Direct electrical heating (DEH) systems offer a reliable and environmentally friendly alternative to the traditional chemical injection methods used to prevent hydrate formation and wax deposition which are observed in subsea flowlines in some field developments.

Potential hazards, like hydrate formation and wax deposition, become more probable for deepwater and high pressure/high temperature (HP/HT) reservoir developments, especially during production shut downs and possible tail production periods.

Traditional chemical injection methods involve considerable operational costs and represent a risk to the environment. So, since 1987, the Norwegian oil industry has been investigating alternative electrical heating methods for the prevention of hydrate and wax plugs. Joint industry projects in Norway have included full-scale qualification testing of both induction and direct electrical heating of single subsea pipelines. Development work carried out between 1996-99 by Nexans, the cable manufacturer, SINTEF, the research organization and Statoil, resulted in the full qualification and application of a DEH system on carbon steel and Cr13 flowlines.

The DEH system has since proved its capability in Statoil's Åsgard oil and gas field where flowlines vary between 3.7 and 5.6 miles (6 km and 9 km) with a total length of 28 miles (45 km). DEH has also been used on a 20-mile (16-km) flowline on the North Sea Huldra field in 2001 and it will shortly be installed in the Norwegian Sea Kristin field, due onstream in 2005.

Metallic

The DEH system is based on the fact that an electric alternating current (AC) in a metallic conductor generates heat. The pipe to be heated is an active conductor in a single-phase electric circuit, with a single core power cable as the forward conductor. Cabling is located parallel with and close ("piggybacked") to the heated pipe. The system is supplied via a dynamic riser cable from the platform power supply. The riser cable includes two power cores, one as a feeder and one as return cable.
One of the power cores in the riser cables is connected to the near end of the pipe, and the other to the forward piggyback cable, which is connected to the far end of the pipe. For safety and reliability reasons, the heating system is electrically connected to surrounding seawater (i.e. it is an "open system") through several sacrificial anodes. The consequence of applying the open system is that the seawater acts as an electric conductor in parallel to the pipe by the direct electric contact between pipe and seawater at both ends of the heated pipe. Current is divided between the pipe and seawater. At the cable connection points the total system current enters the steel pipe. Part of the system current leaves the pipe and is transferred to water through the anodes in the transfer zone.

The length of this transfer zone has been measured directly on a test pipe - a 682-ft (208-m) long, 8-in. diameter carbon steel pipe - and it is typically 164 ft (50 m) at 50 Hz. The current that leaves the individual anodes initially has a radial direction. Apart from the transfer zones, the current in the seawater is parallel to the pipe. Complex electromagnetic calculations are necessary to determine the current distribution between pipe and seawater. These calculations are associated with physical laws such as "neighbouring effect" and "skin effect." The generated heat in the pipe is affected by many factors of which the material properties (electric resistance and magnetic permeability) and dimensions of the pipe are most important. For the efficiency of the heating system, the distance between the pipe surface and the parallel cable is of great importance. The reason is that the spacing significantly affects the magnitude of both the pipe current and the current distribution within the pipe.

For a constant system current, a piggyback installation (where the cable is strapped to the pipe) is the most efficient. Verification tests have shown that if the distance between pipe surface and cable is increased from 1.96-in. to 19.68-in., heat development in the pipe decreases by approximately 30%.
The anodes required for the system have to be 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. Aluminium anodes have been qualified for this purpose and if a sufficient number are applied, then there should be no significant surface corrosion on carbon or Cr13 pipe material. The AC current does not influence the internal corrosion of the pipeline.

General requirements for DEH

The heating system may be designed for the following purposes:

To maintain a steady state pipe temperature above the hydrate formation temperature after planned or unplanned shutdowns. The objective is to start operation of the system just prior to hydrate formation.
Heating of the flowline, which has been cooled down to the ambient seawater temperature (e.g. after a simultaneous process shutdown and electric heating system failure). The system can also be used to maintain the required temperature at low production rates. Continuous heating for some years may be required for a tail production system.

Design

When designing the heating system for a specific subsea flowline, the pipe and seawater current is calculated by finite element methods. These calculations also provide information about the current distribution in the seawater itself and of axially induced voltage in any parallel pipes or umbilicals close to the heated pipe.

The following parameters are required when designing the heating system:

Flowline:

• Thermal, electrical and magnetic data.
• Diameter/wall thickness.
• Pipe insulation: thickness, thermal conductivity (with corresponding U-value and heat capacity).
• Thermal data and dimensions of protection (surroundings/ seabed, including depth of gravel, rock dumping etc.).
• Thermal properties of the pipe fluids in different operation modes.
• Geometry/length of pipeline.

Design criteria:
• Temperature and electrical conductivity of the surrounding seawater.
• Temperature requirements for pipe fluids.
• Required heating time.
• Temperature profile of the pipeline.
• Operational time expected.

Rating is strongly dependent on the thermal characteristics of the pipe contents in the different phases i.e. the composition of oil, gas and water. When calculating the required heat development, "worst case situations" from a thermal point of view are considered for both pipeline and electric cable. The following conditions are assumed: The required heat development in the pipe is calculated when the insulated pipe is located on the seabed. The "worst case" is based on the minimum expected seawater temperature of the field. Also, the cable conductor cross section is determined for a buried/rock-dumped pipe with the maximum depth of backfill or taking into account seabed movements, and the maximum seawater temperature of the field. The thermal rating of the cable must also be considered, if heating is required while the pipe is in production mode.

First

The world's first installation of a DEH system took place in 2000 for Åsgard on the Norwegian continental shelf, where it is designed to heat the flowlines from 42.8?F (6?C) up to 80?F (27?C) to prevent hydrate formation.

Åsgard consists of a floating production unit and subsea production and water injection facilities. There are six 10-in. inter-field subsea production flowlines, heated by DEH. These flowlines are arranged in three groups, with two parallel flowlines to each of the three production templates.

The six production lines from the template to the platform are electrically heated from a point of approximately 98.4 ft to 164 ft (30 m to 50 m) from the template to approximately 98.4 ft to 164 ft (30 m to 50 m) from the production riser base at the platform.

Alternating current to the system is supplied from the Åsgard B topside power system. Maximum system voltage is 5.3 kV and maximum system current is 1,520 Amps - this is for heating from complete cool-down. Current is supplied subsea through dynamic electrical risers.

There are three electrical risers, one for every two flowlines, and each electrical riser includes four electrical power cables - one forward and one return phase - for each flowline.
The riser cable also includes six hydraulic pipes for valve control. The riser cable has an OD 212 mm and a weight in air of 297 lbs per 3.28 ft (135 kg per m).

Subsea ends of the electrical riser cables are connected to the electrical heating feeder and return cable at the subsea termination structure. The feeder cable is piggybacked on to the flowline and connected to the template end of the flowline. The return cable is connected to the flowline at Åsgard B.
Power is fed to the far end of the flowline by the feeder cable and returns in the steel pipe. The pipe is electrically connected to the surrounding seawater and therefore a part of the return current will pass through the seawater. Earthing electrodes (anodes) are mounted at each end of the pipe to form a well-defined, low impedance path for the current to the sea.