  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.
