energize oil & gas – issue 01-2013 - GL Group

integrity composite repairs
methods, suggesting that the circumferential yield
strength of the repair material is the property dominating
the behaviour. Therefore the different materials are not distinguished
in the results presented below.
Figure 6. Example of failure predicted for
a repaired bend (900 NB bend first predicts
failure at bend/pipe transition).
LPF ( = applied pressure in MPa)
LPF / failure pressure (MPa)
50
40
30
20
10
50
45
40
35
30
25
20
15
10
5
0
Limit pressures for repaired bends
600 NB thin
300 NB
1,200 NB
900 NB
600 NB
300 NB
150 NB
50 NB
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Displacement (mm)
Figure 7. Predicted failure pressures for
repaired bends.
LPF unrepaired
LPF repaired
P f
Pmin
undamaged 150 NB
undamaged 600 NB
0
0.05 0.07 0.09 0.11 0.13 0.15
D/(TSy)
Figure 8. Predicted failure pressures from
FEA compared with ISO 24817 spool survival
test pressure (P f ) and Pmin, based on von
Mises’ yield criterion and thick-wall theory.
Discussion
The truncation of the LPF plots in Figure 7 was determined
to be a result of the conservative assumption that the material
model used was elastic-perfectly plastic.
For each geometry, the failure pressure for the repaired
bend is predicted to be higher than the calculated pressure
required for the short-term spool survival test to be classed
as successful. This is illustrated in Figure 8, which shows the
predicted failure pressure for each model against the factor
(D/TSy), compared with the ISO 24817 test pressure P f .
This was considered further, and two major factors contribute
to the difference between the predicted LPF and the
short-term survival test pressure redefining P f :
P f is based on thin-wall shell theory. This assumption is
not valid for use on all the D/T ratios in the FEA.
P f is based on the hoop stress reaching yield strength,
whereas the von Mises yield criterion is used in the Riks
analysis to determine the LPF.
The pipe will be capped at both ends during the test and
therefore will be subject to an axial stress due to internal
pressure, in addition to the hoop stress. Calculation of the
failure pressure based on von Mises stresses will therefore
be more representative.
If both of these factors are taken into account, then
the failure pressure (Pmin) is calculated based on the von
Mises equivalent stress reaching the SMYS and using thick-
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