This DP event occurred on a DP equipment class 2 supply vessel whilst engaged in cargo operations alongside a construction vessel. 

The vessel was operating in open bus configuration, with both generators running and connected and all six thrusters selected to DP. The vessel is fitted with two DGNSS and one laser based reference system, a CyScan. All three reference systems were selected to DP at the time of the event. 

The vessel carried out all necessary pre-500m checklists, made its 500m entry and proceeded to a position underneath the construction vessels main crane, with the vessels laying port to port side. 

The weather conditions at the time of the event had the wind and current both coming from the North and a 2m swell, as shown in Figure 2. 

The vessel entered the final position under the crane at 16:55 and commenced cargo operations. At 17:54 the vessel started to experience position reference system losses, with frequent instability of the two DGNSS. 

During cargo operations when a container was being lifted from the supply vessel to the construction vessel, it obstructed the line of sight between the CyScan unit on the supply vessel and the CyScan reflector on the construction vessel. 

The loss of the CyScan position reference system generated a reference system failure alarm and moments later the vessel began to lose position with the accompanying alarms. 

The DPO decided to initially change to DP Joystick however the system required to synchronise joystick and during this period the vessel movements were towards the construction vessel and risked collision. 

The DPO attempted to change to manual control which requires a two step action – select panel and turn key. The first attempt failed but the second attempt was successful, at which point the DPO pulled the vessel out and clear. 

Figure 1 – Line of Sight

3 Findings 

Investigation of the event concluded that: 

Following analysis of the event it was determined that none of the position reference systems were able to provide suitable position information to the DP system. 

The position in relation to the construction vessel caused a reduction in the visible arc of the horizon, horizontally and vertically, this reduced the line of sight to many satellites. The position of the CyScan reflector relative to the transmitter also caused interruption and failure of the position references. 

Following the vessels ASOG, the operations should have been stopped at the initial instability of the PRS and taken to a clear position whilst they investigated and risk assessed the situation. Crew were reminded of the importance and their right to exercise the Stop Work Authority at all times.