Characterisation and Mitigation of Subsea Cable Induced Currents on Offshore Oil Platform Structures in the Niger Delta

This study characterises the magnitudes, distribution, and corrosion implications of stray electrical currents induced by subsea power cables on the structural steel of offshore oil platforms in the Niger Delta, South South Nigeria, and evaluates cathodic protection design modifications capable of mitigating their impact. Subsea power cables feeding offshore platforms generate alternating magnetic fields that induce currents in nearby metallic structures through electromagnetic coupling, and these currents can interfere with cathodic protection systems designed to prevent corrosion of platform steel jackets in the highly corrosive Niger Delta marine environment. Field survey data on cable routing geometry and water column conductivity profiles were obtained for three offshore field developments in OML 11, OML 27, and OML 60. An electromagnetic interference model was developed using finite element analysis in COMSOL Multiphysics to compute induced current densities on representative jacket structures under the operational cable loading conditions. Electrochemical corrosion rate implications were assessed using the Stern-Geary polarisation resistance method applied to coupon specimens exposed to Niger Delta seawater in a controlled cathodic protection interference simulation. Results confirm that induced AC densities at jacket nodes closest to the cable touchdown points exceed the ISO 18086 threshold above which AC corrosion risk becomes significant. Modified cathodic protection designs incorporating AC mitigation earthing cells and localised sacrificial anode increases at the affected nodes are modelled and shown to reduce effective corrosion rates to acceptable levels. Keywords: subsea cable, stray current, cathodic protection, corrosion, Niger Delta.