Field test study and quasi-static analysis of global characteristics and survivability of a wave energy converter test platform mooring system

This study presents a catenary spread mooring system design of a mobile ocean test berth, named the Ocean Sentinel (OS), and developed by the Northwest National Marine Renewable Energy Center to facilitate the ocean testing of wave energy converters (WECs). The original OS mooring design (which was similar to a conventional WEC point absorber mooring system) was evaluated through a field data analysis using a quasi-static approach. The field data analysis on the measurements of mooring tension and environmental conditions collected during an ocean test of the OS is based on a quasi-static analysis of the analytical catenary equations of mooring chains. Both global and survivability characteristics of the mooring system were evaluated. The global characteristics include the influence of the OS excursion on mooring tension, the directional control of the OS, and the environmental contributions of waves, current, and wind. The relationship between the dynamic mooring tension and the environmental conditions of waves and current was quantified through an empirical equation. It was found that the contribution of current to the mooring tension oscillation increased as the current velocity increased and was significant at high current velocity. The survivability characteristics include the anchor movability and strength capacities of mooring lines. Because anchor movement occurred near the end of the field test, a new systematic procedure of designing a mooring system with adequate anchor resistance was developed and applied to improve the OS mooring system design.