Abstract

This paper will present the design of the Rotary Separator Turbine and skid package used in the first offshore field trial of this technology on the Ram/Powell tension leg platform (TLP) located in the Gulf of Mexico. The RST is a compact gas-liquid separation and power generation device designed to operate on process fluid pressure. This technology offers a potential 50% weight and space savings over conventional vessel separators, and is under development for use on minimum footprint facilities. The purpose of the test was to verify the separation and mechanical characteristics in actual production conditions. An integrated test package installed on the Ram/Powell TLP in June 2000 tested the RST on actual well production.

Rotary Separator Turbine

The Rotary Separator Turbine (RST) enables compact and efficient gas-liquid separation while recovering pressure drop energy. The energy provides liquid pressurization, shaft power, or a combination of the two. Separation efficiency of the gas and liquid meets or exceeds the performance of a conventional vessel separator, with substantial weight and volume reduction. Additional benefits of the RST stemming from the centrifugal separation include foam reduction and improvement in operability due to motion insensitivity.

The RST is a product of high velocity two-phase flow research at the Jet Propulsion Laboratory1. During the 1970's and 1980's several prototype turbines were tested in geothermal power applications. A 30" full-scale unit installed in August 1998 at Cerro Prieto, Mexico operates at a power output of 0.5-1 MW.

The first oil and gas application employed the RST as a centrifugal foam breaker for an offshore application. A reaction design RST tested in an onshore gas facility on natural gas/crude mixture provided complete foam breakdown with a liquid carryover of 30 PPM. Further test work in 1996 used a RST for separation of inert gases and crude oil during offshore loading. A pilot plant unit tested on a North Sea shuttle tanker exhibited good separation and performance unaffected by ship motion. A 1997 Joint Industry Programtested a second RST at a major oil and gas flow facility2.

Operating Principle

Figure 1 shows the basic principles of operation with the liquid and gas flow paths. The four main components identified in the diffuser type RST are the two-phase nozzles, rotor drum, gas path blading, and diffuser scoop3.

Two-Phase Nozzles

The inlet nozzles convert the pressure energy of the two-phase flow mixture into kinetic energy. Due to the density difference, the gas phase accelerates more rapidly than the liquid. Shear forces, exerted by the high velocity gas, break up the liquid into small droplets. In turn, the small liquid droplets have a large surface area to mass ratio. This facilitates efficient momentum transfer between the liquid and the gas during the pressure decline, resulting in an even distribution of fine liquid droplets suspended in a continuous gas phase of the exit jet.

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