砂土上复合重力式基础承载力包络面及系泊点研究

Study on Bearing Capacity Envelope Surface and Mooring Point of Composite Gravity Foundation on Sand

重力式锚固基础具有较好的稳定性和环境适应性,是砂土海床上锚固海上浮式平台时较为可靠的方案.根据现有研究可知,添加裙板可提升重力式基础的各向承载力,对重力式基础打孔可减少基础所受水动力.为提升重力式锚固基础的承载能力和使用性能,本文提出了一种带有裙板和底部打孔的复合重力式基础,可显著减少传统重力式锚固基础的成本、减少基础底部水动力、便于基础贯入土体内部.本文基于有限元极限分析方法,首先研究裙板和打孔对基础竖向承载力的耦合作用,发现裙板可提升基础承载力,打孔率较高时基础承载力降低,在临界打孔率内时基础承载力不变,随后求得临界打孔率内复合重力式基础的三维承载力包络面.考虑复合重力式基础实际运用问题时,基础的实际承载能力与锚固点位置相关,使用基础的三维承载力包络面可求得最佳锚固点位置.基于实际的波浪流参数,求得基础承受的波浪力等外荷载,建立复合重力式基础的受力模型,将复合重力式基础的受力转化为三维承载力包络面中的荷载路径,求解荷载路径与承载力包络面的交点方程式得到基础提供的系泊力,对比不同系泊点位置和系泊角度得到的系泊力,发现在重力式锚固基础底部水平系泊可以为系泊缆提供最大系泊力.

Owing to its better stability and environmental adaptability,the gravity foundation is a reliable scheme for anchoring floating offshore platforms on sand in the seabed.According to the existing research,it is known that adding skirts can improve the gravity foundation’s anisotropic bearing capacity,and bottom perforations can reduce the hydrodynamic force exerted on the foundation.To improve the bearing capacity and service performance of the gravity foundation,a type of composite gravity foundation with skirts and bottom perforations is proposed in this paper,which can significantly reduce cost,reduce the hydrodynamic force,and facilitate penetration.Based on the method of finite element limit analysis,the coupling effect of skirts and perforations on the foundation’s vertical ultimate bearing ca pacity is studied at first,and it is found that skirts can improve the vertical ultimate bearing capacity while a high perforation ratio will reduce this capacity.In addition,the vertical ultimate bearing capacity is not affected in the case of a value within the critical perforation ratio.Then,a 3D bearing capacity envelope surface of the composite gravity foundation within the critical perforation ratio is obtained.Considering that the actual bearing capacity of the foundation is related to the mooring point position in practical applications,the optimal mooring point position can be found by using the 3D bearing capacity envelope surface of the foundation.Based on the actual wave and flow parameters,the external loads such as wave force are calculated,and a force model of the composite gravity foundation is built.The stress of the composite gravity foundation is converted to the load path in the 3D bearing capacity envelope surface,and the mooring force can be obtained by solving the equation of intersection between the load path and the bearing capacity envelope surface.From the comparison of mooring force obtained at different mooring points and mooring angles,it is found that the gravity foundation can provide a maximum mooring force for the mooring rope when it is anchored horizontally at the bottom.