Comments on Recent Code Developments Related to Pipeline Freespan Analysis

During the last 20 years the Dn V code [1–5] has represented the latest word in design recommendations for pipeline free span evaluations. This is true to the extent that important codes, such as API RP 1111 [6], simply refer to Dn V for further guidelines. During most of these 20 years the majority of the design codes which did have anything to say about vibrations due to vortex shedding, including Dn V, had and in general still have, dissociated axial pipeline forces from the vortex shedding problem. It is well known, however, that the natural period of a compressed element increases as does also the axial compressive force. In March, 2002 Dn V released a new free span recommended practice [5], in which an attempt was made to couple the global free span buckling problem and that of vortex induced vibrations. All those of the pipeline engineering market, who have attempted to use it, will certainly agree that this code has greatly improved the pipeline free span ultimate limit state and fatigue design by introducing more general procedures associated to more sophisticated numerical techniques. Later that same year the authors undertook a large project, 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 Dn V code, on the other he was not willing to accept the very short free span limits, which were resulting from the calculations. This does not necessarily imply that the new code was wrong, because it could be, that previous codes had overestimated the allowable spans, but, in addition to the short spans, it was also found that the new code has some limits of applicability, which, basically, exclude a large range of common pipeline sizes. 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 these can be overcome in order to use the Dn V safety approach and still produce larger spans, by properly focusing on the frees pan buckling problem.