Experience in operating World's first Subsea Separation and Water Injection Station at Troll Oil Field in the North Sea
Authors: Horn Terje , Bakke William , Eriksen Gunnar 
Published: October 2008

In May 2000 Norsk Hydro reported, by means of an OTC paper, that Troll Pilot would be ready for start-up in June of that year. The Pilot was indeed started at that time. However, failures in the electrical connector system developed in the subsea installation shortly after start-up. This triggered the Repair and Upgrade Project which involved several interventions on the Troll Pilot subsea station during the summer of 2001. These repair activities, which focused on subject electrical interfaces and corresponding marine operations, were presented by an OTC paper in 2002.

Following the repair operations, Troll-Pilot was started in August of that year and it has now been in continuous operation, reporting 100% availability, since August 2001.

This paper will focus on presenting experience in operating the separator station and relate this to the defined operational and design strategies. Issues to be covered as a spin-off from subsea separation are implications on flow assurance and variance in the well fluid entering the separator system. The paper will also cover risk of installing a subsea separator system and relating this to the regularity experience of the station, giving feedback on main components used on the station.

The Troll Pilot Separator Station is installed on the Troll Field at 340 m WD approx 3,5 km from the Troll-C Production platform. This province of the Troll field will be developed with more than 50 subsea wells. Troll-Pilot is installed in a production loop system with two production templates giving a total capacity of 8 well-slots. At the moment 6 wells has been completed and hence available for the separator station.

Troll Field Location(Available in full paper)

Component/Process strategies established during design stage

Main objectives for the Troll Pilot development were:To improve the water treatment capacity of the Troll C platform and its environment and thus maximise ThroughputTo demonstrate commercial viability of subsea separation and boosting with a view to other applications, i.e. to be a competitor to separator stations installed on a platform. During early design stages of the Troll Pilot Subsea Separator Station, strategies for operating the various components being based on new technology and utilizing existing technology, were developed. The following main components/processes are considered to be the ones which have contributed to the results and good experience gained so far:

Separator with process requirements

The purpose of the process is to isolate the water phase for re-injection into a well completed in the water zone of the reservoir thus directing reduced quantities of water to the topsides facility at Troll C. The gas and oil streams are recombined at the common outlet from the separator and directed to the flow-line.

The objective of achieving a feasible, practical and efficient process plant was the first and dominant thought when the Troll Pilot concept was formed. Both time and budget constraints ruled out development of new separation principles. Thus basically a simple gravity separator, utilizing the same principles as applied in separators on platforms and land-based oil production facilities, was selected. However, the following simplifications was included: