Why do I need a PoF for time-dependent failure mechanisms? The TTF is more relevant since it drives integrity re-assessment schedules and other risk mgmt. efforts.

TTF is indeed the more useful metric for many applications. However, the TTF has a probabilistic aspect that must be understood before its use in, for example, setting an integrity reassessment interval. The TTF calculation uses segment lengths since degradation mechanisms are usually related to surface areas.  A probabilistic TTF per mile is an intermediate calculation […]