Page 71 - Pipelines and Risers

44 Chapter 3

A corrosion defect may reduce the hoop buckling capacity of the pipe. It is here assumed that
this effect can be accounted for by considering the remaining wall thickness, ‘h = t-d‘ (d =
depth of corrosion defect) if the corrosion defect is not too wide or deep. ‘t’ is substituted by
‘h’ in Equation (3.14), except for in the expression for ‘P~,~’. Buckling is an equilibrium
problem and occurs when external loads are higher than or equal to internal resistance over
the cross-section. The cross-section here means a rectangular one, with height of ‘t’ or ‘h’ and
length along the pipe longitudinal direction of (1) unit. Internal resistance is described by the
cross-section with the wall-thickness of ‘h’ (or ‘t’). External loads are the moment and
compression acting on the cross-section. ‘P~,~’ describes the amplification of the external
loads due to a combination of imperfection (i.e. w1) and axial compression acting on the pipe-
wall. The amount of amplification will not be affected by a local corrosion defect unless the
defect is wide and deep. The internal resistance is reduced by the corrosion defect and
therefore ‘h’ is used as a replacement of ‘t’.

Based on the above, Equation (3.14) is modified to Equation (3.15):

(3.15)

in which ‘~~,~r)
is:
3
(3.16)

3.2.3 Bending Moment Capacity

The pipe cross sectional bending moment is directly proportional to the pipe curvature, see
Figure 3.3. The example illustrates an initial straight pipe with low D/t (~60) subjected to a
load scenario where pressure and longitudinal force are kept constant while an increasing
curvature is applied.
M

--_ --__
-_

Figure 3.3 Examples of bending moment versus curvature relation.
Different significant points can be identified from the moment-curvature relationship. When
applyinghncreasing curvature the pipe will fit be subjected to global deformation inside the
material’s elastic range and no permanent deformation occurred. By global deformation is
here meant deformation that can be looked upon as uniform over a range larger than 3-4 times
the pipe diameter. After the LINEAR LIMIT of the pipe material has been reached the pipe