Study On Mechanism For Spudcan During Preloading And Extracting Of Offshore Jack-up Drilling Platform And Bearing Capacity Of Improved Spudcan In Static And Cycling Loads

Offshore jack-up drilling platform have to be used in kinds of soil conditions so that prediction of bearing capacity of spudcan, prevention of punch-through failure and safety of extracting spudcan are key problems in the field. How to avoid these foundation failures of offshore jack-up rigs is resolved in the dissertation, which promote the domestic study on spudcan.The development of a computational procedure about saturated porous media and mixture theories and its application to the analysis of soil-spudcan system model are presented in the dissertation, which help to simulate accurately the processing of preloading and extracting spudcan. The study shows that the limiting bearing capacity factor of spudcan is 10.30 without soil flow while it is 12.25 with soil flow for different uniform and non-homogeneous clay, and the limiting uplift resistance factor is 12.62. Moreover, the formulations of cavity and bearing capacity of spudcan are given in the dissertation, as well as extraction resistance and evolving base suction.To reveal the failure mechanism during punch-through, bearing capacity and the formation of cavity for layered soil are simulated by finite element model. Based on the study, the concept of limiting safety distance for spudcan is presented, and the effect of the strength ratio between the lower and upper soil layers, the thicker relative thickness of the top clay layer, the normalized shear strength and non-homogeneity factor on bearing capacity of spudcan is described in details. Then the engineering formulation of bearing capacity of spudcan in stratified soil profile with a relatively thin layer of stiff clay overlying a weaker layer is given in the dissertation.In the analysis of extracting spudcan, the conception of the uplift force divided into upper resistance and base suction is presented, which is proved to be feasible by the numerical models carried out to evaluate pore pressure changes in the soil around spudcan and the breakout force of spudcan for different soil and working load condition during extraction in different embedded depth. Based on the new conception, the soil failure mode for different depth of embedded spudcan during extraction is derived and how the magnitude of base suction changes with operation period and magnitude of working load, uplift velocity and depth of embedded spudcan is revealed, which shows the change of base suction is from 27%-80% of breakout force. Thus the results can be used to design the uplift capacity and jetting system of spudcan base in engineering.The new style of bucket spudcan foundation is given in the dissertation by equipping the traditional spudcan with skirts, which is proved to have potential benefits by increasing the effective embedded depth and consequently improving the foundation fixity and global stiffness of jack-up drilling platform. Comparing corresponding V-H, V-M and V-H-M yield surfaces in static and V-Vc, V-Hc yield surfaces in cycling loading from swipe events and constant displacement ratio events between traditional spudcan and bucket spudcan foundation, it is found out that the bearing capacity under horizontal load and moment of the new foundation is increased by at least 10%, especially horizontal load being up to 20% in cycling load. Based on the example calculations in limit storming loading condition, the global bearing capacity of the jack-up in stiff clay is increased by about 46% with bucket spudcan foundation.