Pile driving of large diameter monopiles:Current practice and challenges

Peter Meijers

Delft University of Technology, The NetherlandsFactulty of Civil Engineering and Geosciences

Section of Offshore Engineering

11 October 2017

Introduction

Increasing demand for offshore wind energy

Large-diameter monopiles

D ≈ 8 m (2016)

A drivability study is performed based onthe model of Smith1

• Hammer type

• Number of hammer blows

• Induced stress levels

1[Smith, 1962, ]

Pile driving of large diameter monopiles – P.C. Meijers 2 / 14

Introduction

Increasing demand for offshore wind energy

Large-diameter monopiles

D ≈ 8 m (2016)

A drivability study is performed based onthe model of Smith1

• Hammer type

• Number of hammer blows

• Induced stress levels

1[Smith, 1962, ]

Pile driving of large diameter monopiles – P.C. Meijers 2 / 14

Introduction

Increasing demand for offshore wind energy

Large-diameter monopiles

D ≈ 8 m (2016)

A drivability study is performed based onthe model of Smith1

• Hammer type

• Number of hammer blows

• Induced stress levels

1[Smith, 1962, ]

Pile driving of large diameter monopiles – P.C. Meijers 2 / 14

Introduction

Increasing demand for offshore wind energy

Large-diameter monopiles

D ≈ 8 m (2016)

A drivability study is performed based onthe model of Smith1

• Hammer type

• Number of hammer blows

• Induced stress levels

1[Smith, 1962, ]

Pile driving of large diameter monopiles – P.C. Meijers 2 / 14

Mathematical modelling

Classical Axisymmetric shell

z, u

z = L

z = 0

F(t)

hr

F(t)

R, w

z, u

Pile driving of large diameter monopiles – P.C. Meijers 3 / 14

Dispersion curves

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

3.5

4Dispersion curves

Classical

Pile driving of large diameter monopiles – P.C. Meijers 4 / 14

Dispersion curves

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

3.5

4Dispersion curves

ClassicalAxisymmetric shell

Pile driving of large diameter monopiles – P.C. Meijers 4 / 14

Ring frequency

In a cylindrical shell, strong coupling of axial and radial motions occursaround the ring frequency2

fr =1

πD

√E

(1 − ν2) ρ

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

3.5

4Dispersion curves

axial

axial

radial

ClassicalAxisymmetric shell

2[Hodges et al., 1985, Bozich, 1967, ]

Pile driving of large diameter monopiles – P.C. Meijers 5 / 14

Hammer characteristics

Examples of force signals created with the hammer model of Deeks andRandolph.3

0 5 10 150

20

40

60

80

Time signal

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.2

0.4

0.6

0.8

1Normalised amplitude spectrum

Long pulse

3[Deeks and Randolph, 1993, ]

Pile driving of large diameter monopiles – P.C. Meijers 6 / 14

Hammer characteristics

Examples of force signals created with the hammer model of Deeks andRandolph.3

0 5 10 150

2

4

6

8

10

Time signal

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.2

0.4

0.6

0.8

1Normalised amplitude spectrum

Short pulse

3[Deeks and Randolph, 1993, ]

Pile driving of large diameter monopiles – P.C. Meijers 6 / 14

Small diameter pile

Steel pile with D = 0.92 m. fr = 1900 Hz. Ew = 0%

00.51

D = 0.92 m

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

3.5

4Dispersion curves

ClassicalAxisymmetric shell

Pile driving of large diameter monopiles – P.C. Meijers 7 / 14

Small diameter pile

Steel pile with D = 0.92 m. fr = 1900 Hz. Ew = 0%

0 20 40 60 80 100

0

5

10

1510 -3

Pile driving of large diameter monopiles – P.C. Meijers 7 / 14

Large diameter pile

Steel pile with D = 7.0 m. fr = 247 Hz. Ew = 10%

00.51

D = 0.92 mD = 7.0 m

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

3.5

4Dispersion curves

ClassicalAxisymmetric shell

Pile driving of large diameter monopiles – P.C. Meijers 8 / 14

Large diameter pile

Steel pile with D = 7.0 m. fr = 247 Hz. Ew = 10%

0 20 40 60 80 100

0

5

10

1510 -3

Pile driving of large diameter monopiles – P.C. Meijers 8 / 14

Energy in converted to radial motion

Short pulse:

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 120

5

10

15

20

25

Pile driving of large diameter monopiles – P.C. Meijers 9 / 14

Energy in converted to radial motion

Long pulse:

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 120

5

10

15

20

25

Pile driving of large diameter monopiles – P.C. Meijers 9 / 14

Summary

When?The force signal contains energy around the ring frequency, whichdecreases with increasing pile diameter.

What?

• Energy around the ring frequency is put in radial motion not axialmotion

• Slow propagation of this energy because of low group velocity

• Oscillations of u and w with the ring frequency

Pile driving of large diameter monopiles – P.C. Meijers 10 / 14

Summary

When?The force signal contains energy around the ring frequency, whichdecreases with increasing pile diameter.

What?

• Energy around the ring frequency is put in radial motion not axialmotion

• Slow propagation of this energy because of low group velocity

• Oscillations of u and w with the ring frequency

Pile driving of large diameter monopiles – P.C. Meijers 10 / 14

Open questions/challenges

• What is the effect of the radial motion on• drivability?• bearing capacity?• stress levels?• number of stress cycles?

• Should we avoid it or take advantage of it?

Pile driving of large diameter monopiles – P.C. Meijers 11 / 14

Open questions/challenges

• What is the effect of the radial motion on• drivability?• bearing capacity?• stress levels?• number of stress cycles?

• Should we avoid it or take advantage of it?

Pile driving of large diameter monopiles – P.C. Meijers 11 / 14

Acknowledgments

This work is part of the research programme EUROS, which is (partly)financed by the Netherlands Organisation for Scientific Research(NWO).

Pile driving of large diameter monopiles – P.C. Meijers 12 / 14

References

Bozich, W. F. (1967). The Vibration and Bucklng Characteristics of Cylindrical ShellsUnder Axial Load and External Pressure. Technical report.

Deeks, A. J. and Randolph, M. F. (1993). Analytical modelling of hammer impact for piledriving. International Journal for Numerical and Analytical Methods in Geomechanics,17(5):279–302.

Hodges, C. H., Power, J., and Woodhouse, J. (1985). The use of the sonogram instructural acoustics and an application to the vibrations of cylindrical shells. Journal ofSound and Vibration, 101(2):203–218.

Smith, E. A. L. (1962). Pile-driving analysis by the wave equation. American Society ofCivil Engineers Transactions, 127:1145–1193.

Pile driving of large diameter monopiles – P.C. Meijers 13 / 14

Pile driving of large diameter monopiles:Current practice and challenges

Peter Meijers

Delft University of Technology, The NetherlandsFactulty of Civil Engineering and Geosciences

Section of Offshore Engineering

11 October 2017