IEC 61400-3-2:2025
(Main)Wind energy generation systems - Part 3-2: Design requirements for floating offshore wind turbines
Wind energy generation systems - Part 3-2: Design requirements for floating offshore wind turbines
IEC 61400-3-2:2025 specifies requirements for assessment of the external conditions at a floating offshore wind turbine (FOWT) site and specifies essential design requirements to ensure the engineering integrity of FOWTs. Its purpose is to provide an appropriate level of protection against damage from all anticipated hazards during the planned lifetime.
This document focuses on the engineering integrity of the structural components of a FOWT but is also concerned with subsystems such as control and protection mechanisms, internal electrical systems and mechanical systems.
This first edition cancels and replaces IEC TS 61400-3-2, published in 2019. 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.
Systèmes de génération d’énergie éolienne - Partie 3-2: Exigences de conception des éoliennes en mer flottantes
l'IEC 61400-3-2:2025 spécifie des exigences d'évaluation des conditions externes sur un site d'éoliennes en mer flottantes (FOWT), ainsi que les exigences essentielles de conception, afin d'assurer l'intégrité technique des FOWT. Elle a pour objet de fournir un niveau de protection approprié contre les dommages provoqués par tous les dangers prévus pendant la durée de vie prévue.
Le présent document se concentre sur l'intégrité technique des composants structurels d'une FOWT, mais concerne également les sous-systèmes, tels que les mécanismes de commande et de protection, les systèmes électriques internes et les systèmes mécaniques.
Cette première édition annule et remplace l'IEC TS 61400-3-2 parue en 2019. Cette première édition annule et remplace l'IEC TS 61400-3-2 parue en 2019.
a) le contenu pertinent de l'IEC 61400-3-1 a été transféré dans l'IEC 61400-3-2, faisant de l'IEC 61400-3-2 un document autonome qui ne doit pas être lu directement conjointement avec l'IEC 61400-3-1;
b) plusieurs modifications ont été apportées concernant les conditions océano-météorologiques spécifiées à l'Article 6 en prenant en compte la nature de la FOWT et le site en mer sur lequel la FOWT est installée, notamment: (1) l'importance de la propagation directionnelle des vagues a été soulignée, car elle peut entraîner des charges plus importantes pour la FOWT, y compris l'ajout des nouvelles Annexe O et Annexe P informatives, et (2) la caractéristique de la houle a été expliquée, ce qui peut être pertinent pour certains projets FOWT, y compris l'ajout d'une nouvelle Annexe R informative concernant la caractéristique de la houle;
c) les 7.1, 7.2, 7.3, 7.4 et 7.5 ont été modifiés pour inclure un tableau de DLC (Design Load Case, cas de charge pour la conception) révisé et ses descriptions associées, y compris, entre autres, des exigences mises à jour sur la directionnalité, les conditions de vagues, les cas de contrôle de redondance et de stabilité après avarie, et un cas de contrôle de solidité; d'autres mises à jour sont effectuées concernant les recommandations et les éléments nécessaires fournis sur les calculs de charge et les exigences de simulation;
d) le 7.6 a été mis à jour avec des recommandations relatives à l'évaluation de la fatigue ainsi que des clarifications sur l'analyse de l'aptitude au service et le matériel applicable pour le WSD. L'Annexe L connexe a été mise à jour et une nouvelle Annexe M a été ajoutée pour la clarification des facteurs de sécurité et l'approche de charge et d'effet de charge pour les sous-structures flottantes;
e) le concept de système de commande de flotteur qui interagit avec le régulateur de l'éolienne a été introduit à l'Article 8;
f) l'Article 11 "Conception de la fondation et de la sous-structure" a été renommé en "Conception des ancres" et des exigences relatives aux conditions transitoires ont été ajoutées;
g) un article plus détaillé sur la conception du béton a été ajouté à l'Article 16 ainsi qu'une Annexe Q informative;
h) l'Article 15 a été mis à jour dans le but d'améliorer la facilité d'utilisation, en utilisant l'expérience du pétrole et du gaz et en prenant en compte les caractéristiques uniques des éoliennes. Les mises à jour comprenaient des recommandations pour les TLP, la stabilité après avarie, la stabilité dynamique, les essais et l'ajout de l'Annexe S concernant la manière d'analyser la probabilité de collision.
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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
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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
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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
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