Case History: Design, Implementation, and Results of Separator Retrofits

Abstract

Summary Dubai Petroleum Co. (DPC) performed two in-situ offshore separator retrofit projects during 1989-91. Significant cost savings were achieved over the alternatives of separator replacement, addition, or modification onshore. The retrofits were accomplished without production loss or compromised operating safety. DPC's experience demonstrates that in-situ offshore facility modifications can be done successfully and simultaneously with normal operations. Improved performance after the retrofits strongly supports the need for properly designed internals to realize the full potential of a separator. Although the two retrofits share a common objective, they differ significantly in design, planning, and execution. First, a three-phase separator critical for uninterrupted production was modified during a 6-day scheduled fieldwide shutdown using only "cold work." This was followed by the conversion of a two-phase, double-barrel separator to three-phase service without a field shutdown. The work took place during 6 months with the use of "hot work." The success of both field modifications was attributed to innovative engineering and careful planning. Introduction Production is gathered from 67 platforms offshore Dubai, through a network of subsea multiphase pipelines to the central facilities at Fateh and S.W. Fateh (Fig. 1). Gas is separated and compressed for gas lift and pipeline export. Water is removed, processed, and disposed overboard. Separated oil from the S.W. Fateh central facilities is pumped to Fateh for further stabilization, dehydration, storage, and tanker offtake in three "khazzans." Khazzans are large, openbottom, underwater storage containers that resemble inverted champagne glasses. The oil floats on top of the water within the khazzans and displaces seawater out the bottom as more oil is introduced. During the 6 or more hours that the oil resides in the khazzans, its basic sediment and water (BS&W) is reduced to less than 0.5% for tanker sale. Before 1987, all production separators in the system operated in the two-phase mode. They removed the gas from the produced fluid before it was sent to the khazzans. Oil and water separation took place entirely in the khazzans. This method of water separation and disposal was abandoned for three-phase separation when water production increased to levels that exceeded the capacity of the khazzans.

Optimal Lifetime Design of Primary Hydrocarbon Production Separation Systems

“…Historically many expensive retrofits and unit replacements have taken place as water and gas rates goes up 3 . In some cases it is possible to consider planned retrofit, where equipment is designed and configured such that it can be easily replaced or upgraded in the future.…”