The Study On The Stability Of Various Types Of Foundations Penetrating Near Footprints During Jack Ups' Reinstallation

The penetration and the pulling out process of the spudcan can form a footprint with a diameter of over ten meters.The platform penetrates near the old footprint,and the spudcan may slide into the old footprint,which leads to the poor positioning of the platform and causes the bending of pile leg and even the overturning of platform.For the reason,a large deformation finite element method(coupled Euler-Lagrange)is used to analyze the spudcans penetrating into the footprint in normally consolidated soil,the main contents and conclusions are as follows:(1)The mechanism of horizontal reaction force of spudcan is discussed.The lack of soil in the footprint causes the soil to flow into it during the penetrating,the asymmetry of the flow of the soil results in the horizontal reaction acting on the spudcan.For the traditional spudcan,at the special eccentricity,the bottom of the spudcan is inverted conical,and the side slope of the pile is contacted with each other.The horizontal component of the contact force between the soil and the normal foundation greatly improves the horizontal force of the spudcan.(2)The action mechanism of bending moment of a certain section of spudcan or pile leg is discussed.The bending moment of a certain section of a spudcan or leg is composed of two parts: one is the positive moment caused by the eccentric of the vertical reaction force;the other is the negative force bending moment caused by the horizontal reaction force multiplied by the distance between the two sections.At the penetrating process,when just contacting the seabed,spudcan unilateral contact seabed,vertical force of eccentric moment coincided with the rapid development of the largest,reaching the maximum value;with the piling depth increases,the spudcan completely buried,eccentric vertical force is greatly reduced,when the bending moment caused by the rapid decay,negative moments gradually rehabilitated leading role.The higher the section position,the greater the influence of the horizontal reaction force.(3)The friction property of spudcan-soil interface is given by penalty function,and the influence of friction on spudcan-soil interaction is discussed.The results show that the horizontal reaction force of spudcan H is greatly affected by friction,and the vertical force and bending moment are less affected by friction conditions.(4)The influence of the offset distances and the depth of footprint on the horizontal force and bending moment of spudcan is analyzed,and the range of dangerous operation is determined.With the increase of the eccentricity,the horizontal reaction force and the peak moment of the bending moment all rise first and then decrease.When the eccentricity is 0.5~0.75 D,the peak value of the horizontal force reaches the maximum;when the eccentricity is 0.25~1.25 D,the bending moment peak value is greater.The deeper the footprint is,the maximum horizontal force and maximum bending moment are increased.The peak horizontal force and peak bending moment of various working conditions are given in the charts,which provide the basis for engineering design.(5)The force responses and the failure mechanism of soil flow during the penetration into footprints of flat bottom,traditional and skirted spudcans are compared and analyzed.Compared with the commonly used traditional spudcan,the horizontal counterforce of skirted is large and the bending moment is approximately equal.However,the horizontal reaction force and the peak moment of the moment of the skirted are inserted into the seabed stage at the barrel wall,and the vertical force is smaller at this time.As a result,skirted spudcan have some advantages over foot stability.