Hook-up and tensioning of mooring lines to the floating substructure in its final position.

What it costs

This is included in the floating offshore wind turbine installation contract.

Who supplies them

Boskalis, Bourbon Offshore, DOF Subsea, Maersk and Tronds Marine.

Key facts

A three-column semi-submersible typically has a three-point mooring spread, as a minimum, and at least one of these lines includes an in-line tensioner.

Any sections of mooring line that have not been pre-installed are installed. For example, codes do not allow fibre rope lines to be laid on the sea bed for extended periods, so they cannot be installed a year in advance. ROVs are used to make any remaining connections between mooring line sections and support the connection to the anchor-handling winch to bring the upper section of mooring chain on deck. The upper chain is connected to one of the three mooring points on the floating substructure and the winch slowly feeds the chain back out into the water and is eventually disconnected from the mooring chain. This same process is completed for the second and third mooring lines. At this point there is some slack in the lines.

To tension the system, the free end of the active lower chain section is retrieved from the sea bed with the support of an ROV, as before. Vertical loads are applied to the active side using the winch and crane of the AHV. This gradually increases tension throughout the entire mooring spread as the active side of the third line slowly passes through the tensioner mechanism. The active side is lowered, disconnected, and laid to rest on the sea bed once the required line tensions are reached. The use of an in-line tensioner can significantly reduce the tug bollard pull required to reach the mooring line pre-tension, depending on the geometry of the mooring system and the position of the tensioner.

What’s in it

Guide to a Floating Offshore Wind Farm