As mentioned in Degradation Classification, the DMs can be classified in terms of the failure characteristics (see the figure below):
- Age-Related (AR) failures (e.g., Pitting, general Corrosion), and
- Non-Age-Related (NAR) failures (e.g., Cracking).
Strategy Based (SB) failures fall in a third category which are closely related to NAR failures. Corrosion Under Insolation (CUI) is currently the only predefined Strategy Based failure in IMS.
Degradation Mechanisms can be classified in terms of failure characteristics.The table below outlines the AR and NAR failure characteristics.
Age-Related vs Non-Age-Related failures:
Age-Related failures | Non-Age-Related failures |
|---|---|
The Degradation Mechanism (DM) has been designed in. Damage is controlled through Barrier management and verification. | The control of DM should be addressed in the design; Barriers are designed in. |
It progresses at a steady rate within operating limits. | Time is not relevant. Occurrence is random when operating outside limits. |
Time to failure should be known.
| Time to fail strongly depends on process upsets/excursions. |
Design life is known. | |
Progress can be quantified and (Barrier) verified. | Rapid progress possible once initiated:
|
Failure acceptable within normal design envelope. | Occurrence of failures not acceptable within normal design envelope. Emphasis is on control of the Integrity Operating Window (IOW). |
Verification is done through inspection and IOW monitoring. | Verification is done through IOW monitoring and opportunity inspection. |
The outcome of an AR RBI Analysis is always a Next Inspection Date (NID). | The outcome of a NAR RBI Analysis is typically a Maximum Inspection Interval (MII) or an Inspection Strategy (IS). |