Analysis On Mechanical Stability Of Subsea Wellhead And Bearing Capacity Of Conductor For Deepwater Drilling

As drilling operations move to deep water, subsea wellhead bears complex stress because it joins the marine riser and casing string. The stability of subsea wellhead becomes increasingly concerned. However, there is no theoretical analysis method regarding this problem at present. Therefore, it is essential to run research on the wellhead mechanical stability and the conductor bearing capacity.Based on the theories of pile foundation and mechanics of materials, a bearing capacity analytical model of the conductor and surface casing for deepwater drilling was established. The model considered deepwater drilling conditions, combined axial and lateral loads, variable stiffness of string, and nonlinear response between the string and soil. A numerical method was adopted to solve this model. Results indicate that the skin resistance of conductor occupies the major part of the vertical bearing capacity, and the vertical bearing capacity of conductor increases rapidly along with the conductor into the soil. The effect of lateral load is focused on the upper string section, and greater setting depth of the casing string has almost no influence on its lateral bearing capacity. With drilling vessel offset or TTR's increase, the maximal lateral displacement, bending moment, shearing force and soil reaction along the string increased obviously, and the acting depth of the string increased gradually.Considering the influences such as the marine environmental loads, drilling vessel drifting, riser dynamic disturbance, nonlinear behavior between casing string and formation, a comprehensive stability analysis method and its corresponding numerical solution method were proposed. Results illustrate that the lateral displacement and bending moment of the wellhead increased nearly linear with the mean drifting offset. The greater TTR may cause wellhead instability because the wellhead has a limited bearing moment capacity, and thus it is necessary to disconnect the LMRP and BOP connection promptly. The setting depth of conductor must overrun the shallow soil of the sea bed, but greater setting depth has almost no influence on the stability of wellhead. Such measures as to enhance the conductor bending strength, control scour depth on seabed, acquire the geological sample data of shallow formation can strengthen the stability performance of wellhead.In view of the wellhead stability's dynamic effect problem that was caused by the riser swinging, according to the basic theory of terradynamics, considering the non-linear characteristics of soil, string-section changes and vertical load on the wellhead, the dynamic analytical model and the numerical solution method were established. The analysis results indicate that the deformation and internal force of casing string in the lower frequency region have little difference with the static situation, and the deformation and internal forces began to increase sharply only close to the natural frequency of the string. The outer diameter and wall thickness of conductor have little influence on the natural frequency of the string. The static method of the wellhead mechanical stability and conductor bearing capacity analysis is fulfilling the engineering requirements.Based on the analysis of conductor mechanical characteristics, jetting technique, geotechnics and pile theory, a universal determination method of conductor setting depth considering time effect was proposed. The method can reduce conductor design blindness and drilling cost. Results show that with set-up time increasing, the bearing capacity of conductor increases gradually, and the required setting depth could decrease. However, longer set-up time could cause drilling cost increasing, and therefore the set-up time should be determined according to actual situation. The growth factor of bearing capacity has considerable influence on the result, and it should be selected on the basis of block experimental data.The conventional stability analysis theory of subsea wellhead has been used to analyze the wellhead mechanical stability and the conductor bearing capacity of the SBOP system for deepwater drilling. Results indicate that the setting depth of conductor and the load on the subsea wellhead of SBOP system are smaller than conventional riser system. The riser with larger size will cause subsea wellhead greater bending moment and lateral displacement. It is necessary to increase the riser buoyancy when water depth increases.Based on the above theoretical studies, a software to analyze the mechanical stability of subsea wellhead was developed. This software may realize the mechanical properties analyses of subsea wellhead, the bearing capacity analysis of conductor and surface casing, the reasonable setting depth design of conductor for both conventional and SBOP drilling system.