New separator internals cut revamping costs

ABSTRACT This article describes the deboltle-necking operation of the productionseparators on the Shell Leman AK platform in the southern North Sea. Theoriginal, compactly built separators became overloaded through decreasing wellpressures. Gradually they were forming a serious restriction in the productionflowsheet. Two alternatives were considered to overcome this problem: to extendthe platform and to install new separators or to retrofit the old separatorvessels with state-of-the-art internals. The second alternative was chosenbecause of considerably lower costs (totalling only 10% of first alternative)and considerably shorter downtime. This separator upgrade project was a jointcooperation between the operator of the field, their central research andengineering facilities, and the above mentioned companies andinstitutions. BACKGROUND The debottle-necking operation formed part of a larger project in which gasproduction of an ageing gas field was boosted through, among others, theinstallation of new compressors. The process flowsheet of the a.m. platform isdepicted in fig. 1. The function of the platform is to remove liquids from theproduced gas and to secure transport of the gas onshore through additionalcompression The produced well fluids first pass through the slugcatcher that removes thebulk of the liquid. Downstream of the slugcatcher two parallel separatorsremove the finer liquids, so the gas is ‘fit’ to enter the centrifugalcompressors. In recent times well pressures were at a level of 6,9 bar, withtypical flowrates of 200 MMSCFT/D per vessel. It was doubtful whether undersuch conditions the second stage separators functioned effectively, althoughthe original design stated that the separators could handle 600 MMSCFT/D pervessel. After the planned installation of the compressors, flowrates wereexpected of 175 MMSCFT/D per vessel at pressures of 2,9 bar. The vendor of theoriginal separators re-rated max. flowrates per vessel at these pressures at 72MMSCFT/D, and it became clear that the separators would not sufficiently guardthe compressors against incoming free liquids. Therefore, it was considerednecessary before installing the new compressors to improve the performance ofthis separation stage. ORIGINAL SEPARATOR DESIGN The original separators (fig. 2) consisted of horizontal vessels of 1520 mmdiameter, 3630 mm length with 11 horizontally mounted 8" cyclone tUbes in thetop half of the vessel. Although this arrangement functioned well under theconditions the separators were originally designed for, a clear deteriorationof performance was observed with increasing flowrates. In the end, no moreliquid was separated at all (fig. 9). OPTIONS TO IMPROVE SEPARATION PERFORMANCE The two main alternatives to improve the performance of the second separationstage wereto replace the current separators with bigger ones able to handle theincreased flowrates.to remove the internals from the current separators and to install highthroughput retrofit internals. The generally accepted advantage of new vessels was that it would be certainthat performance will improve, disadvantage the lack of space on the platform, requiring a platform extension. Advantages of the retrofit option are mainlythe much lower costs and the much shorter downtime.