循环荷载下吸力锚基础周围孔压响应特性数值研究

Numerical Analysis of Pore Pressure Response around a Suction Anchor under Cyclic Loading

吸力锚基础海上安装方便、定位精确且具有较大上拔承载力,可作为张力腿平台的系泊基础,在深水油气工程应用广泛。服役过程中,作用在吸力锚基础上的荷载一般为一定预张力上的单向循环动力作用,动力荷载可分为波频荷载和二阶低频慢漂荷载,其中波频荷载的幅值较小但周期短,低频荷载幅值较大但周期较长。本文建立多孔介质海床中吸力锚在定常力基础上承受循环上拔荷载的有限元模型,对锚体周围的孔压响应特性进行数值计算与分析,重点分析比较波频荷载和低频慢漂荷载作用下的振荡和残余孔压的变化趋势与影响范围。研究表明,循环上拔荷载作用下吸力锚外侧浅层土体孔压累积显著,降低土体有效应力,弱化筒壁-土体摩擦阻力,有可能引起吸力锚失效模式的转化;循环荷载的幅值和周期都将对孔压响应的大小和分布造成一定影响。

Suction anchors are widely used in deep sea oil and gas engineering as a foundation for spars and TLPs because of their considerable uplift capacity and convenient installation.The suction anchor mainly sustains dynamic loads transmitted from the mooring line except for steady loads.The dynamic loads can be divided into two,wave-frequency loads with smaller amplitude and shorter period,and the second-order slow drift loads with larger amplitude and longer period.When the suction anchor is subjected to dynamic uplift loads with different frequencies and amplitudes,pore pressures increase in the soil around the anchor.In this study,to investigate the development of oscillatory pore-pressure and the accumulation mechanism of residual pore pressure in the seabed soil,a finite element model of a suction anchor embedded in a porous seabed subjected to dynamic uplift forces is constructed.On the basis of a steady uplift load,the pressure distribution and development under dynamic loads with different amplitudes and periods are contrasted.The results indicate that oscillatory and residual pore-pressure responses are significant around the anchor.Negative oscillatory pore-pressures mainly appear at the lower part of the anchor.Residual pore-pressure builds-up in the shallow soil,reducing the soil effective stress and the friction between the anchor and soil.Consequently,the failure mechanism may change and the uplift capacity is reduced.