Internal Corrosion Prevention and Control of Long-Distance Multiphase Mixture Transportation Subsea Pipeline in a Deepwater Gas Field in the South China Sea

A newly-built long-distance multiphase mixture transportation subsea pipeline in a deepwater gas field in the South China Sea has the characteristics of long mileage, high partial pressure of CO₂ and high water content. Due to high corrosion risk and single pipe design, stringent requirements and challenges are presented for later operation and maintenance, such as pigging operation and corrosion inhibitor management. Based on corrosion risk analysis and corrosion calculation, the internal corrosion prevention and control scheme of the subsea pipeline was proposed, in which the first 1.5 km of the pipeline was made of carbon steel lined with 625 alloy composite pipe, and “carbon steel+6 mm internal corrosion allowance+corrosion inhibitor” was used in the 1.5-113 km pipeline section. During operation, to guarantee the corrosion inhibitor efficiency was above 85%, the application effect of anti-corrosive agents should be evaluated regularly, and the pigging should be carried out according to the corrosion monitoring and testing results. Meanwhile, localized corrosion risks caused by H₂S, deposits and microorganisms should be noticed during operation. Dynamic corrosion risk assessment should be conducted promptly based on operation conditions, corrosion monitoring and detection, corrosion inhibitor post-evaluation, and analysis results of the products from pigging operation. Then the subsequent testing and evaluation plans, and corrosion control measures should be formulated according to the corrosion risk assessment results.