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Optimization of floating support structures for deep water wind turbines

Optimization of floating support structures for deep water wind turbines. EWEA 2011 14-17 March 2011, Brussels, Belgium by Petter Andreas Berthelsen MARINTEK. Background and motivation. Growing interest for floating wind turbines (FWT) Limited access to shallow water areas world wide

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Optimization of floating support structures for deep water wind turbines

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  1. Optimization of floating support structures for deep water wind turbines EWEA 2011 14-17 March 2011, Brussels, Belgium by Petter Andreas Berthelsen MARINTEK

  2. Background and motivation • Growing interest for floating wind turbines (FWT) • Limited access to shallow water areas world wide • Can be installed further offshore • in areas with stronger and steadier wind • with less visual impact • Potential is huge provided cost can be brought down to a competitive level • Additional technical challenges for FWT • E.g. exposed to wave induced motions • Conceptual design need to • Limit wave and wind induced motions • Minimize cost There is a need for efficient methods for optimal design of floating offshore wind turbines

  3. Scope of work • Develop a tool for: Optimization of floater and mooring system for a given wind turbine size and given design requirements. Optimization in this context is the same as minimizing the material cost while satisfying functional and safety related design requirements • Based on existing software tools • Consider optimization of Spar type floaters only

  4. Design tool • WINDOPT • Efficient design tool for minimum cost design offloatingsupport structures, includingmooring system and cable connection • Spar type concepts • Building block for WINDOPT • MOOROPT – Optimization tool for mooring- and riser systems • MIMOSA 6.3 – Response analysis in frequency domain • WF+LF motion • Mooring and power cable forces • WAMOF 3 – Hydrodynamic response analysis of slender structures • NLPQL – Nonlinear optimisation with arbitrary constraints • Usefultoolfor conceptual design and parametric studies

  5. Problem formulation • 3 key items for optimization • Objective function Cost function to be minimized: Spar buoy cost + mooring line cost • Constraints Design requirements: Floater motions, heel angle, nacelle accelerations, capacity (safety factors), offset limitations, etc… • Variables Design parameters influencing the objective function and/or constraint related responses: Spar diameter and length, mooring line diameter, lengths, pretension, etc…

  6. Parameterizations and cost model • Need a simplified representation of the spar buoy • Assume that a representative mass and cost figure for an initial structure is available • Mass per unit length governed by depth and diameter • Spar buoy cost • Mooring line cost Diameter dependency Depth dependency

  7. Constraints • Spar buoy: • Maximum spar draught • Maximum tower inclination • Max/min requirements on heave and pitch period • Maximum nacelle acceleration • Mooring lines: • Maximum mooring line tension • Maximum allowable horizontal offset • Minimum static horizontal pretension (for minimum yaw stiffness) • …

  8. Variables • Spar buoy • Cylinder variables • Height and diameter • Diameter of footing • Vertical positionof fairleads • Mooring system • Line direction • Pretension or distance to anchor • Segment length • Segment diameter • Tower and turbinearefixed Col 1 Water plane section Col 2 Transition section Col 3 Main buoyancy section Col 4 Heavy ballast section Footing, Bottom plate Mooring lines Power export cable

  9. Analysis option • Extreme response analysis • Repeated NCASE times, each one with NENV different environments. • Typical cases • Operational (rotor running) • Survival (passive rotor) • Damage (line break, etc…) • Check for design constraints • Fatigue response calculation • Check for fatigue life constraints (mooring lines only) (not included in present work)

  10. Example • Water depth is 320 m • Extreme conditions: Reference case based on: NREL/IEA OC3 5MW turbine

  11. Example: Floater definitions • Initial data: • Material mass- and cost assumption • Consider three different design options

  12. Example: Performance • Constraints Max allowable draught is 120 m • Results

  13. Example: Spar shape

  14. Example: Cost Mooring lines:

  15. Concluding remarks • Useful tool for finding improved solutions for floating support and mooring system • Efficient for parametric studies and valuable for early evaluations of spar concepts • (not intended as detailed simulation tool) • Weight and cost models can be further improved • Realistic design constraints need to be obtained • Further development to be considered are: • Fatigue life constraints (completed, but not included in present work) • Optimize power cable (completed, but not included in present work) • Include tower design • Other types of floaters • Improved description of wind loads

  16. Acknowledgement • This work has been carried out as part of NOWITECH (Norwegian Research Centre for Offshore Wind Technology) which is co-funded by the Research Council of Norway, and participating industrial companies and research organisations (www.nowitech.no)

  17. References • Fylling and Berthelsen (2011), WINDOPT – An optimization tool for floating support structures for deep water wind turbines, OMAE 2011

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