Article discusses the Statoil/SINTEF system utilizes a thermally insulated single pipe flowline warmed by Direct Electrical Heating (DEH). Electrical power is applied directly to the ends of the flowline. Electrical current flows from one end of the pipe to the other with the metal wall of the pipe acting as a resistive heating element thus dissipating the applied electrical power as heat. A/C electrical power is transmitted from topsides to the subsea flowline by two single core riser cables originating at the platform power supply. One of the two single core riser cables is connected to the near end of the pipe, and the other riser cable is connected to a piggyback cable that is strapped to the flowline and is electrically connected to the far end of the pipe. In the Statoil/SINTEF system, the pipe is coated with a thermal insulation. The thermal insulation is not intended to be an electrical insulator, thus the flowline is not electrically isolated from the surrounding seawater.
A good electrical connection to the surrounding seawater is made at each end of the flowline by a series of sacrificial anodes. Because the system is electrically connected to surrounding seawater it is commonly described as an ‘earthed current system’ or an ‘open system’. The seawater acts as an electric conductor in parallel to the pipe with the applied current dividing between pipe and seawater.
At the cable connection points the total system current enters the steel pipe. A portion of the system current leaves the pipe and is transferred to the seawater through the area around the anodes. The area around the anodes is called the transfer zone. The current that leaves the transfer zone initially has a radial direction, then flows parallel to the pipe. The length of the transfer zone has been measured directly on full-scale test pipelines, and is typically 50 meters at a frequency of 50 Hz. The amount of power lost to sea is a function of the distance between the piggyback cable and the heated pipe and is typically less than 10% of the applied power resulting in power transfer efficiencies better than 90%.
The Statoil/SINTEF system design is the result of a joint industry project initiated in 1996 to qualify the Direct Electric Heating Method. Statoil has funded the design and has acted as the managing entity for suppliers that execute the various portions of each installation in the North Sea, much like INTEC has acted for the Shell systems. Nexans is a major player in the project and performed the majority of the subsea engineering work. SINTEF Energy Research of Trondheim, Norway performed the preliminary design work, the electrical and thermal calculations, the measurements on various grades of steel and stainless steel pipes, and development of the current protection system.
Qualification work on the direct electric heating system was performed by SINTEF and funded by various Norwegian oil companies. Feasibility of the system was proven both by scaled tests and on full-scale test installations. The qualification test program included pipe sizes from 8 inch diameter pipe through 18 inch diameter pipe, in carbon steel, duplex steel, and 13% Chrome steel.
There are no licensing requirements or royalty payments required for the use of the Statoil/SINTEF system, however, much of the technology was funded by Statoil, and a business arrangement between Statoil and the end user of the technology would be advantageous to expedite the transfer of technology, lessons learned, and to avoid a repeat of developmental work already performed.
Figure 1 - Schematic of Statoil/SINTEF DEH System
Figure 2 - Schematic of Statoil/SINTEF DEH System at Template/Manifold
Figure 3 - Schematic of Statoil/SINTEF DEH System at Template/PLEM
Complex heating arrangements can be built up with multiple branches and long flowlines. Flowline segments can be separated by equipment sleds or templates. Field tests have confirmed that a cable loop bypassing the template, combined with electromagnetic chokes made of laminated steel core elements, effectively reduced stray currents passing through the template. In one test performed, a system current of 2000 Amps produced a template current of approximately 40 Amps with a measured maximum electromagnetic flux density of approximately 50 microteslas. The measured values of stray currents indicate that the maximum voltage exposure in seawater is less than 1 V/m. An optimal configuration for the electromagnetic chokes was determined to be 5 to 6 cores (500 kg) evenly distributed along the pipe over a length of approximately 10 meters adjacent to the template.
The success of this design has made it Statoil’s preferred solution for hydrate control. Statoil uses electrical heat to warm flowlines, risers, and jumpers. Statoil’s DEH system was first adopted in 2000 when six pipes (up to 8.5 km long) were installed during the second development phase of the Asgard field. Thereafter a 16-kilometer long condensate pipeline was installed at the Huldra field and six flowlines at the Kristin field in 2004. In 2005 the concept was taken a stage further for the Norne field satellite, Urd. The flowline design was upgraded to a cladded pipe, consisting of a thick outer layer of carbon steel and a thin (3 mm thick) inner liner of stainless steel through which the electric current flows. To date fourteen DEH flowlines are in operation; all were constructed from stainless steel or stainless steel lined carbon steel.
Another advantage to the design is that it utilizes the low cost Reel-Lay method. Reel Lay allows the Statoil/SINTEF DEH flowlines to be built up on shore; furthermore, the installation is more economical because the lay rate is faster than with other methods.