Most subsea umbilicals utilise Insulation Monitoring Devices (IMDs) to detect any degradation in the insulation material surrounding the power conductors. IMDs from different manufacturers operate in slightly different ways but fundamentally, they all apply a signal between the copper conductors and ground (i.e. using the sea water and ground as the return path). 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. The signal that the IMD applies between the conductor pair and ground is used to measure the leakage current that is flowing through the insulation material. Ideally, this current should be negligible in magnitude, but will increase as the insulation degrades. In order to fully characterise the cable integrity, the IMD should interpret the current flowing to provide both a resistive and capacitive component measurement (insulation resistance and insulation capacitance).
In multi-channel umbilicals utilising more than one IMD, the multiple IMD voltage sources and common return path means that any inductive and capacitive coupling between channels can result in complex and varying return path currents. Since each IMD is effectively measuring the return path current, the consequence of this coupling or ‘crosstalk’ means that the reported Insulation Resistance or Insulation Capacitance from each IMD will likely be unstable (varying in value) or will simply read too high or too low. The consequence is that the IMD readings will not be accurate and will erroneously report incorrect measurements.
The problem is exacerbated if two power pairs (channels) co-exist as a ‘quad’ within the umbilical and also if the umbilical is long. The closer the proximity of the power pairs and the longer that the pairs run together (length) the worse the capacitive and inductive coupling will be and therefore the worse the crosstalk.
The IMD supplied by Viper Innovations, the V-LIM, has been designed specifically for the subsea applications and umbilicals; it is optimised to measure accurately over a wide range of insulation resistance values, can accommodate the high capacitances of long umbilicals, and in respect of the crosstalk issues discussed in this article, can incorporate an ethernet interconnect between all V-LIMs connected to any one umbilical. This ethernet connection provides precise measurement synchronisation for each V-LIM, ensuring that the V-LIM hardware and software completely eliminates the effects of any crosstalk.
The result of this synchronisation means that the electrical integrity of each power channel can be measured, providing stable and accurate data completely unaffected by any adjacent V-LIM. As a result of the exceptional results obtained, Viper have been busy replacing legacy IMD systems across the world where operators have been reluctantly living with highly inaccurate and varying readings. The synchronisation is bespoke to the V-LIM and so any upgrade that requires the synchronisation function will require all IMDs to be replaced with V-LIMs.