Direct electrical heating of subsea pipelines - technology development and operating experience

Formation of hydrates is a well known problem in subsea production systems for oil and gas. As the unprocessed well-stream cools down, hydrates are formed at temperatures up to 25 °C depending on the water cut and pressure inside the pipeline. Several options are available to solve this problem. Traditionally, chemicals (methanol) have been used. Methanol reduces the critical temperature where hydrates are formed. Alternatively, hydrates can be prevented using thermal insulation in combination with additional heating. In this approach the well-stream is kept above the critical temperature for hydrate formation. This paper presents a method, direct electric heating (DEH), for heating of the pipeline by forcing a high electric current through the pipeline itself. The basic electromagnetic theory for design and dimensioning of the system is presented. The DEH system is a 1-phase system where the heated pipeline is not electrically insulated from the surrounding sea water. Hence, the system current is divided between seawater and the steel pipeline causing a need for additional sacrificial anodes along the pipeline. The design of the anode system for a pipeline with DEH system is given. In case of a fault (earth fault) in the cable, a protection system is needed. If the fault is not cleared in due time, the heat generated at the fault location may damage the pipeline itself causing severe violation on the integrity of the production system. Several options for fault detection and localisation are given. The paper presents comparisons between theoretical analysis and experiments on both full size models and scaled models where several aspects of the system have been tested. There are currently over 100 km pipelines in the North Sea where the DEH system is installed. Operating experience from these installations is presented in the paper.