Noise from Oil & Gas Facilities – “Acoustics 101” and Best Practices for Noise Control

Rob Stevens, HGC Engineering

Noise From Oil & Gas Facilities

“Acoustics 101” and Best Practices for Noise ControlRob Stevens – HGC Engineering

Environmental Noise FromOil and Gas Facilities

1. “Acoustics 101”

2. Best Practices for Noise Control

“Acoustics 101” for Oil & Gas Plants

• Why Care About Noise?• The Science of Sound• Measures for Time-varying Sound• Frequency and Pitch• Octave Bands & A-weighting• Environmental Sound Level Limits• (Sound Pressure versus Sound Power) If time allows

Why care about noise?

Why care about noise?

• Outdoors (“Environmental”) – LEGAL REQUIREMENTS. Avoid adverse impact: disturbance, annoyance, loss of enjoyment of property, interference with sleep

• Workplace Noise Exposure – LEGAL REQUIREMENTS. Avoid risk of hearing loss

• Indoors (Lab, Office, Control Room): – Communications, comfort & productivity, avoiding fatigue

Environmental (Offsite)Noise Impact

• There are legislative noise limits in many jurisdictions, sometimes with reporting/permitting requirements:

• Federal/State Environmental Protect Act(s)

• Municipal codes, ordinances, bylaws

• Requirements of financiers – World Bank, International Finance Corporation, International Development Corporation, sovereign development banks

A Bit about the Science of Sound

Sound = Air pressure fluctuations(in the range of 20 to 20,000 per second)

The Ear is Essentially a Microphone

Sound = Air pressure fluctuations(in the range of 20 to 20,000 per second)

Time

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Amplitude of fluctuations => Loudness

Sound = Air pressure fluctuations(in the range of 20 to 20,000 per second)

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Time

InstantaneousPressure Fluctuations

Sound Pressure Level, Lp

Sound Pressure Level, DecibelsAudible range of loudness is immense – more than a factor of one million, from

20 micro-Pascals (µPa) to more than 20,000,000 µPa

So, we use a logarithmic scale, in units of “decibels”

Sound Pressure Level, DecibelsSound pressure level “LP” in decibels is defined as:

LP = 20log(p/p0)

p0 = 20 micro-Pascals = 0 dB≈ Threshold of Hearing

0.00002 Pa

0.002 Pa

0.2 Pa

2.0 Pa

1 x

100 x

10,000 x

1,000,000 x

Time-varying Sound

Steady sound, the level is obvious...

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Time

Steady Sound

Sound Pressure Level, Lp

What is the level of this sound?

Most Common Descriptor forTime-varying sound: LEQ

• LEQ = Energy equivalent sound exposure level (over a given time period)

• Like an average, but a logarithmic average

• Definition: LEQ is the hypothetical steady sound level that would contain the same energy in a given period of time as the actual time-varying sound

Another Important Descriptor forTime-varying sound: L90

• L90 = “Ninetieth percentile exceedance” sound level

• The sound level that is exceeded 90% of the time in a given period

• Useful for capturing the steady component of the sound, and rejecting short term transient sounds

Time

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InstantaneousPressure Fluctuations

Sound Pressure Level, Lp

Leq

L90

Time

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Steady Sound

Leq

L90

Spurious Noise

The Use of LEQ and L90

• LEQ correlates well with potential of a sound to disturb residents

• LEQ is the basis of most environmental noise legislation

• L90 is useful to isolate the sound of a steady industry from transient background sounds e.g., traffic, birds, voices

The Weighted Day/NightEquivalent Level: LDN

• Used in some jurisdictions: LDN is a 24 hour LEQ with the nighttime-average levels penalized by +10 dBA

• Usually for LDN, daytime is defined as 07:00 to 22:00 and nighttime as 22:00 to 07:00

• LDN is an alternative to having separate daytime and nighttime limits (usually with nighttime being more restrictive)

Acoustic Frequency

Frequency

• In addition to differences in level of sound (loudness), the ear can detect differences in frequency (tone, pitch)

• Frequency = rate of pressure oscillations

• Measured in Hz

• Hz = “cycles per second”

FrequencyS

ou

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Pre

ssu

re

Time

Low Frequency

FrequencyS

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Time

High Frequency

FrequencyS

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Time

Multiple Frequencies

Frequency

• Many “noises” contain an irregular mixture of multiple frequencies

• (Whereas (most) music contains a harmonious mixture of multiple frequencies)

• How to handle the multitude of frequencies contained in a sound or noise, when measuring?

Frequency – Octave Bands

Frequency

• Octave frequency bands can be inconvenient – a spectrum of levels…

• Also, a spectrum does not intuitively describe how loud the combined sound is

• To make simpler and more intuitive, we use the A-weighted sum (single number) particularly for environmental noise (outdoors)

Frequency – A-weighting

• Human ear is not equally sensitive to sounds (or components of a sound) having different frequencies

• Frequency response of the human ear is referred to as the “A curve”

• We apply the A curve weightings to the levels at each frequency and sum the result into a single number

Frequency – A-weighting

-50

-40

-30

-20

-10

0

10

31 63 125

250

500

1k 2k 4k 8k

16 k

Wei

gh

tin

g [

dB

]

Frequency [Hz]

Frequency – A-weighting

• An A-weighted sum (dBA) is a good single-number descriptor of the loudness of a sound

• Levels measured in dBA correlate well with the potential of a sound to cause disturbance or annoyance in a community

• So, environmental noise limits are quoted in units of dBA

• However, for the more detailed purpose of acoustical modeling, analysis and noise control, spectral levels (e.g., octave bands) are necessary

Environmental Noise Limits

Environmental Noise Limits

• Usually legally mandated – Federal/ State/Municipal

• In US, usually Municipal

• Resource of links to noise laws by jurisdiction:www.noise-ordinances.com

Environmental Noise LimitsHow They Vary• Location at which the limits apply?

• Point of reception – e.g., neighboring residence, or• Property line, or• Point of emission – e.g., 25’ from equipment

Environmental Noise Limits

• Quantitative or Qualitative• e.g., “not clearly audible” versus LEQ ≤ 45 dBA

• Time period of limits?• Worst case 1 hour LEQ day/night• Full day/full night Leq – 15 hour / 9 hour• 24 hour period using LDN

• Fixed Limits (e.g., 45 dBA) or relative to background sound?

Environmental Noise Limits

• E.g., New Jersey State Noise Limits:• Point of reception – neighbouring property line• Daytime/Nighttime: 07:00 to 22:00 & 22:00 to 07:00• Duration – Unclear! LEQ? LP? 1 hour? Full period?• Fixed Limits: Residential 65 dBA day; 50 dBA night

Best Practices for Noise Control

Best Practices for Noise Control• Assess noise when contemplating:

• New facility

• Addition or modification to an existing facility

• Periodically, if required by regulator or from time to time as a best practice

• Complaint received

Best Practices for Noise Control• In most cases, assessing noise requires an

Engineering Noise Study by a qualified practitioner

• Except when a contemplated undertaking involves direct replacement of existing equipment or new equipment/activities that are acoustically negligible

• “Qualified practitioner” could be internal, or an outside consultant

Best Practices for Noise Control

• An Engineering Noise Study entails:• Identifying governing laws, codes, guidelines

• Establishing the applicable sound level limits

• Preparing a sound source inventory

• Quantifying the facility-total sound levels

• Evaluating the facility-total sound levels against limits

• If needed, developing noise control recommendations

• Creating a report, perhaps for submission to regulator

Best Practices for Noise Control

• The Equity Engineering Group (E2G) is a firm specializing in outsourcing of technical specifications and Best Practices.

• E2G has a published “Noise Control Best Practice” document for Oil & Gas and Petrochemical facilities

• See: www.equityeng.com

Optional (Time Permitting):Sound Power vs Sound Pressure

Sound Power vs Sound Pressure• Sound pressure level represents the magnitude of

resulting air pressure fluctuations at some defined point in space (the effect).

• Sound power level is the total rate at which a source emits sound energy into the surrounding environment (the cause).

Sound Power vs Sound Pressure

Thermal Analogy

Thermal Power (Watts)

Temperature (degrees)

Sound Power vs Sound Pressure

Sound Power Level

(dB re 10-12 W)

Sound Pressure Level(dB re 20 µPa)

Sound Power vs Sound Pressure

• Sound pressure level depends upon:• Source sound power level• Source-receiver distance• Reverberation/Reflections• Background Sound

• Sound power level is a fundamental quantity, independent of these environmental factors.

Sound Power vs Sound Pressure• Because sound power is a fundamental quantity, it

is preferred for specifying the maximum sound output of equipment (or comparing one item to another)

• A calculation is needed to determine sound pressure from sound power, or vice versa (depends on room, distance, orientation, etc.)

Can Estimate Sound Power byMeasuring Sound Pressure

Fine if there is no background sound or room reverberation

Sound Level Meter

Or Measure True Sound Power UsingSound Intensity Instrumentation & Methods

Sound Intensity Probe

Sound Power vs Sound Pressure

• More information about sound power measurement using sound intensity methods at:

www.SoundIntensity.com

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