A probability-based study on failure mechanism and quantitative risk analysis for buried offshore pipelines subjected to third-party impact loads, exploring the effects of spatial variability of soil strength

•Failure mechanism of pipelines in spatially varied soils under impact loads is explored by a developed RF-LDFEA model.•Failure plane for soil cover was independent of soil variability and two failure modes for pipelines are identified.•A QRA model is derived to capture uncertainties from soil variabilities and other factors simultaneously.•Uncertainties from structure-related factors show significant influences on the failure risk.•Failure probability surfaces corresponding to multiple influential parameters are constructed for safety design. Burial is an effective approach to offshore pipeline protection for impact loads. However, few studies address the influences of inherent soil spatial variabilities on failure behaviour of soil covers and pipelines, causing deviations. Therefore, a random field-large deformation finite element analysis framework is developed to explore the failure mechanisms of buried pipelines in spatially varying soils. The failure mode of soil cover is conformed to a local mode, where the failure path is insensitive to soil variability. The failure mechanism of pipelines depends on the competition mechanism between soil strengths and pipe-soil interactions, based on which two typical failure modes are summarized. Soil variability not only aggravates the impact damage but also stimulates the diversity of structural responses. Correlations between probabilistic damage degrees and multiple influential factors are discussed. Further, inspired by the principle of energy dissipation, an integrated quantitative risk assessment model is derived to reveal the failure risk evolution, where uncertainties from soil variabilities and structure-related factors are considered. The latter shows a significant influence, which may pose an additional failure probability of over 50 %. Different safety design approaches are compared, and spatial failure probability surfaces are configured for burial depth determination.