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

Highlights

• Multi-body dynamics of the offshore wind turbine is modelled using Kane's approach.

• Fatigue design is carried out considering soil-structure interaction and rotor dynamics.

• Long-term performance of the monopile is investigated in terms of characteristics strain.

• Site-specific design curves are generated for the design of monopole in different flow conditions.

Abstract

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.

Introduction

Offshore wind farms have gained popularity in the recent past due to its various advantages, e.g. steady wind flow over an extended period and vast space in the marine environment. Many of these turbines in shallow water (depth around 30$~$50 m) are supported by monopiles, whose dimensions depend on the size and power rating of the turbines, besides other environmental factors. The design of these structures is challenging and requires a high-fidelity model of the complete multi-body rotating system, where soil-structure interaction (SSI) plays a vital role.

Various methods are proposed in the literature to model the soil-structure interaction. For example, a closed-form expression for finding the fundamental frequency of a wind turbine tower under SSI was developed and experimentally validated by Adhikari and Bhattacharya [1]. Finite element analysis of monopile supported offshore wind turbine is a popular approach, where soil properties are modelled by linear or non-linear springs [2]. The dynamic interaction results in an increased response of the combined system. Zuo et al. [3] modelled NREL 5 MW benchmark wind turbine in the finite element framework using ABAQUS and observed a significant reduction of the tower natural frequencies due to SSI. They also noticed that blade and tower responses increased with rotor speed. Thus, monopile design without considering blade rotation led to gross underestimation that could subsequently affect the structure. Bisoi and Haldar [4] studied soft-soft and soft-stiff monopile design and checked the overall safety against serviceability and fatigue limit states. They observed that the length of monopile below its critical depth had a negligible impact on its design.

Rong et al. [5] derived an analytical formula for the natural frequency of monopile supported wind turbine using Euler-Bernoulli beam model with p-y curves for foundation stiffness. In this context, different aspects of offshore foundation design may be found in Bhattacharya [6]. This book explains the importance of soil-structure interaction and multiple methods to evaluate the foundation stiffness. It also provides a comprehensive review of different methods used for long-term behaviour estimation. Banerjee et al. [7] modelled the effect of foundation flexibility using an equivalent spring-dashpot model for monopile below the mudline. They carried out dynamic analysis using four different soil conditions and observed that the total displacement at the tower-top increased with reduced shear wave velocity, especially for lower soil stiffness. Kementzetzidis et al. [8] studied different geotechnical aspects of monopile foundation using 3D non-linear finite element analysis with a significant emphasis on the robustness of soft-stiff design, particularly in the extreme weather. In this context, impact load test on pile [9,10] is a standard tool to estimate its stiffness and in-situ frequency response function for finite element modelling.

Over the last decade, offshore wind turbines witnessed significant growth in its size to meet the power demand. Due to this reason, the load-carrying capacity of monopile has to increase to accommodate more giant turbines. This requirement has led to developing the new methodologies to improve the bearing capacity using stiffener to avoid the abnormal size of monopile [11]. In general, fatigue design focuses on the short-term effects, while long-term effects are equally crucial for serviceability. Katsikogiannis et al. [12] studied the stress due to axial force and bi-directional bending moments for damage estimation. They compared the sensitivity of three soil modelling techniques (viz. macro-element model, linear elastic stiffness model with soil damping and p-y curves) under wind-wave misalignment. Their study concluded that significant fatigue damage might occur for misalignment greater than 30°.

In general, soil type around the monopile affects its design life. Schafhirt et al. [13] investigated this influence on long-term performance due to cyclic loading. They modelled soil-structure interaction by non-linear p-y curves to estimate the fatigue life using the mudline bending moment, which was affected by the softening of the surrounding soil. Besides spring stiffness, aero-elastic and hydrodynamic damping play a significant role in fatigue life and cost-effective design [14,15]. In this context, current industry standards are often inadequate [16] for the upcoming large turbines due to their simplified approach for stiffness and damping characterisation and wind-wave modelling in the light of misalignment and soil-structure interaction. These shortcomings often lead to underestimation of fatigue damage accumulation and subsequent design life of the foundation. Marino et al. [17] compared the linear and non-linear wave models for fatigue load estimation. They concluded that the linear wave model underestimated fatigue load in the non-operating (or parked) condition, where hydrodynamic loads predominate. Simultaneously, the non-linear wave model was less critical in normal operating conditions, where aerodynamic loads dictated the design. Risi et al. [18] investigated the performance of offshore wind turbine under extreme events, e.g. earthquake along with wind and wave loads. In their study, a finite element model of an offshore wind turbine was developed for seismic fragility analysis to investigate its vulnerability, particularly under crustal earthquakes in soft soils. Loken and Kaynia [19] estimated the fatigue life of monopile and caisson foundation of offshore wind turbine using the mudline bending moment, which concluded that fixed bottom boundary condition over-estimated the design length.

Monopiles in the marine environment are often subjected to stress reversal caused by the environmental loads. Thus, fatigue crack propagation in these structures is the subject of various studies [20,21], where residual stress has a significant role in the performance of welds and connections. In this context, structural loads in the marine environment are highly uncertain. Hence, stochastic analysis of monopile has remained a significant area of research, where the overall reliability index is often quantified by approximate models [22] or Monte Carlo simulation [23]. Haldar et al. [24] studied the stochastic response of monopile supported offshore wind turbine in clay using non-linear p-y curves. They observed that the increase of embedded length reduced the mean of maximum tilt at the mudline, while marginally influencing its fatigue life. Teixeira et al. [25] used kriging for stress-cycle analysis of offshore wind turbine at a reduced computational cost without compromising its accuracy. Velarde et al. [26] conducted a sensitivity analysis of fatigue loads due to uncertainty in environmental, structural and geotechnical parameters. They observed that the uncertainties in turbulence intensity and wave load had a significant influence on fatigue loads. A comprehensive review of stochastic fatigue damage analysis for offshore wind turbine may be found in Jimenez-Martinez [27].