Back to basics…… for Foundation design of Monopile Support Structures

1. By Victor Krolis Back to basics…… for Foundation design of Monopile Support Structures 05/12/2007 European Offshore Wind energy conference 2007

2. Monopile design sequence The turbine manufacturers indirectly “shape”the design criteriafor the foundation The foundation takes about 30%of the total costs for one offshore wind turbine

3. Monopile design sequence The turbine manufacturers Correct direction of input of design criteria? Offshore engineers

4. Monopile design sequence The turbine manufacturers Mutual input of design criteria seems to be the way Offshore engineers

5. Why mutual input of design criteria? Future: 5 MW and larger turbines

6. Why mutual input of design criteria? Future: 5 MW and larger turbines Heavier turbines

7. Why mutual input of design criteria? Future: 5 MW and larger turbines Heavier turbines Moving into deeper waters

8. Why mutual input of design criteria? Future: 5 MW and larger turbines Heavier turbines Moving into deeper waters Larger Monopiles (> 5 m.) are needed since this is still an attractive type of support structure economic wise

9. Goal: To quantify the effects of design choices on the total mass (= €)by visualizing the mutual influences of basicdesign parameters such as the natural frequency, soil stiffnessandthe penetration depth

10. So…If larger pile diameters are needed, may the current API design methods be correlated to largediameter piles and still be considered to be an efficient methodof foundation design?

11. So…If larger pile diameters are needed, may the current API design methods be correlated to largediameter piles and still be considered to be an efficient methodof foundation design? API is based on empirical research conducted on pile diameters ranging from 0.2 to 2 meters

12. How due high numbersof cyclic loading effectthese large diameter piles?

13. Shouldn’t we go back to basics and evaluate the basic foundationdesign parametersfor these largediameter piles?

14. Answer:YES!!Why?

15. Scale effects of large diameter monopiles • p-y method can become unconservative for large diameter piles: • University of Duisburg-Essen performed Finite Element simulations for piles ranging from 1 to 6 m.

16. 33 % SWM P-Y method FE SWM P-Y method FE 20 % Scale effects of large diameter monopiles Pile deflection y [m] Depth z [m] Deflection lines of 1m pile according to p-y method & SW method compared to the FE results [University of Duisburg-Essen, K. Lesny])

17. 50 % SWM P-Y method FE 120 % Scale effects of large diameter monopiles Pile deflection y [m] Depth z [m] Deflection lines of 6m pile according to p-y method & SW method compared to the FE results [University of Duisburg-Essen, K. Lesny])

18. Effects of high numbers of cyclic loading • Cyclic soil degradation: decrease of soil stiffness and strength

19. Effects of high numbers of cyclic loading • How can this be quantified for large diameter piles?

20. Simulations for : • Vestas V90 • NREL 5MW • Soil profile: • Loose • Medium dense • Dense Sand Research approach Simulation model: • Monopile: • Various Diameters • Wall thickness – Diameter ratio over whole • Length of pile is: 1:80

21. Research approach Chosen location:

22. Research approach • Environmental data: • Mostly sandy soils • Wave data from the NEXTRA database • Wind data from K13 buoy

23. Scale effects of large diameter monopiles • Suggestion of a modified factor for the • initial coefficient of subgrade modulus k : [University of Duisburg-Essen, K. Lesny]

24. Effects of high numbers of cyclic loading • Cyclic soil degradation: decrease of soil stiffness and strength • Structural ‘shakedown’: stabilizing of permanent deflections after N number of cycles. If not…the pile will fail

25. Effects of high numbers of cyclic loading • Cyclic soil degradation: decrease of soil stiffness and strength • Structural ‘shakedown’: stabilizing of permanent deflections after N number of cycles. If not…the pile will fail

26. KsN (z) [N/m] 0 Increasing number of load cycles N [-] Effects of high numbers of cyclic loading • Cyclic soil degradation: decrease of soil stiffness and strength

27. Effects of high numbers of cyclic loading • Important parameters to account for: • Type of cyclic loading: • one-way • two way cyclic loading t t

28. Similar effect as wind load Conservative approach Effects of high numbers of cyclic loading • Important parameters to account for: • Type of cyclic loading: • one-way

29. Effects of high numbers of cyclic loading • Important parameters to account for: • Type of cyclic loading • Numbers of cyclic loading • Magnitude of cyclic loading

30. Effects of high numbers of cyclic loading • Methods studied to quantify effects of soil stiffness degradation: • API 2000 (= p-y method) • Deterioration of Static p-y Curve (DSPY) method

31. Effects of high numbers of cyclic loading • Methods studied to quantify effects of soil stiffness degradation: • API 2000 (= p-y method)

32. Effects of high numbers of cyclic loading • API 2000 (= p-y method)

33. Effects of high numbers of cyclic loading • Difference between API & DSPY method: • API recommends a factor of A = 0.9 to reckon with stiffness degradation:

34. Effects of high numbers of cyclic loading • Difference between API & DSPY method: • API recommends a factor of A = 0.9 to reckon with stiffness degradation: • Lateral pile deflection according to API:

35. Effects of high numbers of cyclic loading • Difference between API & DSPY method: • API recommends a factor of A = 0.9 to reckon with stiffness degradation: • Lateral pile deflection according to API:

36. Effects of high numbers of cyclic loading Difference between API & DSPY method: Lateral pile deflection according to API:

37. Effects of high numbers of cyclic loading • DSPY: • KhN= horizontal subgrade modulus at N cycle [N/m²] • KhN = horizontal subgrade modulus at first cycle [N/m²] • t = factor that takes into account the • type of cyclic loading, installation method, soil • density & precycled piles

38. Effects of high numbers of cyclic loading Simulation approach: 1. Model with environmental data available 2. Simulate for static load case  determines static APIp-y curves and static lateral deflections 3. Determine cyclicp-y curves with DSPY method 4. Simulate cyclic load case  determines cyclicAPIp-y curves

39. Esoil Effects of high numbers of cyclic loading Simulation approach: 5. Compare cyclicAPI p-y curves with cyclic DSPYp-y curves  rate of degradation of Kh can be determined for both cases and compared

40. Effects of high numbers of cyclic loading Simulation approach: 6. Simulate relative pile-soil stiffness ratio as a function of number of cycles

41. Numerical model for parametric studies • Basic design parameters considered are: • Natural frequency • Soil stiffness (= subgrade modulus) • Penetration depth

42. Numerical model for parametric studies Beam on Elastic Foundation Monopile Offshore Wind Turbine

43. L3, D3, t3 MSL L2, D2, t2 L1, D1, t1 k*(z) Numerical model for parametric studies • The model: • Three sections with various diameter, wall thickness and length • Modified subgrade modulus included • Variationof mass turbine

44. Analytical model for parametric studies Approach: Perform parametric studies for existing offshore wind turbines such as the Vestas V90 and futureturbines NREL 5MW

45. Analytical model for parametric studies Make 3D diagrams in which the effect of the diameter on the natural frequency, soil stiffness and penetration depthis visualized

46. Analytical model for parametric studies With this approach the ability will emerge to constantly relate the preliminary design choices with the rotational frequency ranges

47. Acknowledgement This research is sponsored by Geodelft From January 2007 it will be incorporated in Deltares www.Deltares.nl

48. THANK YOU!!