Transcript Back to basics…… for Subsoil design of Monopile Support

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 turbine
manufacturers
indirectly “shape”
the design criteria
for the foundation
The foundation takes
about 30% of the
total costs for one
offshore wind
turbine
Monopile design
sequence
The turbine
manufacturers
Correct direction of
input of design
criteria?
Offshore engineers
Monopile design
sequence
The turbine
manufacturers
Mutual input of
design criteria seems
to be the way
Offshore engineers
Why mutual input of design criteria?
Future:
5 MW and larger turbines
Why mutual input of design criteria?
Future:
5 MW and larger turbines
Heavier turbines
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
How due high numbers of cyclic loading
effect these large diameter piles?
Shouldn’t we go back to basics and
evaluate the basic foundation design
parameters for these large diameter
piles?
Answer:
YES!!
Why?
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
Pile deflection y [m]
Depth z [m]
33 %
SWM
SWM
P-Y method
P-Y method
FE
FE
20 %
1m
Deflection lines of
pile according to p-y method & SW method compared to the
FE results [University of Duisburg-Essen, K. Lesny])
Scale effects of large diameter monopiles
Pile deflection y [m]
Depth z [m]
50 %
SWM
P-Y method
FE
120 %
6m
Deflection lines of
pile according to p-y method & SW method compared to the FE results
[University of Duisburg-Essen, K. Lesny])
Effects of high numbers of cyclic loading
Cyclic soil degradation: decrease of soil
stiffness and strength
Effects of high numbers of cyclic loading
How can this be quantified for large
diameter piles?
Research approach
Simulation model:
Simulations for :
• Vestas V90
• NREL 5MW
Monopile:
• Various Diameters
• Wall thickness – Diameter ratio over whole
• Length of pile is: 1:80
Soil profile:
• Loose
• Medium dense
• Dense Sand
Research approach
Chosen location:
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 :
 zref
k ( z )  Es ( z ) / z  k 
 z
*



1a
[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
KsN (z) [N/m]
• Cyclic soil degradation: decrease of soil
stiffness and strength
0
Increasing number of load cycles N [-]
Effects of high numbers of cyclic loading
Important parameters to account for:
• Type of cyclic loading:
one-way
t
two way cyclic loading
t
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
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
• 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:
 k s ,0 .z

p( y , z )  A. pu ( z ).tanh
.y 
 A. pu ( z ) 
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:
 k s ,0 .z

p( y , z )  A. pu ( z ).tanh
.y 
 A. pu ( z ) 
Effects of high numbers of cyclic loading
Difference between API & DSPY method:
Lateral pile deflection according to API:
 k s ,0 .z 
p( y , z )  A. pu ( z ).tanh
.y 
 A. pu ( z ) 
Effects of high numbers of cyclic loading
• DSPY:
K hN ( z )  K h1( z ).N
t
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
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
Monopile Offshore Wind
Turbine
Beam on Elastic
Foundation
Numerical model for parametric studies
The model:
• Three sections with
various diameter, wall
L3, D3, t3
thickness and length
• Modified subgrade
modulus included
MSL
L2, D2, t2
• Variation of mass
turbine
k*(z)
L1, D1, t1
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
THANK YOU!!