Nick Terdre, Contributing Editor
Direct electrical heating (DEH) has become one of Statoil’s preferred methods for securing flow assurance in multiphase pipelines. The technology is installed on the Åsgard, Huldra, Kristin, and Urd fields and also is being applied in the Tyrihans, Alve, and Morvin developments. A retrofit system has been qualified for the Ormen Lange field.
Nexans Norway and Sintef Energy Research in Trondheim have been partners in developing the technology since it was first established through a joint industry project in the mid 1990s. Since then, it has been further developed to meet ever more demanding applications.
“The concept has developed from something small into an important tool for our flow assurance engineers,” says Atle Børnes, a Statoil DEH specialist.
How it works
DEH involves passing an alternating current through the pipeline wall. The current is supplied through a cable piggybacked to the pipeline and connected to it via terminations at each end. The cable is strapped to the pipeline on the pipelay vessel, and the terminations are made up at the same time.
Installation of the Tyrihans production pipeline with DEH system attached. View from the stinger of the lay bargeAcergy Piper.
Additional anodes are attached to the pipe wall in the current transfer zone, which extends about 50 m (164 ft) each side of each termination, to allow current to leak into the sea in a controlled and safe fashion and to avoid the risk of AC-induced corrosion.
The power requirement for heating a pipeline to the desired temperature depends on several parameters, including the dimension of the pipeline and the efficiency of the thermal insulation. In the DEH systems installed to date, the power rating has been on the order of 1-2 MW, and the current requirement has been up to 1.5 kA. For the Tyrihans application, the DEH power demand will be about 10 MW, due to the increased length of the pipeline.
Topsides equipment includes a transformer especially designed for DEH duty. A degree of operational flexibility must be incorporated in the transformer to enable it to produce a range of voltages to meet the requirement for passing different currents through the pipe.
Operational experience with DEH systems has been good, according to Børnes. Problems requiring some rectification have arisen only during installation. Once in operation, the systems all have functioned as intended. The Huldra system has seen the most use, being activated some 60 times since it was commissioned in May 2002.
There are two main functional requirements for DEH systems: Firstly, to maintain the temperature of the pipeline fluids above the hydrate formation temperature during shutdowns; and secondly, to raise the fluid temperature from ambient to above the hydrate formation temperature in the event that the fluids have cooled completely (for example, because the DEH system was unavailable for some reason during a shutdown).
The systems on Huldra and Tyrihans additionally have been designed to operate continuously during the tail-end period of production, as it is expected that the volume of fluids will be reduced to the point where they will not maintain a temperature above the hydrate formation level without assistance.
While Sintef has taken care of DEH rating calculations, Nexans has been responsible for developing the power cables and all of the subsea accessories. One of the initial challenges was to take a traditional XLPE (cross-linked polyethylene) submarine cable and put it into a completely new application with quite different operating modes, says Torunn Clasen, Nexans project manager for DEH projects.
Such a cable normally has armor to enable it to take the loads generated during installation and operation. The problem is that armoring is not compatible with the working principle of DEH.
A specific mechanical protection was designed for the Åsgard project. It is integrated into the cable to achieve the impact requirement. The protection is a “sandwich” of three protective sheets made of plastic material that does not affect the efficiency of the DEH system.
The cable termination was also completely new and took a lot of engineering to develop, Clasen says. It had to be designed in such a way that if the installed cable failed, a replacement could be connected.
Innovative evolution
Subsequent projects have thrown up new challenges. Kristin, with a reservoir temperature of 170° C (338° F), presented the challenge of high temperature. Kristin also needed to have trawl-board protection, a need that could not be met by the existing solution. A specific mechanical protection was designed to address these issues. Nexans, in cooperation with Statoil, designed an external protection profile known as the mechanical protection system (MPS). The MPS consists of a segment that sits on the pipeline and contains a channel or groove for the cable. An upper part is fitted after the cable has been laid in the channel. The MPS is strapped to the pipeline.
This system has become standard and has been used on the Urd and Tyrihans DEH systems. It also has been specified for upcoming projects.
Installation of the Urd pipeline. The DEH cable is contained in the mechanical protection system that is strapped to the pipeline.
This solution has the advantage of coping well with thermal expansion of the pipeline when hot contents are flowing through it. A 20-km (12.4-mi) section of pipeline, for example, can elongate by as much as 30 m (98 ft). If the cable is strapped directly to the pipeline, it is exposed to high tensile forces during thermal expansion. The force generated at the cable termination can be as much as 15 metric tons (16.5 tons), Børnes says. With the MPS system, the cable is installed with low back-tension, which makes it possible to feed in a cable of greater length than the pipeline it is piggybacked to. This provides extra length that can be taken up during thermal expansion.
The next major challenges came with Tyrihans. Here, the pipeline, at 43 km (27 mi), is much longer than the previous longest, which was 16 km (10 mi) on Huldra, while the main pipeline diameter is 18 in. (45.7 cm), compared with the previous maximum of 12.6 in. (32 cm) on Urd. Due to the long length, a DEH cable rated for 52 kV had to be developed and qualified for Tyrihans.
In the earlier DEH piggyback cables, the metal screen through which charging currents drain to ground is made of stainless steel. As length increases, the screen voltages become too high for the outer insulating sheaths to handle. An alternative method for draining the charging currents was required for Tyrihans.
One alternative considered was to divide the line in two and install a separate DEH system on each part. That way, the screen voltage in each section would correspond to that for half the length. Unfortunately, this option resulted in much higher costs due to the increased requirement for feeder cable and the need for terminations and anodes in the mid-section current transfer zone.
The solution involved the use of a semi-conductive outer sheath on the cable, which allows the charging currents occurring along the length of the cable to be continuously drained into the sea.
The Tyrihans DEH cable is expected to be in operation for five years during the field’s expected 20-year production life - two years in terms of accumulated shutdowns during the first 17 years, and then continuously during the last three years. A long-term aging program was carried out for the Tyrihans DEH cable. The program began in 2003 and concluded in 2006.
The next step
A 20-km (12-mi) DEH system has been qualified for retrofit use on Ormen Lange. The giant gas field, which has been developed by Norsk Hydro, relies on MEG injection as the primary means of flow assurance. DEH likely will be deployed in a novel way to assist plug removal operations in either of the two 30-in. (76-cm) production pipelines.
It is intended for use in the 20-km (12-mi) stretch nearest the field, where the pipelines run up steep slopes from a water depth of 850-550 m (2,789-1,804 ft), and the temperature of the water is below freezing.
In this application, the temperature will be raised above the ice melting point but not the hydrate melting point, as is the case with a conventional DEH system. Statoil, a partner in Ormen Lange, was responsible for the DEH qualification activities.
The Ormen Lange application expands the operating envelope of the DEH system in several ways. The water depth is one. Previous systems were limited to depths of 500 m (1,640 ft). The 30-in. (76 cm) pipeline size is another. The cable must take much greater loads than a cable piggybacked on a pipeline. And the extreme water depth adds to the loads. This is also the first application in which ice plug melting has been assessed.
Another factor is the lack of thermal insulation on the Ormen Lange lines, which means a higher current is required for heating. The electrical requirements for the cable comprise a current demand of 2.6 kA, system voltage of 52 kV, and power demand of 6.4 MW.
A completely new cable design has been qualified, for which Nexans is seeking a patent. The design includes a steel core to take the installation loads. Because of its length, it has a semi-conductive outer sheath, similar to the Tyrihans DEH cable.
In case of cable damage, and to avoid having to retrieve the whole cable to the surface, a wet repair method is being developed to excise damaged sections and to splice in new lengthsin situ, Clasen says. All of this obviously has to be done using ROVs.
There are other directions in which DEH technology could be developed. One idea involves combining a DEH cable with a conventional subsea power cable, for example, for running subsea pumps, Børnes says. When normal production is under way with the subsea pumps in operation, the DEH system is not needed. When the DEH is needed, the pumps are not. So savings could potentially be made by having one cable serve both functions.
Developing the technology for use with ever longer step-outs and in ever greater water depths also would enhance its value as a subsea tool. Within a few years, DEH could be qualified for pipeline lengths in excess of 100 km (62 mi) and water depths greater than 2,000 m (6,561 ft), Børnes says.
For Nexans, the DEH experience has provided valuable lessons, in Clasen’s view. “We’ve had very good cooperation with Statoil on these projects,” she says. “It’s great that we have some oil companies that push the technology and challenge us as a supplier. The DEH technology pushes the development of subsea power cables forward. We learn more about the cables, and that knowledge can be applied elsewhere.”
Only Statoil has applied DEH technology so far, Børnes says, but other oil companies are showing interest. While some of the parts, such as cable designs, have been patented, there is no patent on the method itself, so others are free to develop their own versions. In the not-so-distant future, it would not be surprising to see others bring it into use, he says.
