Subsea Hydrocarbon Processing and Treatment: Twister Subsea

Twister™ is an innovative gas conditioning technology which is now generally accepted for natural gas applications. Condensation and separation at supersonic velocity provides some unique benefits. The extremely short residence time (∼ 2ms) prevents hydrate problems, and eliminates the requirement for chemicals and associated regeneration systems. The simplicity and reliability of a static device, with no rotating parts, operating without chemicals, ensures a simple, environmentally friendly facility, with high availability. Previous studies have confirmed that Twister is a key enabler for subsea gas conditioning, enabling direct sales gas export, without the need for topside facilities. The objective of this paper is to present the results of a recent study which evaluates the technical feasibility of using Twister technology for subsea dehydration and hydrocarbon dew-pointing. Four Twister subsea gas production scenarios were studied. Different gas compositions were used to simulate both lean and rich gas reservoirs. Four different process schemes were simulated using UniSIM© and evaluated for each subsea scenario. The process schemes include a Chemical-free, MEG inhibited, a Hybrid and a Liquid Re-injection scheme. The Chemical-free and Hybrid schemes use electricity to prevent hydrate blockage. The MEG inhibited scheme utilizes MEG (Mono-Ethylene Glycol) for hydrate inhibition. This scheme requires some 85% less MEG than wet-gas evacuation under the same production conditions. The Hybrid scheme combines these two processes, and injects MEG after bulk water removal. The Hybrid system can operate using significantly less MEG than that required for the MEG inhibited process, and some 95% less than wet-gas evacuation. Liquid Re-injection produced on-spec gas, but wastes part of the value in the gas stream, as well as MEG. It is concluded that the Hybrid and Chemical-free schemes require some additional subsea technology, which is not yet available. The MEG inhibited scheme, however, uses available subsea technology and can be applied to create on-specification gas ready for direct subsea pipeline export, using available technology. Subsea compression will be required somewhat earlier when compared to wet-gas evacuation. This apparent disadvantage is however overcome by the elimination of platform required for production, the elimination of long multiphase gas lines with inherent pressure losses and overall flow assurance improvement.