H2Tech - Q3 2022 - 33

MAINTENANCE AND RELIABILITY
represent its final critical flaw size.
* The initial flaw could grow from
being an embedded flaw to reach
its final critical flaw size and still
be embedded.
* The initial flaw could grow from
being a surface flaw, to through
the wall to then reach its final
critical flaw size.
The assessments focus on the state
of the vessel before leakage occurred; in
other words, the crack may keep growing
as an embedded or surface flaw before the
depth penetrating the wall thickness of the
vessel. Therefore, under current operating
loads, the remaining life of a vessel was
defined as the remaining time before the
most critical flaw reached either its critical
flaw size (i.e., outside of the safe area in
the FAD) or prior to being developed as a
through-wall thickness crack (i.e., before
leakage occurs). The most critical defects
from each vessel, as summarized in TABLE
7, were considered for the remaining life
assessment of the vessels.
Cyclic stresses. Based on the maximum
and minimum operating stresses shown
in TABLE 4 of the FEA results section, the
cyclic stresses required for the fatigue
crack growth assessment were calculated
and are summarized in TABLE 8.
Fatigue crack growth model. The fatigue
crack growth analysis was carried
out using the Paris law (Eq. 1):
(da/dN) = C(ΔK)m
where:
da/dN = fatigue crack growth rate,
mm/cycle
ΔK = Stress intensity range, MPa √m
C, m = Paris law coefficient and
exponent.
C and m are the material parameters
available from API 579 and BS 7910,
which are applicable to weld steel joints in
air environment conditions up to 100°C.
C = 1.29 × 10-12
and exponent, m = 2.88.
Results of remaining life assessment.
TABLE 5 summarizes the remaining
life assessment results from 12 vessels
(D01-D12) in terms of remaining
operating cycles, days and years. The
remaining life in days and years were calculated
based on the operating condition
of 100 cycles per day.
In general, most of the PSA vessels
(1)
FIG. 6. An FAD of vessel D01.
FIG. 5. Generic geometry of flaws: (A) an elliptical embedded flaw and (B) a semi-elliptical
surface flaw.
FIG. 7. An FAD of vessel D10.
H2Tech | Q3 2022 33

H2Tech - Q3 2022

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