Function
The assembly, pre-commissioning, and storage of offshore wind turbines and foundations that are ready for tow-out and installation.
Who is involved
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.
Key facts
The turbine tower is pre-assembled onshore and transported with the nacelle and blades for final assembly offshore. Tower sections are typically preassembled onshore with any internal components and the completed structure is transported vertically to site for installation.
Turbine installation methods vary depending on the turbine supplier and the relative size of turbine and vessel. Installation methodologies aim to reduce as far as practical offshore operations. Three variations in the rotor installation process have been used repeatedly:
- Placing the nacelle on the tower then lifting the pre-assembled rotor in one piece to mate with the nacelle (a single rotor lift)
- Mounting the hub and two blades on the nacelle “bunny ears” at port, before mounting the nacelle on the tower onsite and then fitting the final blade, and
- Placing the nacelle plus hub on the tower then lifting individual blades to mate with the hub, turning the rotor each time to repeat the same lift three times.
The third method is current preferred practice, even though this involves more offshore operations.
Offshore wind developers have become used to assembly and installation rates of about two turbines per week for, enabling the turbines for a 1 GW offshore wind farm to be installed in a single season.
Major turbine components have such high mass that they are normally stored, and pre-assembly work carried out, on specially reinforced pedestals. The mass of a primary structure, typically more than 3,500 tonnes, 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 then assembled in a process known as final assembly or turbine integration. Assembled in situ with an installation vessel for fixed.
Towers are assembled one section at a time, to avoid creating a load on the lifting equipment that is even greater than the nacelle weight. The next stage is to install the nacelle and finally the turbine blades, normally one at a time.
Notable differences for floating
Installation times for floating wind farms are longer than developers are used to with fixed wind farms. An output rate of at least one floating offshore wind turbine per week is needed for a 1 GW wind farm with 30 turbines to be installed in two seasons, given typical constraints including weather. Semi-submersible floating substructures need pre-assembly at the construction port.
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 seabed improves its stability for lifting activities.
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.
