Simulation of noise generation when driving large monopiles · Simulation of noise generation when driving large monopiles João Saraiva and Hendrik Versteele . Abaqus Benelux Users

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


• 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


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.


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


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


(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)


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



• 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;


Water

(acoustic medium)

Sand

(elastic medium)

28m

40m

64m

30m



• 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;


Water

(acoustic medium)

Sand

(elastic medium)

28m

40m

64m

30m



• Mesh:


26 m3.9 m

17.3 m



• 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).



Noise simulation – water pressure


Dynamic water pressure – outside pile

T = 0ms


T = 4ms



T = 6ms



T = 8ms



T = 12ms



T = 16ms



T = 20ms



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


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


Pressure wave – at 18m outside

Increasing distance – time delay + attenuation

Pile-soil

interface18.0 m depth


Pressure wave inside and outside

Softened blow, MHU2100

Inside

Outside

Pressures (noise) higher inside and frequency content is different


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


Pressure wave – radial acceleration

Correlation of pressure wave and radial acceleration indicates way forward to

predict source level noise from wave equation simulations.


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:


• 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

Documents

Simulation of noise generation when driving large monopiles · Simulation of noise generation when driving large monopiles João Saraiva and Hendrik Versteele . Abaqus Benelux Users