Abstract
This paper presents simplified finite-element analysis procedures based on geometrical nonlinearity and ductile Mohr–Coulomb–Davis plasticity for analysis of bending behaviour of steel pipes subjected to lateral soil loading. A simple, and easy to implement, user-defined subroutine to represent soil stiffness using the Janbu model is also presented and discussed. The development of a three-dimensional (3D) finite-element model is presented, and its evaluation against experimental measurements is discussed. Data are presented for different burial depths of the pipe, including soil loading on the pipe as well as 3D responses, longitudinal bending deflections and pressure distribution along the pipe. It was shown that numerical analyses which include soil modulus dependency on confining pressure lead to effective 3D calculations of pulling forces, bending moments along the pipeline and flexural deformations, based on measured soil parameters. The 3D analysis model requires the use of lower order (linear displacement) elements, which overestimated peak mobilized load. However, those 3D calculations effectively provided the progress of both the load–deflection and longitudinal bending response of the steel pipe at embedment ratios up to 5 where most energy pipelines are buried.
Highlights
Oil and gas transmission lines can cross zones of soil instability and may need to be designed to resist differential ground movements
A constant value for the friction angle of shearing resistance of the soil (f ) was used, to avoid issues associated with modelling of strain softening. Both the peak friction angle and the constant volume friction angle were used in separate analyses to examine the calculated soil behaviour for each of these limits. These choices were made because the current study focuses on responses up to peak lateral loads, and because methods developed to capture post-peak behaviour for 2D plane strain problems have not been demonstrated for the 3D calculations needed to calculate the longitudinal pipe bending, where the emphasis is given in the current study
The effect of soil modulus that varies with depth based on the Janbu stress function on the flexural pipe response is illustrated in figure 17 using load–displacement curves and bending moment distributions for soil zones where soil modulus is uniform with depth according to equation (3.4) (Es 1⁄4 803 kPa used for H/D 1⁄4 3), and the analyses reported earlier based on soil with the ‘Janbu initial’ modulus
Summary
Oil and gas transmission lines can cross zones of soil instability and may need to be designed to resist differential ground movements. Soil instabilities can result from natural phenomena such as soil creep, slope failures, landslides and seismic excitations in the. C cohesion of the soil D pipe diameter. H burial depth to the pipe spring line. Ko the coefficient of lateral Earth pressure at rest L pipe length. Nqh horizontal bearing capacity factor for sand
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