Bearing Capacity Of Offshore Foundation

The demand for oil and gas has been increasing over the recent decades, which impulse the world to exploit offshore or even in deep water. Abundant reserves have been discovered offshore and in deep water of the South China Sea. This brings broad application prospect to jack-up platforms and Floating Production Storage and Offshore System (FPSO) in exploitation. The foundations such as spudcan foundation for jack-up platforms, anchor plate and suction anchor for floating production platforms will also be widely used. However, techniques and experience for spudcan foundation, anchor plate and suction anchor are far from mature. So much work is in great need for the application of such foundations.The vertical load induced by self-weight, the horizontal force and bending moment by waves and currents are that the spudcan foundation mainly sustain. Therefore, it is of great significance to investigate its bearing capacity under these combined loads. The pullout bearing capacity has been the focus of research on anchor plate and suction anchor. In more than forty years, much work has been done to investigate the bearing capacity of anchor plate and suction anchor with model tests, limit analysis and finite element method(FEM). However, it is costly and time-consuming for model tests and field tests, and limit data can be gotten. As for limit analysis and limit equilibrium analysis, they are not so applicable in condition of complicated structures and complex boundary conditions, because the failure mechanism, stress field or the failure surface should be determined ahead. Besides, the bearing capacity determined by limit equilibrium analysis is neither upper bound nor lower bound and it is always not the true solution. Though FEM is used widely in numerical analysis with many advantages, there has been no uniform standard to judge limit state in analysis of geotechnical limit bearing capacity. Especially, when the non-associated flow rule is involved, local instability zone still exists though the stress state is within the ultimate limit envelope. So, it is meaningful to advance a new standard to determine the limit state for geotechnical materials. The present dissertation focuses on the bearing capacity and failure mechanisms of spudcan foundation, anchor plate and suction anchor and the main work is as follow:1. Pullout bearing capacity of deeply embedded anchor plate in soft clay is investigated. For strip anchor plate and circular anchor plate in undrained homogeneous clay, two simple kinematically allowable velocity fields are established, and then the pullout bearing capacity is obtained with upper bound method of plasticity. The velocity fields and bearing capacity results are verified by nonlinear FEM.The validation of the proposed velocity fields is proved. And because of their simplicity and reasonability, it is easy to be used for practical projects.2. The bearing capacity of spudcan foundation in homogeneous soft clay and normally consolidated soft clay under different loading (simple loading and combined loading) are studied based on ABAQUS.The failure envelope in V-H,V-M,H-M and V-H-M loading spaces are developed and compared with related empirical formula.3. The method of combining of FEM and Hill's stability condition to determine the limit state is implemented. Firstly, several criterions for material failure in geotechnical engineering are briefly introduced, and their relationship is investigated. The application of Hill's stability condition in geotechnical engineering is taken as focus. Then ABAQUS is secondarily developed to combine the FEM and Hill's stability condition, the procedure is detailed.4. The feasibility of combination of FEM and Hill's stability condition is verified. Two examples, namely the bearing capacity of strip foundation and the pullout bearing capacity of strip anchor plate, are selected to prove the feasibility, and the advantages of combining FEM and Hill's stability condition is described in detail.5. The combination of 3D elastoplastic finite element method and Hill's stability condition is adopted to obtain the limit bearing capacity and the potential failure region for suction anchor in Coulomb material. The influence factors are taken into consideration, which include the loading point, dilatancy angle, cohesion, internal friction angle and the unit weight of soil. A regressive formula for bearing capacity envelope is fitted in V-H space, which can be a reference for design of suction anchor in Coulomb material.