Pipelaying equipment short circuit causes thrusters to trip – DP undesired event

Comments:

The vessel maintained position throughout.  The ROV was restricted to the tether management system (TMS) until the vessel was confident of the cause of the thrusters’ unexpectedly stopping.

All damaged sections of the ‘lay spread’ DC bus bars which fed in to the spare drive section of frame A were removed from service and all cabinets cleaned and dried out with air driers.  Blown fuses were replaced; the vessel was confident that the fault was localised to the bus bar, and not the connected equipment.  The set point of the air conditioning unit was adjusted from 21°C up to 28°C and signage placed on the AC units stating ‘Not to be adjusted’.  All external doors have been closed and signage placed on the doors stating ‘To be kept closed at all times’.

Considerations from the above event:

• The DP system was configured for open bus tie operation;
• Pipelaying (project) equipment should be on a separate switchboard to critical DP auxiliary equipment;
• Ideally, main and standby equipment should be fed from separate switchboards;
• Four DGNSS were online – this is contrary to International Maritime Organization (IMO) DP equipment class 2 requirements of requiring three position reference systems of which at least two should operate on different principles.