Reliability Case Notes No. 7. Risk assessment of the hot-water drilling system for accessing subglacial Lake Ellsworth

The exploration of Subglacial Lake Ellsworth is a high profile project funded by the Natural Environment Research Council. Clean access to the lake will be provided by a hot-water drilling (HWD) system developed by the British Antarctic Survey. The HWD system is designed to provide a 36 cm diameter borehole through 3.2 km of ice. Drilling to this depth with the HWD system has never been attempted before. This report aims to quantify the risks of the HWD system deployment. A formal assessment of the technical risks is presented. Our analysis was conducted in two parts. First we estimated the probability of failure for all components and processes that take part in each phase of the deployment. In the second part we estimate the availability of the HWD system in light of all potential failure modes. Availability is the probability of the system being available given that it is needed at a given moment. Seventy five potential failure modes have been identified. The assessments for all these failure modes are presented in this report. The probabilities of failure for the top three critical failure modes are: • Boiler failure at 0.048; • Damaged surface hydraulic pipes at 0.0185; • Driller reeler human error at 0.0168. Our availability analysis concluded that, once water circulation has been established, the probability of successfully creating the main hole is 0.83. This will enable the deployment of the probe. Once the main hole has been created, the probability of successfully reaming the main hole, enabling the deployment of the second probe, is 0.89. This is a very high probability of success. These figures take into account the drilling rate and the uncertainty associated with the effectiveness of the drilling process. The current HWD system design has incorporated design changes to avoid previous weaknesses identified during field campaigns. In particular, the current design does not contain hose couplings. This decision has reduced the risk of not achieving the target depth by 15%.

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