LH4-1 oil field is a subsea tie-back which was developed by CNOOC in 2012. After a short period of production, the subsea control umbilical started with low Insulation Resistance (IR) of its electric control quads. Extensive trouble shooting work immediately started. The trouble was identified to be on the main umbilical. The subsea electric control was swapped to the spare quad. After about a year, the spare quad also had zero IR. In order to resume the subsea control, feasibility of using the spare Medium Voltage (MV) power cable triads as control channel was studied immediately. The idea of using three core triad to establish dual communication channels with two frequency bands was thoroughly studied and it was found theoretically feasible. Originally the communication was one loop controls 4 wells, now with the new arrangement one loop controls 8 wells, 3-core with one as common return makes two loops. So the new MV cable communication configuration can still achieve subsea control with 100% redundancy. Subsea special bridge flyleads were designed and manufactured to convert the 70mm2 3-core MV flylead to 10mm2 4-core communication flylead. Offshore intervention work was done in 2014. After installation, subsea communication to all 8 wells was re-established as expected. The signal qualitywas much better that the original umbilical possibility due to the large MV copper cores.

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