H2Tech - Q3 2022 - 31

MAINTENANCE AND RELIABILITY
was not designed to assess structural integrity
of the entire vessel, such as inside
the vessel, skirt, vessel support, etc.
Views of a cross-section of the PSA
vessel model for the hydrotest load case,
hoop stress, von Mises stress and maximum
principal stress are shown in FIG. 2.
The locations for the extraction of the
stresses for the FFS assessment of the vessel
for weld locations are N02 (top nozzle),
N01 (bottom nozzle), H01 (top head
weld), H02 (middle body weld), H03
(bottom head weld), V01 (upper vertical
weld) and V02 (lower vertical weld), as illustrated
in FIG. 3.
The through-thickness stress distributions
of the welds selected for the assessment
are shown in FIG. 4. The stress
component was chosen depending on
the crack orientation with respect to the
weld location (e.g., a hoop stress component
for transverse cracks in circumferential
welds and an axial stress component
for transverse cracks in seam or vertical
welds). These results have been summarized
in TABLE 4 in terms of membrane
and bending stresses, which are required
for the fracture mechanics assessments.
FFS assessment. The FFS assessment
and remaining life assessment for the
crack-like defects found in the welds were
carried out in accordance with guidance
given in API579 and BS7910. The assessment
included two major components:
* Determining the criticality of the
defects using a failure assessment
diagram (FAD)
* Calculating the remaining life of the
most critical defect from each vessel.
Flaw geometry, location and orientation.
The most critical defects were selected
from the inspection reports based
on the defects' height and the ligament.
The assumptions made for the FAD assessment
were:
* The length of defects was same as
the weld size (as shown in TABLE 5),
i.e., wall thickness as the flaw lengths
were not available.
* The defects are located at the
highest stress area within the weld.
* Subsurface or embedded defects
are characterized as elliptical cracks,
and surface defects are characterized
as semi-elliptical cracks.
The generic geometry of the elliptical
embedded flaw and semi-elliptical surFIG.
4. Through-wall stress extraction path for cracking at the weld location: (A) top nozzle, N02;
(B) bottom nozzle, N01; (C) top head weld, H01; (D) middle body weld, H02; (E) bottom head
weld, H03; (F) upper vertical weld, V01; and (G) lower vertical weld, V02.
H2Tech | Q3 2022 31
face flaw used in the FAD assessment are
presented in FIG. 5.
In this assessment, the lengths (2c as
indicated in FIG. 5) were considered having
the same size as the weld size, and
the crack depths (labelled 2a as an embedded
crack, labelled " a " as a surface
crack) were taken from the inspection
report. Flaw locations and orientations
were as specified by the inspection report.
The flaw orientation was assumed
normal to the weld.
Primary and secondary stresses. The
primary stresses used for this assessment
come from the " hydrotest " loading condition
shown in TABLE 4 of the FEA results.
The PSA vessels were subjected to
post-weld heat treatment (PWHT);
therefore, secondary weld residual stresses
in accordance with BS7910 for PWHT
conditions have been applied. For flaws
that are aligned parallel to the weld, the
residual stress can be assumed as 30% of
the yield strength of the parent material
at room temperature. For flaws that are
aligned transverse to the weld, the residual
stress can be assumed as 20% of the
lesser of the yield strength of the parent
and weld materials at room temperature.

H2Tech - Q3 2022

Table of Contents for the Digital Edition of H2Tech - Q3 2022