DEH: Going with the flow

DEH cable being placed into the traditional protection system, which is a labor and time intensive process. Images from Nexans.

Exploration and production in harsher environments and deeper waters, far from the shore, pose flow assurance difficulties for operators.

The mix of high pressure and low temperature, in deepwater environments, make pipelines susceptible to hydrate and wax formation, leading to plugs in the flowline and slowing or potentially blocking the flow of oil and gas.

Wax is found in most oil and gas fluids. The cold temperature of the pipe wall causes deposits of the wax to harden and stick to the surface. These deposits increase the viscosity of oil, eventually creating a blockage.

Hydrates are crystalline, ice-like water molecules composed of water and gas, such as methane or carbon dioxide.

They form from the substantial amount of water that is typically present in the untreated well steam— ranging from 10% to as high as 80% in a tail production period. At high pressures they can start to precipitate at temperatures as high as +25°C.

Problems start when they clump together into a slush-like material. They typically form and join together during shutdown situations, but may also form during normal operation, especially if there is a long tieback.

Existing solutions to maintain flow assurance when hydrate and wax forma-tion is possible, is to use a combination of chemicals and pressure evacuation. This involves injecting chemicals into the well stream and removing them top- side. Thermodynamic chemical inhibi-tors work by lowering the temperature at which hydrates form.

Other chemical inhibitors can be used in smaller volumes, but are less effective if the fluid has a long way to travel through the pipeline. For example, kinetic hydrate inhibitors, which benefit from being both cheap and effec- tive, can only prevent hydrate formation for a short period of time and are more suited to moderate water temperatures.

Nexans’ DEH system, showing the piggy back cable, which completes the circuit, strapped to the flowline. Image by Nexans.

Direct electrical heating (DEH) systems offer an alternative solution to the problem of wax and hydrate formation, reducing the use of chemicals.

Originally started as a joint industry project, which sought to find solutions to hydrate formation using direct heat rather than chemicals, DEH was first tested at the Åsgard field in the Norwegian Sea in 2000, following a long development phase in cooperation between Statoil, SINTEF, Nexans and others.

How does it work?

In a DEH system, an electric alternat- ing current (AC) in a metallic conductor generates heat. The flowline to be heated serves as an active conductor in the elec- tric circuit, formed by the dynamic riser cable, the feeder cable, the piggyback cable (PBC), and the flowline.

AC current is supplied from the topside power system via the riser cable. Subsea, the riser cable is connected to the feeder cable in the subsea junction box. Using the feeder cable, one of the conductors in the riser cable is connected to the flowline near end, while the other conductor is jointed to a PBC. The PBC is strapped to the flowline along the entire length to be heated, and con- nected to the flowline at the far end. The flowline, then becomes the primary return conductor in the system, and is heated due to its own electrical resistance.

The new integrated protection system.

For safety and reliability reasons, the flowline is electrically connected to the surrounding seawater (i.e. it is an “open system”) through several sacrificial anodes. These aluminum anodes are rated for both corrosion protection and sufficient grounding of the system during the expected lifetime of the flowline and the service life of the heating system. The AC current does not influence the internal corrosion of the flowline.

During operation, power is fed to the far end of the flowline through the PBC and returns through the steel flowline and seawater. At each end of the flow- line, where the current enters and leaves the pipe, additional anodes are mounted to form a well-defined, low impedance path for the current to the sea, known as the current transfer zone.

The current that needs to be supplied in a DEH system will vary according to many different criteria, including the temperature that needs to be maintained and the magnetic and electrical proper- ties of the pipe. For the Åsgard installa- tion for Statoil the system current is over 1000 A.

DEH systems can be used continuously to maintain the temperature of the flow- line above the hydrate formation tem- perature, for example in wells that are far from the platform or in deep waters. DEH systems can also be used intermit- tently, for example during a planned or unplanned shutdown, when the tempera- ture in the flowline decreases, increasing the risk of wax and hydrate formation.

Examples

The Åsgard field is in the Norwegian Sea some 200km west of Nord-Trøndelag. At Åsgard, there are six pipelines heated by DEH and tied back to a floating platform. DEH is used to heat the flowlines from +6°C up to +27°C in order to prevent hydrate development.

Nexans supplied a dynamic riser cable with four power cores rated for 12kV with 1600sq mm copper conductor and hydraulic tubes. The system included the world’s first coupling between high voltage cable and pipeline on the seabed, according to Nexans.

The DEH system on Skarv.

The next installation took place in 2001, at the Huldra field, also operated by Statoil, on a 16km-long flowline. Here the DEH system is used to maintain the flowline above +37°C in order to prevent wax formation.

In 2007-2008, Nexans supplied the world’s longest DEH system to the 42km long Tyrihans flowline for Statoil. New S-lay methodology was used for installation and a coaxial design was developed for the riser cable and the feeder cable. Also a new concept for fault detection by optical fiber break monitoring was developed and introduced in the piggyback cable in the Tyrihans project.

Protecting your DEH system

Most DEH systems today have been installed in particularly demanding locations, such as the North Sea, where the sea conditions are rough and the weather is severe. Therefore, the cables in these systems need to be able to withstand severe mechanical loads, particularly from trawl impacts which are common in the North Sea.

Nexans has developed a polymer-based integrated mechanical protection system (IPS) for the piggyback cable. A polymer IPS has a long design life, even when immersed in seawater. It is made up of interlocking segments of polymer with hollow cores to absorb heavy impacts, such as trawling and dropped objects.

Future projects

In March 2014, Nexans announced the award of the DEH systems contract from BP for the Shah Deniz field, in the Azerbaijan sector of the Caspian Sea. DEH systems will be provided for 10 subsea flowlines. Altogether 130km of subsea pipelines will be heated. First delivery of the system will be in July 2014. The second delivery will be in June 2016.

Nexans is taking DEH into deeper waters at Chevron’s Lianzi field. The DEH system for Lianzi will be deployed for the subsea pipeline serving the Lianzi oil field devel- opment, between the Republic of Congo and the Republic of Angola. Lianzi is tied back to the Benguela Belize Lobito Tomboco, in Angola block 14m, in 390-1070m water depth, making it the deepest DEH system installed.

The contract, which is with Subsea 7, covers the delivery of the complete DEH system, including riser cable, feeder cable, a 43km-long piggyback cable, and all associated subsea accessories. Delivery is scheduled for this summer.

Conclusion

Overall, Nexans has delivered more than 225km with DEH Cables to heat 19 flow- lines around the world. Nexans-produced systems provide flow assurance for 10 of the 11 DEH fields in operation today globally.