Cross talk effects in subsea umbilicals are not always recognised.
If you look up ‘crosstalk’ in an English dictionary, you will find a definition similar to: ‘unwanted signals in one channel of a communications system as a result of a transfer of energy from other channels’.
From Maxwell’s equations, we know that electric currents in a conductor create electric and magnetic fields, and they can interfere with signals in other conductors. Cable designs try and eliminate cross-channel interference through the use of screened twisted pairs. The use of digital communications also means that the signals are less susceptible to crosstalk interference.
Therefore, crosstalk is not anticipated to be a problem in a subsea umbilical, however this can occur when circuits share a common signal return path.
Most subsea umbilicals use Insulation Monitoring Devices (IMDs) to detect any degradation in the conductor insulation material. IMDs apply a signal between the copper conductors and ground (the sea water is the return path). The IMD interprets the current flowing in this return path to provide both an insulation resistance and insulation capacitance measurement. If an umbilical carries more than one power pair, which is the norm, then there would typically be an IMD monitoring each power pair separately. However, each IMD will use the same ‘earth’ return path.
In umbilicals using more than one IMD, the common return path means that any inductive and capacitive coupling between channels can result in complex and varying currents. The consequence of this ‘crosstalk’ means that the IMD readings will not be accurate.
The problem is exacerbated if two channels co-exist as a ‘quad’ within the umbilical and also if the umbilical is long. The close proximity of the power pairs and the longer the length, the worse the coupling will be and, therefore, the crosstalk.
The IMD supplied by Viper Innovations, the V-LIM, has been designed specifically for subsea applications; it is optimised for a wide range of insulation resistance values and, in respect of the crosstalk issues discussed here, can incorporate an ethernet interconnect between all V-LIMs connected to any umbilical. The ethernet provides precise synchronisation for each V-LIM measurement cycle, ensuring that the effects of any crosstalk is completely eliminated by the V-LIM.
This means that the measurements are stable and accurate and are completely unaffected by any adjacent V-LIM. Viper have been replacing legacy IMD systems across the world where operators have been reluctantly living with inaccurate and varying readings.