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.