Optimized future floating wind turbines (FWTs) are expected to be both larger and relatively lighter than conventional offshore platforms, thus more flexible. The common practice of modeling the platform as a rigid body in coupled dynamic simulations of FWTs can then be questioned. Specifically, natural frequencies of the elastic modes of large flexible platforms can be close to the frequency range of excitation loads. Additionally, platform flexibility can have a significant effect on the natural modes involving significant tower deformation. Considering the platform’s flexibility in coupled simulations of large FWTs requires distributing the hydrodynamic and hydrostatic pressure loads on the flexible model of the platform, instead of the traditional approach of lumping the loads at a single point. This work presents a rational method to evaluate the first-order added mass, radiation damping, and excitation coefficients for a multi-body representation of the platform and develops an energy-conserving distributed formulation for the hydrostatic loads. Assuming small flexible deformations, a decoupled radiation damping matrix is used to model radiation loads for better computational efficiency, while a fully coupled infinite-frequency added mass matrix is used to ensure a stable model. The decoupled radiation coefficients can either be obtained from a single-body or a multi-body diffraction/radiation analysis. A case study of the INO OptiFLEX 22MW semisubmersible FWT is used to illustrate and verify the implementation of the proposed approach. Compared to a baseline model with rigid floater, the results show that introducing platform flexibility significantly affects the high-frequency dynamics of the tower and can also potentially affect mooring line tensions. Moreover, platform flexibility was shown to influence roll and pitch dynamics. These findings highlight the need to model platform flexibility in coupled simulations when analyzing and designing future large FWTs, which can be achieved through the proposed methodology.
During deep-sea launch and recovery, the non-metallic umbilical is subjected to sustained equipment weight and deep-sea high-pressure. Due to the viscoelastic nature of non-metallic armor-layer materials, cumulative creep deformation may occur, causing umbilical elongation, stiffness degradation, reduced load-bearing capacity, and even umbilical failure or equipment loss. The coupled mechanism between sustained seawater hydrostatic pressure and the viscoelastic–viscoplastic response of aramid-reinforced composite lines remains unclear in existing creep-recovery studies. In this study, aramid-reinforced composite lines in the non-metallic armor layer of the umbilical are the research object. A novel experimental device that couples a constant tensile load with seawater pressure has been developed. 72 h creep–recovery behavior was investigated in air, water, and various seawater pressure conditions. A creep-recovery constitutive model considering the effect of seawater pressure was proposed. Results show that the water increases the total creep strain and non-recoverable permanent strain by 8.13% and 21.7%, respectively, compared to air. Under seawater pressures of 20–80 MPa, the total creep strain remained at 2.25%–2.45%, while the non-recoverable permanent strain increased by 21.7%–48.9% relative to ambient-pressure water conditions. The proposed model predicts with errors within 12%, effectively describing creep–recovery behavior under seawater pressures and providing a theoretical basis for evaluating and optimizing non-metallic umbilical load-bearing structures.
Design optimization of offshore wind turbine (OWT) monopiles presents a high-dimensional challenge, as the pile is discretized into multiple segments (typically 2–3 m in length), each with independent diameter and wall thickness. Furthermore, the optimization is governed by various design constraints (ultimate, fatigue, and serviceability limit states), numerous design load cases (DLCs), and complex multi-physics involving aero-hydro-servo-elastic-soil interactions. These factors necessitate computationally expensive time-domain simulations. Conventional optimization methods are susceptible to local optima and struggle with the high-dimensionality, multiple constraints, and numerous load cases inherent to this problem. Moreover, current monopile designs are seldom optimized using integrated multi-physics simulations, which can lead to suboptimal or potentially over-conservative designs. This study proposes an efficient global optimization framework for high-dimensional, multi-constrained, integrated design of monopiles under various load cases. The framework comprises three key components: (i) an integrated aero-hydro-servo-elastic-soil model for providing high-fidelity dynamic responses; (ii) the subset simulation optimization (SSO) algorithm for efficient global exploration of the high-dimensional design space; and (iii) a high-fidelity multivariate metamodel based on Gaussian process regression (GPR) for accelerating the optimization process. The effectiveness of the proposed framework is demonstrated by optimizing a monopile for a 15 MW OWT. Benchmarking against a suite of optimization algorithms (GD, GA, PSO, SA) demonstrated the superiority of SSO, which achieved a 9% reduction in mass and reduced the required computational time by 38%. The results identify fatigue, rather than ultimate strength, as the governing constraint for monopile optimization. The framework equips designers with a computationally efficient, globally optimal route to lighter offshore wind turbine foundations, cutting costs while sustaining structural integrity.
During the preliminary design phase, accurate and efficient calculation of the platform’s natural frequency is crucial for preventing resonance between the semi-submersible platform and waves and for enhancing stability. In this study, a modified orthogonal strip theory model was proposed through orthogonal slicing and multiple linear regression, and the results were compared and validated against the commercial software ANSYS AQWA for hydrodynamic coefficient calculation. Based on a semi-analytical and semi-empirical approach, this study proposed simplified formulas for added masses in the heave, roll and pitch directions of a semi-submersible platform with square columns, accounting for variations of column section dimension, column spacing and draft depth. Equivalent conversion relationship between the added masses of square-section and circular-section column was investigated. Furthermore, based on two mass distribution assumptions, a quantitative study was conducted on variation of the natural frequency of photovoltaic module within the geometric parameter space. This study provides a relatively efficient analytical method for calculating the frequency-domain characteristics and conducting preliminary dimension design of multi-column semi-submersible photovoltaic modules.
This study investigates the occurrence of gap resonance in side-by-side structures with sharp corner from the perspective of vortex dynamics. Potential flow analysis is first employed to determine the resonant and non-resonant frequency. A RANS–VOF frame based CFD model is applied to investigate the vortex dynamics during the gap resonance phenomenon. Vortex Energy Conversion (VEC) mechanism is newly discovered during the whole process of gap resonance. The VEC process consists of two consecutive stages. In the first stage, the sharp corner accumulate vorticity and turn the horizontal flow into vertical flow toward the gap region. Large amount of flow energy is captured by the newly formed vortex and transported into the gap region. In the very early second stage, the vortex shedding occurs and further accumulate field energy. Then the vortex size grows and keeps the energy in the region. In the late period of second stage, the vortex dissipates and the energy is transformed into flow kinetic energy, generating large velocity in the vertical direction and bringing in the well-known gap resonance phenomenon.
Catamaran salvage ships are perfect for marine rescue, wreck salvage, ocean cleanup, and other tasks. The salvage ships have to arrive quickly at the salvage site, thus catamarans with good hydrodynamic performance are best. This paper proposes a parametric automatic optimization design method to obtain a new type of catamaran salvage ship with good resistance and to forecast its seakeeping performance during salvage operations. In this paper, the underwater hull of the catamaran salvage ship is constructed using full parameterization, and its design parameters are analyzed for sensitivity. The SOBOL algorithm is used for spatial sampling, and the optimization variable is the parameter with the largest correlation to the total resistance value for multiple conditions in the light-load and full-load conditions. Finally, the Tsearch algorithm is used to build a set of parametric catamaran salvage ship automatic calculation frameworks based on numerical simulation. According to the sensitivity analysis and automatic optimization results, the Catamaran salvage ship with good resistance performance is obtained, and the seakeeping performance of the ship under different salvage angles is forecasted. This article provides reference and practical guidance for the design optimization method of Catamaran salvage ships and its seakeeping performance analysis for salvage in waves.
Scour significantly impacts the dynamic behavior of monopile-supported offshore wind turbines (OWTs) subjected to marine environmental loads. It is vital to accurately simulate the effect of scour on soil-pile interaction to investigate the dynamic responses of monopile-supported OWTs under scoured conditions. In this study, a simplified analytical model for soil-pile interaction considering scour effects has been developed based on the Winkler foundation method. The developed model thoroughly accounts for the influence of different scour types on soil-pile interaction, with numerical validation confirming that the simplified analysis method can be effectively applied to analyze the dynamic responses of monopile-supported OWT under scoured conditions. A parametric study is conducted through numerical simulations under varying scour conditions, incorporating wind, wave, and seismic actions. The results show that scour reduces the bearing capacity of monopile foundation, and local scour amplifies the dynamic responses of OWT under wind, wave and seismic actions. These responses are primarily dominated by the local scour depth, with minimal impact from the morphology of the local scour hole. Additionally, global scour reduces seismic responses while amplifying the dynamic responses under combined wind-wave loading. Accordingly, the practical impacts of different scour types on the dynamic performance of monopile-supported OWTs should be systematically considered in engineering design.
A bi-normalized input energy spectrum for energy-based seismic design of structures subjected to offshore ground motions (GMs) is presented. The bi-normalized input energy spectrum, Eeq, is obtained by dividing the vibration period of single-degree-of freedom (SDOF) systems by the predominant period and by dividing the input energy equivalent velocity by the spectral pseudo-velocity. A total of 200 offshore GMs obtained from 273 earthquakes and 369 onshore GMs obtained from 263 earthquakes that have occurred in the Japan Sagami Bay Region with magnitude greater than five were employed to assess Eeq. The computer program IDARC2D was used to determine the nonlinear time-history analysis of inelastic SDOF systems for computing the Eeq spectra. The Eeq variation with offshore ground-motion characteristics, the normalized period, displacement ductility ratio, postyield stiffness ratio and hysteretic law is investigated. The differences in Eeq of offshore and onshore GMs are also highlighted. The Eeq was found to be not sensitive in seafloor stations, significant duration, magnitude and epicentral distance. In comparison to onshore GMs, offshore GMs can increase mean Eeq of structural systems, with the magnitude of this increase potentially reaching approximately 20%. Analytical estimates of Eeq were finally introduced to accommodate the extension of the Energy-Based Seismic Design via bi-normalized input energy to structures subjected to offshore GMs.
The present paper addresses newly developed finite elements that addresses the prediction of stresses in conductors of dynamic power cables applied for connecting offshore wind turbines to the grid. To describe relevant physical effects during all phases including manufacturing, installation, and operation, 3D analysis is required. As a result, the development has been based on beam and beam contact formulations that enhance computational efficiency. This is followed by a numerical study that serves several purposes; verification, shedding light on different aspects of conductor behavior, and providing sufficient detail to be applied as a reference example for other scholars. Good correlations have been found between the proposed models and other models either obtained from literature or by applying other software.
This paper presents an experimental investigation on the hydrodynamic and energy-conversion performance of heaving-type oscillating water column (OWC) wave energy converters integrated with a parabolic energy-focusing breakwater. A comprehensive set of 1:15 scale physical model tests was performed under both regular and irregular wave conditions to characterize the hydrodynamic response, rigid-body heave motion, chamber air pressure evolution, and wave-to-wire (WtW) efficiency of single-OWC and triple-OWC array configurations. The influences of spring stiffness, motion–pressure coupling, and inter-device array interaction were systematically examined. Experimental results reveal that spring stiffness exerts a governing effect on the coupled hydrodynamic behavior of the heaving OWC. An appropriately selected stiffness enhances energy extraction by intensifying transient negative acceleration near motion reversal and accelerating chamber pressure buildup. Relative to the fixed OWC, the heaving configuration yields distinct advantages under short-period waves, characterized by an inertia-driven spike-type pressure response that favors high instantaneous power output and peak WtW efficiency. Under irregular waves, this fundamental mechanism persists and manifests as burst-like energy conversion modulated by energetic wave groups. Correlation analysis confirms that chamber pressure peaks exhibit a stronger association with heaving velocity and acceleration than with displacement. For the triple-OWC array with two wave-focusing levels, efficient cooperative energy capture is achieved in the short-period regime, where secondary devices contribute substantially to total power production. In realistic irregular seas, the array configuration suppresses extreme instantaneous power peaks and elevates mean power output by redistributing hydrodynamic responses over broader energetic wave-group intervals. These findings elucidate the motion-induced pressure generation mechanism and array interaction effects in heaving OWC systems, demonstrating clear performance superiority over fixed counterparts. The outcomes provide robust guidance for hydrodynamic tuning, motion optimization, and array layout design of multifunctional marine structures that synergistically combine protection and wave energy utilization.
Optimization presents a promising route to cost reductions and performance enhancements of floating offshore wind turbines (FOWTs) and the National Laboratory of the Rockies (NLR)’s Wind Energy with Integrated Servo-controls (WEIS) is the leading open-source multi-disciplinary FOWT optimization toolset. An objective of the EU’s FloatFarm project was to expand WEIS by enabling the use of DTU Wind and Energy Systems’ HAWC2 aero-elastic-servo-hydro solver within the framework as an alternative to NLR’s OpenFAST. This work demonstrates the capabilities of the coupled framework, which includes attractive features such as control co-design (CCD) and parallel design load case (DLC) generation, execution and analysis. An optimization and CCD case study is presented, which serves to demonstrate the functioning WEIS-HAWC2 framework, showcasing the ability for a HAWC2 model to be updated according to iterative changes in a FOWT design, and whose simulation outputs can be used to guide optimization problems to convergence. The case study performed modifications to the blades, tower, moorings and floating platform of the IEA 15 MW VolturnUS-S FOWT with the aim of minimizing the total structural mass, which was ultimately reduced by 3.2%, relative to the initial design, while adhering to prescribed constraints, when simulated under normal operating conditions. The implemented CCD algorithm was also applied to ensure that controller gains were suitable to iteration-specific structural designs. The coupled WEIS-HAWC2 framework provides HAWC2’s established and expanding user-base with opportunities to perform multi-disciplinary optimization of FOWTs, and is intended to be a valuable tool for the FOWT research community.
A new bucket foundation design, comprising two skirted semicircular sections and a skirted rectangular center segment, has been developed for breakwater applications. Nevertheless, the interaction between the skirt and soil, along with the penetration resistance during jacking installation, remains poorly characterized, hindering its broader adoption in sandy soils. To bridge this gap, >20 large-scale laboratory tests were performed to examine the jacking process of this foundation in sand, and static jacking was adopted rather than suction or negative pressure in all the tests. Results delineate a jacking process where soil settlement progressed linearly before evolving into nonlinear, loading-rate sensitive behavior upon soil plug formation, with a definitive inflection point at lid contact. Soil pressures demonstrated a steady, elevation-dependent increase with penetration depth. The internal-to-external stress ratio, characterized by a critical threshold, was accurately described by a power function model. Analysis of the penetration force components showed tip resistance to be predominant (≈60%), substantially exceeding internal skirt friction (≈35%) and external friction (≈5%). The sharpest rate of increase in internal friction was ascribed to the constricted deformation and plugging within the compartments. Owing to the small loading-rate effect observed across the load increment range, the generalized updated methods incorporating triaxial friction angles or cone penetration test resistances are established and discussed, which provides accurate predictions of penetration resistance and a practical jacking design approach for novel bucket foundations in sand.
As the jacket-type offshore wind turbine (JOWT) is increasingly deployed in seismically active regions, ensuring its seismic resilience has become a critical technical challenge. This study proposed a novel multi-stage self-centring JOWT (MSCJOWT) design by introducing a multi-stage self-centring transition piece comprising a rocking tower base system and hybrid self-centring dampers. The MSCJOWT incorporates multi-stage energy dissipation and controlled rocking mechanisms to mitigate seismic actions and enable post-earthquake self-centring. To assess the seismic advantages of the MSCJOWT design, nonlinear time-history analyses were conducted on the validated numerical models of a benchmark 5-MW JOWT and nine MSCJOWTs featuring diverse hysteretic behaviours. Furthermore, incremental dynamic analyses and seismic fragility analyses were performed considering both serviceability and ultimate limit states. The results demonstrated that the MSCJOWT exhibited distinct multi-stage behaviour, effectively improving serviceability under frequent earthquakes and collapse prevention capacity under rare earthquakes, while enabling post-earthquake self-centring. The seismic performance of the MSCJOWT can be optimised by adjusting its key design parameters, offering considerable design flexibility.
Mechanically lined pipes (MLPs) are increasingly employed in offshore engineering owing to their superior resistance to corrosion and cost efficiency. Nevertheless, their structural performance under the combined actions of bending and external pressure remains insufficiently understood. This study explores the buckling behaviour of MLPs under combined bending and external pressure through numerical simulations and machine learning methods. First, a dedicated finite element model was constructed to examine the buckling responses of MLPs under combined loading, and its accuracy was validated against available experimental data. Subsequently, a comprehensive parametric analysis was conducted, generating a large and reliable dataset that reflects the influences of key geometric, material, and manufacturing parameters. Based on these data, six machine learning models were established to conduct the correlation analysis and capacity predictions. The results indicate that outer pipe thickness dominates the ultimate bending capacity, with increased thickness significantly enhancing ultimate bending capacity. External pressure is identified as the most critical factor reducing bending resistance, whereas the influence of liner properties is comparatively minor. Finally, a set of refined formulas was suggested to assess the ultimate bending capacity of MLPs, demonstrating markedly improved accuracy compared with the existing DNV design formula.
Ultra-large outfitting ships are especially vulnerable during extreme weather due to their shallow draft, large wind-exposed area, and absence of self-propulsion. This study investigates the mooring performance of a 15,000 TEU outfitting container ship through a validated numerical model, benchmarked against a 1:62 scale physical model test. Time and frequency domain analyses were applied to assess ship motions, mooring line tensions, and wharf structural responses under extreme conditions. Transverse motions were found to dominate, with resonance amplification and sway-roll coupling occurring when the wave period approaches the ship's natural roll period, greatly increasing mooring tensions and impact forces. Structural analysis indicated that transverse beams were most critical, and that long-period waves could cause longitudinal displacements exceeding design limits. To improve system stability, three mooring optimization strategies were evaluated: increasing the number, diameter, and pretension of mooring lines. Among them, increasing the number of mooring lines was most effective in reducing ship motions, alleviating peak line tensions, and constraining longitudinal displacement. Based on the project-specific operational control limit of 75% of the minimum breaking load and the validation-bias correction, the allowable environmental combinations were identified for outfitting ships under extreme conditions. These findings provide a reference for mooring arrangement selection, pre-event reinforcement, and operational safety assessment of similar outfitting ships under forecasted extreme conditions.
Significant thermal load is generated in submarine power cables during power transmission, leading to thermal expansion and thermal stress in each layer of the cable structure. The resulting interlayer contact forces modify the contact state and slip behavior between the helical armor wires and adjacent layers, thereby influencing the nonlinear bending behavior of the cable. To address this, a theoretical model is developed to investigate the thermo-mechanical bending behavior of submarine cables. Based on thin rod theory and thick wall theory, an axisymmetric model under thermal loading is first established. A bending model that incorporates interlayer shear deformation is then formulated to characterize the nonlinear bending hysteresis response under combined thermal and bending loads. The axisymmetric and bending responses predicted by the theoretical model are validated against finite element (FE) simulations. The results show that the temperature rise increases the contact force between the armor layer and adjacent layers, which in turn raises the critical slip curvature and enlarges the bending-moment range in the hysteresis curve. Parametric analysis further reveals that a larger lay angle significantly reduces the stick stiffness and the moment range, whereas the critical slip curvature decreases and then increases as the lay angle grows. This work provides a theoretical framework and key insights into the intrinsic relationship between nonlinear bending behavior and thermo-mechanical effects in submarine cables.
As a key piece of equipment in subsea power transmission systems, cross-linked polyethylene (XLPE)-insulated submarine cables are renowned as the “nerve lines” of such systems, featuring a multi-layered, multi-material composite structure. In practical engineering applications, these cables are susceptible to substantial tensile loads during laying process or when forming exposed suspended spans, which may ultimately lead to structural damage and failure. Based on the helically wound and thick-walled cylindrical structural characteristics of the submarine cables, a tensile mechanical model considering radial contraction was established to theoretically analyze the tensile mechanical properties. Meanwhile, a specialized experimental platform for submarine cables was constructed, and systematic tensile mechanical experiment was conducted under room-temperature condition. Compared with the tensile stiffness of experimental results, the theoretical analysis exhibited an error margin of <8.97%, validating the reliability and rationality of the proposed tensile mechanical model at room temperature. Building on this foundation, further theoretical derivations were performed by accounting for the high-temperature environment of the conductor layer within the submarine cable during power transmission. The results revealed that a bearing capacity reduction effect occurred under high-temperature conditions, and the tensile mechanical properties of the submarine cable exhibited coupling behavior, with a significant reduction in tensile stiffness during the loading stage. This study is expected to provide valuable technical references for the structural design and mechanical property analysis of the submarine cables.
Global navigation satellite system real-time kinematic (GNSS-RTK) technology, especially GNSS-RTK with different sampling rates, has limited applications in real-time dynamic displacement extraction and modal identification of offshore platforms. Furthermore, its monitoring accuracy is severely affected by multipath low-frequency and random high-frequency noise. Hence, this study innovatively extends the low-rate and high-rate (i.e., 10 Hz and 50 Hz data sampling frequency) GNSS-RTK to offshore platform applications under rapidly changing environmental excitations based on a static test. A novel integrated approach combining the improved variational mode decomposition-successive variational mode decomposition (IVMD-SVMD) with the random decrement technique (RDT) is developed to reduce noise and estimate modal parameters. The results show that the low-, mid-, and high-frequency noise of high-rate GNSS-RTK in complex environments are distributed in 0 to 0.02 Hz, 0.02 to 0.5 Hz, and higher than 0.5 Hz ranges, respectively. Simulation and field experiments verify that the IVMD-SVMD approach effectively mitigates noise and has better denoising performance. Low-rate GNSS-RTK at 10 Hz is robust in first-mode frequency identification, with a relative error of 1.89%. The maximum relative error of high-rate GNSS-RTK with 50 Hz in higher-mode frequency estimation is 1.26%. Frequency extraction has slightly higher accuracy in GNSS-RTK measurements compared to damping ratio extraction. This study thoroughly demonstrates the suitability and practicality of GNSS-RTK with different sampling rates in dynamic monitoring and modal analysis of fixed offshore platforms.
High-piled wharves, as critical marine infrastructure, are subjected to harsh environmental conditions, such as corrosion, wave loading, and ship impacts. While structural redundancy has been extensively studied for bridges and buildings, its application to wharves remains largely unexplored. This study addresses this gap by proposing a comprehensive Pre-damage Structural Redundancy (PSR) evaluation framework tailored to high-piled beam-slab wharves (HBWs). Moving beyond traditional measures of static indeterminacy, the framework introduces component-specific redundancy indicators based on bearing capacity. An incremental live load method is adopted to enhance practical applicability. Finite element simulations of five case-study wharves validate the proposed method. Results indicate that system-level PSR ranges from 18% to 24%, allowing temporary load increases during emergency operations without structural strengthening. The ranking of component PSR values corresponds to the failure sequence under overload conditions, enabling proactive maintenance planning. PSR primarily originates from design practices such as crack width control, pile length rounding, and reinforcement detailing. The framework is generalizable to other pile-supported marine structures, including offshore platforms and wind turbine foundations. This study fills a critical research gap and provides quantitative guidance for both design optimization and emergency response planning.
