Experimental analysis of impact loading during installation of Jack-up units

Jack-ups, self-elevating mobile offshore units, are being extensively used for offshore construction projects such as oil rigs and offshore wind turbines. The jack-up can be towed from site to site and therefore has to withstand different environmental conditions at each site, such as wind and waves. This work is concerned with a sudden and uncontrolled touchdown of a leg on the seabed due to wave action occurring during the lowering phase while the jack-up is still in a floating mode. This impact can cause seabed instabilities and induce leg damage. Due to a strict limit of significant wave height for the installation phase, the available working weather window for a jack-up unit is small. In order to extend the weather window for installation of the jack-up unit, the impact problem has been investigated. Small scale 1-g experiments are performed in a tank to gain better understanding of the impact problem. Several leg/spudcan systems are designed and manufactured. The leg/spudcan system is allowed to impact the seabed under its own weight. The impact forces on the subsoil, total stresses and pore water pressures are measured for different drop heights, masses, spudcan diameters and different inclinations to the vertical. By analyzing the test results an equation to calculate impact forces from dimensionless parameters is obtained by the experiments. The proposed expression can assess the impact force on subsoil with very dense sand.