Leman Gas Compression Project Southern North Sea

ABSTRACT The paper is a case history of the design, construction and installation of a gas compression station in the Southern North Sea - U.K. Sector. It is one of the three compression platforms serving the Leman Field and is operated by Amoco (U.K.) Exploration Company on behalf of the East Leman Unit. The station is located at the inlet of, a 30-inch diameter gas transmission line. The nominal capacity of the station and line is 900 MMSCFD at a compressor discharge pressure of 1350 psia. The station is designed for four turbine/compressor units with two units initially installed. INTRODUCTION The Leman gas field is located in the U.K. Sector Southern North Sea approximately 40 miles from shore. The reserves are located mainly in two license blocks 49/27 and 49/26, with smaller areas extending into adjoining blocks. The geology and reservoir performances of the Leman field have been published in previous technical papers (see references 1 – 2). The eastern part of the field is operated by Amoco on behalf of the East Leman Unit and the western part by Shell on behalf of the Shell/Esso Group. The field was discovered in 1966 and the first platform was put into operation in 1969. Field development since 1969 consisted of adding drilling and production platforms in order to fulfill the field's plateau rate (seller's delivery capacity) of 2400 MMSCFD. This was accomplished with a present development scheme consisting of six Amoco drilling-production complexes (see references 3 &4) and four Shell drilling-production complexes. Separate operations of the single field by Shell and Amoco called for co-operation .agreements and these exist to the extent that each company owns and operates a gas transmission line while the third line is jointly owned and operated by Shell. Gas is taken to shore by the three 30-inch diameter transmission lines to Bacton in Norfolk where both Shell and Amoco have their gas processing facilities. This is illustrated on the simplified map Fig. 1. The gas is sold at Bacton to the British Gas Corporation and fed into the National Gas Grid. Contractural delivery specification includes -10° F water dew point and 1000 psia delivery pressure. The produced gas is sweet, mostly methane, with approximately 1 1/2 barrels of condensate per million cu.ft. of gas. This condensate is sold separately. Initially, wellhead flowing pressures were on the order of 2400 psia which permitted free flow of the gas to shore. At maximum capacity, the transmission line requires an inlet pressure of 1350 psia. Due to reservoir depletion the wellhead flowing pressures have declined to the point that compression is required to maintain plateau rates. The compressor stations are designed for a capacity of 900 MMSCFD each this being the nominal flowline capacity - at a compressor discharge pressure of 1350 psia.