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Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Cathie Associates
Simulation of noise generation when driving large
monopiles
João Saraiva and Hendrik Versteele
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
• Background
• Simulation of sound (water) pressure waves caused
by impact pile driving
– Abaqus: Axisymmetric FEA model
• What can be done during pile design to reduce
noise generation?
– Effect of hammer signature
– Geotechnical engineering options
Overview
Simulation of noise generation when driving large monopiles
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Background
• Steel monopiles are increasingly being
used for windturbine offshore foundations;
• Pile installation using impact hammers:
- Hammer strikes the pile;
- Compression wave propagates through the
pile;
- Wave is radiated to adjacent areas (water
and soil);
- Wave is reflected from the pile tip.
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Background
• German regulations:
< 190dB peak sound level at 750m;
< 160dB SEL (sound exposure level) at
750m.
• Monopile foundation sizes are
increasing (>7m diameter):
– Increased driving resistance;
– Increased hammer size;
– Increased noise levels due to hammer
energy;
– Increased noise levels due to diameter
effect.
• What can be done?
50kPa=214dB
200kPa=226dB
1kPa=180dB
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Background
• Mitigation measures:
– Bubble curtains;
– Sleeves;
– Etc.
• What else can be done?
– Noise source reduction measures;
– Geotechnical measures.
RAVE/PAV - Restemeier et
al., 2012
Koschinski & Lüdemann, 2014
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
(in addition to “conventional” measures)
What can be done to reduce noise?
1. Reduce noise source (change stress wave in the
pile)
2. Reduce soil resistance during driving
3. Installation by vibration (not topic of this
presentation)
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise source simulation
• Monopile
– 8m OD x 100mm WT;
– Total length 64m;
– Embedded length 32m;
– Solid elements: CAX4R;
– 15cm width x 1cm height (10 elements
across pile wall);
– Material: Linear elastic (steel);
– Unit Weight = 78.5 kN/m3;
– Young’s Modulus = 210 GPa;
– Poisson’s ratio = 0.3;
Axisymmetric FEA model
Water
(acoustic medium)
Sand
(elastic medium)
28m
40m
64m
30m
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise source simulation
• Soil:
– Solid brick elements: CAX4R;
– 15cm width and height;
– Material: Linear elastic;
– Unit weight = 10 kN/m3;
– Young’s Modulus = 260 MPa;
– Poisson’s ratio = 0.3;
• Pile-soil interface:
– Displacement vs Stress coupling;
– Elastic perfect plastic;
Axisymmetric FEA model
Water
(acoustic medium)
Sand
(elastic medium)
28m
40m
64m
30m
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise source simulation
• Water:
– Accoustic elements: ACAX4;
– 15cm width and height;
– Material: Accoustic medium;
– Unit weight = 10 kN/m3;
– Bulk Modulus = 2.2 GPa;
– No cavitation modelled;
• Water-pile interface:
– Acceleration – Pressure coupling;
Axisymmetric FEA model
Water
(acoustic medium)
Sand
(elastic medium)
28m
40m
64m
30m
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise source simulation
• Mesh:
Axisymmetric FEA model
26 m3.9 m
17.3 m
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise source simulation
• Loads:
– Different impact hammer signals are
evaluated.
• Boundary Conditions:
– Displacement boundaries;
– Absorbing boundaries: not considered.
• Analysis steps:
– Geostatic step (pile-soil-water equilibrium);
– Apply hammer blow (dynamic implicit).
• Calculation time = 3.5 hours (4 cpus + gpu).
Axisymmetric FEA model
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Noise simulation – water pressure
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Dynamic water pressure – outside pile
T = 0ms
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 4ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 6ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 8ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 12ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 16ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
T = 20ms
Dynamic water pressure – outside pile
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave – control points
depth 0.2m
depth 6.5m
depth 12.5m
depth 23.5m
depth 27.8m
depth 18.0m
range 24m
1.0m from wall
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave – at 1m outside
Time delay as depth increases
Pile
-so
il in
terf
ace
1.0 m away
from pile wall
Peak suction higher than compressive wave
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave – at 18m outside
Increasing distance – time delay + attenuation
Pile-soil
interface18.0 m depth
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave inside and outside
Softened blow, MHU2100
Inside
Outside
Pressures (noise) higher inside and frequency content is different
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave – softened hammer
blow
Softened blow, MHU2100
Control point at 18m depth, 1m away from the pile
Cushioned blow reduces peak pressure and changes frequency content
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Pressure wave – radial acceleration
Correlation of pressure wave and radial acceleration indicates way forward to
predict source level noise from wave equation simulations.
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
Driving down the SRD
Measure Potential to reduce SRD
Friction reducer 25 %
Low friction coating 20 %
Pressure differential 25 %
Combined effects >50 %
• Based on the literature and from the FE results, mitigation measures will be
required to meet the German regulations;
• A lower Soil Resistance during Driving (SRD) leads to a reduced hammer
size/energy;
• Potential measures to reduce the noise during impact driving:
Abaqus Benelux Users Meeting 2015 | 29 - 30 October, 2015 Brussels Paris Newcastle London Hamburg Milan
• Water pressure waves propagation can be studied using
Abaqus;
• Effect of hammer signature:
– useful reductions in acoustic pressure can be obtained;
– What is the effect of the pressure wave inside the pile and in
soil?
• Geotechnical approaches to reduce SRD for monopiles:
– Driving shoe;
– Low friction coatings?
– Suction assisted installation;
>50% reductions in SRD by combination of geotechnical measures
seems feasible.
• Vibration…not to be forgotten.
Conclusions
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