Atle Harald Børnes

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

The formation of hydrates in the subsea production of oil and gas is a well-known problem. As the unprocessed well stream cools down, hydrates start to form around 25 degC, depending on the water cut and pressure in the pipeline. Several solutions are available to solve this problem. Generally, chemicals (i.e., methanol) have been used. Methanol reduces the critical temperature where hydrates are formed. Alternatively, hydrates can be prevented by using thermal insulation in combination with direct electrical heating (DEH). Thus, the well stream is kept above the critical temperature for hydrate formation. DEH heats the pipeline by forcing a large electric current to flow through the pipeline steel. The system model for design and sizing of the system is presented. DEH uses a single-phase system where the heated pipeline is electrically connected to the surrounding sea water. Thus, the system current is divided between sea water and pipeline, requiring additional sacrificial anodes on the pipeline. The anode system for a pipeline with DEH is discussed. There are currently more than 100 km of DEH pipelines on the Norwegian Continental Shelf. The operating experience from these installations is discussed. This paper presents the research and development for application of the system for pipelines with lengths up to 50 km

Adjustable frequency drive system for North Sea gas pipeline

The paper provides a brief history and overview of the 100 million cubic meters of gas per day Troll Project, the largest gas project in Europe with an expected lifetime of 50 years. A system description details the selection of five, 40 MW, 12 pulse variable speed drives for the onshore plant and their accompanying turbo rotor synchronous motors and 5 harmonic filters. We detail our lessons learned during selection of the drives, back to back testing, and compressor string testing on through startup of the unit trains.

DEH Cable System Preventive Protection With Distributed Temperature and Strain Sensors

Hydrate and wax formation in subsea flowlines is a major cause of production impairment. Among various approaches used to minimize the risk, Direct Electrical Heating (DEH) is being applied. DEH is based on passing a current through the pipe wall to mitigate heat losses from the fluid to the surroundings during events which require flow assurance measures. The Piggyback Cable, a high voltage cable attached to the DEH pipeline, is during operation exposed to thermal and mechanical loads which may be critical for the integrity of the DEH system. The overall safety requirement is that any potential Piggyback Cable fault is detected and disconnected from the power source before damage is caused to the pipeline.Conventional cable fault detection methods based on current measurements give adequate protection for the main part of the pipeline. However, for the far end of the Piggyback Cable complementary fault detection is required. A method based on fiber break monitoring has been qualified for this purpose. The new method is implemented in the North Sea on two DEH pipelines operated by Statoil, 43 and 21 km long respectively. The protection is facilitated by standard single-mode fibers integrated into the DEH cables.Although not basis for the design the integrated fibers open up possibilities for temperature and strain sensing using stimulated Brillouin scattering. Sensing has been performed on a 43 km DEH pipeline using the DITEST AIM (Distributed Temperature and Strain Asset Integrity Monitoring). Analysis of the sensing results reveal that distributed fiber optic sensing is capable of pin-pointing thermal events and strain induced loads for an object of this length.Copyright