Remnant Life (RL) and Next Inspection Date (NID) Calculations

Corrosion Schedules are recurring. When a field inspection is completed, it must be determined when this inspection needs to be repeated in the future. This is the NID.

There are different types of NIDs that are calculated and displayed in the Calculation Summary. Only the IMS Calculated NID and a Time-Based NID takes part in the horserace for the Corrosion Schedule’s NID. The Recommended NID is always the one with the most conservative interval!

• IMS Calculated NID – determined from the corrosion data.
• Time-Based NID – determined from RBI, the User, or from Industry Codes (e.g. maximum interval for Vessel Corrosion Schedules is 10 years (MII can also be defined via Special Emphasis)).
• Recommended NID – it is either the Calculated or Time-Based NID, whichever is the earliest date. Time-Based will be used if the Ignore Calc Interval checkbox on the Circuit Detail window is applied.

If the Ignore Calc Interval is checked, the IMS Calculated NID will be ignored in the horserace.

Note: If EVA was enabled for the Circuit, this can also take part in the horserace.

S-RBI methods are often used to help determine the NID, but it is important to understand what the IMS Calculated NID is in comparison with S-RBI NID. Consider the IMS calculated NID as a safety net when S-RBI is used.

To determine the IMS Calculated NID we must first determine the Maximum Inspection Interval (MII). This it determined from the Remnant Life (RL) and the Total Interval Factor (TIF), also called the Governing IF.

Then the IMS calculated NID can be determined by adding the Last Inspection Date (LID) to the MII.

The RL, which is the time remaining until the pressure boundary reaches the Renewal Thickness (Tmin), is calculated from the Remaining Corrosion Allowance (RCA) and the CR. RCA is equal to the wall thickness of the last reading minus the Tmin. The CR is the maximum of Long CR, Short CR, Linear Regression CR, Suggested CR, and (depending on the selected Calculation Method - see Corrosion Rate Calculations – Details) the Average Long-term CR and Average Short-term CR.

The TIF is made up of multiple parameters: Circuit IF (user value or S-RBI IF), DSCF IF and PERF IF (see Tab: Corr Rates IF). The selected Calculation Method will determine which parameters are used in the calculation.

Example

Assume the TIF is 0.4. Use the below table to calculate the NID for CML 5.

Thickness readings (metric - mm):

The RL is calculated from the highest CR. In this case it is the Short CR.

In this way IMS will calculate the NID for every CML/MP. The earliest NID then becomes the IMS Calculated NID. This Driving CML/MP’s data is display in the Calc Summary.

The Driving CML for the IMS Calculated NID, is shown.

 

Special Emphasis - Setting a MII

Take note that in case of Pipe and Pipe System Equipment, the Special Emphasis in the Circuit's Details section can be populated (when there is no pending event). The classification chosen will determine the Maximum Inspection Interval (MII) allowed for the Calculation Summary and Corrosion Schedule. This is done by setting the Time-Based interval in the Circuit Calc Summary to the Special Emphasis time interval.