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

摘要

Direct electrical heating (DEH) has shown to be a verypromising method to prevent wax and hydrate formation insubsea pipelines in the North Sea (flowlines up to 16 km). Theheated pipeline is an active conductor in a single-phase ACelectric circuit, together with a single core high voltage extrudedcable strapped (piggybacked) to the heated flowline. For newprojects with longer flowlines (exceeding 40 km), the insulatingouter sheath and the metallic screens can be replaced by a semiconductiveouter sheath for a continuous transfer of capacitivecurrents to the environments. This can be essential to avoid highvoltages subjected to the metallic screens. Due to the relative high temperatures of the crude oil, thermalinsulating properties of the clay and heating of the cableconductor, relative high temperatures can be subjected to thecable materials.This paper presents results from measurements of mechanicaland electrical properties of different candidate semi-conductivecable outer sheath materials aged in relevant service conditionsand measurements of electrical resistivity and water content ofclay samples taken from relevant oil fields.The results are very promising and indicate good mechanicaland electrical properties as well as long endurance of thecandidate semi-conductive outer sheath materials. The tests alsoshow that approximately the same resistivity was measured forthe semi-conductive outer sheath and clay making continuouslytransferring of AC currents to ground possible.