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.


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.


FINO 1: An offshore research platform in the North Sea, off Germany.


Flag State: The administration with which the unit is registered (flagged).


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.


Floating Unit: A hull structure and its integral marine systems together with propulsion system (where fitted) and essential machinery.


Foundation (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.


Foundation (Monopile Foundation): A type of foundation with a cylindrical tube (normally steel) that is normally driven tens of meters into the sea bed, although it can also be inserted into pre-drilled holes.


Foundation (Non-monopile Steel Foundation): also called Jacket Foundation): Collective term used to describe all steel foundations other than monopiles. Includes braced, welded, space-frame structures (collectively called ‘jackets’), tripods and tripiles.


Foundation (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.


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.


Full LLoad Hours: The turbine’s average annual production divided by its rated power. The higher the number of full load hours, the higher the tubine’s production at the chosen site.


Furling: A passive overspeed control mechanism which functions by reducing the projected swept area, by turning the rotor out of the incident wind direction.


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.


Giromill (or cycloturbine): A vertical axis H configuration wind turbine with articulating straight blades.


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.


Grid-connected: A wind turbine is grid-connected when its output is channelled directly into a national grid (see also stand-alone system).


Grid Reinforcement: A weak grid can be reinforced by up-rating its connection to the rest of the grid. The cost of doing this may fall to the wind farm developer.


High Voltage (HV): typically 100 to 150 kV.


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 Axis Wind Turbine (HAWT): a wind turbine whose rotor axis is substantially parallel to the wind flow.


Horizontal Directional Drilling (HDD): Horizontal directional drilling is a low impact (trenchless) method of installing underground cables using a surface-launched drilling rig.


Hub: the rotating component of the wind turbine to which the rotor blades are fixed.


Hub Height: The height of the rotor axis above the ground.


Hybrid Power Systems (HPS) are combinations of renewable technologies (such as wind turbines or solar photovoltaics) and conventional technologies (such as diesel generators) that are used to provide power to remote areas.


IEC - International Electrotechnical Commission.


Independent Power Producer (IPP): a privately owned and operated electricity production company not associated with national utility firms.


Installed Capacity: The total MW of operational generation plant of a given technology.

Institutional capacity building refers to the process of creating more effective institutions through the increase of shared knowledge resources, relational resources and the capacity for mobilisation. It is usually related to the capacity to facilitate open policy- and decision-making processes (at national and local levels) that provide access to relevant stakeholders and room for various types of knowledge resources.


Interconnected system: two or more individual electric systems that normally operate synchronously and are physically connected via tie-lines (see also: synchronous area).


Interconnection: A transmission link (such as a tieline or transformer), which connects two control areas.


Intermedial Load: It refers to those electricity-generation technologies contributing to satisfy the demand in a range between the base load and peak load of the electricity system. A generating unit that normally operates at a constant output (amount of electricity delivered) takes all or part of the base load of a system. In contrast, a peak load unit is only used to reach specific peak periods of a few hours when the demand is high.


Internationally Recognized Standards: Technical Codes, specifications, recommended practices, etc., issued by competent authorities and recognized by the Regulatory Authorities.


ITC – Investment Tax Credit


Jacket Foundation: See Non-monopile steel foundation.


Kilohertz (kHz): A unit of measurement of frequency. It is a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one thousand hertz (1000 Hz).


Kinetic Energy: The energy of motion that powers wind turbines.


Learning Rate: A learning curve parameter. It is estimated on available data for wind turbines and it indicates the achieved reduction in specific production costs.


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. In other words, the present-day average cost per kWh produced by the turbine over its entire lifetime, including all costs – (re-)investments, operation and maintenance. Levelised costs are calculated using the discount rate and the turbine lifetime.


Liftboat: Unit with a buoyant hull (generally either triangular or pontoon shaped) with moveable legs capable of raising the hull above the surface of the sea and designed to operate as a sea bed-stabilized unit in an elevated mode. The legs may be designed to penetrate the sea bed, or be attached to a mat or individual footings which rest on the sea bed. In general, installation and maintenance activities would be undertaken in the jacked-up condition. Generally self propelled.


Load: An end-use device or customer that receives power from the electricity system. Load should not be confused with demand, which is the measure of power that a load receives or requires.


Low Voltage (LV): below 1000 V.


Low-voltage Ride-Through (LVRT): see fault ride-through.


MARI-PARS – Massachusetts and Rhode Island Port Access Route Study


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.


Medium Voltage (MV): typically 10 to 35 kV.


Megawatt (MW) and Megawatt hour (MWh): Unit of power and unit of energy.


Met Mast: A mast or tower which carries meteorological instrumentation (typically wind speed transducers at several heights and wind direction, air temperature and pressure transducers).


Microvolts/cm (μVcm−1) : A unit of measurement of electrical fields.


Millitesla (mT) : A unit of measurement of static magnetic fields.


Minigrid: A distribution network usually operating only at low voltage and providing electricity supply to a community. It is supplied by one or more diesel generators, wind turbines, mini-hydro generators or solar photovoltaics.


Monopile Foundation: A type of foundation with a cylindrical tube (normally steel) that is normally driven tens of meters 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. Also called Jacket Foundation.


NAICs Codes – North American Industry Classification System codes


N-1 Criterion: A rule that requires elements remaining in operation after the failure of a single network element (such as a transmission line/transformer or generating unit, or in certain instances a busbar) to be capable of accommodating the change of flows in the network caused by that single failure.


(N-1)-safety: Any single element in the power system may fail without causing a succession of other failures, leading to a total system collapse. Together with avoiding constant overloading of grid elements, (N-1)-safety is a main concern for the grid operator.


National Authority: The specified controlling coastal state administration in whose territorial waters the unit will operate. These may include both marine and industrial administrations.


Net Metering: A policy implemented by some states and utilities to ensure that any extra electricity produced by an on-site generator, such a small wind system, can be sent back into the utility system, and where the owner is charged for energy on the basis of the net import.


Net Transfer Capacity: The maximum value of generation that can be wheeled through the interface between the two systems without leading to network constraints in either system, taking into account technical uncertainties about future network conditions.


Non-Destructive Examination (NDE): Visual examination and Non-Destructive Testing (NDT).


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.


OSW: Offshore Wind


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.


Operations, Maintenance and Service (OMS): for either wind farm OMS or onshore transmission OMS, consisting of:


* 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 programs 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.


Network Power Frequency: It defines the sensitivity, given in megawatts per hertz (MW/Hz), usually associated with a (single) control area/block or the entire synchronous area, which relates the difference between scheduled and actual system frequency to the amount of generation required to correct the power imbalance for that control area/block (or, vice versa, the stationary change of the system frequency in the case of a disturbance of the generation/load equilibrium in the control area without being connected to other control areas). It should not be confused with the K-factor (K). The network power frequency characteristic includes all active primary control and self-regulation of load and changes, due to modifications in the generation pattern and demand.


NIMBY: Acronym for ‘Not In My Back Yard’ and refers to an explanation of the local rejection to technological projects. Recent research is questioning the traditional explanation of local rejection to technological projects based on the NIMBY concept, as this may be giving an incorrect or partial explanation of all the variables at stake.


Numerical Weather Prediction (NWP): weather forecasting by computational simulation of the atmosphere.


NYSERDA – New York State Energy Research and Development Authority


Offshore Facility: Any facility located in the inland navigable waters of the United States, including the Great Lakes, or in the coastal waters of the United States, including the territorial seas of the United States, the exclusive economic zone of the United States, and the outer Continental Shelf of the United States.


Offshore Wind Developments: Wind projects installed in shallow waters off the coast, in a marine environment.


Onshore Wind Developments: Wind farms installed on land.


OREC - Offshore Renewable Energy Credit


P3 – Public-Private Partnership


Photovoltaic Generation (PV): The generation of electricity from sunlight or ambient light, using the photovoltaic effect.


Point of Common Coupling (PCC): The point on the public electricity network at which other customers are, or could be, connected. Not necessarily the same location as point of connection.


Point of Connection (POC): The point at which the wind farm electrical system is connected to the public electricity system.


Power Curve: It depicts the relationship between net electric output of a wind turbine and the wind speed measured at hub height on a 10-minute average basis.


PPA – Power Purchase Agreement


Qualified Offshore Wind Property: An offshore facility (see) using wind to produce electricity.


Rated Wind Speed: The lowest steady wind speed at which a wind turbine can produce its rated output power.


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.


Reynolds Number: A dimensionless number describing the aerodynamic state of an operating aerofoil. The number is used along with the angle of attack to describe the limits of a particular aerofoil’s lift-to-drag ratio and the conditions at which stall occurs. Small wind turbine aerofoils typically operate in a low Reynolds number range, from 0.150 to 0.5 million.


RTO – Regional Transmission Operator


Rural Electrification: Regular supply of electricity to rural residents. It implies the extension of power lines to rural areas, or the use of stand-alone or isolated power systems.


Sea Bed-stabilized: A unit that is designed to operate under normal operating and survival conditions while the footings, mat or pontoons rest on the sea bed.


Self-elevating (or Jack-up) Unit: Floating unit which is designed to operate as a sea bed-stabilized unit in an elevated mode. These units have a buoyant hull (generally either triangular or pontoon shaped) with moveable legs capable of raising the hull above the surface of the sea. The legs may be designed to penetrate the sea bed, or be attached to a mat or individual footings which rest on the sea bed. Generally not fitted with a propulsion system.

Self-propelled: The unit is designed for unassisted sea passages and is fitted with propelling machinery in accordance with the (Class’) Rules.


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