A Versatile Multi phase Two-Screw Pump Package for Subsea Deployment
Authors: Cooper P. , Prang A.J. , Thamsen P.U. , Mair J. 
Published: October 2008

A constant-speed, motor-driven pump package is presented for subsea installation horizontally on the sea bottom. The motor and pump are field proven components, which are built, integrated and furnished by the same supplier. A sister company provides the subsea engineering, installation, and maintenance of this package. The motor is taken from a line of submersible, liquid-filled AC units of power levels up to 5,000 hp. These motors are currently employed in subsea trenching and other applications including offshore rigs in multistage centrifugal pump services. The pump is of the timed two-screw variety, currently being widely applied in land-based surface installations to handle mixtures of oil, water, gas, and solid contaminants in various combinations of steady and unsteady flow. As an example, one of these pumps has been operating successfully for more than a year, ingesting multiphase fluid at a gas void fraction (GVF) of 0.75 to 0.90 (Le., 75 to 90 percent gas) from 50 wells. Prior to shipment, the same pump demonstrated its ability to handle 100-percentgas slugs of prolonged duration. Integration of the motor and pump into a subsea package incorporates pressure compensators that maintain the liquid in the motor cavity and the oil for the timing gears and inboard bearing at the suction pressure of the pumpage, insuring adequate mechanical seal life. An alternate, lower-maintenance, advanced-technology version eliminates the seals and utilizes a motor, bearings and gears that operate in the pumpage. Subsea engineering, Installation and maintenance are accomplished via state-of-the-art techniques. The package extends the life of older declining fields while providing attractive paybacks.

Introduction

As remote subsea wells deplete, boosting is not cost effective if the pump requires mostly liquid in order to function. The small amounts of oil involved, say 1000 bpd or less, cannot be economically recovered unless a multiphase pump is located in the vicinity of the well. This concept is illustrated in Figure 1. Multiple well applications can be manifolded together to feed one pump, the piping arrangement providing for a flow check of each pump individually. Several of these multiphase pumps in turn delivering product to centrally located separation equipment on a platform or onshore appears to be a practical way to extend the life of wells that would otherwise have to be abandoned. These wells normally produce mixtures of gas, oil and water in varying proportions that can vary considerably with time at the pump inlet. Gas void fractions (GVF) of 0.95 (i.e., 95% gas by volume) - and higher - are fairly typical. GVF is related to the more frequently quoted gas-oil ratio (GOR) or the mass of gas in standard cubic feet per barrel of oil (scf/bbl) as follows:(mathematical equation)(available in full paper) where GLR is the volume flowrate ratio of gas QG to liquid QL and is given by(mathematical equation)(available in full paper)

T is absolute temperature and p is pressure. Standard temperature and pressure are 15 C and 14.7 psia respectively; so that Tstd =(273.15+15) K. This mixture must be pumped to as much as 50 bar or 700 psi. To date, practically all multiphase pumps have been located on the surface and generally onshore 1, where the installation costs are smaller and the frequent maintenance often needed for new concepts can be carried out with relative ease. In fact, as one would infer from Figure 1, to install and maintain a pump subsea requires a considerable infusion of deepwater technology, which is as sophisticated as the design of the pump package itself.2 As more successes are achieved in dealing with the technical and relia