This report summarizes acoustic surveys undertaken by the University of Washington at the US Navy Wave Energy Test Site (WETS) from 2015 – 2019. WETS is located in Kaneohe, HI on the windward side of the island of Oahu and is within the restricted waters surrounding Marine Corps Base Hawai’i. WETS is a grid-connected wave energy test site with test berths located in 30 m, 60 m, and 80 m water depth. The seabed in the area is predominantly bedrock, in some places overlaid with a thin layer of sand. At the 60 m and 80 m berths, there are permanent moorings for wave energy converters (WECs) under test. These consist of surface floats that are connected to WECs and placed under tension by hawser lines, forming a three-point catenary mooring. A similar arrangement exists at the 30 m site, though the floats are sub-surface.
Over the five years of observations, two wave energy converters were deployed. The first, the Azura by Northwest Energy Innovations, occupied the 30 m berth during two distinct periods. Between these periods, the WEC was altered to improve its power capture. The more detailed survey reports referenced in this document refer to the initial configuration as ‘Azura’ and the altered configuration as ‘Azura 2.0’ to distinguish between these configurations. The second WEC, the BOLT-class Lifesaver by Fred. Olsen, initially occupied the 60 m berth, then was redeployed to the 30 m berth for a second round of testing.
Acoustic surveys utilized two types of instrumentation packages: fixed and drifting. The fixed packages (Sea Spiders) were deployed on the seabed for multi-month duration and their hydrophones were configured to record on a duty cycle. The drifting packages (DAISYs) were deployed from surface vessels for short-term observations, typically on the order of a few hours. These modes of observation provided distinct information. Fixed packages allowed for the study of temporally varying characteristics over long periods, while drifting packages allowed for observation of spatial characteristics. Data from fixed and drifting platforms were also utilized in a complementary manner. Drifting surveys rapidly identified the frequency range of acoustic emissions from a WEC. With this information, the recording frequency on the fixed hydrophones could be restricted to the range of interest for WEC emissions, increasing the allowable duty cycle and, consequently, the opportunity for observations in a wider range of sea states. This remainder of this report is organized into five sections:
- A summary of Sea Spider deployments and lessons learned, including an index of underlying reports;
- A summary of DAISY drifts and technology development, also including an index of underlying reports;
- A short discussion of the soundscape at WETS and the contributions of WECs to this soundscape;
- The impact of these surveys on the understanding of acoustic emissions of WECs; and
- Next steps for data collection and instrumentation development.