EN 61400-3:2009

SLOVENSKI STANDARD
01-januar-2010
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Wind turbines -- Part 3: Design requirements for offshore wind turbines
Windenergieanlagen – Teil 3: Auslegungsanforderungen für Windenergieanlagen auf
offener See
Eoliennes -- Partie 3: Exigences de conception des éoliennes en pleine mer
Ta slovenski standard je istoveten z: EN 61400-3:2009
ICS:
27.180 Sistemi turbin na veter in Wind turbine systems and
drugi alternativni viri energije other alternative sources of
energy
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61400-3
NORME EUROPÉENNE
April 2009
EUROPÄISCHE NORM
ICS 27.180
English version
Wind turbines -
Part 3: Design requirements
for offshore wind turbines
(IEC 61400-3:2009)
Eoliennes -  Windenergieanlagen -
Partie 3: Exigences de conception Teil 3: Auslegungsanforderungen
des éoliennes en pleine mer für Windenergieanlagen auf offener See
(CEI 61400-3:2009) (IEC 61400-3:2009)

This European Standard was approved by CENELEC on 2009-04-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: avenue Marnix 17, B - 1000 Brussels

© 2009 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61400-3:2009 E
Foreword
The text of document 88/329/FDIS, future edition 1 of IEC 61400-3, prepared by IEC TC 88, Wind
turbines, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61400-3 on 2009-04-01.
This European Standard is to be read in conjunction with EN 61400-1:2005.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2010-01-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2012-04-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61400-3:2009 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60034 NOTE  Harmonized in EN 60034 series (partially modified).
IEC 60038 NOTE  Harmonized as HD 472 S1:1989 (modified), with the following title “Nominal voltages for
low-voltage public electricity supply systems”.
IEC 60146 NOTE  Harmonized in EN 60146 series (not modified).
IEC 60204-1 NOTE  Harmonized as EN 60204-1:2006 (modified).
IEC 60204-11 NOTE  Harmonized as EN 60204-11:2000 (not modified).
IEC 60227 NOTE  Is related to HD 21 series (not equivalent).
IEC 60245 NOTE  Is related to HD 22 series (not equivalent).
IEC 60269 NOTE  Harmonized in EN 60269 series (modified).
IEC 60364 NOTE  Harmonized in HD 384/HD 60364 series (modified).
IEC 60439 NOTE  Harmonized in EN 60439 series (partially modified).
IEC 60446 NOTE  Harmonized as EN 60446:1999 (not modified).
IEC 60529 NOTE  Harmonized as EN 60529:1991 (not modified).
IEC 61000-6-1 NOTE  Harmonized as EN 61000-6-1:2007 (not modified).
IEC 61000-6-2 NOTE  Harmonized as EN 61000-6-2:2005 (not modified).
IEC 61000-6-4 NOTE  Harmonized as EN 61000-6-4:2007 (not modified).
IEC 61310-1 NOTE  Harmonized as EN 61310-1:1995 (not modified).
IEC 61310-2 NOTE  Harmonized as EN 61310-2:1995 (not modified).
IEC 61400-21 NOTE  Harmonized as EN 61400-21:2002 (not modified).
__________
– 3 – EN 61400-3:2009
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60721-2-1 1982 Classification of environmental conditions -

+ A1 1987 Part 2-1: Environmental conditions HD 478.2.1 S1 1989
appearing in nature - Temperature and
humidity
IEC 61400-1 2005 Wind turbines - EN 61400-1 2005
Part 1: Design requirements
IEC 62305-3 2006 Protection against lightning - EN 62305-3 2006
(mod) Part 3: Physical damage to structures and + corr. September 2008
life hazard + A11 2009
IEC 62305-4 2006 Protection against lightning - EN 62305-4 2006
Part 4: Electrical and electronic systems + corr. November 2006
within structures
ISO 2394 1998 General principles on reliability for - -
structures
ISO 2533 1975 Standard atmosphere - -

ISO 9001 2000 Quality management systems - EN ISO 9001 2000
Requirements
ISO 19900 2002 Petroleum and natural gas industries - EN ISO 19900 2002
General requirements for offshore structures

ISO 19901-1 2005 Petroleum and natural gas industries - EN ISO 19901-1 2005
Specific requirements for offshore
structures -
Part 1: Metocean design and operating
considerations
ISO 19901-4 2003 Petroleum and natural gas industries - EN ISO 19901-4 2003
Specific requirements for offshore
structures -
Part 4: Geotechnical and foundation design
considerations
1) 2)
ISO 19902 - Petroleum and natural gas industries - Fixed EN ISO 19902 2007
steel offshore structures
ISO 19903 2006 Petroleum and natural gas industries - Fixed EN ISO 19903 2006
concrete offshore structures
1)
Undated reference.
2)
Valid edition at date of issue.

IEC 61400-3 ®
Edition 1.0 2009-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Wind turbines –
Part 3: Design requirements for offshore wind turbines

Eoliennes –
Partie 3: Exigences de conception des éoliennes en pleine mer

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XF
CODE PRIX
ICS 27.180 ISBN 2-8318-1025-2
– 2 – 61400-3 © IEC:2009
CONTENTS
FOREWORD.5
INTRODUCTION.7
1 Scope.8
2 Normative references.8
3 Terms and definitions .9
4 Symbols and abbreviated terms .15
4.1 Symbols and units .15
4.2 Abbreviations.16
5 Principal elements .17
5.1 General .17
5.2 Design methods.17
5.3 Safety classes .19
5.4 Quality assurance.19
5.5 Rotor – nacelle assembly markings.20
6 External conditions .20
6.1 General .20
6.2 Wind turbine classes .21
6.3 Wind conditions .21
6.4 Marine conditions .22
6.5 Other environmental conditions.31
6.6 Electrical power network conditions.32
7 Structural design.33
7.1 General .33
7.2 Design methodology .33
7.3 Loads .33
7.4 Design situations and load cases .34
7.5 Load and load effect calculations .51
7.6 Ultimate limit state analysis.54
8 Control and protection system.57
9 Mechanical systems.57
10 Electrical system.58
11 Foundation design .58
12 Assessment of the external conditions at an offshore wind turbine site .59
12.1 General .59
12.2 The metocean database.59
12.3 Assessment of wind conditions.60
12.4 Assessment of waves .62
12.5 Assessment of currents.63
12.6 Assessment of water level, tides and storm surges.63
12.7 Assessment of sea ice .63
12.8 Assessment of marine growth .64
12.9 Assessment of seabed movement and scour.64
12.10 Assessment of wake effects from neighbouring wind turbines.65
12.11 Assessment of other environmental conditions .65

61400-3 © IEC:2009 − 3 −
12.12 Assessment of earthquake conditions .65
12.13 Assessment of weather windows and weather downtime.65
12.14 Assessment of electrical network conditions.65
12.15 Assessment of soil conditions .66
13 Assembly, installation and erection .67
13.1 General .67
13.2 Planning .68
13.3 Installation conditions.68
13.4 Site access .68
13.5 Environmental conditions .68
13.6 Documentation.69
13.7 Receiving, handling and storage.69
13.8 Foundation/anchor systems.69
13.9 Assembly of offshore wind turbine.69
13.10 Erection of offshore wind turbine .69
13.11 Fasteners and attachments .69
13.12 Cranes, hoists and lifting equipment.70
14 Commissioning, operation and maintenance .70
14.1 General .70
14.2 Design requirements for safe operation, inspection and maintenance .70
14.3 Instructions concerning commissioning .71
14.4 Operator’s instruction manual .72
14.5 Maintenance manual.74
Annex A (informative) Key design parameters for an offshore wind turbine.76
Annex B (informative) Wave spectrum formulations.79
Annex C (informative) Shallow water hydrodynamics and breaking waves .84
Annex D (informative) Guidance on calculation of hydrodynamic loads.92
Annex E (informative) Recommendations for design of offshore wind turbine support
structures with respect to ice loads. 105
Annex F (informative) Offshore wind turbine foundation design. 116
Annex G (informative) Statistical extrapolation of operational metocean parameters for
ultimate strength analysis. 117
Annex H (informative) Corrosion protection . 123
Bibliography . 127

Figure 1 – Parts of an offshore wind turbine.10
Figure 2 – Design process for an offshore wind turbine.19
Figure 3 – Definition of water levels.29
Figure 4 – The two approaches to calculate the design load effect.55
Figure B.1 – PM spectrum .80
Figure B.2 – Jonswap and PM spectrums for typical North Sea storm sea state .81
Figure C.1 – Regular wave theory selection diagram.84
Figure D.1 – Breaking wave and cylinder parameters.96
Figure D.2 – Oblique inflow parameters .96
Figure D.3 – Distribution over height of the maximum impact line force (γ = 0°) .98

– 4 – 61400-3 © IEC:2009
Figure D.4 – Response of model and full-scale cylinder in-line and cross-flow (from
reference document 4) .100
Figure E.1 – Ice force coefficients for plastic limit analysis (from reference document 6) . 110
Figure E.2 – Serrated load profile (T = 1/f or 1/f ) . 113
0,1 N b
Figure G.1 – Example of the construction of the 50-year environmental contour for a 3-hour
sea state duration. . 118

Table 1 – Design load cases .36
Table 2 – Design load cases for sea ice .50
Table 3 – Partial safety factors for loads γ .56
f
Table 4 – Conversion between extreme wind speeds of different averaging periods .61
Table C.1 – Constants h and h and normalised wave heights h as a function of H .87
1 2 x% tr
Table C.2 – Breaking wave type .90

61400-3 © IEC:2009 − 5 −
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINES –
Part 3: Design requirements for offshore wind turbines

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61400-3 has been prepared by IEC technical committee 88: Wind
turbines.
This part is to be read in conjunction with IEC 61400-1:2005, Wind turbines – Part 1: Design
requirements.
The text of this standard is based on the following documents:
FDIS Report on voting
88/329/FDIS 88/338/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

– 6 – 61400-3 © IEC:2009
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of IEC 61400 series, published under the general title Wind turbines, can be
found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
61400-3 © IEC:2009 − 7 −
INTRODUCTION
This part of IEC 61400 outlines minimum design requirements for offshore wind turbines and is
not intended for use as a complete design specification or instruction manual.
Several different parties may be responsible for undertaking the various elements of the
design, manufacture, assembly, installation, erection, commissioning, operation and
maintenance of an offshore wind turbine and for ensuring that the requirements of this
standard are met. The division of responsibility between these parties is a contractual matter
and is outside the scope of this standard.
Any of the requirements of this standard may be altered if it can be suitably demonstrated that
the safety of the system is not compromised. Compliance with this standard does not relieve
any person, organization, or corporation from the responsibility of observing other applicable
regulations.
– 8 – 61400-3 © IEC:2009
WIND TURBINES –
Part 3: Design requirements for offshore wind turbines

1 Scope
This part of IEC 61400 specifies additional requirements for assessment of the external
conditions at an offshore wind turbine site and it specifies essential design requirements to
ensure the engineering integrity of offshore wind turbines. Its purpose is to provide an
appropriate level of protection against damage from all hazards during the planned lifetime.
This standard focuses on the engineering integrity of the structural components of an offshore
wind turbine but is also concerned with subsystems such as control and protection
mechanisms, internal electrical systems and mechanical systems.
A wind turbine shall be considered as an offshore wind turbine if the support structure is
subject to hydrodynamic loading. The design requirements specified in this standard are not
necessarily sufficient to ensure the engineering integrity of floating offshore wind turbines.
This standard should be used together with the appropriate IEC and ISO standards mentioned
in Clause 2. In particular, this standard is fully consistent with the requirements of IEC 61400-
1. The safety level of the offshore wind turbine designed according to this standard shall be at
or exceed the level inherent in IEC 61400-1. In some clauses, where a comprehensive
statement of requirements aids clarity, replication of text from IEC 61400-1 is included.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60721-2-1:1982, Classification of environmental conditions – Part 2-1: Environmental
conditions appearing in nature. Temperature and humidity
Amendment 1:1987
IEC 61400-1:2005, Wind turbines – Part 1: Design requirements
IEC 62305-3:2006, Protection against lightning – Part 3: Physical damage to structures and life
hazard
IEC 62305-4:2006, Protection against lightning – Part 4: Electrical and electronic systems
within structures
ISO 2394:1998, General principles on reliability for structures
ISO 2533:1975, Standard Atmosphere
ISO 9001:2000, Quality management systems – Requirements
ISO 19900:2002, Petroleum and natural gas industries – General requirements for offshore
structures
61400-3 © IEC:2009 − 9 −
ISO 19901-1:2005, Petroleum and natural gas industries – Specific requirements for offshore
structures – Part 1: Metocean design and operating conditions
ISO 19901-4:2003, Petroleum and natural gas industries – Specific requirements for offshore
structures – Part 4: Geotechnical and foundation design considerations
ISO 19902, Petroleum and natural gas industries – Fixed steel offshore structures
ISO 19903: 2006, Petroleum and natural gas industries – Fixed concrete offshore structures
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply in addition to
those stated in IEC 61400-1.
3.1
co-directional (wind and waves)
acting in the same direction
3.2
current
flow of water past a fixed location usually described in terms of a current speed and direction
3.3
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
3.4
designer
party or parties responsible for the design of an offshore wind turbine
3.5
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 wind turbine behaviour
3.6
external conditions (wind turbines)
factors affecting operation of an offshore wind turbine, including the environmental conditions,
the electrical network conditions, and other climatic factors (temperature, snow, ice, etc.)
3.7
extreme significant wave height
expected value of the highest significant wave height, averaged over 3 h, with an annual
probability of exceedance of 1/N (“recurrence period”: N years)
3.8
extreme wave height
expected value of the highest individual wave height (generally the zero up-crossing wave
height) with an annual probability of exceedance of 1/N (“recurrence period”: N years)

– 10 – 61400-3 © IEC:2009
3.9
fast ice cover
rigid continuous cover of ice not in motion
3.10
fetch
distance over which the wind blows constantly over the sea with approximately constant wind
speed and direction
3.11
foundation
part of an offshore wind turbine support structure which transfers the loads acting on the
structure into the seabed. Different foundation concepts are shown in Figure 1 together with
the other parts of an offshore wind turbine
Rotor-nacelle assembly
Tower
Tower
Support
Platform
structure
Water level
Sub-structure
Sub-structure
Pile
Sea floor
Pile
Seabed
Foundation
IEC  001/09
Figure 1 – Parts of an offshore wind turbine
3.12
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

61400-3 © IEC:2009 − 11 −
3.13
hindcasting
method of simulating historical (metocean) data for a region through numerical modelling
3.14
hub height (wind turbines)
height of the centre of the swept area of the wind turbine rotor above the mean sea level
3.15
hummocked ice
crushed ice and ice floes piled up into ridges when large ice floes meet with each other or with
a rigid obstacle, for example an offshore wind turbine support structure
3.16
ice floe
sheet of ice in size from metres to several kilometres, not rigidly frozen to a shore, still or in
motion
3.17
icing
build-up of a cover of ice or frost on parts of an offshore wind turbine that can result in added
loads and/or changed properties
3.18
land-locked waters
waters almost or entirely surrounded by land
3.19
load effect
effect of a single load or combination of loads on a structural component or system, for
example internal force, stress, strain, motion, etc.
3.20
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
3.21
manufacturer
party or parties responsible for the manufacture and construction of an offshore wind turbine
3.22
marine conditions
characteristics of the marine environment (waves, sea currents, water level, sea ice, marine
growth, seabed movement and scour, etc.) which may affect the wind turbine behaviour
3.23
marine growth
surface coating on structural components caused by plants, animals and bacteria
3.24
mean sea level
average level of the sea over a period of time long enough to remove variations due to waves,
tides and storm surges
– 12 – 61400-3 © IEC:2009
3.25
mean zero crossing period
average period of the zero-crossing (up or down) waves in a sea state
3.26
metocean
abbreviation of meteorological and oceanographic
3.27
multi-directional (wind and/or wave)
distribution of directions
3.28
offshore wind turbine
wind turbine with a support structure which is subject to hydrodynamic loading
3.29
offshore wind turbine site
the location or intended location of an individual offshore wind turbine either alone or within a
wind farm
3.30
pile penetration
vertical distance from the sea floor to the bottom of the pile
3.31
power collection system (wind turbines)
electric system that collects the power from one or more wind turbines. It includes all electrical
equipment connected between the wind turbine terminals and the network connection point.
For offshore wind farms, the power collection system may include the connection to shore
3.32
reference period
period during which stationarity is assumed for a given stochastic process, for example wind
speed, sea elevation or response
3.33
refraction
process by which wave energy is redistributed as a result of changes in the wave propagation
velocity due to variations in water depth and/or current velocity
3.34
residual currents
components of a current other than tidal current. The most important is often the storm surge
current
3.35
rotor – nacelle assembly
part of an offshore wind turbine carried by the support structure, refer to Figure 1
3.36
sea floor
interface between the sea and the seabed

61400-3 © IEC:2009 − 13 −
3.37
sea floor slope
local gradient of the sea floor, for example associated with a beach
3.38
sea state
condition of the sea in which its statistics remain stationary
3.39
seabed
materials below the sea floor in which a support structure is founded
3.40
seabed movement
movement of the seabed due to natural geological processes
3.41
scour
removal of seabed soils by currents and waves or caused by structural elements interrupting
the natural flow regime above the sea floor
3.42
significant wave height
statistical measure of the height of waves in a sea state, defined as 4 × σ where σ is the
η η
standard deviation of the sea surface elevation. In sea states with only a narrow band of wave
frequencies, the significant wave height is approximately equal to the mean height of the
highest third of the zero up-crossing waves
3.43
splash zone
external region of support structure that is frequently wetted due to waves and tidal variations.
This shall be defined as the zone between
– the highest still water level with a recurrence period of 1 year increased by the crest height
of a wave with height equal to the significant wave height with a return period of 1 year, and
– the lowest still water level with a recurrence period of 1 year reduced by the trough depth of
a wave with height equal to the significant wave height with a return period of 1 year
3.44
still water level
abstract water level calculated by including the effects of tides and storm surge but excluding
variations due to waves. Still water level can be above, at, or below mean sea level
3.45
storm surge
irregular movement of the sea brought about by wind and atmospheric pressure variations
3.46
sub-structure
part of an offshore wind turbine support structure which extends upwards from the seabed and
connects the foundation to the tower, refer to Figure 1
3.47
support structure
part of an offshore wind turbine consisting of the tower, sub-structure and foundation, refer to
Figure 1
– 14 – 61400-3 © IEC:2009
3.48
swell
sea state in which waves generated by winds remote from the site have travelled to the site,
rather than being locally generated
3.49
tidal current
current resulting from tides
3.50
tidal range
distance between the highest astronomical tide and the lowest astronomical tide
3.51
tides
regular and predictable movements of the sea generated by astronomical forces
3.52
tower
part of an offshore wind turbine support structure which connects the sub-structure to the rotor
– nacelle assembly, refer to Figure 1
3.53
tsunami
long period sea waves caused by rapid vertical movements of the sea floor
3.54
uni-directional (wind and/or waves)
acting in a single direction
3.55
water depth
vertical distance between the sea floor and the still water level
NOTE As there are several options for the still water level (see 3.44) there can be several water depth values.
3.56
wave crest elevation
vertical distance between the crest of a wave and the still water level
3.57
wave direction
mean direction from which the wave is travelling
3.58
wave height
vertical distance between the highest and lowest points on the water surface of an individual
zero up-crossing wave
3.59
wave period
time interval between the two zero up-crossings which bound a zero up-crossing wave
3.60
wave spectral peak frequency
frequency of the peak energy in the wave spectrum

61400-3 © IEC:2009 − 15 −
3.61
wave spectrum
frequency domain description of the sea surface elevation in a sea state
3.62
wave steepness
ratio of the wave height to the wave length
3.63
weather downtime
one or more intervals of time during which the environmental conditions are too severe to allow
for execution of a specified marine operation
3.64
weather window
interval of time during which the environmental conditions allow for execution of a specified
marine operation
3.65
wind profile – wind shear law
mathematical expression for assumed wind speed variation with height above still water level
NOTE Commonly used profiles are the logarithmic profile (equation 1) and the power law profile (equation 2).
In()z z
V()z = V(z )⋅ (1)
r
In()z z
r 0
α
⎛ ⎞
z
⎜ ⎟
V()z = V(z ) ⋅ (2)
r
⎜ ⎟
z
⎝ r⎠
where
V(z) is the wind speed at height z;
z is the height above the still water level;
z is a reference height above the still water level used for fitting the profile;
r
z is the roughness length;
α is the wind shear (or power law) exponent.
3.66
zero up-crossing wave
portion of a 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
4 Symbols and abbreviated terms
For the purposes of this document, the following symbols and abbreviated terms apply in
addition to those stated in IEC 61400-1:
4.1 Symbols and units
A Charnock’s constant [-]
C
d water depth [m]
–1
f wave spectral peak frequency [s ]
p
g acceleration due to gravity [m/s ]

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h thickness of sea ice [m]
h thickness of sea ice with a recurrence period of N years [m]
N
h ice thickness equal to the long term mean value of the annual
m
maximum ice thickness for winters with ice [m]
H individual wave height [m]
H breaking wave height [m]
B
H design wave height [m]
D
H individual wave height with a recurrence period of N years [m]
N
H significant wave height [m]
s
H significant wave height with a recurrence period of N years [m]
sN
H reduced wave height with a recurrence period of N years [m]
redN
k wave number [-]
K accumulated freezing degree-days [°C]
max
s sea floor slope [°]
p(V ) probability density function of hub height wind speed [-]
hub
S single sided wave spectrum [m /Hz]
η
R design value for component resistance [-]
d
R characteristic value for component resistance [-]
k
S design value for load effect [-]
d
S characteristic value for load effect [-]
k
t time [s]
T wave period [s]
T design wave period [s]
D
T peak spectral period [s]
p
T mean zero-crossing wave period [s]
z
U sub surface current velocity [m/s]
ss
U wind generated current velocity [m/s]
w
U breaking wave induced surf current velocity [m/s]
bw
V  expected extreme wind speed (averaged over 10 min), with a
N
recurrence period of N years [m/s]
V reduced extreme wind speed (averaged over three seconds), with a
redN
recurrence period of N years [m/s]
η sea surface elevation relative to SWL [m]
κ von Karman’s constant [-]
λ wave length [m]
θ wave direction [°]
w
θ mean wave direction [°]
wm
θ  current direction [°]
c
σ sea surface elevation standard deviation [m]
η
τ temperature [°C]
4.2 Abbreviations
COD co-directional
61400-3 © IEC:2009 − 17 −
CPT cone penetration test
DLC design load case
ECD extreme coherent gust with direction change
ECM extreme current model
EDC extreme direction change
EOG extreme operating gust
ESS extreme sea state
EWH extreme wave height
EWLR extreme water level range
EWM extreme wind speed model
EWS extreme wind shear
HAT highest astronomical tide
LAT lowest astronomical tide
MIC microbiologically influenced corrosion
MIS misaligned
MSL mean sea level
MUL multi-directional
NCM normal current model
NSS normal sea state
NTM normal turbulence model
NWH normal wave height
NWLR normal water level range
NWP normal wind profile model
RNA rotor – nacelle assembly
RWH reduced wave height
RWM reduced wind speed model
SSS severe sea state
SWH severe wave height
SWL still water level
UNI uni-directional
5 Principal elements
5.1 General
The engineering and technical requirements to ensure the safety of the structural, mechanical,
electrical and control systems of an offshore wind turbine are given in the following clauses.
This specification of requirements applies to the design, manufacture, installation and manuals
for operation and maintenance of an offshore wind turbine and the associated quality
management process. In addition, safety procedures, which have been established in the
various practices that are used in the installation, operation and maintenance of an offshore
wind turbine, are taken into account.
5.2 Design methods
This standard requires the use of a structural dynamics model to predict design load effects.
Such a model shall be used to determine the load effects for all relevant combinations of

– 18 – 61400-3 © IEC:2009
external conditions and design situations as defined in Clause 6 and Clause 7 respectively. A
minimum set of such combinations has been defined as load cases in this standard.
The design of the support structure of an offshore wind turbine shall be based on site-specific
external conditions. These shall therefore be determined in accordance with the requirements
stated in Clause 12. The conditions shall be summarized in the design basis.
In the case of the rotor – nacelle assembly, which may have been designed initially on the
basis of a standard wind turbine class as defined in IEC 61400-1, 6.2, it shall be demonstrated
that the offshore site-specific exter
...