Top of the line (TOL) corrosion presents a major challenge for many oil and gas operating companies, especially those producing or exporting gas. It is known to occur during multiphase flow, such as transport of wet gas, as a result of water vapour condensing on the upper, internal surfaces of the pipe which may not be protected by conventional corrosion inhibitors. The dissolution of corrosive gases present in the gas stream (mainly CO2 and/or H2S) into the condensed water can result in severe general or localised corrosion. The design and testing of inhibitors to protect against TOL corrosion is a key area of development, and as yet no industry standard test methodology is available for measuring TOL corrosion and assessing inhibitor performance under these conditions.

In order to combat TOL corrosion, effective inhibitors are required to possess two somewhat contradictory properties: namely, to establish stable films on the steel surface while also possessing sufficient volatility to be transported to all locations where protection is required. Recently, we have developed a laboratory test method involving a relatively rapid screening stage followed by testing at more field-representative conditions, to determine TOL corrosion rates as well as to qualify the effectiveness of corrosion inhibitors designed to provide effective control against both TOL and bottom of the line (BOL) corrosion.

The method, which includes techniques for ambient-pressure testing as well as alternative techniques for elevated pressures, has been shown to reliably determine the effectiveness of TOL corrosion inhibitor formulations. Both approaches have been validated by comparison with field cases.

The application of this test methodology is already providing improved inhibitor selection for TOL corrosion, complementary to more standard test methods for inhibitor selection for BOL corrosion.

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