On the survivability of WECs through submergence and passive controllers

Abstract

The survivability of WECs during extreme seas and heavy storms has proven to be challenging during the deployment of devices as they often fail during extreme storms. The survival mechanism is often inherited with the device design and mode of operation. It is not economically and logistically viable for devices to be taken back onshore in case of storms. Some devices lock their PTOs during heavy storms and others lock all the moving parts all together, in the case of CETO, its design has an advantage where it can be submergepad. CETO is a buoyant actuator WEC composed of a buoy submerged close to the surface. Three tethers connect CETO to the sea-bottom through rotary PTOs, allowing the device to be wound down, submerged, and therefore, less exposed to the extreme loads at the sea surface during large storms. This paper will study the survivability of the device during extreme sea-states and will examine the required depth to bring its response back to operational conditions. This work will also look at the alteration of some passive controllers, such as a conventional spring to minimize the response of the device instead of maximising the power capture. With the PTO objective altered in extreme sea-states to minimize the response instead of capturing power, the possibility of harvesting power during extreme sea-states with the device submerged will be checked. Finally, the survivability strategy of CETO through submergence will be showcased with wave tank experiments conducted at IHC as part of the Europewave program.