Page 92 - Pipelines and Risers

Limit-state based Strength Design 65

- The effect of out-of-roundness on moment capacity and strain criteria is included;
- The pigging requirements and repair systems, are met, and;
- Cyclic loads induced out-of-roundness have been considered.

Finite element analysis may be performed to calculate the increase in out of roundness during
the life cycle of a pipeline. The analysis is to include fabrication tolerances and a11 loads
applied through the pipelines life-cycle such as point loads, bending against a surface, axial
load and repeated pressure, temperature and bending cycles.

4.3 Bursting

4.3.1 Hoop Stress vs. Equivalent Stress Criteria
An analytical study by Stewart (1994) and a finite element analysis have demonstrated that
for a pipe under combined internal pressure and bending:

If a pipeline section is in a displacement controlled situation, then a hoop stress criterion
provides a good control of bursting.
If a pipeline section is in a load controlled situation then an equivalent stress criterion may
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be applied to ensure sufficient burst strength for pipes under combined internal pressure
and axial loads (the influence of bending is yet to be investigated).

For pipelines in operation, it is generally conservative to apply the equivalent stress criteria to
control bursting since the dominating load is internal pressure combined with bending.

The bursting failure mode is governed by the tensile hoop stress. To ensure structural strength
against bursting, the hoop stress is to fulfil the following conditions:

yielding limit state, oh I q,.SMYS, where qs is usage factor for SMYS (Specified
Minimum Yield Stress)
bursting limit state, oh I q,,.sms, where q,, is usage factor for SMTS (Specified
Minimum Tensile Stress)

For load-controlled situations, special consideration shall be made to bursting. It has been
chosen to use an equivalent and longitudinal stress criterion according to the results from the
analytical study and the finite element analysis.
4.3.2 Bursting Strength Criteria for Pipeline

The hoop stress due to pressure containment is not to exceed the following criteria:
D-t
(pi - pe)- 5 qs min[SMYS(T),0.87 * SMTS(T)] (4.2)
2t