The dynamic positioning (DP) classification notations (DPS-1 to DPS-3) reflect the equivalent International Maritime Organization (IMO) equipment classes which represent increasing levels of redundancy. The criteria given and the use of Failure Modes and Effects Analyses (FMEA) have formed the industry's basis for evaluating DP. However, other important measures, such as position keeping capability and reliability of the designs, are not reflected in these current notations.

This paper investigates the use of another well established risk tool - the fault tree which allows for the interaction of various components and their failures to be mapped from the vessel's design. The technique provides for the use of the tree design and failure data to be combined so as to give numerical values to the probabilities of (time between) failures of system components.

The values that come from the fault tree approach can then be used in the development of an Enhanced Reliability (ER) notation (e.g. DPS-2, ER-60, 40kn - indicating 60 days between failures that result in loss of redundancy in a 40 knot ‘environment’). This enhancement to DP criteria is expected to be the most significant since the IMO Guidance was originally published.


The progression of offshore operations into deeper waters was inevitably accompanied by the increased adoption of DP systems as traditional mooring systems became less viable for small units or for short periods of operation.

Entities interested in using vessels with DP capabilities need a mechanism for comparing different units against their specific operational requirements. In addition, DP system vessel owners want their assets to be easily identified for the capabilities (and investments) they have to offer.

Results, Observations, and Conclusions

Procedures on the development and the requirements of the Enhanced Reliability for DP systems will be presented.

Significance of Subject Matter

Compliance with the Enhanced Reliability requirements would provide a means for owners and operators to have a consistent and quantitative measure of the capability relative to the reliability of the DP system.

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