A novel leg design for reducing consolidation settlement of deeply embedded spudcan foundations in clayey seabed soils

Abstract

Field monitoring data has demonstrated excessive spudcan settlement during jack-up rig operation. However, there are no methods available for operators to reduce the consolidation settlement development. This paper presents a centrifuge and numerical model investigation aimed at reducing spudcan settlement. Initially, two centrifuge model tests were carried out to realistically replicate spudcan "penetration-unloading-consolidation" stages in an offshore field. The test results show that the spudcan consolidation settlement was partially reduced by the presence of an existing lattice leg extending from the top of spudcan footing. Based on this, it is suspected that a sleeve of a certain length extending from the top of the spudcan footing could be used to further reduce spudcan consolidation settlement. This work was accomplished by conducting small strain finite element analysis, the feasibility and reliability of the numerical model were first verified by comparing with centrifuge model test results. Subsequently, numerical models of spudcan installation and consolidation were conducted for both plain and sleeved-spudcan footings, under various soil strengths, operational loads, and embedment depths. Numerical results show that the top-mounted sleeve helped to prevent soil above the spudcan footing from moving downward, thereby reducing spudcan settlement. Based on the numerical results, an explicit equation assessing sleeved-spudcan consolidation settlement was established, as a function of sleeve area ratio and correlated well with that of a plain spudcan. This study provides a feasible and reliable numerical method for assessing spudcan consolidation settlement as well as an effective leg design to reduce settlement.