A novel approach for evaluating pipe stress in buried pipes



    Originally published in 11th International Pipeline Conference 2016, 10th November (Volume 2: Pipeline Safety Management Systems; Project Management, Design, Construction and Environmental Issues; Strain Based Design; Risk and Reliability; Northern Offshore and Production Pipelines)

    All buried pipes experience some form of loading attributed to the weight of soil burden. When pipelines cross under railways, roads, car parks or construction sites, the pipes also experience live surface loading from vehicles on the ground. The outcome of this surface loading is through-wall bending in pipes, which in turn results in both hoop stress and longitudinal stress.

     Today’s standards limit the stresses to maximum values in terms of hoop stress, longitudinal stress and combined biaxial stress. Dating back to the 1940s, stress and deformations were estimated in Spangler’s works however many models have since been developed.
    Guidance for the design of pipeline crossings of railway lines and highways is provided for in API RP 1102. This recommended practice cites the Cornell model which was developed exclusively based on experiments on bored pipes crossing under a railway or a highway at a near-right angle. The live surface loading distribution is also limited to the wheel layout typical of trains and motor vehicles. Other existing models typically focus purely on hoop stress.
    This article aims to discuss a new method for evaluating stress in buried pipes under surface loading irrespective of the type of vehicle or equipment or the position of such in relation to the pipe.
    The article makes a comparison of different assessment approaches and presents that the Kiefner approach provides conservative estimates in the majority of cases and in more cases than the API RP 1102 approach. The approach developed by Kiefner serves as a universal tool to assess a broader range of parameters as compared to the API RP 1102 which was developed based on more specific criteria with a more narrow range of input parameters.
    In conclusion, Kiefner’s approach to estimating the stress in buried pipes under surface loading is presented in this paper, considering both hoop stress and longitudinal stress resulting from surface loading. This approach also accounts for the stiffness effect of internal pressure and the support provided by soil around the pipe. The new approach is a universal tool that is able to handle a wide range of loading scenarios and when compared to the API RP 1102 approach, the new Kiefner approach showed superior performance.
    Access the full article here.