Underwater wellheads are the basis for deepwater oil and gas field development and carry the weight of various pipe strings and tools in the well. The suction pile wellhead designed by combining a cylindrical foundation with the underwater wellhead can greatly improve the bearing capacity of the wellhead, and ensure stability and operational safety. Based on pile foundation bearing theory and the ultimate foundation reaction method, the wellhead bearing capacity of the suction pile was analyzed, and the calculation model of the suction pile wellhead bearing capacity under different soil conditions was established, and the on-site operation suction pile wellhead bearing capacity chart was obtained. The finite element method was used to analyze and verify the change in bearing capacity of the suction pile under the action of seepage. The results show that considering the undrained shear strength of the shallow soil in the South China Sea, a 6-m diameter suction pile at a penetration depth of 12 m, can reach a bearing capacity of up to 6000 kN. The results of this study can provide a theoretical basis for deepwater oil and gas wellhead design.
Deepwater oil and gas exploration and development is an important part of the world oil and gas exploration and development. There are many issues still need further study. During deep-water operations, the penetration of surface casing brings great disturbance to shallow layer soft soil, which can lead to the shock and subsidence of the wellhead. Insufficient load bearing capacity of the wellhead negatively affects the stability and safety of deep-water operations (Yunjin Xu et al, 2017). The suction underwater pile wellhead technology uses the cylindrical foundation as a load bearing structure (Yanbin Wang, 2014). Compared with penetrating into shallow soil for well construction, the suction underwater pile wellhead technology has smaller requirements of shallow layer soft soil and enables marine conductors to be mass installed into the sea. The strong bearing capacity of the suction underwater pile improves wellhead stability and shortens well construction period. For hydrate production tests, the suction underwater pile wellhead technology is able to conquer the difficulties of building a long horizontal interval of horizontal wells and putting completion casings in place (Guan Zhichuan, 2009; Vaitkunaite E, 2016; Beck R. D,1991). The suction underwater pile wellhead adopted in the second hydrate production test in the South China Sea was 67.5m in diameter, 12m in length, 96t in weight, guaranteeing the horizontal deflecting capacity in shallow soil and preventing the wellhead from sinking or overturning throughout the operation process (Philippe Jeanjean, 2002). There have been some analyses about the bearing characteristics and influencing factors of the main cylindrical foundations, but the studies about the bearing capacity of suction underwater pile wellhead under complicated geological conditions and wellhead dynamic loads in deep water are few and far between.
