The International Design Standard for Offshore Wind Turbines: IEC 61400-3

J. F. Manwell, Prof.Wind Energy Center

Dept. of Mechanical & Industrial Engineering

Univ. of Mass., Amherst, MA 01003

IGERT SeminarFebruary 21, 2013

Why Are Standards Necessary?

• Without proper design standards, failures are much more likely

• Offshore presents particular challenges!2

Who Cares About Standards?

• Regulators

• Banks

• Insurance companies

• Designers

• Project developers/owners

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The Larger Context

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Design Standards

Component Certification

Type Certification

Project Certification

Analytical Assessment of Components

Component Testing

Analytical Assessment of Entire Turbine

Prototype Testing

Analytical Assessment of Project

Design Standards Process: IEC 61400-3 International Electrotechnical Commission (IEC)

• 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

• Defined here

• Includes:– Rotor/nacelle

assembly (RNA)– Support structure

• Tower

• Substructure

• Foundation

s u b - s t r u c t u r e

p i l e

f o u n d a t i o n

p i l e

p l a t f o r m

t o w e r t o w e r

s u b - s t r u c t u r e

sea floor

s u p p o r t s t r u c t u r e

rotor-nacelle assembly

seabed

water level

RNA

Substructure

Foundation

Tower

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Common Types of Fixed Bottom Support Structures

• Monopiles

• Gravity base

• Jackets

• Others – Tripods– Suction bucket

http://www.theengineer.co.uk/in-depth/the-big-story/wind-energy-gets-serial/1012449.article

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Parts of Floating Offshore Wind Turbine

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RNA

Floating substructure (hull)

Tower

Mooring system (moorings and mooring lines

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 site-specific 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 – 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

Meteorological /oceanographic or “Metocean” Conditions

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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:– 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 correlations19

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

• Reasonable range must be considered

• Includes tidal range, storms

HSWL highest still water levelHAT highest astronomical tideMSL mean sea levelLAT lowest astronomical tideCD chart datum (often equal to LAT)LSWL lowest still water levelA positive storm surgeB tidal rangeC negative storm surgeD maximum crest elevationE minimum trough elevation

A

B

C

E

D

HSWL

HAT

MSL

LAT

CD

LSWL

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Sea Ice

• Sea ice may seriously affect design of support structure– Special consideration, such

as ice cones may be needed

• Detailed information given in standard

Cone breaks ice

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. mussels and barnacles) and “soft” (seaweeds and kelps)

Barnacles on ship

http://www.dsdni.gov.uk/index/urcdg-urban_regeneration/nomadic/dsdin_nomadic_gallery.htm

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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/HTML%20version/EIA%20Report/Section4.htm

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Situations

1. As in 61400-1– 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…

• Wind conditions

• Waves

• Wind and wave directionality

• Sea currents

• Water level

• Other conditions

• Type of analysis

• Partial safety factor26

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

• Find characteristic material resistance, fk, from literature (or other source)

• Find design material resistance, Rd, using material safety factor

• Acceptable

kfd SS

kmd RR /1

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dd RS

Assessment of Metocean External Conditions

• 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)

• 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 in-situ 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

• 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…

• General corrections

• Wave models

• Hurricanes/cyclones

• Wind shear as affected by waves

• Floating ice

• Boat (service vessel) impact

• Soil characterization

• Vortex induced vibrations35

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…

• Foundation design (soil/structure interaction)

• Material properties

• Offshore data collection

• Environmental impact

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