The Gulf of Mexico oil spill has a tangible $100b liability for BP (and 32 million Google hits for “Gulf of Mexico oil”). The Blow out Preventer (BOP) failed despite having a low catastrophic failure rate (4 tests failed in around 90000 tests). Even if we assumed the MTBF (mean time between failures) to be around 100 year, this is still a high risk of $100b per failure / 100 years/failure = $1b/year. A risk worthwhile mitigating. There are reports suggesting that further redundancy should be considered (i.e. to remove the single points of failure and to minimise common cause failures). Assuming that this redundancy could have cost $10m, the simple payback of this is $10m / ($1b/year) = 4 days. Interestingly, now, a compensating provision of $10m does not seem to be a lot.