Semi-Submersible Wind Turbine Hull Shape Design for a Favorable System Response Behavior

TL;DR

This study optimizes the hull shape of semi-submersible floating wind turbines to achieve favorable dynamic responses in harsh conditions, reducing fatigue and loads, and introduces a new design criterion for system behavior.

Contribution

It presents a novel integrated optimization approach for semi-submersible wind turbine hulls, focusing on dynamic response reduction and a new indicator for optimal behavior.

Findings

Optimized hull shape results in low fatigue and small rotor motion.

The turbine exhibits a favorable response to wave loads, rotating about a point near the rotor hub.

The methodology is validated with high-fidelity models.

Abstract

Floating offshore wind turbines are a novel technology, which has reached, with the first wind farm in operation, an advanced state of development. The question of how floating wind systems can be optimized to operate smoothly in harsh wind and wave conditions is the subject of the present work. An integrated optimization was conducted, where the hull shape of a semi-submersible, as well as the wind turbine controller were varied with the goal of finding a cost-efficient design, which does not respond to wind and wave excitations, resulting in small structural fatigue and extreme loads. The optimum design was found to have a remarkably low tower-base fatigue load response and small rotor fore-aft amplitudes. Further investigations showed that the reason for the good dynamic behavior is a particularly favorable response to first-order wave loads: The floating wind turbine rotates in pitch-direction about a point close to the rotor hub and the rotor fore-aft motion is almost unaffected by the wave excitation. As a result, the power production and the blade loads are not influenced by the waves. A comparable effect was so far known for Tension Leg Platforms but not for semi-submersible wind turbines. The methodology builds on a low-order simulation model, coupled to a parametric panel code model, a detailed viscous drag model and an individually tuned blade pitch controller. The results are confirmed by the higher-fidelity model FAST. A new indicator to express the optimal behavior through a single design criterion has been developed.