Study On Bearing Capacity Of Suction Anchors With Taut Mooring Systems In Soft Clay Under Cyclic Loading

Suction anchor with taut mooring system is an important deepwater floatingplatform foundation. In this dissertation, the failure modes and bearing capacities ofsuction anchors with taut mooring system are researched under static and cyclic loadsin soft clay. The research includes the following aspects.The model tests were conducted to research the failure modes and bearingcapacities of suction anchors under static load. The results show that the displacementalong the mooring direction is about0.6times as large as the diameter of anchor whenthe failure mode of anchor is vertical pullout, and the displacement along the mooringdirection is about0.3times as large as the diameter of anchor when the failure modeof anchor is lateral failure. By optimizing the aspect ratio of anchor, the bearingcapacity of suction anchor plays better under the premise of saving cost. The changeof soil conditions affects the failure modes of anchor, causing the correspondingvariations of bearing capacity.Failure modes and bearing capacityies of suction anchors were also studied undercombined average and cyclic loads by model tests. Results are as follows.When the failure mode of anchor is vertical pullout under static load, the failuremode is still vertical pullout under combined average and cyclic loads in the sameloading direction. When the failure mode of anchor is lateral failure under static load,the failure mode depends on degradation of vertical and lateral resistances undercombined average and cyclic loads..When the number of cycles to failure is more than a certain value, the cyclicbearing capacity of anchor is less than the static ultimate bearing capacity. For aspecified loading direction and the number of cycles to failure, the cyclic bearingcapacities increase with the increase of average load. The cyclic load undertaken byanchors is the maximum when the average load ratio is about0.5.The effect of frequency change of cyclic load on the cyclic bearing capacity ofanchor depends on the number of cycles to failure. With the frequency being reducedfrom0.1Hz to0.01Hz, when the number of cycles to failure is between100and1000,the cyclic bearing capacity can be decreased by about8%at most. When the numberof cycles to failure is between1000and2000, the cyclic bearing capacity can be decreased by about4%at most. When the number of cycles to failure is more than2000, the cyclic bearing capacity reduces to about1%at most, and then the effect offrequency on bearing capacity can be neglected.The method to analyze the bearing capacity of suction anchor is researched byfinite element calculations. Results are as follows.The interaction between anchor and soil should be determined by thefailure mode of anchor. When the failure mode of anchor is vertical pullout, it isproper to use critical shear stress friction model to describe the interaction betweenanchor and soil. When the failure mode of anchor is lateral failure, it is proper to usethe Coulomb friction model to describe the interaction between anchor and soil.The value of elastic modulus has effects on the ultimate bearing capacity anddepends on the failure modes of anchor if the undrained stress-strain relationship ofsoft clay is described by the perfect elastic-plastic model obeying Mises failurecriterion and the ultimate bearing capacity is determined based on the displacementstandard. The elastic modulus is determined using150times of the undrained shearstrength of soil when the failure mode of anchor is vertical pullout. The elasticmodulus is determined using75times of the undrained shear strength of soil when thefailure mode of anchor is lateral failure.The effect of loading direction on the reverse end-bearing resistance of anchor isanalyzed by using finite element method. A modified relation is suggested, whichreflects the effects of loading directions on the reverse end-bearing resistance ofanchor.The feasibility estimating average displacement of anchors is also researched bypseudo-static elasto-plastic finite element calculations using the cyclic shear strengthof soft clay under combined average and cyclic loads. The comparison betweencalculation and the results of model tests shows that variations of averagedisplacement can be estimated.Finally, a pseudo-static method determining the cyclic bearing capacity of suctionanchor with taut mooring system is developed using the cyclic shear strength of softclay and the upper bound plasticity analysis theory. The comparison betweencalculation and the results of model tests shows that this method is feasible to analyzethe cyclic bearing capacity of anchor.