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Monopile
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Back to basics…… for Foundation design of Monopile Support Structures
By Victor Krolis
05/12/2007 European Offshore Wind energy conference 2007
Monopile design sequence
The foundation takes about 30% of the total costs for one offshore wind turbine
The turbine manufacturers indirectly “shape” the design criteria for the foundation
Monopile design sequence
Offshore engineers
The turbine manufacturers
Correct direction of input of design criteria?
Monopile design sequence
Offshore engineers
The turbine manufacturers
Mutual input of design criteria seems to be the way
Why mutual input of design criteria? Future:
5 MW and larger turbines
Heavier turbines
Moving into deeper waters
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
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
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?
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
Shouldn’t we go back to basics and evaluate the basic foundation design parameters for these large diameter piles?
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.
Scale effects of large diameter monopiles
Deflection lines of 1m pile according to p-y method & SW method compared to the FE results [University of Duisburg-Essen, K. Lesny])
SWM P-Y method FE
33 %
20 %
SWM P-Y method FE
Pile deflection y [m]D
epth
z
[m]
Scale effects of large diameter monopiles
Deflection lines of 6m pile according to p-y method & SW method compared to the FE results [University of Duisburg-Essen, K. Lesny])
50 %
120 %
SWM P-Y method FE
Pile deflection y [m]D
epth
z
[m]
Effects of high numbers of cyclic loading Cyclic soil degradation: decrease of soil stiffness and strength
Research approach Simulation model:
Monopile:• Various Diameters• Wall thickness – Diameter ratio over
whole • Length of pile is: 1:80
Simulations for : • Vestas V90 • NREL 5MW
Soil profile:• Loose• Medium dense • Dense Sand
Research approach Environmental data:
• Mostly sandy soils
• Wave data from the NEXTRA database
• Wind data from K13 buoy
Scale effects of large diameter monopiles • Suggestion of a modified factor for
the initial coefficient of subgrade modulus k :
a1ref
s*
zz
kz/)z(E)z(k
[University of Duisburg-Essen, K. Lesny]
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
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
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]
0
Effects of high numbers of cyclic loading Important parameters to account for:
• Type of cyclic loading: one-way
two way cyclic loading t
t
Effects of high numbers of cyclic loading Important parameters to account for:
• Type of cyclic loading: one-way
Similar effect as wind loadConservative approach
Effects of high numbers of cyclic loading Important parameters to account for:
• Type of cyclic loading
• Numbers of cyclic loading
• Magnitude of cyclic loading
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
Effects of high numbers of cyclic loading Methods studied to quantify effects of soil stiffness degradation:
• API 2000 (= p-y method)
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:
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:
y.
)z(p.Az.k
tanh).z(p.A)z,y(pu
0,su
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:
y.
)z(p.Az.k
tanh).z(p.A)z,y(pu
0,su
Effects of high numbers of cyclic loading Difference between API & DSPY method:
Lateral pile deflection according to API:
y.
)z(p.Az.k
tanh).z(p.A)z,y(pu
0,su
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
t1hhN N).z(K)z(K
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
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
Effects of high numbers of cyclic loading Simulation approach:
6. Simulate relative pile-soil stiffness ratio as a function of number of cycles
Numerical model for parametric studies Basic design parameters considered are:
• Natural frequency
• Soil stiffness (= subgrade modulus)
• Penetration depth
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, t1k*(z)
MSL
Analytical model for parametric studies Approach:
Perform parametric studies for existing offshore wind turbines such as the Vestas V90 and future turbines NREL 5MW
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
Analytical model for parametric studies With this approach the ability will emerge to constantly relate the preliminary design choices with the rotational frequency ranges
Acknowledgement
This research is sponsored by Geodelft
From January 2007 it will be incorporated in Deltares
www.Deltares.nl