3. DEFINITIONS, ACRONYMS AND UNITS

Abbreviation / Acronym

Definition

Environmental conditions

Characteristics of the environment (wind,
waves, sea currents, water level, sea ice, marine growth, scour and overall
seabed movement, etc.) which may affect the device behaviour.

External conditions

Factors affecting operation of a device,
including environmental conditions, the electrical network conditions, and
other climatic factors (temperature, snow, ice, etc.)

hindcasting

Method of simulating historical
(metocean) data for a region through numerical modelling.

Hub
height (wind turbines)

Height of the centre of the swept area of
the wind turbine rotor above the mean sea level.

Offshore
wind turbine (OWT)

Wind turbine with a support structure
which is subject to hydrodynamic loading.

MES

Marine Energy Systems

MEC

Marine Energy Converter

WEC

Wave Energy Converter

TEC

Tidal Energy Converter

DLC

Design Load Case

MIS

Misaligned

MUL

Multi-directional

PDF

Probability Density Function

CDF

Cumulative Distribution Function

POT

The Peak Over Threshold method (POT)
is an extreme values analysis method that relies on extracting from a
continuous record the peak values reached for any period during which values
exceed a certain threshold.

IFORM

Inverse First Order Reliability Method (See
Annex G de IEC 61400-3)

N (subscript)

Recurrence period in years.

COD

Co-directional: Acting in the same
direction, for example wind and waves.

MUL

Multi-directional: Distribution of
directions, for example wind and waves.

UNI

Uni-directional: Acting in a single
direction, wind and/or waves.

Reference period

Period during which stationarity is
assumed and for a given stochastic process, for example wind speed, sea
elevation or response.

F

Fatigue

U

Ultimate strength

N

Normal

A

Abnormal

T (partial safety factor)

Transport and erection

*

Partial safety factor for fatigue.

T_R

Marine operation time length.

T_POP

Planned marine operation time.

T_C

Bias of
the marine operation time.

Abbreviation / Acronym

Definition

Wind profile –wind shear law

Mathematical expression for assumed wind
speed variation with height above still water level

NWP

Normal Wind Profile (see IEC 61400-1)

NTM

Normal Turbulence Model (see IEC 61400-1)

EWM

Extreme Wind speed Model (see IEC
61400-1)

EOG

Extreme Operating Gust (see IEC 61400-1)

ETM

Extreme Turbulence Model (see IEC
61400-1)

EDC

Extreme Direction Change (see IEC 61400-1)

EDC₁

Extreme Direction Change (see IEC
61400-3)

ECD

Extreme Coherent gust with Direction
change (see IEC 61400-1)

EWS

Extreme Wind Shear (see IEC 61400-1)

RWM

Reduced Wind speed Model (see IEC
61400-3)

RNA

Rotor-nacelle assembly: Part of an
offshore wind turbine carried by the support structure

V(z)

Mean wind speed at height z.

z

Height above the still water level.

V_hub

Mean wind speed at hub height.

V_in

Mean wind speed into a device.

V_out

Mean wind speed exiting a device.

V_r +/- 2 m/s

Sensitivity to all wind speeds in the
range shall be analysed.

k₁

Extreme Direction Change (see IEC
61400-3)

V_ref

Reference Wind Speed. Basic parameter for
wind speed used for defining wind turbine classes. A turbine designed for a
wind turbine class with a reference wind speed V_ref, is designed
to withstand climates for which the extreme 10 min average wind speed with a
recurrence period of 50 years at turbine hub height is lower than or equal to
V_ref. (See IEC 61400-1)

V_eN

Hub height extreme wind speeds with a
recurrence period of N years.

V_N

Expected extreme wind speed (averaged
over 10 min), with a recurrence period of N years.

V_redN

Reduced
extreme wind speed (averaged over three seconds), with a recurrence period of
N years.

Abbreviation/Acronym

Definition

Sea State

Condition of the sea in which its statistics
remind stationary.

Wave height

Vertical distance between the highest and
lowest points on the water surface of an individual zero up-crossing wave.

Wave period

Time interval between the two zero
up-crossings which bound a zero up-crossing wave.

Zero up-crossing wave

Portion of time history of wave elevation
between zero up-crossings. A zero up-crossing occurs when the sea surface
rises (rather than falls) through the still water level. 

Wave spectral peak frequency

Frequency of the peak energy in the wave
spectrum.

Wave spectrum

Frequency domain description of the sea
surface elevation in a sea state.

H

Individual wave height.

H_S

Significant wave height: Statistical
measure of the height of waves in a sea state, equal to the mean height of
the highest third of the zero-crossing waves.

H_N

Individual/extreme wave height with a
recurrence period of N years.

H_rated

Wave height at which the WEC produces
rate power output.

H_SN

Extreme significant wave height with a
recurrence period of N years.

H_s,in

Wave heights at which the WEC starts to
produce power.

H_s,out

Wave heights at which the WEC shuts down.

H_s,rated

Device rated extreme wave height.

H_s,T

Limiting value of H_s for
transport, erection and/or maintenance.

T

Wave period.

T_e

Energy period.

T_p

Peak spectral period.

Design wave

Deterministic wave with a defined height,
period and direction, used for the design of an offshore structure. A design
wave may be accompanied by a requirement for the use of a particular periodic
wave theory.

Mean zero crossing period

Average period of the zero-crossing (up
or down) waves in a sea state.

tsunami

Long period sea waves caused by rapid
vertical movements of the sea floor.

Wave direction

Mean direction from which the wave is
travelling.

NSS: Normal Sea State

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

NWH=H_NWH: Normal
Wave Height

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

SSS: Severe Sea State

For Offshore Wind design uses, see IEC
61400-3.

SWH: Severe Wave Height

For Offshore Wind design uses, see IEC
61400-3.

ESS: Extreme Sea State

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

EWH: Extreme Wave Height

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

RWH: Reduced Wave Height

For
Offshore Wind design uses, see IEC 61400-3.

Abbreviation / Acronym

Definition

current

Flow of water past a fixed location
usually described in terms of a current speed and direction.

Tidal current

Current resulting from tides.

Tides

Regular and predictable movements of the
sea generated by astronomical forces.

NCM: Normal Current Model

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

ECM: Extreme Current Model

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses, see
IEC TS 62600-2.

NTM: Normal Turbulence Model

For Marine Energy System design uses, see
IEC TS 62600-2.

ETM: Extreme Turbulence Model

For Marine Energy System design uses, see
IEC TS 62600-2.

Water level

The variation in water level at the
site shall be taken into account in the calculation of the hydrodynamic
loading of a device.

NWLR: Normal Water Level Range

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses,
see IEC TS 62600-2.

EWLR: Extreme Water Level Range

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses,
see IEC TS 62600-2.

MSL:
Mean Sea Level

For Offshore Wind design uses, see IEC
61400-3.

For Marine Energy System design uses,
see IEC TS 62600-2.

SWL

Still Water Level: Abstract water level
calculated by including the effects of tides and storm surge but excluding
the variations due to waves. SWL can be above, at, or below MSL. 

Storm
surge

Irregular movement of the sea brought
about by wind and atmospheric pressure variations.

HAT

Highest Astronomical Tide: Highest still
water level that can be expected to occur under any combination of
astronomical conditions and under average meteorological conditions. Storm
surges, which are meteorologically generated and essentially irregular, are
superimposed on the tidal variations, so that a total still water level above
highest astronomical tide may occur.

LAT

Lowest Astronomical Tide: Lowest still
water level that can be expected to occur under any combination of
astronomical conditions and under average meteorological conditions. Storm
surges, which are meteorologically generated and essentially irregular, are
superimposed on the tidal variations, so that a total still water level below
lowest astronomical tide may occur. 

Tidal
range

Distance between the highest astronomical
tide and the lowest astronomical tide.

U

Current velocity.

U_1, U_5, U₅₀

Current velocity with a recurrence period
of 1, 5 and 50 years respectively.

U_in

Current velocity into a device.

U_out

Current velocity exiting a device.

U_rated

Device rated current velocity.

U_T

Limiting
value of current velocity for transport, erection and / or maintenance.