Tow-out of the assembled floating offshore wind turbine from the construction port to the offshore wind farm site.

What it costs

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

The tow-out operation is usually completed by three AHVs that have a combined day rate of about £130,000.

Who supplies them

Boskalis, Bourbon Offshore, DOF Subsea, Maersk and Saipem.

Key facts

The floating offshore wind turbine is towed out to site using a primary towing AHV with minimum bollard pull of around 200 t.

These operations are usually supported by two trailing AHVs during tow-out and positioning activities at site.

The operation is usually completed in fair weather with a maximum significant wave height (Hs) of between 1 and 1.5 m and mean wind speeds below 14 m/s. A weather window of sufficient length is required, so that the assembled floating offshore wind turbine could be towed back to port in the case of problems with hook-up. Transit speeds of between 3 and 4 kn are used. This is limited by a number of factors including vessel fuel consumption, towing capacity and weather.

Transit distance is a limiting factor in the tow-out of floating offshore wind turbines and this is a key consideration for construction port and wind farm site selection.

Prior to tow-out, the floating substructure is normally ballasted to add stability in a location with adequate water depth for the increased draft. As the reference design is a steel semi-submersible, this involves pumping water onboard.

A constraint during transportation and installation is the nacelle acceleration limit defined by the turbine supplier to avoid damaging the turbines and invalidating warranties. This is typically about 0.5 g.

The channel depth between the port and the project site is one factor which influences the choice of substructure type and construction material.

Guide to a Floating Offshore Wind Farm