Field experimental investigation of punch-through for different operational conditions during the jack-up rig spudcan penetration in sand overlying clay

Abstract

The jack-up rig is the most widely applied mobile platform in the shallow-water offshore drilling. Since several predetermined target wells are in the soil stratum of sand overlying clay, the punch-through accidents occur frequently during the jack-up rig spudcan penetration leading to disastrous consequences and sometimes even casualties. Although a few centrifugal model experiments and indoor large-scale experiments have been conducted to simulate the spudcan penetration, these experiments expose significant knowledge gaps that deviate from the actual field operations due to the limitations of a constant displacement speed plan instead of multi-stage preloading plan. In this study, seventeen (17) field experiments simulating spudcan installation in sand overlying clay in in-situ soil strata in the intertidal sea of the Bohai Bay are reported to address the knowledge gaps. A systematic study of the spudcan penetration is presented through key influencing factors such as soil strength ratio of upper sand to lower clay, geometric ratio of spudcan diameter to upper sand thickness, ballasting speed and holding period time. Results indicate that the ultimate (peak) soil bearing capacity increases almost quadratically with the increase in strength ratio of upper sand to lower clay; the final spudcan penetration resistance decreases as a power law relationship with the increase in geometric ratio of spudcan diameter to upper sand thickness; the instantaneous spudcan penetration velocity increases as a logarithmic function with the increase in ballasting speed and the pore water pressure decreases as an exponential function with the increase in holding period time. From these experiments, three novel schemes are proposed to prevent the punch-through. Lastly, this study provides a design basis and theoretical guidance for precise calculations of the ultimate bearing capacity, accurate analysis of punch-through risk and effective prevention of the punch-through accidents in the real jack-up rig.