Novel piece-wise linear system for Floating Offshore Wind Turbines (FOWTs) wet-towing operations

This paper evaluates the dynamic effects of a novel piece-wise linear towing system on the T-Omega Wind Floating Offshore Wind Turbine (FOWT), when connected between a tugboat and the wind turbine to perform wet-towing operations. The novel towing device consists on two rigid connectors that include an elastic coupling aiming to reduce variable connection forces transferred between the tug and the turbine, that could result into turbines’ undesired responses. Therefore, a numerical study of a 14 DOF towing system under regular sea waves is simulated with the state-of-art real multiphysics Marine Simulator at the National Decommissioning Centre (NDC) at the University of Aberdeen. The towing system performance is evaluated for “Low” to “Moderate” Sea States with a variation of wave parameters, such as, wave height ($H$), period ($T$) and incident angle ($\alpha$). The effects on the FOWT dynamics are evaluated for different towing system parameters (i.e. coupling compliance, ${\chi }_i$) and various constant towing velocities ($V_i$). System trajectories are computed and RAOs are evaluated for the turbines’ heave, roll and pitch DOF, showing that the novel system effectively reduces the pitch RAOs up to a 36% for wave excitation periods close to the pitch resonance. Moreover, the coupling shifts the pitch resonance to higher periods, which vary proportionally with the system rigidity and the wave height. The effects on turbine’s heave and pitch hydrodynamics when wet-towed at velocities lower than 2.5 [m/s] can be neglected for head seas. Finally, the towing device relative displacements are evaluated as a function of the forces applied to choose most optimal device compliance for “Low” Sea States. The study reveals some nonlinear effects induced on the turbine, such as towing “Fishtailing”, and discloses the strong nonlinear turbine dynamics induced by towing and environmental parameters.