Line of sight

The vessel was holding position when a crane movement blocked the path from the Fanbeam to the reflector on the turbine.

The DP system registered the drop in reference system, with appropriate alarms and held position on the two remaining reference systems.

However, when the crane moved away from the path of the Fanbeam, the Fanbeam reacquired a connection, and a 4.5m move was experienced.

The SDPO changed over to joystick control and pulled safely out of the field to carry out troubleshooting.

Troubleshooting of the event concluded that:

• Once the path was clear, the Fanbeam reacquired connection with a different reflector, which had a similar range, bearing and signal level as the original reflector.
  
  
• As the wrong reflector was acquired, the DP system repositioned the vessel according to the position of the secondary Fanbeam reflector.
  
  
• The vessel recreated the scenario, and the same outcome was experienced with each repeat. Without deselecting the Fanbeam from DP, after the signal loss, the Fanbeam reacquired the wrong reflector, and the vessel experienced a position move.
  
  
• During an alternative scenario, when the Fanbeam lost connection, the DPOs deselected the Fanbeam from the DP desk. Only when the Fanbeam became available and stable did they reselect the Fanbeam back into DP, and the position remained stable.
  
  
• Once the crane covers the line of sight between the Fanbeam and reflector and a prediction error is generated, the DPO should deselect it from DP.

Figure 5 – Fanbeam array