Many of the undiscovered oil and gas fields are found in vulnerable areas and in deep waters. The recent Deepwater Horizon accident in the Gulf of Mexico in April 2010, has clearly demonstrated that drilling operations in such areas are demanding. The society and the authorities are therefore concerned with the reliability performance of barrier functions that have a dedicated role in the prevention of environmental releases. For example, well monitoring and kick detection are important barrier functions to prevent unexpected influx of reservoir fluid to the well (kick). Unfortunately, the standards that apply to drilling related barriers give little attention to the selection of adequate reliability performance measures and the need to establish and follow up reliability performance during operation. The main objective of this paper is therefore to propose a set of reliability performance indicators that are suitable for trending the performance of the most critical barrier functions during drilling. To support the selection of indicators, a model has been developed to (i) identify the main barrier functions that are used in response to a kick, (ii) identify possible outcomes of blowouts and (iii) evaluate the barrier function importance in lieu of the blowout severity. Many of the discussions raised in the paper are generic, rather than application specific and the approach may therefore be applicable beyond the scope of drilling operations. The paper builds on an ongoing research project that is performed in close collaboration with the Norwegian Research Council and Norwegian petroleum industry.