Mooring dynamics for wave energy applications

Doktorsavhandling, 2017

This work aims to increase the modelling accuracy of two important problems for the wave energy industry. One concerns the mooring dynamics in the presence of snap loads (shock waves in cable tension). The other is related to nonlinear effects in the resonance region of moored wave energy converters (WECs). The thesis describes the development of Moody, a high-order discontinuous Galerkin (DG) model for mooring dynamics aimed at capturing snap loads. Two different DG formulations are presented. The first version uses the local DG method, while the second is based on the Lax-Friedrich (LF) approximative Rie- mann solver. Exponential convergence for smooth cases and excellent agreement with experimental data are shown for both versions. The LF-based solver is ex- tended to include shock-identifiers, slope limiters and hp adaptive mesh refinement. Computational results show good shock resolution in both linear and non- linear cable materials. We further develop an automated program interface to provide dynamic mooring response to volume of fluid Reynolds averaged Navier- Stokes (VOF-RANS) simulations of WECs in OpenFOAM®. Model scale experiments of a moored vertical cylinder are carried out with the partial aim to provide validation data for the coupled VOF-RANS-Moody model. The validation shows very good agreement between experimental and numerical results of surge and heave motion, from which we conclude that the coupling is working as expected. We get a good match in mooring force response, but the pitch response is shown to be more sensitive to model input parameters. Valida- tion of high-fidelity models puts tough requirements on experimental data quality, which are difficult to meet in small scales. The experiments also assess the performance of three types of mooring con- figurations for WECs. Results show how cable slack-snap events are important for the dynamic range of mooring force response in survival conditions. As the steepness of the waves increase, the response amplitude per wave height decreases in the resonance region for all three configurations. This nonlinear effect is con- sistently seen throughout the results from experiments and coupled simulations of both model scale and full-scale geometries. Although paid for with a substantial computational effort, we conclude that the high-fidelity VOF-RANS-Moody model is able to predict the fully nonlinear response of WECs with good accuracy.