High-order mooring simulations for increased accuracy in wave energy applications

The prevailing simulation technique for floating wave energy converters (WECs) is to use the linear hydrodynamics in convolution form to solve for the motion of the floating structure. This allows for very fast time domain simulations with the possibility to include non-linear reaction forces from e.g. moorings and power take off (PTO). Although its accuracy for very large motion amplitudes is debated (Yu and Li 2015, Palm 2015), it is the best method available to make long-term simulations of WEC response. Brown and Mavrakos (1999) showed a big difference in mooring response depending on how the mooring cables of marine structures where modeled. As WECs are subjected to large motions in relatively shallow water, the differences between different methods are pronounced compared to traditional oil and gas structures, and the uncertainty in model choice is still to be properly quantified. We present a coupling between the open source code WECSim (NREL), and a finite element code for mooring cable dynamics that specializes in accurately capturing snap loads. The in-house mooring model is based on the discontinuous Galerkin method and provides very low numerical diffusion of load propagation. We also present a comparison between our mooring model and using the native, lumped mass mooring model of WECSim, which is a standard method for mooring dynamics. The effect of changing mooring model is evaluated in terms of structural WEC response, peak, mooring load and its potential impact on the fatigue damage