environmental concerns

Figure 1. Gas compression site.

For some time, fixed permanent noise monitors havebeen installed near major airports to identify aircraftoperating with excessive noise during takeoff and land-

ing. These monitors provide a reliable means of investigatinga complaint and, ultimately, of controlling neighbourhoodnoise. The extension from monitoring airport noise to moni-toring the noise from an industrial site is a logical step.However, while the application may be similar, the designparameters are vastly different.

This article explores the use of permanent noise moni-tors on fixed industrial sites, such as oil and gas facilities. Itdescribes how noise on these sites can be measured, howthe results should be interpreted, and how both the owner ofthe facility and the regulatory authority can benefit from cap-turing these data. The article provides casestudy examplesfrom a monitor installed at a coastal gas compression site inthe UK (Figure 1). The noise monitor is set at a distance ofapproximately 50 m from both the compressor house, whichcontains four gas turbine driven compressors, and the after-coolers.

Noise monitorsA noise monitor consists of a sound level meter and ameans of recording the noise data at a fixed position on ornear the site. The value of the monitor is in recordinginstances of unacceptable noise levels and being able to

relate these instances to some event. In some instances, anoise monitor is used to refute claims that have alreadybeen made. By showing no significant change in the noisedata, it can be used as proof against allegations of noise nui-sance or a breech of permit conditions.

In other cases, results are collected and stored untilsome future occasion when they are needed. In either case,the data captured by a noise monitor should be protected.Although the results from the noise monitor are seldomrequired, they should be secured and readily available for aslong as necessary to respond to a noise complaint. After anappropriate period of time (one month, for example), theresults can be archived, but available for recovery shouldthey be needed in the future. Since the amount of datastored each day is relatively small, a year’s results easily fiton a single CD. The data should also be protected from unau-thorised use and treated as company confidential so thatcontrols are in place to handle the processing and distribu-tion of the information.

Noise regulationIn the 25 states of the European Union, the noise impact ofmajor industrial facilities is regulated under Council Directive96/61/EC ‘concerning integrated pollution prevention andcontrol’; it is widely known as the ‘IPPC Directive’. The pur-pose of this Directive is to control the impact of industry on

John Henderson andDavid Richardson, CB&IJohn Brown, UK, discuss

the use of fixed monitors for

regulating noisefrom an oil and

gas facility.

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the environment outside the site of the facility. This Directiveapplies to all new facilities, and existing ones will be broughtin by 2007. The Directive also covers all aspects of ‘pollu-tion’ in the industrial context: emissions of gases, solids andliquids; emissions of noise and vibration; and the use of rawmaterials and energy.

The IPPC Directive requires the ‘best available tech-niques’ (BAT) to be used to control these factors, with costconsiderations included in the meaning of ‘best available’.When it comes to noise, this means controlling the noise lev-els emitted from the site as a result of plant operations (con-struction noise is not covered by IPPC, but is typicallyincluded in the local development permit). The interpretation

of ‘best available techniques’ is central to IPPC. Bothoperator and regulator are able to draw on a range ofEuropean BAT Reference Notes (BREF) and national guidance publications (which are based on the BREF).This guidance provides examples of BAT and sets outthe ground rules for determining emission limits; inthis way it provides consistency within states andacross the European Union. The exact conditions foreach site, or ‘installation’ (the term used by the direc-tive) are set on a case-by-case basis and generallytake local conditions and local opinions into account.These conditions consist of a set of limit values forpollution characteristics, including noise emitted, andare determined in the installation’s permit. In general,the limit values should correspond to the use of BAT;but where environmental quality standards are set(for example, neighbourhood noise limits), these stan-dards may govern the permit conditions, and addi-tional or different techniques to those established inBAT may be required.

The Directive explicitly requires emission monitor-ing and the reporting of results to the regulator. Thelocal environmental regulator specifies the measure-ment methodology, frequency and evaluation proce-dure in the IPPC permit. In practice, local regulationsrequire all these details to be developed by the oper-ator in their permit application and the permit thenrefers back to it.

Data interpretationTo use noise monitoring effectively at an industrialsite, results should be able to demonstrate whetheror not a noise that results in a complaint was causedby an event on the site. To be able to make this deter-mination, the noise monitor must be located withcareful attention to many critical details for accuratelycollecting and interpreting the data. Several factorsmust be considered, including noise level variationscaused by individual cars, aircraft, changes in winddirection, local activity such as farming and in coastalareas, the tide.

Inevitably, the monitor will measure the combina-tion of background and site noise. The success of themeasurement depends on the difference between thebackground sound level and the noise from the site atthe monitor location. Background sound level is theaverage minimum sound level and is measured at thequietest time, usually at night. A noise model can bedeveloped to help site the noise monitor at a newfacility. It is important to determine the optimal loca-tion for measuring the site noise. If the monitor islocated too close to equipment, the monitor will onlymeasure the noise from the closest item. If it is toofar away, the site noise will be masked by ambient

noise.When the weather is good and the monitor is working per-

fectly, the monitor will provide a trace similar to the graphshown in Figure 2. The measurements are one minute induration and measure the statistical value LA90 (the back-ground) and the equivalent sound level LAEQ (an average ofthe sound level).

This graph illustrates the need to understand thesources of interference when the data recorded by the mon-itor are interpreted. In the case of the coastal gas compres-sion site, the occasional peaks during night time are due tonearby road traffic and operators on their nightly rounds.During the day, fighter aircraft from a nearby squadron fly over

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environmental concerns

Figure 2. Typical normal time history for a day.

Figure 3. Effect of the onset of rain on the measurement.

Figure 4. Noise measurements in high winds.

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the site, causing peaks of noise.Local weather conditions are also fre-

quently responsible for providing exception-ally high readings. On the site in question,wind speed and direction were recorded.Generally speaking, it is important to mea-sure wind speed, wind direction and rainfallas a minimum. Temperature measurementscan also be helpful in that they can give anindication of a temperature inversion with itsassociated anomalous transmission ofnoise.

The way in which meteorological eventsinfluence the data collected on monitors isbest shown by measurements taken duringrain and high wind. Figure 3 shows the effectof the onset of persistent and heavy rainfall,and Figure 4 shows the effect of wind that isblowing steadily at approximately 20 knotswith gusts of up to 40 knots.

The rain impacts the ability of the micro-phone to collect other sounds and effectively masks themonitor’s ability to capture other noise readings. The windnoise likewise blocks the monitor’s ability to record othersounds and eliminates the possibility of extracting noiseinformation about the site.

Pure tone noiseThe site used for this casestudy contains centrifugal gascompressors. Such compressors sometimes produce a puretone noise as the result of a malfunction or other unplannedevent. In industrial facilities, an audible pure tone can be thereason for widespread dissatisfaction and noise complaintsfrom neighbours in the area. At an early stage in establish-ing noise monitors in this facility, it was recognised that thenoise monitor should be able to identify and measure a puretone to help correct a malfunction should one arise. Themost reliable test for a pure tone is a listening test.Measurement of a pure tone is possible using narrow onethird octave band measurements. The third octave band con-taining the pure tone is expected to stand above the adja-cent bands by more than 5 dB.

In this case, the ability to measure the noise in one thirdoctaves was a major factor in the selection of the specificequipment installed. The one third octave band measure-ment was carried out each hour, on the hour, in place of oneof the background measurements. The result of the mea-surement is shown in Figure 5. The three graphs given arefor normal operation, operation during high winds and opera-tion during a period when a pure tone was emitted from thecompressor pipework. The measurement of the frequencywas central in determining the cause of the noise, whichturned out to be debris in the compressor suction filters.Once the cause was determined, the facility operators wereable to move quickly to solve the problem.

Future enhancementsAdvances in the design of noise measurement now allow thecollection of more than the two units measured in this case.These advances provide a greater freedom to select a rangeof different statistical levels. There is even a means to useevents to trigger specific types of measurements, or to sup-press specific noises, such as jet aircraft flying over the site.

Clearly all of these are benefits if they help to establisha real situation that could cause excess noise and resolve ajustified complaint quickly. The disadvantage in adding mea-

surements is the amount of additional analysis necessary toestablish the cause of the problem. There is no substitute toforecasting where the problem is likely to come from andselecting the measurement to identify the specific problemsanticipated. This keeps the data simple and effective.

Advances in communication technology allow the use ofmobile phone technology to interrogate the monitor, to setthe measurement type and to allow the monitor to make atelephone call if pre-set parameters are exceeded. The free-dom to move the monitor around the site and the simplicityof not having additional cabling is an obvious advantage.However, this does mean that there is a need for an activemobile phone on a site where one may not be currently per-mitted, and the potential interference with other control sys-tems must be considered, along with the possibility of tam-pering or of the data being intercepted.

Benefits of noise monitoringThe regular automatic monitoring of community noise isexpensive: a network of automatic monitors has a high cap-ital cost and is prone to interference of all kinds. Regularmanual monitoring is also expensive, it is labour intensiveand measurements need to be made by day and night, andin various suitable weather conditions to be meaningful.However, a third option, the combination of continuous auto-matic measurement onsite and validated computer model-ling of environmental levels has advantages for owner, regu-lator and the community. In the UK, this technique was widelyaccepted for larger industrial plants before the IPPC Directivewas implemented and can be used effectively to fulfil theDirective requirements for BAT and noise emission monitor-ing, providing a cost effective approach under the new regu-lations.

So, while noise monitors today are generally installedat the request of the regulator, there is an advantage tothe site if one is installed by the owner and used in con-junction with the regulator to analyse any complaint thatmight be made.

As technology continues to provide better techniques forcapturing and understanding the data collected by noisemonitors, these devices will increase in their value to owners and operators of oil and gas facilities. This will beachieved through their ability to correct noise problems andto validate noise levels in the event of an unjustified com-plaint. Both facility owners and the regulatory authority canbenefit from the installation of these devices.

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environmental concerns

Figure 5. The one third octave measurement for high and low wind and for thepure tone.

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