Direct Electrical Heating of a Flexible Pipe

Abstract

This paper describes a new direct electrical heating method which offers unique flow assurance opportunities in flexible pipe systems. The method effectively expands the operational envelope of flexible unbonded pipes by offering prevention/removal of hydrate plugs, mitigation of vax precipitation and allowing for the transportation of viscous fluids over longer distances by distributed heating along the entire pipe length. Furthermore, the direct heating system can be applied continuously or intermittent as required by operation, e.g. after prolonged shut down periods. The direct electrical heating solution for flexible pipe takes advantage of the electrical insulation properties of the polymeric pressure sheath in a flexible pipe. The system is effectively operated as a coaxial cable, where electrical current is passed through the carcass and returned via the tensile armor layers. Normally, the electrical resistance of the carcass is significantly above that of the tensile armor resulting in heat being primarily generated in the innermost carcass layer and in direct contact with the bore fluid. Thus, the presented system is a highly effective heating solution of the transported fluid which minimizes heat loss to the ambient structure. Furthermore, it is also highly beneficial that the existing type-approved pipe structure can be used without any modifications. A full-scale demonstration of the new heating concept is presented here, where a 125 m long flexible pipe with a 12 in. bore diameter was placed in a water tank and subsequently heated electrically. Optical fibers for distributed temperature measurements were integrated in the pipe structure to monitor and regulate the bore temperature in a feedback loop with the power system.