Analysis of the Fatigue Characteristics of Mooring Lines and Power Cables for Floating Wave Energy Converters

Doctoral thesis, 2018

To reduce greenhouse gas emissions and expand the energy mix, there is a pressing need for the exploitation of renewable sources of energy, such as biomass, hydropower, solar power, waves, and wind. This thesis focuses on ocean wave energy and its applications. Ocean wave energy is abundant and geographically widespread and has one of the highest energy densities among renewable energy sources, presenting a great opportunity for securing an emission-free energy supply. However, one challenge associated with existing wave energy technology is to ensure and verify the reliability and long-term performance of wave energy converter (WEC) systems; these aspects are fundamental to achieving wave energy at a cost that is commercially competitive in the long term.

The main objective of this thesis was to develop a complete numerical analysis procedure for assessing the fatigue characteristics of the mooring lines and power cables used in floating WEC systems. Both the moorings and cables must be designed to survive under cyclic loading and not fail due to fatigue, which would endanger the safety and functionality of the WEC system. However, due to the wide variety of WEC concepts that have been proposed, it remains challenging to identify which numerical methods are most appropriate for the reliable prediction of the mechanical service life of the moorings and cables. Starting from a hydrodynamic and structural response analysis of a WEC system or an array of WECs, the research presented herein contributes to a systems perspective in which the fatigue performance of moorings and cables is predicted, the power performance is estimated, and the levelised cost of energy is calculated with consideration of the interaction effects among WECs in an array.

It was found that a coupled analysis approach should be used to simulate the hydrodynamic and structural responses of WECs because it captures best the mechanical coupling and hydrodynamic interaction effects of WEC systems. A wave-height/wave-period matrix of fatigue was designed as a visualisation tool to illustrate the influence of environmental loads on fatigue damage accumulation in the moorings and cables. Numerical simulations of multi-WEC array farms showed that hydrodynamic interactions among WECs in an array farm strongly affect fatigue damage in moorings, which in turn influence the related cost assessment of a WEC system. Compared with a biofouling-free condition, it was shown that the presence of biofouling on the WEC system not only reduces the power absorption of WECs but also decreases the fatigue life of the moorings and cables. The results obtained from the numerical simulation were validated against a model-scale ocean basin laboratory experiment and compared with measurement data from a full-scale WEC installation. The findings showed that the simulation model can satisfactorily predict the dynamic motion response of a WEC system under moderate sea state conditions and under non-resonant conditions.