HV cable design applicable for direct electrical heating of very long flowlines

Direct electrical heating (DEH) has shown to be a very promising method to prevent wax and hydrate formation in subsea pipelines used in the North Sea. Up to now the system has been installed on flowlines of lengths between 5 and 15 km. In this system the heated pipeline is an active conductor in a single-phase electric circuit, together with a single core high voltage extruded cable strapped (piggybacked) to the heated flowline. A specially designed cable with an insulating outer sheath has been proven feasible for lengths up to 15 km. However, for new projects with flowlines exceeding 40 km a new cable design is essential to limit high voltages subjected to the screen caused by the capacitive and inductive currents. This paper presents electrical and thermal rating of the DEH system for long flowlines, measurements of mechanical and electrical properties of different semiconductive 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 show a long endurance of the semiconductive cable sheath materials at the calculated service temperatures. The tests confirm that a sufficient low resistivity of the semiconductive outer sheath and clay for continuously transferring capacitive currents to ground was measured. The results indicate that the DEH system can be used for particular long flowlines to prevent wax and hydrate formation