A Novel Gas/liquid Separator to Enhance Production of Deepwater Marginal
Authors: Di Silvestro Roberto , Abrand Stephanie , Shaiek Sadia , Butin Nicolas , Riou Xavier , Kathleen Decrin Marie 
Published: May 2011

Today, Subsea processing is recognized to be an efficient way for oil production enhancement, especially for fields having challenging reservoir characteristics or laying in very deepwater. These marginal fields must be developed with cost efficient solutions and innovative technologies to allow the economical recovering of the in place hydrocarbons as the conventional solutions are not viable in such cases.

The subsea gas/liquid separation associated with the boosting of produced liquids is one of the possible configurations for the depletion of the marginal fields, as the reduced back pressure at the wellhead can allow a larger recovery of the in place hydrocarbons and simplify the hydrate preservation strategy of the production flowlines during shut down.

Being installed in deepwater, the subsea processing systems shall address the mechanical and functional constraints that are imposed by the deepwater installation and operation along with the obvious reliability requirements.

In order to provide a solution to this challenge, Saipem has developed a subsea gas/liquid separation and boosting station integrating a gravity separator made of pipes, specifically designed for the deepwater environment: the Vertical Multi-Pipe Separator is composed of an array of vertical pipes that provide the required separation and liquid hold up volume. The reduced diameter and wall thickness of the vertical pipes, as compared with the equivalent single separation vessel, is particularly suited in deep and ultra-deep water applications and/or high pressure services. Furthermore, the system relies on the gravity separation whose efficiency is less prone to the input flow rate and the un-steady regimes than dynamic separation processes.

To demonstrate the reliability and effectiveness of the multi-pipe separator, an extensive and in depth qualification and testing program has been performed for the validation of the concept and for the confirmation of anticipated performances. In particular, the validation of the separation performances was carried out, within the framework of a JIP sponsored by BP and Total, in a pressurized multiphase loop handling crude oil, natural gas and water.

Introduction

The Operators are more and more interested in the development of reservoirs laying in ultra deep waters, or in the tie back of remote or marginal fields to existing production facilities. In particular, the maturation of the Oil Industry's experience in the deep water technology was traditionally made on Oligocene reservoirs characterised by high pressure light oil able to grant large production flow rates in natural depletion. However large Miocene reserves remain available for depletion but characterised by heavy viscous oil in low pressure reservoirs.

In all the above cases, the subsea boosting of the produced liquid is required to allow the economical development of the fields with acceptable level of oil recovery. The subsea separation of the associated gas and the subsea boosting of the liquid through pumps is one of the most interesting solution in deep and ultra deep water, allowing longer tie back distances.

The installation of a subsea separator is also beneficial in the management of the slugs that may be generated in the subsea flowline network and in certain flowing conditions. The capability of handling large slug volumes is in many cases the sizing criteria for the subsea separators that shall also provide the needed residential time to the gas and liquid phase to separate.

The combination of large volume and diameter separators in deep water is always associated to very thick wall thickness of the separator shell that shall resist to the collapse when operating in low pressure or in depressurised condition. The present paper introduces a novel approach to the deep water separation with the aim of avoiding costly and long lead pressure vessels and of making use of line pipes to provide the required separation and slug handling volumes.