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51317--9713-Assessing Corrosion Risk and Selection of Appropriate Testing Programs for Gas and Gas-Condensate Pipelines

There is uncertainty about the best way to determine the corrosion risk for gas-condensate pipelines, and use of chemical inhibitors as mitigation strategy. We present considerations when devising corrosion mitigation and inhibition strategies, as well as a recommended test for inhibitor qualification.

 

 

 

Product Number: 51317--9713-SG
ISBN: 9713 2017 CP
Author: Caroline Simpson
Publication Date: 2017
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There has long been uncertainty about the best way to determine the corrosion risk for gas-condensate pipelines and the effectiveness of chemical inhibitors as part of the mitigation strategy. This is because corrosion is so affected by flow and transport phenomena which are typically complex. Many horizontal lines experience flow regimes that result in transient periodic high wall shear stress (WSS) which is known to affect both inhibited and uninhibited corrosion rates. Where phases are stratified condensation driven by temperature gradients can result in localized top of the line (TOTL) corrosion in some locations but with slugging occurring in others due to periodic liquid accumulation in topographical synclines in turn leading to localized high WSS. To this must be added the effect of operational uncertainty in nominally dry lines (with respect to water dew point) where corrosion should not be expected under normal conditions but may occur during excursions.Excluding wet lines where glycol loops are required to control hydrates the most common mitigation strategy other than controlling water content is the use of corrosion inhibitors. However this requires effective chemical transport to all regions subject to corrosion which can be challenging to treat TOTL corrosion in stratified conditions unless the inhibitor is volatile and may require specific partitioning behaviour between hydrocarbon and water phases and may be particularly problematic in lines where little or no liquid phase is present at the injection point. In this paper we present some considerations when devising both corrosion mitigation and inhibition strategies and a recommended test approach for inhibitor qualification. This approach accounts for effects of chemical transport and WSS addresses chemical compatibility with other species in the system and includes cases where either free liquid is present at pipeline entry or only appears further downstream. In addition we also highlight some advances made in test methodologies now available particularly when considering the more challenging case of TOTL corrosion inhibitor chemical performance testing.

Key words: Top of line corrosion, TOL, wall shear stress, stratified flow, field to laboratory testing

There has long been uncertainty about the best way to determine the corrosion risk for gas-condensate pipelines and the effectiveness of chemical inhibitors as part of the mitigation strategy. This is because corrosion is so affected by flow and transport phenomena which are typically complex. Many horizontal lines experience flow regimes that result in transient periodic high wall shear stress (WSS) which is known to affect both inhibited and uninhibited corrosion rates. Where phases are stratified condensation driven by temperature gradients can result in localized top of the line (TOTL) corrosion in some locations but with slugging occurring in others due to periodic liquid accumulation in topographical synclines in turn leading to localized high WSS. To this must be added the effect of operational uncertainty in nominally dry lines (with respect to water dew point) where corrosion should not be expected under normal conditions but may occur during excursions.Excluding wet lines where glycol loops are required to control hydrates the most common mitigation strategy other than controlling water content is the use of corrosion inhibitors. However this requires effective chemical transport to all regions subject to corrosion which can be challenging to treat TOTL corrosion in stratified conditions unless the inhibitor is volatile and may require specific partitioning behaviour between hydrocarbon and water phases and may be particularly problematic in lines where little or no liquid phase is present at the injection point. In this paper we present some considerations when devising both corrosion mitigation and inhibition strategies and a recommended test approach for inhibitor qualification. This approach accounts for effects of chemical transport and WSS addresses chemical compatibility with other species in the system and includes cases where either free liquid is present at pipeline entry or only appears further downstream. In addition we also highlight some advances made in test methodologies now available particularly when considering the more challenging case of TOTL corrosion inhibitor chemical performance testing.

Key words: Top of line corrosion, TOL, wall shear stress, stratified flow, field to laboratory testing

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