Pull-out capacity and failure mechanism of suction anchors in clay considering trench configuration and local scour effects

Seabed trenching is a common geological and geotechnical hazard encountered in offshore engineering, potentially compromising the pull-out capacity of suction anchors and posing risks to mooring systems over time. Currently, to ensure conservative design and facilitate modelling convenience, many relevant studies simplify the trench configurations compared to their in-situ conditions which may lead to inaccurate estimation of trench influence. Moreover, the combined effect of trench and local scour is rarely addressed. Therefore, this paper aims to investigate the impact of various trench transversal and longitudinal geometric configurations on undrained pull-out capacity and failure mechanisms of suction anchors in clay considering local scour phenomena. Two distinct downward trends in the pull-out capacity for trench width expansion and trench depth extension are revealed. Comparisons between simplified and realistic trench configurations are conducted and it is recommended to model the realistic trench configurations for accurate estimation of capacity loss. Scenarios involving simultaneous trenches and local scours result in a more significant reduction in pull-out capacity compared to individual trenches. Based on the analyses results, a systematic calculation framework is proposed to estimate the pull-out capacity during trench development. Furthermore, two design considerations are examined with the conclusions that relocating padeye depth towards shallower positions to reduce trench depth cannot achieve higher capacity and selecting suction anchors with larger aspect ratios should be preferred choices considering material cost. These findings can assist in estimating capacity loss and optimizing anchor design when encountering trench-related issues in engineering practice.