DP run-off incident – Class 2 DP vessel

IMCA has received a report of a DP run-off caused by operational changes to the DP system which led to the loss of all reference systems. The DP class 2 vessel was alongside a platform and suffered a loss of reference systems, requiring the vessel to be taken into manual control. The vessel was recovered to a safe location without mishap. No damage was incurred to the platform or the vessel.

An initiating cause was the deterioration of the performance of one of the position reference systems. The DP operator on watch selected alternative reference systems to maintain DP positional integrity, in accordance with normal accepted procedures, whilst the problems with the deteriorating reference system were addressed.

The configuration of the vessel linked the faulty reference system with a number of other reference systems, with the result that the loss of one system caused the loss of others, including a vertical reference system (VRS).

The voting algorithm in the DP system was based on three VRS units, rather than two VRS units, as actually installed onboard the vessel. This caused the second VRS to be rejected by the DP system, with the secondary effect that all other reference systems were also rejected.

In addition, sensor offsets to the centre of gravity had been omitted, which caused erroneous calculations of movement, and errors in wiring compensating the pitch had occurred either at commissioning of the vessel or during maintenance of the DP system. This caused pitch motions to be amplified rather than compensated. There was insufficient documentation of these wiring changes onboard the vessel.

It should be noted that the errors could normally have been picked up during commissioning or annual FMEA DP trials, provided that the FMEA trial was not carried out in calm water, where the corrections for the vessel’s movements would have been too small to be distinguished. However, since FMEA trials test function and not configuration, the potential for error could remain undetected in the system.

Further, it should be noted that records of all system configurations, offsets, drawings, changes etc should be held onboard the vessel, and kept up to date.

The following actions were proposed:

1. DP reference inputs should be correctly selected to mitigate loss;
2. DP system configuration should be checked, as every vessel is different;
3. All DP issues should be documented, together with the close-out issues, to aid the DP maintenance engineers as required;
4. Configuration of individual reference systems should be modified such that VRS units cannot be inadvertently affected through failure of those systems.