Bearing Characteristics Of Suction Caisson Foundation Under Vertical Cyclic Load

The suction caisson foundation can be used as the anchoring foundation of the tension leg platform(TLP).In offshore engineering,the suction caisson foundation is not only subjected to the vertical load caused by buoyancy of the upper floating platform during service,but also the cyclic load caused by the environmental factors,such as wind,wave and current.The strength of the soft clay will be weakened under cyclic load,which will result to the shortening of operation life and decrease of bearing capacity of suction caisson foundation,leading to influence on the dynamic response and security of upper structure.In order to ensure the stability of floating structure,the study taking the mechanical characteristics of anchoring system into consideration was carried out.In this dissertation,the uplift bearing characteristics and the variation regular of ultimate uplift bearing capacity of suction caisson foundation are researched under static and cyclic loads in soft clay.The research includes the following aspects.(1)The laboratory model tests were conducted to research the uplift bearing characteristics of suction caisson foundation in clay under monotonic load,including the failure modes,the monotonic ultimate bearing capacity,the variation regular of pore water pressure and soil pressure at the top and bottom of different caissons.In addition,different failure modes were also derived.The laboratory model tests were also conducted to study the uplift bearing characteristics of suction caisson foundation under cyclic load,including the effects of average load ratio,cyclic load ratio and number of cycles on the ultimate bearing capacity after cycling,the pore water pressure at top and bottom,the displacement of caisson,and the undrained shear strength of soil.It showed that high average load ratio and cyclic load ratio would result in high weakening rate of soil strength,leading to high reduction of uplift ultimate bearing capacity.(2)Based on traditional theoretical analysis on failure modes,besides the local shear failure mode,bottom tension failure mode and general shear failure mode,the top tension failure mode was derived by a series of laboratory model tests in clay.The distinguishing method of bottom tension failure mode and top tension failure mode was also put forward.The calculation formula of static uplift ultimate bearing capacity was modified according to top negative pressure on the friction force of side wall.Another modified relationship was also suggested,which reflects the effects of weakening coefficient of undrained shear strength on the ultimate bearing capacity of caisson after cycling.(3)The undrained shear strength acquired by model test after the whole cycling,instead of tracing attenuation process,was applied to the finite element numerical calculation.The feasibility of applying finite element simulation to the calculation of ultimate bearing capacity after cycling is studied by comparing the result of numerical calculation,model tests and theoretical calculation.It is found that the finite element simulation can give a reasonable range for the attenuation ratio of the ultimate bearing capacity after cycling.(4)The method to calculate the average load and cyclic load on the foundation of Huarui6.0MW offshore wind turbine-floating platform-mooring system is researched by the commercial software Sesam.In this finite element simulation,the 3D pseudo-static elasto-plastic method is applied to the calculation of ultimate bearing capacity and the design of suction caisson foundation under different average load ratio and cyclic load ratio.It is found that the reduction ratio of ultimate bearing capacity after cycling is not only related to average load ratio,but also to the length-diameter ratio of caisson,including the length and diameter of caisson.It is feasible to optimize uplift bearing characteristic after cycles by numerical analysis.