Revamps 2022 Issue

FEA. The result is shown in Figure 3, given as years in ser- vice since the last inspection, assuming continuous opera- tion and four pressure cycles per hour. The time in service has been divided into three zones:  The safe operating zone  The ‘critical zone’ where the crack is growing at an ele- vated rate  The ‘high-risk zone’ where the crack is predicted to be beyond the limiting size. It is recommended that the reinspection interval is estab- lished using a ‘half-life’ approach. For example, in this case, the time to reach 80% through-wall is four years, and the reinspection should be after two years (half life). Measure peaking using a template An example of the importance of which model (global or local peaking) is employed in the assessment when using template measurements across the seam weld is shown in Figure 4 . The number of pressure cycles to reach 80% through-wall from the undetected crack that is 1.5 mm in height and 10 mm in length is shown as a function of the measured peaking for local and global peaking models, as given in API 579, assuming local peaking will result in a much shorter life as compared to global peaking because of the higher calculated stresses. This highlights the importance of the assumptions used in the analysis and why it is essential to obtain a laser scan of the vessel and use that for the finite element stress analysis. Operators often face the challenge of conducting an internal inspection during turnaround, which requires the absorbent to be removed and the vessel to be scaffolded on the inside. This can be very costly, considering there are often 6-10 vessels in total. Conducting the laser scanning and the subsequent stress analysis helps identify the areas of high stress in the vessel, which in many cases eliminates the need for an internal inspection. The likely locations for crack initiation when considering the weld peaking are on the ID of the vessel at the weld seam, and these can be ultrasonically inspected from the OD while the vessels are in service.

SCF

Number of cycles to failure from BS7910 (Mean +2SD)

API 579 Global peaking API 579 Local peaking

1.24 1.56 1.73

612,000 381,000 296,000

FEA (laser scan)

The effect of hydrogen is hugely important on the fatigue crack growth rate, and it is easy to become overly conser- vative when conducting the fatigue crack growth analysis. This can result in determining a remaining life that does not represent experience seen in service and unmanageable short inspection intervals. Published fatigue crack growth rates 4 were for tests con- ducted close to the operating conditions for many of the PSA vessels in operation in terms of hydrogen pressure, cycling frequency, and load ratio. Therefore, these crack growth rates have been used in this analysis. The inspection and life management strategy should consider the potential that a fatigue crack can initiate due to excessive weld peaking. Therefore, it is recommended to establish an inspection interval based on the time it takes for the largest crack that could be missed during an inspec- tion to grow to a limiting crack size. While limiting crack sizes should be calculated employ- ing procedures either in Part 9 of API 579 or BS 7910, in most cases for the seam welds, the limiting depth of the crack will be 80% through-wall, which is the limit depth for a surface breaking flaw in accordance with API 579. The reason the cracks will reach 80% is because the vessels are subjected to post-weld heat treatment, so welding residual stress does not contribute greatly to the fracture ratio. Assuming an undetected fatigue crack in the seam weld of 1.5 mm in height and 10 mm in length, the time for this crack to grow to 80% through-wall has been calculated for the case shown in Figure 3 based on the stress from the Table 3 Fatigue life for the two peaking models compared with the FEA laser scanning

100,00 400,00 300,00 200,00 500,00 600,00 700,00 800,00 900,00

Safe zone of operation

Critical zone of operation

High risk zone of operation

API 579 Global peaking API 579 Local peaking

Half life for next inspection (2 years)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0123456789 Measured peaking value (mm) 0

Estimated remaining life since last inspection

Figure 3 Operational management and inspection interval requirement (assuming four pressure cycles per hour and continued operation)

Figure 4 Difference in number of pressure cycles to reach 80% through-wall from the last inspection for local and global peaking models

Revamps 2022