Integrated Modeling of Offshore Wind Turbines

Abstract

In order for the offshore wind industry to be competitive, the cost has to be driven down. One of the major factors that contribute to the overall cost is the over-dimensioning of the substructure. The response of the structure under various loading conditions can provide valuable insight in the design phase so as to provide a structural design that is cost effective yet reliable, and can withstand the loads that are considered to act on the structure.

The objective of the present thesis is the development of a 1D finite element model, that allows for a dynamic analysis of an offshore wind turbine under the combined actions of wind and wave or wind and ice. For this purpose, different models have been combined and improved or extended. Through this model the importance of accounting for non-linear and breaking waves, the effect of the kinematic stretching on the response and the manner in which the misalignment of the load affects the response can be investigated.

A detailed design of the NREL-5MW offshore wind turbine supported by a monopile is subjected to wind, wave and ice action. The aerodynamic action is evaluated through a model valid for the above rated regime when pitch control is active, using a turbulent wind signal resulting from the Kaimal spectrum. The hydrodynamic action is calculated either with the Morison equation or the MacCamy and Fuchs equation with the use of either linear or nonlinear water particle kinematics. An approach towards the calculation of the load from a breaking wave is considered accounting for the wave skewness and asymmetry during such an event. The ice action is calculated through a model that evaluates the force while in the crushing regime. The soil is represented with linear soil springs.

The structure’s response is investigated for all the loads separately at first. The next step is the combined analysis. Aligned and misaligned cases are considered. Results show that wind load is dominating the response in the aligned and misaligned wind and wave case regardless of the method used to calculate the hydrodynamic load in the case of a small wave height. In the case of a larger wave height, using Stokes theory and the Morison equation, the hydrodynamic load is contributing to the resulting response. Concerning the ice loading, the intermittent crushing and the continuous brittle crushing regimes occur for the turbine. The response to the combined wind and ice action appears to be affected by both loads in all examined cases.