Revamps 2022 Issue

or global peaking assumptions. However, the difference in the calculated bending stress can be significantly dif - ferent, depending on whether you are using a local or global peaking model, and consequently, the calculated fatigue life can be misleading. The difference in the local and global deformation as copied from API 579 is shown in Figure 1 . The deformation behaviour is most accurately cap - tured by 3D laser scanning and imported into an FEA model to determine the associated through-wall stress for the fatigue analysis. The displacement of a PSA shell subjected to a laser scan and consequently analysed using an FEA is shown in Figure 2 . In this case, it is interesting to note that if a template had been used, the value of δ (see Figure 1 ) had been measured to approximately 3.5 mm. However, in this case, when looking at the laser scan data, the defor- mation more resembles the global ovality. This, in turn, resulted in stresses across the seam weld that were more severe than those measured if a template had been used. A comparison is shown in Table 1 . Rb given in Table 1 is the membrane to bending ratio that needs to be considered in the fatigue assessment. The deformation from the original fabrication shown in Figure 2 is only one example. There are examples where peaking occurs locally across the seam weld, which can be severe and cause failures due to fatigue cracking. 5 In most cases, the issue with peaking is not taken directly into consideration during the design phase. In some cases, the peaking or deformation will be life-limiting for the PSA vessels. Therefore, it is recommended that a midlife assess- ment is carried out to evaluate the consumed life fraction based on laser scan data. Take, for example, the PSA sub - jected to laser scanning shown in Figure 2, with the bending stress of 69 MPa, assuming that the vessels were designed for 20 years (or 700,000 pressure cycles). Currently, it has been in operation for eight years and has been exposed to 280,000 pressure cycles. Therefore, the next step is to use an appropriate fatigue curve to determine the life fraction based on the stress defined by the FEA. BS 7910: 2019 (Section 8.8.1) offers a fatigue assess - ment for welded joints with misalignment based on the stress concentration factor caused by the misalignment. A joint quality category equivalent to those given in BS 7608 is derived based on the additional bending stress from the misalignment. The results are summarised in Table 2 . The quality curves in BS 7910 provide design life (mean plus two standard deviations). The fatigue life for mean plus 2 SD has been compared with that estimated using mean fatigue data. However, it should be noted that since the fatigue curves are developed for cyclic behaviour in air, it is

U, Magnitude

+1.397e-01 +1.233e-01 +1.069e-01 +9.054e-02 +7.414e-02 +5.775e-02 +4.136e-02 +2.497e-02 +8.574e-03

Figure 2 Displacement contour plot of 3D laser scanned PSA shell being subjected to an FEA (units in inch), peak displacement across the weld is 3.5 mm, while the ovality measured in one cross-section is 11 mm

Bending Membrane stress (MPa) stress, (MPa)

API 579 Global peaking API 579 Local peaking

0.24 0.56 0.73

23 53 69

FEA (laser scan)

recommended not to rely on mean properties as there is no account for the hydrogen environment. In summary, for this case, the most severe stress range due to deformation would not have been detected if a template across the seam weld had been used. Developing the life management strategy for PSA vessels using fracture mechanics The fatigue performance and crack growth rate for a PSA depend on several factors: • Hydrogen partial pressure • Frequency • Stress intensity factor range, ΔK, which is a function of crack size and stress level • Load ratio (Min/Max) • Material (base metal, weld or HAZ). Several published papers have investigated the effects of hydrogen on fatigue crack growth. However, very few have managed to simulate the service conditions of PSA vessels. Table 1 Bending stress as a function of different models (3.5 mm) for global and local peaking and compared with results from the FEA

Membrane

Bending

SCF (km) 1 + σ b/ σ m

Current number

Mean plus 2SD

Mean

stress

of cycles

Cycles to failure Life fraction

Cycles to failure

Life fraction

94MPa

69MPa

1.73

280,000

296,000

95%

744,000

38%

Table 2 Life fraction calculations based on the FEA

Revamps 2022