Experimental Study On The Global Behaviour Of Jack -Up Unit Reinstalled Near A Footprint

Spudcan is the foundation of the jack-up unit, and the depression on the seabed left by previous spudcan penetration and extraction was called the footprint. Footprint-spudcan interaction is one of the causal sources of jack-up unit accidents. The depression creates a localized eccentric loading and bending moment on the spudcan and platform legs that invariably cause the tilting of the platform. This paper presents a set of experiments carried out at 1g with a three-legged jack-up unit model equipped with a flat-based spudcan in clay. A rotary joint connection was adopted, and results were compared against the responses of fixed head condition. In all of the experiments, an initial footprint was created at the outside of the forward leg. The global dynamic behaviour, induced bending moment and horizontal loads on three legs and pressure distribution under the spudcan were recorded. It is found that the peak moment on the forward leg occurs at the spudcan base level, but the maximum on the stern leg is just under the hull. The leg failure is most prone to occur at the top of the stern leg at a shallow penetration depth due to excessive stress. The critical reinstallation location was found to be at 0.5 times the equivalent diameter of the spudcan and the influence of the footprint can be negligible when the offset distance larger than 1.5D. The non-uniform pressure distribution under the spudcan can intuitively reflect the eccentric loading at the bottom of the spudcan, and the pressure discrepancy on the two sides of the spudcan during the penetration process is highlighted.