The springing responses of a tension-leg-platform wind turbine (TLPWT) excited by the third-harmonic force of an extreme regular wave are investigated using an integrated (Aero-Hydro-Mooring) numerical model developed and presented in this paper. The model comprises a hybrid hydrodynamic model, which employs fully nonlinear potential theory (FNPT) for wave kinematic prediction and nondiffracting potential theory (NDPT) for wave force prediction, to simulate extreme wave and predict associated wave forces accurately and efficiently. Numerical simulation is carried out for the interaction of a floating TLPWT with waves. The focus is on the TLPWT motions, principally excited by higher-order harmonic wave forces. In particular, the springing responses generated by the third-order force at the triple wave frequency in regular waves are investigated, together with the wind turbine responses and tensions in the mooring lines.
The offshore wind farm development began in shallow water areas by placing wind turbines on fixed (seabed mounted) structures. However, most of the global offshore wind resource is available in the location, where water is much deeper and deploying wind turbines on fixed support structures becomes economically infeasible. Therefore, it is strongly desired to develop a cost-effective floating platform to support a wind turbine that can compete with other energy sources.
The prime requisite for floating wind turbine systems is their ability to withstand environmental forces, albeit with some degrees of oscillations. If the floating platform allows the system to tilt appreciably, the behavior of a wind turbine would be affected even to such an extent that possible operating limitations and/or energy output reductions would have to be considered for estimating LCOE. One of the best promising concepts for a floating wind turbine is deemed to be a mooring line stabilized tensionleg-platform (TLP) that experiences minimal tilting movement.
Moreover, it is lightweight and has fewer mooring footprints as compared to its counterpart. Favorable indications for the adoption of the TLP system have also been given by technical and economic results produced by a separate study carried out by Musial et al. (2003) and Italian Electrical System (Casale, et al., 2010). TLPs are generally designed to keep their natural frequencies in heave, pitch, and roll degrees of freedom, several times above the dominant wave frequencies, whereas in the surge, sway, and yaw degrees of freedom below them. Though their natural frequencies are far away from the dominant wave frequencies, the non-linear effect can give rise to force components near their natural frequencies. Even if these force components are small, their effect on the dynamic response can be significant.