Design standards for floating wind turbine structures

1. Design standards for floating wind turbine structures EWEA 2011 - Brussels

2. Outline of presentation • Background – why is a floater standard needed? • Existing relevant DNV documents • Plans for development of full-fledged standard • Key issues to be covered in development of standard

3. Background for initiating development of floater standard • Existing standards are in practice restricted to bottom-fixed structures only: • IEC61400-3 • DNV-OS-J101 • GL (IV Part 2) • With regard to use for design of floaters, shortcomings of these standards exist with respect to: • Stability • Station keeping • Site conditions (related to LF floater motions) • Floater-specific structural components (tendons, mooring lines, anchors) • Accidental loads • ALS design in intact and damaged condition • Other: Simulation periods, higher order responses, safety level... • DNV guideline 2009 (technical report): • Addresses some of the issues not dealt with in existing standards Photo: Trude Refsahl, Statoil

4. Current DNV documents

5. Next step • Development of full-fledged design standard for floating support structures • Approach • Joint Industry Project – industry involvement • Quality assurance by • Technical reference group • Internal and external hearings

6. Technical issues to be covered by design standard • Safety philosophy and design principles • Site conditions, loads and response • Materials and corrosion protection • Structural design • Foundation design • Stability • Station keeping • Control and protection system • Mechanical system and electrical system • Transport and installation • In-service inspection, maintenance and monitoring K. Ronold

7. Safety philosophy and design principles; safety class approach • Safety philosophy as for fixed wind turbine structures in DNV-OS-J101 • Safety class methodology; three classes are considered depending on severity of failure consequences: • Low • Normal • High • Target failure probability is set depending on required safety class • Design principles • Partial safety factor method • Requirements for partial safety factor are set depending on required target failure probability • Safety class • It is important to determine/decide an adequate safety class for the various structural components of floating wind turbine structures L.C. Nøttaasen

8. What shall the safety level be for floating support structures? • The target safety level of the existing standards is taken as equal to the safety level for wind turbines on land as given in IEC61400-1, i.e. normal safety class • The scope for this fixed target safety level has been expanded several times: • Extrapolation from smaller turbines to larger turbines • Extrapolation from onshore turbines to offshore turbines • Extrapolation from turbine+tower to support structure • Extrapolation from individual structures to multiple structures in large wind farms • Cost-benefit analyses would likely show a need to go up one safety class, from normal to high, at least for some structural components • The DNV guideline for floating wind turbine structures recommends design of station keeping system to high safety class (with a view to consequences of failure) • Target safety level is likely to depend on the number of turbines in the wind farm

9. Site conditions – particular issues for floaters Special issues to be considered relative to current requirements in existing codes: • Adequate representation of wind in low frequency range • Adequate representation of dynamics may require more thorough/improved representation of simultaneous wind, waves and current • Gust events based on gust periods in excess of 12 sec must be defined; must cover expected events and reflect frequencies encountered for dynamics of floaters • For floaters which can be excited by swell, the JONSWAP wave spectrum is insufficient and an alternative power spectral density model must be applied • For tension leg platforms, water level and seismicity may be of significant importance

10. Loads – particular issues for floaters Special issues to be considered relative to current practice for bottom-fixed structures: • Simulation periods to be increased from standard 10 min to 3 to 6 hrs • Purpose: Capture effects of nonlinearities, second-order effects, slowly varying responses • Challenge: Wind is not stationary over 3- to 6-hr time scales • Loads associated with station keeping system include permanent loads • Pretension of tendons (permanent load) • Pretension of mooring lines (permanent load) • Ship impact loads (from maximum expected service vessel) need more thorough docu- mentation than for bottom-fixed structures • Larger consequences of ship collision • Motion of two bodies with different motion characteristics Photo: Øyvind Hagen, Statoil

11. Loads – continued • Additional load cases to be defined, accounting for • Changes necessitated by new/additional gust events • The fact that the control system is used to keep turbine in place by preventing excitations • Accidental loads to be considered. Examples: • Dropped objects • Change of intended pressure difference • Unintended change in ballast distribution • Trawling • Collision impact from unintended ship collisions • Explosions and fire A. Grimsby

12. Structural design • Reliability-based calibration of partial safety factor requirements for design of structural components not covered by DNV-OS-J101 • Examples: tendons, mooring lines • Existing design standards from other industries may be capitalized on: • DNV-OS-C101 and DNV-OS-C105 for tendons • DNV-OS-E301 for mooring lines • Difficulties because of different definition of characteristic loads • Shortcomings because of rotor-filtrated wind loads are not covered by existing standards • Need for data to define a representative set of design situations for safety factor calibrations • Load and response data for various structural components • Full scale data (example Hywind) • Model scale data • Data from analytical models

13. Stability • Sufficient floating stability is an absolute requirement • In operation phase and in temporary phases • In intact as well as in damaged condition • Additional compartmentalization is usually not required for unmanned structures • The need for a collision ring in the splash zone depends on • Manned/unmanned • Substructure material (concrete/steel/composites) • Size of service vessel and resistance against ship impacts • Location and design of manholes and hatches to be carried out with a view to avoid water ingress • Dropped objects and ship collisions may pose threats to stability C.F. Salicath

14. Station keeping • Three types are foreseen: • Catenary or taut systems of chain, wire or fibre ropes • Tendon systems of metal or composites for restrained systems such as TLPs • Dynamic positioning • Various issues for catenary and taut moorings: • Mooring system is vital for keeping wind turbine in position such that it can produce electricity and maintain transfer of electricity to receiver • Optimization of mooring systems may lead to non-redundant systems where a mooring failure may lead to loss of position and conflict with adjacent wind turbines • Sufficient yaw stiffness of the floater must be ensured • Various issues for tendon systems: • Systems with only one tendon will be compliant in roll and pitch • Floaters with restrained modes will typically experience responses in three ranges of frequencies • High frequency, wave frequency, low frequency • More complex to analyse than other structures • Terminations are critical components, regardless of whether tendon is metallic or composite

15. Needs for information • Load/response data for various structural components • tendons • mooring lines • structural components in floater from • analysis models • model tests • full scale measurements • Wind data for definition of new gust events • Wind data in low frequency range • Ship impact load data • Data for accidental loads and frequencies of accidental events causing damage of wind turbine structure Courtesy: Statoil

16. Acknowledgments Illustration contributions from • Statoil • C.F. Salicath • L.C. Nøttaasen • A. Grimsby are gratefully acknowledged

17. Thank you Thank you for your attention

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