Case Study – Masking of PRSs – SIMOPS

1 Overview of Event

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 4 PRS’s selected into DP Control all with automatic weighting, the PRS’s 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’s 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.

2 What can be concluded?

• The Pipelay Vessel was using the smaller crane on the port side. The large main crane located on the transom was in the upright position.  This crane is a large capacity crane and as such, has a high structure when in the upright position.
• At the startup of this operation, the DPO on the OSV should make a concentrated effort to indicate the blind sector for the DGNSS satellites in the DGNSS equipment, via the HMI. This function is available in modern DGNSS equipment and should form part of the DP ops setup check list.
• The cargo lifts caused the line of sight to be broken for both the relative position references. An alarm was raised at the DP Operating Station and accepted by the DPO, in the knowledge that the DGNSS weighting would increase and compensate for the loss of the two relative units.
• SIMOPS was being carried out with the Pipelay Vessel co-ordinating the OSV to move at the same time as the Pipelay Vessel (25m). Cargo operations continued and so did the intermittent masking of the relative signals.
• Poor practice due to safety concerns – mismatch of vessel positioning when moving – to continue to use the crane for loading pipes when both vessels were moving, especially when there was also a heading change.
• The two vessels did not move at the same speed relative to each other. Not only did the main crane move relatively to the OSV, the whole pipelayer moved (hull, accommodation, deck equipment, mission equipment, etc).  The blind sector was dynamic and was able to mask the satellite signal to the 2 DGNSS’s located on the OSV, an alarm was raised which the DPO accepted.
• At the point of the first overshoot there had been approximately 20 cargo moves during the continuous pipelay operation with the drop out alarms being accepted as a normal part of the operation. However, when the masking of the Relative PRS’s and the masking of the Absolute PRS’s happened at the same time, the DPO did not realise the significance of the alarms (even though a specific alarm for all PRS drop out would have been generated) as they had been accepting them all through the operation.

3 Additional Comments

• 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.

4 Guidance that would be relevant

The following IMCA Guidance would be relevant to this case study:

• IMCA M203 – Guidance on Simultaneous operations (SIMOPS)
• IMCA M220 – Guidance on operational planning
• IMCA M252 – Guidance on position reference systems and sensors for DP Operations
• IMCA M242 – Guidance on satellite-based positioning systems for offshore applications