FFS Appendix 1. Calculations

LimitState/MAT

The LimState is the MAT (Minimum Acceptable Thickness) of a corroded pipeline for a specific corrosion morphology and defect assessment code.

The following types of corrosion morphology are distinguished:

• General corrosion.
• Grooving corrosion.
• Pitting corrosion.

Pitting corrosion is characterized by Defects having a length/depth ratio ≤ 20 (pits, short but relatively deep).

Grooving corrosion is characterized by Defects having a length/depth ratio > 20 and ≤ 80 (scars, somewhat longer length, but not that deep).

General corrosion is characterized by losing wall thickness all over the pipeline (length/depth ratio > 80).

MAT calculation for a specific corrosion morphology 

 The corrosion morphology factor sets a certain length for which to check the MAT.

The factors for pitting and grooving can be modified by the administrator.

The generic formula to calculate the MAT is:

The different Assessment codes use different variable assignments for calculating the Remaining Strength Factor (RSF) and bulging factor (M).

If you are working at a Shell site, we recommend you also reference SR.14.13139.

Diameter conversion

There is a conversion from a ‘named’ nominal diameter to the actual outer diameter in mm (which should be used in calculation). A ’10 inch’ line for example has an actual outer diameter of 273.1 mm (and not 254 mm).

Conversion from named nominal diameter to the actual outer diameter: