Taut mooring system design

A novel taut mooring system is used to anchor the turbine to deadweight anchors located fore and aft on the seabed. The use of a semi-submersible structure, a vented turbine cover and a third-party procured anchor-cable shock absorber (Supflex) ensures that the wave impact forces are within design parameters.

The anchoring system on the turbine consists of a cylindrical member (mooring post) with a submerged anchor-chain pulley at its midpoint, connecting the upper and lower parts of the turbine. The mooring posts are fully retractable. Because several are used to moor the turbine, each can be serviced onsite, thereby facilitating the IMO's five-yearly anchor inspection requirement without the need for dry-docking. Following disconnection from the mooring post, the anchor chain assembly can be fully recovered without the need for divers.

Each deadweight anchor is constructed from reinforced concrete and floated into position in a similar way to bridge caissons. Once in position, it is sunk in a controlled manner by partially flooding the deadweight anchor. After confirmation that the anchor is correctly located, it is fully flooded with seawater and then filled with locally sourced ballast to complete the ballasting process. The decommissioning process reverses this procedure: air is pumped inside the deadweight anchor to remove the ballast and seawater and thereby refloat it to the surface before towing it to an appropriate disposal or recycling facility. This approach sets new environmental standards by minimising the environmental impact on the local benthic (seabed) community during the installation and decommissioning process.

The anchor chains are fed through the deadweight anchor body via holes fore and aft. The anchor is a cylindrical plate fastened to the end of the anchor chain. Its diameter is greater than that of the holes in deadweight anchor. Recovery of the anchor chain, once the shock absorber is disconnected, is achieved by connecting a recovery line to an eyelet on the anchor plate and pulling the anchor chain back through the deadweight anchor body.

The mooring system has been specifically designed to have a very low anchor-cable angle, thereby minimising the uplift forces induced by anchor-cable tension and the additional buoyancy requirement at the turbine.