Numerical Modelling of the Effects of Liquefaction on the Upheaval Buckling of Offshore Pipelines Using the PM4Sand Model

Abstract

The buckling tendency of an offshore pipeline buried in a liquefiable soil aggravates under earthquake situations. Moreover, to understand the upheaval displacement behavior of an offshore pipeline under dynamic loading, it is crucial to understand the variation of liquefaction potential within the soil bed. Thus, in the present study, the variation of the liquefaction potential within the soil body and its effect on the pipeline upheaval displacement (u) and post-shake uplift resistance (Vup) is investigated using a finite element package called PLAXIS 2D. The study was performed for different seismic and soil conditions. To define the soil, two advanced constitutive models are used. The static stages are modelled with the ‘Hardening Soil Model with small strain’ (HSS model), while the dynamic stage is modelled with the PM4Sand model. Moreover, the problem is defined as a 2d plane strain problem. The pipe is considered to be covered with Nevada sand. Several parameters such as a sand-density index (Dr), pipe embedment depth (H), seismic frequency (f) and amplitude are varied to study the variation of the soil liquefaction potential, the pipe upheaval buckling and the post-shake uplift resistance. The model is validated with past studies and a considerable match is obtained. The liquefaction potential is shown using the shadings of a user-defined parameter called a pore water pressure ratio (ru). Moreover, the variation of pipe upheaval displacement (u) and pipe uplift resistance (Vup) are shown using various plots. Thus, it is observed that the liquefaction potential is reduced with an increase in the frequency and the amplitude of the seismic signal. Moreover, the peak upheaval buckling, and the duration of earthquake loading to reach the peak upheaval buckling, decreased with an increase in the earthquake signal frequency. Again, the variation of post-peak uplift resistance of the buried pipeline with the pipe embedment depth is observed to be independent of the signal parameters. However, the variation of uplift resistance of the pipeline with the soil relative density is influenced by the signal parameters.