Operators are often faced with the challenge of operating pipelines having unintended free spans. These free spans may be discovered during routine inspections or after significant environmental events. Managing the spans requires an engineering assessment of the current conditions as well as ongoing monitoring, since continued erosion can further increase the span length. Operators are often unsure of how to assess these spans or what issues need to be addressed. Analytical approaches exist for addressing free spans that are based on the calculation of longitudinal stresses using classical beam theory. These calculations require little detailed information and can provide span-length guidelines that are applicable to a number of similar spans. However, these methods are often very conservative and result in limited span lengths. In most cases, finite element assessments provide an improved and less restrictive estimate for the strength of the pipeline. Finite element assessments may be based on traditional elastic approaches or take advantage of strain limits in the ASME B31.4 and B31.8 pipeline codes.
For submerged pipelines at water crossings, factors other than gravity-induced loads often influence the allowable span length. Hydrodynamic drag forces, potential debris impact, and vortex-induced vibration must also be considered. This paper presents case studies for several free spans. The case studies illustrate what methods are available for assessing free spans and present the strengths and weaknesses of each method as a means for evaluating pipeline integrity. Finally, the paper also discusses key issues surrounding the application of strain-based methods and the assessment of vortex-induced vibration.
Dotson, R., Matta, L., “Key Considerations in the Assessment of Pipeline Spans”, Proceedings of the PPIM 2015, February 9-12 2015, Houston, TX, USA