Analysis Of Offshore Wind Turbine Research Articles

Analysis of Offshore Wind Turbine by Considering Soil-Pile-Structure Interaction: Effect of Sea-Wave Load Duration

ABSTRACT Offshore Wind Turbines (OWTs) confront different types of environmental loads during their lifetime. One of the most significant loads is the cyclic sea-wave load which affects the OWT system during the approximate 25 years of design life. This lateral cyclic load can influence the response of the OWT system because of accumulated permanent displacement and excess pore water pressure generation in soil. This procedure can change pile-soil stiffness. However, the behavior of OWT and its foundations is mostly studied under short-term cyclic loading, and the effects of duration of the cyclic loads on pile-soil interaction and performance of the foundation are not well understood and documented. Therefore, there is a lack of guidance in codes for the duration effects of cyclic loads on the structural and geotechnical response of OWTs. In this regard, the current study considers the effects of duration of the cyclic loads on serviceability and performance of the OWT system by considering soil-foundation-structure interaction using a 2-D finite element method. The behavior of the OWT system is evaluated based on the internal forces and deformation of the monopile foundation, shear strain, and excess pore water pressure ratio in the surrounding soil. Besides, liquefaction susceptibility in the sandy soil layer at the vicinity of the monopile and its effect on the performance of the foundation is investigated. Finally, the results can provide guidance on estimation of the dynamic performance of the OWT system during long-term cyclic loads. They can be used to specify the need for consideration of the duration of cyclic lateral loads for the design of OWT structures.

Preface

Fourth Conference of Computational Methods & Ocean Technology (COTech 2023) November 30 – December 1, 2023, University of Stavanger, Stavanger, Norway The COTech (Computational Methods & Ocean Technology) conference started as part of the research and dissemination activities of the Program Area for research “COTech - Computational methods in Offshore Technology” at Faculty of Science and Technology, University of Stavanger (UiS) with its first event in 2017. This Program Area for Research was founded in 2015 with seven professors, four associate professors, two adjunct professors and five research (PhD) students from the Department of Mechanical and Structural Engineering and Materials Science (IMBM), whose expertise and competence lies primarily within use of computational methods such as finite element methods, boundary and volume element methods, computational fluid dynamics and the like in marine and subsea technology, marine operations, design and analysis of mechanical systems, integrity and reliability of offshore structures and mechanical systems, renewable energy and wind engineering. In the ocean-related engineering area in particular, numerical computation approach is nowadays not only serving as a means to cultivate and realize innovative ideas, but also it is becoming the primary choice to solve complex engineering problems for the harsh and unfriendly environment in the Arctic.Like the previous conference, this year’s COTech conference is conducted as part of the dissemination activities of the Institute Strategic Program (ISP) entitled “Computational Methods and Ocean Technology” established under the research activities of the Department of Mechanical and Structural Engineering and Materials Science. This three-year program (2021 - 2023) intends to bring researchers of the department under four selected research areas:1. Ocean Energy Conversion - installations & dynamic analysis of offshore wind turbines, structural health monitoring, corrosion assisted fatigue, ocean wave energy.2. Aquaculture Technology - efficient farming techniques, advanced numerical modeling and computation, fluid-structure interaction, design effective maintenance programs for aquaculture.3. Marine and Subsea Technology - sediment erosion (scour), scour prediction CFD models, structural integrity and fatigue life of offshore structures, adaptive control of ROVs in subsea application, predictive maintenance of subsea structures4. Innovative Solutions - 3D printing based innovative solutions, inspection drones, augmented reality for maintenance training,…In addition to the thematic areas covered in the last three conferences, the 4th COTech conference accommodated some of the emerging technologies and applications such as machine learning techniques, additive manufacturing technologies, 3D printing in healthcare technology, smart energy storage and bio-inspired design.In general, the conference is intended to provide a platform for academics and professionals working within diverse forms of the computational methods, Ocean/Offshore technology and Oil and Gas technologies to come together, present their recent works in the area, exchange ideas, and establish professional networks. It will serve as a forum for multidisciplinary research and bring together enable them to exchange their research experience and disseminate their results within the involved fields.List of Conference Organizing Committee, Volume Editors, Invited Keynote Speakers, Additional plenary session speakers, Technical Committee Members and Reviewers are available in this Pdf.

An integrated linguistic Pythagorean fuzzy decision-making approach for risk analysis of offshore wind turbine

The evaluation of risk factors within the context of condition-based maintenance (CBM) for offshore wind turbines has been a challenging issue. In this paper, a novel risk analysis method that combines the fuzzy weighted with zero inconsistency (FWZIC) technique and the DEMATEL method under the linguistic Pythagorean fuzzy environment is proposed. Firstly, 75 risk factors linked to offshore wind turbines are identified by leveraging analysis and prior literature, which are modeled using linguistic Pythagorean fuzzy sets (LPFSs) to facilitate the complexity of expert opinions in the face of uncertainty. Then, the hybrid expert weight calculation method and the FWZIC-based criteria weight calculation method are introduced to determine the weights of the experts and the criteria while considering uncertainty. Next, the extended DEMATEL method under the linguistic Pythagorean fuzzy environment is adopted to determine the significance and influence of the pinpointed risk factors, and an illustrative cause–effect diagram is crafted to dissect the interconnections between these factors. Through this intricate analysis, the cause factors and effect factors specific to offshore wind turbines are identified, and the results obtained from the proposed method are validated through sensitivity analysis and comparative analysis. The results of this study could provide valuable recommendations and references for the implementation of CBM in offshore wind turbines.

Fatigue life analysis of offshore wind turbine under the combined wind and wave loadings considering full-directional wind inflow

Fatigue damages of an offshore wind turbine (OWT) would be caused by long-term wind and wave loads, necessitating the importance of fatigue life analysis to ensure the in-service behaviors of OWT. This study evaluates the fatigue damage and predicts the fatigue life of OWT considering full-directional wind inflow under the coupling effects of non-Gaussian wind field and wave. The forces and bending moments are assessed in three types of the wind field, i.e., Gaussian, softening non-Gaussian, and hardening non-Gaussian, according to the Kaimal spectrum by using OpenFAST. The dynamic responses of OWT are implemented by a three-dimensional finite element model (FEM) established by ABAQUS. The fatigue life is assessed in the tower base section and the pile of OWT under different loading combinations. Results show that the crack initiation life is greatly affected by the non-Gaussian wind field, while the crack propagation life is not that sensitive. The fatigue life of OWT under a full-directional wind inflow is approximately three times longer than that under a single-directional wind inflow. It is recommended to consider full-directional wind inflow in the non-Gaussian wind field for OWT design.

Dynamic Analysis of Offshore Wind Turbine Supported by Jacket Substructure under Wind and Wave Loading

Burning of fossil fuel for the production of energy causes severe global warming effects. Renewable energy sources like solar, wind and tidal etc. are the alternative renewable energy sources which contribute in the reduction of adverse global warming effects. Wind turbines are being used for extracting wind energy from several years. Wind blow is continuous with limited disturbance in the offshore region when compared with main land. Offshore wind energy extraction is in research stage at many locations and implemented in European countries. Prediction of response of wind turbine supporting systems is essential in the design to withstand the environmental loads such as wind, wave, current and seismic etc. In the present study, a horizontal axis offshore wind turbine (HAWT) supported on an offshore jacket structure is considered and the response studies are performed. The jacket is considered at a water depth of 51m, thus total height of the jacket is 61m with a free board of 10m. A wind turbine of 5MW capacity is considered to be on top of jacket structure. The height of the wind tower is assumed as 70m, and a transition structure of 4m height is positioned in between jacket and tower. A free vibration analysis is performed to estimate the natural frequencies and mode shapes of the jacket supported wind turbine. The modal analysis is carried out using ANSYS static structural module. The response analysis under wind, wave, current and aerodynamic drag loads is performed using SACS 13.2 software. Wind force is estimated based on API 2005 provisions. The aerodynamic forces on the wind turbine blades are evaluated using Betz Theory. Wave loading is calculated using Morison equation and linear Airy’s wave theory. A parametric study is carried out by varying wave period from 6s to 20s. As the structure is symmetric about longitudinal and lateral directions, a wave directional analysis is also carried by considering 0o and 45o wave directions. The structural responses are studied for the combined wind, wave and current loads. Cut-in, rated, cut-out and storm conditions are simulated by modelling wind and aerodynamic loads on the tower, wind interacting area of the jacket and blades. Wave period and direction are varied to simulate different wave conditions. It is observed that the structural response is increasing as the wind velocity is increasing and wave period is decreasing.

Wind and soil model influences on the uncertainty in fatigue of monopile supported wind turbines

Several alternative engineering models are available for the use in analysis of offshore wind turbines. However, it is not always clear which of the models will yield the most accurate or sufficiently conservative results. This paper investigates the effect of using two alternative soil-structure interaction models and two wind coherence models. The focus is on assessing how these modelling choices influence the predicted long-term fatigue damage in the support structure. The two soil models are a macro-element model and a p-y-curve model with Rayleigh damping. This gives differences in both the damping and stiffness properties of the turbine model. The differences between the two soil models tend to decrease as the turbine size increases. The wind coherence models considered are the Kaimal spectrum with exponential coherence and the Mann uniform shear turbulence model. The Kaimal model predicts the highest response at low frequencies, while the Mann model gives the highest response predictions at higher frequencies. Which turbulence model predicts the highest long-term fatigue damage is then determined by the natural frequencies, rotor and blade passing frequencies of the different turbines.

Dynamic analysis of offshore wind turbines subjected to the combined wind and ice loads based on the cohesive element method

Ice loads are an important and decisive factor for the safe operation of offshore wind turbines (OWTs). In severe environment load cases, it shall lead to prominent ice-induced vibration and ice-induced fatigue failure of OWT structures. Based on the cohesive element method (CEM) and considering the pile–soil interaction used by nonlinear distributed springs, the full interaction model of the ice and monopile OWT structure with an ice-breaking cone in a cold sea region is established in this study. Furthermore, the Tsai-Wu failure criterion and the empirical failure formula of maximum plastic failure strain are used to describe the mechanical behavior of ice bending failure in the collision simulation tool LS-DYNA, and the dynamic ice loads under different ice velocities and cone angles are statistically analyzed. Finally, according to the interaction process between sea ice and OWT containing the ice-breaking cone, the dynamic response of OWT under the combined wind and ice loads is studied, and the most reasonable ice-breaking cone angle is determined. The results show that the method adopted in this paper can well simulate the bending failure process of sea ice. Concurrently, the cone angle has a significant impact on the dynamic response and damage of the OWT, and the recommended optimal cone angle is 60.

An efficient fatigue assessment model of offshore wind turbine using a half coupling analysis

Fatigue failure is an important issue for offshore wind turbine support structures. Especially, fully coupled time domain analysis will lead to high computational burden. Therefore, accurate and efficient fatigue evaluation methods have always been the common goal pursued by the industry and researchers. This paper evaluates fatigue damage under combined wind and wave loading, using a fixed jacket offshore wind turbine as an example. A half coupling model (HCM) with distinct advantage by considering aerodynamic damping and structural damping is developed for dynamic response analysis of offshore wind turbine. The precision and effectiveness of the developed HCM is verified to be reasonable by comparing with numerical simulated results. A linearized Morison equation is proposed to achieve frequency domain analysis of wave response which is embedded as a programming module in ANSYS. Based on the developed model, dynamic response analysis is carried out under wind and wave loading, and then combined stress spectra at the location of different tubular joints (X-joint and K-joint) are obtained. To consider the interaction of wind and wave, a spectral fatigue combination rule based on Han and Ma's method is used to calculate fatigue damage. The developed combination rule is assessed by compared with time domain fatigue result based on rainflow cycle counting. Analysis using the newly developed model and method demonstrates a high accuracy and efficiency for fatigue damage prediction.

Fatigue design sensitivities of large monopile offshore wind turbines

AbstractThe input for fatigue analyses of offshore wind turbines is typically chosen based on design values provided by design standards. While this provides a straightforward design methodology, the contribution of different input parameters to the uncertainty in the fatigue damage estimates is usually unknown. This knowledge is important to have when improving current designs and methodologies, and the parameters governing the uncertainty is typically found through a sensitivity analysis. Several sensitivity studies have been performed for monopile‐based offshore wind turbines, typically focusing on specific turbines and engineering disciplines. This paper performs a sensitivity study for three monopile‐based offshore wind turbines (5 MW, 10 MW, 15 MW) using parameters from several engineering disciplines. The results show that the fatigue utilization is primarily governed by the uncertainty in the SN curves and fatigue capacity. Following this, the uncertainty in the environmental conditions is the dominating uncertainty, with wind loads becoming increasingly important as turbine size increases. Additionally, the effect of modelling uncertainties is investigated. The wind‐related model uncertainties dominate in the tower top, while uncertainties in the wave and soil models dominate in the tower base and monopile. Designers wanting to reduce the uncertainty in a design are recommended to focus on the environmental conditions, and using as accurate models as possible. All modelling uncertainties are significant, but research should particularly be focused on wave directionality and soil models.

Influence of Pile-Soil Interaction on Dynamic Properties and Response of Offshore Wind Turbine with Monopile Foundation in Sand Site

In this study, a sophisticated 3D finite element model of NREL 5MW wind turbine (WT) superstructure is developed using Abaqus, which includes necessary functional features, such as rotor rotation, drivetrain, pitch, servo-control, etc. A multi-step validation process has been conducted in order to verify the reliability of the blade, the Rotor-Nacelle Assembly (RNA) and the entire WT model. In the time domain, a software framework of ABA-OWT is developed for the dynamic analysis of offshore wind turbine (OWT) based on the Abaqus WT structure model and FAST submodules. The framework is capable of computing wind loads, wave loads, and servo-control commands in a fully coupled way. The pile-soil interaction (PSI) is modeled by nonlinear p-y, t-z and Q-z springs and the dashpots parallel with them. A typical sandy site is considered, while several identical analyses are performed for both numerical models including fixed base and PSI. After calculating the eigenvalues of the WT structure and analyzing its dynamic response to the combined action of wind and wave, three representative wind-wave load cases are chosen for subsequent seismic response study. The important conclusions are as follows. Firstly, the PSI reduces the natural frequencies of FA1, SS1 and FA2 by about 12% and SS2 by 23.5%. Secondly, under the combined action of wind and wave, the PSI significantly increases the tower-top peak motion responses in normal operating cases, but its influence on the mudline peak internal force responses is almost negligible; For parking cases, the PSI dramatically enlarges the peaks of the tower-top motion responses and the mudline bending moment, and the effect of PSI on the structural peak responses in parking cases is greater than that in normal operating cases. Finally, under the conjoint action of wind, wave and seismic, the PSI increases the tower-top peak motion responses and reduce the mudline peak internal force responses, and because of the aeroelastic effects, the influence of PSI on the structural peak responses in normal operating cases is always the smallest. In a word, the PSI has a remarkable impact on the natural frequency, dynamic response and seismic response of offshore wind turbine (OWT) with monopile foundation, which should be considered in a reasonable way in the practical offshore wind power project.

A new method for structural safety and reliability analysis of offshore wind turbines

With the motivation to overcome the shortcomings of the Rosenblatt inverse-first-order reliability environmental contour method (IFORM), in this study, the use of bivariate kernel density estimation with smoothed cross-validation bandwidth selection method is proposed for generating more accurate environmental contour lines. The environmental contour lines at a chosen offshore site obtained by using the proposed new method were compared with those obtained by using the Rosenblatt IFORM, and the accuracy and effectiveness of the proposed new method have been fully and clearly substantiated. Next, the 50 year extreme structural dynamic responses of a monopile-supported 5 MW offshore wind turbine installed at this chosen offshore site based on the proposed new method and the Rosenblatt IFORM were calculated. Analysing the calculated results, it can be found that the 50 year extreme fore−aft shear force value based on the 50 year extreme sea state obtained using the proposed new method is 78.9% larger than the corresponding value obtained based on the Rosenblatt IFORM. The calculation results in this paper were further systematically analysed and compared, and the necessity and importance of using more realistic environmental contour lines (such as those generated using the proposed new method) have been finally highlighted.

Structural performance degradation identification of offshore wind turbines based on variational mode decomposition with a Grey Wolf Optimizer algorithm

The performance degradation assessment of offshore wind turbine (OWT) structures plays a crucial role in ensuring the safe operation of the structures. This paper presents a method for assessing the performance degradation of OWT structures based on an optimized variational mode decomposition (VMD) algorithm. The method introduces the technique of multisensor data fusion and the Grey Wolf Optimizer (GWO) algorithm to optimize the setting of VMD parameters and then enables the assessment of the structural performance degradation of offshore wind turbines by extracting the structural performance degradation signature of the structure from noise-reduction data. To demonstrate the correctness and advantages of the method in this paper, a 4-degree-of-freedom (4-DOF) system under the action of white noise is used. Numerical calculations show that the developed method can accurately identify the 5%, 10%, 15% or 20% reduction in the structural stiffness and the coefficient of variance of the optimal structure is decreased to 3.6% of the initial design. To further investigate the performance of the proposed strategy, physical tests of the offshore wind monopile structure are conducted. As the loss of the overall structural performance is simulated by removing different numbers of bolts from the connected device, it is demonstrated that the proposed method can effectively assess the structural performance. Finally, field measurements are carried out on a monopile OWT located near Rudong County, Jiangsu Province, in the Yellow Sea of China. The impact of typhoons on the performance of the structure is effectively assessed by analyzing measurement data before and three days after the typhoon In-Fa through the wind farm, the potential application of the method in the structural safety assessment and analysis of OWTs is validated.

Data‐driven virtual sensing and dynamic strain estimation for fatigue analysis of offshore wind turbine using principal component analysis

AbstractVirtual sensing enables estimation of stress in unmeasured locations of a system using a system model, physical sensors and a process model. The system model holds the relationship between the physical sensors and the desired stress response. A process model processes both the physical sensors and the system model to synthesise virtual sensors that ‘measure’ the desired stress response. Thus, virtual sensing enables mapping between the physical sensors (input) and the desired stress response (output). The system model is a mathematical model of the system based on knowledge or data of the system. Here, the data‐driven system model is constructed directly on data analyses for the specific system. In this paper, supervised learning and data‐driven system models are applied to strain estimation of an offshore wind turbine in the dynamic range through a novel use of principal component analysis (PCA); 40 min of training data is used to establish the data‐driven system model that can estimate the dynamic strain response with high precision for 2 months, while the estimated fatigue damage averaged out to −1.76% of the measured strain response.

Fragility analysis of offshore wind turbine expose to near-field pulse-like ground motion

Fragility analysis of offshore wind turbine expose to near-field pulse-like ground motion

Analysis of Offshore Wind Turbine by considering Soil-Pile-Structure Interaction: Effects of Foundation and Sea-Wave Properties

ABSTRACT Prediction of the dynamic performance of an offshore wind turbine (OWT) requires consideration of many different design parameters. Besides the superstructure, the OWT foundation also plays an important role both functionally and financially in the design. In this study, numerical dynamic analyses of an offshore wind turbine with a monopile foundation are performed under wave loading that may lead to soil liquefaction around the pile due to cyclic stresses induced by the pile displacements using the open-source program, OpenSees. Effects of foundation properties such as monopile diameter, pile embedment depth, and sea-wave characteristics such as its period, sea-water depth, duration, and level of the loading on the dynamic performance of the system are investigated. The results in terms of deformations, excess pore water pressure, and inertial forces are presented and discussed. The findings are considered as valuable guidance on the estimation of the dynamic performance and liquefaction susceptibility of the offshore wind turbine foundations under cyclic sea-wave loads.

Seismic fragility analysis of monopile offshore wind turbines considering ground motion directionality

The necessity of performing a detailed seismic fragility analysis of offshore wind turbines (OWTs) is well established in the wind power industry. This paper presents a seismic fragility analysis for large-scale monopile offshore wind turbines (MOWTs) considering ground motion directionality. Based on recommendations by the Eurocode EN 1993-1-6, DNVGL-ST-0126 and literature study, the critical displacement, rotation, local buckling strength for structural components were proposed. Fragility curves under different angles of ground motion incidence were obtained by using the truncated incremental dynamic analysis (TIDA) and compared between the first scenario of the wind turbine parked in a calm sea and the second scenario of the wind turbine in normal operation at the rated wind speed of the turbine. The results show that ground motion directionality has obvious influence on the seismic fragility of the MOWT. Wind and wave loads increase the probability of the MOWT exceeding limit states and amplify the ground motion directionality effects on the structural fragility. Neglecting aerodynamic damping leads to an overestimation of the structural fragility. The excessive displacement at the tower top, local buckling at the bottom of the tower and in the monopile at the mudline section, and tension fracture in the grouted connection are the critical failure modes of the MOWT being subjected to earthquakes.

Dynamic analysis of OWT foundation with large diameter monopile under transient storm loading

ABSTRACT To investigate the stability of monopile foundation under dynamic loading, a comprehensive numerical model for the analysis of offshore wind turbines (OWT) foundation under a general transient storm loading is presented in this study. The dynamic stiffness and soil deformation around the large-diameter monopile is simulated using this method. During the numerical analysis, a dynamic boundary surface model of soil is derived instead of the empirical strength degradation. Along the axis direction of the monopile, an intensive study about deformation law of the seabed soil is analysed, moreover, some parameters which may affect the OWT stability and the dynamic stiffness are discussed. Some conclusions can be drawn that the dynamic stiffness and the lateral displacement of the monopile foundation can be obviously improved by increasing the buried depth than the diameter, and the proposed failure mode can well describe the failure law of soil around the monopile due to the dynamic loading.

Multi-body dynamic analysis of offshore wind turbine considering soil-structure interaction for fatigue design of monopile

Fatigue design of monopile foundation for an offshore wind turbine using multi-body dynamic analysis is the theme of this paper. For this purpose, a benchmark turbine is modelled in Kane's approach considering soil-structure interaction for a site at the North Sea. Each soil layer is modelled using the p-y curve, and the dynamic response of the combined system is simulated using aero-hydro-servo-elastic analysis. The fatigue life is estimated following IEC 61400-3 guidelines. The numerical analysis presented in this study shows stress-based fatigue analysis using rainflow algorithm in the light of soil-structure interaction for short-term damage accumulation. Besides short-term effects, the study also investigates serviceability criteria over the lifespan of the structure, i.e. long-term effects. Finally, design curves are developed for a given power rating. Overall, the study highlights the importance of soil-structure interaction and its impact on the fatigue life of offshore wind turbines.

Gaussian process regression for fatigue reliability analysis of offshore wind turbines

The fatigue limit state (FLS) often drives the design of offshore wind turbine (OWT) substructures in European waters. Assessing fatigue damage over the intended design life of an OWT is computationally expensive, primarily as dynamic structural analyses have to be run for a large number of stochastic wind and wave loading conditions. This makes structural reliability assessment for the FLS a challenging task. In addition to evaluating load-induced fatigue damage, simulation-based structural reliability analysis also requires sampling of random variables that model uncertainties in the capacity of OWT structural components. To this aim, we develop and validate a computational framework for OWT fatigue reliability analysis that relies on Gaussian process (GP) regression to build surrogate models of load-induced fatigue damage. We demonstrate that the proposed approach can reduce the computational effort required to evaluate FLS reliability with high accuracy through application to three plausible offshore wind farm sites in Europe. The sensitivity of various goodness-of-fit metrics to different model assumptions is investigated to further reduce the computational effort required to perform GP regression/predictions. The results from this study can provide guidance for practical applications of the proposed framework in OWT projects.

Dynamic analysis of offshore wind turbines

For large-scale 5MW offshore wind turbines, the discrete equation of fluid domain and the motion equation of structural domain with geometric nonlinearity were built, the three-dimensional modeling of the blade considering fluidstructure interaction (FSI) was achieved by using Unigraphics (UG) and Geometry modules, and the numerical simulation and the analysis of the vibration characteristics for wind turbine structure under rotating effect were carried out based on ANSYS software. The results indicate that the rotating effect has an apparent effect on displacement and Von Mises stress, and the response and the distribution of displacement and Von Mises stress for the blade in direction of wingspan increase nonlinearly with the equal increase of rotational speeds. Compared with the single blade model, the blade vibration period of the whole machine model is much longer. The structural coupling effect reduces the response peak value of the blade displacement and Von Mises stress, and the increase of rotational speed enhances this coupling effect. The maximum displacement difference between two models decreases first and then increases along wingspan direction, the trend is more visible with the equal increase of rotational speed, and the boundary point with zero displacement difference moves towards the blade root. Furthermore, the Von Mises stress difference increases gradually with the increase of rotational speed and decreases nonlinearly from the blade middle to both sides. The results can provide technical reference for the safe operation and optimal design of offshore wind turbines.