Offshore Wind Turbines Design Standards

Transcription

1 Offshore Vindmøller Udenoms faciliteter Offshore Wind Turbines Design Standards By Jan Behrendt Ibsø DNV Global Wind Energy

2 DNV Global Wind Energy DNV Global Wind Energy located 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 offshore windturbine structures 25 employees in Denmark and growing Technical co-operation with Risø Research Laboratory

3 DNV history of Offshore Wind Farms - Denmark DENMARK Vindeby, 5 MW, Installation Year: 1991 SEAS/Bonus Tunø Knob, 5 MW, Installation Year: 1995 Elsam/Vestas Middelgrunden 40 MW. Installation Year: 2000 SEAS and Bonus Samsø, 25 MW Installation Year: 2002 Hydro Soil Services Project Certification. Frederikshavn, 4 prototype WT Installation Year: 2002/2003 Elsam Horns Rev, 160 MW Installation Year: 2002/2003 Elsam/Eltra Rødsand, 165 MW Installation Year: 2002/2003 Energi E2/SEAS

4 DNV history of Offshore Wind Farms - Germany GERMANY Wilhelmshaven, 4.5 MW Installation Year: 2004 Enercon/Bladt Industries Borkum West, 60 MW Installation Year: 2004 Prokon Nord Pommersche Bucht Winkra North Sea WSD SKY MW Installation Year: 2004 EON Butendiek, 240 MW Installation Year: 2005 Offshore Bürgerwindpark Butendiek Arkona Becken Südost AWE (EON) Adlergrund Umweltkontor

5 DNV history of Offshore Wind Farms UK UK North Hoyle, 60 MW Installation Year: 2003 Vestas Celtic Ltd Kentish Flats, 90 MW Installation Year: 2004 GREP Rhyl Flat, 60 MW Installation Year: 2005 National Wind Power Barrow Offshore Wind Farm, 110 MW Installation Year: 2005 Vestas Celtic Wind Technology Ltd. Teeside Offshore Wind Farm, 110 MW Installation Year: 2006 LPC. Lynn + Inner Dowsing, 250 MW Installation Year: 2004 AMEC. NETHERLANDS Egmond an Zee, 100 MW Installation year: 2004 BCE

6 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

7 The Integrated Model Definition of integrated model for offshore wind turbines: An aeroelastic model that includes the dynamic influence of wave loads and foundation/soil stiffness and damping. New topics when moving offshore Stiffness and damping Soil stiffness and deflection, u Wave spectre and 1st tower/pile bending frequency Movements at tower top du/dt Structural analysis of the foundation and soil Load cases Traditional load cases are combined with wave loads

8 How offshore wind farms affect support structure design Support Structures (tower + foundation) reaches % 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

9 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

10 Standards and Codes Offshore Wind Turbines Currently no national nor international Standard/Codes covering Offshore Wind Turbines as an integrated System: IEC and IEC WT01 covers onshore wind turbines but not foundation IEC (not finalised) covers offshore wind turbines but not foundations Eurocodes cover onshore buildings (High Safety Class) not wind turbines IS and API covers offshore oil-and gas fixed structures (High Safety Class (wind turbines not covered) Danish Recommendation for Offshore Wind Turbines, December 2001 Offhore support structure design not covered specifically and based on DS codes

11 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 DNV-OS-J103: Offshore Wind Turbine Electrical Systems, DNV-OS-J104: Offshore Wind Turbine Gear Boxes, 2005

12 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

13 DNV Offshore Standard DNV-OS-J101 New design standard for design of support structure for offshore wind turbines Basis for new DNV rules DNV standard is based on experience from more than 24 offshore wind projects & general rule development from maritime and offshore industries for decades Status Internal and external hearing finalised 16 April 2004 Will be published in June 2004

14 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

15 DNV Rules for Offshore Wind Turbine Structures DNV-OS-J101 Standard covers: Site specific design of wind turbine, support structure and foundation considered 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

16 DNV Rules for Offshore Wind Turbine Structures Content of DNV Rules Design principles Safety levels Site conditions Loads Structural design Materials Corrosion Manufacturing Transport Installation Maintenance Decommissioning Life cycle approach

17 Foundation Solutions Various foundation design covered Focus on Gravity foundation Mono-piles Tripods Other concepts included: Jackets, Hybrids, Floating foundations, Suction buckets etc.

18 DNV Rules for Offshore Wind Structures

19 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

20 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 with 10 5 (β=4.2) being the aim for designs to the normal safety class and 10 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.

21 1) Target Reliability Index Limit state Target reliability index β (design working life) Target annual reliability 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.

22 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.

23 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.

24 Local Joint Flexibility (LJF) Spring characteristics may be found from Buitrago. Beam element Stiff offset element Rotational and/or axial LJF spring Application of joint flexibility will give a more correct distribution of member forces.

25 Influence from mean stress (crack closure) Welded structural details:

26 Fracture Mechanics Fatigue Calculations K = F S F E F T F G S πc c F S = b F E = c b 2 F T = sec( πc / 2t) / 2 c/b = 0.2

27 Fracture Mechanics Fatigue Calculations K ( S F S ) m+ b b πc = Fm S m = tensile membrane geometric stress range component S b = outer-fibre bending geometric stress range component c = crack depth Notch stress S b = S b o 1 t c Geometric stress Nominal stress A B C Section A-A Section B-B Section C-C

28 Fatigue Life Improvement Methods Tubular Girth Weld Profile Grinding: Improved to SN-curve Curve C /Curve 125 (Normally Curve D /Curve 90 without weld profile grinding) Grinding as for tubular joints Large radius grinding removing weld caps and weld toe undercut