Reliability assessment of drag embedment anchors and laterally loaded buried pipelines

Aslkhalili, Amin (2020) Reliability assessment of drag embedment anchors and laterally loaded buried pipelines. Masters thesis, Memorial University of Newfoundland.

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Abstract

Drag embedment anchors and buried subsea pipelines are two important elements of the offshore field developments that are used for station-keeping of floating facilities and transferring the hydrocarbons, respectively. The lateral soil resistance against the drag anchors and pipelines are mobilized in a similar fashion with identical conventional design equations. This is fundamentally caused by similar lateral projection of the anchor and pipe geometries. The reliability assessment of the drag embedment anchors as a key component of mooring systems, and the lateral response of trenched pipelines as crucial structural elements are significantly important due to a range of uncertainties involved in the design process. Despite the similar design equations for lateral soil resistance against the moving anchor and pipe, these elements are subjected to different kinds of loadings and uncertainties that are expected to affect their reliability indices. In this study, the reliability of drag embedment anchors and laterally displaced pipelines were conducted and compared to investigate the extent of similar fashions in the lateral response of these two elements to large displacements. Both uniform and non-homogeneous soil domains were considered and compared to evaluate the impact of more realistic design scenarios. Macro spreadsheets were developed for iterative limit state and kinematic analyses and obtaining the holding capacity of drag embedment anchors. The lateral force-displacement responses of the buried pipelines were extracted from published centrifuge model tests and incorporated into finite element models in ABAQUS. Automation Python scripts were developed to perform a comprehensive series of numerical analyses and post-process the outputs to construct the required databases. Response surfaces were developed and probabilistic analyses were conducted by using the first order reliability method (FORM) to obtain the reliability indices and failure probabilities. Comparative studies were conducted to obtain an equivalent annual probability of failure between the pipelines and drag anchors. The study showed that the similar conventional approaches for modeling of the anchors and pipelines lateral displacement might be acceptable for homogeneous soil domains. However, the reliability indices were significantly affected by defining non-homogenous soil domains. It was observed that the magnitude of the reliability indices in the layered soil strata and trenched/backfilled conditions could be significantly reduced. This, in turn, revealed the need for improving the current design codes to incorporate more realistic conditions. The proposed probabilistic approach was found robust to optimize the subsea configuration of the anchors and pipelines and improve the reliability indices. The study revealed several important trends in anchors and pipeline-seabed interactions and provided an in-depth insight into its impact on reliability assessment and a safe and cost-effective design.

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