Direct Impedance Heating of Deepwater Flowlines (1999)

Abstract Direct heating of flowlines to prevent formation of hydrates and wax is becoming an attractive alternative to conventional technology. As the water depths increase in offshore oil and gas exploration world wide, reliable flowlines and risers are extremely important to economically recover these deepwater fields. Conventional technology relies on enough thermal insulation on the flowline to allow steady state operation outside the hydrate region or to permit sufficiently long cool-down periods for typical shutdown operations. With the advent of severe flow assurance problems, direct heating can solve many of these problems by eliminating or reducing the costly back-up systems required. This paper examines the details of an ongoing development project funded by the DeepStar program, demonstrating a direct heated system with respect to thermal and electrical performance. Facts from the completed engineering phase regarding system description, calculation examples, test results and cost comparisons are presented and discussed. Currently, approximately ¼ mile of a thermally insulated flowline with n electrical cable is being field tested in shallow (100 ft.) Norwegian waters. The overall project will be completed in August 1999. Introduction As offshore oil and gas production pushes into deeper water, the risk of hydrate plugging of pipelines and flowlines continues to grow, as does the cost of remediating any such plugs. Conventional methods of preventing hydrate plugs, such as blowdowns, hot oiling and methanol injection are costly and not entirely reliable. For example, at locations where the subsea manifold is higher than the riser base, or locations where the flowline route has substantial high and low spots to trap gas, the process of venting gas is very complex. Conventional remediation techniques, requiring pressure blowdown, are also uncertain and it may take months to melt a hydrate plug. Electric heating can be a very attractive alternative for both prevention and remediation of hydrate plugs having potentially high reliability and little adverse operational impact. Improved reliability of flowlines is also critical in terms of operational costs. Direct heating may improve reliability and substantially reduce operational costs of subsea fields. Electric heating may also prove effective in preventing or remediating paraffin plugging. In this case it may be possible to reduce capital costs by replacing conventional pigging loops with single heated flowlines. Two direct heated systems are considered 1) the fully insulated system, requiring complete electrical insulation of the flowline from the seawater, and 2) the earthed current system, requiring electrical communication with the seawater through anodes or other means. For both systems, current is passed directly through the flowline pipe to provide heating. An earthed current system will be implemented at the Statoil Åsgard field in the North Sea. The system is to be operational from October 2000. Although direct heating can be applied and operated in several modes, this paper will concentrate on the main task within DeepStar, i.e. the fully insulated (closed) system.