Pearl GTL Offshore Materials, Corrosion, Chemicals and Integrity - Challenges and Solutions

Abstract

This reference is for an abstract only. A full paper was not submitted for this conference. Abstract PEARL GTL (Gas To Liquids) 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 at Ras Laffan, producing 140,000 barrels per day of GTL products and 120,000 barrels per day of natural gas liquids. Two unmanned offshore platforms will be installed in the North Field and gas will be delivered to the onshore GTL plant via two subsea pipelines. There will be no processing facilities on the platforms - all produced fluids will be transported to shore via the main pipelines for treatment onshore. Although the platforms will normally be unmanned, regular visits will be scheduled for activities such as preventive and corrective maintenance, supply of consumables, etc. The gas contains quantities of H2S and CO2 and is consequently corrosive to carbon steels that are commonly used for such facilities elsewhere. During the winter season, sea temperatures on the seabed can be below hydrate formation temperature and it will therefore be necessary for part of the year to inject kinetic hydrate inhibitor chemical at the inlet of the pipeline to avoid hydrate formation. The challenging combination of 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 has meant significant challenges in terms of materials selection and chemical treatments that are suitable for ensuring integrity of the facilities for the required asset design life at a realistic cost. Corrosion resistant alloys have been specified where necessary but carbon steel with corrosion inhibition of very high availability has been selected for the offshore pipelines. The challenges and solutions identified in the materials selection, corrosion control and chemicals selection required to ensure integrity will be discussed. This paper explains the philosophy for corrosion control of the Pearl offshore facilities, including: downhole completions - materials testing and qualification for tubing and casing, controls and limitations on completion and workover fluids topsides equipment - selection of materials for pressure vessels and pipework, including requirements for internal cladding and other protective measures export pipelines - materials selection, coatings and chemicals treatments onshore reception facilities - materials selection for onshore slugcatcher. Implications for how the asset is operated will be presented, related to: limits set on well clean-up criteria before allowing fluids into the pipeline limits imposed on minimum flowrates and chloride levels during operations and the implications for pigging and batch inhibition schedules process monitoring and control required to ensure integrity of the corrosion control system operator awareness and training fit for purpose procedures inspection activites, including intelligent pigging ongoing integrity assessment The overall Pearl story will be summarised to show how technical integrity is achieved throughout the operating life of the asset.