1. Installation and commissioning:
All installation and commissioning of balance of plant and turbines, including land and sea-based activity.
Today, the typical process for installation is to install foundations and cables in separate processes one year then install and commission turbines the next year.
Developers vary in strategy but contracts are usually let for the cable-laying (export and array), substation installation, foundation installation and turbine installation.
Components: Export cable-laying. Foundation installation. Array cable-laying. Construction port. Offshore substation installation. Sea-based support. Turbine installation. Commissioning.
1.1 Export cable-laying:
Installation of the cable connecting the onshore and offshore substations.
Export cables are laid in as long sections as possible, of up to 70km in length, to avoid subsea connections.
Cables are typically buried to 1.5-3m below sea bed to avoid disturbance, for example by fishing vessels or ship anchors. Simultaneous lay and burial using a cable plough is normal for a variety of soil conditions. This is likely to be preferred.
A two-stage process may also be undertaken. Here a cable is laid on the sea bed, after which a trenching ROV, supported by a trenching vessel, undertakes the burial. Export cable-laying is viewed as a significant constraint with a limited number of vessels currently available and few companies with the expertise.
Main suppliers: Global Marine Systems, Nexans, Prysmian, Subocean and Visser & Smit.
Components: Trenching vessel. Cable-laying vessel.
1.1.1 Trenching vessel:
Undertakes cable burial post-laying of the cable on the sea bed. Vessels are typically 90m DP2.
Post-lay burial is undertaken using a trenching ROV using a high pressure jetting system to fluidize the sea bed and allow the cable to sink to the required depth. Jetting sleds can also be utilized.
Components: Trenching ROV
18.104.22.168 Trenching ROV:
Uses specialist equipment to form a trench in which to lay and bury the cable. A high pressure jetting system is used to fluidize the sea bed and allow the cable to sink to the required depth. Jetting sleds can also be utilized.
Cutting systems are used to form a trench in clay or rock sea bed.
Main manufacturers (vessel): IHC Engineering Business, Perry Slingsby, SMD
Components: Power supply. Propulsion system. Control system. Pressure and flow water jetting system. Lighting system. Trench cutter.
1.1.1 Export cable-laying vessel:
This lays the cables between the offshore and onshore substation.
Historically, cable installation has been undertaken with barges equipped with a carrousel, tensioners and haulers.
Array cables at some wind farms have been installed using a dynamically positioned (DP2) vessel. A dynamically positioned vessel provides faster set up and installation time and can remain on station in higher sea states and wind conditions compared with a barge.
The same vessels are sometimes used for export and array cable installation, although export cable-laying vessels will typically have larger carousels to accommodate longer cables. They may need to have a shallow draft to install the cables close to shore.
Main suppliers: Global Marine Systems, Nexans, Prysmian, Subocean and Visser & Smit.
Components: A carousel for cable storage, a cable installation spread, a cable plough and a work class ROV. Heave compensation for plough launch and recovery.
22.214.171.124 Cable plough:
A cable plough is normally used to simultaneously lay and bury a cable.
Cable ploughs can bury cable down to 3-4m below sea bed level. The plough will generally require a tow force of approximately 150 tons to pull the plough through the soil. Using a barge, this force is supplied by a tow tug. For a dynamically positioned vessel, a specialist vessel with an appropriate bollard pull is required.
Main manufacturers: IHC Engineering Business and SMD.
Components: It can have high pressure jetting nozzles at the leading edge. Skids, which maintain the plough at the required depth.
126.96.36.199 Work class ROV:
Subsea ROVs have many uses including visual inspections of subsea structures such as cable J-tubes and foundations, feeding the cable through the J-tubes and monitoring operations such as grouting of piles.
Cable installers will seek to avoid using an ROV to minimize costs. However, the use of ROVs avoids the high costs associated with the use of divers to work at depths requiring specialist equipment and extended decompression.
Main manufacturers: Perry Slingsby, Saab Seaeye and SMD.
Components: Remote camera. Power supply. Propulsion system. Control system. Lighting system.
1.1 Foundation Installation:
Transport and fixing of foundation in position. The process involved varies with the foundation technology employed. Monopiles typically are driven from a jack-up vessel but can be drilled and can be installed using a floating vessel. Jacket and tripod foundations may be installed by floating cranes. Gravity base foundations may use floating cranes or specialist barges to support float out.
Monopiles (up to 6m diameter) are driven into the sea bed using a hammer and anvil system before mounting transition pieces and feeding the cable into the foundation.
For larger turbines, jackets or tripods in steel or concrete gravity foundations are typically used. For jacket and tripod foundations pin piles are driven into the sea bed and the foundation lowered onto the pile heads and grouted into position.
Concrete gravity foundations can weigh significantly more (3,000 tons) and may be floated out to position before being sunk. The sea bed must be leveled to receive such foundations.
Offshore substation foundations may be installed in a similar way to turbine foundations but are significantly larger. Cables are drawn from the sea bed through a J-tube into the foundation base to feed up to the wind turbine.
Components: Foundation installation vessel.
1.1.1 Foundation installation vessel:
Transports the foundations from the quayside fabrication facility to the site and secures them to the sea bed. Systems include self-propelled jack-up vessels, towed jack-up barges, floating sheerleg or catamaran cranes.
To date, the vessel of choice for monopile installation has been the self-propelled jack-up (e.g MPI Resolution). Floating cranes such as Samson, Rambiz and Stanislav Yudin have also been used for jacket, tripod and GBS foundations.
Vessels have a range of onboard tooling, depending on type of foundation to be installed. For monopiles, on-board hammer and anvil systems are used to drive the piles. On-board drilling systems are used where hammering is not possible due to ground conditions or environmental restrictions. Monopiles are then grouted into position.
To assist with positioning monopiles, an upending tool can be used to help lift, rotate and lower the pile into position on the sea bed. A handling tool is used for guiding the pile during pile driving.
Typical specifications for a large turbine:
Steaming speed: up to 11 knots
On-board capacity: around 1,000 tons
Components: On-board crane. Dynamic positioning. Propulsion systems. Jack up system. Specialized foundation installation tooling.
1.1 Array cable-laying:
Installation of the power cables between the turbines and the offshore substation.
Cables may be laid in a spider arrangement with a small number of turbines connected to a single cable that provides the connection to the substation, or in a series of chains with around 6 to 10 turbines on each.
It is often not possible to plough close to the turbine and substation. A trenching ROV may be used to bury the cable close to these structures.
MPI has introduced a remotely operated subsea cable-laying “tractor”, which can carry the 1km stretch of cable required between turbines, laying and burying it as it goes. It is equipped with a chain cutter and jetting tool.
Main suppliers: CTC, Global Marine Systems, MPI, Subocean and Visser & Smit.
Components: Array cable-laying vessel.
1.1.1 Array cable-laying vessel:
Lays the power cables between the turbines and the offshore substation. Historically, cable installation has been undertaken with barges equipped with a carousel, tensioners and haulers.
Array cables have been installed using a dynamically positioned (DP2) vessel. A DP2 vessel provides faster set up and installation time and can remain on station in higher sea states and wind conditions compared with a barge.
Main suppliers: Subocean, Visser & Smit and Global Marine Systems.
Main components: A carousel, cable tank or reels for cable storage, a cable installation spread, a cable plough and / or a trenching ROV.
1.1 Construction port:
The base for pre-assembly and construction of the wind farm. Separate locations may be used for feeding foundations and the wind turbines to a wind farm. Location is critical as it affects the time spent in shipment and sensitivity to weather windows.
Large areas of land are required due to the space taken when turbines are stored lying down on the ground. Two turbines take up nearly 2 hectares of space.
Construction port requirements:
- At least 8 hectares suitable for lay down and pre assembly of product;
- Quayside of length 200–300m length with high load bearing capacity and adjacent access;
- Water access to accommodate vessels up to 140m length, 45m beam and 6m draft with no tidal or other access restrictions;
- Overhead clearance to sea of 100m minimum (to allow vertical shipment of towers); and Sites with greater weather restrictions or for larger scale construction may require an additional lay-down area, up to 30 hectares.
Components: Quay. Lay-down area. Cranes. Workshops. Personnel facilities.
1.1 Offshore substation installation:
Transfer of the substation from its quayside fabrication site and installation on the foundation. This is a heavy lift (1,000 tones plus) and will typically be carried out by floating crane.
The substation is floated out of port on a barge. This may be equipped with a heavy lift crane or a separate vessel will lift the substation onto the foundation.
Main contractors: Fluor, Hochtief, Per Asleff, Bilfinger Berger, MT Højgaard. Ballast Nedam, Geosea.
Components: Substation installation vessel.
1.1.1 Substation installation vessel:
Transport and lift of offshore substation, in order to position it on pre-installed foundation. Floating cranes may be of the sheerleg or catamaran type. Heavy lift vessels used in offshore wind include Rambiz, Stanislav Yudin and Samson. Crane ratings are from 900 tons to over 3,000 tons.
Main suppliers: Bonn & Mees, DBB, Huisman, Scaldis Salvage and Seaway Heavy Lift.
1.1 Sea-based support:
A number of vessels are used to support the installation process. These may include crew vessels, anchor handling, barges, dive support, ROV handling.
Specialist vessels are used to take crew to the wind farm for installation and commissioning tasks. These are typically 15-20m catamarans.
I7. Turbine installation
ROV support vessels are 80-100m DP2 vessels with a moon pool and deck crane. Dive support vessels have a similar specification to ROV support vessels. They may require saturation diving systems to enable lengthy and deep water (over 50m) dives.
Main operators: Holyhead Towing, MPI Offshore, Offshore Wind Power Marine Services and Williams Shipping, Windcat.
Main manufacturers: Alicat, Arklow Marine Services, Alnmaritec and South Boats.
1.2 Turbine installation:
Turbine installation involves transporting the turbine components from the construction port and installing the turbine on the foundation.
Installation methods vary and include assembly of turbine tower, nacelle and blades at sea through to transfer of complete turbines from land, though this method has to date only be used on a demonstration project.
In some cases, towers have been mounted vertically on the vessel and one or more blades joined to the hub before shipment. Some methods may involve the construction of the turbine at sea, lifting nacelle and blades to the top of the tower; others will involve the transportation of fully constructed turbines to minimize the time and work content at sea.
Some innovative solutions may involve the construction of the turbine at sea, lifting nacelle and blades to the top of the tower; others will involve the transportation of fully constructed turbines to minimize the time and work content at sea.
Main suppliers: A2SEA, Ballast Nedam, Geosea, Jack-Up Barge, Marine Construct International, MPI
Offshore, Scaldis Salvage.
Components: Turbine installation vessel. Turbine installation vessel plant and equipment.
1.2.1 Turbine installation vessel:
Supports the erection of the turbine on the foundation. Similar vessels may be used to those for foundation installation. They are often designed with jack-up legs to create a rigid lifting platform.
Main vessel operators: A2SEA, Ballast Nedam, Beluga-Hochtief, Geosea, Jack-Up Barge, Marine
Construct International, MPI Offshore, Scaldis Salvage and Seajacks.
Length: 130m, Beam 38m, Draft 5m
Crew berths: 100
Crane: 1000 tons
Tonnage: 9,300 tons
Transit speed: 11 knots
Jack-up depth: 35m
No. of wind turbines capacity: 6
No. of jack up legs: 4- 6
Jack up speed: 1m/min
Dynamic positioning system
Components: On-board crane. Dynamic positioning. Propulsion systems. Jack-up system. Specialized turbine transport and installation frames.
After installation, commissioning is the process of safely putting all systems to work and addressing punch lists before handover.
The key steps in commissioning the offshore substation and cabling include visual inspection, mechanical testing, protection testing, electrical insulation testing, pre-energisation checks, and trip tests and load checks.
Assuming grid connection to the turbine is complete, key steps in turbine commissioning include:
- Check of installation activity and documentation.
- Energizing of all subsystems.
- Testing of each link in safety and emergency system chains.
- Exercising of all safety-critical and auxiliary systems.
- Slow rotation of the rotor to confirm balance and smooth operation of the drive train.
- Over-speed sensor and other safety-critical checks.
- First generation and checks on normal operation of all systems.
- Checks on critical components and connections after a period of attended operation, then after a longer period of unattended operation.
Even after first generation, it is routine to have significant punch lists for each turbine and substation containing outstanding issues that need to be addressed before handover to the customer and O&M teams. Handover will also require demonstration of performance and reliability over an agreed length of time.
The Crown Estate. “A Guide to an Offshore Wind Farm”. Provides technical and policy-related information on offshore wind turbines. This is the source used for all the information in this document.