A comparison of elastic mooring systems for floating wave energy converters

Examensarbete för masterexamen
Chen, Ho-Ann
Wave energy is a widespread and abundant source of renewable energy, yet the technology of harnessing energy from waves is still in the pre-commercial stage. One of the key challenges in the development of wave energy converters (WEC) technology is to ensure the long-term performance and reliability of the mooring system, which must be designed to survice under cyclic loading, not fail due to fatigue, at a commercially competitive cost. This thesis takes WaveEL, a heaving point-absorbing WEC developed by Waves4Power, as the reference case to investigate the performance of WEC systems with different elastic mooring systems in regards to fatigue life and power absorption. As the electricity production of the heaving point-absorber WEC relies on the cyclic heave motion of the WEC device, the long-term deployment of the WEC systems under irregular ocean loads poses high fatigue damage on the mooring lines. The snap loads that act on the mooring lines also decrease the mechanical life of the mooring system. Studies have shown that by adding tether components to the mooring system, the fatigue life of the system can be increased, thus making the WEC systems more commercially feasible. A tether component is a mechanical system that reduces the responding force range and absorb the snap loads in mooring lines. It is this thesis’ objective to develop numerical models to represent tether components, and to evaluate its impact on the WEC performance by integrating the elastic tether component into the mooring system. This thesis investigates two mooring configurations: a 3-leg mooring system with floating buoys between the WEC device and the anchor to decreases the stress loading on the mooring lines, and a 2-leg mooring system with a deeper design water depth so that the system can benefit from the elasticity of the mooring lines. Comparisons are made between the two original mooring configurations and the same mooring setup with the tether component implemented. Coupled hydrodynamic-structural simulations are carried out with different mooring configurations under operational loads. The heave motions of the WEC device and the force response in the mooring lines under operation conditions are obtained from the simulation results and used to calculate the fatigue life of the mooring system and the absorbed power of the WEC system. It was found that the tether component greatly improves the fatigue life of the mooring system, whilst not negatively affecting the power absorption.
wave energy converter, mooring line, coupled simulation, elastic mooring tether, fatigue, power absorption, heaving point absorber
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