The response of spudcan foundations during penetration through a soft clay layer toward a sand layer is investigated through large deformation finite element analyses. Relationships for the penetration resistance taking account of the evolving soil failure mechanisms are explored. The findings show that the substantial increase in the penetration resistance is associated with the squeezing mechanism, as the softer soil that is trapped beneath the spudcan base and the stronger sand layer is squeezed horizontally. A simplified expression is proposed to predict the spudcan penetration resistance between the depth at which the squeezing prevails and the clay-sand interface, with its performance compared with centrifuge test results and conventional design approaches.

Introduction

Mobile jack-up rigs are widely used for offshore drilling in shallow to moderate water depths up to 150 m. A typical oil and gas jack-up rig consists of a buoyant triangular platform and three independent retractable legs, with each leg resting on a large 10–20 m diameter spudcan (Menzies and Roper, 2008). Jack-ups have more recently been used, with modified designs, as heavy lift vessels for the installation of offshore wind turbines. When used as installation vessels, jack-ups usually have a more rectangular hull design and smaller size spudcan foundations, possibly equipped with more than three legs. Jack-ups can be self-installed once on-site. During the installation stage, the spudcan foundations are first pushed into the seabed through self-weight before being preloaded by pumping sea water into ballast tanks in the hull. This proof load is 50%–100% higher than that expected during operations, which ensures the footings can withstand extreme environmental loads in any extreme storm design event (International Organization for Standardization (ISO), 2016).

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