D Decommissioning
Function

Removal or making safe of offshore infrastructure at the end of its useful life, plus disposal of equipment.

What it costs

The decommissioning of a 1GW offshore wind farm will cost around £300 million (gross, excluding any resale value of equipment removed).

Who supplies them

Contractors will be similar to those used for installation.

Likely also other offshore operators will enter the space, including players with offshore oil and gas decommissioning experience who will not get involved with new product installation.

Key facts

At the end of the nominal design life of an offshore wind farm, there are a number of options:

  • Extend the operational life of existing assets through a programme of risk assessments, inspections, addressing regulatory aspects, and some component replacement.
  • Repower the site with new (larger) turbines, likely meaning decommissioning of existing turbines, foundations and array cables, with the possibility to extend the life of electrical transmission assets. The MW rating of the wind farm and transmission system may be unchanged as the area of the wind farm and density of turbines (MW installed per square km) are likely to be unchanged.
  • Fully decommission the site.

Properly financed decommissioning plans typically are required as part of gaining planning approval to construct the wind farm. In practice, permission is likely to be sought to deviate from decommissioning plans as the sector matures decommissioning techniques. UK Government acts as decommissioner of last resort so is ultimately responsible. As a result, it takes security for decommissioning.

Turbine decommissioning will require complete removal of the structure. For nacelle and tower components, the potential for recycling is considerable. There is no current process for recycling composite materials such as those used in the blades and nacelle cover, but is likely that methods will emerge by the time a large volume of offshore wind turbine commissioning is required.

The process for foundation decommissioning will depend on the technology adopted and its sea bed connection.

Decommissioning has only been carried out on a number of small, early offshore wind farms overseas.

Environmental surveys are typically required before and after decommissioning, along with post-decommissioning management of the site in line with the Energy Act 2004.

D.1 Turbine decommissioning
Function

Complete removal and shipment to shore of turbine rotor, nacelle and tower.

What it costs

Around £45 million for a 1GW offshore wind farm.

Who supplies them

Turbine installation contractors such as A2Sea/GeoSea (DEME Group), Seajacks and Van Oord Offshore Wind can provide turbine decommissioning.

Likely also other offshore operators will enter the space, including players with offshore oil and gas decommissioning experience.

Key facts

The process will be a reverse of the installation process, such as individual blades being removed, then hub and nacelle then finally tower.

In some cases where it is determined that the remaining life if sufficient, there will be a market for reuse of second-hand components, either as spares for other operating wind farms or possibly for re-installation elsewhere.

Similar lifting frames and arrangements will be used at installation to enable work in the largest envelope of weather conditions. For complex lifting operations, the same level of planning and health and safety management as at installation is required, with the added assessments and method statements to deal with the risks of damaged components or seized interfaces.

In general, the removal process may be quicker than for installation because minor damage to components will be less critical. If components are to be recycled rather than reused, then in some cases less care needs to be taken to preserve the delicate aerodynamic surfaces and the condition of other components, potentially enabling the use of different equipment or enabling operations in a wider operating environment.

D.2 Foundation decommissioning
Function

Removal and shipment to shore or cut-off at sea bed level and making safe.

What it costs

Around £75 million for a 1GW offshore wind farm.

Who supplies them

Foundation installation contractors such as Boskalis, Geosea (DEME Group) and Jan de Nul can provide foundation decommissioning.

Likely also other offshore operators will enter the space, including players with offshore oil and gas decommissioning experience.

Key facts

Decommissioning plans may define specific requirements for removal of components below the mud line which in turn may drive the choice or design of foundations and installation methods. It is possible that in some circumstances, structures could be left in position where they support ecosystems that in the opinion of the regulator must be safeguarded.

For monopiles or jackets, all elements above the sea bed will probably need to be removed with piles cut off at an agreed height (typically 1m below the top of the sea bed). Initially, the process is likely to draw heavily on the oil and gas industry’s experience of removing subsea structures and then be optimised for the offshore wind industry.

Removal of foundations is likely to involve the use of a work-class ROV fitted with a range of cutting and drilling tools including guillotine saws, hydraulic hole cutting tools (for making lifting holes) and abrasive waterjet cutting. Gravity base manufacturers stress the ease of decommissioning as the structures can be de-ballasted and lifted/floated off to be broken down or used as breakwaters, the basis for artificial reefs or similar.

The use of suction caissons has been put forward as a means of reducing fabrication and installation costs. Decommissioning could also be straightforward, using the suction system in reverse to raise the foundation from the sea bed. Early trials have shown positive results, but removal after many years of fatigue loading has not been tried yet in wind.

D.3 Cable decommissioning
Function

Removal and shipment to shore.

What it costs

Around £140 million for a 1GW offshore wind farm.

Who supplies them

Cable installation contractors such as Boskalis, Global Marine and Subsea 7 can provide cable decommissioning.

In addition, companies such as CRS Holland, Pharos Offshore and Subsea Environmental Services can carry out subsea cable recovery.

Key facts

The value, especially of the main conductor material in array and export cables, is such that it is likely to remain worth removing the cable, rather than leaving it buried.

Cables will be disconnected each end then pulled from the sea bed and wound on to drums or chopped into short sections for storage on the decommissioning vessel. The method of gripping and pulling the cable will depend on the ground conditions and burial depth. For sandy conditions the approach is likely to involve fluidising the sea bed while the cable is pulled. The industry is likely to develop new tools for the process.

Particular care will be needed at cable crossings (power or telecommunications) to avoid damage to functioning assets.

D.4 Substation decommissioning
Function

Decommissioning plans typically are required as part of gaining approval to construct. These may define specific requirements for removal of components below the mud line which in turn may drive the choice or design of substation foundations and installation methods.

What it costs

Around £65 million for a 1GW offshore wind farm.

Who supplies them

Substation installation contractors such as Boskalis, Saipem and Seaway Heavy Lifting (Subsea 7) can provide substation decommissioning.

Key facts

The process is likely to be a reverse of the installation process, although it may prove cheaper to cut the substation into sections for removal to enable a series of smaller lifts that can be undertaken by a lower cost vessel.

In some cases, there will be a market for reuse of second-hand electrical components, after refurbishment, as spares or for other applications.

If the remaining life of the substation structure and equipment, after refurbishment, is sufficient, the substation could be left in-situ and reused for a repowered wind farm of the same capacity.

D.5 Decommissioning port
Function

Port where equipment removed is offloaded and marshalled for next stage of processing.

What it costs

Included in decommissioning contract for each of the components.

Who supplies them

Similar to installation ports, but also including facilities with less stringent requirements and locations dedicated to decommissioning. Examples in the UK include Belfast, Great Yarmouth and Hull and specialist decommissioning facilities at Seaton. Non-UK ports include Eemshaven, Esbjerg and Vlissingen.

Ideally, decommissioning ports will have salvage and processing facilities on site and some ports may develop expertise in handling certain types of materials. In some cases, some specialisations developed as part of oil and gas decommissioning may be valuable, even if this involves additional transit time from the wind farm site.

Key facts

Facilities similar to those used for installation will be required. Large structures that will be broken up are likely to be transported to facilities dedicated to such activity.

What's in it
D.6 Reuse, recycling or disposal
Function

Once equipment is onshore, there is a motivation to extract maximum value via reuse, recycling or disposal.

What it costs

Overall, net positive value.

Who supplies them

Contractors that provide salving/recycling include Delta Marine, DUC Marine Group and Scaldis Salvage & Marine.

Key facts

Currently, different parts of decommissioned onshore wind turbines are reused, recycled or disposed of, depending on age, condition and material content. There is an established second-hand market for onshore turbines known to be robust and reliable and determined to have sufficient fatigue life remaining. In this case, the turbines are refurbished and installed on new foundations for operation up to 50% beyond the design life period.

Offshore costs and financing are such that it is unlikely that offshore turbines will be decommissioned with sufficient fatigue life remaining and so be re-installed offshore, Turbines are typically disassembled for recyclable scrap, with relatively low proportion of nacelle and tower mass having no residual value and requiring safe disposal. There is typically a high content of a range of known steel grades and cast iron; also valuable amounts of copper, aluminium and in future, permanent magnet materials.

Today, the wind turbine component that cannot be cost-effectively recycled are the composite blades and nacelle cover, but is likely that methods will emerge by the time substantial offshore wind turbine decommissioning is required, as there are a range of projects underway in this area.

Blades are typically made from a combination of glass- and carbon-fibre in epoxy- or polyester-based resin matrices, along with polyethylene terephthalate (PET) or balsa foam. At the root end, there are steel inserts to provide bolted connection to the blade bearing. Other than this, there is typically a copper-based lightning protection system.

So far, blades have been cut up and either sent for burning (in waste to energy or district heating plant) or to landfill.

Most foundations and substation topsides typically have high steel content, so can be broken down and recycled as input to the manufacture of new steel components. Some substation components may be reused; others can be recycled as for turbines, again with relatively low proportion requiring having no residual value and requiring safe disposal.

The cable conductor can be readily processed and reused in a range of sectors, XLPE may be cleaned, dried and ground and recycled as filler for new power cables or as insulation in lower voltage cables or accessories.

View All