What is often not fully appreciated is the benefit of being able to monitor the integrity of the insulation material of the electrical cables from the initial installation, when the insulation is known to be good, and not just monitoring it when the cable is close to failure (a low insulation resistance). A new cable is ideally commissioned with an IR value in excess of 1GΩ. Over-time there will be some IR degradation due to the hygroscopic properties of some of the insulation material and also due to any manufacturing flaws or inclusions coupled with the fact that the cable is in a seawater environment. By measuring the full range of IR, a declining IR trend can be identified earlier in the degradation process.
The majority of LIMs on the market, have a relatively low range of measurement with a maximum measurement capability of 10MΩ – clearly unable to measure any degradation until the insulation resistance has dropped to a level below this maximum measurement capability. Newer offerings to the market have far higher ranges, but generally, there is a trade-off with measurement accuracy and measurement repeatability at the lower end of the measurement range. Measurement at the low end is where it is critical to both production and personnel safety. Ideally, therefore, the LIM should have the high-performance capability at the ‘low end’ of its measurement range but have a capability to measure insulation resistance at 1GΩ, albeit at reduced accuracy. For the monitoring of subsea installations, a measurement range of higher than 1GΩ provides very little added value since this would typically be the level of insulation resistance presented to the LIM for a newly installed subsea system. Any change in measurements above 1GΩ could be as a result of the LIM measurement accuracy just as much as a change in the condition of the insulation. In addition, no decisions or plans would be made in practice as long as the insulation reading was in the hundreds of MΩ. It is therefore important that the LIM selection is made on the needs of the monitoring function rather than just selecting on the highest measurement value quoted on the data sheet – ‘bigger isn’t better’ when it comes to LIMs.
Existing topside LIMs only provide a single insulation resistance measurement for the complete subsea system. When the IR drops, the LIM provides no information on the number of faults or their location. The IR alone does not tell the operator which component is at fault and resolution of an issue often requires a costly and time-consuming diagnostic campaign. However, modern technology offers operators a solution to this challenge. LIM devices are now available that can be installed subsea to form a distributed LIM network that communicates back to a topside monitoring device. These units measure the IR of the entire system but critically they can also determine the IR upstream and downstream of their location. By placing these in strategic locations in the subsea network, a complete subsea distribution system electrical integrity map is created; giving operators not only information about the general insulation integrity beneath the water but also the information required to reduce the cost of maintenance and repairs. Indeed, having the visibility of which leg of the subsea system is failing offers a huge financial gain. Knowing where a fault lies, also reduces the number of disconnects required during the fault-finding process and so reduces the risk of introducing additional faults or weaknesses in the system.