HOW LOW CAN YOU GO?.
There are actually four factors that should be considered when deciding when to act once it is apparent that the insulation resistance is deteriorating. Firstly, the safety of topsides personnel in the vicinity of the electrical power unit whose output is connected to the umbilical: low insulation resistance that affects both conductors in the cable can lead to stray currents that are hazardous to personnel and can potentially be fatal.
The level of insulation resistance at which these stray currents can become dangerous needs to be calculated and considered in the plan of action. There is a threshold below which, the application of V-LIFE is unlikely to be successful, this is recognized as the second factor that needs to be considered when deciding how low to let the insulation resistance drop to. This threshold is a function of the actual failure mode, but what this means is that if the insulation resistance is allowed to drop too low before pro-active action is taken, V-LIFE is no longer an option. The third factor that any operator should be aware of is that the degradation of the insulation can accelerate quickly – it is regularly seen that even after months of slow degradation, all of a sudden the insulation resistance falls and ‘drops off a cliff’. A slow decline in insulation resistance should not be ignored. Finally, the fourth consideration is the loss of copper from the conductors as a result of the monitoring measurement and/or the application of electrical voltage to the system; the lower the insulation resistance the more severe the copper loss, which could ultimately lead to fusing of a conductor and resultant open circuit.
As a result of these factors and risks to personnel and system operability, Viper recommends that proactive planning is taken as soon as the insulation resistance starts to fall and in any case as soon as the absolute value of insulation resistance reaches 1MΩ. Action should be taken whilst the insulation resistance is still in the 100s of kΩ.