PL External Corrossion Assessment

Take note

This article explains how to use and interpret the External Corrosion Assessment module in IMS PLSS and its role in calculating the Remaining life of External sections. The module allows you to specify both Past and Future Corrosion Rates, which are essential inputs for the RBA. By utilizing these Corrosion Rates, the RBA assessment can determine the Remaining life of the external sections of pipelines. Additionally, this module provides access to CP data, CIPS data, Rectifier station data, Wet insulation data, and DCVG data. The External Corrosion Assessment module is available only for the following Equipment Groups of Pipe Type Carbon steel and CRA:

• Pipeline

• Pipeline Jumper

• Flowline

To open the External Corrosion module:

• Navigate to the Equipment Main Screen.

• Find your Equipment.

• Click on the Equipment ID.

• In the header of the applicable Equipment Details Page, click on the External Corrosion button.

External Corrosion Assessment

The External Corrosion Assessment section is divided into five sub-sections:

ECH

In this grid, an overview of previous assessments will be shown.

Example

In the image below, one Assessment was performed in January 2015.

The Indicator is colored based on the comparison of the Worst Future CR compared to the Design CR which has been calculated in the RBA module. The indicator is colored:

• Red: if the Worst Future Corrosion Rate is more than four times the Design Corrosion Rate or if the Past and/or Future Corrosion Rate is missing.

• Orange: if the Future Corrosion Rate is between one and four times the Design Corrosion Rate.

• Green: if the Worst Future Corrosion Rate is less or equal to the Design Corrosion Rate.

A black circle around the Indicator means the ECH has been Final Approved.

Note: The Condition date is the Assessment date that will be used in the RBA module to calculate the Remaining Life.

Adding External Corrosion Assessment

• Click the Plus button.

  

• Enter the required data in the pop-up window.

• Click Save when done.

On the ECH Detaiils Page of the ECH you created, the "GENERAL" checklist will already be selected and under Actions performed "EXTERNAL CORROSION ASSESSMENT" option will be set to "Yes". The Checklist section displays the Internal Corrosion Assessment Graph with the Sectioning section and Confidence Rating questionaire. 

Note: In case you do not see the sectiones mentioned above, on the ECH Details Page, in the Checklist section, under Actions performed click on the Grid button.

Adding Pipeline Sections to the ECH

Next you have to create Pipeline sections and add their Past and Future Corrosion rates and Corrosion Morphology type.  If there are previous Assessments, the Pipeline Sectioning will be copied from the last Assessment but the Past CR and Future CR have to be specified.

To add Sections manually:

• On the ECH Details Page of the newly created ECH, scroll to the Checklists section.

• Click the Plus button under the Graph.

• In the Start At field, enter the distance from the very start of the pipeline to where this section begins.

• In the Past CR field, enter the past corrosion rate for that section of the Pipeline.

• In the Future CR field, enter the future corrosion rate for that section of the Pipeline.

• From the Corrosion Morphology drop-down menu, select the applicable option.  

  

• Click Save.

• Repeat the process to add additional Sections.

Next, you will have to fill in the Confidence Rating questionnaire: 

• Click Edit.

• Tick the checkbox(es) next to the applicable questions.

• Enter a Comment if needed.

• Click Save.

Lastly, Final Approve the ECH.

More details can be found in the PL External Corrosion Assessment Appendix 1.

External Corrosion Assessment Graph

In the Graph subsection, the External Defects are shown in pink. Different Corrosion Rates are shown as described in the legend. The orange lines denote the different sections of the Pipeline.

The Corrosion Rate and Morphology grid

This subsection contains a grid with data related to different Sections of the Pipeline and their associated Morphologies.

In the table below most of the columns from the grid are explained.

Confidence Rating - External

For the Confidence assessment, a questionnaire is provided. Read carefully the statements made. The tick mark provided (or not provided) should be backed with arguments in the Comment option.

The image below, illustrates the Questionaire for Onshore Pipelines:

For Offshore Pipelines, the Questionnaire is the same as for internal corrosion:

The Confidence Rating is calculated based on selected questions. The default Confidence Rating is Medium. There are five questions in total and upon checking the box for each question Confidence Rating changes.
The default/starting level is “Medium”, and the 1st and 5th questions are negative. The other questions are positive. Checking a negative question will lower the level of confidence by one class and checking a positive question will raise the level of confidence by one class. The levels are: Very Low, Low, Medium, High, and Very High.

The Confidence assessment is performed for the entire Pipeline, not per Section. In the Design data, the user specifies whether the Pipeline/Flowline is "Onshore", "Offshore" or both. For Internal Corrosion, the questionnaire is the same. For External Corrosion, different questionnaires are available if the Pipeline is Onshore or Offshore.

The table below provides explanations for the questions in the Confidence Rating Questionnaire for Offshore Pipelines.

Corrosion Morphology Plot

This chart shows three categories: “Pitting”, “Grooving” and “General”. The categorization is based on the ratio of defect length to depth:

• Defects with a ratio less than 20 fall under the category "Pitting."

• Defects with a ratio greater than 80 are classified as "General."

• Defects falling between these values are categorized as "Grooving."

In this plot, the blue lines, L/d = 20 and L/d = 80, divide the graph into a Pitting, Grooving and General area.

By clicking on a row in the Morphology Grid above, the Defects of the selected Section will be shown in the Corrosion Morphology Plot.

The Remnant Life Plot

The Remnant Life plot illustrates the Wall Thickness trends over time, featuring the Nominal Wall Thickness, estimated values at Assessment dates, Limit State, and the depth of the most severe defect found during Inspection. The accompanying table presents the Assessment dates with their corresponding estimated Wall Thickness values and indicates the projected date when the Wall Thickness will reach zero.

In this graph:

• The blue dots represent the Nominal Wall Thickness, and the Estimated Wall Thickness at the Assessment dates.

• The red line shows the LimState if there are no Defects in the section, otherwise it shows the Minimum Required Thickness.

• The red dots represent the Corrosion Tolerance based on the Defect.

The accompanying table presents the Assessment dates with their corresponding estimated Wall Thickness values and indicates the projected date when the Wall Thickness will reach zero.