夹层土上自升式钻井平台穿刺机理的离心模型试验研究

Centrifugal model tests on mechanism of spudcan penetration of jack-up drilling platform in egg-shell layered soil

自升式钻井平台是一种海洋石油开采中常用的平台结构形式。桩脚穿刺是引发自升式钻井平台事故的最主要的原因,目前对于穿刺机理的研究都集中在桩靴达到或者进入到硬层以后,即认为只有桩靴在达到或进入硬层后,穿刺行为才开始启动。在中国南海海域普遍存在硬夹层分布土层,即软—硬—软,俗称"鸡蛋壳"土层分布,现场情况显示,在这种土质条件下,现有的方法很难对穿刺行为做出准确的预测。采用离心模型试验以及有限元的方法对上述土层分布下的穿刺机理进行了研究。离心模型试验显示:在桩靴贯入过程中所受到的土阻力的大小沿深度的分布存在两种不同的模式。当上层软土强度与硬层强度之比在0.5左右时,土阻力沿深度的分布存在两个峰值点;当上层软土强度与硬层强度之比在0.2左右时,土阻力沿深度的分布与现有计算模式的情况相同。有限元计算分析显示造成上述现象的主要原因是塑性区在土体中的展开模式的区别。在钻井平台预压插桩过程中应该充分考虑土体中土阻力的分布特点,合理设计每级预压荷载的大小,预防穿刺的发生。

A jack-up drilling rig is a type of mobile platform which is commonly used in offshore engineering. Spudcan installation in stiff clay overlying soft clay is subjected to potential penetration through hazard. Penetration failure occurs when the applied load exceeds the maximum bearing resistance of the layered soil causing the spudcan to plunge into the underlying clay. In the South China Sea, the layered soil called 'egg shell' or 'sandwich layered soil' which is composed of thick soft-thin stiff-thick soft clay can be found. The methods for calculating penetration often used today are difficult to make right prediction for this layered soil in practice. To provide a better understanding of the spudcan bearing resistance in 'egg shell' soil,centrifugal model tests are undertaken to investigate the development of the bearing failure mechanism and bearing resistance-depth profile. At the same time, the finite element method is also used to study the mechanism of penetration in the layered soil. The centrifuge tests show that the failure mechanism is governed mainly by the undrained shear strength ratio between the two layers. When the ratio of the undrained shear strength of soft clay to that of stiff clay is around 0.5, the spudcan resistance-depth profile has two peak points; when the ratio is about 0.2, the spudcan resistance-depth profile is the same as that provided by the other researchers. The FEM analysis shows that under different undrained shear strength ratios, the plastic zones around spudcan expand in different ways. When jack-up drilling rigs are preloaded in 'egg shell' layered soil,different bearing failure mechanisms should be considered properly to prevent platform from penetration through failure.