The UPS and downs

4.1   Overview

A DP equipment class 2 vessel was transiting within an offshore wind farm. Four of its five thrusters were selected to DP, with three of four generators running, the bus ties closed and two redundant groups available. Two GNSS position reference systems were selected to DP, together with three gyros, three wind sensors and three motion reference sensors. Visibility was 12 nautical miles, with a significant wave height of 1.5 m, wind speed of 22 knots and a real current of 0.6 knots.

4.2   What happened?

At approximately 07:53 hrs, while the vessel was transiting in DP mode, the DPO observed the loss of Gyros 2 and 3 together with alarms from a 230V backup power supply. The telephone system remained powered but was inoperable. The Master contacted the engineering team for a status update while the bridge team stopped the vessel and maintained control in DP1. The client was informed of the delay and the vessel’s downgraded DP status.

The ETO had been following up a previous recurring UPS alarm and, in accordance with the manufacturer’s instructions, attempted to reduce demand on the unit by enabling ECO mode. During the changeover, the batteries did not provide the required power, and a momentary interruption caused dependent systems, including the telephone system and gyros, to reboot.

The vessel informed shore management and moved out of the field to restore full DP2 status. All systems were reinstated approximately one and a half hours later, after which the DP setup checklist was completed before the vessel re-entered the field.

Sequence of events during the incident

4.3   Findings

The event form identified the following key findings:

• The loss of Gyros 2 and 3 first alerted the bridge team to the event.
  
  
• The event was categorised as a power-distribution failure, with an electrical component failure and battery condition recorded as underlying factors.
  
  
• The ETO did not inform the bridge before changing the UPS to ECO mode because a smooth transition was expected.
  
  
• Changing the UPS operating mode caused an unexpected interruption to dependent systems, including the gyros, and reduced the vessel to DP1.
  
  
• The manufacturer’s instructions did not indicate that changing to ECO mode could cause a power fluctuation or interruption while the UPS remained supplied by normal power.
  
  
• The event form recorded a hardware failure associated with lack of maintenance and identified the need for repair and procedural changes.
  
  
• Following the event, the vessel was moved to a safe position and a function test was completed using the DP setup checklist. The event was closed out and the lessons learned were shared internally and externally.
  
  

4.4   Conclusion

The event resulted from work on a critical UPS being undertaken without the bridge team being informed or the effect on dependent equipment being fully understood. Although the ETO intended to reduce demand on a UPS whose batteries required replacement, the change to ECO mode caused an unexpected interruption that affected the gyros and other dependent systems, reducing the vessel to DP1.

The event demonstrates that no work, equipment status change or operating-mode change should be undertaken on UPS equipment supplying DP-critical or communication systems during critical DP activity without prior notification to the bridge, confirmation of the equipment supplied by the UPS, and an assessment of the operational consequences. Even a change perceived as routine, such as selecting a different UPS mode, can interrupt dependent systems and affect DP capability. Battery condition, maintenance arrangements and manufacturer guidance should also be reviewed so that similar interruptions are identified and controlled before any change is made.

4.5   Primary references

• IMCA M103 – Guidelines for the design and operation of dynamically positioned vessels
  
  
• IMCA M117 – Code of practice for the training and experience of key DP personnel