Wave Loading Research Articles

Investigation of Fatigue Assessment Method for Glass Fiber Reinforced Composite Hull Structures Based on Stiffness Degradation

A study was conducted on the fatigue assessment method for composite ship structures under complex marine environments, and a fatigue assessment method based on the principle of stiffness degradation was proposed. Fatigue tests were performed on the composite material of the target ship to obtain the stiffness degradation parameters under tension–compression loading. Four fatigue hotspot areas in the midsection of the hull were selected, and mesh refinement was applied to these locations to accurately capture the variations in stress gradients. The structural stress response transfer function was calculated, and the short-term and long-term distribution data of wave loads were obtained. Finally, the fatigue life of the target ship hotspots was predicted by combining spectral fatigue analysis with the stiffness degradation theory. The results indicate that the connection between the bulkhead stiffener and the inner bottom plate has the shortest fatigue life, and its dimensions were optimized.

On a river-barge numerical strength analysis in head design wave loads

On a river-barge numerical strength analysis in head design wave loads

Impacts of Transition Piece Designs on the Resilience of Large Offshore Wind Turbines Subject to Combined Earthquake, Wind and Wave Loads and Soil‐Structure Interaction

ABSTRACTThe urgent global drive to mitigate greenhouse gas emissions has significantly boosted renewable energy production, notably expanding offshore wind energy across the globe. With the technological evolution enabling higher‐capacity turbines on larger foundations, these installations are increasingly situated in earthquake‐prone areas, underscoring the critical need to ensure their seismic resilience as they become a pivotal component of the global energy infrastructure. This study scrutinises the dynamic behaviour of a 15 MW offshore wind turbine (OWT) under concurrent earthquake, wind and wave loads, focusing on the performance of the ultra‐high‐strength cementitious grout that bonds the monopile to the transition piece. Employing LS DYNA for numerical simulations, we explored the seismic responses of four OWT designs with diverse transition piece cone angles, incorporating nonlinear soil springs to model soil‐structure interactions (SSIs) and conducting a site response analysis (SRA) to account for local site effects on ground motion amplification. Our findings reveal that transition pieces with larger cone angles exhibit substantially enhanced stress distribution and resistance to grout damage, evidenced by decreased ovalisation in the coned sections of the transition piece and monopile, and improved bending flexibility. The observed disparities in damage across different cone angles highlight shortcomings in current design guidelines pertaining to the prediction of grout stresses in conical transition piece designs, with the current code‐specified calculations predicting higher stresses for transition piece designs with larger cone angles. This study also highlights the code's limitations when accounting for grout damage induced by stress concentrations in the grouted connections under seismic dynamic loading conditions. The results of the study demonstrate the need for refinement of these guidelines to improve the seismic robustness of OWTs, thereby contributing to the resilience of renewable energy infrastructure against earthquake‐induced disruptions.

A multi-fidelity approach for the evaluation of extreme wave loads using nonlinear response-conditioned waves

In the present study, a multi-fidelity methodology based on response-conditioned waves (RCW) is demonstrated for the probabilistic assessment of wave-induced loads and responses of offshore structures. The methodology consists of two steps: (i) the RCW are determined using a surrogate response model and (ii) they are reproduced in an experimental or CFD-based numerical wave tank (NWT) to obtain the fully nonlinear response, as a high-fidelity evaluation. The main novelty of the proposed response-conditioning technique is that it uses a fully nonlinear wave solver that calculates the wave propagation inside an NWT. In this work, the extreme Vertical Bending Moment (VBM) of a zero-speed containership is investigated. It is found that the proposed approach provides nonlinear wave sequences that can be explicitly and exactly reproduced experimentally, up to very extreme sea states (Hs=17 m, Tp=15.5 s). Moreover, through comparison of the obtained results with an experimental Monte Carlo approach, it is shown that the overall framework can predict the short-term distribution of the VBM accurately and efficiently. Finally, given that model tests are costly and not widely accessible, the implementation of the high-fidelity response evaluation within a CFD-based NWT is also demonstrated and validated against the available experimental results.

Recursive Bayesian estimation of wind load on a monopile-supported offshore wind turbine using output-only measurements

Offshore wind turbine structures experience combined wind and wave loading during their lifetime, and the cyclic characteristics of these loads significantly impact the fatigue life of the support structure. Continuous monitoring of stress-related quantities, such as strain time histories at hotspot locations of the structure, can help to estimate the fatigue life. However, sparse measurements from the substructure necessitate using a digital twin as a tool for structural health monitoring by creating a virtual model. This virtual model relies on real-time input loads for virtual sensing and stress-related quantities prediction. However, the exact loading on these structural systems is often unknown or estimated with considerable uncertainty. This paper implements a recursive window-based Bayesian estimator for input load estimation and virtual sensing of acceleration and strain time history at unmeasured critical locations using output-only measurements. The estimator is numerically and experimentally validated in the context of input load estimation for an offshore wind turbine, and the results are compared with a traditional augmented Kalman filter and a linear regression approach for the quasi-static component of loading. The method is first demonstrated through a numerical study using a finite element model of a 6 MW offshore wind turbine, where input wind load time histories are accurately estimated. Then, the framework is applied to the real data measured from an offshore wind turbine in the North Sea. The study demonstrates the window-based Bayesian input estimator as a promising tool for the digital twinning of offshore wind turbines, demonstrating superior accuracy in input load estimations and full response predictions compared to the augmented Kalman filter and linear regression methods without the constraint of collocated measurements at input load locations.

Study on the influence of submergence depth on the hydrodynamic and wave load characteristics of semi-submersible structures induced by a solitary wave

The submergence depth directly affects the safety of semi-submersible marine structures due to that the submergence depth significantly impacts on the hydrodynamic characteristics and wave loads of structures excited by extreme wave. This paper studies the influence of submergence depth on the hydrodynamic and wave load characteristics of semi-submersible structures by establishing a numerical model of the interaction between solitary waves and semi-submersible structures based on the SPH model and Rayleigh theory. Furthermore, equations for transmission coefficient, reflection coefficient, and wave load are fitted. The calculated wave heights of solitary wave propagation test case are in good agreement with the theoretical values. The maximum relative error of the wave peak is 8.4%. The calculated wave loads of submerged horizontal plates test case has a consistent trend with the experimental data. The maximum relative error of wave load peak and valley is 54% (absolute error 0.37 N). Furthermore, the interaction between solitary waves and structures with different submergence depths is investigated by using the meshless numerical model. It is found that the reflection coefficient first increases and then decreases with increasing submergence depth, and reaching a maximum value of 0.39 at the submergence depth equal to 0.0 m. On the contrary, the transmission coefficient decreases first and then increases with the increase of submergence depth. The minimum value of transmission coefficient is 0.36 with the submergence depth of 0.3 m. As the submergence depth increased, the horizontal wave load peak of the structure gradually increases, and the maximum value of 0.13 is obtained at the submergence depth of 0.7 m. The peak of vertical wave load rapidly increases with the increase of submergence depth and then gradually decreases while the trough gradually decreases with increasing submergence depth.

Assessment of typhoon-induced wind and wave effects on fatigue crack growth life in offshore wind turbines

ABSTRACT Fatigue and structural failures in OWT towers are common due to sustained aerodynamic and wave loads. This study predicts the remaining service life of 2.1MW jacket-type Offshore Wind Turbine (OWT) towers, focusing on fatigue crack growth life (FCGL). Using one-way fluid-structure interaction (FSI) simulations with Typhoon "Muifa" parameters, it analyzes stress under wind, wave, and current effects. FNK theory and linear elastic fracture mechanics are applied to calculate FCGL under various loads. The results indicate that the participation of waves and currents significantly affects the stress distribution in both the tower and the tower frame. During the crack propagation process, cracks initially propagate along the thickness direction and then extend along a straight path after penetrating the thickness. Under typhoon conditions, waves and currents cause a significant reduction in the FCGL of the tower, decreasing from 2.46 years to 1.48 years numerically.

Numerical study on hydroelastic responses and bending-torsion coupled loads of a ship in oblique regular waves

In this paper, the CFD-FEM two-way coupled method is extended to simulate the asymmetric motions and wave load responses of a ship in oblique regular waves considering hydroelasticity effects. The numerical results of ship motions and loads including time series and response amplitude operators are validated by comparing with the tank experimental results. Then the ship motions, vertical bending moment, horizontal bending moment and torsion moment in oblique waves are analyzed in both time domain and frequency spectra; and the influence of wave height, wave length and ship speed on the results are systematically analyzed. Moreover, the correlation between horizontal bending moment and torsion moment as well as their component loads is analyzed by cross-spectral analysis method. The effect of springing responses corresponding to the bending-torsion coupled mode on the sectional loads is also discussed.

A Multiscale Model to Assess Bridge Vulnerability Under Extreme Wave Loading

A multiscale model is proposed to assess the impact of wave loading on coastal or inland bridges. The formulation integrates various scales to examine the effects of flooding actions on fluid and structural systems, transitioning from global to local representation scales. The fluid flow was modeled using a turbulent two-phase level set formulation, while the structural system employed the 3D solid mechanics theory. Coupling between subsystems was addressed through an FSI formulation using the ALE moving mesh methodology. The proposed model’s validity was confirmed through comparisons with numerical and experimental data from the literature. A parametric study was conducted on wave load characteristics associated with typical flood or tsunami scenarios. This included verifying the wave load formulas from existing codes or refined formulations found in the literature, along with assessing the dynamic amplification’s effects on key bridge design variables and the worst loading cases involving bridge uplift and horizontal forces comparable to those typically used in seismic actions. Furthermore, a parametric study was undertaken to examine fluid flow and bridge characteristics, such as bridge elevation, speed, inundation ratio, and bearing system typology. The proposed study aims to identify the worst-case scenarios for bridge deck vulnerability.

Faster than real-time, phase-resolving, data-driven model of wave propagation and wave–structure interaction

A machine learning time-series prediction approach is proposed for wave propagation and wave load prediction. Under unidirectional wave conditions and variable bathymetry, given a wave gauge upstream, a model is shown to reproduce wave elevation or wave forces downstream under irregular steep and either non-breaking or breaking conditions. Attempts to perform the opposite calculation, predicting upstream conditions from downstream measurements, results in higher error, likely due to information loss under breaking conditions. For choice of machine learning approach, comparisons show that the Time-series Dense Encoder (TiDE) approach results in a good balance between model complexity, stability, computational time, and error. Over a flat bottom, time-series of wave elevation can be predicted up to 10 wavelengths away, though with a degraded accuracy compared to shorter distances. Similar results are shown for time-series of forces on a vertical cylinder, showing better results than a simple Morison approach, as used in engineering tools such as OpenFAST, but with a similarly fast computational time. Generalizations show that training on irregular wave data permit extrapolations to periodic wave cases. Finally, the same method also is also demonstrated at field-scale, comparing results between two offshore buoys.

Fatigue study of unbonded flexible risers for ocean thermal energy conversion based on the joint distribution of wind, wave, and current

This paper investigates flexible risers attached to the base of a semi-submersible platform. Using meteorological and oceanographic data, the wind-wave-current boundary distribution was fitted, and the c-vine copula method established the joint probability density function of wind, wave, and current. An improved grid-based method was used to account for these loads, selecting conditions that meet the confidence level by calculating occurrence probabilities. Time-domain analysis yielded the flexible riser stress time history, and the Miner linear damage accumulation method calculated damage at critical points. The overall lifespan pattern of the riser was analyzed with the joint probability distribution. Results indicate that large heave response amplitude and higher current speeds significantly reduce the riser's fatigue life. The improved grid-based method effectively reduces computational load, providing a feasible approach for designing the fatigue life of ocean thermal energy conversion risers under wind, wave, and current loads.

Fatigue analysis of offshore wind turbines with soil-structure interaction and various pile types

Monopile offshore wind turbines (OWTs) are susceptible to fatigue damage, and time domain fatigue analysis is time-consuming and unsuitable for simulating various loading conditions. In this study, frequency domain fatigue analysis of OWTs with soil-structure interaction (SSI) and different monopile designs (the conventional monopile, monopiles with an extension, a wheel, and four spoiler ribs) is conducted. First, detailed 3D finite element models of OWTs are developed. The tower, monopile, and soil are modelled using solid elements, and SSI is accounted for using the Mohr-Coulomb failure criterion and surface-to-surface contact. Next, the power spectral densities of wind and wave loads on the tower and monopile are derived. Subsequently, the fatigue damages of the OWTs are evaluated under individual and combined wind and wave loads. The results show that damage increases with wave height, peaking at the rated wind speed and wave frequency close to the first frequency of the OWTs. Wind-induced damage is significantly greater than wave-induced damage, and the combined wind and wave loads produce more damage than the sum of the individual loads. Additionally, alternative monopile designs, particularly the monopile with a wheel, can reduce fatigue damage, though stress concentration effects on the wheel and ribs require careful consideration.

Influence of Foundation–Soil–Foundation Interaction on the Dynamic Response of Offshore Wind Turbine Jackets Founded on Buckets

This study investigates the impact of soil–structure interaction (SSI) and foundation–soil–foundation interaction (FSFI) on the dynamic behaviour of jacket substructures founded on buckets for offshore wind turbines. A parametric analysis was conducted, focusing on critical load cases for conservative foundation design. Different load configurations were examined: collinear wind and wave (fluid–structure interaction) loads, along with misaligned configurations at 45° and 90°, to assess the impact of different loading directions. The dynamic response was evaluated through key structural parameters, including axial forces, shear forces, bending moments, and stresses on the jacket. Simulations employed the National Renewable Energy Laboratory (NREL) 5MW offshore wind turbine mounted on the OC4 project jacket founded on suction buckets. An additional optimised jacket design was also studied for comparison. An OpenFAST model incorporating SSI and FSFI considering a homogeneous soil profile was employed for the dynamic analysis. The results highlight the significant role of the FSFI on the dynamic behaviour of multi-supported jacket substructure, affecting the natural frequency, acceleration responses, and internal forces.

A Quasi Time-Domain Method for Fatigue Analysis of Reactor Pressure Vessels in Floating Nuclear Power Plants in Marine Environments

The reactor pressure vessel (RPV) in onshore nuclear power plants is typically analysed for fatigue life by considering the temperature, internal pressure, and seismic effects using a simplified time-domain fatigue analysis. In contrast, the frequency-domain fatigue analysis method is commonly employed to assess the fatigue life of ship structures. The RPV of a floating nuclear power plant (FNPP) is subjected to a combination of temperature, internal pressure, and wave loads in the marine environment. Consequently, it is essential to effectively integrate the frequency-domain fatigue analysis method used for hull structures with the time-domain fatigue analysis method for RPVs in FNPPs or, alternatively, to develop a suitable method that effectively accounts for the temperature, internal pressure, and wave loads. In this study, a quasi-time-domain method is proposed for the fatigue analysis of RPVs in FNPPs. In this method, secondary components of marine environmental loads are filtered out using principal component analysis. Subsequently, the stress spectrum induced by waves is transformed into a stress time history. Fatigue stress under the combined influence of temperature, internal pressure, and wave loads is then obtained through a stress component superposition method. Finally, the accuracy of the quasi-time-domain method was validated through three numerical examples. The results indicate that the calculated values obtained by the quasi-time-domain method are slightly higher than those obtained by the traditional time-domain method, with a maximum deviation of no more than 24%. Additionally, the computation time of the quasi-time-domain method is reduced by 98.67% compared to the traditional time-domain method.

Time-domain simulation of nonlinear motions of moored floating body in irregular waves using rankine source method

This study introduces a three-dimensional time-domain Indirect Boundary Integral Equation Method (IBIEM) for simulating the motion of the floating body in both regular and irregular waves. The method systematically addresses second-order wave loads, focusing on low-frequency motions induced by low-frequency wave loads, crucial for the stability of offshore structures. Unlike traditional methods, the time-domain IBIEM simplifies the analysis by simultaneously considering all wave components of irregular waves, making the calculation of nonlinear wave loads more straightforward and intuitive. An approximate second-order equation of motion is proposed, simplifying the computational process by eliminating the need for solving boundary value problems for second-order radiation forces. The IBIEM is coupled with a quasi-static mooring model based on the elastic catenary equation, accurately capturing the influence of mooring systems on the response of the floating body. Validation against model tests and established numerical methods demonstrates the accuracy of the developed code in predicting the dynamic behavior of offshore structures. Applied to various scenarios, including a truncated cylinder, the barge, H2FPSO, and K-Semi, the results show strong agreement with experimental data, confirming the reliability and effectiveness of the method in simulating complex wave-structure interactions, making it a valuable tool for offshore structure design and analysis.

Effects of foundation conditions on failure modes of jacket type offshore platforms using incremental irregular wave dynamic analysis

Quantitative assessment of environmental conditions and their loading effects is crucial for the optimal design of marine structures, especially offshore platforms subject to extreme environmental conditions. Wave load, particularly irregular wave load, significantly influences the structural response and ultimate strength of jacket-type offshore platforms (JTOPs) throughout their operational life. The dynamic nature of these irregular wave loads necessitates the use of advanced time history analysis to accurately capture their impact. This study aims to enhance the understanding of how foundation conditions impact failure modes and the structural response of JTOPs through an innovative incremental irregular wave dynamic analysis procedure. By focusing on an existing steel jacket-type platform in the Persian Gulf, this research evaluates six models with varying soil profiles and pile conditions to assess their effects on structural performance. The application of incremental irregular wave dynamic analysis provides a more precise and realistic modeling of wave impacts compared to traditional methods. The findings demonstrate that stronger soil profiles and improved pile conditions significantly enhance structural integrity, particularly under extreme loading conditions. Specifically, optimizing pile configurations is critical for enabling the structure to endure larger waves, whereas variations in the yield stress of steel material have a lesser impact on the platform's response. Additionally, the dynamic results are compared with pushover analysis, which serves as a benchmark to evaluate the accuracy and effectiveness of the pushover analysis compared to incremental irregular wave analysis method used in this study.

Numerical modeling of seismic behavior of Caisson foundation for Offshore Wind Turbines taking into account wave and marine currents forces

It is crucial to simulate the seismic behavior of offshore wind turbines, especially when dealing with foundations on non-cohesive soil. There is a risk of liquefaction occurring, which highlights the need to obtain values of excess pore water pressure. In this study, we created a three-dimensional model of a caisson foundation for an offshore wind turbine on loose sandy soil from the Syrian coast. The Mohr-Coulomb constitutive plasticity model was used to analyze two scenarios. The first scenario involved applying wind and earthquake loads, while the second scenario included marine currents and wave loads in addition to the wind and earthquake loads. We used Coupled acoustic-structural medium analysis after confirming its effectiveness on the soil through comparison with simulation results of the FLAC3D program from a previous study. The numerical modeling results indicated that it is possible to use Coupled acoustic-structural analysis in soil and water modeling. The study monitored the values of excess pore water pressure and found that liquefaction occurred in the soil due to the earthquake. The analysis also highlighted the importance of considering wave and marine currents loads in analyzing these structures. While these factors had a slight impact on excess pore pressure values, they significantly affected the directions and values of displacements.

Experimental Investigation of Nonlinear Forces on a Monopile Offshore Wind Turbine Foundation Under Directionally Spread Waves

Abstract Accurate prediction of nonlinear wave loading is crucial for designing marine and offshore structures, yet it remains a challenging task. Prior research has primarily focused on uni-directional extreme sea states, revealing that linear loading cannot accurately represent the total wave forces acting on offshore wind turbine foundations, with significant contributions from high-order harmonics. This study broadens the scope to include multi-directional and bi-directional wave interactions with monopile offshore wind turbine foundations. We use a phase-based harmonic separation method to isolate harmonic components in the presence of complex wave scenarios. This approach allows for the clear delineation of individual harmonics from the total wave force by controlling the phase of incident focused waves. Remarkably, this method shown effective even with multi-directional and bi-directional spreading. The clean separation of individual harmonics enables the estimation of contributions from each harmonic. Our findings are in line with previous research, showing that nonlinear loading can constitute up to 40% of the total under certain wave conditions. We have also observed that wider wave spreading reduces nonlinear high-order harmonics, and uni-directional waves induce the most severe nonlinear forces. These insights emphasise the importance of accounting for high-order nonlinear wave loading in offshore structure design.

An Improved Direct Forcing Immersed Boundary Method With Integrated Mooring Algorithm for Floating Offshore Wind Turbines

Abstract An upgraded direct forcing immersed boundary method is implemented in the open-source hydrodynamic framework REEF3D::CFD for simulating the six-degrees-of-freedom (6DOF) motions of a floating offshore wind turbine (FOWT) based on the OC5 semi-submersible design. The direct forcing method is enhanced with a new density interpolation method across the fluid-structure interface that removes unphysical spurious phenomena and ensures stable and accurate wave load calculations on floating objects. A quasi-static algorithm is used for modeling the mooring system of the OC5 platform and restraining its motions in waves. The Navier-Stokes equations are solved on a staggered structured rectilinear grid for the hydrodynamic simulations. The level-set method is used to capture the free surface of the ocean waves. A ray-casting algorithm is employed to get inside-outside information near the fluid-solid interface while maintaining the underlying Cartesian grid in the hydrodynamic domain. The performance and accuracy of the mooring algorithm are compared to the widely-used mooring model MoorDyn, which is coupled to the hydrodynamic solver in REEF3D::CFD. The study demonstrates that the enhanced direct forcing method with the integrated quasi-static mooring algorithm in REEF3D::CFD provides a robust and accurate tool, suitable for the numerical analysis of the state-of-the-art FOWT in ocean waves.

Scale effects on wave loads and slamming loads in ship hydroelasticity simulation by CFD-FEM method

A partitioned CFD-FEM two-way coupled method is employed to simulate the wave loads and hydroelasticity of a 310-m-long ship at different scales and the scale effects are investigated. Four different scales, including three model scale 1:100, 1:50, 1:25 and one full-scale 1:1, of the ship under equivalent wave conditions are calculated for comparison. The results of the resistance in waves, global motions, wave loads, slamming and green water pressures for ships at different scales are comparatively analyzed from the aspects of time series, frequency spectra and statistics values to investigate the scale effects. The research interest mainly lies in the scale effect on slamming and green water pressures on the flexible ships. The scale effects on ship seakeeping and slamming pressures by pure CFD simulation are also comparatively studied. It can be concluded that the scale effect has little influence on ship vertical motions and loads, while it has more influence on slamming pressure loads and green water on deck, but the deviation caused by other uncertainty factors can be comparable with that of scale effects.