Long distance gas pipelines compressor stations play an important role in providing natural gas transportation capacity at the same time that allow the pipeline capacity expansion in conjunction with the option of installing loop lines along its length. This subject has attracted the attention of many authors (Santos, 1997 and 2000; Kurz et all, 2003) with papers addressing the pros and cons of adopting a specific compressor station units arrangement, whether series, parallel or parallel-series, with conclusions not always comprehensive and converging. This paper brings back this important subject to our attention and presents an evaluation of the best arrangement for a compressor station not as a theoretic and isolated design but as part of an entire gas transportation system. A case study is presented base on the expansion design for the Bolivia-Brazil Gas Pipeline Project - Gasbol for the Station #5, in Brazil side. Gasbol pipeline has a design capacity of 30 MMm3/d (1,059 MMcfd) resulted from a ND 32", 1813 km (1127 miles) pipeline and 14 compressor stations with 4 units, each of them originally designed with 7000 hp ISO, starting at Rio Grande, Bolivia up to São Paulo, Brazil. This paper also covers the Gasbol capacity expansion studies from 34 (1201) and then to 40 (1413), 50 (1766) and 68 MMm3/d (2402 MMcfd) by means of retrofitting its compressor stations and by adding looping on incremental basis. The subject of which arrangement to adopt has been considered and simulated at the design phase of the project and was again performed by the time we started the expansion design. Such analysis has even considered the opportunity of using variable speed driver with electric motor as an alternative against gas turbine drivers (Santos, 2000). Failure analysis in transient mode was also performed to identify the pros and cons of the compressor station units arrangement This paper presents a technical and economic approach to help deciding on which compressor station arrangement to adopt.
When the Bolivia-Brazil Gas Pipeline design has started in 1993 there were some uncertainties to be addressed such as the certified volumes for the gas reserve, closure of transportation agreements, creation and implementation of gas distribution companies, gas selling agreements and market growth projections. The design started assuming a transportation capacity of 16 MMcm/d (565 MMdfd) from a 28" pipeline diameter that proved to be unfeasible. The design capacity was defined latter on to be 30 MMcm/d (1059 MMcfd) from a 32" pipeline diameter with extension of 1813 km (1127 miles) and 14 compressor stations. The expansion project based solely on new compressor stations design has improved the capacity from 30 (1059) to 34 MMm3/d (1201 MMcfd).
As part of the design phase we adopted a methodology that proved to be of fundamental importance for the project success that includes the following steps: