Subsea wellhead stability study of composite casing for deepwater drilling

摘要

A composite casing is a new kind of pipe string composed of an upper conductor and a lower casing. It reduces the installation time considerably and can be applied to hard formations. However, the wellhead stability when using a composite casing is not the same as that of a conventional conductor, and it is not yet well understood. A mechanical model was established to evaluate the wellhead stability, considering the effects of the cement, waiting time, and end-resistance force of the joint. The results showed that the composite casing structure had better stability than previous conventional jetting conductors. Sensitivity analysis was also conducted. First, in the horizontal direction, the influence of the load was mainly within 30 m below the mudline, while in the vertical direction, the influence of the load was as deep as 80 m. Second, the end-resistance force of the joint was large and thus could not be ignored. Third, the composite casing composed of a conductor with a larger size had relatively good stability and lower horizontal displacement and vertical load; the top tension of the riser should be designed to be relatively large. Finally, the waiting time, which was short, was sufficient to obtain the required stability, and it should be designed by considering other factors. The results of this article will contribute to the drilling design of composite casings in the future.