Site-specific controller design for monopile offshore wind turbines

Abstract

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 10MW monopile OWT located in 30 meter 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 meter) variation in water depth.

Wave disturbance rejection for monopile offshore wind turbines

“…In order to reduce the computational effort, the environmental conditions are lumped into fatigue equivalent environmental load cases (ELCs). Following the procedure described in Smilden et al, the metocean data are distributed into a three‐dimensional collection of bins based on mean wind speed at hub height, significant wave height, and wind/wave misalignment. After removing the environmental conditions with probability less than 1 × 10 −4 , the total number of bins becomes 138.…”

“…In adherence with the standard DNV‐ST‐0437 the OWT is assumed to be available 90 % of the lifetime. The fatigue life of monopile OWTs may be significantly reduced by unavailability conditions . In idling situations, the aerodynamic damping is negligible.…”

“…A downward pointing arrow indicates that a reduction of the performance parameter is desirable, and vice versa …”

“…A fatigue utilization of 1.2 in the baseline case suggests that the foundation will fail during the 20‐year lifetime. There are many uncertainties related to the present analysis, and a maximum fatigue utilization near 1.0 indicates that the considered design case represents a realistic foundation design. With tower feedback control, the fatigue utilization is significantly reduced.…”

Post‐installation adaptation of offshore wind turbine controls

“…Furthermore, the ultimate and serviceability load effects depend on the turbine's operational settings, such as the cut‐out and rated wind speed. A number of control strategies are available for reducing load effects in offshore wind turbine (OWT) support structures . However, regardless of the control strategy used, there are unavoidable adverse side‐effects, such as wear and tear of various wind turbine components, and power output degradation in terms of both quality and quantity .…”