Cost-Effective Span Analysis Methodology for Different Pipeline Applications

A pipeline laid on an uneven seabed may result in free spanning, which may cause strength and fatigue problems under static and dynamic loads, leading to vortex-induced vibrations. This can potentially result in expensive remediation and/or intervention works. An assessment of criticality of the spans can be complicated and time consuming due to complexity of the seabed profile, unknown pipeline residual tension, variability of the supporting soil stiffness and other uncertainties inherent in the problem. In this paper, a methodology for the span analysis is presented. It comprises a simple screening, with intermediate screening followed by detailed finite element analyses. Each of these analyses requires a different level of modeling effort and input data. Therefore, performing an efficient span analysis is important, because it not only ensures that all span issues are captured but also minimizes unnecessary calculations. Without proper planning of the analysis, critical pipeline spans may be overlooked or the number of analyses to be performed can be overwhelming. The analysis methodology described in the paper comprises the three levels of analysis mentioned above, and addresses how the work scope and specific application will control the required input and analysis levels. The applications discussed include existing versus new pipelines, selection of pipeline route versus as-installed analysis, pipelines with and without thermal buckles, and an analysis with complete versus incomplete seabed survey data. The applications also include scenarios with multiple spans, where resource allocation is of significance. The methodology presented in this paper presents the in-house experience and can serve as a guideline for a cost-effective span analysis.