海流作用下平板锚的抗拉承载力

Uplift capacity of plate anchors under loop current loads

吸力贯入平板锚是一种系泊深海浮式结构的新型基础,设计中通常假定平板锚的抗拉承载力等于不排水条件下的短期抗拉力。但在季节性海流等长期荷载作用下,由于海床中超静孔压扩散导致吸力消减,平板锚的长期承载力可能远小于短期抗拉力。提出一种基于网格重分的大变形耦合有限元方法模拟海流引起的平板上拔过程,采用修正剑桥模型描述正常固结黏土海床的力学性质。大变形耦合计算得到的短期承载力系数与不排水精确解吻合,验证了所提数值模型的的可靠性。针对不同的海流荷载幅值,计算平板锚达到失稳所需时间,建立海流容许持续时间与荷载幅值及海床渗透系数之间的关系。当海流持续时间一定时,长期承载力随渗透系数的增加而显著降低。对于渗透系数10-8m/s的海床,如果海流持续3个月,保证平板不失稳的海流荷载不能超过短期抗拉力的60%。

The suction embedded plate anchor is a new foundation to moor floating structures in deep water. The short-term uplift capacity of plate anchors under undrained conditions is usually taken as the design capacity in practice. However, the sustained uplift capacity of the plate anchors subjected to seasonal loop currents may be much lower than the short-term capacity. The reason is that suction is diminished gradually due to dissipation of the excess pore pressures in clayey seabed. Here a large deformation coupled finite element approach based on frequent mesh regeneration is proposed to reproduce the pull-up of the plate anchors to loop currents, in which the modified Cam-clay model is adopted to describe the constitutive behaviour of normally consolidated clay. The large deformation coupled approach is validated by comparing the numerical short-term capacity factors with the limit solutions under undrained conditions. The allowable hold time of sustained loads against different loading amplitudes is investigated, and the dependence of allowable hold time on soil permeability is studied as well. For a given hold time, the long-term capacity may be reduced significantly with increasing permeability. If the hold time of the loop currents is 3 months and the permeability of soil is 10-8 m/s, the current loading amplitude needs to be as low as 60 % of the short-term capacity to keep the stability of plate anchors.