”Offshore Vindmøller – Udenoms faciliteter”

Offshore Wind TurbinesDesign Standards

ByJan Behrendt Ibsø

DNV Global Wind Energy

DNV Global Wind Energy

DNV Global Wind Energylocated in Denmark

Network with DNV offices: UK, Germany, Spain, US, the Netherlands, Taiwan and India

Wind turbine type certification, wind farm project certification, State of the art rules for off-shore windturbine structures

25 employees in Denmark and growing

Technical co-operation with Risø Research Laboratory

DNV history of Offshore Wind Farms - Denmark

Frederikshavn, 4 prototype WTInstallation Year: 2002/2003Elsam

Horns Rev, 160 MWInstallation Year: 2002/2003Elsam/Eltra

Rødsand, 165 MW Installation Year: 2002/2003Energi E2/SEAS

DENMARK

Vindeby, 5 MW, Installation Year: 1991SEAS/Bonus

Tunø Knob, 5 MW, Installation Year: 1995Elsam/Vestas

Middelgrunden 40 MW.Installation Year: 2000SEAS and Bonus

Samsø, 25 MW Installation Year: 2002 Hydro Soil ServicesProject Certification.

DNV history of Offshore Wind Farms - Germany

GERMANY

Wilhelmshaven, 4.5 MWInstallation Year: 2004Enercon/Bladt Industries

Borkum West, 60 MWInstallation Year: 2004Prokon Nord

SKY 2000 – 150 MWInstallation Year: 2004EON

Butendiek, 240 MWInstallation Year: 2005Offshore Bürgerwindpark Butendiek

Arkona Becken SüdostAWE (EON)

AdlergrundUmweltkontor

Pommersche BuchtWinkra

North SeaWSD

DNV history of Offshore Wind Farms – UK

UKNorth Hoyle, 60 MWInstallation Year: 2003Vestas Celtic Ltd

Kentish Flats, 90 MWInstallation Year: 2004GREP

Rhyl Flat, 60 MWInstallation Year: 2005National Wind Power

Barrow Offshore Wind Farm, 110 MWInstallation Year: 2005Vestas Celtic Wind Technology Ltd.

Teeside Offshore Wind Farm, 110 MWInstallation Year: 2006LPC.

Lynn + Inner Dowsing, 250 MWInstallation Year: 2004AMEC.

NETHERLANDSEgmond an Zee, 100 MWInstallation year: 2004BCE

Site Specific Integrated Structural System

Site specific, e.g.:

- Wave height- Water depth- Tide and Current- Soil Conditions- Wind+Wave Loads- Optimised project

specific support structure design

The Integrated Model

Definition of integrated model for offshore Definition of integrated model for offshore wind turbines:wind turbines:An An aeroelasticaeroelastic model that includes the model that includes the dynamic influence of wave loads and dynamic influence of wave loads and foundation/soil stiffness and damping. foundation/soil stiffness and damping.

New topics when moving offshore New topics when moving offshore •• Stiffness and dampingStiffness and damping

•• Soil stiffness and deflection, uSoil stiffness and deflection, u•• Wave spectre and 1st tower/pile Wave spectre and 1st tower/pile

bending frequencybending frequency•• Movements at tower top Movements at tower top du/dtdu/dt•• Structural analysis of the foundation Structural analysis of the foundation

and soiland soilLoad casesLoad cases

•• “Traditional” load cases are “Traditional” load cases are combined with wave loads combined with wave loads

How offshore wind farmsaffect support structure design

Support Structures (tower + foundation) reaches 30-40 % of the total investment. This has the following impact:

• Design- Differentiated foundation types and or sizes- Further offshore i.e. harsh environment yielding high requirements to strength- Large water depth resulting in significant dynamic influence of foundation to the WT- High requirements to installation phase as critical when far out to sea- Simple and robust solutions in favor to high-tech non-proven solutions.

• Insurance – Investors- Requirements to reliable investments, thus independent project certification

• Critical issues during project- e.g. procurement of steel, sea transport, installation vessels, installation

Design Standards – Key Areas

New key areas for design standards for offshore wind turbines

including support structures as compared with onshore:

- Geotechnical modeling and analysis

- Aerodynamic modeling and analysis

- Hydrodynamic modeling and analysis

- Structural modeling and analysis

- Corrosion aspects

- Practical issues such as procurement, manufacturing and installation

- Combination of the above

Standards and Codes – Offshore Wind Turbines

Currently no national nor international Standard/Codescovering Offshore Wind Turbines as an integratedSystem:

IEC 61400-1 and IEC WT01 covers onshore wind turbines but not foundationIEC 61400-3 (not finalised) covers offshore wind turbines but not foundationsEurocodes cover onshore buildings (High Safety Class) not wind turbines IS0 19902 and API covers offshore oil-and gas fixed structures(High Safety Class (wind turbines not covered) Danish Recommendation for Offshore Wind Turbines, December 2001 Offhoresupport structure design not covered specifically and based on DS codes

DNV Offshore Wind Turbine Standards

DNV-OS-J100: Offshore Wind Turbines

- DNV-OS-J101: Offshore Wind Turbine Structures- DNV-OS-J102: Wind Turbine Blades, Nov. 2004- DNV-OS-J103: Offshore Wind Turbine Electrical Systems, 2005 - DNV-OS-J104: Offshore Wind Turbine Gear Boxes, 2005

DNV Offshore Standard DNV-OS-J101

Special new topics covered in the DNV Standard:

• Minimum soil investigations • Determination of design waves• Combined loads (wind-waves and wind-ice)• State-of-the-art fatigue design of tubular joints • Grouted connections in mono-piles• Grouted connections - pile to jacket sleeve• Composite design - steel tubular in concrete shaft• Suction bucket foundation

DNV Offshore Standard DNV-OS-J101

New design standard for design of support structure for offshore wind turbines

Basis for new DNV rulesDNV standard is based on experience from more than 24 offshore wind projects & general rule development from maritime and offshore industries for decades

StatusInternal and external hearing finalised 16 April 2004 Will be published in June 2004

DNV Offshore Standard DNV-OS-J101

The standard focuses on• structural design • manufacturing • installation • follow-up during the in-service phase

for the support structure• i.e. all structural parts below the nacelle

including the soil

DNV Rules for Offshore Wind Turbine Structures

DNV-OS-J101 Standard covers:Site specific design of wind turbine,support structure and foundationconsidered as an integrated structure.Site specific parameters e.g.: • Soil conditions • Wind conditions• Water depth• Wave height• Current • Combined Wind-Wave,Wind-Ice• Corrosion • Structural stiffness

DNV Rules for Offshore Wind Turbine Structures

Content of DNV Rules

• Design principles• Safety levels• Site conditions• Loads• Structural design• Materials

Life cycle approach

• Corrosion• Manufacturing• Transport• Installation• Maintenance• Decommissioning

Foundation Solutions

Various foundation design covered

Focus on• Gravity foundation• Mono-piles• Tripods

Other concepts included: Jackets, Hybrids, Floating foundations, Suction buckets etc.

DNV Rules for Offshore Wind Structures

Design Principles DNV-OS-J101

1) Design by the LRFD Method (linear combination of individual load processes)

2) Design by direct simulation of combined load (direct simulation of combined load effet of simultaneously acting load processes)

3) Design assisted by testing

4) Probability-based design

Design Principles – Target Safety Level

The rules are an offshore standard, providing an overall safety level corresponding to low to normal safety class:

The target safety level for structural ultimate limit state (ULS) designs is to the lower end of normal safety class corresponding to an annual probability of failure in the range 10−5-10−4 with 10−5 (β=4.2) being the aim for designs to the normal safety class and10−4 (β=3.7) being the aim for designs to the low safety class. This range of ultimate limit state (ULS) target safety levels is the range aimed at for structures, whose failures are ductile with no reserve capacity.

Target Reliability Index

Limit state Target reliabilityindex β (design working life)

Target annualreliability index β (one year)

Ultimate 3,1 (EC:3.8) 3,7 (EC:4.7)

Fatigue 1.2-3,1 (EC:3.8) *) -

*) Depends on inspectability, reparability and damage tolerances.

)1

DNV Offshore Standard DNV-OS-J101

The rules are an offshore standard, providing an overall safety level corresponding to low to normal safety class.

The DNV-OS-J101 account for the fact that the structures are unmanned and the risk for pollution of the environment is limited.

Fatigue Design of Offshore WT Structures

In order for offshore wind turbines and their support structures to be economically feasible, optimisation of the design, including the fatigue design needs to be carried out.

Local Joint Flexibility (LJF)

Spring characteristics may be found from Buitrago.

Rotational and/or axial LJF spring

Stiff offset elementBeam element

Application of joint flexibility will give a more correct distribution of member forces.

Influence from mean stress (crack closure)

Welded structural details:

Fracture Mechanics Fatigue Calculations

K = FS · FE · FT · FG · S · cπ

FS = 1.12 – 0.12 ·bc

FE= 2/165.1

25945.41

−

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛+

bc

FT = ( )tc 2/sec π

c/b = 0.2

Fracture Mechanics Fatigue Calculations

( ) c ·S · F S · bbm π+= mFKSm = tensile membrane geometric stress range component Sb = outer-fibre bending geometric stress range componentc = crack depth

Section A-A Section B-B Section C-C

Notch stress

Geometric stressNominal stress

A B C

Sb = Sbo ⎟

⎠⎞

⎜⎝⎛ −

tc1

Fatigue Life Improvement Methods

Tubular Girth Weld Profile Grinding:

Improved to SN-curve Curve ‘C’/Curve ’125’(Normally Curve ‘D’/Curve ’90’ without weld profilegrinding)

Grinding as for tubular jointsLarge radius grindingremoving weld caps and weldtoe undercut