A FATIGUE ANALYSIS OF THE NO-WEC MOORING SYSTEM AT THE U.S. NAVY WAVE ENERGY TEST SITE OFF O‘AHU, HAWAI‘I

Abstract

The Wave Energy Test Site (WETS), off the coast of Marine Corps Base Hawai‘i, provides a unique location for the full-scale validation of Wave Energy Conversion (WEC) devices in the USA. WETS has three separate berths, allowing for the simultaneous testing of three WEC devices with up to 1 MW power transmission to shore. Two WEC devices have been tested at WETS (two deployments each), and many other devices are planned to be deployed in the coming years. Since the 2014 installation of the mooring systems at the 60 m (Site A) and 80 m (Site B) berths, some of the original mooring lines have failed– the lines failed due to this lack of pretension in the mooring lines during much longer than anticipated period during which no WEC was deployed, which resulted in substantial link-to-link collisions and failure of joining links in catenary "thrash zone" of the chain. The new mooring system at the two deeper WETS berths is a 3-point spread catenary system consisting of 2.75" ground chain and 4” (60m berth)/3.5” (80m berth) riser chains, leading from the anchors to three separate surface buoys. From the surface buoys, hawsers connect a WEC device to the rest of the mooring system. When no WEC is deployed, a no-WEC hawser system keeps the system in tension to reduce fatigue damage and wear on the moorings. This study focuses on understanding the fatigue damage to the mooring chain at the 60 m berth when the mooring system is in the no-WEC configuration. This analysis is desirable for understanding the fatigue for the no-WEC configuration during the extended WETS idle periods. For the fatigue damage analysis, typical sea states are identified based on analyzing a 41-year wave hindcast and validated with 20 years of buoy data. The fatigue damage analysis is based on a frequency-domain analysis of the no-WEC mooring system’s responses to typical sea states. The long-term fatigue damage calculation is performed by considering the probability of occurrence of these typical short-term sea states. The long-term fatigue analysis shows that the new mooring lines installed at the 60 m berth at WETS can withstand failure due to fatigue for at least 41 years when they are in the no-WEC configuration, which is well past the estimated decommission date for WETS.