The outcomes of reliability based design methods applied to wave energy converters (WECs) in the early stages of development depend heavily on the uncertainty in the variables used to determine the loads acting on, and responses of, devices. Open source software has been developed in recent years to aid in the response modelling of devices such as NEMOH, WEC-Sim and OpenFOAM. Uncertainty in the estimation of the viscous drag forces acting on a device has been identified as a key source of error in diffraction and radiation based hydrodynamic models. Different methods of determining the drag coefficient are compared in the case of 2 heaving point absorbers in the absence of PTOs.
This study uses physical model decay tests to try and quantify the viscous damping for the X-MED buoy and Uppsala University’s model WEC, a bottom referenced point absorber similar to the full scale Seabased device. The approach is then evaluated by applying the values obtained for the drag coefficients to the hydrodynamic model WECSim which is then compared to physical model tank tests in regular waves.