Underwater noise generated by offshore
pile driving
Dr. ir. Apostolos Tsouvalas
Delft University of Technology
Faculty of Civil Engineering
& Geosciences
Section of Structural Mechanics
23-8-2016
1
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Practical motivation
2
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Foundation types
EWEA (2014)
3
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
2-8m
o Length
40 - 80m
o Wall thickness 3 - 12 cm
Pile driving
4
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Methods of installation
Vibratory device
Impact hammer
5
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Environmental assessment per project, seal scarers…
Germany
SPL = 190 dB re 1µPa at 750 m
SEL = 160 dB re 1µPa at 750 m
http://www.phys.unsw.edu.au/jw/hearing.html
7
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise mitigation
Noise prediction
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Wave radiation by impact piling
- impact duration ~ 5 ms
- energy input ~ 900 kJ
t= 6cm
7m
5m
20m
33m
Soil medium consists of 2 layers
and is water-saturated
15
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Wave radiation by impact piling
f(t)
water
Soil layer 1
Soil layer 2
16
Project description
Introduction
Noise prediction
Noise mitigation
Conclusions
“Supersonic” analogy
V~400 m/s
V~0.7 m/s
Water
Soil
Mach 0.7
Mach 1.0
Mach 1.4
17
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Wave radiation from vibro-piling
t= 6cm
7m
fm = 20Hz
5m
20m
33m
Soil medium consists of 2 layers
and is water-saturated
18
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Wave radiation from vibro-piling
f(t)
19
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Concepts of noise mitigation
21
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Principle of noise mitigation
hydraulic hammer
Noise mitigation screen or
air-bubble curtain
direct pressure
waves
receiver
Scholte waves
The primary noise path is
blocked
Secondary noise path
largely unaffected …
(re-radiation through the soil
& interface waves)
Secondary noise
path
22
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
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
(Va=1% & αr=1mm)
23
Introduction
Project description
Noise prediction
Noise mitigation
Conclusions
Concluding remarks
I.
Wave radiation due to impact pile driving consists of:
o
pressure waves in the water
“primary noise path”
o
shear and compressional waves in the soil
o
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