An assessment was performed in order to understand better the integrity status of a carbon steel N. Sea pipeline which transports hydrocarbon fluids with a high CO2 content (5.7-6.2 mole%) and H2S (ca. 600 ppm), receiving continuous injection of a combined scale/corrosion inhibitor. The review was triggered by high corrosion rates, with readings up to 0.9 mmpy measured by intrusive topsides corrosion probes coupled with low confidence in the applied corrosion mitigation and management controls.
The assessment concentrated on the following parameters: flow velocities and wall shear stress rates using field production data; calculation of uninhibited corrosion rates using NORSOK M506; estimation of uninhibited wall loss; review and trending of available corrosion monitoring data; corrosion inhibitor application; review of chemical qualification testing; compliance of inhibitor injection with designated target rate and correlation with the corrosion probe readings; and review of historical production data to ensure that the velocities / shear stress values used for chemical testing were comparable to the flowing regime in place.
The assessment identified that numerous performance metrics were either not in place or not followed up correctly, e.g., inhibitor selection, application, field optimisation and interaction with other chemicals, corrosion monitoring and correlation with inspection data, impact of sand production on inhibitor availability and efficiency, management of change procedures, and issues around internal and third-party communication and interfaces. The latter are of particular significance in operating environments where there are multiple entities involved in operating a system – especially integrity management.
This paper presents a summary of the identified root causes for the observed metal loss spanning a review period of two years; highlights elements of ineffective integrity management practice; and provides a list of pertinent parameters that ought to be assessed so as to obtain a pragmatic understanding of a pipeline's condition, including amendment of applied mitigation measures.