Abstract
A cost-benefit analysis or Life Cycle Costing (LCC) of flowline replacement was recently undertaken for a major oil producing company in the Middle East because of frequent corrosion failures in the pipeline network. The company has more than 2,000 production wells (consisting of oil wells, water wells, injectors and disposal wells) distributed throughout the country. Thousands of kilometers of 101.6 mm (4-inch) and 152.4 mm (6-inch) bare carbon steel (API 5L Grade B) flowlines are used to convey products to some 20 Gathering Centers (GCs). A number of different alternative replacement options and/or materials in lieu of bare carbon steel may be considered. However, as stipulated by the company, the cost-benefit analysis was limited to the following 4 items: (a) Carbon steel with corrosion inhibitor, (b) In-situ coatings, (c) In-situ HDPE linings, and (d) Corrosion resistant alloys (CRAs). Alternative materials such as GRP, GRP-lined steel, and composites were not considered.
The cost-benefit analyses showed that when equal life expectancies of 25 years are considered for the various options, the in-situ coating option is the most economical based on present worth (PW). However, on an annualized basis, bare carbon steel is by far the most economical. Hence, in situations where the flowline is not exposed to very corrosive conditions (e.g. corrosion rates of <5 mpy), the use of bare carbon steel would be adequate and most cost-effective. In environments where carbon steel corrodes at about 10 mpy or greater, the use of an alternate option is advisable. According to the annualized costs, the use of HDPE linings when used alone appears to be the most economical followed by in-situ coatings when used alone. The LCC calculations further showed that the use of corrosion inhibitors or CRAs as replacement options for flowlines do not appear to be attractive from an economic standpoint.
