Measures to Prevent Subsea Wellhead Instability for Deepwater Drilling

Subsea wellhead connects a drilling riser and casing and plays a pivotal role in deepwater drilling system. The wellhead vertical or horizontal bearing capacity directly determines its stability and the safety of deepwater drilling system. Therefore, the bearing stability of subsea wellhead has become one of the most important issues in deepwater drilling operation. According to the influence of marine environmental loads, drilling operation conditions, and shallow seafloor soil parameters, the theoretical models to analyze the wellhead stability under the dangerous conditions are conducted. Then the evaluation indexes of the bearing capacity of the subsea wellhead were determined. The fuzzy comprehensive evaluation method, analytic hierarchy process method can be used to analyze the wellhead stability. Several improved measures can be used to maintain wellhead stability, such as select a reasonable riser tension, control the drilling platform drift, reduce the riser weight, improve the conductor bending stiffness, get the seafloor soil sample, control mud line scouring and install an anti-instability device.