Cathodic overprotection and interference condition on a gas transportation pipe encased in a casing pipe in the presence of water in the annulus

The Italian ministerial decree (D.M. 4 April 2014) foresees that at rail crossings and parallelisms, pipes for liquid and gas transportation shall be encased in a well-sealed, coated steel pipe called casing pipe. Laboratory tests have been carried out to investigate the protection and interference condition of both the carrier pipe and the casing in the case where water is present in the annulus. Cathodic protection was applied to the gas pipe by an impressed current system, cell voltage was increased and the IR-free potential of both pipes was measured. Different configurations were considered for the casing: bare tube with and without a through hole; coated tube with a defect on the internal and external surface; bare tube electrically connected to the internal pipe by a shunt. In the presence of a perfect watertight seal, no interference takes place on the casing pipe, as expected. If an electrolyte is present in the annulus, overprotection conditions on the internal pipe are reached in all tested conditions, except for test performed on the connected pipes. In the presence of a bare casing, interference effects are negligible but overprotection condition on the internal tube is reached at the lowest cell voltage.