Site-specific controller design for monopile offshore wind turbines

The fatigue life of offshore wind turbine (OWT) support structures is sensitive to variations in site-specific conditions such as the water depth and soil properties. Site condtions may vary significantly within a wind farm, and they may change throughout the lifetime of the OWT. This paper analyses how control strategies for fatigue life extension can compensate for differing fatigue loads due to varying site conditions. Control strategies applicable for both power production and idling situations are analysed, and methodology to reduce undesirable side-effects is proposed. The design case is a 10 MW monopile OWT located in 30 m water depth at the Dogger Bank in the North Sea, and results are based on time-domain simulations performed using an aero-hydro-servo-elastic simulation tool. The results show that, when all the investigated control strategies are utilized, a fatigue damage reduction over the 20-year lifetime of approximately 50% is possible. Furthermore, it is shown that adverse side-effects such as wear of pitch actuators and fluctuations in the power output can be significantly reduced by limiting the use of control strategies to some predefined situations. With only moderate cost to other system components, the control system is able to compensate for 20% variation in soil stiffness, and 5% (1.5 m) variation in water depth.