Abstract:

 Suction anchors with taut mooring system are important anchor foundations for the floating platforms in deep water where shallow sediments are mostly saturated soft clays. The cyclic bearing capacity of the suction anchor under a combination of average and cyclic loadings is vital factor for the design of the anchor in soft clay. The paper assumes that the average shear stresses of the soil for different failure zones are uniform and the ratio of the average shear stress to the cyclic shear strength is equal to the ratio of the static load to the sum of static and cyclic loads applied to the anchor based on the failure mode of the suction anchor under inclined loadings at the optimal load attachment point. Undrained cyclic shear strengths of the soil for failure zones are determined based on curves of normalized cyclic shear strength versus normalized average shear stress obtained from laboratory tests of the soft clay samples. Soil resistances for the different failure zones are analyzed. An approach for calculating the interface depth between upper sliding wedge failure zone and deep plane flow failure zone is developed according to the horizontal stress continuity condition. Further, a simplified limiting equilibrium method is proposed for evaluating the cyclic bearing capacity of the anchor. The proposed simplified calculation method is employed to predict the model tests of the cyclic bearing capacity of suction anchors with both vertical and horizontal failure modes. The predictions are found to be in agreement with the experimental results. The smallest and greatest deviations are 0.79 % and 16.08 % respectively and the average deviation is 5.74 %. The predicted results could preferably reflect the variation between the cyclic bearing capacity and the number of cycle to failure, which verifies the feasibility of the proposed simplified method for calculating the cyclic bearing capacity of the suction anchor.

Key words: suction anchor, cyclic bearing capacity, cyclic shear strength, model tests, simplified limiting equivalent method