Offshore Wind (OSW) Terminology & Definitions

Updated: Jul 3, 2020

ACPARS – Atlantic Coast Port Access Route Study


Active Power: A real component of the apparent power, usually expressed in kilowatts (kW) or megawatts (MW), in contrast to reactive power.


Adequacy: A measure of the ability of the power system to supply the aggregate electric power and energy requirements of the customers within component ratings and voltage limits, taking into account planned and unplanned outages of system components. Adequacy measures the capability of the power system to supply the load in all the steady states in which the power system may exist, considering standard conditions.


Aeolus: In Greek mythology, is a name shared by three mythical characters. Aeolus (/iːˈoʊləs/; Ancient Greek: Αἴολος, Aiolos [a͜ɪ́olos], Modern Greek: [ˈe.o.los] "quick-moving, nimble") was a son of Hippotes and is mentioned in Odyssey and the Aeneid as the Keeper of the Winds. Accordingly, on occasion, wind energy is referred to as Aeolic Energy.


Ancillary Services: Services such as provision of reactive power, or frequency response, necessary for control or operation of the power system, and which it may be beneficial to provide from generators or other system users.


Annualized Net Metering: The same as net metering, but in this case the regulator averages a user’s net electricity consumption or production over the span of one full year, rather than a shorter period.


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.


AWEA – American Wind Energy Association


Balance of Plant (BoP): Includes all the components / infrastructure of the wind farm except the turbines, including transmission assets built as a direct result of the wind farm, civil works, SCADA and internal electrical system. It may also include elements of the grid connection.


Barge Unit: Surface-type unit without primary propelling machinery.


Black Start Capability: Some power stations have the ability to start up independently of a power grid. This is an essential prerequisite for system security, as these plants can be called on during a blackout to re-power the grid.


Department for Business, Energy and Industrial Strategy (BEIS): In the UK, 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 (Load Factor): Ratio of annual energy production to maximum energy production if the turbine / wind farm ran at rated power all year; the ratio between the average generated power in a given period and the installed (rated) power.


Capacity: The rated continuous load-carrying ability of generation, transmission or other electrical equipment, expressed in megawatts (MW) for active power or megavolt-amperes (MVA) for apparent power.


Capacity Credit: a wind turbine can only produce when the wind blows and therefore is not directly comparable to a conventional power plant. The capacity credit is the percentage of conventional capacity that a given turbine can replace. A typical value of the capacity credit is 25 per cent (see capacity factor).


Cape Wind Farm: America's first offshore wind farm, located on Nantucket Sound. Once built, it will include 130 offshore wind turbines capable of producing enough electricity to provide 75 percent of the electricity necessary to power Cape Cod and the Islands of Martha’s Vineyard and Nantucket.


Certification: Generally prescriptive application of recognized Classification Rules and Regulations and or Codes and Standards by authorized ‘Certifying Authorities’, but acceptance of equivalents by risk-based justification may also be allowed by agreement with the Classification Society and or the Certifying Authority.


Classification: The development and worldwide implementation of a set of published Rules and Regulations which set and maintain standards of quality and reliability. A unit is ‘in class’ when the relevant Rules and Regulations have, in the opinion of the classification society, been complied with, or when it has been granted special dispensation from compliance.


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.


Column-Stabilized Unit: Unit with a working platform supported on widely spaced buoyant columns. The columns are normally attached to buoyant lower hulls or pontoons. These units are normally floating types but can be designed to rest on the sea bed.


Consent: Planning permission.


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.


D: The wind turbine rotor diameter (measured in meters).


Darrieus Rotor: A sleek vertical axis wind turbine developed by French inventor G. J. M. Darrieus in 1929 based on aerodynamic profiles.


dB(A): The human ear is more sensitive to sound in the frequency range 1 kHz to 4 kHz than to sound at very low or high frequencies. Therefore, sound meters are normally fitted with filters adapting the measured sound response to the human ear.


Decibel (dB) is a unit of measurement that is used to indicate the relative amplitude of a sound or the ratio of the signal level such as sound pressure. Sound levels in decibels are calculated on a logarithmic scale.


Diffuser: A downwind device that diffuses the wind stream through a rotor.


Direct Drive: A drive-train concept for wind turbines in which there is no gearbox and the wind turbine rotor is connected directly to a low-speed electrical generator.


Distributed Generation: Single or small clusters of wind turbines spread across the landscape, in contrast to the concentration of wind turbines in large arrays or wind power plants.


Doppler Shift Principle: When a source generating waves moves relative to an observer, or when an observer moves relative to a source, there is an apparent shift in frequency. If the distance between the observer and the source is increasing, the frequency apparently decreases, while the frequency apparently increases if the distance between the observer and the source is decreasing. This relationship is called the Doppler effect (or Doppler shift) after Austrian physicist Christian Johann Doppler (1803–1853).


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 minimizing the cost of power electronics. An electrical machine concept in which variable-speed operation is provided by using a relatively small power electronic converter to control currents in the rotor, such that the rotor does not necessarily rotate at the synchronous speed of the magnetic field set up in the stator.


Decommissioning Expenditure (DECEX): Spend on removal or making safe of offshore infrastructure at the end of its useful life, plus disposal of equipment.


Efficiency: For a turbine, it describes the amount of active electrical power generated as a percentage of the wind power incident on the rotor area.


Electricity Demand: The total electricity consumption in GWh consumed by a nation annually.


Energy Payback: The time period it takes for a wind turbine to generate as much energy as is required to produce the turbine in the first place, install it, maintain it throughout its lifetime and, finally, scrap it. Typically, this takes 2–3 months at a site with reasonable exposure.


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.


Equivalent Sound Level (dBLeq): It quantifies the environmental noise as a single value of sound level for any desired duration. The environmental sounds are usually described in terms of an average level that has the same acoustical energy as the summation of all the time-varying events.


ETSU: Energy Technology Support Unit of the UK Government.


EWEA: European Wind Energy Association.


Export Cable: Electrical cable that connects the onshore and offshore substations, or between an AC offshore substation and a DC converter substation.


External Costs: Those costs incurred in activities which may cause damage to a wide range of receptors, including human health, natural ecosystems and the built environment, and yet are not reflected in the price paid by consumers.


Fault ride-through (FRT): A requirement of many network operators, such that the wind turbine remains connected during severe disturbances on the electricity system, and returns to normal operation very quickly after the disturbance ends.