The novel technique of applying electrical heating to prevent hydrate formation in multiphase pipelines has been successfully applied on the Norwegian Continental Shelf. The first pipelines where direct electrical heating (DEH) is applied have recently been put into operation. DEH is based on applying electrical current through the steel wall of the pipeline and heat is hence generated due to the resistive losses of current. By applying DEH the fluid temperature can be maintained above the hydrate equilibrium temperature (HET). Operational experience has proven that the concept is successful with regard to temperature control. It has been shown that the DEH system is capable of heating the pipeline inventory from the seabed temperature to a temperature above the HET.
In this paper the DEH-system installed on the Huldra field development is described in detail and the application of the DEH system with respect to hydrate prevention and possible hydrate remediation is discussed.
In November 2001 production commenced on the Huldra field. Huldra is a gas/condensate field located in blocks 30/2 (PL051) and 30/3 (PL052) in the North Sea at a water depth of 125 meters.
The Huldra field originally had an estimated in place gas reserve of 19.4 GSm3 and a condensate reserve of 7.4 MSm3. Measured reservoir conditions are 675.5 barg and 136 degrees Celsius.
The field is developed with a not-normally-manned wellhead platform. As from July 2002, Huldra has become remotely controlled from Veslefrikk. The well streams are routed to a one-stage production separator on the Huldra platform. Gas from the production separator is cooled and liquids are removed in a scrubber. The gas saturated with water is fiscally metered, inhibited with MEG and routed to Heimdal for further treatment. Liquid production, i.e. condensate and produced water, from the production separator and the scrubber are mixed and transported to Veslefrikk, for further treatment. Figure 1 illustrates the infrastructure.
Fig.1. Huldra infrastructure(Available in full paper)
A schematic view of the topside process on the Huldra platform is shown in Figure 2.
Fig.2. Huldra topside process(Available in full paper)
At Veslefrikk a three-phase inlet separator receives the flow from Huldra. The capacity of this vessel is limited to handle 250 m3/h of condensate, 40 m3/h of water and 25000 Sm3/h of hydrocarbon gas. The gas is routed to the existing 2. stage compressor and the condensate to the existing 2. stage separator at Veslefrikk B. There is installed a separate produced water treatment system for the Huldra water due to high scaling potential upon mixing with Veslefrikk water. For the topside process the hydrate control philosophy on the Huldra platform is based on the use of insulation and heat tracing. Parts of the process that are not covered by heat tracing are designed with slope towards areas that are heat-traced.
The rich gas pipeline to Heimdal is inhibited using continuous injection of MEG. The dew point of the export gas is controlled to 35°C before being exported and the amount of MEG used for inhibition is based on the water dissolved in the gas.
