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

2. Monopile design sequence
The turbine manufacturers indirectly “shape” the design criteria for 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 basic design parameters such as the natural frequency, soil stiffness and the penetration depth

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

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

12. How due high numbers of cyclic loading effect these large diameter piles?

13. Shouldn’t we go back to basics and evaluate the basic foundation design parameters for these large diameter 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. Scale effects of large diameter monopiles
Pile deflection y [m]
33 %
SWM
P-Y method FE
Depth z [m]
SWM
P-Y method FE
20 %
Deflection lines of 1m pile according to p-y method & SW method compared to the FE results [University of Duisburg-Essen, K. Lesny])

17. Scale effects of large diameter monopiles
Pile deflection y [m]
50 %
Depth z [m]
SWM
P-Y method FE
120 %
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. Research approach Simulation model: Simulations for : Vestas V90
NREL 5MW
Soil profile:
Loose
Medium dense
Dense Sand
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. Effects of high numbers of cyclic loading
Cyclic soil degradation: decrease of soil stiffness and strength
Increasing number of load cycles N [-]
KsN (z) [N/m]

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

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 API p-y curves and static lateral deflections
3. Determine cyclic p-y curves with DSPY method
4. Simulate cyclic load case  determines
cyclic API p-y curves

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

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

44. Analytical model for parametric studies
Approach:
Perform parametric studies for existing offshore wind turbines such as the Vestas V90 and future turbines 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 depth is 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

48. THANK YOU!!