Subsea Water Separation and Injection: A Solution for Hydrates
Authors: Alary V. , Marchais F. , Palermo T. 
Published: October 2008

As production systems go deeper and deeper, flow assurance becomes a major issue for offshore operators. Wax, asphaltene, hydrates are the every day concerns of production teams on the surface. Subsea water separation and injection is a solution for hydrates prevention when the water cut increases in a cold deep environment. The DIPSIS module (Deep Integrated Production Separation and Injection System) has been developed to be part of an antihydrate strategy in deep-water (1500 m or more) allowing treatment with Low Dosage Additive of a reasonable quantity of produced water. The DIPSIS processing unit has been designed to separate the water from the hydrocarbons as close as possible to the subsea production wellheads. Only a maximum of 10% water cut is expected downstream the module. The design of DIPSIS takes into account the environmental conditions of deep offshore subsea units: high external pressure, modular installation and retrieval, R.O.V. access and interfaces, high reliability by redundancy of critical systems, modular subsea control system.

DIPSIS project is being carried out by a group of partners (operators, suppliers, and designers) gathering all expertise to produce the most suitable design to deep offshore operators needs.

Introduction

As shown in the first part of this paper, the understanding of the hydrate formation process is important but does not answer all the questions. How fast hydrates will form, whether or not they will form, whether they will aggregate or remain dispersed, how accurate is the model compared to in field conditions are questions highly dependent on the fluid composition and other parameters. Although operators have means to predict, prevent or remedy the formation of hydrate plugs, current prevention policies for deep-water fields are expensive and still not completely satisfactory. The review, in the second part of the paper, of the existing strategies, together with their advantages, drawbacks and limitations will confirm that the only flexible, safe and easy to implement solution could well be the continuous treatment with chemical hydrate inhibitors associated with subsea separation. In the third part of the paper, several strategies involving the DIPSIS subsea separation module are reviewed and key features of the DIPSIS module are evaluated. Other advantages of subsea separation are introduced in the final part of the paper, to demonstrate how a DIPSIS module is a cost effective and technically sound solution for hydrates prevention in deep water environments.

The problem of hydrates

For several years hydrates properties have been studied worldwide in laboratory and loop test facilities. The objectives are to better understand the mechanism of formation, the physical properties of hydrate structures but also to try to develop means of preventing formation of hydrate plugs. This question has become all the more crucial since deepwater fields have been discovered or put in production. These fields are perfect candidates to encounter hydrates forming conditions.

Hydrate structure.

Gas hydrates are ice-like crystalline compounds that form whenever water contacts with molecule of gas such as low weight molecular hydrocarbon molecules (C1, C2, etc.) or others: N2, CO2 or H2S.