noise and vibrations in the petroleum industry

Working togetherfor a safer world

Noise and vibrations in the petroleum industry

IPTC, Doha, 21st January 2014


Consultancy in Lloyd’s Register

• Lloyd’s Register started out as a classification society 250 years ago

• Now Lloyd’s Register covers a wide range of services to the marine and energy business including consulting.

• Consulting includes among several areas Engineering Dynamics and hereunder Noise and Vibrations


Measuring Noise

• We use state of the art tools for measuring and analysing noise.

• What can we measure?

• Noise level

• Sound power of machines

• Reverberation time of rooms

• Absorption

• Insulation

• Etc.

Power: Watts(sound)

Sound Pressure: dB


Measuring Vibrations

• What can we measure

• Vibration levels

• Mobility

• Natural frequencies

• Impact test or operational modal analysis (OMA)


Pre-construction analysis

• Finding and preventing the problems before they occur.

• Internal noise

0

5

10

02

46

810

1214

1618

00.5

1

90

91

92

93

94

95

96

97

98

99

100


Lp,1 Lp,2 = ?

Calculating acoustic absorption and insulation needed for living quarters


Outdoor noise modelling


Modelling of structure-borne noise

Lv1

Lv2 Lv3

Lv4

LW2

LW1

LW4

LW3


Reducing vibrations from machinery

• Finite Element Modelling (FEM) to calculate mobility of support structures

Excitation points:


Modelling as part of troubleshooting

• Modelling coupled with measurements is a powerful tool for solving many different problems


Case study

• High levels of tonal noise from 45 t/h steam boiler

• Preventing delivery of an oil tanker

• Tones increase in frequency with increase in boiler load

• Increase in flow speed and temperature

• Tones appear very suddenly following a small increase in load

• And disappear just as suddenly at higher loads

• Several tones observed simultaneously


Measurements: Waterfall plot


• Fan tone• Frequency doesn’t change with fan speed

• Combustion instability• Frequency too high• Unlikely with oil fired nearly stoichiometric burn

• Side branch resonance in exhaust stack• Frequency too high• No vibrations around side branch

• Flow-structural interaction in heat exchanger• Change in frequency with load too great

• Flow-acoustic interaction in heat exchanger…


Theory: Flow-acoustic interaction


• Scaling analysis • Flow induced noise

• Important variables: flow speed U (function of temperature in the boiler), pin diameter a

• [U] ~ ms-1, [a] ~ m, [f] ~ s-1 so St = fa /U• Acoustic resonance

• Important variables: sound speed c (function of temperature), size of boiler d

• [c] ~ ms-1, [d] ~ m, [f] ~ s-1 so He = f d / c

• U and c are functions of temperature in the boiler• Sweep


1StU

fa

2StU

fa

20

HeTc

fd

40

HeTc

fd

30

HeTc

fd

10

HeTc

fd


• Frequency of tones scale with sound speed• Acoustic, correspond to eigenfrequencies of the cavity• Frequency of tones are determined by acoustic response

• Existence of tones corresponds to a certain range of Strouhal numbers

• As the boiler load increases• The flow speed increases

• The Strouhal number of a given tone falls into the unstable range when it appears, and eventually to below the unstable range, where it disappears

• The temperature increases• The sound speed increases and • The frequency of a given tone increases


• Simple solution is to fit a silencer

• But…

• Large silencer = large back pressure

• Effects boiler efficiency

• No guarantee a larger boiler (or indeed a smaller) won’t fail

• Motivation for a long term solution


• Scale model built

• 0.9 x 0.6 x 2.5 metres

• Cold air up to ca. 20m/s

• 132 dB(A) 1m from outlet

• Complaints received from golf course 7 km away

• Staff threatened strike

• Tests stopped by police


• Unsteady flow behind pipes generates fluctuating lift on pins

• This is a source of sound


• Acoustic response of cavity characterized by resonant acoustic modes• Interaction between noise generated by unsteady flow and acoustic resonances• Enhances the noise generating capability of the flow (nonlinear phenomenon)• Synchronization• Correlation• Source amplification• Not simply a case of broadband excitation exciting an acoustic resonance (linear

phenomenon)• Resulting in flow-acoustic self-sustaining oscillations, a.k.a. Boiler tones

• Ordinary aeroacoustic prediction gives background noise level of 70 dB with broad peaks up to about 90 dB.

• Peaks are too narrow and too high


An analogy: The Millennium Bridge


• Opened 10 June 2000• Closed 12 June 2000

• Bridge basically well engineered• Calculated the structural response of 100,000 people per day walking across

• Assume stochastic, uncorrelated excitation (foot steps)• Huge safety margin

• But possibly a little soft

• But when the bridge starting swinging, the people on the bridge started swinging too• Walking in time with the swing of the bridge

• Excitation no longer random• Synchronized at the resonant frequency of the bridge• Correlated in space because everyone sways the same way


• Same story in the boiler

• The unsteady flow behind the pins corresponds to the people on the bridge

• Simple (70 dB) model assumes the flow behind each pin is uncorrelated and stochastic

• But the flow “feels” the acoustic field and starts “swaying” in time to the response

• Excitation becomes correlated and synchronized

• New prediction 140 dB

• Measured 120

• Not fully correlated everywhere

• Nonlinear damping mechanisms

• Flow-acoustic self-sustaining oscillation


Numerical analysis: Test boiler at 50%


Treatment

• Geometrical modification• Reduce source power by modifying geometry

• Acoustic attenuation• Increase dissipation in critical modes by introducing absorbing materials

• High temperatures – standard fibrous solutions will melt• Perforated plates

• Located at velocity maxima of critical modes• Hole sized scaled to account for changes in acoustic conditions with temperature

and gas properties• Significant levels of attenuation• Self-cleaning


• If you are interested in learning more about Noise and Vibration control click here.

Lloyd’s Register and variants of it are trading names of Lloyd’s Register Group Limited, its subsidiaries and affiliates.Copyright © Lloyd’s Register [Entity]. 2013. A member of the Lloyd’s Register group.

Gorm DannesboeSenior ConsultantEnergyT +974 445 699 52 M +974 550 992 91 E [email protected]

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Lloyd’s Register Qatar LLCRegus Building, D Ring Road, Doha, State of Qatar

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noise and vibrations in the petroleum industry

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