As the Industry of Oil and Gas advanced in deeper waters exploring, the drilling units positioning technique alternated from anchors to dynamic positioning (DP). Nowadays, even in deep or shallow waters, a large number of DP drilling units has been used. In addition, it was noticed that technology has been improved and was incresead redundancy and hardware in diferentes subsytems of the vessels. No matter how many redundancys a DP drillship contractor can provide to his own vessel, the faults, in the system and subsystem, have been experienced over the years, either for equipment failure or for human errors, and also could lead a of positioning and disconnect from the well.

Taking a deeper look into all DP Petrobras incidents experienced, it was observed that the most of them are related to power system in many differents types of failure. As of knowlodge, there are two main types of switchboards mode: single busbar and split busbar, each of them has yours own vantages and disvantages, but based in its past experienced, few years ago, Petrobras strongly recommended to all its drilling contractors to operate in split busbar instead of a single busbar mode, in order to prevent major failures.

Once that most part of the DP drilling units operating in split switchboards, the worst case failure experienced in the power system, is a partial blackout, but others could be linked a DP incidents. The events assessed, from the Petrobras database DP only, showed that human failures and faults arising from maintenance processes in critical equipment are, currently, the major factors causing DP incidents. Considering the faults occurred, they, usually, do not result in major events, such drift-off and emergency disconnect from the well. In fact, a partial blackout itself does not even lead to a yellow alarm, by Petrobras definitions; even though, knowing the main type and root cause of the failures will help to prevent similar events.

Keeping in mind how importante is to find the root cause of all incidentes, associanting it to a systematic collection and analysis of numerical data helps to investigate the relationships among incidents and to explain and control their occurrence. In other words, a statistical analysis gives a full picture of all incidents and recurrences, also provides substantial informations to estabilished proper barriers to avoid similar ocrurrences; and, especially, when is talking about switchboards modes, adds another decision parameter in the choice which is better.

This is paper will present a statistical analysis of the failures occurred and non productive time during operation in split busbar due to dynamic positioning incidents related to power system failures. Evaluate and compares performance over time of semi-submersible and drillship rigs.

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