Common Failures of Submerged Umbilicals & Cables
The failure of submerged cables can occur due to a variety of reasons, typically involving open circuits, short circuits and electrical insulation failure. The most common cause of subsea electrical failures is the ingress of water into the cable insulation, which decreases the IR and may produce short circuits or leakage to earth. These faults often lead to loss of power and/or communications to subsea equipment and have the potential to halt production from subsea wells. It has also been demonstrated that prolonged operation of subsea power circuits with particularly low Insulation Resistance can lead to the loss of copper from the conductor at the fault site which in turn can lead to the complete loss of operability of the system.
Insulation resistance needs to be maintained at safe levels to not only shield against loss of production but also to protect against electric shocks to humans. When your cables, connectors and umbilicals are new, the electrical insulation should be in excellent condition. Nevertheless, insulation is subject to many effects which can cause it to fail – electrical stresses, mechanical damage, moisture or general ageing. As pinholes or cracks develop, water will penetrate the surfaces of the insulation, providing a low resistance path for leakage current.
Sometimes the drop in insulation resistance is sudden, but usually, it drops gradually, giving plenty of warning, if checked periodically. Good insulation has high resistance; poor insulation, relatively low resistance, but what really matters is the trend in readings over a time period, showing lessening resistance and warning of impending problems.