Compact Subsea Separation System With Integrated Sand Handling
Authors: Fantoft Rune , Hendriks Toine , Chin Robert 
Published: October 2008

Subsea separation is an emerging technology that requires new and optimised solutions. A subsea separator should be reliable to ensure successful operation in a wide range of 3-phase flow regimes, without need for maintenance - but still be retrievable in case of unforeseen events. A particular challenge is the associated production of solids that may cause clogging of equipment as well as damage to downstream systems including water injection pumps and water injection wells.

To meet the requirements for this application, a concept for subsea separation with integrated handling of solids has been developed. The system utilises state-of-the-art components with excellent experiences from topside applications - but are arranged in a new way to meet the requirements for robustness, flexibility and compactness.

This paper is based upon a qualification programme initiated in the first quarter of 2003, covering a field specific subsea produced water separation and solids management system, and describes the work and subsequent results.

Introduction

Water separation combined with water re-injection on the seabed is a new technology available for the oil and gas producers. One installation has already experienced several years of operation through the Troll Pilot. Great potential to improve the economics of many other offshore oil and gas production facilities has been demonstrated through extensive utilization studies of subsea processing technology.

A subsea water separation station in its simplest form comprises a separator and water injection pumps as the main components. Most of the water is removed from the oil and gas phases near the well-head in the separator and re-injected back to a reservoir. Figure 1 shows a typical layout of a subsea water separation system for such an application, where all main components and subsystems are made separately retrievable.

Figure 1: Subsea water separation station.(Available in full paper)

A challenge for subsea water separation systems is that the selected solution must meet strict requirements with respect to both performance and reliability. This has set the focus for the technology development and qualification work. It is especially noteworthy that the separator must provide a satisfactory clean water outlet phase to avoid damage to the downstream injection systems. It is also important that the system be cost effective with respect to both first time installation and modification work in case of any unforeseen failures during operation.

Many oil fields produce, in addition to the oil, gas and water, substantial amounts of associated solids (sand). This introduces some challenges for subsea separation systems, including possibilities for:Clogging of the subsea processing equipment, such as filling of the separator with sand that can eventually degrade its performance.Damage of downstream equipment, with the water injection pumps and well(s) being most sensitive. There are also challenges as to where to deposit the sand. While the natural destination for the oil and gas is the topside facility and the water is being re-injected, the sand is an undesired component that must be treated effectively.

To meet these requirements for both water separation and sand handling, the next generation subsea separation concept with integrated sand handling has been established.