The Three-Stroke Separation and Transportation System

Abstract

This paper describes a system based on using high pressure gas as the energy source to boost the liquid outlet pressure from a subsea separation system. The system consists of three vessels operating in a cycle which provides a constant wellhead back pressure and a relatively uniform flow to the export pipelines. INTRODUCTION The development of the North Sea oil provinces has now entered the mature phase, with an attendant increase in interest in the development of marginal fields. Due to the relatively great water depths, particularly in the Norwegian and British sector, in which many of these fields are located, subsea production systems tend to be the favoured development option. Since the early eighties considerable advances have been made in the development of subsea production systems. The main advances have been in the area of inspection, maintenance and repair (IMR), and control systems rather than fluid transport technology. In the mjddle eighties, as the oil companies began to evaluate the possibilities of developing marginal fields beyond the typical 15-20 km transport limit possible with an unseparated and unboosted wellstream, a search for new technical solutions began. Proposed systems to date can be roughly divided into two groups:separator systems with or without pressure boostingmultiphase pumping systems Both types of systems are viable from a pure process standpoint. Their area of application partially overlaps. Despite their apparent simplicity multiphase pumping systems must overcome formidable problems in such areas as; efficiency at high gas liquid ratios, materials technology, pipeline hydrate formation, and slugging. Separator systems without pressure boosting offer some increase in viable transport distance. The limitation is in the liquid transport distance, not the gas transport distance. At typical available flowing wellhead pressures, the gas from a simple separator system can be transported distances up to 70 km with reasonably economic pipeline sizes, and the liquid up to 35 km. Previously proposed combined separator and booster systems for subsea use have employed rotodynamic pumps for boosting the liquid phase, with an electric motor as the prime mover. These pumps have a high overall efficiency but as with all high power subsea rotating machinery, present power transmission, mechanical and IMR problems. The Aker Booster avoids the problems associated with subsea rotating machinery by using high pressure gas as the energy source. It consists of three vessels which operate cyclically as separators and direct acting gas driven pumps. Its gas outlet pressure is only slightly less than flowing wellhead pressure, while its liquid outlet pressure is determined by the drive gas pressure. Gas diffusion into the liquid during the boost stage is negligible. Typically the liquid phase is discharged into the pipeline at approximately double the stabilisation pressure. This combined with the inevitable temperature drop in the pipeline means that multiphase flow development either does not occur, or is insignificant. PROCESS DESCRIPTION The Aker Booster is a combined separator and direct acting gas driven pump.