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Field Compressors for Coal Seam Gas Gathering SystemsScrew or reciprocating compressors. . . . how to choose? A decision analysis tool for the owner's management. . .

Field compressor selection3Introduction1 7

Different technologies4

Intangible dataFor CSG projects designed to deliver gas into highpressure transmission pipelines, the design flowingwellhead pressure (FWHP) at peak flow - as specifiedby the owner's petroleum engineers - has a significantbearing on cost of downstream compression. Field (or"nodal") compressors are required to move the gasfrom the wellheads to the central plant. ConnectingHDPE gathering lines are sized to satisfy the designpressure constraints of P1 (maximum desired backpressure on most distant well) and P2 (minimumdesired field booster compressor suction pressure)

When a large number of compressors are employed,resultant capital, operating & maintenance costsimpact on profitability. Engineers and consultantsevaluate competing bids and make recommendations.This is a complex task due the need to incorporatejudgements on "intangible factors". The responsibilityfor final decision-making normally rests with theproject owner's management, who normally have moreunderstanding of these intangible or strategic factorsthan do external consultants.

P1

P2

Where each vendor offers the same brand or type ofcompressor, the bid evaluation task is more straightforward. With new or unfamiliar technology, there isuncertainty or subjective factors that bear on theevaluation process. The owner's managementtherefore needs a pragmatic method of addressinguncertainty and reaching a decision through consensus.

Task for owner's management:-Step 1: List your project-specific concerns or priorities, forexample a) delivery period/impact on schedule; b) local servicesupport; c) risk profile of supplier; e) operability of package; f)ease of future relocation; g) turndown; h) compression ratioStep 2: Agree on a weighting factor (0-100) to reflect theperceived project impact of these intangible factors.Step 3: Use investigative skills (and assistance from engineeringconsultants if necessary) to allocate a score (1-5) that eachvendor proposal is judged to receive against the aboveselection criteria. A single "advantage factor" (0-100) is thencalculated for each vendor proposal by means of weightedsummations of the allocated scores.Minimising uncertaintyDifferent advantage factors may be received from differentengineers and consultants, due to individual biases, lack of timefor proper investigations, orinfluence by a vendor's marketingpeople. The owner's management are in a position to addressthese deviations, ideally, but not always, to achieve consensus.

Fig. 1 Example network simulation of a CSG well "pod"

Fig. 2 Field compressor stations are replicated over the field

Decision analysis methodology5Fig. 3 High speed screw (left) and slow speed integral (right)

Tangible data6

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2 Number of boosters required

"Choosing by Advantages" was introduced to theundersigned by Clifton C. Hefner, Adjunct Professor(Project Management), Queensland University ofTechnology (2008). Contact Cliff on telephone on +61419 029 622 for further information. The procedureoffers a rational approach for collecting and presentingdata from competing proposals. It enables considerationof both tangible and intangible selection criteria.

A field compressortakes gas at pressureP2 and sends it bytrunk line to a centralgas plant for finalcompression,dehydration andmetering.

The number of units at each field compressor stationdepends on several factors, the most significant of whichis capacity of each unit at P2. A standard size (1,000 hp)field compressor has a capacity of 5-8 TJ/day dependingon P2. Approximately 170 TJ/day of CSG field productionis required for every million tonnes of LNG to beproduced. Hence the large number of field compressorsrequired for CSG to LNG projects.

Where each vendor offers the same brand or type ofcompressor, the bid evaluation task is more straightforward. With new or unfamiliar technology, there isuncertainty or subjective factors that bear on theevaluation process. The owner's managementtherefore needs a pragmatic method of addressinguncertainty and reaching a decision through consensus.

This consists of capital, operating and maintenance(O&M) cost estimates. These estimates are convertedto a net present value (NPV) cost over the life of theproject. The owner's FEED consultants are generallybest equipped to generate the input data required toestimate NPV cost for technically acceptable options.

Task for owner's management:-Step 1: List your project-specific concerns or priorities, forexample a) delivery period/impact on schedule; b) local servicesupport; c) risk profile of supplier; e) operability of package; f)ease of future relocation; g) turndown; h) compression ratioStep 2: Agree on a weighting factor (0-100) to reflect theperceived project impact of these intangible factors.Step 3: Use investigative skills (and assistance from engineeringconsultants if necessary) to allocate a score (1-5) that eachvendor proposal is judged to receive against the aboveselection criteria. A single "advantage factor" (0-100) is thencalculated for each vendor proposal by means of weightedsummations of the allocated scores.Minimising uncertaintyDifferent advantage factors may be received from differentengineers and consultants, due to individual biases, lack of timefor proper investigations, orinfluence by a vendor's marketingpeople. The owner's management are in a position to addressthese deviations, ideally, but not always, to achieve consensus.

The final bid evaluation results are ploted on a single chart thatsummarises all estimates and engineering judgements.Accountability and traceability of the decision-making process isdocumented in accordance with principles of good governence.

"Production Management Best Practices for CBM Fields"SPE Applied Technology Workshop, 30 March-2 April 2009

Contact: geoff@gamckee.com +61 4186 777 51

Management question:"Are we prepared to pay X million

dollars NPV to increase theadvantage factor from Y to Z?"