Risk Assessment Method of Subsea Wellhead Instability in Consideration of Uncertain Factors in Deepwater Drilling

Abstract

In deepwater drilling, the subsea wellhead is in a complicated stress state and carries the risk of instability. The mechanical stability of subsea wellhead has gradually become one of the key factors in the design and construction of deepwater drilling. In this paper, through the analysis of stress and deformation of subsea wellhead system in deepwater drilling, the characteristic parameters that characterize the mechanical stability of subsea wellhead were determined. On this basis, the influence of different factors on characteristic parameters was analyzed based on dimensionless processing and sensitivity analysis. Factors that were more sensitive and had greater impact on the wellhead mechanical stability of the instance well were screened. Aiming at the problem of greater uncertainty of resistance coefficient, the risk assessment method of subsea wellhead instability considering of uncertain factors was established. Using this method, the wellhead instability risks could be quantitatively evaluated in the drilling process, construction parameters and monitoring values of environmental loads which meet the wellhead safety requirements that could also be proposed. Example shows that: according to the selected parameters such as center value of drag coefficient $\mu = 0.8$, fluctuation coefficient of drag coefficient $\sigma = 0.05$, safety limit of subsea wellhead displacement $S_{\mathrm{wm}} = 0.37\,\hbox {m}$, and safety limit of subsea wellhead deflection angle $\theta _{\mathrm{wm}} = 3.1{^{\circ }}$, there was 24% probability of occurrence of wellhead instability for the target well. Meanwhile, the platform drift $S_{\mathrm{w}}$ should be < 43 m, and the maximum marine current velocity $u_{\mathrm{w}}$ should be < 1.188 m s$^{-1}$.