The Interaction of Free Span and Lateral Buckling Problems

The traditional design approach for most engineering problems, of which pipelines are no exception, is to segment the project and to present design solutions for each of these design items. When setting up a pipeline schedule, therefore, one will find an item called free span analyses and another called global buckling, which covers both lateral and upheaval buckling problems. This has been justifiable so far, because freespan vibrations have traditionally been treated totally dissociated from the axial force on the pipe, while lateral buckling is a problem to which, only recently, the industry has turned its attention. DNV has a tradition of being the regulatory agency, which has a lead on vortex shedding problems. This tradition has recently been confirmed, when they issued a new freespan vibration guideline [1], in which they are now considering the interaction of axial forces in the calculation of the pipeline vibration frequency. Shortly after this code was issued, the authors undertook three large pipeline projects, in which the use of the aforementioned code was a contractual requirement. If on one hand, however, the owner insisted upon the use of the new DNV code, on the other he was not willing to accept the very short free span limits, which were resulting from the calculations. Because of this, the authors were forced to look at the problem in further depth, thus resulting interesting conclusions, which will be presented in this paper. These point out some conservative aspects of the code, and make suggestions as to how this conservatism can be overcome, in order to use the DNV safety approach and still produce larger spans, by properly focusing on the freespan buckling problem. In addition to this, the authors have concluded that the freespan buckling problem cannot be dissociated from global buckling, because, in general, it was found that the pipe not seldom moves from a local span buckling mode to a global lateral buckling mode, thus giving the free span problem a completely different emphasis. The experience gained during these projects will be shared in this paper.