Thorough preparation is key

With 4 of 8 diesel generators running and connected to a closed ring bus, and 7 of the 8 thrusters selected to DP, a DP equipment class 3 MODU was set up in Green mode whilst on standby.

The weather at the time was poor, with wind speeds of 43 knots and 2.2 knots of current. The rig was positioned in a drift-off position, with the nearest vessel approximately 1.9 NM away.

Generators 1, 3, 5 & 7 were operating at approx. 45% when the DPO initiated a 10-degree heading change to provide better protection of the rig against the weather.
During the heading change, the rig was struck by a large wave, which resulted in an instant load spike and generator fault alarm on all running generators.
The power management system initiated automatic start of the first standby generator ‘DG 6’. DG 6 started, paralleled and the breaker closed on to the ring, before the power management system opened the breakers and shut down DGs 1, 3, 5 & 7.
Auto starts were initiated for the three remaining stand-by engines, but before they could synchronise and connect, DG6 ramped to 100% and tripped on overload. The MODU blacked out, and the blackout recovery system was initiated.
Due to the standby generators already going through the start-up & synchronise process at the time of the blackout, the vessel was able to restore power within 12 seconds, the MODU recorded a 7m shift in position.

Figure 1 - Forced out the green circle

Following investigation, the following was found:

• The DP system automatically switched temporarily to high precision control mode, as designed to when a position or heading change is made in Green Mode.
  
  
• The system was set to high gain.
  
  
• The large wave during the heading move forced the vessel out of the green area.
  
  
• The sudden power load increase initiated the auto-start system, as required.
  
  
• The system was set to Droop mode, and it was later identified that the droop curve limits were too tight to cater for the large load step changes of the generators.
  
  
• The power management system disconnected multiple generators when the first standby
  generator connected.