Distributed temperature sensing fibre optic cable

A system for monitoring the temperature of a remotely operated vehicle (ROV) (806, fig. 8) umbilical cable (800, fig. 8) in an oil well or a subsea pipeline. The umbilical (800, fig. 8) supplies power to the ROV (806, fig. 8) in order to perform a particular task and contains an optical fibre 702 which is coupled to a laser 706. Backscattered light is detected by a spectrometer 708 and a distributed temperature profile T(z) along the optical fibre 702 is measured. Overheating or weaknesses in the umbilical cable (800, fig. 8) and improvements in power delivery can be identified.