What is the difference between CIM's Corrosion Growth Rate Calculations (Fixed, Half Life, Pit to Pit) when running an analysis?

 

Fixed Corrosion Growth Rate

Cognitive Integrity Management ‘Fixed Corrosion Growth Rate’ method will apply the known predetermined fixed rate (i.e. 7mpy for ET) to calculate/extrapolate out remaining life to use for growth conditions being evaluated.

Half Life Corrosion Growth Rate

Cognitive Integrity Management ‘Half-Life Corrosion Growth Rate’ method is applying industry accepted practices which uses the reported depth along with installation/construction year to derive a corrosion growth rate (metal loss depth across an elapsed time) and will then divide by 2 to account for initiation time factor.

Pit-to-Pit Corrosion Growth

Cognitive Integrity Management Pit-to-Pit corrosion growth rates are determined according to change in depth across an elapsed time. This method takes the anomaly depth in the current assessment versus the anomaly depth of the most recent match for that anomaly in an earlier assessment (which may or may not be the previous assessment). If the current anomaly matches multiple anomalies in that earlier assessment, it will take either the minimum, maximum or average change in depth based on whether the user selects Pit-to-Pit Minimum, Maximum, or Average in the analysis. CIM then takes the change in depth, divides by the time elapsed between the two measurements, and converts that into an annualized growth rate.

Pit2Pit Positive Growth Example:

2019 Anomaly has a depth of 51%. It has no match in the 2018 Assessment but matches two 2017 anomalies of 13% and 49%.

• If the user chose Pit-to-pit Maximum, the depth change is 38% over 2 years, or 19% per year, multiplied by wall thickness of 250 mils = 47.5 MPY.
• If the user chose Pit-to-Pit Minimum, the depth change is 2% over 2 years.
• If the user chose Pit-to-Pit Average, the depth change is 20% over 2 years.

In addition, if the matched anomalies result in negative or zero growth, the system will fall back to a previously defined “floor” using our Fixed Corrosion Growth Rate attribute. Doing so allows the system to carryon on with remaining life calculations and projections while factoring in customer defined level of conservatism. If an anomaly has no previous match and is considered to be a net new anomaly, the system will use Half Life Corrosion Growth Rate to determine the growth rate for that anomaly.

Pit-to-Pit Negative growth, zero growth, and net new anomaly Examples:

• 2019 anomaly has a depth of 51% and 2015 anomaly had a depth reported at 55%. The system recognizes as negative growth and will apply fixed corrosion growth rate (floor).
• If the 2015 anomaly had also been reported at 51%, the system would recognize as zero growth and will apply the fixed corrosion growth rate (floor).
• If the anomaly had previously never been reported and we do not have a positive match, the system would treat as a net new anomaly and would then apply the half life corrosion growth rate.

Historical Growth Trend Method

Cognitive Integrity Management ‘Historical Growth Trend’ will derive a corrosion growth rate from a multi point trend line generated from more than 2 historical depth values. 

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