Qualification of a Subsea Separator With Online Desanding Capability
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25367, ’Qualification of a Subsea Separator With Online Desanding Capability for Shallow-Water Applications,’ by M.D. Olson, E.J. Grave, and J.C. Juarez, ExxonMobil Upstream Research, and M.R. Anderson, ExxonMobil Development, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.

In recent years, companies have executed project-specific qualification programs for subsea-processing technologies. This paper summarizes the results of a qualification program that included a multiphase, subsea-separation system for shallow-water applications. The intent of this qualification program was to develop subsea-separation technologies for the global subsea portfolio, rather than for a specific project. To meet this goal, a separator design was chosen that would meet performance targets over a wide range of operating conditions.

Introduction

Subsea processing is not a new concept; however, recent economic considerations have led to more applications, ranging from simple single-phase or multiphase boosting to separation/boosting to future compression projects. There has been a modest number of subsea-separation applications in the Norwegian North Sea, in the Gulf of Mexico, off the west coast of Angola, and most recently in the Campos basin of Brazil. Future subsea projects that have been announced include two compression and liquid-boosting units that will be installed in the Norwegian North Sea. These two projects, and a few of the installed units, use the simplest form of subsea separation: two-phase gas/liquid separation.

The most notable projects that have installed three-phase subsea separators, which remove a produced-water stream, include the Troll C pilot unit, Tordis in the North Sea, and, most recently, Marlim offshore Brazil.

Shallow-Water, Three-Phase Separator Design

In the preliminary separator design, bulk separation was provided by two inlet vane diffusers (IVDs) installed on the two inlet nozzles. In this design, the IVDs diffuse the momentum of the inlet in a gradual manner such that the liquid phases are not sheared into smaller droplets, which can lead to liquid droplets entrained in the gas or the formation of stable oil/ water emulsions. Downstream of the inlet section, a series of perforated baffles was provided to straighten the flow paths in the oil/water phases in an attempt to maximize the separation length and minimize recirculation or stagnant zones. In the preliminary separator design, there were no separation internals downstream of the perforated baffles in the settling section. High-efficiency oil/ water-separation internals, such as platepack coalescers or vessel-based electrostatic grids/coalescers, were avoided because of reliability concerns.

A water-retaining weir was included in the design to separate the oil/water outlet compartments. With this design, a single level detector, such as a nucleonic device, can be installed upstream of the weir and can be used to measure both the gas/liquid- and the oil/water-interface levels.