Electrical and mechanical properties of semi-conductive sheath materials aged at high temperatures in Seawater

Direct electrical heating (DEH) has shown to be a very promising method to prevent wax and hydrate formation in subsea pipelines in the North Sea. Up to now the system has been installed on flowlines of lengths up to 15 km. In this system the heated pipeline is an active conductor in a single-phase AC electric circuit, together with a single core high voltage extruded cable strapped (piggybacked) to the heated flowtine. For new projects with flowlines exceeding 40 km, the insulating outer sheath and the metallic ground screens can be replaced by a semi-conductive outer sheath for a continuous transfer of capacitive currents to seabed, or to clay for buried sections. This can be essential to avoid too high voltages subjected to the metallic ground screens. Due to the relative high temperatures of the crude oil, thermal insulating properties of the clay and heating of the cable conductor, relative high temperatures can be subjected to the cable materials. This paper presents results from measurements of mechanical and electrical properties of different candidate semi-conductive cable outer sheath materials aged in relevant and accelerated service conditions, and measurements of electrical resistivity and water content of clay samples taken from relevant oil fields. The results are very promising and indicate good mechanical and electrical properties as well as long endurance of the candidate semi-conductive outer sheath materials. The tests also show that approximately the same resistivity was measured for the semi-conductive outer sheath and clay making continuously transferring of AC currents to ground possible.