In-House Design & Modifications Improve Sour Gas Processing in an Indian Offshore Gas Field – A Case History

Abstract

Abstract BXY field is the largest gas field of India, located in offshore, 80 km from Mumbai city. The field produces about 32.5 MMscmd sour gas using two process complexes BXA & BXB. The field was put on production in September’1988 with initial gas production of about 2 MMscmd. The field attained a gas production of 10 MMscmd during 1994 and this was the period when some operational problems were observed at one of the process complexes i.e. BXA. The sour gas processing (H2S >100 ppm) at BXA involved cooling of well stream fluids (from 95 to 43 °C) in shell-tube type well stream coolers, three phase separation of well fluid and finally transportation of gas & condensate to shore terminal at a distance of about 230 km. Initially each complex was designed for lesser capacity which was upgraded to 15 MMscmd in 1995-96 after progression of field development. No problem was observed in BXB complex. In BXA complex, the processing temperature exceeded the normal temperature of 43 °C. This resulted in rising trend of moisture content in sales gas, high consumption of TEG (2-3 times more than normal value) in dehydration unit and increased load on operational staff for monitoring. Frequent failures of pipe & fittings of gas cooling system were observed. Well stream coolers had problem of air locking & cooling water flow across coolers came down to half quantity. Overall, severe operational problems were observed in the plant. An in-house analysis was carried out to resolve these problems. The analysis of total processing system viz. gas cooling system, gas dehydration system, condensate stabilization system was made. Based on analysis, it was found that gas cooling water system is the bottleneck. The various components including material of construction of various equipment of this unit were analyzed and causes of operational problems resolved. A new cooling water system known as "cooling water segregation system" was conceived, designed and installed at BXA complex & other suitable measures undertaken. These efforts have streamlined the operating conditions of plant and are now processing 16.5 MMscmd gas without any problem. The present paper describes stepwise identification of problems, design, review, installation of newly designed system and other issues to resolve the operational problem in this major processing complex. The improved design on implementation resulted in smooth operations in the plant and is working since then without any problem.