Direct electrical heating flows with the current

At the forefront of innovative technology, Nexans cable and cable system experts have designed an efficient, cost-effective and environmentally-friendly direct electrical heating (DEH) system to pass electrical current over the production flowline for oiland gas pipelines, thereby reducing wax
and hydrate formation without the use of monoethylene glycol (MEG) chemical injection.
Four integral components form the DEH system: the riser cable, which supplies power from the topside to the seabed; the feeder cable, which connects the DEH riser cable to the pipeline and piggyback cable; the piggyback cable, which is the current conductor; and the integral protection system, which is wrapped around the piggyback cable to protect it from external overloads. “On a high level it’s a very simplistic system where you are running a high-voltage AC current to the far end of the production flowline, and then the electrical current is turned on and uses the pipeline as a current resistor. The pipeline heats up using its own internal electrical resistance; that’s the basic functionality of the system,” Nexans project engineer Audun Haglo said. The first system to be tested and run was for Statoil, an international energy company, in 2001. Since then, Nexans has developed the technology for several of Statoil’s projects and, more recently, for BP. “BP installed the company’s first system in 2011. After BP bought the system, we are seeing a lot more interest from around the world, including Australia,” Mr Haglo said. “The system is more environmentally effective as MEG chemicals are not being injected to keep the pipeline heated for better flow production; instead you are using an electrical current to heat the production line. “The DEH system works differently: itheats up the flowline so you don’t need to inject anything as long as your system is designed to operate under normal production.
We have thermal insulation on the flowline and you can run production. If you have a shutdown you have a certain amount of time before you need to mitigate,” Mr Haglo said. The power generated can also be utilised to run the topside facility when theproduction line is shut down. “That’s where the DEH is such a cost-effective system and environmentally friendly, because you are not actually doing anything ‘til you need it. Then when you have a shutdown you free up a lot of the power use to topside and you can use that to run your DEH system to keep the fl owline warm. When you are ready to start production you turn off the DEH system and start up production.” “The sheer volume of MEG liquid that is needed has started to become expensive, topside is expensive and for some of the new platform designs you don’t have much space topside.
“The amount of MEG normally needed on flowlines is so much that you need to look into separate tankers or storage facilities,” Mr Haglo said. “The DEH system is effective on anything that’s temperature dependent because it heats up flowlines so it’s qualified forhydrates and wax as well. To my knowledge it has only been used for hydrates: mainly gas production fields,” Mr Haglo said. The system does, however, have limitations: it works well for single-length flowlines but not as well for major networks of subsea flowlines. “It is very challenging, if even possible, to heat a spiderweb of flowlines with a lot of different valves or production units because you’d have to run that high current through all the systems. When you get the more complex structures, some form of chemical in the flowline is the way to go,” Mr Haglo said.
For now, Nexans can offer systems up to 120km long; the longest tested DEH system to date measures 42km and was installed in 2007-2008 on Statoil’s Tyrihans project in the Norwegian Sea. “Not only is Tyrihans the longest DEH system in operation, it isalso heating the fl owline with the largest diameter,” Mr Haglo said.
The only other issue for the subsea oil andgas pipelines is the AC corrosion rate. A way to mitigate this is to design and plan for it so it doesn’t become a significant problem. “The current philosophy is to apply enough anodes to reduce your current density to approx 40A/m2 [amps per square metre] or lower.
With current densities below 40A/m2, AC corrosion is not believed to occur or, at least, is considered insignificant. Unfortunately, this phenomenon is not yet fully understood and values are likely to change again in the future.”
The lifetime design of the DEH system is between 20 to 30 years and the run time is estimated at between three to five years. The short run time is because it gets very hot. Nexans is developing new ground with pipeline installations systems in deeper depths of water. In June 2011, Nexans
presented a qualified high-voltage dynamic riser cable that could go as deep as 3000m; the previous record was 1700m. “The challenges with larger depths are usually mechanical issues due to the fact that the cables become very heavy, and the topside hangoff loads and installation loads become very high. The DEH system is not qualified to go down that far yet, but Nexans is working on it,” Mr Haglo said.


By Zana Kaic

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