In 2012 Cape Horn Engineering was appointed by the global energy company Repsol to conduct CFD simulations on two types of off-shore wind turbine platforms. Floating platforms are exposed to the challenging environment of high winds, strong currents and high sea states. This investigation was carried out to predict the behaviour of the two types of platforms in these conditions.
The CFD simulations run by Cape Horn Engineering take into account most of the physics of the dynamics problem, although at a much higher computational cost than traditional, simplified methods of potential theory with viscous corrections such as panel codes like WAMIT or methods based on the Morison equation. The CFD simulations are set-up with the Reynolds-Averaged-Navier-Stokes (RANS) equations and include the free-surface and dynamic interaction between the fluid and structure.
For each of the platform types investigated, two types of simulations were run: forced oscillating motions in calm water and freely floating platform motions in waves and wind under the constraints of the mooring lines.
Forced Motions: These first type of simulations were used to compute hydrodynamic coefficients, i.e. added mass and damping, which Repsol compared to results obtained by the code WAMIT for solving the radiation problem.
The simulations were run forcing the motion in each of the six degree of freedom at the time and at six different frequencies (periods 4, 5, 6, 20, 25 and 20 seconds). Subsequently, the values of added masses and damping were obtained computing a Fast Fourier Transform of the last two periods of the output oscillation.
Free Motions: In these types of simulations the platforms were free to move under the wind, current and waves in the six degrees of freedom, and are only constrained by the mooring system. The waves are modelled as fifth order harmonic waves.
Since the RANS simulations considered the flow turbulence and viscosity, all dynamic effects associated with added mass and viscous damping of the platform, including many geometrical details like secondary tubes and damping plates, are taken into account. The total mass, centre of gravity and moments of inertia of the full scale platform were considered.
The aerodynamic forces acting on the tower, nacelle and rotating blades were approximated by an aerodynamic model provided by the client that takes into account the exact attitude of the wind turbine with respect to the wind direction.
For each mooring system a model was developed that in each time instant determined the reaction forces at the attachment point of the line with the platform. A few simplifications of the mooring system were inevitable: the reaction force direction was assumed to be in a vertical plane that passes through the anchor and attachment points. Lift, drag, added masses and damping, i.e. all hydrodynamic forces on the lines, and the aerodynamic forces for the portions above the water surface, were not considered. The bending and torsional stiffness of the line was neglected. Stresses in the line and attachment points were not computed.
The computational matrix consisted of eight different combinations of waves (height, period and direction), current (speed and direction) and wind (speed and direction) for each platform type. The output of the simulations were the time history of the motions, forces and accelerations.