Control Design for Floating Offshore Wind Turbines

Floating systems provide an opportunity to expand the available tidal stream energy resource and reduce the levelised cost of energy (LCOE) by increasing the number of viable deployment sites; simplifying the installation, maintenance and decommissioning, and; by accessing greater flow speeds near the free surface. However, the proximity of the free surface raises concerns over both the power delivery and the survivability of these systems, due to the presence of waves and the associated excitation of the floating structures. Without an accurate prediction of the power output and greater confidence in the resilience of these systems, the risk to investors is too high to gain significant support for the industry. This has led to the development of a coupled and fully-nonlinear numerical model within the open-source CFD environment, OpenFOAM, capable of evaluating the performance and behaviour of full floating tidal systems [1]. The model solves the incompressible ReynoldsAveraged Navier-Stokes equations for a two-phase fluid [2], tracks the motion of the system in six degrees of freedom, and uses expression-based boundary conditions for wave generation [3]; a two-way coupled, actuator method to represent the turbine [4]; and a static catenary formulation for the moorings [5]. The model has previously been shown to agree with industry standard codes in relatively benign conditions, but has demonstrated additional complexities are present in more realistic conditions and these are not captured by simpler approaches. This work details the continuing development of the numerical model, and, in particular, focuses on validation against the Modular Tide Generator’s (MTG) floating tidal platform concept, which consists of a catamaran style hull, catenary mooring system and a submerged horizontal axis tidal turbine. The simulation results are compared with a series of 1:12 scale physical experiments, conducted in the COAST laboratory’s Ocean Basin at the University of Plymouth. The behaviour of the full system has been explored in a range of wave, current, and wave-current conditions, both with and without the turbine. In each case, the accuracy of the numerical model predictions has been assessed against multiple criteria, including the motion of the barge, tension in each of the mooring lines and the thrust on the turbine. The results imply that the model successfully captures some of the key coupled properties of the problem, including relative increases in turbine load due to the motion-thrust coupling. However, further work is required to improve turbine load calculations in reversing flows, and to introduce dynamic catenary mooring line functionality.