The International Marine Contractors Association (IMCA) has issued at Safety Flash about an incident that saw a dive support vessel (DSV) lose power while conducting diving operations.
During diving operations, the vessel experienced a power loss scenario in dive control where both circuit breakers after the uninterruptible power supply (UPS) tripped, causing a loss of power to the electronic components of the dive system. Power was re-established using the UPS bypass switch. The dive was aborted to carry out an investigation. No-one was harmed.
What went wrong? What were the causes?
During setup of the dive system the UPS was set to work on approximately 220 VAC input (the input from a vessel generator power supply typically fluctuates). The UPS was also set up on the assumption that the power from the vessel would not drop below 85% of the expected supply voltage (a drop to 187 VAC).
Investigation confirmed that this is in fact what happened – the supply voltage did indeed drop below the minimum threshold of 187 VAC. When this happened, an automatic and internal bypass in the UPS kicked in to keep power supplied to the outlet side. Therefore, the low voltage was transferred through the internal UPS bypass and into the circuit breakers that protected all the electronic equipment in dive control.
When attached to a fixed output load (such as electronic equipment), a voltage drop will necessarily cause an increase in current flow. The current increased to the point where the circuit breakers tripped to further protect the electronic equipment. This disconnection of the power to the electronics shut them down without the UPS taking over or giving any sort of alarm condition.
This voltage drop may have been gradual and therefore was not noticed. It wouldn’t have affected any other circuit breakers as there wouldn’t have been any power spike. However, since the UPS corrects to 85% of the input voltage, once that limit was reached it automatically transferred to the bypass, thereby immediately causing a power spike on the output side as the voltage changed from 220 VAC (UPS corrected) output to <187 VAC (UPS bypassed output).
Further investigation revealed that the low voltage was a result of a large current draw, in turn as a result of other equipment on the same power bus being used at the same time. In this case, it was discovered that the remotely operated vehicle (ROV) was being recovered from the water at the time of the power loss. On this vessel, the dive system power is taken from the same power bus as that which is used for the ROV.
What actions were taken? What were the lessons learned?
Immediately after the incident, action was taken to verify that the ROV BUS tie breaker on the vessel 440 VAC power distribution panel was in the open position. This circuit breaker needs to remain in the open positon at all times during diving and chamber operations. The UPS on all similar vessels was set to prohibit the use of the internal bypass again. This means that if at any point a low voltage scenario occurs again, the UPS will switch onto the battery backup, give an audible alarm and the internal bypass will never be used. The dive supervisors should be made aware that this alarm may occur at any time a high draw/low voltage situation is experienced again and all they need to do is to call the dive techs to confirm the UPS status.
A daily ‘health’ check of the UPS system has been added to the planned maintenance system for all company UPS systems. Due to the nature of the issue, the manufacturer was contacted during the investigation to seek advice on the UPS. The manufacturer had not reported similar occurrences, but a report will be sent to them for their records.