The concept of increasing the damping ratio to decrease the response amplitude is applied to the highly nonlinear problem of the vertical installation of subsea equipment. It is known that, depending on the characteristics of the equipment and of the excitation, the installation cable can become slack, which is succeeded by elevated tension peaks known as snap loads. Due to the nonlinear aspects of the installation, such as the cable stiffness becoming null when the cable is not stretched, different dynamic behaviors can occur when the slackness phenomenon happens. This work shows that increasing the damping to cause a double-periodic response to become quasi-harmonic can greatly reduce the tension peaks on the cable. However, further increasing the damping past this threshold only results in slight decreases in the tension peaks. Thus, this work proposes the use of the damping ratio capable of causing the response to become quasi-harmonic, which is referred to as the safe damping factor, to increase the safe operation window of the installation.
The present article investigates the collapse performance of steel pipes fabricated by the high-frequency welding (HFW) method. Two 16-in. outer diameter HFW line pipe specimens are subjected to full-scale external pressure testing in order to determine their collapse pressure and propagation pressure. Prior to testing, one of the pipes has been externally coated, which introduced a mild heat treatment to the pipe, while the other pipe remained intact (noncoated). The full-scale test procedure is modeled with a computationally efficient finite element methodology, which accounts for the actual geometry and material properties, and employs a simple and efficient technique for simulating pipe collapse and buckle propagation. The numerical results compare very well with the full-scale experimental results, in terms of the values of collapse and propagation pressure, the collapsed shape and history of local strains at critical locations. Both experimental and numerical results show a very good performance of HFW pipes against collapse, indicating that those pipes can be strong candidates for offshore applications. The analytical procedure for predicting the collapse and propagation pressure in DNV-ST-F101 is also evaluated and shows that its conservative predictions are improved if an appropriate value of fabrication factor is used.
The stress concentration factor (SCF) for a hotspot at the root of a pipe girth weld due to high-low misalignment offset (hi/lo) is used in fatigue and fracture assessments, since some such offset is inevitable due to fabrication and welding alignment tolerances. Where the hi/lo is not already accounted for by the SN fatigue resistance curve used, an SCF is needed. Following established practice, the SCF is determined to account for bending stresses across the wall thickness of the pipe, but does not account for stress concentrations arising from the reentrant corners that can arise at both the root and the cap of the girth welds. This SCF arises from a linear distribution of stress across the wall thickness, which is statically equivalent (in terms of membrane force and bending moment) to the actual stress distribution. A simple approximation to determine this SCF using the line of tension from axisymmetric shell theory and considering the actual details of the geometry at the girth weld leads to the same result as in the Recommended Practice DNV-RP-F108 at the cap, but at the root, the value of SCF-1 is found to be more than double that from DNV-RP-F108. The result is confirmed by axisymmetric finite element analysis.
Thermoplastic composite pipes (TCPs) offer superior corrosion resistance, fatigue performance, low weight, and spoolability compared to steel alternatives. A typical TCP wall comprises an inner thermoplastic liner, a continuous fiber-reinforced unidirectional (UD) thermoplastic tape reinforcement layer, and an outer thermoplastic jacket, thermally fused into a fully bonded structure. A theoretical model based on three-dimensional anisotropic elasticity, complemented by finite element analysis, is developed to quantify through-thickness stress distributions under internal pressure. Burst pressure-defined as the maximum sustainable internal pressure-is predicted by applying the Hashin fiber tensile failure criterion to the computed stress state. Parametric sensitivity analysis reveals that inner diameter, reinforcement layer thickness, and UD tape longitudinal tensile strength are the dominant factors. Focusing on the optimal +/- 55-deg winding architecture, a burst pressure database is generated to derive a compact empirical predictive formula. Validation against experimental results for glass fiber-reinforced pipes demonstrates the formula's accuracy. The resulting formulation provides a rapid and practical tool for the preliminary design and screening of TCPs.
It is well recognized that girth welds of carbon steel pipe lined with a thin layer of a corrosion-resistant alloy constitute a weakness. Plastic bending to levels such as those imposed by reel-lay installation leads to stress concentration due to the mismatch of properties between the carrier steel, the liner alloy, and the weld. Furthermore, the constraint of the weld causes a local periodic separation of the liner from the carrier, which triggers wrinkling and subsequently large-amplitude buckles (Yuan and Kyriakides, 2015, "Liner Wrinkling and Collapse of Girth-Welded Bi-Material Pipe Under Bending," Appl. Ocean Res., 50, pp. 209-216. 10.1016/j.apor.2015.01.018). The present analysis shows that replacing the contact stress of manufacture by a low level of constant internal pressure does not alter the induced disturbance or its consequences under bending, and that the growth of liner separation accelerates when the liner achieves a moment maximum (critical curvature). In addition, the presence of small geometric imperfections in the neighborhood of the weld was shown to reduce the curvature at which the stability of the liner becomes critical. Cyclic bending causes progressive accumulation of liner separation adjacent to the girth weld. The evolution of events as the number of cycles, N, increases is similar to that of monotonic bending, with N replacing curvature. The rate of growth of liner separation depends on the amplitude of the imperfection, the internal pressure, and the curvature of the bending cycle. It was observed that, when the liner separation reaches the level at which the instability becomes critical under monotonic bending, its rate of growth per cycle accelerates. Thus, monitoring liner separation during cycling can guide design.
Abstract Concurrent wave and vortex-induced vibration (VIV) loads, along with platform motion-induced vibrations for marine risers and subsea power cables, lack sufficient understanding. The evaluation of fatigue damage under simultaneously acting loads is often simplified when using frequency-domain tools. The time-domain load model, VIVANA-TD, has been developed as part of the Lazy Wave Riser Joint Industry Project. This model extends the Morison equation by incorporating vortex shedding force terms. Its synchronization model introduces phase coupling between the force and response, effectively capturing how the local vortex shedding frequency adjusts to achieve lock-in. The model’s advancements in handling structural nonlinearities and time-varying flows improve VIV predictions. This study validates the combined cross-flow and in-line time-domain VIV load model using data from Equinor’s truncated steel catenary riser (SCR) and steel lazy wave riser (SLWR) model tests. The results show that the predicted maximum fatigue damage is within a factor of 5 of the measured values in most cases. This demonstrates that VIVANA-TD accurately captures the key characteristics of VIV responses under oscillatory flow conditions. The findings enhance understanding of VIV driven by platform motions, with practical implications for both the oil and gas and renewable energy sectors.
The steel lazy-wave riser (SLWR) is a good alternative for deepwater oil and gas exploitation, as its buoyancy section alleviates the top tension and TDP (touchdown point) movements. Although studies exist on dynamic compression instability in SCRs (steel catenary risers), no work has been found on dynamic compression instability in SLWRs. The differences in geometry and dynamic behavior between the two configurations do not enable extrapolation of results from SCRs to SLWRs. In SCRs, dynamic compression instability occurs near the touchdown zone; in SLWRs, the critical zone is near the top. This article investigates whether, despite this difference, the analytical formulation for dynamic instability in catenary risers available in the literature can be applied to SLWRs. To check and validate the analysis, the proposed methodology was implemented and verified for catenary risers. Once the results were checked against the literature, the same procedure was applied to a steel lazy-wave riser case study. Then, the influence of torsion was verified, as its presence can lead to out-of-plane buckling at a significantly lower load. The results showed that the model developed for catenary risers, disregarding torsion, did not always get accurate results for the lazy-wave configuration, but was always on the safe side. When torsion was present, as the instability occurred at the top, the observed critical load remained unchanged, but the instability always occurred out-of-plane. The article provides a project guideline for using the analytical critical load value to ensure no instability occurs.
This study presents insights gained from wave flume experiments conducted to investigate the hydrodynamic responses of a SPAR floating offshore wind turbine (FOWT) platform supporting a 5 MW wind turbine under combined wind and wave loading conditions. The significance of this work lays in the methodology in which a FOWT model was subjected to regular and irregular waves under aerodynamic loading introduced as rotor thrust force for below-rated and rated wind speeds to determine platform-level dynamics. First, free decay tests were conducted to obtain the natural periods and damping ratios of the SPAR FOWT system. Furthermore, the wave elevations, displacements, rotations, and rotor thrust were measured under the influence of varying regular, irregular waves, and thrust forces. Response amplitude operators (RAOs), statistical and spectral analyses, were carried out to determine the response behavior of the SPAR FOWT under aero-hydrodynamic loading. It could be noted that the surge was affected by the rotor thrust force. A coupled surge-pitch phenomenon was observed during the wind-wave interaction on the platform, especially for higher wave periods. The rotor thrust influence on heave was negligible. However, the aerodynamic damping resulted in reduced pitch responses. The rotor thrust exhibited noticeable fluctuations under wave conditions, highlighting the clear coupling between platform motions and aerodynamic loading and became more pronounced with increasing wave heights, primarily due to the amplified platform responses under larger waves. These observations highlight the importance of including aerodynamic loading effects in understanding the dynamics of FOWT and suggest that the proposed aero-hydrodynamic framework could be a robust design approach for effective FOWT analysis.
Marine wave energy has the potential to make a substantial contribution toward achieving a carbon-free society. This study investigates the hydrodynamic responses of point absorber wave energy converters (PA-WECs), which are set in linear arrangements perpendicular to the wave direction. Owing to the interaction among the individual PA-WECs in an array, the configuration of the array significantly influences the overall system efficiency. In our previous study, we evaluated the power generation for the optimal control and arrangement of PA-WECs. Building on that foundation, the present study analyzes the hydrodynamic interactions among floating structures and PA-WEC arrays, as well as the corresponding arrangement and control parameters, based on the boundary value problem of potential flow theory. Waves diffracted by the PA-WEC array could be analyzed and optimized for efficiency; the lower the diffracted wave amplitude, the higher the efficiency. Furthermore, the theoretical value of the q-factor was determined to evaluate the performance enhancement achieved through optimal array design. The wave field analysis revealed that, as the number of WECs increases, the array exhibits a pronounced damping effect on the wave amplitudes on the lee side, indicating effective absorption of incident wave energy.
A drillship is a floating offshore structure used to perform drilling operations, marking the initial stage of oil field development. At the center of the drillship is a moonpool, through which the drilling riser is lowered to conduct drilling activities. Violent flows often occur inside the moonpool due to moonpool resonance, and these abnormal oscillations can directly affect operator safety and increase operational downtime. Therefore, minimizing violent flow within the moonpool is crucial for ensuring safety and maximizing operational efficiency. To achieve this objective, optimization of the recess deck geometry is essential in moonpool design. This study investigates the fluid dynamic characteristics inside the moonpool under variations in recess deck height. Changes in free-surface elevation and flow behavior were closely examined at specific locations within the moonpool for with and without recess deck configuration. Furthermore, the study compares and analyzes the impact forces and flow characteristics acting on the splash plate with and without the recess deck. Results show that variations in recess deck height lead to significant changes in the internal flow pattern. From the comparative analysis of free-surface elevation, an optimal recess deck height that effectively mitigates violent flow inside the moonpool is proposed. Additionally, model test results reveal distinct differences in fluid impact forces on the splash plate between configurations with and without a recess deck. Overall, this study provides valuable experimental and numerical data and fundamental insights for optimizing the moonpool geometry of drillships. The findings are expected to contribute to future research and design efforts aimed at improving moonpool performance and operational safety in similar offshore structures.
The present study examines the scattering and trapping behavior of obliquely incident small-amplitude water waves by a floating front-curved, thick, porous breakwater in shallow water. The breakwater, curved toward the seaward side, directly interacts with incoming waves, significantly influencing wave attenuation. The wave motion through the permeable structure is modeled employing the Sollitt and cross formulation under finite-depth conditions. A numerical scheme based on the multidomain boundary element approach is formulated to derive the associated boundary value problem, and the results are validated with existing experimental data and analytical results. The hydrodynamic characteristics of the breakwater are evaluated in terms of wave reflection, transmission, surface elevation, and forces on both the breakwater and an adjacent impermeable wall. The study found that the front-curved thick porous breakwater increases the wave reflection and attenuates the wave transmission and force compared to the vertical-edge porous breakwater. The results indicate that the radii of curvature play a critical role in improving the redirection of wave energy. Larger curvature radii are found to reduce wave transmission effectively, while in wave trapping scenarios, wave reflection is enhanced and exhibits an oscillatory trend in intermediate water owing to the dynamic redirection of wave energy.
The steady-state response of an ice sheet floating above a trench-shaped sea-bed and subjected to a concentrated load moving at a constant speed is investigated. The fluid domain is divided into three virtual regions corresponding to the geometry of the sea-bed, and the required matching conditions between two adjacent regions are provided. To ensure the structural continuity of the ice sheet, conditions on deflection, slope, shear force, and bending moment are imposed at the virtual boundaries on the upper surface, together with appropriate matching conditions at the instantaneous position of the moving load. The dispersion relation of flexural gravity waves is used to study the phase speed and group speed in shallow and deep water regimes, providing insight into their dispersive properties. The analysis is based on a plane-wave approximation, which is deemed appropriate for the present parameter regime where the propagating-mode effects dominate. The ice deflection is described in a piecewise analytical form across various regions by solving the governing equations using standard ordinary differential equation techniques in a moving coordinate framework. The steady-state deflection profile of the ice sheet is then obtained, highlighting the combined influence of the sea-bed geometry, trench depth, and load speed on the wave propagation. The results further demonstrate that increasing trench depth leads to a reduction in the ice sheet deflection by weakening the ice-water interaction, whereas higher load speeds intensify wave generation and amplify the dynamic response of the ice sheet, particularly within the critical speed range.
In engineering design, it is common to estimate N-year values of environmental variables. When based on small datasets, such estimates often show unconservative bias due to statistical uncertainty in distribution parameters. We study here the bias associated with various distributions used to characterize extremes. Simple analytical bias corrections, suitable for spreadsheet calculations, are established for these distribution models. Results from traditional distribution models (Gumbel/Weibull) are compared with those from asymptotic theory; i.e., generalized extreme value (GEV) and generalized Pareto (GPD) distributions. These generalized models are found to yield notably larger statistical uncertainty effects (e.g., bias) than traditional models.
This study presents a numerical investigation of the structural behavior of subsea pipe-in-pipe (PiP) systems designed for transporting cryogenic fluids. The investigation focuses on assessing the effective axial force (EAF) and the onset of structural instabilities, such as lateral buckling and pipeline walking, under varying operational and geometric conditions. The developed numerical model was validated against reference studies and applied to scenarios with temperatures as low as -253 degrees C. Results indicate that anchor length is a critical factor in achieving a fully restrained EAF. The inclusion of intermediate bulkheads facilitates load redistribution between the inner and outer pipes, while employing Invar material for the inner pipe that significantly mitigates thermal effects, potentially allowing residual compression due to the double wall mechanism inherent to pipe-in-pipes, even at low temperatures. Introducing geometric imperfections revealed susceptibility to lateral buckling triggered by the hydrostatic pressure test. Thermal and pressure cycling, simulating operational shutdowns and restarts, demonstrated interaction between lateral buckling and walking, particularly on sloped seabeds, with stabilization occurring after a few cycles. The authors noted that resizing the inner pipe wall thickness could eliminate buckling and walking, confirming that the compressive axial force magnitude of the coupled system is the primary trigger for these instabilities. One of the conclusions that arises is that appropriate wall thickness design, combined with materials exhibiting low thermal expansion, is essential to ensure the structural integrity of cryogenic PiP systems. These findings enhance understanding of the underlying mechanisms which are not straightforward.
Deep-water drilling risers experience vortex-induced vibration (VIV) under nonlinear shear currents, while buoyancy modules (BMs) modify the mass distribution and hydrodynamic loading. This study numerically investigates the VIV response of a BM-fitted riser using a modified wake oscillator-based structural model implemented within an in-house matlab framework. The formulation captures coupled cross-flow (CF) and in-line (IL) motions under exponential shear flow with depth-varying axial tension, and the model predictions are validated against published benchmarks. Parametric analysis quantifies the isolated effects of sea-surface velocity, top tension ratio (TTR), riser wall thickness, and internal fluid density on displacement envelopes, root-mean-square (RMS) profiles, trajectory patterns, and frequency spectra. Results show that the magnitude of the sea-surface current is the dominant excitation factor, while higher TTR enhances IL-CF coupling and promote multimode participation. The buoyancy layout strongly influences resonance susceptibility, whereas increased wall thickness and internal fluid density provide passive suppression through stiffness- and inertia-based damping. The findings clarify how hydrodynamic forcing, buoyancy distribution, and structural properties jointly govern the VIV response, providing guidance for preliminary fatigue assessment for buoyancy-assisted deep-water risers.
Remote subsea operations of oil and gas installations could be a complex undertaking related to the inherent risk in the operating processes. This systematic literature review consolidates current knowledge on risks and mitigation strategies in remote subsea operations for offshore oil and gas systems, focusing on water injection, oil–water separation, and oil transfer processes. Addressing the research questions, the study identifies critical risk hotspots, including equipment failures, human–machine interface errors, external events (e.g., environmental conditions), and challenges in real-time monitoring and decision-making. Comprehensive searches were performed in Scopus and Web of Science for studies published between 2000 and 2024, drawing from a final analysis of 44 articles selected. Studies were selected based on predefined inclusion and exclusion criteria relevant to risk assessment in remote subsea operations. The screening process involved independent reviewers and included both title/abstract and full-text review stages. Additionally, a data analysis was performed regarding parameters such as the evolution of publications over the years, the most explored techniques, areas of application, among others. The study concludes that traditional static risk assessment methods are insufficient for real-time remote operations, and the implementation of dynamic models, combined with comprehensive training and robust equipment design, is essential for effective risk mitigation and operational safety enhancement. The effectiveness of Bayesian networks in dynamic risk analysis was highlighted and complemented by other methodologies such as decision trees and event trees. Advancements in monitoring technologies, big data analytics, and machine learning indicate promising pathways for the evolution of risk management practices.
Abstract Efficiently solving ordinary differential equations (ODEs) is of importance in simulation-based digital-twin solutions for different marine and offshore industrial applications. Many existing simulation codes in this field adopt a uniform time-step approach in solving ODEs. The complexity of a simulation is influenced by the number of time-steps. To minimize the required number of total steps within a simulation, an adaptive-time-step explicit ODE solver can offer potential improvement by adjusting time stepping dynamically. However, this solution can still encounter inefficiencies, especially when operations like convolution integrals are repeatedly computed within each major time-step for evaluating the next state. To address this challenge, a relaxation scheme within adaptive-time-step explicit ODE solvers is proposed in this study. The relaxation scheme dynamically smooths over repeated calculations with a below-threshold filtering mechanism on time-consuming parts such as convolutions. This enables efficient solving within any major step. A case study with a floating vessel for aquacultural cultivation is performed to guide the choice of threshold value. The improved performance in calculation speed is demonstrated by comparing with results using uniform time-step solvers. The proposed relaxation scheme offers reduced computational complexity and improved solving speed. In addition, numerical results demonstrate that this approach maintains good accuracy for numerical simulations of marine dynamic systems. This study serves as a foundation for further advancements in the improvement of ODE solving, particularly for applications where different categories of environmental loads are involved. This numerical scheme may also enable efficient time-domain simulations for multiple-floater dynamics.
Marine ecosystem restoration increasingly relies on artificial reefs (ARs) as critical tools for enhancing biodiversity and sustaining fishery resources. While ARs generate ecologically beneficial hydrodynamic features through upwelling and wake regions, the resulting near-seabed flow patterns can induce sediment scouring that compromises structural integrity and ecological functionality. Here, a systematic investigation of triangular ARs is presented. The hydrodynamic and ecological performance of these ARs (characterized by upwelling, wake region, and sediment scouring) is governed by three interdependent structural parameters: base angle (alpha), height (h), and length (l). Through three-dimensional computational fluid dynamics (CFD) simulations performed using openfoam, combined with analysis of existing experimental scour data, we quantify the relationships between these structural parameters and three critical performance indices: upwelling index (I-u), wake index (I-w), and scour index (I-s). Generalized Linear Model (GLM) analysis reveals that alpha exerts dominant control over both I-u and I-s, while h demonstrates limited influence on these indices. By developing a comprehensive performance index and employing Kriging interpolation with Bayesian optimization, we identify an optimal triangular AR configuration (alpha = 30.3 deg, h = 9.7 cm, and l = 33.4 cm) that maximizes ecological benefits while minimizing scour effects. Our findings establish a quantitative framework for AR design optimization, advancing the development of sustainable marine infrastructure.
The L-shaped caisson has been extensively utilized in marine infrastructure projects, particularly for deep-water ports and artificial islands. Numerical studies were conducted in this article to investigate the deformation and stability of the L-shaped caisson quay wall on sandy soil under live load. The accuracy of the numerical model was validated by the laboratory model tests in the literature. Parametric studies were conducted to examine the effects of caisson dimensions, live loads, and backfill soil properties on the stability of the L-shaped caisson quay walls. It is found that the heel length of the L-shaped caisson, the loading distance and width of the loading plate, and the effective friction angles of the soil heavily influence the stability and deformation of the L-shaped caisson quay wall. The horizontal displacements decreased by 49% with increasing heel length and by 92% with increasing loading distance. Four failure modes can be categorized based on the shapes and numbers of failure surfaces generated in the sand backfill. A method for determining failure modes was developed based on a statistical analysis of 130 numerical simulation cases. The predictive accuracy of the proposed classification criteria was verified to be 94% for the failure Mode I, 77% for Mode II, and 82% for Mode III.
