A development program to further increase the competitiveness of the direct electrical heating technology has been initiated. By increasing the power frequency from 50/60 to 100-200 Hz, less current is needed to develop the same heat generation in the pipe. A test program is running with the purpose of studying AC corrosion on the cathodically protected steel pipelines at higher frequencies. This article presents results from initial tests, showing that corrosion on steel specimens are present at 60 Hz, but that zero or very little corrosion is present at 100 and 200 Hz. On the aluminum specimens, corrosion rate is reduced by 55 % from 60 to 100 Hz. and is somewhat further reduced at 200 Hz.
When a hydrocarbon production pipeline is shut down, it gradually cools to a temperature level where hydrates or wax may be formed. If hydrates and wax are formed in a pipeline, these may restrict the flow and may in a worst case situation block the entire pipe. Removal of hydrates is very difficult and potentially hazardous. One of the traditional methods for hydrate or wax prevention is to add chemical inhibitors to the well-stream. In continuous operation this method has considerable operational costs, (Hesjevik, 2006). Chemical treatment requires storage tanks, inhibitor regeneration plants, piping and pumps on the platform or vessel that occupy space and deadweight tonnage, as well as a sizeable subsea piping infrastructure. One of several alternative qualified methods to prevent hydrates and wax formation is direct electrical heating (DEH).
A 3-year development program for further increase of the competitiveness of the DEH technology was initiated in 2016. An innovative step by increasing the power frequency is proposed. Prestudies indicate clear benefits by going from a DEH-system operating at normal power frequency of 50/60 Hz to a system operating at a frequency ranging between 100 and 200 Hz, (Lervik, 2016). In a DEH-system, electric alternating currents of some hundred amperes are transferred from the steel pipe to the surrounding seawater. Thus, AC corrosion aspects must be considered. At present, DEH systems are only qualified for use at 50 and 60 Hz power frequency. One of the outstanding issues is related to AC corrosion.