Pile driving of large diameter monopiles: Current practice and challenges

Transcription

1 Pile driving of large diameter monopiles: Current practice and challenges Peter Meijers Delft University of Technology, The Netherlands Factulty of Civil Engineering and Geosciences Section of Offshore Engineering 11 October 217

2 Introduction Increasing demand for offshore wind energy 1 [Smith, 1962, ] Pile driving of large diameter monopiles P.C. Meijers 2 / 14

3 Introduction Increasing demand for offshore wind energy Large-diameter monopiles D 8 m (216) 1 [Smith, 1962, ] Pile driving of large diameter monopiles P.C. Meijers 2 / 14

4 Introduction Increasing demand for offshore wind energy Large-diameter monopiles D 8 m (216) A drivability study is performed based on the model of Smith 1 1 [Smith, 1962, ] Pile driving of large diameter monopiles P.C. Meijers 2 / 14

5 Introduction Increasing demand for offshore wind energy Large-diameter monopiles D 8 m (216) A drivability study is performed based on the model of Smith 1 Hammer type Number of hammer blows Induced stress levels 1 [Smith, 1962, ] Pile driving of large diameter monopiles P.C. Meijers 2 / 14

6 Mathematical modelling Classical Axisymmetric shell F(t) F(t) z = R, w z, u h r z, u z = L Pile driving of large diameter monopiles P.C. Meijers 3 / 14

7 Dispersion curves Dispersion curves Classical Pile driving of large diameter monopiles P.C. Meijers 4 / 14

8 Dispersion curves Dispersion curves Classical Axisymmetric shell Pile driving of large diameter monopiles P.C. Meijers 4 / 14

9 Ring frequency In a cylindrical shell, strong coupling of axial and radial motions occurs around the ring frequency Dispersion curves Classical Axisymmetric shell f r = 1 E πd (1 ν 2 ) ρ axial 1 radial.5 axial [Hodges et al., 1985, Bozich, 1967, ] Pile driving of large diameter monopiles P.C. Meijers 5 / 14

10 Hammer characteristics Examples of force signals created with the hammer model of Deeks and Randolph. 3 Time signal Normalised amplitude spectrum Long pulse 3 [Deeks and Randolph, 1993, ] Pile driving of large diameter monopiles P.C. Meijers 6 / 14

11 Hammer characteristics Examples of force signals created with the hammer model of Deeks and Randolph Time signal Normalised amplitude spectrum Short pulse 3 [Deeks and Randolph, 1993, ] Pile driving of large diameter monopiles P.C. Meijers 6 / 14

12 Small diameter pile Steel pile with D =.92 m. f r = 19 Hz. E w = % D =.92 m Dispersion curves Classical Axisymmetric shell Pile driving of large diameter monopiles P.C. Meijers 7 / 14

13 Small diameter pile Steel pile with D =.92 m. f r = 19 Hz. E w = % Pile driving of large diameter monopiles P.C. Meijers 7 / 14

14 Large diameter pile Steel pile with D = 7. m. f r = 247 Hz. E w = 1% D =.92 m D = 7. m Dispersion curves Classical Axisymmetric shell Pile driving of large diameter monopiles P.C. Meijers 8 / 14

15 Large diameter pile Steel pile with D = 7. m. f r = 247 Hz. E w = 1% Pile driving of large diameter monopiles P.C. Meijers 8 / 14

16 Energy in converted to radial motion Short pulse: Pile driving of large diameter monopiles P.C. Meijers 9 / 14

17 Energy in converted to radial motion Long pulse: Pile driving of large diameter monopiles P.C. Meijers 9 / 14

18 Summary When? The force signal contains energy around the ring frequency, which decreases with increasing pile diameter. Pile driving of large diameter monopiles P.C. Meijers 1 / 14

19 Summary When? The force signal contains energy around the ring frequency, which decreases with increasing pile diameter. What? Energy around the ring frequency is put in radial motion not axial motion 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 1 / 14

20 Open questions/challenges What is the effect of the radial motion on drivability? bearing capacity? stress levels? number of stress cycles? Pile driving of large diameter monopiles P.C. Meijers 11 / 14

21 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

22 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

23 References Bozich, W. F. (1967). The Vibration and Bucklng Characteristics of Cylindrical Shells Under Axial Load and External Pressure. Technical report. Deeks, A. J. and Randolph, M. F. (1993). Analytical modelling of hammer impact for pile driving. International Journal for Numerical and Analytical Methods in Geomechanics, 17(5): Hodges, C. H., Power, J., and Woodhouse, J. (1985). The use of the sonogram in structural acoustics and an application to the vibrations of cylindrical shells. Journal of Sound and Vibration, 11(2): Smith, E. A. L. (1962). Pile-driving analysis by the wave equation. American Society of Civil Engineers Transactions, 127: Pile driving of large diameter monopiles P.C. Meijers 13 / 14

24 Pile driving of large diameter monopiles: Current practice and challenges Peter Meijers Delft University of Technology, The Netherlands Factulty of Civil Engineering and Geosciences Section of Offshore Engineering 11 October 217