ABSTRACT
High reliability is not always the optimum solution. This fact is demonstrated by presenting an analysis performed in connection with the design of the process instrumentation on a Norwegian offshore installation.
Unnecessary production shutdowns can result from spurious trip signals coming from the large number of level and pressure sensors installed on oil/gas separation equipment. The frequency of occurrence of spurious trips can be reduced by, for example, introduction of multiple sensors and voting logic. The decision for or against measures to increase reliability should be based on a complete and quantitative assessment of cost (investment, operation, maintenance) and benefit (increased oil production), integrated over the whole lifetime of the installation. Such a decision will not always be in favour of increased reliability. In the example presented the preferred solution was, in fact, simple 1-out-of-l logic, partly based on the outcome of this analysis.
