Page 74 - Pipelines and Risers

Buckling/Collapse of Deepwater Metallic Pipes 47

3.2.7 Pure Compression
A pipe subjected to increasing compressive force will be subjected to Euler buckling. If the
compressive force are additional increased the pipe will finally fail due to local buckling. If
the pipe is restraint except from in the longitudinal direction, the maximum compressive force
will be close to the tensile failure force.
I;; =SMTS.A

3.3 Pipe Capacity under Couple Load

3.3.1 Combined Pressure and Axial Force
For pipes subjected to single loads, the failure is dominated by either longitudinal or hoop
stresses. For the combination of pressure, longitudinal force and bending the stress level at
failure will be an interaction between longitudinal and hoop stresses. This interaction can
(neglecting the radial stress component and the shear stress components) be described as:

(3.20)
where (31 is the applied longitudinal stress, oh the applied hoop stress and (311 and bhl the limit
stress in their respective direction. The limit stress may differ depending on if the applied load
is compressive or tensile. a is a correction factor depending on the ratio between the limit
stress in the longitudinal and hoop direction respectively.

For pipes under combined pressure and tension, Eq. (3.20) may be used to find the pipe
strength capacity. Alternatives to Eq. (3.20) are Von Mises, Tresca's, Hill's and Tsai-Hill's
yield condition. Experimental tests have been performed by e.g. Corona and Kyriakides
(1988). For combined pressure and longitudinal force, the failure mode will be very similar to
the ones for single loads.

In general, the ultimate strength interaction between longitudinal force and bending may be
expressed by the fully plastic interaction curve for tubular cross-sections. However, if Dlt is
higher than 35, local buckling may occur at the compressive side, leading to a failure slightly
inside the fully plastic interaction curve, Chen and Sohal (1988). When tension is dominating,
the pipe capacity will be higher than the fully plastic condition due to tensile and strain-
hardening effects. Based on finite element results, the critical compressive or tensile force
related to bending has been found to be:
F, = 0.5. (SMYS + SMTS). A (3.21)
where O.Sx(SMYS + SMTS) is longitudinal stress at failure.

As indicated in Figure 3.1, pressure and bending both lead to a cross sectional failure.
Bending will always lead to ovalisation and finally collapse, while the pipe fails in different
modes for respectively external and internal overpressure. When bending is combined with
external overpressure, both loads will tend to increase the ovalisation, which leads to a rapid
decrease in capacity. For bending combined with internal overpressure, the opposite is seen.