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Direct Electric Heat Pipe-in-Pipe System

Published on 18 Feb 2015 by Lana Hajovsky


This article discusses the Shell/INTEC system consists of a pipe-In-pipe (PIP) flowline warmed by direct electrical heat (DEH). With DEH, the electrical output of a generator is applied to the ends of a metal pipe and the pipe acts as a resistive heating element. Single phase AC electrical power is fed into the inner and outer pipes via an insulating bulkhead joint located at either the center of a flowline segment or at the topsides end. Power flows down the pipeline and then returns via an electrical connection between the two pipes at the end of the pipeline segment. The applied electrical power is divided between the inner and outer pipes. 45% of the electrical power goes into heating the inner pipe and 55% of the applied power goes to heating the outer pipe. The inner pipe must be adequately supported within the outer pipe and this is accomplished by insulating centralizers. The amount of driving voltage that can be applied is proportional to the distance between the inner and outer pipes. The annulus between the two pipes must also be very clean and dry, conditions which are very difficult to attain on the best of lay barges. The Shell/INTEC system is installed using the J-lay method.

System Description

The Shell Serrano and Shell Oregano flowlines are end driven. Electrical power is applied at the topside end and a bulkhead fitting completes the circuit at the opposite end.

The Shell Nakika flowline segments are center driven. Electrical power is applied at an electrical insulating joint located at the center of a flowline segment and an electrical bulkhead joint completes the circuit at each end.

One of the design limitations of the first systems that Shell qualified was the voltage limit of the wet-mate-able subsea electrical connectors. At the time of system qualification, the maximum continuous voltage permissible was 1.9 kV and the maximum current was 1,100 A. Tronic has an entire product line of ROV wet mate-able connectors and the design that was used on the original Shell systems is good for 3.3 kV with the same current limit.

The other limitation on voltage drive is the annular gap. The cleanliness, moisture content, and centralization of the annular gap influences the maximum voltage that can be applied. In practice, the dimensional tolerance of the annular gap is maintained by centralizers. The stiffness and spacing of the centralizers determines how close the inner pipe will be to the outer pipe once the installation is complete, and once thermally induced strains are present.

The Shell/INTEC system is installed by the J-lay method. Offshore pipe lay operations are conducted outdoors, on the deck of the lay barge. Weather conditions such as rain and humidity, and manufacturing operations such as water-cooling during welding pose a risk of getting water in the annulus during PIP J-lay. Pipe lay operations are controlled to minimize the risk of water, and electrical inspections of each joint were made to assure that no water entered the annulus. Conditions that might cause electrical shorting are thus avoided during the PIP pipelay operations. If shorting is discovered, the pipe laying operation reversed and replaced the pieces causing the short. Desiccant is placed in the annulus at intervals to absorb condensed moisture. A five-foot-long polyurethane “plug” is cast in the annulus during pipelay every third quad joint. The insulation in the form of 3 pounds per cubic foot polyurethane foam is applied to the outer surface of the inner carrier pipe. A gap is left in the annulus between the insulation and the outer carrier pipe, so that the inner and outer pipes could be assembled and welded separately in the J-lay assembly process.

The Shell system is covered under a series of United States Patents including US‑6,292,627, US‑6,142,707, and US‑6,688,900. Exclusive license to the Shell patents has been granted to INTEC. Payment of a license fee of USD350K is required for each use of the design. The definition of what makes up a “licensable use” of the design is a point of negotiation.

Installations

The described system has been successfully installed on Shell Serrano, Shell Oregano, Shell Nakika, and on Shell Habanero as listed below.

Project

Serrano

Oregano

Nakika

Habanero

In Service Date

2002

2002

2003

2003

Flowline diameter

6” x 10”

6” x 10”

10” x 16”

6” x 10”

Water Depth

1040 m

1040 m

2000 m avg.

610 m

Flowline Material

Carbon Steel

Pipe-in-pipe

Carbon Steel

Pipe-in-pipe

Carbon Steel

Pipe-in-pipe

Carbon Steel

Pipe-in-pipe

Flowline description

1 segment

1 segment

6 segments

6 segments

Flowline total length

10 km

12 km

37 km

39 km

  • Shell Serrano and Oregano - Two 6”x10” PIP flowlines, one is 10 kilometers long and the other is 12 kilometers long. The system is used for hydrate remediation and to assist during cold startups. The system is powered through electrical insulation joint topsides. Cold startup requires 400 to 500 kW (50 W/m). Power requirement to maintain fluid temperatures is 250 kW (25 W/m).
  • Shell Nakika - One 10”x16” PIP flowline loop, 37 kilometers long, divided into six segments, the longest segment being 13.2 kilometers. The 13.2-kilometer segment requires approximately 60 W/m, which is 1,000 kVA at a power factor of 0.8. Voltage applied at the midline connector is as high as 950 volts for the 13.2-kilometer segment, and as low as 140 volts for the shortest segment of 1.6 kilometers in length. The system is used for hydrate remediation, and is activated by ROV intervention. Subsea ROV electrical connectors are provided at an electrical insulation joint at the center of each segment.
  • Shell Habanero - Two 6”x10” PIP, 11 miles long. The 6” inner pipe is insulated with 1.3” thick-jacketed polyurethane foam.



Tags: Direct Electric Heat Pipe-in-Pipe

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