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Calculation of Inspection Intervals for an Ammonia Storage Tank Based on Design Modifications, RBI, Commissioning Inspection, and FEA

One of the most critical welds in the design of a large storage tank is the weld between the bottom annular plates and shell wall. The change in direction of the material introduces a biaxial stress, and possible stress concentration. This paper evaluates a method of determining a required inspection interval based on using a combination of conservative design modifications, risk based inspection (RBI) assessments, on site construction verification, and FEA analyses. In the case study presented, a design modification was made to specify this traditionally dual fillet weld as a full-penetration fillet weld. While this exceeds the typical design requirements, exceeding the minimum requirements, inspection codes do not provide acceptance criteria for the design. A more thorough volumetric inspection was conducted, in addition to the typical magnetic particle inspection, to confirm the full penetration of the weld was achieved. In making conservative changes to the design, more robust inspections, and performing RBI methodology, a longer inspection interval can be determined due to the increased information available on the outset. These small changes at the start of the construction can lead to increased operating time without an intrusive inspection through the lifespan of the tank.