The effects of crosstalk in submarine umbilicals are not always recognized.
If you search for “crosstalk” in an English dictionary, you will find a definition similar to: “unwanted signals in a channel of a communication system as a result of transfer of energy from other channels”.
From Maxwell’s equations, we know that electric currents in one conductor create electric and magnetic fields, and they can interfere with signals in other conductors. Cable designs attempt to eliminate inter-channel interference through the use of shielded twisted pairs. Using digital communications also means that the signals are less susceptible to crosstalk interference.
Therefore, crosstalk should not be an issue in an underwater umbilical, but it can occur when circuits share a common signal return path.
Most submarine umbilicals use insulation monitoring devices (IMDs) to detect any degradation of the conductor insulating material. IMDs apply a signal between copper conductors and land (seawater is the return path). The IMD interprets the current flowing in this return path to provide both a measurement of insulation resistance and insulation capacity. If an umbilical carries more than one power pair, which is the norm, then there would usually be an IMD monitoring each power pair separately. However, each IMD will use the same “land” return path.
In umbilicals using multiple IMDs, the common return path means that any inductive and capacitive coupling between the channels can result in complex and varying currents. The consequence of this ‘crosstalk’ means that IMD readings will not be accurate.
The problem is exacerbated if two channels coexist in a ‘quad’ within the umbilicus and also if the umbilical is long. The closer the power pairs are and the longer the length, the worse the coupling will be and hence the crosstalk.
The IMD provided by Viper Innovations, the V-LIM, was designed specifically for subsea applications; it is optimized for a wide range of insulation resistance values and, with respect to the crosstalk issues discussed here, can incorporate an Ethernet interconnect between all V-LIMs connected to any umbilical. The Ethernet provides precise timing for each V-LIM measurement cycle, ensuring that the effects of any crosstalk are completely eliminated by the V-LIM.
This means that the measurements are stable and precise and are absolutely unaffected by an adjacent V-LIM. Viper replaced old IMD systems across the world where operators reluctantly lived with inaccurate and variable readings.
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