Since this is an unmanned operation, the calculation is fairly straightforward, balancing:

1. the cost of failure 2. the probability of failure 3. the cost of reducing the probability of failure

If these can be accurately calculated, then a certain failure rate can be quite acceptable. Of course, it's still worthwhile to determine the cause of any failures, and hence the cost of eliminating that cause and a reevaluation of points 1..3.