Important PRS considerations when operating close to an asset that is not rigidly fixed to the seabed

This information note reproduces item 1 of IMCA DP Bulletin 03/23 and is issued to widen distribution and awareness on this topic.

From the DP Station Keeping Event reports received by IMCA, it is clear that the set up and mixing of position reference systems (PRSs) have contributed to these events. The IMCA DP Committee wants to promote understanding to reduce incidents of this nature. Therefore, instead of identifying a PRS as either absolute or relative, this bulletin defines absolute and relative DP positioning and provides examples of how the different PRSs can be used in both circumstances.

IMCA DP Station Keeping Event reports, in recent years, confirm that the following issues were contributory factors:

♦ set up and mixing of relative & absolute position reference sensors
♦ lack of special functionality within the DP Control Systems (DPCSs) to cope with the demands of relative position-keeping of moving targets
♦ lack of focused training on the perils of mixing absolute and relative PRSs without special DP functionality installed or in use
♦ lack of training in use of such special functionality
♦ improper PRS management along with operator error.