The assembly, pre-commissioning, and storage of floating offshore wind turbines that are ready for tow-out and installation.
What it costs
About £31 million for a 450 MW floating offshore wind farm.
Who supplies them
This work is usually contracted to either the wind turbine supplier, or to a wind turbine installation and commissioning contractor. The contractor normally provides supervisory input and subcontracts the work to a technician services company.
Floating offshore wind turbine final assembly taking place at port. Photo of the Kincardine Offshore Wind Farm project courtesy of Principle Power.
Offshore wind developers have become used to assembly and installation rates of about two turbines per week for fixed wind farms, enabling the turbines for a 1 GW fixed offshore wind farm to be installed in a single season. An output rate of at least one floating offshore wind turbine per week is needed for a 450 MW wind farm with 30 turbines to be installed in one season, given typical constraints including weather.
Some designs of semi-submersible floating substructures need pre-assembly at the construction port. The mass of a primary structure, typically in excess of 3,500 t, is greater than the maximum lift capacity of the largest mobile cranes. Rail systems or self-propelled modular transporters (SPMTs) are options for moving on land. Ring cranes, vessel-mounted cranes, or semi-submersible barges can be used to move a primary structure from land into water. A dry dock addresses both issues but large dry docks are scarce. Other components, such as secondary steel or transformers, may be pre-assembled at the construction port too.
The major turbine components are moved to the quayside, normally using SPMTs. Some pre-assembly work is expected to be performed at this stage, for example installing electrical equipment in the base of the turbine tower. Major turbine components have such high mass that they are normally stored, and pre-assembly work carried out, on specially reinforced pedestals.
The floating substructure is brought from wet storage to the quayside using harbour tugs.
The major turbine components are then assembled onto the floating substructure in a process known as final assembly or turbine integration. This activity can either be completed with a landside crane located on the quayside or by a temporary jack-up crane vessel alongside the quay. Ballasting the substructure so that it rests on a mattress laid on the sea bed improves its stability for lifting activities.
We expect towers to be assembled onto the floating substructure one section at a time, to avoid creating a load on the lifting equipment that is even greater than the nacelle weight. If lift capacity is not a restriction, the tower could be assembled on the quayside and lifted as one unit. The next stage is to install the nacelle and finally the turbine blades, normally one at a time.
The assembled floating offshore wind turbine is pre-commissioned at port to the greatest possible extent to reduce offshore commissioning work. This involves mechanical and electrical testing of the various subsystems.
Wet storage is required prior to tow-out of assembled floating offshore wind turbines. Developers typically plan for a stock of around 20% of the completed project in wet storage, so that offshore installation can proceed smoothly and take best advantage of weather windows. This is in addition to the wet storage required for inbound floating substructures.