Page 75 - Pipelines and Risers

48 Chapter 3

Here the two failure modes work against each other and thereby “strengthen” the pipe. For
high internal overpressure, the collapse will always be initiated on the tensile side of the pipe
due to stresses at the outer fibers exceeding the material limit tensile stress. On the
compressive side of the pipe, the high internal pressure will tend to initiate an outward buckle,
which will increase the pipe diameter locally and thereby increase the moment of inertia and
the bending moment capacity to the pipe. The moment capacity will therefore be expected to
be higher for internal overpressure compared with a corresponding external pressure.
3.3.2 Combined External Pressure and Bending

Bai et al. (1993, 1994, 1995 and 1997) conducted a systematic study on local
buckling/collapse of external pressurized pipes using the following approach:

review experimental work
validate finite element (ABAQUS) models by comparing numerical results with those
from experimental investigation
conduct extensive simulation of buckling behavior using validated finite element models
develop parametric design equations accounting for major factors affecting pipe buckling
and collapse
estimate model uncertainties by comparing the developed equations with the design
equations

Details of the equations are given in the above listed papers, in particular Bai et al. (1997).

The ultimate strength equations for pipes under combined external over-pressure and bending
(e)a +(g (3.22)
is proposed by Bai (1993) as the following function:
1
=

In experimental tests, sets of W) and (P/Pc) at failure are recorded. The exponents a and B
can be optimized by identifying which values of a and p will provide the most stable and
consistent probabilistic description of the model uncertainty in terms of mean value
(preferably close to l.O), CoV (as low as possible) and distribution type (preferably with a
distinct lower bound).

Based on finite element results, Bai et al. (1993) proposed that a = p=1.9. In DNV’96
pipeline rules, a round number 2.0 is adopted.

In order to develop simple criteria for bucklingkollapse of pipelines under simultaneously
axial force, pressure and bending, formulation described in Section 3.4 is used (Hauch and
Bai, 1999). See ABS(2000).