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PETROMAKS2-Stort program petroleum

Next Generation Electrical Heating for Flow Assurance

Alternative title: Neste generasjon av elektrisk røroppvarming

Awarded: NOK 10.3 mill.

Project Number:


Project Period:

2016 - 2018


Direct Electrical Heating(DEH) is an established cost efficient and environmental friendly method for preventing wax and hydrate formation in subsea flowlines. The technology has its origin in Norway, and is currently in use for heating more than 20 flowlines ranging up to 43 km in length, and has been installed in water depths down to 1070 m. In a DEH system, an electrical current is conducted by the flowline steel, generating heat. By using the steel pipe as an electrical conductor heat is supplied to the oil/gas flow, and the temperature can be maintained above the critical temperature for formation of hydrates and wax. When increasing the power frequency in a DEH System, the heat generation in the flowline increases and hence it is possible to achieve the required temperature by a lower supply current. A lower current allows the use of smaller and more cost-effective cables, which is beneficial with respect to reel size, transportation and installation. Smaller and lighter cables also reduce the hang-off weight on platform, which is essential for the development of deepwater fields. A laboratory test program and computer simulations have been carried out to study how the DEH design and material properties influence the heat development in the pipeline and rating of DEH for the power frequency range from 50 to 200 Hz. The analyzes show that the DEH cable conductor cross section may be reduced with 25% at 100 Hz and 50% at 200 Hz (compared to 50 Hz) for flowline lengths up to 30 km. For longer lengths than 50 km, the benefits of increasing the power frequency are diminishing. New knowledge has been gained in the field of AC corrosion. Tests have been carried out showing that corrosion on cathodically protected pipe steel (X65) is independent of current density and power frequency in the tested range. The results indicates that steel can be subjected to significantly higher current density than previously known without corroding. For anodes (which are made in aluminum), corresponding tests show that applying higher frequency significantly reduces corrosion rate, even for higher current density than what is used today. This knowledge results in cost saving on anodes and will also make engineering of complex subsea systems simpler and safer. Installation considerations for deepwater fields have been carried out in the project. To reduce needed cable, ease the installation and save offshore time, a wet joint for connecting power cables underwater is considered. Removing the need for dry-splicing cables means that cables can be significantly shorter, especially for deepwater applications where excess cable lengths for dry-splicing, may amount to several kilometers. A prototype joint has been tested successfully at low pressure and testing at high pressure is ongoing. Deepwater cable solutions have been considered in the project. One of the main challenges with a subsea power cable in deepwater fields is the high topside tension caused by the weight of the cable, and the risk of overloading the internal elements. To reduce the cable weight and increase the axial load capacity of a cable, the traditional steel wire armour may be replaced by carbon fiber (CF) rods. Using CF rods as armor introduces new challenges to the mechanical termination of the cable topside. A conventional hang-off arrangement with armour block, locking cones and press plate, cannot be used for a CF armour as such solution will crush the fiber rods. A development program to qualify a CF armour is ongoing and a concept for the termination arrangement has been selected based on small scale testing. A prototype must be built and tested to qualify the solution. The cost of DEH projects can be considerably reduced when using higher power frequency as smaller and lighter cables can be used and less anodes are required. Evaluations show that there is a clear cost-saving compared to a 50 Hz system. However, for lengths longer than 50 km, increasing the frequency is not beneficial. Reduction of cross-section area of copper in the cable up to 50% has a positive environmental effect due to a similar reduction in carbon footprint. A typical project of 30 km may hence have its needed copper reduced with 138 ton resulting in a similar reduction in mining and exploration needed. Further benefits of reduced cable weight and volume can be seen for transportation and handling of these products.


Direct Electrical Heating (DEH) of offshore pipelines is an established cost efficient and environmental friendly method for flow assurance with its origin in Norway. Consequently, for existing field developments the major part of the engineering work, manufacturing, supplies and installations has been based on contracts with Norwegian companies. At present the method is far from fully exploited at the same time as it is being challenged by new heating methods under development. Therefore, to further increase the competitiveness of the DEH technology an innovative step is proposed. By going from a DEH system operated at the standard power frequency of 50/60 Hz to a system operated with a frequency up to-200 Hz, clear benefits on several of the involved components as well as on the whole system are realized. A key is the reduction of DEH current which directly reduces the size and weight of the power cables. Furthermore, this results in lower energy losses, reduced interference with neighbouring installations, and less complex DEH grounding system. Operation at higher frequencies is believed to directly result in better system utilization and less AC corrosion. Although some of the involved DEH components are already well suited up to200 Hz operation in the expanded operational environment, more research and development are needed to realize the innovation: Determination of the relationship between AC corrosion and frequency under different operating conditions Exploration of the influence of thermal and mechanical stresses on the electromagnetic properties of steel pipe surfaces Development of DEH cables suitable for operation at frequencies up to 200 Hz and in ultra deep waters (3000 m) For the already existing application area, the increased competitiveness given by the new DEH system is anticipated to maintain the level of activity, even in the challenging market situation. For the expanded application area, new sales are anticipated.

Funding scheme:

PETROMAKS2-Stort program petroleum