Investigation of Corrosion Rates and Material Selection for Subsea Flowline Applications in Deepwater Blocks Offshore Cross River State

Deepwater developments offshore Cross River State represent an emerging frontier in Nigeria's offshore petroleum sector, and the corrosion management of subsea flowlines in deepwater environments is a critical engineering challenge due to the combined effects of elevated CO2 partial pressures, low temperatures, and complex multiphase flow conditions. This study investigates internal corrosion rates and evaluates material selection criteria for subsea flowline applications applicable to deepwater blocks in the south-eastern offshore margin. Corrosion rate predictions were developed using the de Waard-Milliams CO2 corrosion model modified for deepwater temperature and pressure conditions, incorporating produced water chemistry data from analogue deepwater fields in the Nigerian sector of the Gulf of Guinea. Three material options were evaluated: carbon steel with corrosion inhibitor injection, 13Cr stainless steel, and duplex stainless steel 2205. Life cycle cost analysis was performed using a Net Present Value approach over a twenty-year design life, incorporating inspection and maintenance cost estimates. Results indicate that carbon steel with continuous corrosion inhibitor injection provides the most cost-effective solution for sweet service environments with CO2 partial pressures below 0.5 bar, while duplex stainless steel offers superior performance in mixed sour-sweet environments. Recommendations for cathodic protection design and inspection interval optimisation are provided. Keywords: subsea flowline, corrosion, deepwater, material selection, Cross River State.