Development of Corrosion Inhibitor and Kinetic Hydrate Inhibitor for the Pearl GTL Project
Authors: McFarland Sam , Pit Halimah , Unsal Refah , de Reus Han 
Published: November 2008

PEARL GTL (Gas To Liquids) project is a fully integrated project that will take 1.6 billion cubic feet per day of unprocessed gas from Qatar's North field into onshore gas processing plants, producing 140,000 barrels per day of GTL products and 120,000 barrels per day of natural gas liquids.

There will be no processing facilities on the two offshore platforms - all produced fluids will be transported to shore via the main pipelines for treatment.

The gas contains H2S and CO2 and is consequently corrosive so continuous injection of corrosion inhibitor into the pipelines is necessary. Sea bottom temperatures can be below hydrate formation temperature so it is necessary for part of the year to also inject kinetic hydrate inhibitor into the inlet of the pipeline to avoid hydrate formation.

The challenging combination of highly corrosive conditions, minimal offshore intervention and very high plant uptime requirements means that corrosion control and flow assurance of the highest effectiveness is required.

This paper explains the philosophy for corrosion control and flow assurance for the pipelines. The selection process for corrosion inhibitor and kinetic hydrate inhibitor will be described and the laboratory test protocols and results discussed. The specific and unique requirements for monitoring and control of the inhibition system will be addressed, along with other measures such as specific pigging requirements to ensure continued pipeline integrity. Operational implications will be outlined.

Introduction

The Pearl GTL project, currently in development by Qatar Shell GTL Limited, will build the world's largest GTL plant, to create value and growth for both Qatar and Shell. The scope of a project of this magnitude demands meticulous attention to detail to each module and component to ensure that operations begin flawlessly and smoothly. Shell's expertise and breadth of experience has been brought into play to meet the challenges of this endeavour in the North Field, offshore Qatar.

The current offshore development consists of two blocks in the North Field based on the concept of identical single wellhead platforms in each block with no processing facilities. The platforms will supply produced fluids that will be transported to shore via dedicated pipelines. By striving for a solution where the risk of failure, under specified operating conditions, that would endanger the safety of personnel, the environment or asset value, has been reduced to as low as reasonably practicable (ALARP) during the complete asset lifecycle. Shell and Qatar Petroleum intend to successfully achieve, maintain and demonstrate the technical integrity of these critical offshore facilities.

The fields will produce wet and sour gas with high levels of CO2 and H2S. To counter the effects of corrosion, a number of solutions based on various operating scenarios were considered. By basing the choice on technical acceptability first, and then cost, carbon steel protected by continuously injected corrosion inhibitor (CI) was selected as the corrosion control option for the pipelines. In recognition of some of the operating scenarios that were considered, supplementary measures such as operational pigging and injection of batch corrosion inhibitor (BI) will also be implemented to minimize the effects of corrosion.