There – but not really there

The supply vessel was operating with two DGNSS, one radar-based and one laser-based reference system accepted into the DP system.

The vessel was operating in poor weather conditions and experiencing wind and waves on the starboard bow, with a heavy swell of 3.2m and around 28 knots average wind. The vessel had been experiencing position excursions of around 3-5m quite frequently and also heading was dropping around 3-5 degrees when losing position.

As the vessel was working on the drift-off (lee-side), the Master and DPOs considered this acceptable.

At 10:08, when the vessel's position and heading deteriorated with around 2 metres and 2 degrees excursion, out-of-limit warnings were generated. Over the following couple of seconds, one by one, prediction error alarms for each of the reference systems were generated but not fully realised by the DPO.

The vessel commenced drifting aft and losing its heading due to the wind and waves. The DPO was monitoring the operator station and awaiting the thrusters to react and start thrusting to return to position; however, the vessel continued moving further away from position. On the operator station, the DPO reported that the reference systems were still appearing to be online, though the weighting was not clarified.

When the vessel's speed began increasing away from the asset, the DPO disengaged sway and surge to take control on DP joystick, at which point, a position dropout alarm was triggered due to no reference systems accepted into the system. The DPO manoeuvred the vessel away from the asset on the DP joystick and out of the 500m zone.

Figure 2 – Screenshot to show difference exceeding 5 m

Investigation of the event concluded that:

• Due to only one line of DP alarms being visible at a time on the operator station screen, the DPO did not visually see the reference prediction error alarms activated. When many alarms activate in a short period, the second DP operator station must be used to show the full alarm list.
  
  
• Multiple rapid alarms had been accepted, and the alarm drop-down had not been expanded to check entirely those activated. If they had, it would have been realised that the ‘No reference system’ alarm had been activated and that the system was using the estimated position only.
  
  
• From analysis of the DP system history, it was seen that the vessel was hit by a wave train. The wave train's effect is that the position reference systems' measure position is moving away from the estimated position.

Figure 3 – History analysis

• The wave train hit the vessel, and this ‘unknown external force’ pushed the vessel rapidly out of position, causing deviation between the measured position (reference systems) and the estimated position (model position). The deviation exceeded the prediction error limits of 5 m and caused all reference systems to be rejected.
  
  
• The DP software version onboard the vessel had a known weakness regarding position drop out of the DP reference systems during rapid movements caused by wave trains/ several waves in a row.