The combined wellhead has the advantage of high bearing capacity and broad prospects for application in the construction of deep water surface wells; therefore, it is important to conduct relevant research to guide the construction of combined wellheads. The aim of this paper is to study the mechanical characteristics of the combined wellhead in the longitudinal direction and establish the method for real-time calculation of the bearing capacity of the combined wellhead taking into account the effects of installation. The variations of external wall pressure and internal soil pressure during installation and the waiting process of the combined wellhead were investigated by indoor experiments. The installation efficiency of the combined wellhead at different negative pressure conditions was also compared. The experimental results show that the pressure on the outer wall of the combined wellhead facility gradually increases as the depth of the combined wellhead facility increases, and the closer it is to the bottom, the greater the soil pressure. The pressure on the inner soil will increase rapidly during the installation process to almost the same level, while the pressure on the outer soil will increase slightly. During the waiting process of the combined wellhead, as the waiting time increases, the soil damaged during the installation process gradually recovers, and the external wall pressure gradually rises. Moreover, the pressure on the outer wall gradually increases, the pressure on the inner soil decreases slightly leveling off within about 24 h. An appropriate increase in suction pressure facilitates the installation of the combined wellhead, and approximately 2.0 L/min is the optimal option for installation. The computational model developed in this paper and the experimental measurements corroborate each other. These combined wellheads significantly improve the stability of deep water soft-soil well construction and provide a theoretical benchmark for developing oil, gas and hydration in deep water.

The effective cementing of oil and gas wells in deep water weakly consolidated formation is vital for the stable supply of world energy. During the cementing operation, the cement slurry was injected into the formation under pressure differential, and thereby forms a strength transition zone in the adjacent area of wellbore. To properly guide the formation cementing operation, a unified model for predicting the diffusion distance of cement slurry in weakly consolidated formation considering different diffusion patterns was proposed in this paper. The unified model described the diffusion process of cement slurry by the governing equation of compaction grouting, penetration grouting and fracture grouting. The criterion to identify the diffusion pattern was proposed as well. Results show that, when the pump pressure is below the threshold for penetration, compaction grouting is the sole diffusion form, with its distance influenced by the wellbore radius, the shear modulus of the soil, and the stress within the formation boundaries. Exceeding this pressure allows for both compaction and penetration grouting to coexist, where the penetration grouting's diffusion distance depends on the rheological properties of the cement slurry, the formation's physical properties, and operational conditions. Upon reaching the initial cleavage pressure, significant cracking occurs, and the diffusion of the cement slurry extends to the length of these cracks, with the fracture grouting model being based on the Drucker-Prager criterion and influenced by the formation properties and operational factors. The proposed model was validated by numerical simulation results, which showed good performance to predict the diffusion distance of cement slurry. This model provides a costeffective approach to guide the cementing operation of weakly consolidated formation in deep water.

In order to study the stress and deformation characteristics of the PLC construction method pile cofferdam structure, this paper takes the deep-water foundation construction of a certain project as the background. The main bridge of the project adopts (90+180+90) m continuous beam arch, and the lower structure of the main bridge adopts a bearing platform and pile group foundation. The plane size of the cofferdam is 29.8mx22.35m, the overall cofferdam is composed of steel pipe piles, Larsen VIW shaped steel sheet piles, purlins, and internal supports. Using finite element software to establish a comprehensive model of the cofferdam space, considering the effects of load combinations such as soil pressure, static water pressure, water flow force, and wave force on the cofferdam, it is divided into 5 working conditions for loading calculation according to different construction stages, and the most unfavorable working conditions are obtained. The structural stress and deformation of the cofferdam are analyzed. The results indicate that the strength and deformation of the deep-water foundation cofferdam meet the requirements. The lateral deformation at the center of the cofferdam structure shows a trend of first increasing and then decreasing. For the purlin and internal support system, the force on the lower support is greater than that on the upper support, and the force on the middle position is greater than that on the two ends. To ensure safe construction, the lower purlin and internal support can choose steel with larger moment of inertia and yield strength.