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)
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