Lateral Buckling: Trying to be Less Conservative

Over the last few years, lateral buckling has gone from a secondary issue to one of the major concerns in pipeline design. Not many years ago pipelines were thought of as flexible elements, which could absorb virtually any kind of displacement, but due to the trend to increase fluid temperatures and after a few major environmental accidents, it seems that lateral buckling became the major design issue. Burial was then assumed to be the only safe solution. This is the normal reaction to every traumatic experience, but now that there is alertness and that accidents are being prevented, it is once again time to re-evaluate and see where caution has become excessive and what cheaper alternatives can be used. Several papers have been written over the last 3 or 4 years addressing this issue, [10–13] for instance, and, in general, one could say that there is a consensus regarding the need to know more about how pipelines move, when they buckle laterally, and to what extent they should be allowed to do so. Still another issue, which has been discussed along with this one, is related to how cyclic motions (due to cycles of heating and cooling) can aggravate the problem. Attention is drawn to the fact that the buckling analyses are usually performed based on models conceived to simplify the design, while, on the other hand, construction and installation pay a penalty, because of unnecessary conservatism, which could be avoided if a bit more effort was put into the design. Just to illustrate what is being said, let us consider a typical lateral buckling problem and how the practice has decided it should normally be treated: 1. Determine the pipeline embedment length (that for which the pipeline will build up sufficient axial friction to anchor the axial force due to the temperature variation). 2. Build a model twice that size, embedded at both ends; using a program, which can model axial and lateral friction (this is a nonlinear analysis). 3. Build a prop type lateral installation deviation at the center of the model. 4. Analyze the pipeline assuming that the expansion from both sides will build into that deviation, thus causing the pipeline to buckle at that section. In spite of having become a traditional design approach, it is associated with a conservative model. The bottom is assumed flat, the soil model is a simple elasto-plastic spring and, also, that there is only one lateral imperfection, which will concentrate all the axial expansion. The object of this paper is to show that it can be very advantageous to spend more time and money, on the design, using more detailed analysis models, in order to save much more on the construction and installation. This will be done by sharing the experience gained based on the use of such models, during several recent pipeline projects, handling over 300km of pipes, with diameters varying between 6 and 34 inches in water depths ranging from 0 to 2000m.