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: 27.180 - Wind turbine energy systems
• ICS: 31.240 - Mechanical structures for electronic equipment
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: MT 3-2 - TC 88/MT 3-2
• Current Stage: DELPUB - Deleted Publication
• Start Date: 22-Jan-2025
• Completion Date: 30-Apr-2021

Relations

• Revised: IEC 61400-3-2:2025 - Wind energy generation systems - Part 3-2: Design requirements for floating offshore wind turbines
• Effective Date: 05-Sep-2023

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
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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

– 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

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

– 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

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

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

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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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
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• the required support cannot be obtained for the publication of an International Standard,
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• 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.

– 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.
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.
– 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.

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

– 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 entry: Refer to Figure 1.
3.79
anchor
device attached to the end of the mooring line or tendon and partially or fully buried in the
seabed to limit the movement of the mooring line or tendon and to transfer loads to the
seabed
Note 1 to entry: Available options for anchoring floating structures include drag anchors, anchor piles (driven,
jetted, suction, torpedo/gravity-embedded and drilled and grouted), and other anchor types such as gravity anchors
and plate anchors.
3.80
catenary mooring
mooring system where the restoring action is provided by the distributed weight of mooring
lines
3.81
floating substructure
part of a FOWT support structure that floats above the sea floor, connects to the tower and
station-keeping system, and consists of a buoyant structure for supporting operational loads.
Note 1 to entry: A floating substructure can also be referred to as a hull. Different floating substructure concepts
are shown in Figure 1 together with the other parts of an offshore wind turbine.

Rotor – nacelle assembly
Tower
Platform
Water level
Floating
substructure
Mooring line
Tendon
Mooring line
(catenary)
Sea floor
Anchor
Pile
Seabed
IEC
From left to right: Spar, TLP, and Semi.
Figure 1 – Parts of a floating offshore wind turbine (FOWT)
3.82
mooring system
passive type of station-keeping system that typically comprises mooring lines, anchors,
connectors and hardware and may include other components such as buoys, clamped weights,
turrets, disconnecting system, etc.
3.83
recognized classification society
member of the International Association of Classification Societies (IACS), with recognized
and relevant competence and experience in floating structures
3.84
redundancy check
design situation where a FOWT has reached a new position after one mooring line or tendon
has broken and is now held in position by the remaining mooring lines or tendons
3.85
scantling
sizing of plates, girders and stiffeners of floating substructures
3.86
station-keeping system
system capable of limiting the excursions and/or accelerations of the FOWT within prescribed
limits and maintaining the intended orientation
Note 1 to entry: A station-keeping system may differ from a mooring system in the case of active thrusters,
tendons, etc.
Floating
substructure Tower
Support structure
– 12 – IEC TS 61400-3-2:2019 © IEC 2019
3.87
taut-line mooring
mooring system where the restoring action is provided by elastic deformation of mooring lines
3.88
tendon
collection of components of a station-keeping system that forms a vertical link between the
TLP-type floating substructure and the foundation on and beneath the sea floor for the
purpose of providing station-keeping and floating stability to FOWTs
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 and IEC 61400-3-1:
4.1 Symbols and units
f upper end of low-frequency range  [Hz]
low frequency
L velocity component integral scale parameter [m]
k
S.F. safety factor  [-]
σ allowable stress   [N/mm or MPa]
allowable
σ allowable buckling stress   [N/mm or MPa]
buckling
σ critical compressive buckling stress or shear buckling stress [N/mm or MPa]
cr
σ specified minimum yield strength  [N/mm or MPa]
y
4.2 Abbreviations
For the purposes of this document, the following abbreviated terms apply in addition to those
stated in IEC 61400-1 and IEC 61400-3-1:
FOWT floating offshore wind turbine
IACS international association of classification societies
IMO international maritime organization
RCS recognized classification society
TLB tension-leg buoy
TLP tension-leg platform
WSD working stress design
5 Principal elements
5.2 Design methods
The design methodology summarized in IEC 61400-3-1:2019, Subclause 5.2, is basically able
to be applied to FOWT, with the following modifications, illustrated in Figure 2.
The design of the FOWT support structure shall include the design of the station-keeping
system per Clause 14 and consider floating stability per Clause 15.
Due to the additional compliance of the station-keeping system of FOWTs relative to fixed
offshore wind turbines and the changed dynamic response (including couplings to the RNA), it
may be less likely that an RNA initially designed as a standard wind turbine class as defined
in IEC 61400-1:2019, Subclause 6.2, is suitable for use in a FOWT.

It is necessary to demonstrate that the FOWT support structure and the site-specific offshore
conditions do not compromise the RNA structural integrity. The demonstration shall comprise
a comparison of loads and deflections calculated for the specific FOWT support-structure and
the specific site conditions with those calculated during the initial RNA design.
The potentially increased dynamic response of FOWT relative to fixed offshore wind turbines
also has implications for the design of the control and protection system (see Clause 8),
mechanical systems (see Clause 9), and tower.
In lieu of testing described in IEC 61400-3-1, data from model-scale testing may be used to
increase confidence in predicted design values and to verify structural-dynamics models and
design situations (see Annex K).
Design initiated
Site-specific external
conditions (6)
RNA design
(e.g. IEC 61400-1,
Standard wind turbine class)
Design basis for FOWT
RNA design
Support structure design,
including the station-keeping
system (Clause 14) and
floating stability (Clause 15)
Design situations and
load cases (7.3)
Load and load effect
calculations (7.4)
Limit state analyses (7.5)
Structural
integrity
OK?
Design completed
IEC
Figure 2 – Design process for a floating offshore wind turbine (FOWT)

– 14 – IEC TS 61400-3-2:2019 © IEC 2019
5.6 Support structure markings
The following information, as a minimum, shall be prominently and legibly displayed on the
indelibly marked FOWT support structure (including floating substructure) nameplate:
• model and serial number;
• production year;
• displacement;
• draft marking (combined load line markings for operating in sea operation);
• company and registered owner identification;
• manufacturer and country;
• owner;
6 External conditions – definition and assessment
6.1 General
6.1.1 In addition to IEC 61400-3-1, the external conditions described in this clause shall be
considered in the design of a FOWT. The external conditions defined in IEC 61400-3-1:2019,
Clause 6 can basically be applied, but due to the FOWT support structure being a floating
system, some additional aspects regarding wind, wave and other external conditions have to
be considered.
6.1.2 Wind conditions
It shall be ensured that the representation of the wind in the low-frequency range is adequate.
This includes, but is not limited to, an adequate representation of power spectral density in
the low-frequency range as well as adequate models for representation of gust events. In
particular, this affects the extreme operating gust (EOG) as defined in IEC 61400-1:2019,
Subclause 6.3.2.2.
The upper end of the low-frequency range (in Hz) is indicated by the frequency
V
hub
(23)
f =
low frequency
6L
k
where V is the wind speed (in m/s) at hub height averaged over 10 min and L is the
hub k
velocity component integral scale parameter (in m), defined in IEC 61400-1:2019, Annex B. A
turbulence model often applicable at low frequencies is described in ISO 19901-1:2015,
Clause A.7.4.
Gust event durations for EOG, EDC, ECD defined in IEC 61400-1:2019-–, Subclause 6.3.2,
may be inadequate for FOWT design due to possible FOWT motion natural frequencies that
are considerably lower than for fixed systems and shall be evaluated, if relevant. For methods
to address longer EOG periods (see Annex O).
6.3.3 Marine conditions
The loading and response of the floating substructure is typically more driven by waves than
by wind; care should be taken when the combined wind and wave input to the load
simulations is defined in order not to disregard important sea states.
___________
For fixed offshore wind turbines, the waves are often taken dependent on the wind speed; this may be
insufficient for FOWTs.
Swells can be of importance in conjunction with low-frequency responses of FOWTs. Wind-
wave misalignment cases leading to bi-directional wave loading may require specific attention
for FOWTs and shall be taken into account. This may be particularly important for load cases
driving tower-base fatigue.
6.3.5 Other environmental conditions
Specific attention should be paid to the assessment of the seismic analysis in the case of
TLP/TLB-type floating substructures (see Annex J).
7 Structural design
7.1 General
The FOWT shall be designed in accordance with this clause. Additional requirements relevant
to the design of floating substructures shall follow ISO 19904-1.
The station-keeping system shall be designed as per Clause 14.
7.3 Loads
7.3.2 Gravitational and inertial loads
IEC 61400-3-1:2019, Subclause 7.3.2 is generally applicable. Inertial loads, including
gyroscopic loads, are of special importance to FOWTs due to their potentially additional
compliance and increased dynamic response from aerodynamic and hydrodynamic loading.
7.3.3 Aerodynamic loads
IEC 61400-3-1:2019, Subclause 7.3.3 is generally applicable. The aerodynamic interaction
between the airflow and the FOWT is of special importance due to their additional compliance
and increased dynamic response. The interaction of potentially large translational and
rotational motions of the floating substructure with the aerodynamic loading of the RNA and
tower shall be considered, including aeroelastic effects and the associated global and local
dynamic and unsteady aerodynamic effects (e.g. dynamic inflow, oblique inflow, skewed wake,
unsteady airfoil aerodynamics including dynamic stall, blade-vortex interaction). Wind loads
on the floating substructure shall also be considered, where relevant.
7.3.5 Hydrodynamic loads
Air gap should have a minimum value of 1,5 m. The air gap shall be determined by
appropriate model tests and/or calculated using detailed global performance analyses that
account for relative motions between the floating substructure and waves. When assessing
the air gap requirement, consideration shall be given to the effect of wave run-up and motions
of the floating substructure. The wave run-up is principally affected by the geometry of the
structure, wave height, and wave steepness and is typically determined through model tests.
Local wave crest elevation shall be taken into account as appropriate, refer to API RP 2FPS.
As a minimum, the requirement of air gap shall be checked for the DLCs associated with
extreme storms with a return period of 50 years.
Additionally, to IEC 61400-3-1, strength for wave impact load including slamming, sloshing
and green water in accordance with ISO 19904-1 shall be assessed.
7.3.6 Sea/lake ice loads
Annex D of IEC 61400-3-1:2019 does not apply to FOWTs. See Annex D.

– 16 – IEC TS 61400-3-2:2019 © IEC 2019
7.3.7 Other loads
Wake effects from neighbouring FOWTs during power production shall be considered. The
assessment of the suitability of a FOWT at a site in an offshore wind farm shall take into
account the deterministic and turbulent flow characteristics associated with single or multiple
wakes from upwind machines, including the effects of the spacing between the machines, for
all ambient wind speeds and wind directions relevant to power production.
The wake behind a wind turbine introduces a wind velocity deficit that tends to meander plus
additional turbulence from what is present in the natural free stream. When wakes from one or
more upwind machines partially impinge on a downwind rotor, large asymmetric loads
(including yaw loads) can be produced on the downwind rotor. These loads and resulting
response from dynamic motion may be especially important in FOWTs that are soft in yaw as
a result of their station-keeping system configuration. The floating substructure motion shall
be accounted for when applying wake models described in IEC 61400-1.
Mooring, tendon and power cable loads have important effects. See Clause 14.
Hydrostatic loads acting on the floating substructure because of internal and external static
pressures and resulting buoyancy shall be taken into account where appropriate, including
time-varying contribution from hydrostatic pressure due to heave, roll and pitch displacements
of the structure from its mean position.
Regarding the effect of earthquake loading for floating structures, see Annex J.
For sites prone to tsunamis, a tsunami shall be generally considered as variance of water
surface elevation and horizontal current; see Annex L. If a suitable tsunami warning system is
in place to shut down the wind turbine, the tsunami condition can be analysed without
considering additional loading from the operating turbine.
7.4 Design situations and load cases
IEC 61400-3-1:2019, Subclause 7.4 shall be applied where applicable to FOWT systems.
Additional considerations from particular aspects of FOWT shall be considered.
For each DLC, if wind, wave, swell and current misalignment can lead to higher loading for
FOWT, this misalignment shall be considered, including in cases where IEC 61400-3-1:2019,
Subclause 7.4 specifies co-directionality.
The extreme operating gust (EOG in DLC 2.3, 3.2 and 4.2) shall be additionally investigated
with a longer duration for FOWTs (see Annex O).
In the case of fault conditions (DLC 2.1 to 2.6 and 7.1 to 7.2), for FOWTs with active control
systems in the support structure (e.g. active ballast or stationkeeping systems with active
thrusters), faults of such systems shall be considered.
In addition to the design load cases defined in IEC 61400-3-1, the specific load cases for
FOWT in Table 2 shall be considered.

Table 2 – FOWT specific design load cases
Design DLC Wind Waves Wind and wave Sea Water Other Type of Partial
Situation condition directionality currents level conditions analysis safety
factor
2) Power 2.6 NTM SSS MIS, MUL NCM NWLR Fault of sea- U A
production state limit
V < V
plus in hub protection
occurrence out system
...