Shun-Han Yang

Experimental and numerical investigation of a taut-moored wave energy converter—a validation of simulated buoy motions

This study presents an experimental and numerical investigation of a taut-moored wave energy converter system with a point-absorber type of wave energy converter. The wave energy converter system consists of a buoy, a unique three-leg two-segment mooring system with submerged floaters, and a power take-off system designed for the current experiment as a heave plate. The main objective of the study is to validate a numerical simulation model against experiments carried out in an ocean basin laboratory. Two physical models in model scales 1:20 and 1:36 were built and tested. The detailed experimental testing programme encompasses tests of mooring system stiffness, decay tests, and different sea state conditions for ocean current, regular, and irregular waves. A numerical model in the model scale 1:20 was developed to simulate coupled hydrodynamic and structural response analyses of the wave energy converter system, primarily using potential flow theory, boundary element method, finite element method, and the Morison equation. Several numerical simulations are presented for each part of the experimental testing programme. Results for the wave energy converter buoy motions under operational conditions from the experiments and the numerical simulations were compared. This study shows that the simulation model can satisfactorily predict the dynamic motion responses of the wave energy converter system at non-resonant conditions, while at resonant conditions additional calibration is needed to capture the damping present during the experiment. A discussion on simulation model calibration with regard to linear and non-linear damping highlights the challenge to estimate these damping values if measurement data are not available.

The influence of biofouling on power capture and the fatigue life of mooring lines and power cables used in wave energy converters

This study presents an analysis of a wave energy converter (WEC) system consisting of a buoy, a mooring system and a power cable connected to a hub. The investigated WEC system is currently under full-scale testing near Runde in Norway. The purpose of the study was to investigate the characteristics of the entire system, primarily with regard to energy performance and the fatigue life of the mooring lines and power cable, considering the effects of marine biofouling and its growth on the system’s components. The energy performance of the system and the fatigue life of the mooring lines and the power cable were systematically studied via parameter variation analysis, considering different mooring configurations, biofouling conditions, and environmental loads (current and sea state conditions), among other factors. Hydrodynamic and structural response simulations were conducted in a coupled response analysis using the DNV-GL software SESAM. Energy performance analyses and stress-based rainflow counting fatigue calculations were performed separately using an in-house code. The results show that, for a WEC system which has been deployed for 25 years, biofouling can reduce the total power absorption by up to 10% and decrease the fatigue life of the mooring lines by approximately 20%.

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

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.

Analysis of mooring lines for wave energy converters – a comparison of de-coupled and coupled simulation procedures

Mooring systems for wave energy converters (WEC) have to be designed to survive the cyclic loads and motions they are subjected to as a result of the wave load-WEC interaction and the motions of the WEC in the random elevation of the sea surface. The current study compares four simulation procedures for the analysis of fatigue of WEC moorings. The objective is to recommend the type of simulation procedure that can be used to make reliable fatigue design of WEC mooring systems at a reasonable computational effort. A cylindrical floating WEC with four spread mooring lines is chosen for case study. The mooring dynamics of the WEC is simulated using coupled and de-coupled approaches in the time domain. In total, four types of simulation procedures are compared using the commercial simulation software DNV DeepC and an in-house code MOODY. A parameter sensitivity analysis of environmental conditions, model and numerical parameters is presented. The results are compared with respect to fatigue damage calculated using a stress-based approach and the rainflow counting method. It is found that a de-coupled approach, using DNV DeepC to simulate the buoy’s motions and cable response, is recommended since (i) it gives reliable results in terms of motion and stress responses of the buoy, mooring lines and accumulated fatigue damage, (ii) it requires the least model preparation by the engineer and the computation time is reasonable.