Investigation On Perforation Drilling To Mitigate On The Sliding Risk During Spudcan Installation Close To Footprints And Punch-through Potential In Sand Overlying Soft Clay

Jack-up units usually provide foundation support by inserting spudcan into the seabed.In the process of perforation,once the hazardous geological conditions such as stiff-over-soft soil or inclined seabed are encountered,accidents such as punch-through or sliding may occur,resulting in different degrees of damage to the platform structure and even overturning of the whole platform.In the existing engineering practice and research,a variety of measures have been proposed to mitigate the risk of jack-up unit insertion accidents.Among them,"perforation drilling" has been successfully conducted in engineering practice to mitigate the sliding risk of jack-up units installed around footprints and the punch-through potential in stiff-over-soft clays.However,the current research on the mechanism of "perforation drilling" is still insufficient,especially in sand-over-clay stratum which usually shows high potential of punch-through.In addition,there is a lack of a numerical model that can simulate the effect of "perforation drilling" on the process of jack-up units’ insertion.Based on the coupled Eulerian-Lagrange method implemented in ABAQUS software,a reasonable numerical model is established,and the model is validated against a centrifugal test.The mechanism of eliminating two risks by "perforation drilling" is analyzed in detail,and the influence rules of the number,depth,diameter and scope of drilling holes on the effect of risk reduction are studied.The results are as follows:(1)Validated against an indoor perforation test,the model is adopted to investigate the mitigation mechanism and effects of the borehole diameter,number,depth,and the drilling range."Perforation drilling" can effectively mitigate the horizontal component of the velocity vector of the soil under the spudcan,reduce the uneven stress of the footprint,and thus reduce the risk of sliding.(2)The simulations reveal that the mitigation efficiency increases with the borehole diameter,number,and depth.However,it shows little improvement if the borehole depth increases beyond double footprint depth.The influence of drilling range on bending moment of spudcan is small,but it has great influence on vertical reaction force and horizontal sliding force.And the semi-drilling at the outer side of the footprint is a little more effective than the full-drilling at both the inner and outer sides of the footprint.(3)According to the engineering practice reported in this paper,it is found 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 of a diameter D+2/3zF circle concentric with the spudcan.Perforation drilling is a promising technique to mitigate the sliding risk of jack-up units installed around footprints and punch-through risk in sand-over-clay stratum.Based on the numerical simulation results,an evaluation method is proposed to evaluate the effect of perforation drilling to eliminate the sliding risk of jack up platform,which can effectively evaluate the optimal scheme of drilling,and has a great significance for guiding the future engineering practice and ensuring the safe operation of jack-up drilling units.