Operational Aspects of Offshore Multiphase Gas-Condensate Pipelines

ABSTRACT To describe how multiphase flow and hydrate prevention may affect the design of a multiphase gas condensate p1peline the TOGI 20" pipeline is use a as an example. It is shown how the pipeline has been designed to operate safely without regular pigging and with a small slug catcher capacity. Also the potential of hydrate formation and how to handle loss of injection of hydrate inhibitor is considered in detail. INTRODUCTION Development of offshore gas reservoirs has shown that it is often desirable to transport unprocessed well fluid to an already existing installation for processing and export. In particular such transport is attractive when deep water fields are to be developed. In the North Sea a number of developments using multiphase transport have materialized over the last decade. Examples are subsea production systems like North East Frigg, East Frigg and Tommeliten in the Norwegian sector and platform production systems with limited processing like the Viking development in the UK sector. In 1986 it was decided to develop a subsea production unit at around 300 meters of water depth draining a small portion of the Troll Field in the Norwegian sector of the North Sea (Fig 1.). The gas was to be transported to the Oseberg Field Center installat10n 48 KII away where it would be used for gas injection to improve the overall oil recovery by around 10 %. This development called for solution of major challenges in a number of areas including diverless installation and maintenance at deep waters in the rough North Sea environment, Remote control of equipment 48 KII away and control of multiphase flow and hydrate formation in a long large diameter pipel1ne crossing one of the steepest and roughest areas of the North Sea (Fig. 2). The project was named the Troll- Oseberg Gas Injection (TOGI) project. The project is presently preparing for installation, and is schedule for start up In 1991. In the following a discussion will be given to how the multiphase flow and hydrate prevention has been handled in the design) phase and how these to areas have influenced the overall design. HULTIPHASE FLOW Transportation of oil and gas in multiphase pipes has been with us for a long time. It is only in the recent years however that such transport has been attempted over long distances and in pipes of large diameter. The real challenge of multiphase flow transport appears when lines are long, pipe diameters are large and liquid accumulation in the line may severely overload the downstream process facilities. Also when the inlet of the (pipeline is located on a remotely operated subsea station pigging, becomes a costly operat1on and hence excessive liquid accumulation in the pipeline may be problematic.