Dynamic Response Analysis of a Spar Type Floating Wind Turbine

Abstract

Wind power is a large natural source for renewable energy, and many countries have shown interest in establishing floating offshore wind parks. There are many advantages with floating offshore wind turbines, but also many challenges connected to them.

This report focuses on establishing a model for wave-wind induced loading on a Spar type floating wind turbine named OC3-Hywind. The OC3-Hywind is a modified version of an original design developed by Statoil. Two fellow MSc students have developed models for a tension leg platform (TLP) and a semi-submersible, and a comparison between the three concepts have been done.

The model has been constructed by the use of the well known software tools HydroD and DeepC. In addition, a DLL extension, TDHMILL3d, was used to obtain thrust force. HydroD was used in calculations of the hydrodynamic coefficients of the floater. DeepC was used to run coupled floater and mooring analysis in time domain.

Important parameters like hydrodynamic coefficients and natural frequencies compare well to data from the literature. Simulations showed that the Spar is mostly influenced by wind loads in the operational conditions. At rated wind speed of 11.4 m/s the pitch motions of the Spar was showed to be large. To reduce these motions a simple filter was used to extend the turbine control system. In situations where wind and waves have different directions, the Spar experiences large yaw motions. Several factors that may contribute to these motions have studied. To achieve good accuracy in statistical parameters, 10 or more simulations with different seeds were needed.The original depth of 320 m was changed to 160 m. Only minor changes to the mooring system were needed to obtain similar performance as the base case.

In the comparison part typical trends of the different floaters was studied. Based on these trends, positive and negative response characteristics were discovered. The semi-submersible had the largest pitch and surge motions, while the TLP had the largest accelerations below rated. This may result in difficulties in maintenance operations. Only the Spar showed sign of excessive yaw motions. The semi-submersible had the lowest nacelle accelerations and STD for all load cases. In addition, the semi-submersible is the most versatile when it comes to water depth.