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The paper presents an assessment of the design fatigue limits of a pressure vessel that has been in service for hydrocarbon gas separation on an offshore installation. The pressure vessel was designed to the BS 1515 standard that is now superceded. The fatigue limits of welded regions of the vessel have been calculated using the modern design methods of EN 13445 and PD 5500 based on equivalent stress and maximum principal stress, considering both surface extrapolation and through wall linearisation to the fatigue hot spot.

The assessment provides an opportunity to make a comparison between the fatigue lives predicted using modern design methods. There are differences in approach and treatment of stresses between PD 5500 and EN 13445 yet the effect of these differences on the fatigue life for a given weld are not well understood. Since the condensate flash vessel contains design features that are typical for many pressure vessels it enables a comparison of the fatigue lives for different features to be made.

Modern pressure vessel construction standards contain provisions for fatigue assessment of parent material and welds. PD 5500 (Annex C) and EN 13445 (Clause 18) contain these provisions for vessels designed to these standards. The principles of these methods are well established [4], but a summary of the approach for welds is given here to assist the reader in conjunction with this paper.

Differences between PD 5500 and EN 13445 arise in the stress range that is to be used for fatigue assessment in a multi-axial stress system. PD 5500 is based on the range of the maximum principal stress only. In contrast, the main approach advocated in EN 13445 is the use of either a Tresca or von Mises equivalent stress range, according to established German practice; the use of maximum principal stress range is also permitted but specified in Annex P.

The Tresca stresses are generally the highest and the von Mises stresses are the lowest. The maximum principal stresses are similar to the Tresca except in regions of high shear where Tresca dominates. Note that on the head side of the knuckle weld, the maximum principal stress is zero at the outside free surface because this is a region in compression. This can be seen in Figure 4.

The fatigue lives of the three welded areas described above were assessed using the design methods of PD 5500: 2009 Annex C which represent a probability of failure of approximately 2.3% and BS EN 13445-3: 2009 Clause 18 and Annex P which represent a probability of failure of approximately 0.14%. The allowable fatigue cycles of each weld were determined as a function of pressure range using the Class of S-N curve appropriate to the weld detail indicated in Table C.2 of PD 5500 and Table 18.4 and the Table of Annex P of EN 13445. Assessments to EN 13445 assumed the vessel was constructed to Testing Group 1 and 2.

A thickness effect was taken into account for sections where the material thickness e > 22mm (PD 5500) and e > 25mm (EN 13445). The stress ranges obtained from the finite element model were divided by the factor (22/e)1/4 and (25/e)1/4 respectively. In practice this correction was very small.

The weld detail was a full penetration butt weld made from both sides and free from significant defects. With respect to the seam welds in the shell, the lowest fatigue life would be from weld toe cracking at the longitudinal seam under the action of the hoop stress. The calculated stresses were higher on the inside of the vessel therefore the fatigue analysis was performed at this location. The effect of misalignment was not included in the fatigue analysis at this stage. The fatigue assessment was performed using both Class D and Class E curves according PD 5500 (as no information was available concerning the overfill profile) and the Class 80 curve according to EN 13445.

The design fatigue limits for the seam weld at the inside location according to PD 5500 and EN 13445 are shown in Figure 5 in terms of the number of allowable cycles as a function of the pressure range. Fatigue analysis performed using von Mises equivalent stress range (EN 13445) produced a higher fatigue limit than the use of Tresca equivalent stress range (EN 13445) and maximum principal stress range (PD 5500, EN13445). The design fatigue limit based on von Mises equivalent stress range with the Class 80 curve (EN 13445) exceeds that based on the Tresca equivalent stress range with Class 80 curve (EN 13445) and that based on the maximum principal stress range and the Class E curve (PD 5500) by a factor of 1.5 on cycles.

The design fatigue limits from using the Class D curve of PD 5500 and maximum principal stress range are very close to that from using the Class 80 curve of EN 13445 and von Mises equivalent stress. The fatigue limit based on the maximum principal stress range of PD 5500 and EN 13445 are almost identical due to the closeness of the Class D and Class 80 curves and the effect of the plate thickness correction used by each standard.

Figure 6 shows that the fatigue analysis performed using maximum principal stress range and the Class D curve (PD 5500) gives the highest fatigue limit and exceeds the lowest based on Tresca equivalent stress range and Class 80 curve by a factor of 1.5 on cycles.

The weld detail considered was cracking from the toe of the fillet weld into the head. This detail has a nominal Class F fatigue strength according to PD 5500, and a Class 63 based on the maximum principal stress range and a Class 63 or 32 based on the equivalent stress range according EN 13445. The hot spot stress range at the weld toe was determined using surface extrapolation of maximum principal, Tresca and von Mises stresses. Since the hot spot stress incorporates the stress concentration due to the structural discontinuity, PD 5500 allows the use of the Class E curve with the maximum principal hot spot stress range. As no information was available on the size of the weld throat, both EN 13445 Class 63 and 32 curves were used for the fatigue assessment based on the equivalent stress range.

The results (Figure 7) showed that the fatigue analysis performed using the maximum principal stress range (Class E) according to PD 5500 gave the highest design fatigue limit. Use of the Class 32 curve and the Tresca equivalent stress range gave the lowest fatigue limit. The fatigue limit from using the Class E (PD 5500) curve and the maximum principal stress range exceeds that from using the Class 63 (EN 13445) curve and either the Tresca equivalent stress range or the maximum principal stress range by a factor of approximately 1.9 on cycles.

Figure 8 compares the fatigue limits predicted using three different methods for calculating the structural hot spot stress at the compensation plate weld toe. The assessment was based on using the PD 5500 Class E curve. The results showed that the nodal force (NF) method gives better design fatigue life than the surface stress extrapolation (SSE) and through wall integration (TTI) which gave similar fatigue behaviour.

Comparing the fatigue analyses, we observe that PD 5500 can give a higher fatigue limit than EN 13445 for the same weld detail where it allows a different classification. We also observe that the fatigue analyses using Tresca equivalent stress range and maximum principal stress range are more conservative than using von Mises equivalent stress range for the same weld Class.

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