Support requirements for unburied spans on existing pipelines can be difficult to assess. An approach for investigating the adequacy of support of unburied spans is presented. It begins with a screening based on theoretical longitudinal stress evaluation. An estimate of the allowable free span lengths between supports is calculated, considering the weight of the pipe and its contents, longitudinal stresses from internal pressure, thermal stresses, and steady wind loading. Existing span lengths are compared to the allowable length and any segments exceeding the limit are flagged for further study. Review by finite element analysis and comparison with survey data for the identified spans is then performed. Comparison of the FEA model deflections and survey data can help determine whether “pre-existing stresses” or plastic deformations are present, due, for example, to installation procedures or ground movement. The FEA results provide a factor of safety that can be used to help identify potential safety issues and prioritize mitigation efforts.
Matta, Lawrence M. and Dotson, Rhett, “Analysis of Unburied Pipeline Spans”, Proceedings of the 9th International Pipeline Conference, September 2012, Calgary, Canada.