About Skipped Measurements

Some reasons that you may want to skip measurements include:

• The physical location of certain TMLs may be difficult to reach or may require a shutdown in order to take a measurement.
• Your organization may have a strategy that calculates based on a certain percentage of TML measurements for an asset.
• Resource limitations may make it difficult or impossible to take measurements at all TMLs on an asset or its components.

In order to skip measurements in Thickness Monitoring, the must be selected on the TM Admin Preferences page. Then you will need to create MNT records to represent any measurement that was skipped. After you have enabled this option and created MNT records, Corrosion Analyses containing skipped measurements will be updated to take those MNT records into account.

If you enable the option to skip measurements in TM, you will also have the option to enable a feature that allows you to skip measurements consecutively. When you do this, you will be able to create successive MNT records.

Example: Skipping Measurements Consecutively

Suppose that the TML TML01 is difficult to reach. As a result, your organizational policy dictates that while readings for all other TMLs on the associated piece of equipment must be taken every six months, a reading for TML01 must be taken only once per year, as illustrated in the following table.

TML	Scheduled Readings
TML01	June 1, 2014 June 1, 2015
TML02	June 1, 2014 December 1, 2014 June 1, 2015 December 1, 2015
TML03	June 1, 2014 December 1, 2014 June 1, 2015 December 1, 2015

Suppose that you create an MNT record when a reading is skipped for TML01 on December 1, 2014. Next, suppose that, due to budget constraints at your facility, the scaffolding necessary to gain access to TML01 is unable to be built on June 1, 2015 (the date on which an actual thickness measurement reading is scheduled to be taken for this TML). In this case, you may want to create another MNT record for TML01, assuming that an actual measurement will be taken when possible.

When active Thickness Measurements and MNT records exist in a Corrosion Analysis, all calculations, with the exception of Interval Next Inspection Date, are calculated as if MNT records are not included in your Corrosion Analysis.

Consider the calculation for Short Term Corrosion Rate. Short Term Corrosion Rate is calculated using the Short Term Loss and Short Term Interval, where:

Short Term Corrosion Rate = Short Term Loss / Short Term Interval

...where:

Short Term Loss = Near Measurement Value - Last Measurement Value

...and:

Short Term Interval = Last Measurement Date - Near Measurement Date

Example: Corrosion Analyses that Include Skipped Measurements

Consider the following measurements.

Reading Date Measurement (inches)

Reading	Date	Measurement (Inches)
Base	1/1/1985	.2
N/A	1/1/1987	<Measurement Not Taken>
Near	1/1/1990	.195
N/A	1/1/1993	<Measurement Not Taken>
Last	1/1/1995	.19
N/A	1/1/2000	<Measurement Not Taken>

Short Term Loss = .195 - .19

Short Term Loss = .005

Short Term Interval = 1/1/1995 - 1/1/1990

Short Term Interval = 5 years

Short Term Corrosion Rate = .005 / 5

Short Term Corrosion Rate = .001

The Short Term Loss was not calculated using a Last Measurement Value of 1/1/2000, the date in the MNT record that is associated with the most recent inspection of this TML. There is no value associated with this date (or the date in ANY MNT record) and therefore, these records are ignored in all calculations with the exception of Interval Next Inspection Date.

When MNT records are included in a Corrosion Analysis, Interval Next Inspection Date is calculated using the following equation:

Interval Next Inspection Date = Last Measurement Date + Default Inspection Interval

Where Last Measurement Date is the most recent value in the Measurement Taken Date field of both the active Thickness Measurement and active MNT records linked to a given TML.

Example: Interval Next Inspection Date

Consider a Default Inspection Interval of 60 months (i.e., 5 years) and a TML that is linked to records with the following Measurement Taken Dates:

Record	Measurement Taken Date
Thickness Measurement	1/1/2004
Measurement Not Taken	1/1/2005
Thickness Measurement	1/1/2006
Measurement Not Taken	1/1/2007

In this case, the Interval Next Inspection Date would be calculated as:

Interval Next Inspection Date = 1/1/2007 + 60 months

Interval Next Inspection Date = 1/1/2012

When the most recent date is from an MNT record, and when the Corrosion Rate of a TML is less than the Controlling Corrosion Rate of the asset, the following fields in the TML Corrosion Analysis are populated with data based on the Controlling Corrosion Rate:

• Asset Average Corrosion Rate
• Asset Average Corrosion Rate Remaining Life
• Asset Average Corrosion Rate Retirement Date
• Asset Average Corrosion Rate Next Inspection Date

This differs from when the fields are populated in a Corrosion Analysis that does not include skipped measurements.

Example: Skipped Measurements in TML Corrosion Analysis Fields

Consider an asset with:

• An Asset Corrosion Analysis with a Controlling Corrosion Rate of 2.9 Mils/year.
• A TML Corrosion Analysis for a TML linked to the asset, with a Corrosion Rate of 2.2 Mils/year. The most recent measurement date is from an MNT record.

In this case, the Asset Average Corrosion Rate would be set to 2.9 Mils/year.

This Asset Average Corrosion Rate value could be the result of any method that is specified to determine the asset controlling corrosion rate (Maximum, Average, or Formula). This differs from a Corrosion Analysis that does not include skipped measurements. When a Corrosion Analysis does not include skipped measurements, the Asset Average Corrosion Rate field is populated only when certain settings are specified, and this value can be the result only of an Average or Formula method.

Asset Average Corrosion Rate is an input to some of the calculations that are used to populate the Asset Average Corrosion Rate Remaining Life, Retirement Date, and Next Inspection Date fields. These calculations are performed using the same formulas that are used when a Corrosion Analysis does not include skipped measurements. The results of the same calculations in a Corrosion Analysis that includes skipped measurements versus a Corrosion Analysis that does not include skipped measurements, however, may differ because of the values that will be used to populate the Asset Average Corrosion Rate field in each of these Corrosion Analyses.

In every case, the value selected will be the most conservative (i.e., highest) corrosion rate, whether from the TML Corrosion Analysis Corrosion Rate field, or the Asset Corrosion Analysis Controlling Corrosion Rate field.