IEC TS 61400-3-2:2019

Wind energy generation systems - Part 3-2: Design requirements for floating offshore wind turbines

IEC TS 61400-3-2:2019 specifies additional 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 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.
A wind turbine is considered as a FOWT if the floating substructure is subject to hydrodynamic loading and supported by buoyancy and a station-keeping system. A FOWT encompasses five principal subsystems: the RNA, the tower, the floating substructure, the station-keeping system and the on-board machinery, equipment and systems that are not part of the RNA.
The following types of floating substructures are explicitly considered within the context of this document:
a) ship-shaped structures and barges,
b) semi-submersibles (Semi),
c) spar buoys (Spar),
d) tension-leg platforms/buoys (TLP / TLB).
In addition to the structural types listed above, this document generally covers other floating platforms intended to support wind turbines. These other structures can have a great range of variability in geometry and structural forms and, therefore, can be only partly covered by the requirements of this document. In other cases, specific requirements stated in this document can be found not to apply to all or part of a structure under design. In all the above cases, conformity with this document will require that the design is based upon its underpinning principles and achieves a level of safety equivalent, or superior, to the level implicit in it.
This document is applicable to unmanned floating structures with one single horizontal axis turbine. Additional considerations might be needed for multi-turbine units on a single floating substructure, vertical-axis wind turbines, or combined wind/wave energy systems.
This document is to be used together with the appropriate IEC and ISO standards mentioned in Clause 2. In particular, this document is intended to be fully consistent with the requirements of IEC 61400-1 and IEC 61400-3-1. The safety level of the FOWT designed according to this document is to be at or exceed the level inherent in IEC 61400‑1 and IEC 61400-3-1.

General Information

• Status: Published
• Publication Date: 04-Apr-2019
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 27.180 - Wind turbine energy systems
• ICS: 31.240 - Mechanical structures for electronic equipment
• Technical Committee: TC 88 - Wind energy generation systems
• Current Stage: PPUB - Publication issued
• Completion Date: 05-Apr-2019

Standards Content (Sample)

IEC TS 61400-3-2
Edition 1.0 2019-04
TECHNICAL
SPECIFICATION
colour
inside
Wind energy generation systems –
Part 3-2: Design requirements for floating offshore wind turbines
IEC TS 61400-3-2:2019-04(en)
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IEC TS 61400-3-2
Edition 1.0 2019-04
TECHNICAL
SPECIFICATION
colour
inside
Wind energy generation systems –
Part 3-2: Design requirements for floating offshore wind turbines
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180 ISBN 978-2-8322-5986-3

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TS 61400-3-2:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 5

INTRODUCTION ..................................................................................................................... 7

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 9

4 Symbols and abbreviated terms ..................................................................................... 12

4.1 Symbols and units................................................................................................. 12

4.2 Abbreviations ........................................................................................................ 12

5 Principal elements ......................................................................................................... 12

5.2 Design methods .................................................................................................... 12

5.6 Support structure markings ................................................................................... 14

6 External conditions – definition and assessment ............................................................ 14

6.1 General ................................................................................................................. 14

6.1.2 Wind conditions .................................................................................................... 14

6.3.3 Marine conditions ................................................................................................. 14

6.3.5 Other environmental conditions ............................................................................. 15

7 Structural design ........................................................................................................... 15

7.1 General ................................................................................................................. 15

7.3 Loads.................................................................................................................... 15

7.3.2 Gravitational and inertial loads ...................................................................... 15

7.3.3 Aerodynamic loads ........................................................................................ 15

7.3.5 Hydrodynamic loads ...................................................................................... 15

7.3.6 Sea/lake ice loads ......................................................................................... 15

7.3.7 Other loads .................................................................................................... 16

7.4 Design situations and load cases .......................................................................... 16

7.5 Load and load effect calculations .......................................................................... 18

7.5.1 General ......................................................................................................... 18

7.5.2 Relevance of hydrodynamic loads .................................................................. 18

7.5.3 Calculation of hydrodynamic loads ................................................................. 18

7.5.4 Calculation of sea/lake ice loads .................................................................... 19

7.5.6 Simulation requirements ................................................................................ 19

7.5.7 Other requirements ........................................................................................ 21

7.6 Ultimate limit state analysis................................................................................... 21

7.6.1 General ......................................................................................................... 21

7.6.3 Fatigue failure ............................................................................................... 22

7.6.6 Working stress design method ....................................................................... 22

7.6.7 Serviceability analysis ................................................................................... 23

8 Control system .............................................................................................................. 23

9 Mechanical systems ...................................................................................................... 24

10 Electrical systems .......................................................................................................... 24

11 Foundation and substructure design .............................................................................. 24

12 Assembly, installation and erection ................................................................................ 24

12.1 General ................................................................................................................. 24

12.2 General ................................................................................................................. 24

12.3 Planning ............................................................................................................... 25

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IEC TS 61400-3-2:2019 © IEC 2019 – 3 –

12.13 Floating specific items .......................................................................................... 25

13 Commissioning, operation and maintenance .................................................................. 25

13.1 General ................................................................................................................. 25

13.3 Instructions concerning commissioning ................................................................. 25

13.4 Operator’s instruction manual ............................................................................... 25

13.4.1 General ......................................................................................................... 25

13.4.6 Emergency procedures plan .......................................................................... 25

13.5 Maintenance manual ............................................................................................. 25

14 Stationkeeping systems ................................................................................................. 25

15 Floating stability ............................................................................................................ 26

15.1 General ................................................................................................................. 26

15.2 Intact static stability criteria .................................................................................. 26

15.3 Alternative intact stability criteria based on dynamic-response .............................. 26

15.4 Damage stability criteria ....................................................................................... 26

16 Materials ....................................................................................................................... 27

17 Marine support systems ................................................................................................. 27

17.1 General ................................................................................................................. 27

17.2 Bilge system ......................................................................................................... 27

17.3 Ballast system ...................................................................................................... 27

Annex A (informative) Key design parameters for a floating offshore wind turbine ................ 28

A.1 Floating offshore wind turbine identifiers ............................................................... 28

A.1.1 General ......................................................................................................... 28

A.1.2 Rotor nacelle assembly (machine) parameters ............................................... 28

A.1.3 Support structure parameters ........................................................................ 28

A.1.4 Wind conditions (based on a 10-min reference period and including

wind farm wake effects where relevant) ......................................................... 29

A.1.5 Marine conditions (based on a 3-hour reference period where relevant) ........ 29

A.1.6 Electrical network conditions at turbine .......................................................... 30

A.2 Other environmental conditions ............................................................................. 30

A.3 Limiting conditions for transport, installation and maintenance .............................. 31

Annex B (informative) Shallow water hydrodynamics and breaking waves ............................ 32

Annex C (informative) Guidance on calculation of hydrodynamic loads ................................ 33

Annex D (informative) Recommendations for design of floating offshore wind turbine

support structures with respect to ice loads .......................................................................... 34

Annex E (informative) Floating offshore wind turbine foundation and substructure

design ................................................................................................................................... 35

Annex F (informative) Statistical extrapolation of operational metocean parameters

for ultimate strength analysis ................................................................................................ 36

Annex G (informative) Corrosion protection ......................................................................... 37

Annex H (informative) Prediction of extreme wave heights during tropical cyclones ............. 38

Annex I (informative) Recommendations for alignment of safety levels in tropical

cyclone regions ..................................................................................................................... 39

Annex J (informative) Earthquakes ...................................................................................... 40

Annex K (informative) Model tests ....................................................................................... 41

Annex L (informative) Tsunamis ........................................................................................... 43

L.1 General ................................................................................................................. 43

L.2 Numerical model of tsunami [3],[4] ........................................................................ 43

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– 4 – IEC TS 61400-3-2:2019 © IEC 2019

L.3 Evaluation of variance of water surface elevation and current velocity [5] ............. 45

L.4 Reference documents ........................................................................................... 46

Annex M (informative) Non-redundant stationkeeping system .............................................. 47

Annex N (informative) Differing limit state methods in wind and offshore standards ............. 48

Annex O (informative) Application of non-standard duration extreme operating gusts .......... 50

Bibliography .......................................................................................................................... 51

Figure 1 – Parts of a floating offshore wind turbine (FOWT) .................................................. 11

Figure 2 – Design process for a floating offshore wind turbine (FOWT) ................................. 13

Figure L.1 – The calculated result of Equation (L.8) .............................................................. 45

Table 2 – FOWT specific design load cases .......................................................................... 17

Table 4 – Safety factor for yield stress .................................................................................. 23

Table N.1 – Mapping of limit states and load cases in ISO 19904-1, Table 4 and load

cases from IEC TS 61400-3-2 ............................................................................................... 49

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IEC TS 61400-3-2:2019 © IEC 2019 – 5 –
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

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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

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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.

The main task of IEC technical committees is to prepare International Standards. In

exceptional circumstances, a technical committee may propose the publication of a technical

specification when

• the required support cannot be obtained for the publication of an International Standard,

despite repeated efforts, or

• the subject is still under technical development or where, for any other reason, there is the

future but no immediate possibility of an agreement on an International Standard.

Technical specifications are subject to review within three years of publication to decide

whether they can be transformed into International Standards.

Technical Specification IEC TS 61400-3-2 has been prepared by IEC technical committee 88:

Wind energy generation systems.

This part is to be read in conjunction with IEC 61400-1:2019, Wind energy generation systems

– Part 1: Design requirements and IEC 61400-3-1:2019, Wind energy generation systems –

Part 3-1: Design requirements for fixed offshore wind turbines.
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– 6 – IEC TS 61400-3-2:2019 © IEC 2019

From Clause 2 forward, this document does not replicate text from IEC 61400-1 and

IEC 61400-3-1; instead, the section headings (including numbering) and text from

IEC 61400-3-1 apply to this document except where noted. Exceptions include additions,

deletions, or changes in requirements for FOWT relative to fixed offshore wind turbines. New

clauses, subclauses, annexes, equations, tables, and terms and definitions in this document

are numbered sequentially following the last corresponding number from IEC 61400-3-1.

The text of this technical specification is based on the following documents:
DTS Report on voting
88/649/DTS 88/673/RVDTS

Full information on the voting for the approval of this technical specification can be found in

the report on voting indicated in the above table.

This document 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 energy

generation systems, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
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IEC TS 61400-3-2:2019 © IEC 2019 – 7 –
INTRODUCTION

This part of IEC 61400 outlines minimum design requirements for floating offshore wind

turbines (FOWT) 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

document are met. The division of responsibility between these parties is a contractual matter

and is outside the scope of this document.

Any of the requirements of this document may be altered if it can be suitably demonstrated

that the safety of the system is not compromised. Compliance with this document does not

relieve any person, organization, or corporation from the responsibility of observing other

applicable regulations.
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– 8 – IEC TS 61400-3-2:2019 © IEC 2019
WIND ENERGY GENERATION SYSTEMS –
Part 3-2: Design requirements for floating offshore wind turbines
1 Scope

This part of IEC 61400, which is a technical specification, specifies additional 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 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.

A wind turbine is considered as a FOWT if the floating substructure is subject to

hydrodynamic loading and supported by buoyancy and a station-keeping system. A FOWT

encompasses five principal subsystems: the RNA, the tower, the floating substructure, the

station-keeping system and the on-board machinery, equipment and systems that are not part

of the RNA.

The following types of floating substructures are explicitly considered within the context of this

document:
a) ship-shaped structures and barges,
b) semi-submersibles (Semi),
c) spar buoys (Spar),
d) tension-leg platforms/buoys (TLP / TLB).

In addition to the structural types listed above, this document generally covers other floating

platforms intended to support wind turbines. These other structures can have a great range of

variability in geometry and structural forms and, therefore, can be only partly covered by the

requirements of this document. In other cases, specific requirements stated in this document

can be found not to apply to all or part of a structure under design. In all the above cases,

conformity with this document will require that the design is based upon its underpinning

principles and achieves a level of safety equivalent, or superior, to the level implicit in it.

This document is applicable to unmanned floating structures with one single horizontal axis

turbine. Additional considerations might be needed for multi-turbine units on a single floating

substructure, vertical-axis wind turbines, or combined wind/wave energy systems.

This document is to be used together with the appropriate IEC and ISO standards mentioned

in Clause 2. In particular, this document is intended to be fully consistent with the

requirements of IEC 61400-1 and IEC 61400-3-1. The safety level of the FOWT designed

according to this document is to be at or exceed the level inherent in IEC 61400-1 and IEC

61400-3-1.
2 Normative references
Replacement of Clause 2 of IEC 61400-3-1:2019.
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IEC TS 61400-3-2:2019 © IEC 2019 – 9 –

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements 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 61400-1:2019, Wind energy generation systems – Part 1: Design requirements

IEC 61400-3-1:2019, Wind energy generation systems – Part 3-1: Design requirements for

fixed offshore wind turbines

ISO 19901-1:2015, Petroleum and natural gas industries – Specific requirements for offshore

structures – Part 1: Metocean design and operating conditions

ISO 19901-4:2016, Petroleum and natural gas industries – Specific requirements for offshore

structures – Part 4: Geotechnical and foundation design considerations

ISO 19901-6:2009, Petroleum and natural gas industries – Specific requirements for offshore

structures – Part 6: Marine operations

ISO 19901-7:2013, Petroleum and natural gas industries – Specific requirements for offshore

structures – Part 7: Stationkeeping systems for floating offshore structures and mobile

offshore units

ISO 19904-1:2006, Petroleum and natural gas industries — Floating offshore structures —

Part 1: Monohulls, semisubmersibles and spars

ISO 19906:2010, Petroleum and natural gas industries – Arctic offshore structures

IMO Resolution MSC.267(85), International Code on Intact Stability, 2008 (2008 IS CODE)

API RP 2FPS: 2011, Recommended Practice for Planning, Designing, and Constructing

Floating Production Systems

API RP 2T (R2015): 2010, Recommended Practice for Planning, Designing, and Constructing

Tension Leg Platforms
3 Terms and definitions

For the purposes of this document, the following terms and definitions apply in addition to, or

replacing, those stated in IEC 61400-1 and IEC 61400-3-1.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.54
splash zone

external region of the FOWT support structure that is frequently wetted due to waves, tidal

variations and floating substructure motions
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– 10 – IEC TS 61400-3-2:2019 © IEC 2019

Note 1 to entry: To define upper and lower limits of the splash zone, the following parameters shall be

superimposed where applicable to the specific FOWT support structure type:

• the highest still water level with a return 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,

• the lowest still water level with a return 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,
• draft variation, and
• vertical motions (heave, roll, pitch) of the floating substructure.

Note 2 to entry: While splash zone is not explicitly mentioned in this document, the definition given in this

document replaces the definition found in IEC 61400-3-1, which affects the interpretation of IEC 61400-3-1 for

FOWT.
3.58
support structure

part of a FOWT consisting of the tower, floating substructure, and stationkeeping system

Note 1 to
...