Underwater noise generated by offshore pile driving

Transcription

1 Underwater noise generated by offshore pile driving Dr. ir. Apostolos Tsouvalas Delft University of Technology Faculty of Civil Engineering & Geosciences Section of Structural Mechanics

2 Practical motivation 2

3 Foundation types EWEA (2014) 3

4 From fabrication to installation Upending the monopile Steel monopile compiled of several cross sections welded together to form a single piece Typical dimensions: o Diameter 2-8 m o Length 40-80m o Wall thickness 3-12 cm Pile driving 4

5 Methods of installation Vibratory device Impact hammer 5

6 Environmental impact A high indeed sound pressure can be harmful for the marine life the Dutch government allows to pile only during 6 months of the year other countries impose similar restrictions 6

7 Underwater noise regulation All countries recognize that noise is a problem but every country has its own regulations Regulations in various countries The Netherlands Hammering only at half of the year,no simultaneous construction of more than one OWF, environmental assessment per project UK Germany Environmental assessment per project, seal scarers SPL = 190 dbre 1µPa at 750 m SEL = 160 dbre 1µPa at 750 m 7

8 Aim and objectives Aim of the FLOW project Analyse the underwater noise generated by impact piling Objectives Develop a model for the prediction of the underwater noise Analyse data from measurements and validate the model Investigate theoretically several noise mitigation techniques 8

9 Content of the Presentation A. Noise prediction o Basics of underwater noise generation o Modelling techniques& semi-analytical modelling o Wave radiation due to impact piling & vibro-piling B. Noise mitigation o Available noise mitigation techniques o Modelling the air-bubble curtain 9

10 Content of the Presentation A. Noise prediction o Basics of underwater noise generation o Modelling techniques& semi-analytical modelling o Wave radiation due to impact piling & vibro-piling B. Noise mitigation o Available noise mitigation techniques o Modelling the air-bubble curtain 10

11 Generation of underwater noise Stress wave in the pile Direct sound radiation from the vibrations of the pile (primary noise path) Propagation of waves in the water-saturated soil medium Noise that leaks back into the water from the soil (secondary noise path) Noise that propagates along the seabed-water interface (secondary noise path) 11

12 Modelling techniques I. Finite Element method (near-field) II. Finite Element method (near-field) +acoustic propagation models (far-field) III. Boundary Element method (near-field + far-field) IV. Semi-analytical method (near-field + far-field) 12

13 Modelling techniques I. Finite Element Method (near-field) II. Finite Element Method (near-field) +acoustic propagation models (far-field) III. Boundary Element Method (near-field + far-field) IV. Semi-analytical method (near-field + far-field) gain insight into the physics of noise generation computationally fast for acoustic purposes 13

14 The soil-water-pile model External force at the pile head Thin shell theory including high order effects like shear deformation & rotational inertia 3D inviscid compressible medium (no shear waves!) 3D elastic continuum which allows the co-existence of both shear & compressional waves 14

15 Wave radiation by impact piling 7m t= 6cm -impact duration~5 ms -energy input ~ 900 kj 5m 20m 33m Soil medium consists of 2 layers and is water-saturated 15

16 Wave radiation by impact piling f(t) water Soil layer 1 Soil layer 2 16

17 Supersonic analogy V~400 m/s V~0.7 m/s Water Soil Mach 0.7 Mach 1.0 Mach

18 Wave radiation from vibro-piling 7m 5m t= 6cm f m = 20Hz 20m 33m Soil medium consists of 2 layers and is water-saturated 18

19 Wave radiation from vibro-piling f(t) 19

20 Noise mitigation A. Noise prediction o Basics of underwater noise generation o Modelling techniques and semi-analytical modelling o Wave radiation due to impact piling & vibro-piling B. Noise mitigation o Available noise mitigation techniques o Modelling the air-bubble curtain 20

21 Introduction Project description Noise prediction Noise mitigation Conclusions Concepts of noise mitigation 21

22 Principle of noise mitigation hydraulic hammer Noise mitigation screenor air-bubble curtain The primary noise pathis blocked direct pressure waves receiver Scholte waves Secondary noise path Secondary noise path largely unaffected (re-radiation through the soil & interface waves) 22

23 Modelling the air-bubble curtain Noise radiation in the free-field air-bubble curtain Noise radiation with an air-bubble curtain positioned at10m from the pile (V a =1% & α r =1mm) 23

24 Concluding remarks I. Wave radiation due to impact pile driving consists of: o o o pressure waves in the water primary noise path shear and compressional waves in the soil Scholte waves along the seabed-water interface secondary noise path II. Noise mitigation should be targeted towards eliminating both the primary and the secondary noise paths III. Models have been developed for the prediction of the underwater noise levels for a wide range of hammers and installation configurations(with& without noise mitigation) 24