Sufficient independence between barriers

The Norwegian Petroleum Safety Authority (PSA) has requirements for risk reduction described in the Management regulation §5 Barriers. The second paragraph in §5 is stating that Where more than one barrier is necessary, there shall be sufficient independence between barriers. In this paper ice management (IM) barriers are described by means of event trees with Boolean barrier events. The number of barriers may be based on the risk related to the end events of the barrier system, but the basic requirement is the single failure requirement. In order to analyze and verify sufficient independence there is a need for precise descriptions of the barrier elements and the logic relations between the elements. Four type of barrier dependency properties are described and related by studying dependencies in a two-barrier system based on 16 truth tables and 16 related Boolean operators. The relations between the four barrier dependency properties are compared by inserting the truth tables in a diagram where the barrier properties are related to different subsets of the diagram. The two-barrier system is then enhanced by an additional common cause. A Boolean model of the enhanced system is motivated by an established FMEA analysis of the single failure requirement of redundant systems. The model will be extended with a formal Boolean method for describing the systems and the associated barrier elements events, thus establishing barrier models where dependencies and common causes may be included, analyzed and verified to be acceptable.