Analysis of Fatigue Characteristics in Mooring Lines and Low Voltage Cables for Wave Energy Converters

Licentiate thesis, 2016

To reduce both carbon emission and fossil fuel consumption, there is a pressing need for the exploitation of renewable sources of energy such as biomass, hydropower, solar power, waves, and wind. This thesis addresses the application of wave energy, which has a large potential to contribute to the world’s renewable emission-free energy supply. However, the challenge for current wave energy technology to make a commercial impact is to reduce its levelised cost of energy and, as part of this task, to ensure the long-term reliability and durability of the mooring lines and power cables used in wave energy converter (WEC) systems. This thesis contributes to the development of a numerical analysis procedure for assessing the fatigue characteristics of these mooring lines and power cables. The main objective of this thesis is to develop a complete numerical analysis procedure for the assessment of mooring lines and power cables used in WEC systems. A WEC system consisting of a heaving-point-absorber WEC, a catenary mooring chain system, and a low voltage power cable was chosen as the subject of this case study. The numerical study was conducted based on a first-principles approach. The simulation methodology and analysis procedure consisted of a hydrodynamic and structural analysis, a stress and fatigue damage analysis, a parametric study, an energy performance analysis, and an assessment of the influence of biofouling on the WEC energy performance and the fatigue of the moorings and power cable. Coupled and de-coupled simulation procedures were compared using DNV DeepC. It was found that the coupled procedure should be used to simulate the hydrodynamic and structural response of the WEC system to capture the coupling effect in the system. A stress-based rainflow counting fatigue analysis was developed, which enabled the identification of fatigue-critical locations along the mooring lines and power cable under varying environmental conditions. A fatigue-wave height-wave period matrix and a simulation matrix were designed as tools to visualise the fatigue damage to the mooring lines and power cable under various environmental conditions. In comparison with the biofouling-free condition, it was shown that biofouling on the WEC system can reduce the time-averaged power absorption of the WEC by up to 20% and reduce the fatigue life of the mooring lines by approximately 80%. Hence, it is recommended that biofouling is considered during the early stage of WEC system design. The influence of biofouling on the fatigue life of the power cable was found to be negligible. However, considering the long fatigue life calculated for the power cable, it was concluded that it is necessary to develop a more detailed model of the power cable.