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Relative PRS – A Reflection

1 Overview

A PSV was conducting cargo operations alongside an FPSO which was moving around its moored position.  Selected into DP Control were two DGNSS absolute references and one Laser based relative reference, there were no other available relative references for the DPO to select.  The DP Control system was selected to ‘Follow Target’ mode.

During a lift from the deck of the PSV, the vessel thrusters suddenly ramped up and the vessel started to move off position.  The DPO switched the DP Control into Joystick manual heading control to stabilise the vessel and move out of the 500m Zone.

Figure 4 – 1 Position of Vessels

2 What happened?

The laser based relative position reference had switched to another target, in this instance the reflective tape on the crane hook.  As there was only one relative reference available and selected this caused the DP Control to move the vessel off the original position.

3 What can be concluded?

The following can be concluded:

  • There was no redundancy for relative position reference
  • The Laser target appears to have been positioned in the wrong location.  Targets should be located clear of crane operations and personnel with reflecting jackets moving around

4 Additional Comments

  • If the DP control system was in Follow Target mode, the DGNSS should have been in “monitoring” and Relative PRS  in “Mobile” configuration.  The actual setup during the event in unclear.
  • The DP event report did not discuss the use of any dicision support tools such as ASOG.
  • IMCA M 252, Guidelines on position reference systems and sensors for DP operations, section 4.6, details specific operational considerations for laser position references including placement of the sensors and reflectors and the hazards to consider.

5 Guidance that would be relevant

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

  • IMCA M117The Training and experience of key DP personnel
  • IMCA M220Guidance on operational planning
  • IMCA M252Guidance on position reference systems and sensors for DP Operations

The following case studies and observations have been compiled from information received by IMCA. All vessel, client, and operational data has been removed from the narrative to ensure anonymity.

Case studies are not intended as guidance on the safe conduct of operations, but rather to assist vessel managers, DP operators and DP technical crew in appropriately determining how to safely conduct their own operations. Any queries should be directed to IMCA at [email protected]. Members and non-members alike are welcome to contact IMCA if they have experienced DP events which can be shared anonymously with the DP industry.

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