High Voltage Cable Design for Direct Electrical Heating of Very Long Flowlines

摘要

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 (flowlines up to 16 km). 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 flowline. For new projects with longer flowlines (exceeding 40 km), the insulating outer sheath and the metallic screens can be replaced by a semi-conductive outer sheath for a continuous transfer of capacitive currents to the environments. This can be essential to avoid high voltages subjected to the metallic 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 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.