Reduced model design of a floating wind turbine

Abstract

Floating platform concepts offer the prospect of harvesting offshore wind energy at deep water locations for countries with a limited number of suitable shal- low water locations for bottom-mounted offshore wind turbines. The floating spar-buoy concept has shown promising experimental and theoretical results. Al- though various codes for a detailed simulation exist the purpose of this work is to elaborate a reduced Floating Offshore Wind Turbine (FOWT) model that mainly reproduces the overall nonlinear low-frequency behaviour of the system with a significant saving in simulation time. One objective is to extend the model predictive control algorithm that has previously been developed for onshore wind turbines to the FOWT for motion control and load reductions. Another objective is a fast dynamic assessment of new concepts during design phase with respect to load cases defined by the International Electrotechnical Commission (IEC). The platform and wind turbine structure is modelled as a three-dimensional multibody system consisting of four rigid bodies with nine degrees of freedom. That is, unconstrained platform motion, tower bending in two directions and variable rotor speed. The coupled nonlinear system of equations of motion is calculated symbolically using the Newton-Euler formalism that takes Coriolis- and centrifugal forces into account. Complex disturbances on the system arising from aerodynamics and hydrodynamics are simplified along with the model as efficiently and accurately as possible. Wind loads are predicted by reducing the three-dimensional turbulent wind field to a scalar rotor-effective wind speed also considering restoring torques resulting from oblique inflow. Linear wave theory provides the wave kinematics and wave loads are calculated using the relative formulation of Morison’s Equation. An approach is presented to estimate wave loads on the floating structure based only on real-time wave height measurements. This allows also for an analytical calculation of wave loads in time-domain with- out iterative or recursive algorithms so that a significant saving in computational time is achieved. The presented disturbance reduction to simple and measurable inputs for wind and waves is a precondition for the implementation of an opti- mal control algorithm. The reduced nonlinear model is compared to the certified aero-hydro-servo-elastic FAST model in time and frequency domain. The results are promising as there is good agreement in static and dynamic response.