Subsea Separation and Reinjection of Produced Water
Authors: Bringedal B. , Ingebretsen T. , Haugen K. 
Published: October 2008

Abstract
Some of the most significant recent development in subsea design has been within subsea processing, in particular the removal of produced water from the oil closer to the reservoir and subsequent reinjection of this water. Significant effort is being put into both downhole and seabed based separation and re-injection systems for produced water. This paper addresses seabed based solutions.
Although subsea processing has been about for at least the last decade, it is first now (with Troll Pilot) oil companies are gaining enough confidence in the technology to actually consider it a viable field development option. A big question mark is still set on the long-term operational experience. Will the system give satisfactory availability without frequent costly interventions? Major challenges are related to rocess/flow assurance and other operability issues. This paper starts with a brief discussion of these challenges in general and concludes with a typical case (Troll Pilot) where this is discussed in more details. The Troll Pilot case shows how significant effort went into process design of the system in order to minimize the uncertainties and risks associated with this novel subsea processing system.

Introduction
Until recently the injected water used for reservoir pressure support has normally been seawater. The produced water has been treated and disposed to the sea with resulting pollution from remaining oil and production chemicals. In the last few years the trend has rapidly changed towards using produced water instead of seawater for reservoir pressure support, because of a number of benefits such as: Reduced pollution, less problems with scaling due to seawater and produced water mixing and finally less risk of H2S formation in the reservoir.
With increasing focus on the environment, some oil companies even consider to reinjects all produced water into dedicated disposal wells regardless of any improved recovery. Together with the trend towards subsea wells at increasing sea-depths and offsets, it seems obvious that the produced water should be separated from the oil and reinjected close to he subsea wells to avoid long transport distance to and back from the platform.
The worlds first system of this kind will be installed this year on the Troll Field. ABB Offshore Systems (ABBOS) is the main contractor for the operator of the field, Norsk Hydro. The system is developed to separate and re-inject the produced water from 4-8 subsea wells. It is called Troll Pilot because an important objective is to get operational experience before it will be considered a standard field development option for future projects.
To give subsea separation concepts a chance for future field developments, it is important to design the system for operational flexibility, high availability and minimum need for maintenance. To achieve this ABBOS and ABB Corporate Research have given large focus to process operational aspects both for the more general SUBSIS project (Subsea Separation and Reinjection) and the Troll Pilot project. In this paper Troll Pilot is used as an example to outline some of the main operational challenges and how they can be solved. Main emphasis is on special problems related to Subsea Separation and Reinjection even though some of these may also be relevant for topside systems.

Subsea Removal of Produced Water
As the offshore industry moves into increasingly deeper and remote waters and there at the same time is an increased focus on low oil prices and environmental impact of oil and gas production, the traditional field development concepts needs to be challenged in order to provide sound field economics. One clear trend in this picture is