The International Design Standard for
Offshore Wind Turbines: IEC 61400-3
IGERT Seminar
February 21, 2013
J. F. Manwell, Prof.
Wind Energy Center
Dept. of Mechanical & Industrial Engineering
Univ. of Mass., Amherst, MA 01003
Why Are Standards Necessary?
• Without proper design standards, failures are
much more likely 
• Offshore presents particular challenges!
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Who Cares About Standards?
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Regulators
Banks
Insurance companies
Designers
Project developers/owners
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The Larger Context
Design Standards
Analytical Assessment of
Components
Component Certification
Component Testing
Type Certification
Analytical Assessment of
Entire Turbine
Prototype Testing
Project Certification
Analytical Assessment of
Project
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Design Standards Process: IEC 61400-3
International Electrotechnical Commission (IEC)
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Prepare preliminary design (“PD”)
Develop structural dynamic model of PD
Specify external conditions
Specify load cases
Determine structural loads and stresses
Check that stresses are acceptable, given chosen
material
• Adapt design if necessary and repeat
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Antecedent: IEC 61400-1
• The offshore wind turbine design standard
started with conventional, land-based turbine
design standard, IEC 61400-1
• IEC 61400-1 is still directly relevant,
especially to the rotor nacelle assembly
(RNA), and IEC-61400-3 is compatible with it
to the extent possible
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What is an Offshore Wind Turbine?
• A wind turbine shall be considered as an
offshore wind turbine if the support structure
is subject to hydrodynamic loading.
• Note! 61400-3 is not sufficient for floating
offshore wind turbines
– But an IEC working group is presently developing
guidelines for floating OWTs
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Parts of an Offshore Wind Turbine
RNA
• Defined here
• Includes:
– Rotor/nacelle
assembly (RNA)
– Support structure
• Tower
• Substructure
• Foundation
rotor-nacelle assembly
Tower
tower
tower
support
structure
platform
water level
Substructure
sub-structure
sub-structure
pile
sea floor
pile
seabed
Foundation
foundation
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Common Types of Fixed Bottom
Support Structures
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Monopiles
Gravity base
Jackets
Others
– Tripods
– Suction bucket
http://www.theengineer.co.uk/in-depth/the-bigstory/wind-energy-gets-serial/1012449.article
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Parts of Floating Offshore Wind Turbine
RNA
Tower
Floating substructure
(hull)
Mooring system
(moorings and
mooring lines
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Scope of 6100-3
• Requirements (beyond IEC 61400-1) for:
– Assessment of the external conditions at an
offshore wind turbine site
– Essential requirements to ensure the structural
integrity of offshore wind turbines
– Subsystems such as control and protection
mechanisms, internal electrical systems and
mechanical systems
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Design Methods
• Requires the use of a structural dynamics
model of PD to predict design load effects
• Load effects to be determined for all relevant
combinations of external conditions and design
situations
• Design of support structure to be based on sitespecific external conditions
• Design of RNA to be based on IEC 61400-1
(to extent possible)
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Structural Dynamics Model
• Example: FAST
– From US National Renewable Energy Laboratory
– FAST is “an aeroelastic computer-aided
engineering tool for horizontal axis wind
turbines…[it] models the wind turbine as a
combination of rigid and flexible bodies.”
– FAST for offshore also includes modules for
incorporating effect of waves
• Accompanying software: TurbSim (turbulent
wind input), BModes (dynamic properties)
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External Conditions
• Wind conditions
• Marine conditions
Meteorological /oceanographic
or “Metocean” Conditions
– Waves, sea currents, water level, sea ice, marine
growth, seabed movement and scour
• Other environmental conditions
• Soil properties at the site
– Including time variation due to seabed movement,
scour and other elements of seabed instability
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Occurrences of External Conditions
• Normal
– Recurrent structural loading conditions
• Extreme
– Rare external design conditions of greater than
normal magnitude or effect
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Wind Turbine Classes
• Follows that of IEC 61400-1
– Based on: wind speed and turbulence parameters
(I, II, II) and special conditions (S)
– Design lifetime: at least 20 years
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Wind Conditions
• Normal:
– More often than once per year
• Extreme:
– Recurrence of once per year or per 50 years
• For RNA, use wind conditions as in 61400-1,
with some differences:
– Wind shear, inclination of mean flow
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Marine Conditions
• Assumed to primarily affect support structure
• Conditions include at least:
– Waves, sea currents, water level, sea ice, marine
growth, scour and seabed movement
• Normal:
– More often than once per year
• Extreme:
– Recurrence of once per year or per 50 years
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Waves
• Stochastic wave model assumed
• Design sea state:
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Wave spectrum, S (f) (m2/Hz)
Significant wave height, Hs (m)
Peak spectral period, Tp (s)
Mean wave direction, wm (deg)
• Normal, severe, extreme conditions
– Breaking waves
• Wind/wave correlations
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Sea Currents
• Sub-surface currents generated by tides, storm
surge, atmospheric pressure variations, etc.
• Wind generated, near surface currents
• Near shore, breaking wave induced surf
currents running parallel to the shore
• Current models:
– Normal, extreme
• See standard for details
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Water Level
D
• Reasonable range
must be considered
• Includes tidal
range, storms
HSWL
A
HAT
B
MSL
LAT
C
CD
LSWL
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HSWL
HAT
MSL
LAT
CD
LSWL
A
B
C
D
E
highest still water level
highest astronomical tide
mean sea level
lowest astronomical tide
chart datum (often equal to LAT)
lowest still water level
positive storm surge
tidal range
negative storm surge
maximum crest elevation
minimum trough elevation
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Sea Ice
• Sea ice may seriously
affect design of support
structure
Cone breaks ice
– Special consideration, such
as ice cones may be needed
• Detailed information given
in standard
http://www.nrc-cnrc.gc.ca/eng/projects/chc/model-test.html
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Marine Growth
• Marine growth may
influence hydrodynamic
loads, dynamic response,
accessibility and
corrosion rate of the
structure
• Classified as “hard” (e.g.
Barnacles on ship
mussels and barnacles)
and “soft” (seaweeds
http://www.dsdni.gov.uk/index/urcdgand kelps)
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urban_regeneration/nomadic/dsdin_nomadic_gallery.htm
Seabed Movement and Scour
• Seabed soil may move due to currents
• Protection (“rip-rap”) may be needed around
structure
http://sc.epd.gov.hk/gb/www.epd.gov.hk/eia/register/report/eiareport/eia_1772009/HTM24
L%20version/EIA%20Report/Section4.htm
Situations
1. As in 61400-1
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Power production
Power production plus occurrence of fault
Start up
Normal shut down
Emergency shut down
Parked (standing still or idling)
Parked and fault conditions
Transport, assembly, maintenance and repair
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For Each Situation…
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Wind conditions
Waves
Wind and wave directionality
Sea currents
Water level
Other conditions
Type of analysis
Partial safety factor
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Types of Loads
• As in 614000-1
– Ultimate (U)
• Normal (N), abnormal (A), or transport and erection (T)
• Consider: material strength, blade tip deflection and
structural stability (e.g. Buckling)
– Fatigue (F)
• Fatigue loads/fatigue strength
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Method of Analysis
• Characteristics loads predicted by design
tools (e.g. computer codes)
• Method of partial safety factors
• Expected "load function (effect)," multiplied
by a safety factor, must be less than the
"resistance function”
• Design properties for materials from
published data
• Safety factors chosen according to
established practice
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Ultimate Strength Analysis
• Find characteristic load effect, Sk, from analysis
• Find design load effect, Sd, using load safety factor
Sd   f Sk
• Find characteristic material resistance, fk, from
literature (or other source)
• Find design material resistance, Rd, using material
safety factor
Rd  1 /  m Rk
• Acceptable
Sd  Rd
Assessment of Metocean External
Conditions
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Wind speeds and directions
Significant wave heights, wave periods and directions
Correlation of wind and wave statistics
Current speeds and directions
Water levels
Occurrence and properties of sea ice
Occurrence of icing
Other parameters: air, water temperatures, densities;
water salinity; bathymetry, marine growth, etc
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Assessment of External Electrical
Conditions (examples)
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Normal voltage and range
Normal frequency, range and rate of change
Voltage imbalance
Method of neutral grounding;
Method of ground fault detection / protection;
Annual number of network outages;
Total lifetime duration of network outages;
Auto-reclosing cycles;
Required reactive compensation schedule;
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Assessment of Soil Conditions
• Geological survey of the site
• Bathymetric survey of the sea floor including
registration of boulders, sand waves or
obstructions on the sea floor
• Geophysical investigation
• Geotechnical investigations consisting of insitu testing and laboratory tests
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Scope of IEC 61400-3: 2nd Edition
• Consideration of comments from national
committees during pre-publication review
• Comments from others and from EU’s Upwind
research program
• Incorporating recent experience of the design
of offshore wind turbines and their support
structures
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General Areas of Interest
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Load calculations and simulations
External conditions
Assessment of external conditions
Support structure and foundation design
The various annexes on design approaches
Text referring to issues treated by IEC 61400-1
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Changes Likely…
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General corrections
Wave models
Hurricanes/cyclones
Wind shear as affected by waves
Floating ice
Boat (service vessel) impact
Soil characterization
Vortex induced vibrations
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Issues for US
• Wind/wave conditions (e.g. hurricanes)
– 100 yr vs. 50 yr events
• Role of American Petroleum Institute (API),
other US standards
• Role of Bureau of Ocean Energy Management
(BOEM)
• Other standards referenced by 61400-3
– US vs. European or international
– English units vs. metric (SI) units
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Little or No Detail…
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Foundation design (soil/structure interaction)
Material properties
Offshore data collection
Environmental impact
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