Integrated production umbilical avoids hydrate, wax formation

Aug. 1, 2000
Kværner Oilfield Products (KOP) has passed an important milestone with the recent completion of the first stage of qualification of the integrated production umbilical (IPU).
The IPU termination is based on KOP's remote tie-in and connection system.
Click here to enlarge image

Kværner Oilfield Products (KOP) has passed an important milestone with the recent completion of the first stage of qualification of the integrated prod-uction umbilical (IPU). According to Ole Heggdal, project manager for the devel-opment, it should be ready for commercial application at the end of the year.

KOP surveyed oil companies' requirements in terms of key parameters such as length of lines and pressures before embarking on the development, in which its subcontractors have also been closely involved. It now plans to carry out further qualification of the IPU in cooperation with interested oil companies. It has held discussions with several, and some specific field applications are under consideration.

The IPU combines the normal umbilical function with a production flowline. The product is an extension of KOP's integrated service umbilical, which delivers chemicals for well or flowline injection through a steel tube center-line, around which are located power cables, hydraulic lines and other lines as required. The various lines are kept separate by PVC packing, and the whole is contained in a polyethylene sheath.

In the IPU, the central line is used for transporting production flow from a subsea well or wells. This means that its diameter has to be substantially increased, from the 2-3 in. typical of a service line up to 8-10 in., sufficient to transport the output of a couple of subsea wells, or a flow of up to 30,000 b/d of liquids. For the purpose of the development project, lengths of 8-15 km were stipulated. The main limitation on the length of a single IPU is imposed by the size of reel which can be accommodated on the installation vessel. Longer step-outs can, however, be catered for by joining two lengths of umbilical during installation.

The IPU concept offers a series of potential benefits, Heggdal says. By combining the flowline and the umbilical, installation operations are significantly reduced. The insulation which the umbilical packing provides for the flowline slows the cooling of the flowline contents, and by means of a heating system problems of hydrate and wax formation can be eliminated.

This, in turn, eliminates the need for operations which would otherwise be required to control such problems - like methanol injection, pigging, and flaring the pipeline contents as part of hydrate plug removal - which can be costly and environmentally undesirable. The flowline is well protected against impacts, and the temperature of its contents can be monitored, giving improved control over the flow and higher production availability. In sum, the IPU offers potentially significant reductions in both capital and operating costs.

However, to realize these benefits, reliability and operability have to be demonstrated in a number of areas. The issue of flow assurance is a key one - preventing the cooling of the flowline contents to the point where wax will form - around 40° C - or hydrates -- around 20° C. Though the umbilical packing provides insulation, a heating system is required if the temperature of the flowline contents is to kept above the danger zone during times of flowline shutdown. The heating system can use either electric power, or if it is available on the host platform, hot water.

For qualification of the IPU, a 60-meter test length was constructed at KOP's umbilical factory at Moss in southeast Norway and installed in the sea alongside the factory quay. It has an 8-in steel-tube central-line, and an overall diameter of 14 in. It includes both methods of heating - three high-voltage heating lines and three hot water lines are located equidistantly along the center-line. The electrical heating system, developed by Raychem, uses the principle of skin effect heat tracing, which is suitable for long pipelines. Heating is provided through a single insulated conductor in a tube which is attached to the pipe to be traced. Continuous monitoring of the temperature of the flowline contents is performed by a fiber-optic system developed by I.D. Fos Research and BFi Optilas, which uses a sensor specially developed for the project. The fiber-optic chain is housed in a stainless steel capillary 2 mm in diameter, which is installed almost in contact with the centerline.

Installation of the IPU test length at KOP's umbilical factory in Moss, Norway.
Click here to enlarge image

Testing was carried out using seawater as the pipeline medium. The results indicated that both heating systems were able to maintain the temperature of the flowline contents within the required limits. The data has also been used to create a computer simulation model which will be further developed and used for evaluating IPU applications under different conditions.

Further stages of qualification are likely to include weld testing and cycle testing. Repair procedures will also be qualified, but according to Heggdal this should not prove too difficult as they will build on repair procedures already qualified for the integrated service umbilical.

Installation is another important issue. Reeling the umbilical does not present problems as the tubes are laid helically and the pipeline is not rigidly held within the PVC packing. However, it is important that the IPU can be laid by existing installation vessels. When contacted, the four leading installation contractors all confirmed the feasibility of using their vessels for the operation. As an example, Coflexip Stena's Deep Blue can handle reels of 20-30 km lengths, and DSND's Skandia Navica lengths of 10-14 km.

KOP's Moss factory is able to build IPUs with large diameter center-lines. But if the level of demand justifies it, the company will consider building a dedicated IPU factory, though a more detailed market survey of possible future needs will be carried out before a location is decided.

The benefits offered by the IPU will be even greater if the product is used in combination with other new technologies, Heggdal points out. If used in conjunction with subsea separation, an IPU with an 8-in center-line could transport the same volume of hydrocarbons as two 10-in flowlines carrying a wellstream with a 50% water cut.

Often technology offering commercial benefits has an environmental downside, but the IPU does not involve such a trade-off, Heggdal stresses. "This is good green technology," he says. "It's both environmentally clean and commercially attractive."