Fatigue assessment of offshore wind turbines

Abstract

Offshore wind turbines are complex marine structures that are exposed to stochastic dynamic environmental actions such as wind and waves which result in high cycle fatigue conditions. The accuracy of estimated residual life for such structures is directly linked to the various numerical modelling approach. An accurate action is therefore required to ensure a reliable fatigue life estimation and hence the structural safety of all components affecting the modelling technique and analysis set-up under different environmental conditions.

In order to have an idea of the performance of different numerical modelling, a monopile wind turbine case study is selected that is located in the North Sea at a water depth of 30 m. Firstly, a scaled prototype of the monopile is tested in the wave tank of Queen’s University Belfast and the same case created in OpenFoam for the Computational Fluid Dynamics study of hydrodynamic loads. Seven different load cases are selected and generated both in the wave tank and numerical model. The wave characteristics are selected in the regions that the body is slender relative to wavelength. The water depth is considered as intermediated water depth and wave regions are in the non-linear region as described by Stoke’s Second Order or higher.

In this study, the objective is to perform a code-to-code fatigue life sensitivity analysis under operational and non-operational conditions using time-domain coupled aero-hydro-servo simulations. The comparison is performed for a reference 5-MW floating wind turbine configuration (OC4 semi-submersible) using OpenFAST developed in NREL and an FE structural dynamic model (FlexCom) developed in Wood Plc. The accumulated damage in the tower base, tower top is calculated and the effects of various modelling factors, including wind and wave simulation by considering 50%, 65%, 75% and 85% percentile of wave height and wave period in the fatigue damage occurrence are examined.

Thesis is embargoed until 31 December 2025.

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Madjid Karimirad (Supervisor), Trevor Whittaker (Supervisor) & Charles Young (Supervisor)