Term Definition
Annual energy production (AEP) The amount of energy generated in a year.

Gross AEP is the predicted annual energy production based on the turbine power curve, excluding losses.

Net AEP is the metered annual energy production at the offshore substation, so includes wind farm downtime, wake, electrical and other losses.

Array cable Electrical cable that connects the turbines to each other and the offshore substation.
Availability The percentage of time the assets are available to produce / transfer power if the wind speed is within the operational range of the turbine.
Balance of plant (BoP) Includes all the components of the wind farm except the turbines, including transmission assets built as a direct result of the wind farm.
Department for Business, Energy and Industrial Strategy (BEIS) Government department that is responsible for business, industrial strategy, science and innovation and energy and climate change policy.
Cable protection system (CPS) Cable protection systems protect the subsea cable against various external aggressions. Systems include bend restrictors and bend stiffeners where the cable may be subject to increased loading.
Capacity factor Ratio of annual energy production to maximum energy production if the turbine / wind farm ran at rated power all year.
Capital expenditure (CAPEX) Spend on all activities up until works completion date.
Contract for difference (CfD) Contract where government agrees to pay the wind farm owner the difference between an agreed strike price and the average market price of electricity (reference price). If the difference is negative the wind farm owner pays the difference to the government.
Crew transfer vessel (CTV) A vessel used to transport wind farm technicians and other personnel to the offshore wind farm turbines either from port or from a fixed or floating base. Vessels operating today are typically specially designed catamarans that accommodate around 12 passengers.
Cross-linked polyethylene (XLPE) A thermoset material widely used as electrical insulation in power cables.
Doubly-fed induction generator (DFIG) An electrical arrangement where part of the wind turbine generator power passes via slip rings and convertors to enable a limited variable speed operating range whilst minimising the cost of power electronics.
Decommissioning expenditure (DECEX) Spend on removal or making safe of offshore infrastructure at the end of its useful life, plus disposal of equipment.
Environmental impact assessment (EIA) Assessment of the potential impact of the proposed development on the physical, biological and human environment during construction, operation and decommissioning.
Engineer, procure, construct and install (EPCI) A common form of contracting for offshore construction. The contractor takes responsibility for a wide scope and delivers via own and subcontract resources.
Export cable Electrical cable that connects the onshore and offshore substations, or between an AC offshore substation and a DC converter substation.
Front end engineering and design (FEED) Front-end engineering and design (FEED) studies address areas of wind farm system design and develop the concept of the wind farm in advance of procurement, contracting and construction.
Final investment decision (FID) The point at which a developer has in place all the consents, agreements and major contracts required to commence project construction (or these are near execution form) and there is a firm commitment from equity holders and debt funders to provide funding to cover the majority of construction costs.
Floating foundation A buoyant foundation structure anchored to the sea bed via mooring lines. The term includes several foundation types including spar buoys, tension leg platforms and semi-submersibles.
Gas insulated switchgear (GIS) Gas-insulated switchgear is often chosen for its compactness and increased reliability over than air insulated switchgear, but has higher cost.
Gigawatt (GW) and Gigawatt hour (GWh) Unit of power and unit of energy.
Gravity base foundation A type of foundation designed to be transported offshore as a (normally concrete) hollow structure that is later fixed to the sea bed with the addition of ballast.
High voltage alternating current (HVAC) An electric power transmission system that uses alternating current for the bulk transmission of electrical power. Alternating current is the form in which electric power is generated by wind turbines and delivered to an end user.
High voltage direct current (HVDC) An electric power transmission system that uses direct current for the bulk transmission of electrical power. For long-distance transmission, HVDC systems may offer lifetime cost advantages over HVAC systems over long transmission distances. They are currently only used for point-to-point connections.
Highest astronomical tide (HAT) The highest tidal height predicted to occur under average meteorological conditions and any combination of astronomical conditions.
Horizontal directional drilling (HDD) Horizontal directional drilling is a low impact (trenchless) method of installing underground cables using a surface-launched drilling rig.
Jacket foundation See Non-monopile steel foundation.
Levelised cost of energy (LCOE) Levelised cost of energy is a commonly used measure of the cost of electricity production. It is defined as the revenue required (from whatever source) to earn a rate of return on investment equal to the WACC over the life of the wind farm. Tax and inflation are not modelled.
Mean high water springs (MHWS) The average tidal height throughout the year of two successive high waters during those periods of 24 hours when the range of the tide is at its greatest.
Mean sea level (MSL) The average tidal height over a long period of time.
Megawatt (MW) and Megawatt hour (MWh) Unit of power and unit of energy.
Monopile foundation A type of foundation with a cylindrical tube (normally steel) that is normally driven tens of metres into the sea bed, although it can also be inserted into pre-drilled holes.
Non-monopile steel foundation Collective term used to describe all steel foundations other than monopiles. Includes braced, welded, space-frame structures (collectively called ‘jackets’), tripods and tripiles.
Offshore substation (OSS) The structure used to transform and transfer the energy collected by the wind turbines to land in the most efficient manner. It may involve increasing the voltage, providing reactive compensation and converting the current from AC to DC. Some wind farms may have more than one offshore substation and equipment may be located on a number of smaller structures and potentially on one or more turbine transition pieces.
Offshore Transmission Owner (OFTO) An OFTO, appointed in UK by Ofgem (Office of Gas and Electricity Markets), has ownership and responsibility for the transmission assets of an offshore wind farm.
Operational expenditure (OPEX) Spend on all activities from works completion date until decommissioning.
Operations, maintenance and service (OMS) OMS comprises wind farm OMS and onshore transmission OMS.

Definitions of O, M and S are as follows:

  • Operation: day-to-day management including all the work not covered under maintenance and service. For wind farm OMS, this includes cost for port facilities, buildings, management personnel, environmental monitoring and community engagement.
  • Maintenance of assets: scheduled (that is, planned a long time in advance) maintenance, that may be based on suppliers' recommendations or owner's experience. It includes condition-based or time-based maintenance programmes and planned health and safety inspections.
  • Typical maintenance includes inspection, checking of bolted joints and replacement of wear parts (with design life less than the design life of the project).
  • Service of assets: unscheduled interventions in response to events or failures. Interventions may be proactive (before failure occurs, for example responding to inspections or condition monitoring (CM) or reactive (after failure that affects generation has occurred). Also included are interventions due to major components not lasting the full turbine design life, even if intervention was planned prior to construction.
  • Service operations include both on site repair and replacement of large and small components.
Remotely operated vehicle (ROV) ROVs are remotely guided subsea mobile devices. They are usually deployed from a vessel. ROVs can be used for inspections or to carry out handling and repair.
Service operation vessel (SOV) A vessel that provides accommodation, workshops and equipment for the transfer of personnel to turbine during OMS. Vessels in service today are typically up to 85m long with accommodation for about 60 people.
Significant wave height (Hs) The wave height (trough to crest) of the highest third of the waves over a given period.
Supervisory Control and Data Acquisition (SCADA) system Data acquisition, transmission and storage system covering all wind farm assets. The SCADA system enable individual wind turbines, the wind farm substations and associated wind farm equipment to communicate operational status including faults. This allows operators to remotely diagnose faults and issue commands to stop, start and reset turbines and other equipment. The SCADA system keeps a full operating history of the wind farm.
Transition piece A part of the foundation that provides the connection between the foundation and the wind turbine tower. For monopiles, it is usually installed after piling. For non-monopile steel or gravity base foundations, the transition is connected to the main structure before installation.
Turbine rated power The nominal maximum power output from a wind turbine. Sometimes this is referred to as capacity. The wind turbine is limited to this power output, which typically applies when the wind speed at the hub height exceeds about 12m/s and continues until about 25-30m/s when the wind turbine stops generating to avoid excessive loading. In more benign operating conditions characterised by ambient temperature, main component temperatures, wind speed, turbulence level and grid voltage levels, the output may be allowed to exceed the rated power by about 5%.
Unexploded ordnance (UXO) Explosive weapons that did not explode when they were released and remain a risk to seabed users.
Weighted average cost of capital (WACC) The weighted average rate of return a wind farm owner expects to compensate itself and its internal and external investors over the life of a project.
Wind shear The degree to which wind speed changes with height.
Works completion date (WCD) Date at which construction works are deemed to be complete and the wind farm is handed to the operations team. In reality, this may take place over a period of time.