Next Generation Offshore Cooling System To Deliver a Step-Change Improvement in Both Topsides and Subsea Gas Processing
Authors: Moore Richard John 
Published: April 2012

Cooling is a particularly important processing step for efficient gas fielddevelopment. The lack of good cooling options can lead to excessive liquidhandling requirements, overly expensive MEG regeneration systems, pipelinecorrosion issues, and difficult flow assurance challenges.

This paper describes a next generation offshore cooling system which can leadto a step change in both topside and subsea processing, and hence deliverimproved gas field development concepts. The technology has further potentialapplication for cooling HP/HT oil production and, in specific cases, foronshore processing terminals & LNG plants.

Introduction

Cooling oil & gas flows represents a fundamental hydrocarbon processingstep. It is particularly important on offshore gas processing facilities wherecooling is employed, usually prior to dehydration, for water and condensateremoval in order to meet export pipeline specifications. Cooling is also neededin association with compression.

In the current age of big gas developments for LNG, process cooling demands arehigh and a new driver is emerging. Where mono-ethylene-glycol (MEG) is used forhydrate prevention subsea or on unmanned wellhead platforms with no processing, large MEG volumes can result, with high costs incurred for liquids handling& MEG regeneration. Cooling followed by water removal prior to MEGinjection will result in greatly reduced MEG volumes, with a subsequentreduction in expensive MEG regeneration facilities.

In subsea applications, cooling, when attempted, is achieved by passive systemsonly which rely on direct heat loss from the hydrocarbon piping system. Thereis currently no means of controlling temperature variation with flowrate. Thismeans that processing that involves precise temperature control is currentlynot available subsea. This represents a huge, mostly unrecognized, limitationto subsea processing.

Initially this paper will discuss topsides cooling solutions and the differencethat a next generation cooling system can offer. Subsequently the paper willdescribe how this next generation cooling system can be applied in subseaapplications in the form of the world's first fully controllable subsea coolingsystem. Finally innovate field development concepts which are unlocked via thistechnology will be revealed.

Existing Topsides Cooling Solutions

Several cooling options have traditionally been employed for offshorehydrocarbon cooling systems, including:-Air coolingDirect seawater coolingIndirect seawater cooling

Air cooling has limitations due to deck area and ambient temperatureconstraints. Direct sea-water cooling systems require high alloy process heatexchangers; titanium or equivalent, which are expensive & difficult tofabricate. Additionally, issues such as fouling and corrosion in the sea-watersystem remain. The indirect sea-water system has very few problems in theclosed cooling medium circuit. However, the seawater system is the problemarea, with corrosion & fouling issues. Seawater systems have been labeledas some of the most maintenance intensive systems on an offshore platform.