Search

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

Nowadays, the oil and gas exploration and development have been developed into the deep and complex formations. More and more abnormal pressure is encountered, which gives a great challenge to the safe and efficient drilling. Accurate description of pore pressure is of great significance to avoid drilling risk. The complexity of petroleum geology, the incompleteness of the logging or seismic data, the precision of the mathematical model and other issues can all lead to uncertainty in pore pressure prediction. The uncertainty of pore pressure is one of the fundamental causes for drilling risk. In order to address these challenges, a new quantitative description method for uncertainty of pore pressure was established. Firstly the sources of the pore pressure prediction uncertainty were analyzed. Then the uncertainties of Eaton index and normal compaction trend were, respectively, described. Finally the uncertainty interval of pore pressure was established based on Monte–Carlo simulation and normal information diffusion theory. The pore pressure prediction result obtained in this paper was not a single value, but an interval with probability distribution characteristic. The results of example showed that the measured values of pore pressure were all in pore pressure interval with confidence of 90%. Case study validated the feasibility and effectiveness of the proposed method.