Aquamarine Power has developed the Oyster, a Wave Energy Converter (WEC), the device captures wave energy and converts it into electrical energy (see Figure 1 below). The offshore element of the Oyster 2a device is made up of two hydraulic pump modules, a base frame and a flap. The device will be fixed on to two pre-installed foundation piles which have previously been set into the seabed into pre-drilled rock sockets and grouted into place. The cyclical oscillating action of waves on the flap cause the it to pivot on its hinges and pump hydraulic fluid through a pipeline to a shore based electricity generating turbine. This turbine in turn feeds electrical energy to the grid.
The installation of Oyster 2a (the second generation of the Oyster devices) and foundation piles (two for Oyster 2a, a monopile for Oyster 2b and a monopile for Oyster 2c) is scheduled for the summer of 2011 (works commenced in June 2011) at the European Marine Energy Centre (EMEC) wave device test area in Billia Croo, Orkney.
The design life of Oyster 2a and the foundation piles is 20 years, however it is expected that decommissioning would take place when the current seabed lease expires in 2015 or subject to a seabed lease extension being granted by EMEC, in summer 2026.
The decommissioning of Oyster 2a is described in the following sections of this document. In summary, the process will:
- Commence with mobilisation of the decommissioning vessels to the site.
- Divers will be used to perform and guide the decommissioning operations, however diver safety will be a prime consideration in planning the individual tasks and where possible automated, diverless methods will be employed.
- The Oyster 2a WEC will be in the locked down or closed state.
- The low and high pressure pipelines connecting the device to the shore based elements of the system will be cut, and left on the seabed for later recovery as part of decommissioning activities.
- The Oyster 2a WEC will be removed in one piece by cutting it clear of the seabed at the base of the two foundation piles. This will be done as close to the seabed as possible.
- Towing bridles will be pre-installed onto the device prior to the cutting operation. The remotely operated cutting system will allow the Oyster 2a WEC rise on its own buoyancy when the pile cuts are complete.
- The Oyster 2a WEC will be wet towed back to shore by the decommissioning vessels for disassembly.
- The two Oyster 2a foundation piles will be cut flush with the seabed by divers.
- The two monopiles associated with Oyster 2b and 2c will be cut flush with the seabed by divers.
- All seabed pipe work and cabling will be recovered using vessel based carnage and buoyancy modules and transported back to shore for scrapping.
- Any waste produced during decommissioning operations will be removed from site onboard the construction vessel.
- A post decommissioning seabed survey would be carried out following decommissioning and a report provided to the relevant parties (Department of Energy and the Climate Change (DECC) and Marine Scotland).