Page 78 - Pipelines and Risers

Buc!ding/Collapse of Deepwater Metallic Pipes 51

ocomp now defined as the longitudinal compressive stress in the pipe wall that would cause
is
the pipe material to yield according to the von Mises yield criterion. The value of dcVw is
thereby equal to 01 as determined above with the negative sign before the square root and ore,,,
with the positive sign in front of the square root.

3.4.2 The Location of the Fully Plastic Neutral Axis
For a pipe with a circular and yielded cross section, the axial tension can approximately be
expressed as:
F = AwP- + A-P- + %,a, (3.25)
AMPl =2(w-P)r'J (3.26)
(3.27)

A,, = Z(Z -y)r,t (3.28)

Where Aeompl is the compressed part of the non-defect cross section, Acomp2 is the compressed
part of the defect cross section and A,,, is the part of the cross section in tension, see Figure
3.6.

The plastic neutral axis is defined as the axis that divides the compressive and the tensile part
of the pipe cross section, see Figure 3.4 and 3.6.

t,
0-9
Figure 3.6 Pipe cross section and idealized stress diagram for whole plastified cross section.

Inserting the values for &ompl, AcOmp2 and A,,, in Eq. (6) gives:

F = 2(1y - p)rav tomp + 2pr, o,, + 2(n - Iy)r,, toens (3.29)