Tanks - Degradation Management

For Tanks the RBI Analyses take place on the Component level. Note: We mention this explicitly, as in the past this took place or a Sub-Component level. Sub-Components no longer exist in IMS.

Assumptions and Guidance

• Tank RBI in IMS PEI is applicable to aboveground Atmospheric Storage Tanks (ASTs) for pressure boundary Components.
• For other Tank Components, the Reliability Centered Maintenance (RCM) process is advice.
• We advice to no use RBI for unknown repair conditions. Actual conditions should be defined.
• A 40-year design life can be assumed for no inspection history – to establish a baseline or Design Corrosion Rate (DCR). 40-year design life is based on original floor thickness of 6.4 mm [0.25 in.] with a 2.5 mm [0.100 in.] renewal thickness. This leaves ± 4 mm [0.16 in.] Corrosion Allowance, which gives predicted design CR of 0.1 mm/yr [4 mpy].
• When the methodology conflicts with / is less stringent than country laws/regulations, the country laws/regulations should be applied.

Define Asset (incl. LC2 and LC3 calcs) – Step 1

See Step 1.

Defining Tank Equipment

Defining Tank Equipment:

• A Compliance Code, e.g., "API" or "DEP" must be selected in the Equipment details. Note: This affects the Load Case calculations.

You can select a Compliance code when creating Tank Equipment. 

• Go to the Equipment’s Specs section and enter the specification. The table in Tank Equipment highlights the required specs per Load Case.

Defining Tank Components

The Tank Component can be created from the Tank Equipment's RBI section. Take note that it is mandatory to select a CL with which the Component will be linked. Note: In the past Tank-CL were automatically created, but this is no longer the case.

Creating a Tank Component. 

Defining the Corrosion Loop with Tank-DMs

For Tanks you can define normal DMs as well as T-DMs in your CL:

• In the Degradation Management column, click the Plus button to add a new DM.
• Expand the Tanks dropdown to see all T-DMs.

When the DMs have been defined on your CL, they can be assigned to your Tank Components (see Step 3 below).

To learn more about creating and defining a CL see  Step 2 Identify Threats and Barriers (Define the CL).

Specifying additional info for the Tank Component

Depending on the Component type, you will need to specify some additional information for your Tank Component:

Additional info needed for a Shell Component.

This additional information is used in the Load Case calculations.

Start RBI Analysis and Assign DMs – Step 3

See Step 3.

You can assig normal DMs and Tank-DMs to your Component.

The following must be true for a DM to be available to assign to the Component:

• The DM has to be in the Corrosion Loop already.
• The DM must be a Tank DM (T-DM) or alternatively it has to be mapped to a T-DM.
• The DM must be allowed for the applicable Component Type. Note: The "NON-CORROSIVE" DM will always be available for assigning, as long as it is part of the CL. It thus does not depend on the Component Type.  
  

  T-DMs and normal DMs can be assigned, as long as the normal DM is mapped, as shown here. 

Review Inspection History and Credit DMs – Step 5

See Step 5.

The Inspection History must be used to capture the Actual Wall Thickness at the time of the inspection. This last know thickness, as entered in the Inspection History, is used for the Tank CR (LC 1) calculations as displayed on the RBI Calculator tab for the Tanks Components (see Step 6 below).

The Inspection History must be used to capture the Actual Wall Thickness. 

Estimate RL (LC 1) & Define Lining / Coating – Step 6

See Step 6.

Combined CR MII for Tank Floors

For Tank Floors if the Corrosion Type is set to "Generalized" the corrosion between the Internal and External (Underside) T-DMs are combined.

A Tank Floor with a Combined CR MII. 

Remaining Life - Load Case 1 Calculation

The LC1 calculation results are shown on the Calculator tab and is related to the Remaining Life. For these calculations, you need to provide data for at least one inspection with the Actual Wall Thickness (see Step 5 above). Also, some specs need to be specified. You can use the table in Tank Equipment to see which Equipment Specifications and Component data are required for the LC1 calculations. When the Actual Wall Thickness and required specs are available, the RL will be determined. This is then used to calculate the NID.

The Remnant Life tab shows the LC1 calculations.

This Calculator tab will show the following calculated values:

• Calculated RCA = Calculated Remaining Corrosion Allowance: Actual Wall Thickness from last credited inspection (see Step 5 above) – Allowable Wall Thickness (AWT)
• Calculated CR = Calculated Corrosion Rate: Depends on Roof, Shell, or Floor (and Compliance Code). The basic calculation uses (Thickness as built (TAB) - Actual Wall Thickness) / time from Start Date to last inspection.
• Allowable Wall Thickness (AWT): For Shells (DEP compliance code), it is the Calculated AWT. For Shells (API compliance code), it is the highest value between Thickness as built (TAB) or Calculated Thickness (TMin). For Roofs and Floors, it is the Renewal Thickness (Sub-Component Spec).
• Calculated AWT = Depends on Compliance Code, Pitting, and Sub-Component (Roof/Shell/Floor). For Shells (API compliance code) it is the Calculated Thickness (TMin) but with a threshold (see Tank Load Case 1 Calculations).
• DCA = Design Corrosion Allowance: Difference between the Thickness as built (TAB) and the Calculated AWT.
• Calculated Thickness = Minimum Wall Thickness (TMin): For Shells this is calculated from: Diameter (D) (Equipment Spec); Spec Gravity @ Op Temp (Equipment spec); Design Pressure (Equipment spec); Yield; Joint Efficiency Factor (JEF); and Product Height (PH) – which is calculated from Maximum Filling Height (Equipment spec) and Course Height).

Pitting can be set on the General RBI Specs tab.

Also, the Product Height and Minimum Allowable Wall thickness (= Allowable Wall Thickness) are determined. These are shown on the RBI Specs (General) tab (see above figure), as well as on the Component Details Page.

For more information on the LC 1 calculations see Tank Load Case 1 Calculations.

Lining and Coating

Linings/Coating is applicable to Tanks. Thus follow the applicable steps as explained in Step 6 (Take Credit for Lining or Coating Life).

Note: The Lining and Coating specs should be filled in for the Component when the Asset is being defined in Step 1. Missing data can be filled in on the Lining/Coating Tab.

Confidence – Step 10

See Step 10.

There is an addition 4^th question: Has Corrosion assessment been carried out?

The ”RBI Optimisation” Dashboard shows a query that can be used to find tank confidence ratings that are missing an answer.

IF – Step 11

See Step 11.

The Interval Factor is generated from the “full life” table.