RST's Mission to Mars - The First Commercial Application of Rotary Separator Turbine Technology
Authors: Oxley K.C. , Bennett J.R. , Fremin L.O. , Taylor J.D. , Ross G.D. 
Published: October 2008

This paper will highlight the first commercial1 application of rotary separator turbine (RST) technology, a "game changer" technology with the potential to revolutionize deepwater production system design by significantly reducing the footprint/weight and increasing the operating efficiency of the produced fluid separation, oil dehydration and water treating systems when compared to conventional technology. A 32 thousand BPD liquid / 64 million SCFD gas / ANSI 900# two-phase rotary separator turbine was installed on Shell's Mars Tension Leg Platform (TLP) in the Gulf of Mexico to augment welltest separation capacity. This application follows the successful field test of this technology on Shell's Ram-Powell TLP in 1999-2000. Insights into the performance, operability and reliability of this technology will be shared (Note: at the time of manuscript submission in late January 2003, the platform/skid interface systems were fully commissioned and skid commissioning was underway; performance and operability details between February and May 2003 will be presented at the 2003 OTC Program). While this paper focuses on the application of the two-phase version of this technology, it will comment on the applicability of learnings from this installation to the on-going three-phase RST development effort.

Rotary Separator Turbine Technology Overview

The only force available to separate produced liquids (condensate/oil and/or water) and gas in conventional separation equipment is gravity, which typically requires relatively long residence times and consequently results in large equipment volumes and weight. In contrast, two-phase RST technology (also known as Bi-Phase RST) provides gas/liquid separation in a compact, near isentropic expansion process.

Bi-Phase RST Operating Principle

Figure 1 illustrates the basic operating principle of the Bi-Phase turbine used in this application. The five basic elements of this Bi-Phase RST are:Two-phase inlet nozzles (eight equally spaced around the circumference of the casing)Turbine rotor drum, where separation takes place as the fluids impact near tangentiallyLiquid reaction jets for liquid off-take (four equally spaced around the circumference of the rotor drum)Gas flow passage ways through the rotor for gas off-takeRotor shaft, for energy input to expand the operating envelope during low flow conditions Each inlet nozzle receives a share of the multiphase inlet stream from a distribution header internal to the casing. The inlet nozzles, essentially "Laval" type nozzles2, accelerate the gas/liquid mixture. Due to the density difference, the gas phase accelerates more rapidly than the liquid, and shear forces exerted on liquid droplets by the passing gas will cause them to break up into smaller droplets. In turn, the smaller droplets have a larger surface area to mass ratio, which facilitates an efficient momentum transfer between the liquid and gas. The result is an even distribution of fine liquid droplets suspended in the continuous gas phase at the exit of each nozzle. The shear intensity in each expansion nozzle is considerably less than that generated in a conventional throttle valve. As a consequence of this, the tendency for foaming and emulsion forming is significantly reduced.