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  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    1

    Offshore Wind Turbines: Design Considerations and the IEC

    61400-3 Design Standards

    James F. Manwell Professor and Director

    Univ. of Mass. Wind Energy Center

    April 3, 2009

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    2

    What are Offshore Wind Turbines? • According to IEC 61400-3 (Design Standards):

    “Offshore wind turbines are those wind turbines whose support structures are subject to hydrodynamic loading”

    That means waves!

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    3

    Conceptual Illustration of Offshore Wind Turbine

    Wind Turbine

    Maintenance Vessel

    Installation Crane

    Submarine Cable

    Onshore Staging Area and

    Control Room

    Grid Connection

    Rotor nacelle assembly

    Support structure

    Foundation

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    4

    Support Structures vs. Depth

    Photo: National Renewable Energy Laboratory

    Shallow < 30 m Transitional 30-60 m Deep > 60 m

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    5

    Wind Turbine Support Structure for Shallow and Intermediate Depths

    • Typical offshore wind turbine support structure options

    • Type used will depend on seabed properties

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

    pile

    foundation

    pile

    platform

    t o w er tower

    sub-structure

    sea floor

    supp o r t struct u r e

    rotor-nacelle assembly

    seabed

    water level

    Monopile Multimember Gravity

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    6

    External Design Conditions • Wind:

    – Power production – Rotor/nacelle assembly & support structure: extremes, fatigue

    • Waves: – Support structure: extremes, fatigue

    • Currents: – Support structure, rip-rap

    • Ice: – Support structure

    • Others: – Salinity, temperature

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    7

    Design Considerations

    • Turbine size • Support structure options • Water depth • Soil characteristics • External design conditions • Infrastructure (i.e. ship yards, vessels, etc.) • Environmental concerns • Maintainability • Cost!

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    8

    Monopile Structure

    • Sediment thickness • Lateral soil stiffness

    Thrust due to power extraction

    Weight of Rotor/Nacelle Assembly

    Wave forces

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    9

    Forces on the rotor/ nacelle

    assembly (RNA) and

    Support Structure

    soil

    water

    air

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    10

    Gravity Structure

    • Bearing capacity of soil

    • Resistance to overturning

    • Resistance to sliding

    • Cost of steel vs. concrete

    Photo: Carl Bro A/S

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    11

    Manufacture • Monopile

    – Welded steep tube – Prepared off site

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    12

    Manufacture • Gravity

    – Precast concrete or steel structure

    – Fabricate in dry dock

    Photo: Carl Bro A/S

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    13

    Manufacture • Multimember

    – Tubular steel – Fabricated off site

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    14

    Manufacture of Similar Structures (Offshore Oil & Gas )

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    15

    Installation (1)

    Pile driving Installing tower

    Lifting nacelle

    Photos: Courtesy GE Wind and hornsrev.dk

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    16

    Installation (2)

    Photos: Courtesy GE Wind

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    17

    Installation Gravity

    • Remove soft surface material

    • Place gravel layer • Lower foundation with

    heavy lift vessel • Fill with ballast

    Photo: Carl Bro A/S

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    18

    Multimember Structure Installation • Place in seabed • Secure to seabed with multiple piles

    Photo: http://beatricewind.co.uk

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    19

    Electrical Cables

    Typical cable layoutCable cross section

    Cable laying ship Cable trencher

    Illustrations from www.hornsrev.dk

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    20

    Offshore Wind Turbine Design Standards IEC 61400-3

    • Background – IEC = International Electrotechnical Commission – IEC oversees all wind turbine standards (61400) – Standards ensure safety, financibility, insurability – Standards relate strength of structure to external

    conditions and design load conditions

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    21

    IEC 61400-3 External Conditions • Key external factors

    – Wind – Waves – Other (currents, salinity, floating ice, …)

    • Values chosen to find: – Normal loads, extreme loads, fatigue loads – Under Design Load conditions

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    22

    IEC 61400-3 Process Wind, waves, etc.

    Structural dynamic model of turbine

    Stresses at key locations

    Material properties: allowed stresses

    Stresses OK?

    Design load conditions

    Proceed Yes

    No

    Redesign

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    23

    Design Load Conditions • Normal operation • Start up/shut down • Stationary in high winds • Faults • Transport • Installation

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    24

    Sample Design Load Cases

    Table 1 – Design load cases

    Design situation DLC Wind condition Waves

    Wind and wave

    directionality Sea

    currents Water level

    Other conditions

    Type of analysis

    Partial safety factor

    1.1 NTM V in < Vhub < Vout RNA

    NSS Hs=E [Hs| Vhub]

    COD, UNI NCM MSL For extrapolation of extreme loads on the RNA

    U N (1,25)

    1.2 NTM V in < Vhub < Vout

    NSS Joint prob. distribution of Hs,Tp,Vhub

    COD, MUL No currents

    NWLR or ? MSL

    F *

    1.3 ETM V in < Vhub < Vout

    NSS Hs=E [Hs| Vhub]

    COD, UNI NCM MSL U N

    1) Power production

    1.4 ECD Vhub = Vr – 2 m/s, Vr, Vr + 2 m/s

    NSS (or NWH) Hs=E [Hs| Vhub]

    MIS, wind direction change

    NCM MSL U N

  • Wind Energy Center Department of Mechanical and Industrial Engineering

    University of Massachusetts

    25

    Conclusions • OWT design affected by many factors

    – Water depth – Distance from shore – External design conditions (wind, waves, etc.) – Soil type – Turbine size, details – Available infrastructure – Costs

    • IEC 61400-3 will help avoid problems!

    Offshore Wind Turbines:�Design Considerations and the IEC 61400-3 Design Standards What are Offshore Wind Turbines? Conceptual Illustration of Offshore Wind Turbine Support Structures vs. Depth Wind Turbine Support Structure for Shallow and Intermediate Depths External Design Conditions Design Considerations� Monopile Structure Forces on the rotor/ nacelle assembly (RNA) and Support Structure Gravity Structure� Manufacture Manufacture Manufacture Manufacture of Similar Structu