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

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

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

Effects of high numbers of cyclic loading

How can this be quantified for large diameter piles?

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 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 :

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 • 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

Monopile Offshore Wind Turbine

Beam on Elastic Foundation

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

THANK YOU!!