Multi-objective Control of Floating Offshore Wind Turbines


Table 1: Benefits and Limitations of Offshore Wind Energy Benefits


Limitations Wind


Stronger and steadier Less turbulent


Higher speeds at lower altitudes


Higher annual mean wind speed Location Turbines operate at maximum


Has greater extremes Interacts with waves


Harder to assess potential Increased capital cost due to deeper


efficiency without noise restrictions and stronger foundations Proximity to load centres Reduced visual impact


Integration with the electrical grid


Installation, operation and maintenance affected by weather


appropriate modifications and assess the behaviour. A simple and yet robust controller was implemented on the three main floating platforms. This controller, referred to as the baseline controller, was used as a yardstick to measure the performance improvement/ degradation of multiobjective controllers.


The baseline controller consisted of two independent control loops: a generator torque controller and a collective blade pitch controller. The torque control loop was used to smooth power fluctuations. The collective blade pitch control loop was governed by a gain scheduled proportional-integral (GSPI) controller. The GSPI controller was a single-input single-output (SISO) controller with the objective of regulating rotor speed. The gains were scheduled as a function of blade pitch to account for the change in turbine sensitivity at different wind speeds in order to increase its robustness. The GSPI controller is a simple non-linear controller.


The simulations were carried out using 60 turbulent wind and irregular wave conditions. The main performance metrics were averaged and normalised relative to a 5MW onshore wind turbine. The normalisation was used to put the offshore structural loads in perspective as onshore loads have been well defined. These performance metrics are shown in Figure 2.


With all the performance metrics, the lower the value, the better the result. A value of 1 indicates a performance level similar to the onshore wind turbine, where a value of


To better understand the results, it is best to consider them from the turbine’s point of view. That is, what would happen if the onshore wind turbine was mounted on these platforms?


If the onshore wind turbine was mounted on a TLP, the tower side-to-side, shaft and blade fatigue loads would remain similar, power fluctuations would increase by 15% and tower fore–aft loads would increase by 48%. Compared with the other platforms, the TLP had the


Baseline Controller Simulation Method and Results The baseline controller was simulated using a relatively high-fidelity non- linear simulator Fatigue, Aerodynamics, Structures and Turbulence known as FAST on all three main platform types (barge, TLP and spar-buoy). A generic 5MW wind turbine was mounted on each of the platforms.


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