in terms of lifting operation time (11.20% reduction), total energy consumption (16.50% reduction), ballast water allocation (17.84% reduction), and decision-making time (54.50% reduction). Finally, numerical and experimental validations regarding various lifting scenarios are conducted to certify the efficacy and feasibility of the proposed CMD method, underscoring its potential to realize the optimal matches between RFC's lifting trajectory and ballast water allocation. • A collaborative matching design method for RFC offshore operations is proposed. • The optimization model of the CMD method for the RFC is established. • Superiorities of the CMD method over others is verified by a case study. • Efficacy of the CMD method is validated by numerical and practical experiments. • The CMD method greatly improves the overall operating performance of the RFC. [ABSTRACT FROM AUTHOR]

The presented work studied the structural response of the hydraulic pneumatic tensioner (HPT) in a TLP-TTR system with failure of the tensioner cylinder. A fully coupled hull-tendon-TTR-tensioner model was established in AQWA to simulate the failure numerically. A specific HPT was modeled by considering 4 cylinders and the real-time stroke of each piston. A set of formulas was proposed to calculate the real-time tension including different components, e.g. Stribeck friction, in the tensioner. A riser array including 6 independent production TTRs and their tensioners was also modeled. The production TTR model was stacked up by different specific riser joints. The hydrodynamic force acting on the hull was obtained by using the 3D potential flow theory. The real-time tensions on different tensioner cylinders were obtained by using an in-house-developed program. Different environmental conditions, including a calm sea, regular waves, and extreme sea states, were considered in the simulations. In the results, the behaviors of different cylinders of the failed tensioner were presented. The results show that when an accidental local failure of the HPT occurs, the tension and stroke responses are still far from the designed-limits to induce a progressive failure. • A fully coupled TLP-TTR system is established to simulate the accidental local failure of a hydraulic pneumatic tensioner. • A specific HPT is modeled by considering 4 cylinders and each piston stroke. A riser array including 6 TTRs is also modeled. • An in-house developed FORTRAN subroutine is applied to simulate the multi-cylinder tensioner and its local failure. • A residual response is found to propagate several cycles through the failed tensioner and riser. • It is proved that an accidental local failure of the HPT can hardly develop into a progressive failure of other cylinders. [ABSTRACT FROM AUTHOR]

In deepwater oil and gas field development, a Tension Leg Platform (TLP) consists of a hull, tendons, and top tension risers (TTR). When one or more tendons failure due to extreme sea state, a TLP will be under the working condition which is very different from its design condition, showing structural instability and transient effect. To enhance the safety and global stability of whole TLP system, obtaining failure consequences is necessary. By 3D potential flow theory and panel element method, a series of mooring analyses are performed for the target TLP to predict its coupled hydrodynamic response after tendon one-time failure and, in particular, during progressive failure of several tendons. Different directions of environment load that including extreme wind, wave and current and tendon failure positions are considered. In the results, the motion response of the hull, the force from residual tendons and TTRs are presented. • A progressive mooring failure process is calculated for a TLP hull-tendon-riser coupled system under extreme environment. • TTRs' response is analyzed after the local mooring system breakdown and it is compared with tendons' response. • Comparisons are made between the disconnections simultaneously and progressively of multi-tendon. (The fourth point:) A crucial factor in conversion from local mooring failure to global failure is obtained. [ABSTRACT FROM AUTHOR]

Abstract Multiple flexible cylindrical structures arranged in tandem are commonly used in various fields of engineering. Up to now, limited information is available about the flow-induced vibrations (FIVs) of more than two tandem flexible cylinders. Although some FIV characteristics of multiple tandem flexible cylinders can be studied by considering a tandem flexible cylinder pair, there are other characteristics remain unrevealed which require further investigations. This paper presents an experimental study on the FIVs of three and four flexible cylinders in tandem arrangement. The flexible cylinder models are identical with a mass ratio of 1.9 and a length-to-diameter ratio of 350. The centre-to-centre spacings between the cylinders are kept constant as 6 D. The towing speed ranges from 0.05 m/s to 1 m/s and the corresponding Reynolds number lies in the range of 800–16 000. It is found that the presence and vibrations of the downstream cylinders have almost negligible effects on the responses of the most upstream cylinder. Wake-induced vibration (WIV) type responses are observed at high reduced velocities for the downstream cylinders. Due to the shielding effects of the upstream cylinder/cylinders and the influence of the second WIV regime, the transitions between adjacent frequency synchronisation regions and neighbouring vibration modes of the downstream cylinders are delayed to higher reduced velocities. Most of the cylinder responses are dual resonant except for the most downstream cylinder in the three-cylinder system which has identical dominant frequencies in the in-line and cross-flow directions. Combined standing wave and travelling wave responses and standing wave dominated responses are identified for three and four tandem flexible cylinders undergoing FIVs. Highlights • Towing tank tests of the FIVs of three and four flexible cylinders in tandem arrangement are performed. • The FIV characteristics of the three-cylinder system are obviously different from those of the four-cylinder system. • These results are meaningful to the design of engineering structures involving clusters of long flexible cylinders. [ABSTRACT FROM AUTHOR]

Floating Production, Storage and Offtake (FPSO) is one of the key components of offshore oil development and the single point mooring system is the key technology affecting the safety of the FPSO. Therefore, risk analysis is useful in preventing the failure of the FPSO single point mooring system. However, traditional techniques fall short in terms of quantitative assessment, dynamic control, and handling uncertainty. This study provides a risk analysis approach that can be used to assess the FPSO single point mooring system safety risk. The Bayesian Network and interval type-2 fuzzy sets are used to overcome the difficulties in gathering and evaluating the data required for the calculation of the FPSO single point mooring system safety hazards. The proposed approach allows for the identification of the primary underlying causes of these occurrences. The effectiveness of the proposed methods for risk analysis of FPSO single point mooring systems is proved using a case study. According to the findings, it is believed that this study could be useful in providing a thorough and all-encompassing evaluation of the safety risk in multi-process systems and in suggesting a strategic strategy for reducing the FPSO single point mooring system safety hazards. • Bayesian Network and interval type-2 fuzzy sets methods are proposed for the FPSO single point mooring system. • The PA operator is introduced in Fuzzy Bayesian Network analysis to aggregate the opinions of domain experts. • An improved probability conversion method is proposed for expert elicitation. • The results show the outperformance of proposed method in providing more detailed and realistic results. [ABSTRACT FROM AUTHOR]

Under the influence of various factors, the submarine pipelines will be damaged, resulting in oil and gas leakage accidents. It is essential to perform risk assessments to prevent accidents. However, accurate and sufficient historical data usually can not be obtained. To solve this problem, this study proposes a probabilistic Petri net (PPN) method based on intuitionistic fuzzy evidential reasoning (IFER), which can determine the failure probability of basic events through domain expert experience, and quantitatively evaluate the probability of submarine pipeline leakage failure. The main contributes include: according to DNV-RP-F107, a group of intuitionistic fuzzy numbers is defined and an improved probability conversion method is proposed for defuzzification; an IFER approach is proposed, which uses belief structures as expert linguistic terms and transforms them into IFNs to deal with the uncertainty of expert evaluation; a probabilistic Petri net method is proposed to quantitatively calculate the failure probability of top event and identify the critical events. Finally, the proposed intuitionistic fuzzy probabilistic Petri net (IFPPN) method is illustrated by a case study. By comparing with the existing methods, the feasibility of this method is validated, and the evaluation results can provide a reference for the safe operation of the pipeline system. • An intuitionistic fuzzy probabilistic Petri net methodology is proposed for risk assessment. • The intuitionistic fuzzy evidential reasoning approach is proposed for expert evaluation. • An improved probability conversion method is proposed for defuzzification. • A failure probability analysis of the submarine pipeline leakage is conducted to illustrate the applicability of proposed method. • According to importance analysis, the critical basic events are identified. [ABSTRACT FROM AUTHOR]

Frequent and intense internal solitary waves (ISWs) and ISW packets pose serious threats to the in-service safety of deep-water risers in the South China Sea. In this study, a deep-water steel catenary riser (SCR) under multiple ISWs was investigated along with the dynamic characteristics of the touchdown zone (TDZ), including displacements, pipe-soil interactions, tensions, and bending moments. A nonlinear numerical model was developed with a novel vector form intrinsic finite element method and improved pipe-soil interaction model that considers soil stiffness degradation and trench formation. The ISW flow velocity field was solved using the mixed Korteweg-de Vries (mKdV) equation and the hydrodynamic forces were determined using Morison's equation. The effects of the ISW amplitudes, riser configurations, seabed stiffnesses, and pipe-soil models on the structural dynamic responses were investigated. The results showed that the ISW packets rapidly increased the penetration depth of the TDZ. Trench formation changes the bending moment response of the TDZ in subsequent ISWs, increasing the response of the front section and decreasing the response of the latter section. However, its effect on the riser tension is limited. The steel lazy wave riser (SLWR) has significantly larger displacement and bending moment responses in the TDZ than SCR because of the different riser tension and configuration. Different pipe-soil interaction models exert a significant influence on the penetration depth and the variation of the bending moment in the TDZ. • Dynamic analysis of SCRs under multiple ISWs considering soil stiffness degradation and trench formation. • A nonlinear numerical model based on Vector form intrinsic finite elements and an improved pipe-soil interaction model. • Quantitative analysis of displacements, pipe-soil interactions, tensions, and bending moments of the touchdown zone. • Impact analysis of ISW amplitudes, seabed stiffnesses, pipe-soil models, and riser configurations. [ABSTRACT FROM AUTHOR]

Fluid-structure interactions (FSI) of multiple cylindrical structures frequently occur in crossflows. Due to the different wake interference regions of flexible cylinders with different diameters, the interaction between multiple cylinders is complicated and sensitive to the centre-to-centre spacing ratio and diameter ratio. Limited information has been presented on the flow-induced vibration (FIV) of two unequal-diameter flexible cylinders in a tandem arrangement. In this paper, an experimental investigation was carried out in a towing tank to study the FIV of two tandem flexible cylinders with different diameters that can vibrate in both the cross-flow (CF) and in-line (IL) directions. There were two IL spacing ratios (T / d = 12.0 and 6.0, where T is the centre-to-centre distance between two cylinders, and d is the diameter of the small cylinder), and the diameter ratio (d / D , where D is the diameter of the large cylinder) was nearly 0.5. The aspect ratios/mass ratios of the small cylinder and the large cylinder were 350/1.90 and 181/1.47, respectively. The towing velocity varied from 0.05 to 1.00 m/s with an interval of 0.05 m/s. The Reynolds numbers for the small and large cylinder were 800–16000 and 1600–32000, respectively. The CF vibration of the upstream large cylinder with T / d = 6.0 was suppressed at Vrs = 16.28–18.79, whereas the CF vibration of the large cylinder with T / d = 12.0 was not influenced. The downstream cylinder exhibited more complicated behaviour due to the significant wake effect from the upstream cylinder, which is consistent with the two equal-diameter cylinders. The FIV response of the downstream small cylinder was more sensitive to the IL spacing ratio. Because of the wake shielding effect of the large cylinder, the downstream small cylinder vibrated at low IL dominant frequencies, and the IL to CF frequency ratio (f IL / f CF) was approximately 1.0. Moreover,wake-induced flutter was observed on the downstream small cylinder. • An experimental investigation was carried out to study the FIV of two tandem flexible cylinders with different diameters. • When the large cylinder is located upstream, the small cylinder has a slight effect on the CF and IL FIV responses. • The downstream cylinder exhibits more complicated behaviour due to the significant wake effect from the upstream cylinder. [ABSTRACT FROM AUTHOR]

The effects of combined external pressure and cyclic bending on the pipe buckling mechanism are investigated by experiments and finite element analysis. Ten experiments are conducted and it is found that the loading paths of external pressure and cyclic bending have a great influence on the buckling pressure of thick-walled pipes. The loading path of pressure followed by cyclic bending (p → ck) is more dangerous than that of cyclic bending followed by pressure (ck → p). The material properties of kinematic hardening, loading application and mesh convergence are taken into consideration in rigorous FE models. The reason of the critical loading path is revealed from the aspects of section ovality and stress-strain accumulation. Sensitivity analysis is conducted on the loading patterns and axial tension. With the application of axial force, the pipe laying process is simulated more precisely which shows that pipe buckling capacity is more sensitive to the bending cycles in the presence of axial force. The comparison between numerical results and present specification shows that the API specification is conservative in predicting the collapse pressure of thick-walled pipes (D/t = 15–21) whose underestimation is about 11%~37% under the effect of cyclic bending. • Ten experiments and numerical models are conducted to investigate the different loading paths. • The critical loading path (p.→ ck) is determined which is analyzed from section ovality and hoop strain accumulation. • The effects of cyclic bending are reduced on the pipe collapse capacity with the presence of cyclic hardening behavior. • Quantification of the underestimation of the present API standard is compared with the numerical model. [ABSTRACT FROM AUTHOR]

Flow-induced vibration (FIV) occurs in deep-water engineering when multiple cylinders interact with each other at a closer range and leads to significant fatigue damages. Nevertheless, many studies have focused on the fatigue damage caused by vortex-induced vibration (VIV). Using a group of flexible cylinders is more common in offshore engineering and has attracted extensive concern; however, the features of FIV fatigue damage remain unclear. In this paper, the experimental data of two side-by-side flexible cylinders with high aspect ratios (length to diameter, L/D = 350, where L is the cylinder length and D is the cylinder diameter) and low mass ratios (total structural mass to displaced fluid mass, m * = 1.90) are utilized to calculate fatigue damages based on S-N curves and Miner's rule. Four spacing ratios (centre-to-centre distance to cylinder diameter, S/D = 3.0, 4.0, 6.0 and 8.0, where S is the centre-to-centre distance) are adopted to investigate their effects on fatigue damage. The results are presented in terms of strain responses, fatigue damage distributions and the maximum fatigue damages. The cross-flow (CF) fatigue damages of the two side-by-side cylinders remain almost consistent with that of the isolated cylinder at most of the reduced velocities, while the in-line (IL) fatigue damages are more than 2 times greater than that of the isolated cylinder when S/D = 3.0 and 4.0. In addition, fatigue damage of the two side-by-side cylinders is more serious than that of the isolated cylinder in the IL direction when S/D = 8.0 owing to considerably vigorous interactions. • The fatigue damages of two side-by-side flexible cylinders subjected to FIV were calculated and discussed. • CF fatigue damages of the two cylinders remain almost consistent with that of the isolated cylinder at most of the reduced velocities. • For S/D = 8.0, the enlargement effect still exists and causes more severe fatigue damage in the IL direction. [ABSTRACT FROM AUTHOR]

The flow-induced vibration (FIV) of multiple flexible cylinders typically involves multi-mode and complex flow-structure interactions. The FIV characteristics of two staggered flexible cylinders with unequal diameters are experimentally studied in this paper. The two flexible cylinders have aspect ratios of 350 and 181 and mass ratios of 1.90 and 1.47 and are free to oscillate in both the cross-flow (CF) and in-line (IL) directions. According to the relative positions of the two cylinders, 10 different staggered arrangements are studied (P/d = 6, where P is the centre-to-centre spacing and d is the small cylinder diameter; α = 15°, 30°, 45°, 60°, and 75°, where α is the position angle of the downstream cylinder). The multi-mode FIV behaviours of the two staggered flexible cylinders with unequal diameters were investigated with respect to various aspects, e.g., strain and displacement responses, dominant frequencies and dominant modes. Due to the presence of the small cylinder, the CF vibration of the large cylinder was suppressed at α = 15° and 30° when the large cylinder was upstream or downstream. Furthermore, the CF and IL response amplitudes of the small cylinder were significantly enhanced by flow interactions at various position angles. When the small cylinder was located downstream, the FIV behaviours were most greatly affected at small and large position angles (α = 15° or 75°). In addition, the wake-induced flutter phenomenon was observed in the IL FIV response when the small cylinder was downstream. However, this phenomenon was not observed when the large cylinder was located downstream. • The FIV characteristics of two staggered flexible cylinders with unequal diameters were experimentally studied. • Due to the presence of the small cylinder, the CF vibration of the large cylinder was suppressed at α = 15° and 30°. • The wake-induced flutter phenomenon was observed in the IL FIV response when the small cylinder was downstream. [ABSTRACT FROM AUTHOR]