Worst case failure preparedness

Whilst drilling, the DP Equipment Class 3 vessel was operating with all 6 thrusters and 3 of 6 generators in open bus configuration.

Visibility was good and they were experiencing wind speeds of 21 kts from 021°, 2.4 kts of current to 020° and waves of 2.4 m.

The vessel was fitted with, and selected into DP – 3 DGNSS and 3 HPRs.

An alarm sounded on Thruster 3 and the electrician was notified to investigate. When he arrived on the Bridge, the DPO relayed that an alarm had been sounding on the Thruster panel and so all that the DPO had done was press to mute the alarm for T3.

Rather than muting the alarm, however, the vessel had entered independent joystick mode, and communication from the DP desk to the thrusters stopped. All thrusters were available, but unresponsive.

The vessel began to drift off position and, when it passed the red watch circle, the EDS was initiated.

On investigation, the following was found:

• The DPO had attempted to reinstate the DP system, but was unsuccessful as the system did not react
  
  
• The Manual Thruster Control (MTC) panel had a customised safety feature that required a double push on the desired operation button and the ‘Allow’ button at the same time to change the operational mode. Therefore, to enter independent joystick mode the DPO would have been required to press ‘Allow’ + ‘IJS’ at the same time.
  
  
• The DPO maintained that only the silence button was pressed.
  
  
• After the vessel had performed the EDS and it was over 100 m from set-point, the thrusters re-established and enabled.

The unusual goings on were fully investigated but there was no clear root cause identified from a mechanical/electrical point of view by the manufacturers. The investigation did produce a few recommendations:

1. The thruster utility panels were to be covered with a lid/cover to avoid unintended button push caused by human interaction.
   
   
2. Decommissioning the RBUS ‘Allow’ functionality, as this was not a manufacturer standard solution on the RBUS generation. It was a functionality of the legacy/obsolete SPBUS system and therefore not required anymore.
   
   
3. Emergency drill sessions for all operators to be prepared for worst case scenarios.

It can be frustrating when no clear answers are initially found following events, but what is more frustrating is when operators are not fully prepared for worst case failures. Drills are not just a tick box exercises to be conducted monthly, because a spreadsheet says so. They should be designed to mimic real life scenarios, whether they are live drills or round the table desktop, drills should make people think and be prepared for the expected and unexpected.

Does everyone know the vessel, its systems, its redundancy and its backups?

At no point in this event did the DPO or electrician attempt to change over to the Backup DP system!