Page 72 - Pipelines and Risers

Buckling/Collapse of Deepwater Metallic Pipes 45

will no longer return to its initial shape after unloading, but the deformation will still be
characterized as global. If the curvature is increased further, material or geometrical
imperfections will initiate ONSET OF LOCAL BUCKLING. Pipe imperfections will have an
influence on at which curvature and where along the pipe the onset of local buckling will
occur, but will, as long as they are small, for all practical use not influence the limit moment
capacity. After the onset of local buckling has occurred, the global deformation will continue,
but more and more of the applied bending energy will be accumulated in the local buckle
which will continue until the LIMIT POINT is reached. At this point the maximum bending
resistance of the pipe is reached and a geometrical collapse will occur if the curvature is
increased. Until the point of START OF CATASTROPHIC CAPACITY REDUCTION has
been reached, the geometric collapse will be “slow” and the changes in cross sectional area
negligible. After this point, material softening sets in and the pipe cross section will collapse
until the upper and lower pipe wall is in contact. For pipes subjected to longitudinal force
and/or pressure close to the maximum capacity, START OF CATASTROPHIC CAPACITY
REDUCTION occurs immediately after the LIMIT POINT. The moment curvature relation
for these load conditions will be closer to that presented by the dashed line in Figure 3.3.

The moment curvature relationship provides information necessary for design against failure
due to bending. Depending on the function of the pipe, any of the above-described points can
be used as design limit. If the pipe is a part of a carrying structure, the elastic limit may be an
obvious choice as the design limit. For pipelines and risers where the global shape is less
important, this criterion will be overly conservative due to the significant remaining strength
in the elastic-plastic range. Higher design strength can therefore be obtained by using design
criteria based on the stresdstrain levels reached at the point of onset for local buckling or at
the limit point. For displacement-controlled configurations, it can even be acceptable to allow
the deformation of the pipe to continue into the softening region. The rationale of this is the
knowledge of the carrying capacity with high deformations combined with a precise
prediction of the deformation pattern and its amplitude.

The limit bending moment for steel pipes is a function of many parameters. The main
parameters are given below in arbitrary sequence:

Diameter to wall thickness ratio
Material stress-strain relationship
Material imperfections
Welding (Longitudinal as well as circumferential)
Initial out-of-roundness
Reduction in wall thickness due to e.g. corrosion
Cracks (in pipe andor welding)
Local stress concentrations due to e.g. coating, change in wall thickness
Additional loads and their amplitude