IEC 61400-3-2:2025

IEC 61400-3-2 ®
Edition 1.0 2025-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Wind energy generation systems –
Part 3-2: Design requirements for floating offshore wind turbines

Systèmes de génération d’énergie éolienne –
Partie 3-2: Exigences de conception des éoliennes en mer flottantes
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite ni
utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et
les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

IEC Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC Products & Services Portal - products.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews, graphical symbols and the glossary.
committee, …). It also gives information on projects, replaced With a subscription you will always have access to up to date
and withdrawn publications. content tailored to your needs.

IEC Just Published - webstore.iec.ch/justpublished
Electropedia - www.electropedia.org
Stay up to date on all new IEC publications. Just Published
The world's leading online dictionary on electrotechnology,
details all new publications released. Available online and once
containing more than 22 500 terminological entries in English
a month by email.
and French, with equivalent terms in 25 additional languages.

Also known as the International Electrotechnical Vocabulary
IEC Customer Service Centre - webstore.iec.ch/csc
(IEV) online.
If you wish to give us your feedback on this publication or need

further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Recherche de publications IEC -  IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications, symboles
en utilisant différents critères (numéro de référence, texte, graphiques et le glossaire. Avec un abonnement, vous aurez
comité d’études, …). Elle donne aussi des informations sur les toujours accès à un contenu à jour adapté à vos besoins.
projets et les publications remplacées ou retirées.

Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au monde,
Restez informé sur les nouvelles publications IEC. Just
avec plus de 22 500 articles terminologiques en anglais et en
Published détaille les nouvelles publications parues.
français, ainsi que les termes équivalents dans 25 langues
Disponible en ligne et une fois par mois par email.
additionnelles. Egalement appelé Vocabulaire

Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc

Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 61400-3-2 ®
Edition 1.0 2025-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Wind energy generation systems –

Part 3-2: Design requirements for floating offshore wind turbines

Systèmes de génération d’énergie éolienne –

Partie 3-2: Exigences de conception des éoliennes en mer flottantes

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180  ISBN 978-2-8322-9825-1

– 2 – IEC 61400-3-2:2025 © IEC 2025
CONTENTS
FOREWORD . 8
INTRODUCTION . 11
1 Scope . 12
2 Normative references . 13
3 Terms and definitions . 14
4 Symbols, units and abbreviated terms . 26
4.1 General . 26
4.2 Symbols and units. 26
4.3 Abbreviated terms . 27
5 Principal elements . 28
5.1 General . 28
5.2 Design methods . 28
5.3 Safety level for FOWT . 30
5.4 Safety classes for RNA and tower . 30
5.5 Quality assurance . 30
5.6 Rotor–nacelle assembly markings . 30
5.7 Support structure markings . 31
6 External conditions – definition and assessment . 31
6.1 General . 31
6.2 Wind turbine classes . 31
6.3 Definition of external conditions at a FOWT site . 32
6.3.1 General . 32
6.3.2 Wind conditions . 32
6.3.3 Marine conditions . 33
6.3.4 Electrical power network conditions . 40
6.3.5 Other environmental conditions . 40
6.4 Assessment of external conditions at a FOWT site . 41
6.4.1 General . 41
6.4.2 The metocean database . 41
6.4.3 Assessment of wind conditions . 42
6.4.4 Assessment of marine conditions . 44
6.4.5 Assessment of other environmental conditions . 48
6.4.6 Assessment of electrical network conditions . 49
6.4.7 Assessment of soil conditions . 49
7 Structural design . 50
7.1 General . 50
7.2 Design methodology . 51
7.3 Loads. 51
7.3.1 General . 51
7.3.2 Gravitational and inertial loads . 51
7.3.3 Aerodynamic loads . 51
7.3.4 Actuation loads . 51
7.3.5 Hydrodynamic loads . 52
7.3.6 Sea/lake ice loads . 52
7.3.7 Other loads . 52
7.4 Design situations and load cases . 53

7.4.1 General . 53
7.4.2 Power production (DLC 1.1 to 1.6) . 63
7.4.3 Power production plus occurrence of fault or loss of electrical network
connection (DLC 2.1 – 2.6) . 64
7.4.4 Start up (DLC 3.1 to 3.3). 66
7.4.5 Normal shutdown (DLC 4.1 to 4.3) . 67
7.4.6 Emergency stop (DLC 5.1) . 68
7.4.7 Parked (standstill or idling) (DLC 6.1 to 6.5) . 68
7.4.8 Parked plus fault conditions (DLC 7.1 and 7.2) . 69
7.4.9 Transport, assembly, maintenance and repair (DLC 8.1 to 8.4) . 70
7.4.10 Redundancy check and damage stability (DLC F1.1 to F2.3) . 74
7.5 Load and load effect calculations . 75
7.5.1 General . 75
7.5.2 Relevance of hydrodynamic loads . 75
7.5.3 Calculation of hydrodynamic loads . 76
7.5.4 Calculation of sea/lake ice loads . 77
7.5.5 Overall damping assessment for support structure response evaluations . 77
7.5.6 Simulation requirements . 78
7.5.7 Other requirements . 82
7.6 Limit state analysis . 83
7.6.1 Method . 83
7.6.2 Ultimate strength analysis . 86
7.6.3 Fatigue analysis . 87
7.6.4 Serviceability analysis . 88
8 Control system . 89
9 Mechanical systems . 90
10 Electrical system . 91
11 Anchor design . 91
12 Assembly, transport and installation . 91
12.1 General . 91
12.2 Planning . 92
12.3 Environmental conditions . 92
12.4 Documentation . 92
12.5 Transport, receiving, handling and storage . 93
13 Commissioning, operation and maintenance . 93
13.1 General . 93
13.2 Design requirements for safe operation, inspection and maintenance . 93
13.3 Commissioning . 94
13.3.1 General . 94
13.3.2 Energization . 95
13.3.3 Commissioning tests . 95
13.3.4 Records . 95
13.3.5 Post commissioning activities . 95
13.4 Operator’s instruction manual . 95
13.4.1 General . 95
13.4.2 Instructions for operations and maintenance record . 96
13.4.3 Instructions for unscheduled automatic shutdown . 96
13.4.4 Instructions for diminished reliability . 96

– 4 – IEC 61400-3-2:2025 © IEC 2025
13.4.5 Work procedures plan . 96
13.4.6 Emergency procedures plan . 97
13.5 Maintenance manual . 97
14 Stationkeeping systems . 98
14.1 General . 98
14.2 Catenary, semi-taut or taut stationkeeping systems . 98
14.3 Tendon systems . 99
14.4 Synthetic mooring . 99
14.5 Stationkeeping system hardware . 99
14.6 Dynamic power cable . 99
15 Floating stability . 100
15.1 General . 100
15.2 Intact static stability criteria . 101
15.3 Quasi static evaluation . 101
15.4 Dynamic response evaluation . 102
15.5 Damage stability criteria . 102
16 Materials . 103
17 Marine support systems . 103
17.1 General . 103
17.2 Bilge system . 103
17.3 Ballast system . 103
Annex A (informative) Key design parameters for a floating offshore wind turbine
(FOWT) . 104
A.1 Floating offshore wind turbine (FOWT) identifiers. 104
A.1.1 General . 104
A.1.2 Rotor nacelle assembly (machine) parameters . 104
A.1.3 Support structure parameters . 105
A.1.4 Wind conditions (based on a 10-min reference period and including
wind farm wake effects where relevant) . 105
A.1.5 Marine conditions (based on a 3-hour reference period where relevant) . 106
A.1.6 Electrical network conditions at turbine . 107
A.2 Other environmental conditions . 107
A.3 Limiting conditions for transport, installation and maintenance . 108
Annex B (informative) Guidance on calculation of hydrodynamic loads . 109
B.1 General . 109
B.2 Morison’s equation . 109
B.3 Diffraction and radiation theory . 109
B.4 Slam loading . 110
B.5 Vortex-induced vibrations and motions . 110
B.6 Appurtenances and marine growth . 111
B.7 Global analysis and fatigue analysis methods . 111
B.8 Breaking wave loads . 112
B.9 Air gap . 112
Annex C (informative) Floating offshore wind turbine (FOWT) anchor design . 113
Annex D (informative) Statistical extrapolation of operational metocean parameters for
ultimate strength analysis . 114
D.1 General . 114
D.2 Use of IFORM to determine 50-yr significant wave height conditional on
mean wind speed . 114

D.3 Examples of joint distributions of V and H and approximations to the
s
environmental contour . 116
D.4 Choice of sea state duration . 118
D.5 Determination of the extreme individual wave height to optionally be
embedded in SSS . 119
Annex E (informative) Corrosion protection . 120
E.1 General . 120
E.2 The marine environment . 120
E.3 Corrosion protection considerations . 121
E.4 Corrosion protection systems – Support structures . 121
E.5 Corrosion protection in the rotor-nacelle assembly . 122
Annex F (informative) Prediction of extreme wave heights during tropical cyclones . 123
F.1 General . 123
F.2 Wind field estimation for tropical cyclones . 123
F.3 Wave estimation for tropical cyclones . 124
Annex G (informative) Recommendations for alignment of safety levels in tropical
cyclone regions . 125
G.1 General . 125
G.2 Global robustness level criteria . 125
G.3 Design load cases. 125
Annex H (informative) Earthquakes . 127
Annex I (informative) Model tests . 128
Annex J (informative) Tsunamis . 131
J.1 General . 131
J.2 Numerical model of tsunami [51], [52] . 131
J.3 Evaluation of variance of water surface elevation and current velocity [5] . 134
Annex K (informative) Redundancy of stationkeeping system . 135
Annex L (informative) Differing limit state methods in IEC and ISO standards . 136
Annex M (informative) Application of load and load effect logic to floating substructure
design . 138
M.1 General . 138
M.2 Typical load computation setups . 138
M.3 Applied example . 139
Annex N (informative) Guidance on simulation length and associated parameters . 140
N.1 General considerations . 140
N.1.1 General . 140
N.1.2 Initial transient time . 140
N.1.3 Low-frequency dynamics sampling . 140
N.1.4 Reference period . 140
N.2 Simulations for fatigue limit state analysis . 141
N.2.1 General . 141
N.2.2 Response variance and reference period . 141
N.2.3 Statistical convergence of damage . 141
N.3 Simulations for extreme limit state analysis . 141
N.3.1 General . 141
N.3.2 Characteristic extreme consistency with the reference period . 142
N.3.3 Characteristic value variability . 142

– 6 – IEC 61400-3-2:2025 © IEC 2025
Annex O (informative) Estimation of wave directional spreading by long wave method /
single point measurement . 143
O.1 Background. 143
O.2 Linear free-wave extraction . 144
O.3 Second-order calculation . 144
Annex P (informative) Direction spreading function . 146
Annex Q (informative) Concrete structures design . 147
Q.1 General . 147
Q.2 Design load cases. 147
Q.2.1 Limit states in reinforced concrete design . 147
Q.2.2 ULS, ALS and FLS load cases . 148
Q.2.3 SLS load cases . 148
Q.2.4 Load factors . 148
Q.3 Design criteria . 149
Q.3.1 Material factors . 149
Q.3.2 ULS, ALS, FLS verifications . 149
Q.3.3 SLS: Watertightness verification . 150
Q.3.4 SLS: Crack-opening verification . 150
Q.3.5 SLS: Limitation of stresses . 150
Annex R (informative) Relationship between peak wave period and significant wave
height in the sea areas affected by swell. 151
R.1 General . 151
R.2 Relationship between wave height and wave period in the sea areas affected
by swell . 151
Annex S (informative) Application of damage stability criteria . 152
S.1 Objective . 152
S.2 Scenario of loss of floating stability . 152
S.3 Flow of application of new damage stability criteria . 152
S.4 Definition of target probability of failure (PS) . 153
S.5 Definition of collision probability (P1) . 154
S.6 Definition of total loss probability by ship collision (P2) . 156
S.6.1 Concept of estimation of P2 and PT . 156
S.6.2 Simplification of FEM analysis . 156
S.6.3 Estimation of P2 by limit curve . 158
S.7 Additional countermeasure to reduce P2 . 159
Bibliography . 160

Figure 1 – Parts of a floating offshore wind turbine (FOWT) . 16
Figure 2 – Rigid-body motion degrees of freedom of a floating substructure; illustration
by Alfred Hicks, National Renewable Energy Laboratory . 17
Figure 3 – Design process for a floating offshore wind turbine (FOWT) . 29
Figure 4 – Definition of water levels . 38
Figure 5 – Top-down view of nacelle yaw and nacelle yaw misalignment in a simulation . 62
Figure 6 – The two approaches to calculate the design load effect . 84
Figure D.1 – Example of the construction of the 50-year environmental contour for a 3-
hour sea state duration . 115
Figure J.1 – The calculated result of Equation (J.8) . 133

Figure M.1 – Example of load and load effect workflow for a hybrid "beams" and
"nodes" floating substructure model setup . 139
Figure O.1 – A typical 60-min (full-scale) time history spectrum with Hs = 6,18 m and
Tp = 10,36 s recorded at the Ocean Engineering Wide Tank, University of Ulsan,
Korea (South) . 143
Figure R.1 – The relationship between significant wave height and significant wave
period based on the measurement at Fukushima offshore site [2] . 151
Figure S.1 – Concept flow of application of new damage stability criteria . 153
Figure S.2 – Concept image of the approaching frequency . 155
Figure S.3 – Concept of estimation of P2 and PT in a strict way. 156
Figure S.4 – Concept of a limit curve . 158
Figure S.5 – Concept of the probability of total loss probability by ship collision. 158

Table 1 – Conversion between extreme wind speeds of different averaging periods . 42
Table 2 – Design load cases . 56
Table 3 – Safety factor for yield stress . 87
Table G.1 – Additional load cases for tropical cyclone affected regions . 126
Table L.1 – Mapping of limit states in ISO 19904-1 Table 4 and load cases from
IEC 61400-3-2 . 137
Table Q.1 – Partial factors γ for actions for different limit states . 149
F
Table Q.2 – Material factors γ for different limit states and materials . 149
m
Table Q.3 – Allowable crack-width for different exposure zones . 150
Table S.1 – Annual reliability of offshore structures . 154

– 8 – IEC 61400-3-2:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 3-2: Design requirements for floating 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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61400-3-2 has been prepared by IEC technical committee 88: Wind energy generation
systems. It is an International Standard.
This first edition cancels and replaces IEC TS 61400-3-2, published in 2019. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to
IEC TS 61400-3-2:
a) The relevant contents of IEC 61400-3-1 have been migrated into IEC 61400-3-2, making
IEC 61400-3-2 a self-standing document that does not have to be read directly in
conjunction with IEC 61400-3-1.

b) Several modifications have been made regarding metocean conditions in Clause 6
considering the nature of FOWT and the offshore site where FOWT will be installed,
including: (1) the importance of wave directional spreading has been highlighted as it may
result in larger loads for FOWT, including the addition of the new informative Annex O and
Annex P and (2) the characteristic of swell has been explained, which may be relevant for
some FOWT projects, including the addition of new informative Annex R regarding the
characteristic of swell.
c) Subclauses 7.1, 7.2, 7.3, 7.4 and 7.5 have been changed to include a revised DLC table
and its related descriptions, including amongst others updated requirements on
directionality, wave conditions, redundancy check and damage stability cases, and a
robustness check case; further updates are made related to guidance and necessities
provided on load calculations and simulation requirements.
d) Subclause 7.6 has been updated with guidance on fatigue assessment along with
clarifications on serviceability analysis and the applicable material for WSD; related Annex L
has been updated and a new Annex M has been added for clarification of the safety factors
and load and load effect approach for floating substructures.
e) The concept of floater control system that will interact with the wind turbine controller has
been introduced in Clause 8.
f) Clause 11 has been renamed from "Foundation and substructure design" to "Anchor design"
and requirements for the transient conditions have been added.
g) A more detailed clause regarding concrete design has been added to Clause 16 together
with an informative Annex Q.
h) Clause 15 has been updated with the aim to improve ease of use, using experience from oil
and gas and considering unique wind turbine characteristics; updates included guidance for
TLPs, damage stability, dynamic stability, testing and the addition for Annex S regarding
how to analyse collision probability.
This International Standard is to be read in conjunction with IEC 61400-1, Wind energy
generation systems – Part 1: Design requirements.
The text of this International Standard is based on the following documents:
Draft Report on voting
88/1028/FDIS 88/1050/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
The language used for the development of this International Standard is English.
This publication was drafted in accordance with the ISO/IEC Directives, Part 2, and developed
in accordance with the ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement,
available at www.iec.ch/members_experts/refdocs. The main document types developed by IEC
are described in greater detail at www.iec.ch/publications.
A list of all parts of the IEC 61400 series, published under the general title Wind energy
generation systems, can be found on the IEC website.

– 10 – IEC 61400-3-2:2025 © IEC 2025
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

INTRODUCTION
This part of IEC 61400 outlines the minimum design requirements for floating offshore wind
turbines (FOWT) and is not intended for use as a complete design specification or instruction
manual.
Several
...