Experience Gained From the First U.K. Subsea Separator
Authors: Songhurst B.W. , Eyre G.P. 
Published: October 2008

BOET have installed and tested a Subsea Separator Pilot Unit (SSPU) in Hamilton Brothers' Argyll field in the North Sea. Initial trials were completed by the end of 1989. This paper details the design of the pilot unit and the experience gained in design, construction, installation and operation of the SSPU.

Introduction

Project Objective

The objective of the pilot unit project was to demonstrate that wellfluids could be processed subsea to produce dead crude suitable for loading This would in turn demonstrate the technical.and commercial feasibility of the company's concepts for marginal field development, which are detailed in Reference 1.

Project Status

The pilot unit has been installed in the Argyll field and has undergone a series of tests that have proved the concept. The separator has been proven in an actual operating environment: the hostile waters of the North Sea. Initial trials have now been completed. The pilot unit's control system and pump configuration are now being revised to incorporate the lessons learned during this phase. After completion the pilot unit will be put through an extended running test to prove its reliability to meet the needs ofcommercial operation.

Design

Process

The SSPU, as illustrated by Figure 1, is installed virtually directly beneath the floating production vessel Deepsea Pioneer in the Argyll field. The pilot unit takes the flow from a single well having the following characteristics:Flowrate: 5000 bpd(gross liquid)Water cut: 80%GOR: 400 SCF/bbl

The unit ties into existing facilities in the manner illustrated by Figure 2. Well fluids flow into the unit which can essentially be regarded as a box. Liquid products leave via the eXisting well riser whilst associated gas is flared via a newly installed flexible riser. Figure 3 shows how the pilot unit is configured. It consists of 3 vessels: the HP separator (a 2 phase separator), LP separator (a 3 phase separator) and a storage vessel. Wellfluids can be allowed to bypass the unit or will be processed as follows:Well fluids are first choked prior to passing into the HP separator where basic gas/liquid separation is carried out under pressure. De-emulsifying chemicals can be added immediately downstream of the choke.Oil and water then pass into the LP separator where they are further degessed to atmospheric pressure to produce dead crude. The LP separator includes a small sand trap, an oill water separation compartment and an oil storage compartment.Separated oil and water then pass by gravity to the storage vessel which is split into two separate compartments by a full lengthbaffle.Having degassed the products to atmospheric pressure they then have to be lifted some 100m and then discharged into the Deepsea Pioneer's separation system. This is carried out using a single pump which discharges the products in a batchwise fashion. This operation is carried out automatically by the control system.

Control System

The pilot unit's control system is illustrated by Figure 4.

It essentially comprises these components:Sensing elements: level detectors and pressure TransducersIntelligence: programmable logic controller(PLC)