Page 252 - Pipelines and Risers
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Reliability-Based Strength Design of Pipelines 225
LRFD moment format (DNV’96 Section 5, C300)
ASD (Allowable Stress Design) Format - checks of equivalent- and longitudinal stress, as
an alternative to the moment check. (DNV’96 Section 5, C400). The ASD format is
originally from IS0 DIS 13623.
The ASD criteria define a maximum allowable longitudinal and equivalent stress as a
percentage of SMYS. A pipe may be stressed well beyond the yield stress before it loses its
capability to fulfil its function and the ASD criteria should therefore be rather conservative for
this limit state.
The interaction equation between moment, internal pressure and axial loads for local
bucklinglcollapse in DNV’96 is a yield criterion from Mohareb et al. (1994). The maximum
capacity of the pipe is here defined as the bending moment at which the entire cross-section
yields. The moment criteria in DNV’96 (Section5, C305) is a LRFD format of the Mohareb’s
equation, where partial safety factors are defined based on structural reliability analysis work
et al. (1997)). However, Hauch and Bai (1998) found that the allowable moment given by
DNV’96 for some load conditions is at least 20% lower than that corresponding to the ASD
criteria.
In the following the moment criteria shall be re-visited and the causes of the conservatism will
be identified, together with a newly suggested calibration of safety factors.
13.5.2 Analytical Capacity Equation
Bai and Hauch (1998) modified the yield criterion from Mohareb et al. (1994), to account for
the effect of corrosion defects. The ultimate moment capacity is also defined as the moment at
which the entire cross section yields. The cases considered the defect section is in
compression (case l), in compression and some in tension (case 2), in tension (case 3), in
tension and some in compression (case 4). The four cases are shown in Figure 3.4.
13.5.3 Design Format
For internal-over pressure cases, the design format based on DNV’96 (Section 5 C305) may
be written as:
(13.6)
YFYCMKC + YEME, 5 Mc I YR
where MF,~ is the characteristic functional bending moment; ME,^ is the characteristic
environmental bending moment; M, is the characteristic ultimate bending moment (as given
in Chapter 3); YF is the functional load factor =1.1; YE is the environmental load factor which
is 1.3; yc is the conditional load factor; yRis the strength resistance factor.
13.5.4 Limit-State Function
The limit-state function may be expressed as:
g(z)=M,-(M,+M,) (13.7)