Update on Design Standards for Offshore Wind Turbines

J. F. Manwell, Prof.Wind Energy Center

Dept. of Mechanical & Industrial Engineering

Univ. of Mass., Amherst, MA 01003

Massachusetts Wind Working GroupJanuary 30, 2013

Why Are Standards Necessary?

• Without proper design standards, failures are much more likely

• Offshore presents particular challenges!2

Parts of an Offshore Wind Turbine

• OWT defined here• Includes:

– Rotor/nacelle assembly (RNA)

– Support structure• Tower

• Substructure

• Foundation

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p i l e

f o u n d a t i o n

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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 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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Floating offshore wind turbines outside scope of current standards; guidelines for FOWTs being considered separately

Relevant Offshore Standards/Topics

• International Electrotechnical Commission’s IEC 61400-3 (2009)

• IEC 61400-3, 2nd edition (in preparation)• American Petroleum Institute Recommended Practice for

Fixed Offshore Structures (API RP2A, 1993)• AWEA’s Recommended Practices for Design,

Deployment, and Operation of Offshore Wind Turbines in the United State (2012)

• Referenced documents, e.g. ISO 19000 series• Validating, Testing, Certification (application of standards)

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IEC 61400-3 Process

• Prepare preliminary design (PD)• Develop structural dynamic model of PD (SDM)• Specify external design conditions (EDCs)• Specify design load cases (DLCs)• Using SDM with EDCs and DLCs determine

structural loads, deflections and stresses• Check that stresses etc. are acceptable, given chosen

material • Adapt design if necessary and repeat

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External Design 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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Design Load Case 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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DLCs for Various Situations …

• Wind conditions

• Waves

• Wind and wave directionality

• Sea currents

• Water level

• Other conditions

• Type of analysis

• Partial safety factor10

IEC 61400-3, 2nd Ed. Changes Likely

• General corrections

• Wave models

• Hurricanes/cyclones

• Wind shear as affected by waves

• Floating ice

• Boat (service vessel) impact

• Soil characterization

• Vortex induced vibrations11

Issues for US

• Wind/wave conditions (e.g. hurricanes)– 100 yr vs. 50 yr events

• Role of API, other US standards

• Role of BOEM

• Other standards referenced by 61400-3 – US vs. European or international– English units vs. metric (SI) units

• Should IEC 61400-3 2nd ed. supercede AWEA’s Recommended Practice?

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