All installation and commissioning of balance of plant and turbines, including land- and sea-based activity. For offshore activities, the process starts by transporting components from the nearest port to manufacture to either the I.7 Construction port or straight to site. Activities are complete at the wind farm construction works completion date, where assets are handed over to operational teams.
About £650 million for a 1GW wind farm. This includes the installation of the balance of plant and turbines, with related offshore logistics. It also includes developer’s insurance, construction project management and spent contingency (not itemised in sections below).
Installation: Suppliers listed in relevant sections below.
Full engineer, procure, construct and install (EPCI) services: Boskalis, DEME Group, Jan de Nul, MPI Offshore, Seaway 7, and Van Oord Offshore Wind.
Today, the typical process for installation is to install the wind farm in the following sequence, with overlaps where possible:
The installation period for a 1GW wind farm is typically three years from the start of onshore works.
Weather downtime is a key cost consideration for any offshore activity with a third of time often lost through waiting on weather.
Hs is the most widely used measure of limitation offshore. In reality, this needs to be combined with wave periodicity, direction, persistence (the length and frequency of suitable weather windows), wind speed and direction and tidal flow to define the fraction of workable and non-workable days for different activities.
Sites with deeper water and farther from shore are typically associated with more adverse weather conditions and higher weather downtime.
Increases in the size of turbines and foundations have an impact on the weather downtime unless these are accompanied by developments in equipment and processes.
The opportunity for innovation to reduce costs is substantial and increasing the operating range of offshore operations is key as this increases vessel utilisation and shortens project installation time.
Already, the season for installation is being extended, even though this increases weather downtime.
There can be considerable risk in introducing new processes and technologies to reduce weather downtime and demonstration will be difficult in some cases. A concern is that some innovations in installation aimed at reducing costs tend to push the boundaries of what can be achieved in adverse conditions. Addressing health and safety considerations need to remain a focus.
Installation services are supplied on a day rate or lump sum basis, principally for the vessel or vessels and the crew and equipment onboard. Additional costs are fuel and harbour dues.
Developers vary in strategy but contracts are usually let for the cable laying (separately for subsea export and array, and onshore), offshore and onshore substation installation, foundation installation and turbine installation. They may award a single EPCI but this has been less favoured in the UK, particularly by experienced developers that can manage the interface risks between packages.
Foundation installation consists of the transport and fixing of foundation in position.
About £100 million for a 1GW wind farm.
Boskalis, Cadeler, DEME Group, Fred. Olsen WindCarrier, Jan de Nul, Jumbo Offshore (transition pieces only), Saipem, SAL Heavy Lift (transition pieces only), Scaldis Salvage & Marine, SeaJacks, Seaway 7 and Van Oord Offshore Wind.
The process involved varies with the foundation technology employed. Offshore substation foundations may be installed in a similar way to turbine foundations but are substantially larger.
Monopiles may be installed from a jack-up vessel or a floating vessel. The transition piece is usually lifted and grouted or bolted in place from the same vessel but two-vessel strategies have been used successfully. Conical slip joint connections can avoid grouted and bolted connections. This method relies on friction, with the weight of the transition piece acting downward to provide firmness and stability.
Monopiles (up to 10m diameter) are generally moved into position using the main crane and upending tool and held in position by a gripper tool. They are the driven into the sea bed using a hammer and anvil system before mounting and grouting transition pieces.
Transition pieces are usually carried and installed by the same vessel, although a two-vessel strategy in which transition pieces are installed by a separate vessel has been used on several occasions. This focuses the utilisation of the monopile installation vessel, which is likely to have higher day rates than the transition piece vessel. A disadvantage is the costs of mobilising and demobilising two vessels.
Feeder strategies have been used for monopiles, notably with Van Oord’s Svanen, which has no useable deck space for transporting components. In this case, the monopiles are floated to site using tugs or transported using platform supply vessels.
An approximate timetable for installation once at the wind farm site is:
The full cycle time is 2-3 days per monopile, a figure that takes into account mobilisation and demobilisation, loading and waiting on weather.
Under some ground conditions, monopiles are grouted into a pre-drilled rock socket. Under conditions with boulders, a combination of drilling and driving is required.
Jacket foundations may be installed by floating vessels or jack-ups. Installation usually involves pre-piling using a reusable template. The jacket is then lowered into place over the pin piles and grouted. Post-piling, in which the pin piles are driven (or lowered into pre-drilled sockets) through a sleeve on the jacket legs, may alternatively be used. Pre-piling has the advantage of decoupling the piling and jacket installation, enabling a lower cost vessel to be used for piling and maximising the use of deck space of the main jacket installation vessel.
An approximate timetable for installation once at the wind farm site is:
The full cycle time is 3-5 days per jacket, a figure that takes into account mobilisation and demobilisation, loading and waiting on weather.
Under some conditions, suction buckets may be used as the sea bed connection. This technology offers potentially lower installation costs because less equipment is needed. Suction buckets have been deployed commercially with jacket foundations but can be used with monopiles as well.
Gravity base foundations may be installed by floating crane vessels (such as a sheerleg crane vessel) or specialist barges to support float out.
Concrete gravity foundations can weigh substantially more (3,000 tonnes) than steel foundations and may be floated out to position before being submerged. The sea bed must be levelled to receive such foundations.
Large scale installation of gravity bases has not been attempted in UK waters. Cycle times are likely to be similar to jackets, but floating transportation can result in considerably more weather downtime and requires more onshore manufacturing space.
The installation strategies for floating foundations are still evolving and will vary with the specific foundation concept. In general, the aim will be to install the turbines on the foundations at the quayside or in sheltered waters before being towed to site and moored to pre-installed anchors.
Scour protection is generally provided by dumping of rocks or bags of stones or other materials (such as tyres) around the base of the structure. Rock dumping may use a fall-pipe vessel that are widely used in the dredging industry. Bags are likely to be lowered into position using an offshore construction vessel.
The foundation installation vessel transports the foundations from the quayside fabrication facility or I.7 Construction port to the site and secures them to the sea bed. Heavy lift vessels, floating sheerleg vessels and self-propelled jack-up vessels are all used.
These costs are typically included in the foundation installation contract.
Day rates for high specification floating heavy lift vessels are likely to be about £200,000, depending on market conditions and the vessel type. Rates exclude specific foundation installation equipment and investments (for example hammers, pre-piling templates).
Operators: Boskalis, Cadeler, DEME Group, Fred. Olsen WindCarrier, Jan de Nul, Jumbo Offshore, Saipem, SAL Heavy Lift, Scaldis Salvage & Marine, SeaJacks, Seaway 7, and Van Oord Offshore Wind.
Vessel manufacturers: as for I.6.1 Turbine installation vessel
The foundation installation vessel fleet has overlapped with the turbine installation fleet in the past but the fleets are diverging due to the increasing size and mass of components and the relative merits of jack-ups and floating heavy lift vessels.
Foundation installation has made considerable use of vessels originally built for other sectors, including oil and gas, bridge building and near-shore construction.
With the mass of monopiles increasingly exceeding 1,000t, few jack-up vessels have the necessary lifting capacity. With the jacking process lasting several hours and the lower weather sensitivity of the installation process (compared with turbine installation), a floating installation vessel has notable advantages. Disadvantages have been the relatively high charter rates and low availability of heavy lift vessels with a maximum crane capacity of 1,500t or greater. The recent investments in heavy lift vessels for the offshore wind market by Boskalis, DEME Group, Jan De Nul and OHT are considerable. A typical specification for a latest generation heavy lift vessel is:
Jacket foundations are typically lighter than monopiles and the choice of vessel is driven by a number of factors including deck space and lift capacity. Installation using jack-ups is affected in particular because the position of the legs limits the flexible use of deck space. One of the advantages of three-legged jackets is that they enable better use of deck space.
There is growing interest in suction bucket foundations, particularly under jackets. These are potentially faster and therefore cheaper, to install and avoid the need for expensive noise mitigation.
Foundation handling equipment is used to manoeuvre the foundations into position before driving them into the sea bed.
The crane, upending frame, pile gripper, pile guiding positioning frame and monopile plug are part of the contractor’s equipment. As for the lifting tools, these are usually rented and cost about £10,000 per day.
Suppliers include IHC IQIP, Houlder and Temporary Works Design.
Monopiles are transported in the horizontal position.
A lifting tool grips a flange at the top of the monopile, to which the crane hook is secured.
The base of the monopile is gripped by an upending frame while the monopile is raised into the vertical position.
The crane then lifts the monopile and moves into position for piling. A pile guiding and positioning frame is used to position the monopile accurately and ensure verticality. The pile must be installed within 0.25° of vertical.
If a floating vessel is used for piling, a motion-compensated pile guiding and positioning frame may be used.
If the monopile is floated out to site, a monopile plug is used to maintain buoyancy and provide a towing point.
Jackets are transported in the vertical position if a heavy-lift vessel with deck space is used. If the jacket is to be installed in relatively benign conditions, a sheer leg crane vessel may transport a single jacket to site from port.
Equipment may be designed and built for a specific project or installed more or less permanently on the vessel with the flexibility to be used for a range of different projects.
Equipment may be owned or rented by the installation contractor.
Pile guiding and positioning frame
Foundation installation equipment is used to secure the foundation to the sea bed.
If rented, the rate is about £50,000 per day for all the equipment and third party crew to operate, but excluding other rental rates for bolting tools or grouting spread, generators, survey equipment and ROV.
Piling equipment: Cape Holland, Fistuca, IHC IQIP, Menck and PVE.
Noise mitigation: IHC IQIP and W3G Marine.
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. In such conditions, monopiles are then grouted into position.
A hammer and anvil system may be rated up to 4,000kJ and deliver 30-60 impacts per minute via a steel ram. Hammers can pile up to 9m diameter piles. Larger piles may be tapered at the top to avoid any constraint.
A novel system in development is the use of a water column to drive the pile. The reported benefits are lower noise, fewer moving parts and less installation fatigue. Vibro piling has also been trialled and offers the potential of less noise and faster, lower impact piling.
Turbine locations are typically chosen to avoid areas where piling is likely to be problematic. For some sites, a vessel will be mobilised with drilling equipment to mitigate the risk to the project schedule in cases of pile refusal.
For pre-piled jackets, a reusable piling template is lowered to the sea bed and the pin piles hammered into the sea bed using the same process as for monopiles.
There is concern about the ecological impact of pile driving on marine mammals and the harbour porpoise in particular. Piling restrictions have been most common in Germany and the Netherlands.
There are three approaches to minimising the environmental impact of piling: avoidance, deterrence and mitigation.
For avoidance, the aim is to choose foundation technologies (for example, jackets or suction piles) and installations strategies (for example, timing or vibro piling) that have reduced impact.
For deterrence, the aim is to displace animals from areas of high noise levels by means of a ‘soft start’ or using a deterrence device. ‘Pingers’ emit aversive sounds into the marine environment for about 40 mins before piling. ‘Seal scarers’ are similar but emit higher density sounds for about 30 mins before piling.
The two main mitigation technologies are bubble curtains and noise mitigation screens. Other approaches are foam-wrapped piles, hydrosound dampers and resonator systems.
Mitigation systems are expensive both in terms of equipment and time and there has been a large amount of research to understand the enduing impacts of piling on marine mammal populations. Projects may monitor impacts during installation as part of this activity.
Noise mitigation equipment
Sea fastenings are used during the transport of heavy and costly components from the I.7 Construction port to site.
Included in installation subcontract cost.
Suppliers include Durham Sheet Metal, Mammoet, MC Construction, Semco Maritime and Temporary Works Design.
Sea fastenings are typically designed and fabricated specifically for a project, although there is increasing interest in reusable sea fastenings that reduce cost of design and manufacture of the sea fastenings and shorten mobilisation time. Turbine component sea fastenings are well suited to this approach because the dimensions do not vary substantially for a particular turbine model.
Sea fastenings are typically welded steel structures that are welded to the vessel deck during mobilisation. They have locking devices to ensure safe transit and rapid release at the installation site.
Transition piece cradle
Crane sea fastening
The installation of the offshore substation consists of the transfer of the substation from its quayside fabrication site and the installation on the foundation.
About £35 million for a 1GW offshore wind farm.
The installation often forms part of the substation supply contract.
Marine contractors include Boskalis, Heerema, Saipem, Scaldis Salvage & Marine and Seaway 7.
Offshore substation installation is a heavy lift operation (2,000t plus) requiring vessels with sufficient crane capacity. Vessels with the necessary lift capacity typically do not have the deck space to accommodate a substation platform. The substation is therefore floated out of the substation fabrication facility on a barge, usually directly to the wind farm site.
The substation foundation, which is installed prior to the topside structure, may be a monopile or a jacket and the installation may form part of the turbine foundation installation package.
The substation installation vessel allows the transport and lift of offshore substation, in order to position it on pre-installed foundation.
Included in the substation installation contract.
Day rates for most substation installation vessels are about £180,000.
Semisubmersible vessels may typically have day rates greater than £450,000 but if the oil and gas market is quiet then rates may be more competitive.
Operators include Bonn & Mees, DBB, Huisman, Saipem, Scaldis Salvage & Marine and Seaway 7.
Four main types of vessel may be used:
The choice of vessel is likely to be driven by market factors and, in many cases, the vessels serve other markets. As a result, there has been little investment in vessels for the offshore wind market specifically.
Heavy lift vessels used in offshore wind include Rambiz, Sleipnir, Stanislav Yudin and Samson.
Crane ratings are from 900 tonnes to over 3,000 tonnes.
Dynamic positioning system
The construction of the onshore substation consists of the construction of the infrastructure and the installation of electrical equipment.
About £25 million for a 1GW wind farm.
Balfour Beatty, J Murphy and Jones Bros.
Enabling works to level the site and provide road access are completed early, to ensure that the work can begin promptly. They may also address constraining features of the site, such as the existence of overhead power lines or underground pipes. Subcontracted work may include fencing, curbing, tree cutting and the demolition of existing structures.
This work may form part of the main civil construction contract.
The civil contractor will typically work to an engineering design supplied by the main contractor. About 20-25% of the work will be subcontracted, including steelwork, flooring, fencing and sealing roads and car-parking areas, access tracks, gravel paths and hard-standing.
Local suppliers will generally be used unless there are specialist requirements, as they have valuable knowledge of local contractors and good contacts in the local authority and Environment Agency office.
Contractors will recruit local operatives and hire local equipment if they are operating at large distances from their fleet's base.
Electrical works and commissioning will typically be led by the main electrical supplier but substantial work is likely to be subcontracted to a high voltage electrical contractor.
The installation of the onshore export cable completes the connection between the offshore export cable and the onshore substation.
About £5 million for a 1GW wind farm, depending on distance and complexity of route.
Various construction companies such as Balfour Beatty, J Murphy and Sons.
The subsea cables terminate a short distance inland at the transition joint bay. This could be located on the beach, behind a sea defence, or up to 1km inland.
Onshore cabling is generally underground to address local concerns over the siting of overhead power lines.
There are a range of local services used before and during the cable installation. These include wheel washing, road cleaning, traffic management, signage and temporary bridges over rivers and ditches.
At least one site compound will be established along the cable route. These sites will provide equipment storage, car parking and welfare facilities for staff. Typically, they will be 100m by 100m in size.
Before construction, site investigation and environmental work is undertaken to plan the installation and minimise impact on the surroundings.
A cable corridor is used during installation, which comprises the cable trench, storage for spools and access road.
Installation can be carried out using open trenches, typically around 1 metre wide and up to 1,000 metres in length (depending on the cable) or by placing ducts into the trenches and covering them over more quickly. With ducting, it is typical to use medium density polyethylene (MDPE) ducts which are laid in the trench and the cable pulled through the ducts at a later time in up to 1,000m lengths. This option allows excavation, duct installation and backfilling to be completed in sections of up to 120m in a day. This minimises the amount of excavation left open outside working hours, which can help reduce environmental, and safety concerns.
Where the cable crosses obstacles such as roads or railways or encounters difficult or highly sensitive conditions, directional drilling may be used to route and pull the cable under the obstacle without the need for trenching.
Specialist drilling equipment creates a bore that passes the obstacle and can be up to 1,000m in length. Drilling mud is used as lubrication and this is recycled through a temporary mud lagoon during construction and disposed of after construction. Once drilled, a cable duct is then pulled through and the cable is then pulled through again using specialist equipment.
The cable is tested to ensure a complete circuit is in place. Once fully installed, an energised test is carried out to verify operation at or close to the intended voltage.
Care is taken to reduce the impact on endangered species, including species such as newts, bats and dormice, which might require specialist environmental monitoring and/or mitigation.
The installation of array cables enables the connection of the wind turbines to the offshore substation whilst the installation of the export cable enables the connection between the offshore and onshore substations.
About £220 million for a 1GW wind farm. This includes cable-laying vessel, cable burial, cable pull-in and electrical testing and termination. It also includes survey works, route clearance and cable protection systems (not itemised in sections below).
Marine contractors: Boskalis, DEME Group, Global Offshore, Jan de Nul, Nexans, Prysmian, Seaway 7 and Van Oord Offshore Wind.
Cable installation activities are preceded with a survey to define the route and identify any UXO. This is followed by a pre-lay grapnel run (or alternative method) to clear debris from the cable route.
All offshore cable installation (export and array cables) involves the following activities:
There are different strategies involving one or two vessels, and the chosen approach depends on sea bed conditions and equipment available to the contractor.
Pre-trenching and simultaneous lay and burial using a I.5.2.2 Cable plough is often preferred if the soil is suitable as immediate burial and protection is obtained in a single pass which reduces costs. In other cases, a two-stage process may also be used where the cable is laid on the sea bed, after which a vessel with I.5.2.3 Trenching ROV, I.5.2.4 Vertical injector and jetting sled, undertakes the burial.
Export cable installation starts with the shore pull-in (first-end pull-in). The installation vessel then moves off, laying the cable as it goes. Export cables are laid in as long sections as possible, of up to 70km in length, to avoid expensive subsea joints. At the substation, the cable will be either set down and wet-stored for subsequent pull-in to the substation, or immediately installed by the cable-lay vessel, which is preferred. A more detailed description of this in provided in the cable pull-in box (I.5.3 Cable pull-in).
Array cable installation starts with the first-end pull-in at the substation (subsequent first-end pull-ins are done at each turbine). Array cables are usually installed in a spider arrangement with a series of strings of turbines connected to the substation or in a series of loops (strings connected together away from the substation). Strings of turbines may be 6 to 10 turbines long, depending on cable size and turbine rating. The cables may be carried as a single length then cut offshore, or pre-cut. Using pre-cut lengths can save time offshore but because turbine spacing is not uniform, it limits the order in which the cables are installed. This can lead to important delays if there is a problem at one of the turbine locations.
Cables are typically buried to 1-4m below sea bed to ensure long-term cable integrity and to prevent damage, for example by fishing vessels, ship anchors or sea bed movement. The required burial depth is based on a cable burial risk assessment (CBRA) (and a burial protection index (BPI)). For more details on cable burial (see I.5.2 Cable burial.
Cable protection typically falls within the installer’s scope of work. This consists of bend restrictors or stiffeners to limit fatigue loading on the cables and cable entry systems that lock and seal the cable as it enters the foundation. Other techniques like rock dumping and mattresses are also used to ensure burial and protection on cable crossings.
Export cable manufacturers usually subcontract the cable installation; however, companies are increasingly investing in their own fleet (for example Nexans and Prysmian). For array cables, it is usually the cable installation contractor that subcontracts the cable manufacturing.
The cable-laying vessel lays the cables between the wind turbines and offshore substation and between the offshore and onshore substation.
Included in the offshore cable installation contract.
A typical day rate for a cable-laying vessel is about £90,000.
Marine contractors include Boskalis, CWind (Global Offshore), DEME Group, Jan de Nul, Seaway 7 and Van Oord Offshore Wind.
The same vessels may be used for export and array cable installation, although export cable-laying vessels will typically have larger carousels to accommodate longer cables. The vessels may need to have a shallow draft to install the cables in shallow water.
Simultaneous lay and burial can be carried out with a variety of burial tools. In that case, the cable is buried during the lay to obtain immediate protection. Otherwise, a post-lay burial is required. See I.5.2 Cable burial for more details on cable burial.
Cable-laying vessels are characterised as follows:
Generally equipped with a personnel transfer gangway (for example Ampelmann system) and a helideck.
ROVs have many uses including visual inspections of subsea structures such as cable entry locations on foundations or cable routes, feeding the cable through the J-tubes and monitoring operations such as grouting of piles.
Included in the installation contract.
ROVs are usually provided offshore contractor.
Manufacturers include Forum Energy Technologies, Louis Dreyfus Travocean, Saab Seaeye and SMD.
ROVs are generally used not only to monitor the subsea structures but also to assist the laying and pull-in of the cables during which they carry out a touchdown monitoring.
Cable installation contractors usually seek to avoid using ROVs to minimise costs. In deeper water, the use of ROVs avoids the high costs associated with the use of divers to work at depths requiring specialist equipment and extended decompression.
The cable-handling equipment ensures that the cable is safely deployed from the vessel to the sea bed.
The equipment is usually provided by the cable installation contractor; in that case, it is either part of the vessel or can be rented.
Typical day rates for a 2.5t carousel used for array cables are about £4,500.
Cable-handling equipment is usually provided by the cable installation contractor; in that case, it is either part of the vessel or must be mobilised.
Manufacturers: Aquatic, Ecosse Subsea, Fraser Hydraulic Power, Hulst Cable Equipment, MacArtney, Royal IHC and Sparrows.
Rental: Caley Ocean Systems, CWind (Global Offshore), Demanor, Drammen Yard, Ecosse Subsea, Osbit, RentOcean and Sparrows.
Cable handling equipment is designed to protect the cable’s integrity and to ensure the cable is deployed in a controlled manner and at the correct speed.
The cable is stored either on a carousel, in a static tank or on a reel. To exit the storage area, a tensioner is used to grip and move the cable toward the chute where the cable is deployed onto the sea bed whilst ensuring no bending at less than the minimum allowed bend radius.
During a second-end pull-in or pull-in at the substation, a quadrant is used to deploy the end of the cable on the sea bed before it is pulled in.
Cable storage: carousel, tank or reel
Cable lay equipment: tensioners, cable highway (rollers), chute and quadrant
The cable is buried to a predefined depth under the sea bed to ensure protection from external aggression (for example fishing and anchoring) as well as to prevent exposure due to sea bed mobility.
About £20 million for a 1GW offshore wind farm.
The burial and burial tools are usually provided by the cable installation contractor.
Vessel contractors: Assodivers, Boskalis, Canyon Offshore (Helix ES), Global Offshore, Jan de Nul and Van Oord Offshore Wind.
Manufacturers include Canyon Offshore (Helix ES), Osbit, Royal IHC and SMD.
Burial can be achieved either at the same time as the lay of the cable (simultaneous lay and burial) or afterwards (post-lay burial). If the former method is used, a I.5.2.2 Cable plough is used simultaneously during the cable lay to create a trench in which the cable falls and is immediately buried. In the case of a post-lay burial, the vessel will move along the laid cable, using a I.5.2.3 Trenching ROV or I.5.2.4 Vertical injector and jetting sled to fluidise the sediment and allowing the cable to be buried.
Burial depths are determined based on an industry standard (burial protection index and/or cable burial risk assessment). Generally, cables are buried at a depth of 1-4m below the sea bed.
The cable burial vessel undertakes cable burial post laying of the cable on the sea bed.
These costs are usually included in the cable burial contract.
A typical day rate for a cable burial vessel is about £95,000.
Cable burial vessels are provided by a number of offshore vessel operators such as Canyon Offshore (Helix ES), Global Offshore and Van Oord Offshore Wind.
Burial vessels vary in size based on the required burial tools to be mobilised and the water depth. Generally, most types of vessels can be utilised as long as burial tools can be mobilised. Dynamically positioned vessels are generally used although barges may be used in shallower waters in the case of near-shore burial work.
Crane or A-frame
Personnel transfer gangway
Burial tools and equipment
A cable plough is normally used to simultaneously lay and bury a cable but it can also be used in post-lay burial and pre-trenching.
The provision of the cable plough is usually part of the cable installation contract scope.
When hired, a typical day rate for a cable plough is about £5,000.
The cable plough is usually provided by the cable installation contractor, either as part of the vessel or mobilised specifically.
Manufacturers: ETA Subsea Specialists, Osbit, Royal IHC and SMD.
Rental: ETA Subsea Specialists and Pharos Offshore.
Cable ploughs can bury the cable down to 3-4m below sea bed level.
The plough will require a tow force to pull the plough through the soil depending on the soil conditions and the required burial depth. Using a barge (for shallow water operations), this force is supplied by an anchor or a tow tug. For a dynamically positioned vessel, a specialist vessel with an appropriate bollard pull is required. It is often not possible to plough close to the turbine or substation. In that case, a I.5.2.3 Trenching ROV may be used.
High pressure jetting nozzles
A trenching ROV forms a trench in which to bury the cable. This tool is generally used in post-lay burial but can be used during simultaneous lay.
The provision of the trenching ROV is usually part of the cable installation contract scope.
When hired, a typical day rate for a trenching ROV is about £10,000.
The trenching ROV is usually provided by the cable installation contractor, either as part of the vessel or mobilised specifically.
Manufacturers: Forum Energy Technology, Louis Dreyfus Travocean, Osbit, Royal IHC, SIMEC and SMD.
Rental: Dockstr, Ecosse Subsea and James Fisher Marine Services.
ROVs can have either a jetting system or a mechanical cutter. A high pressure jetting system is used to fluidise the sea bed and allow the cable to sink to the required depth (only in sandy sediments and softer clays). For rocky or hard clay sea bed conditions, a mechanical cutter is used.
Pressure and flow water jetting system and/or mechanical cutter
Power supply and control system
Camera and lighting system
Vertical injectors and jetting sleds are used to bury the cable where the sediment can be fluidised (for example sand, soft clays). Vertical injectors are used for simultaneous lay and burial of the cable. Jetting sleds are mostly used for post-lay burial.
The provision of the vertical injector or jetting sled is usually part of the cable installation contract scope.
When hired, a typical day rate for a vertical injector is about £10,000 and £8,000 for a jetting sled.
The vertical injector or jetting sled is usually provided by the cable installation contractor.
Manufacturers: Miah, Royal IHC and Seatools.
Rental: ETA Subsea Specialists, Global Offshore and Modus.
Vertical injectors can bury the cable down to 10m below sea bed level using a high pressure jetting system in soft sediment. They are generally fixed on the side of the vessel. A vertical injector is made up of a header and extension section as well as a burial section, the shoe injector, which contains the jetting nozzles.
Jetting sleds can bury the cable down to 4m below the sea bed. They are usually equipped with a hydraulic actuation system that ensures the cable is buried at the required depth. An eduction system allows the removal of excess material in the trench once the fluidisation has been carried out. Jetting sleds are deployed using a crane and can therefore be mobilised on a large range of vessels.
High-pressure jetting nozzles
Cable depressor detector
Positioning system receptor
For the array cable, the pull-in consists of the pulling of the cable into the substation or turbine foundation.
For export cables, the pull-in consists of pulling the cable to shore as well as into the substations.
About £7.5 million for a 1GW offshore wind farm.
The cable pull-in is usually provided by the cable installation contractor (see I.5 Offshore cable installation).
The installation of the export cable starts with the beach pull-in. During this, the cable vessel is anchored offshore and the cable winched on floats or through a pre-laid duct to the onshore transition joint pit, where it will eventually be jointed to the onshore cable. The installation vessel then moves off, laying the cable as it goes. Depending on the landfall site, some projects require horizontal directional drilling (HDD) which may extend to the first short length of burial offshore. In other cases, the cable may be transferred to a third party shallow draft barge or amphibious vehicle to bring the cable to shore. At the offshore substation, the cable will be either set down and wet-stored for subsequent pull-in to the substation, or immediately installed by the cable-lay vessel, which is preferred. It may however be necessary to wet store the cable if for example the substation is not installed yet or if the lay vessel is not equipped to conduct the second-end pull-in at the substation.
The installation of each string of array cable starts with the pull-in at the substation. The second-end pull-in consists of pulling the cable into the turbine foundation transition piece. After this, the crews pull in the first end of the next cable at the wind turbine location: a messenger wire is used so that the ROV finds the cable entry hole at the base of the foundation; the cable is then pulled up into the foundation. The vessel then moves off to the next location, laying the cable as it goes and pulling it in once it arrives at the following location. For second-end pull-ins, a quadrant is generally used.
Horizontal directional drilling
The electrical testing is designed to test and prove cable integrity whilst the termination enables the electrical connection between the offshore cable and either the wind turbine, the substation or the onshore cables.
About £6.5 million for a 1GW offshore wind farm.
The electrical testing and termination is usually provided by the cable installation contractor (see I.5 Offshore cable installation).
Manufacturers of electrical testing equipment and termination tools: Baur, Megger Pfisterer, Tekmar and WT Henley.
After the cable is pulled into the substation or wind turbine, a hang-off clamp is fitted and the cores of the cables are stripped back and connected to a termination plug. The plug will then be interfaced into a designated junction box or switchgear using a connector. A similar procedure is conducted for the fibre optic cable.
Prior to the termination, a series of electrical tests are performed to prove the cable’s electrical integrity. These include very low frequency (VLF) tests, insulation resistance (IR) tests, time-domain reflectometry (TDR) tests and optical time-domain reflectometry (OTDR).
After the cable is pulled into the transition joint bay on shore, it is terminated at the beach joint.
Test and diagnostics device
Turbine installation involves transportation of the turbine components from the I.7 Construction port and installation of the turbine components onto the foundation.
About £50 million for a 1GW wind farm.
Cadeler, DEME Group, Fred. Olsen WindCarrier, Jan de Nul, MPI Offshore, Seajacks and Van Oord Offshore Wind.
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. Typically, the turbine tower is pre-assembled onshore and transported with the nacelle and blades for final assembly offshore.
Three variations in the rotor installation process have been used repeatedly:
The third method is current preferred practice, even though this involves more offshore operations.
Tower sections are typically preassembled onshore with any internal components and the completed structure is transported vertically to site for installation. Offshore turbine installation is undertaken by jack-up vessels due to the need for a stable platform to perform offshore lifting operations and mating of components at height.
The installation of a turbine from positioning the vessel at the site to departure takes about 24 hours, depending on location and weather conditions. The cycle time is between 1.5 and 4 days, depending on the project (factoring in mobilisation, demobilisation, loading and waiting on weather).
A constraint during transportation and installation is the acceleration limit defined by the turbine supplier to avoid damaging the turbines and invalidating warranties. This is typically about 0.5g.
Blade installation is constrained not only by the operating range of the vessel but also the wind speeds, and the limit has been gradually increased with innovations in blade lifting equipment. The current maximum is normally 13m/s at hub height and any increases beyond this may be limited by health and safety risks.
Whole turbine installation, in which the complete turbine, including the tower, is assembled onshore, then transported and lifted into place on the foundation, reduces the number of offshore lifts as well as avoiding much of the offshore commissioning process. A number of new concepts are in development. The approach is most commonly associated with concrete gravity bases. The concepts typically involve the investment in a bespoke vessel. Due to the considerable improvements in the installation time when using conventional approaches by the turbine suppliers, the business case for full turbine installation is less strong. There has also been slow progress in the commercialisation of deep water gravity base foundations.
A step change in turbine installation could be achieved through the use of floating vessels for turbine component installation, which could shorten installation times further. The movements of the lifting hook at hub heights greater than 110m on a floating vessel have the potential to be substantial, however. Progress on floating installation methodologies will depend on collaboration between turbine suppliers and installation contractors.
Turbine installation is undertaken jointly by the turbine supplier technicians and the installation contractor. The turbine supplier is usually responsible for the lifts along with mechanical and electrical completion.
The turbine installation vessel transports the turbine components to site and supports the erection of the turbine on the foundation. Similar jack-up vessels are used to those for foundation installation.
These costs are typically included in the turbine installation contract.
Day rates for vessels range between £90,000 and £130,000, excluding fuel, crew and equipment. Depending on the transit distance, fuel can cost up to £20,000 per one-way sailing to wind farm site.
Operators: DEME Group, Fred. Olsen WindCarrier, Jan de Nul, MPI Offshore, Seajacks, Cadeler and Van Oord Offshore Wind.
Vessel manufacturers: generally in China, Korea, Singapore or the Arabian Peninsula.
Recent turbine installation on commercial-scale projects to date has normally been undertaken with a self-propelled jack-up vessel designed primarily for the purpose, though in some cases, jack-up barges have been towed with tugs.
Vessel contracts are typically placed by the wind farm developer or the turbine supplier.
An example of specification for these vessels is:
Most of the vessels in operation have been used for both turbine and foundation installation. Increasingly the fleets are diverging. The increase in turbine capacity (and therefore rotor diameter) is associated with a higher hub height. At the same time, foundation mass is increasing and they can now be installed more rapidly from a floating vessel.
The current fleet of turbine installation vessels was designed to install 6-10MW turbines. Investment in new vessels requires careful consideration due to:
A number of vessel cranes have undergone modification but unless upgrades were considered in the original design, they can have an impact on other aspects of the vessels’ performance.
Feeder vessels could be used to limit the transit time of the main installation vessel but this is only likely to be cost effective if the transfer of turbine components from low cost floating feeder vessels can be achieved without increasing risk and if the feeder vessel has a considerably lower charter rate than the main installation vessel.
Floating vessels are considered a natural next step for turbine installation, offering theoretically faster installation than jack-ups. Hook height movements at 110m or higher can be important, thereby limiting the operability of the vessel for installation work. A floating installation vessel could also be used efficiently for foundation installation thereby reducing investment risk.
Vessels no longer suitable for turbine installation in Europe could be further utilised in the service market and in new installation markets such as Asia, where turbine size has so far lagged behind that in Europe.
I.6.1.1 Turbine handling equipment and sea fastenings
Dynamic positioning system
Turbine handling equipment is used to assist in the lifting and manipulation of turbine components during loading in port and installation offshore. Handling equipment is typically developed by the turbine supplier to be specific to a given task and component. There are several handling tools required for offshore installation including tower handling tools, nacelle handling tools and blade handling tools.
Sea fastenings are used during the transport of heavy and costly components from the I.7 Construction port to site.
Included in turbine installation cost.
Component handling tools are often provided to the offshore contractor by the turbine supplier as the tool is specific to a turbine type and installation methodology.
There are several approaches to reducing the sensitivity of turbine component lifts (especially the blades) to high winds, thereby reducing weather downtime:
Currently, blades can be lifted in winds in wind speeds up to 13m/s. Although this limit could be raised in theory, there comes a point where high winds make work on deck hazardous, even if the turbine installation can, in theory, be continued.
Sea fastenings are structures located on the deck of the installation vessel, which allow for the safe transportation of turbine components from the I.7 Construction port to the installation location. Typically, large steel fabricated structures and frames secured to the main deck of the installation vessel.
Sea fastenings are designed to transfer the load of the component into the vessel structure and keep the component in position without damaging the component or vessel. Sea fastenings must also be designed to allow safe access of technicians both during transportation for inspections and to release the component to allow lifting.
Remotely controlled electro mechanical systems
Remote controls operated by installation technician on deck of the vessel
Blade rack sea fastening
Crane sea fastening
After installation, commissioning is the process of safely completing mechanical and electrical assembly, putting all systems to work and addressing punch lists before handover.
These costs are included in the wind turbine / substation supply contract.
Generally led by the wind turbine supplier and substation supplier.
The key steps in commissioning the offshore substation and cabling include visual inspection, mechanical testing, protection testing, electrical insulation testing, pre-energisation checks, trip tests and load checks.
Assuming grid connection to the turbine is complete, key steps in turbine commissioning include:
Even after first generation, it is routine to have several punch lists for each turbine and substation containing outstanding issues that need to be addressed before handover to the customer and operation, maintenance and service (OMS) teams. Handover will also normally require demonstration of performance and reliability over an agreed length of time.
Electrical testing device
The construction port is 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.
Included in installation contracts.
UK ports used so far include Able Seaton, Barrow, Belfast, Cromarty, Dundee, Great Yarmouth, Harwich, Hull, Mostyn, Nigg and Sheerness.
Non-UK ports used for UK projects include Cuxhaven, Eemshaven, Esbjerg, Ijmuiden, Ostende and Vlissingen.
Construction port requirements are typically:
Sites with greater weather restrictions or for larger scale construction may require an additional lay-down area, up to 30 hectares.
Large areas of land are required due to the space taken when turbines are stored lying down on the ground.
Offshore logistics involves coordination and support of offshore installation and commissioning activities.
About £3.5 million for a 1GW wind farm.
High-level coordination is typically undertaken by the developer.
Construction management services: DNV-GL, K2 Management, LOC Renewables, Natural Power (Fred. Olsen), ODE, RINA and SeaRoc (Fred. Olsen).
Offshore logistics covers all the work needed to ensure that construction proceeds smoothly, safely and on time.
Construction management covers a wide range of services including contract management, health and safety and marine coordination. In many cases, contractors are embedded in the construction management team, and this may include sole traders. In addition, in order to fulfil the insurer’s requirements, a marine warranty surveyor (MWS) has to be appointed. The MWS ensures that all activities are compliant with the approved procedures and delivers the Certificate of Approval (CoA).
Specialist software tools are available to plan and monitor offshore activity.
Weather and metocean forecasting services provide visibility of weather windows a few days in advance. While meteorological buoys are typically owned and operated in the UK by the MetOffice, third-party providers with their own forecasting algorithms also offer services.
Support vessels include guard vessels (potentially drawn from local fishing fleets), crew transfer vessels and accommodation vessels. These vessels may be contracted by the developer of the marine contractor.
About £2.5 million for a 1GW wind farm.
Vessel operators: HST Marine, Mainprize Offshore, Turner Iceni, Offshore Wind Power Marine Services and Windcat Workboats.
Vessel manufacturers: Alicat, Arklow Marine Services, Damen, Diverse Marine and Wight Shipyard.
Specialist vessels are used for crew transfer to the wind farm for installation and commissioning tasks. These are typically 15-26m workboats of the kind regularly used during wind farm maintenance.
ROV support vessels are 80-100m DP2 vessels with a moon pool and deck crane.
Marine coordination is necessary in order to manage heightened marine traffic as well as multi-vessel activity on an offshore construction site.
About £850,000 for a 1GW wind farm.
Marine coordination is usually carried out by the developer or a subcontractor, for example SeaRenergy, SMC and WindandWater.
Suppliers of marine management system software include James Fisher Marine Services, SeaRoc (Fred. Olsen) and Systematic.
A marine coordinator, usually located at the base harbour or operations base [B.5], is responsible for the coordination, control and exchange of information between all contractors working on the site. A marine management software system is used to plan and monitor vessel and personnel movements.
The main tasks of the marine coordinator include:
Communicate with all vessels and helicopters.
Marine management system software
Marine coordination centre
Weather forecasts are needed for short-term planning of offshore activities (for example vessel transfers and lifts) and the closer the forecast is to the activity, the more reliable it gets. Metocean data recordings are used to provide real time data to support offshore activity, to verify forecast tools and to resolve disputes regarding weather downtime.
Key metocean parameters that impact installation and commissioning activities are wind speed, wave height and current.
About £300,000 for a 1GW wind farm.
The weather forecast supplier usually offers several options (both in the number of forecasts per day as well as forecasts for the different locations). For example, forecasts for the base harbour and the offshore site or a complete forecast for base harbour, the offshore site and transit route.
Metocean measurement devices can be rented or purchased.
Suppliers for weather forecast services: Fugro, MetOffice, MetoGroup and StormGeo.
Suppliers for current and wave buoys: AXYS Technologies, Datawell and OSIL.
Suppliers for anemometers and lidars: AXYS Technologies (floating lidar), EOLOS (floating lidar), Gill Instruments (anemometer), Leosphere (Vaisala) (lidar) and ZX Lidars (Fred. Olsen) (lidar).
In addition, the vessel contractor generally provides wind measurements (for example via anemometer mounted on crane boom or lidar).
Weather plays a crucial role in offshore installation and commissioning activities as it has an influence on the sequence and duration of planned activities (which need to be conducted safely that is all offshore activities have weather limits, exceeding these would be unsafe) and may lead to delays, which result in elevated costs.
Weather forecasts are generated through global meteorological models that may be improved in their accuracy with finer resolution local models. Forecasts usually include several different meteorological parameters such as wind speeds at different heights, wave and swell height and period as well as general weather information (for example visibility, lightning risk, fog, water and air temperature and rain).
The forecasts are used to plan activities based on when weather windows are available.
Wind parameters are usually measured with a lidar (on a fixed or floating meteorological station) or an anemometer (rotary or ultrasonic) on a fixed metrological station with tall mast. The advantage of the lidar is that wind speed and direction at different heights can be determined.
Ocean parameters can be measured with a wave buoy or current meter although there is a trend towards complete systems that combine both wave and current measurements.
Weather forecast report (and online access)