Case Study 2
Test Manifold

• 2013 inspection shows 12.2mm wall loss on test manifold. Line 10” Sch 160 (28.58mm nom.)
• Line also reported to have active internal corrosion

• 2013 Incumbent IMS provider carried out a Level 3 assessment using FEA and subsequently recommended a temporary repair using a composite wrap as the pipework was not suitable for design conditions.
• It was assumed that the Level 3 was the least conservative assessment and when the wrap expired the test manifold would need to be replaced
• On review the FEA was very conservative and assumed a flat bottomed rectangular defect equal in size to the maximum length and width of the scabbed area.
• This FEA was in fact only equivalent to a code calculation using tmm & ASME B31. 3.
• In 2017 the wrap was removed and further inspection completed.

• A 25mm x 25mm grid was specified over the previously wrap region

• It was proposed that an API579 Level 2 Local Metal Loss calculation was sufficient to prove the test manifold as FFS
• PSV set pressure was 225 Bar(g) and was set as the MAWP for the pipework.
• The MAWPc for the line was 255 bar(g)
• The FCA was increased until the calculated MAWPr was equal to the MAWP
• The calculated RSF was 0.804 compared to an allowable RSFa of 0.9
• A reduced MAWPr was calculated as MAWPc x RSF / RSFa = 227. 8 bar(g)
• A maximum FCA of 4.5mm was calculated which was deemed sufficient until CoP as Asset was deemed to be in Late Life.
• In this case the Level 2 assessment with the correct inspection results gave a less conservative result than the Level 3 assessment and saved the client a substantial scope of work on a late life asset.

It is important to understand the conservatisms in any assessment performed.  Were conservatism is built in to an FEA when performing a Level 3 assessment this can result in an assessment no better than a code calculation.  Select the most appropriate assessment Level and understand the calculations being performed and don’t just follow a set of rules.