Numerical modeling of upheaval buckling of initially imperfect submarine pipelines buried in loose and dense sand

Arman, Riyadul (2018) Numerical modeling of upheaval buckling of initially imperfect submarine pipelines buried in loose and dense sand. Masters thesis, Memorial University of Newfoundland.

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Abstract

Offshore pipelines are being utilized as one of the most convenient and efficient means of transporting oil and gas. Usually pipelines travel a long distance through a variety of soil conditions. The oil pipelines are operated at high temperature and high pressure to assure the flow and prevent wax formation inside the pipeline. The rise in temperature and internal pressure result in longitudinal expansion of the pipeline, which might cause lateral or upheaval buckling when the longitudinal movement of the pipe is restrained. The pipelines are generally buried to minimize heat loss and interference with other marine activities. For buried pipelines, soil offers resistances to lateral and upheaval buckling. The lateral soil restraint is higher than the uplift resistance and therefore the pipelines are more likely to experience upheaval buckling. Under these circumstances, upheaval buckling may occur that could cause failure in some cases. The resistance to upheaval buckling is provided by submerged weight of the pipe as well as the shearing resistance of the backfill soil. The two most popular burial methods of offshore pipelines, jet trenching and plowing, generally deposit the backfill soil in a loose to medium dense state; however, it could be subsequently densified due to environmental loading. Physical model tests show an increase and post-peak decrease of the uplift resistance with upward displacement of pipelines both for loose and dense sands. In the present study, finite element analysis is conducted to investigate the upheaval buckling behavior of submarine pipelines buried in loose and dense sands. The force–displacement behavior that considers the variation of uplift resistance for a wide range of upward displacement is studied. Different types of buckling (i.e., snap and stable bucking) for various initial imperfection ratios, burial depth and density of sand are thoroughly investigated. The importance of incorporation of the post-peak reduction of uplift soil resistance in the finite element analysis of the upheaval buckling is highlighted.

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