Before any FFS assessment is made, it is absolutely necessary to identify the damage mechanism. Not taking this critical step can lead to failure. Damage defects or flaws that are not identified can lead to false conclusions from a stress perspective. This involves the FFS integrity triangle mentioned in Chapter 1 inspection, materials, and mechanical form the integrity triangle that makes the FFS assessment complete. If we were to remove one of those disciplines from the effort, the entire FFS process would fall apart.

Identifying the damage mechanism requires the appropriate NDE method, an estimate of the future damage rate to find the remaining life, and the proper monitoring and mitigation methods.

A proactive approach is the best defense against failure by damage mechanisms. Questions addressing the potential degradation mechanism, the level of degradation that is tolerable, which remaining safety factor is acceptable, the consequence of a failure, and the consequence of a leak versus a rupture must be addressed. Note that API 579 does not describe how to find corrosion, but what to do when you find it.

General and specific material information including heat treatment, chemistry, and strength level are some of the data required in damage assessment. Material toughness is a function of grain size, which is a function of heat treatment. Most damage mechanisms are chemistry dependent. Normally a good rule of thumb is 5 mils per year for the upper shelf of tolerable corrosion. If this rate is exceeded, then a higher alloy is...