Masking of PRS – SIMOPS

An Offshore Supply Vessel (OSV) was working alongside a pipelay vessel, discharging cargo whilst the Pipelay Vessel was executing moves ahead approximately 25m every 10 minutes. The pipelay vessel was also moving abeam and making minor heading changes as per typical pipelay operations.

The OSV was in Auto DP using four PRS’s selected into DP Control all with automatic weighting, the PRS' selected were:

• x1 laser system
• x1 microwave system
• x2 DGNSS

As the pipelay vessel’s crane hoisted the cargo off the deck of the OSV, it was noted that the cargo would interrupt the laser and microwave signal causing loss of signal and therefore, weighting to fall, making DP control solely reliant on the DGNSS.

As the pipelay vessel moved ahead, the OSV was also instructed to match the move, simultaneously discharging cargo. Alarms continued to appear on the DP Control system for the drop out of the relative PRS’ which, the DPO cancelled as if normal.

With the Pipelay Vessel changing its heading and, the difference in speeds between the two vessels, the effect was to also mask the DGNSS. Alarms were generated and cancelled without assessment.

This happened during a 25m Simultaneous Operations (SIMOPS) move. The OSV failed to come to a stop at the set point and the thrusters were noted to be increasing in speed (starting to ramp up). The DPO took the action to remove the surge functionality to bring the vessel to rest and thereby taking the vessel out of auto station keeping. The total overshoot was 7m.

A discussion was held between the PLV co-ordinator (located on the PLV Bridge), and the bridge team of the OSV, and it was decided to continue the operation.

Another 25m SIMOPS move was executed by both vessels resulting in the exact same effect as the previous move, and a similar overshoot of 7m.

The OSV was ordered outside the 500m to investigate.

• The DP control system on the OSV could be equipped with a “Follow Target” function that allows the OSV to automatically follow both pipelayer’s position and heading to prevent a mix of absolute and relative systems. If they do not have a “Follow Target” function, then they should use only Absolute PRS and make operator initiated position move together with the pipelayer.
• OEM Manuals & training should give advice to how best to manage PRS’s within their system. This can be included into the DP Operations Manual.
• If possible, the OSV should be on the same heading as the pipelayer to give the relative systems better working conditions due to the crane.
• How detailed was the ASOG, was it being followed? From the details it would be reasonable to conclude that either an ASOG was not in place, or it was not being followed.