This gas production rig in Liverpool Bay has three platforms. Two are made of mild steel and the control platform is a jack-up using a high tensile steel. Each platform has its own set of sacrificial anodes but the direct mechanical and electrical connection of the high tensile steel rig to the mild steel ones would result in a corrosion battery. This means that the protection on one rig would partially cancel that on the other, leading to accelerated corrosion of the structure. To combat this, all the bridge components and pipes to the control platform are insulated and a DC voltage source makes up the difference in potential to cancel the corrosion battery.

We were commissioned to design and make the power supply shown here. It has a bi-directional continuous output of up to 2 volts and 100 amps. It can take its control parameter from the measured potential difference between the platforms or from a signal generated by the protection monitoring equipment on the rig.

Maximum limits can be set for input and output parameters. The maximum and minimum limits for the alarm can also be set. Testing can be performed by switching to manual operation mode. The power supply outputs, rig differential potential and the control setting are captured by an analogue to digital converter and logged by the protection monitoring equipment over an optically isolated link.

The supply ran continuously for seven years (60,000 hours) except for a very short period after the rig was hit by a direct lightning strike. The result was two blown sense lead protection fuses. These were quickly replaced and the protection was straight back on line.

What happened after 60,000 hours was that the internal fan bearings of the isolation supply failed, which led to a power semiconductor overheating and a failure of this module. This was replaced and the complete unit was upgraded to allow the operators to optionally limit the output current to one direction only. The unit was fully tested to its limits and given a burn-in test. It is still running after a further 60,000 hours.  Compared to a truck running at an average of 60kph, it did just under 4 million kilometres between services!