PL External Corrosion - Excor

Take Note

This overview will explain the Excor section in the IMS PLSS External Corrosion module and provide explanations for how the data is calculated.

This section contains the results of the Excor model and graphical presentations for each Test Post:

• Protection Effectiveness

• Corrosion rate

• Wall thickness

Excor Grid

In this grid, both the combined Corrosion Rate (Combined CR) and the Intermediate Results used to calculate it are displayed. The Combined CR is a function of the Base Corrosion Rate (CR Base), Cathodic Protection (columns starting with "CP"), and Coating (columns starting with "Coating"). Each row in the grid represents a specific combination of sections from the Onshore Soil / Offshore water conditions sectioning in the Ext. Corr tab on the Equipment Detail Page and its corresponding coating type.

Columns Wall and Design factor

These values are extracted from the PL Design Data section on the Equipment Details Page.

Column Combined CR

The Combined CR is the model (predicted) Corrosion Rate and is a function of the CR Base, the Cathodic protection (columns starting with CP), and the Coating (columns starting with Coating).

The function to calculate the Combined CR:

Example:

If we apply the function to the first row in the image above we get:

.

Column CR Base

CR Base is depending on the soil or water type, which is defined in section Onshore Soil/Offshore Resistivity, and will be determined from the following table:

If the Pipeline is Offshore, or both Offshore and Onshore, then the following additional table will also be available:

In the example above the first section from 0 to 557 meters is in Compact loams, clays which results in a CR Base of 0.1 mm/y.

Column CP Eff Prot

CP Eff protection depends on the specified potential and will be determined from the following tables: The table below, with Cu/CuSO4 will be used for Onshore Pipelines.

The table below is used for Offshore pipelines:

The CP value is calculated for each meter of the pipeline by interpolating the CP values between two Test Posts. From 0 meters to the first Test Post the CP value is equal to the first Test Post. From the last Test Post to the end of the Pipeline the CP value is equal to the last Test Post. The maximum interpolated CP value in the section determines the protection effectiveness.

If CIPS data are available, then the CIPS data will be used if the absolute time difference between the date of recording the CIPS data and the date of recording the test post data is less than six months.

Example: CP Eff prot, Row 1, from 0 to 557 is 100%. Fully cathodic protected.

Row 2, from 558 to 800 contains an interpolation of the potentials of test post ID TP01 and TPA (both measurements -870 mV) and an interpolation of the potentials of test post ID TPA and TP02 (from -870 mV at 650 m to -590 mV at 850 m). The maximum interpolated CP value in the section is just at the end at 800 m: -660 mV. Hence the protection effectiveness is 25%.

Row 3, from 800 to 1020 is 0%.

Row 4 to 6 are fully cathodic protected.

Column CP availability

The Cathodic Protection Effectiveness for the Availability will be determined from the following table:

The availability, in the right upper pane, has been set to 90-99% so column CP avail contains, for each section, the value of 50%.

Column CP Eff

The formula to calculate the CP Eff is:

Example: For the first row:

Column CP CR only

The formula to calculate CP CR only is:

Example: For the first row:

Column Coating Age

The Coating Age is the current date minus the Installation date.

Example: For March 2022, the coating age was then 22.2 years.

Column Coating Eff Pipe and Column Coating Eff Joint

For the calculation of the protection effectiveness for the Main Pipe and the Field Joint the following calculation is used:

Where the variables are derived from this table:

For external coating, the following table is used to determine the coating class:

For Joint Coating, the following table is used to determine the coating class:

For section 1 from 0 to 1020 meters, the External coating for the Main Pipe is “Tape double wrap”, and for the Joint coating it is “Tape”. This gives “Class B” for Coating Eff Pipe and “Class B” for Joint protection. With a Coating Age of 20.5 years (assessment performed July 2020) gives:

%

Column Coating Eff

This column Coating Eff presents the minimum value between the column Coating Eff Pipe and Coating Eff Joint.

Column Coating CR only[mm/y]

The formula to calculate the Coating CR only is:

Example: For the first row we get:

Column Eff

The Eff column represents the combined effectiveness of both Cathodic Protection (CP) and Coating.

Example: For the first row the Eff is:

%

Column CR Base, CP CR only, Coating CR only, and Combined CR

See Intermediate Results.

Column CR IP [mm/yr]

This column shows the Corrosion Rate based on the IP run. In the example above external Defects are only present between 800 and 1020 m. The worst Corrosion Rate between these Defects is shown in the column (see WT Graph for the calculation). Note that the model assumes hardly any Wall Loss in the first 10 years based on the coating conditions.

In this example the ILI run is in 2010, install date 2000 so, a high Corrosion Rate that has to be verified.

Column Rem Wall

The remaining wall thickness (see tab WT Graph for the calculation).

Protection Eff Graph

This graph displays the Protection Effectiveness for Cathodic Protection, Coating and overall effectiveness model for each section from the Install date.

The graph shown corresponds to the selected row in the grid above.

The pink line corresponds to the data in Coating Eff Pipe Colum.
The blue dotted line corresponds to the data in the column CP eff.

The green line corresponds to the Effectiveness of the Model and is calculated as:

Corrosion Rate Graph

This graph displays an overview of the different corrosion rates based on:

• CR base

• CP

• Coating

• Model CR

• IP run

This graph conforms to the WT Graph, plotting the CR for each given moment in time with the sectioning behavior of the WT Graph.

The graph shown corresponds to the selected row in the grid above.

Wall Thickness Graph

This graph displays the Protection Effectiveness of the Cathodic Protection, the Coating, and the Overall Model from the installation date. The graph shown corresponds to the selected row in the grid above. The Wall Thickness displays the wall thickness graph-based corrosion rates from three sources: a) coating performance, b) CP performance, and c) model CR. In addition, if corrosion defects have been found in a specific section, the coating performance curve is scaled to hit the worst corrosion defect.

The cross point of the green line with the vertical dotted line is the Remaining Wall Thickness (Rem.Wall) in the grid Overview. The yellow IP defect is the deepest reported external defect from the IP data, the error bar is the accuracy. In this example the deepest Reported defect depth is 2.8 mm, and a nominal wall thickness of 5.6 mm shows the yellow spot at 5.6-2.8=2.8 mm. The inaccuracy is 10%, so the error bar is from 2.8-0.56 to 2.8+0.56.

There are two ways to calculate the IP CR:

1. The Assessment graph (right lower pane) is linear:

   
   

2. The Modelled IP CR used in the WT graph is linear based on the Combined CR. So, the IP factor = linear IP CR/Combined CR at that moment (2006), then applied to Combined CR gives the IP CR.