Taut mooring system design
A recoverable, low-angle taut mooring system designed to secure the floating turbine while reducing uplift loads, installation impact and the need for subsea intervention.
Station keeping
Compliant mooring system
The turbine is held on station by an opposed taut mooring system connected to deadweight anchors located fore and aft on the seabed.
The system uses long synthetic-fibre mooring lines with controlled axial elongation. The compliant mooring lines allow the turbine to move progressively under tidal, wave and current loading while reducing peak loads transmitted to the turbine and its anchors.
The mooring system has been specifically designed to maintain a very low mooring-line angle. This minimises the uplift forces created by line tension and reduces the additional buoyancy required at the turbine.
Because the mooring lines are approximately 250 m long and operate at a very low angle to the seabed, a large vertical tidal movement does not require an equivalent amount of line extension. Instead, most of the tidal excursion is accommodated through a change in mooring-line geometry, with only a relatively small amount of additional stretch required in the compliant polyester section. This allows the mooring system to accommodate a tidal range of around 20 m while maintaining controlled line tension and limiting uplift loads on the deadweight anchors.
High-tenacity marine-grade polyester fibre rope is the preferred material for the compliant section of the mooring system. The final rope construction, diameter, pretension and stiffness will be determined through detailed mooring analysis and material qualification.
Inspection and maintenance
Retractable mooring posts
The anchoring system on the turbine consists of a cylindrical member, referred to as a mooring post, with a submerged anchor-chain pulley at its midpoint. The post connects 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, 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 requiring divers.
Installation and recovery
Recoverable deadweight anchors
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 filled with locally sourced ballast to complete the ballasting process.
Decommissioning reverses this procedure. Air is pumped into the deadweight anchor to remove the ballast and seawater, allowing it to refloat before being towed to an appropriate disposal or recycling facility.
This approach is intended to minimise disturbance to the local benthic community during installation and decommissioning.
Subsea recovery
Recoverable anchor-chain assembly
The anchor chains are fed through the deadweight anchor body via holes positioned fore and aft. A cylindrical anchor plate is fastened to the end of each anchor chain and has a greater diameter than the corresponding hole through the deadweight anchor.
Once the shock absorber has been disconnected, the anchor chain can be recovered by connecting a recovery line to an eyelet on the anchor plate and pulling the chain back through the deadweight anchor body.
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