Rupture of the pipe wall and inadmissible deformations are the two basic failure modes that can be distinguished in pipelines. Rupture can be the result of (a) the fact that the experienced strains exceed the strain capacity of the pipe wall, including less ductile zones in and near the welds and (b) high cycle and/or low cycle fatigue loading. Examples of inadmissible deformations are collapse, local buckles and excessive ovalizations. Deformations caused by external loads like earth pressure and imposed bending in settlement areas do not affect the burst pressure (failure mode rupture), provided that the pipe wall has sufficient strain capacity. If the strains in local buckles do not exceed the strain capacity, then local buckles do not affect the burst pressure either. The paper summarizes the theory to support this. Burst test results on pipes with external loads and with local buckles are presented. These tests have demonstrated that if ductility is good, even extremely deep buckles do not affect burst pressure. Results of finite element calculations to simulate the bending and local buckling behaviour are presented as well. The consequences of the above findings for the limit state design method are briefly discussed.
In modern limit state design, each failure mode is specifically considered. Examples of failure modes are: burst, collapse, local buckling, excessive ovalization, crack growth due to fatigue, rupturing due to the fact that the strains exceed the strain capacity of the pipe material (plate and weld areas with the possibilities of weld discontinuities). An important advantage of the limit state design method is that it provides a much better insight into the actual behaviour and safety of pipelines than the traditional allowable stress design method. Also, more economical designs are possible.