Investigation on Perforation Drilling to Mitigate Punch-through Potential in Sand Overlying Soft Clay

Abstract

Perforation drilling has been successfully conducted in engineering practice to mitigate the punch-through potential in stiff-over-soft clays. However, it has not been implemented in sand-over-clay stratum which usually shows high potential of punch-through. In this paper, an engineering case successfully perforating the sand layer is reported, in which the borehole was filled with drilling mud. Its mechanism is investigated numerically with a finite element model based on the coupled Eulerian–Lagrangian (CEL) method. And effects of the borehole diameter, borehole depth and the drilling position on the mitigation efficiency are discussed. The preliminary study shows that the mitigation effect of perforating the sand overlying clay is most probably due to the weakening of the sand surrounding the borehole. Simulation also shows that the mitigation efficiency increases with the increase of borehole depth. However, it shows little improvement if the borehole depth increases beyond the sand layer. Drilling directly under the spudcan is more effective than the region expanding outwards from the edge with a ratio of 3:1. The reported engineering case and the numerical findings are of great significance for guiding the future engineering practice and ensuring the safe operation of jack-up drilling units in sand overlying clay.