Methodology Step 6

Step 6 Estimate Remaining Functional Life (AR DMs and CUI): 

Below the Step 6 Methodology is described. See also Software Step 6.

Corrosion Mechanisms are the most common Aged-Related Degradation Mechanisms. For these the Remnant Life (RL) must be determined. For this the Selected Corrosion Rate (SCR) and Remaining Corrosion Allowance (RCA) is required:

Where:
RL = Remnant Life [year]
RCA = Remaining Corrosion Allowance (at last inspection) [mm] or [mils]
SCR= Selected Corrosion Rate [mm/y] or [mpy]

Specify Remaining Corrosion Allowance (RCA)

The RCA is defined as the difference between the Design Corrosion Allowance (DCA) and Wall loss or else Remaining WT and the Minimum Allowable WT. See Corrosion and Remnant Life. If the Minimum Allowable WT is unknown, an engineering assessment should be completed to determine what it is.

RCA = DCA - Wall loss or
RCA = Remaining WT - Min Allowable WT

Cladded or Weld Overlaid Equipment: If a corrosion mechanism applies to Equipment with an internal clad or weld overlay of corrosion resistant alloy (CRA), the RCA should be taken as the cladding or weld-overlay thickness. 

Specify Selected Corrosion Rate (SCR)

Where available, CR should be determined from measured Wall Thickness (WT) data (this happens on the Circuit). However, in some cases this trended CR is not representative of the situation or area to be model with the S-RBI assessment (e.g., corrosion underneath repad, isolated pitted area not monitored with CMLs, tank floor assessment MFL/UT, etc). Therefore, this trended CR should be compared with the theoretical CR (if a theoretical model is available). Also, the historical CR, based on experience, should be considered. From these three, the CR for the RL calculation, should be selected conservatively.

Minimum - Design CR: To avoid excessively long (or infinite) RLs (and thus long inspection intervals), default minimum or design CRs should be defined and documented by the RBI team. The SCR should not exceed the . If not given, the should be calculated from:

Where:
DCA= Design Corrosion Allowance [mm] or [mils]
CR_d= Design Corrosion Rate [mm/y] or [mpy]

SCR can either be calculated in the Circuit, Historical or Theoretical and should be above The design CR.

Note 1: To arrive at realistic assessments of CRs, consultation between the Inspector and the Materials & Corrosion Engineer is often required.

Note 2: Where the CR is based on measured data, care should be taken regarding the validity and inaccuracies of the measurements, as well as inaccuracies originating from trending analysis. If only limited and/or unreliable monitoring/measured data are available to calculate the CR, the predicted rate, derived from modelling or a conservative assessment based on experience/expert knowledge or design, should rather be used.

Take Credit for Lining or Coating Life

One can take credit for Lining/Coating. The Component’s Next Inspection Date (NID) will then be extended (see Corrosion and Remnant Life) by the Lining/Coating RL, adjusted by an Interval Factor (IF) (see Step 11). IMS determines this IF_{Lining/Coating} from the Lining/Coating’s quality of installation. The built-in questionnaires are used to determine this Lining/Coating quality.

NID = LID + RL_steel x IF + RL_{Lining/Coating} x IF_{Lining/Coating}

NID = LID + MII_steel + MII_{Lining/Coating}

The RL for the lining should be the guaranteed lifetime of the lining (by supplier) or the number of years for which enough confidence is present.

The LID comes from the last Inspection History (Step 5) where "Poor", "Average", or "Good" was selected for the Lining/Coating Condition. Note: When "None" was selected in an Inspection History it means that the Lining/Coating was not inspected, and therefore that Inspection History's date cannot be applied as LID to the Lining/Coating. This is explained in below example:

Example:

Here we start with 10 years of lining in good condition:

MII Lining would normally be 10 years. The Inspection history shows that the last Inspection Date for the lining condition was 1 January 2014:

While on 1 January 2016, only one Analysis was inspected, but no Lining Condition was provided:

So for CREVICE-CR INT we will get 10 years of lining, starting in 2014 (as the 2016 inspection does not apply):

However, for CHLOR-PIT, the 2016 inspection does apply as a LID for the DM, but the lining still has to be calculated from 2014, as this was not inspect in 2016. Thus it has 'used up' 2 years of lining already (2016 - 2014) and therefore it has only 8 years of lining remaining:

Specific Guidance Summary

When required, Guidance per Special Emphasis and Guidance per DM provide additional guidance on RL calculations for Special Emphasis Components and DMs (see the table below). For all other DMs, refer to the general information provided in this section.

Specific guidance summary - RL:

Theory

For more background information on any of the above, refer to Corrosion and Remnant Life - Theory.