Numerical method for subsea wellhead stability analysis in deepwater drilling

• A conductor–soil numerical model was established based on the foundation theories.
• Nonlinear spring parameters at different depths were assigned continuously (function).
• The maximum conductor moment appears at 2–5 m underneath mudline.
• The influence of loads on conductor stress and moment close to 15 m underneath mudline.

A numerical model of under water wellhead stability analysis in deepwater drilling was established using the pile element and nonlinear spring element of ANSYS. Based on the pile foundation theories, assume the constitutive relation of seabed soil obeys ideal elastic–plastic model. The relationships between undrained shear strength and ideal elastic–plastic spring stiffness were given. In the application case, the D–F parameters of nonlinear spring element were calculated by pile foundation theories. Nonlinear spring parameters in different depth were assigned continuously using the APDL language. The results show that when the drilling vessel’s offsets are constant, the wellhead lateral displacement, rotation angle and the conductor bending moment increase with increasing top tension ratio. The higher the wellhead lateral loads, the deeper the maximum moment depth. When the depth reaches 15 m under the mudline, the displacement and moment are close to zero. With the drilling vessel’s offset increasing, the wellhead displacement, rotation angle and the conductor body moment increase. Meanwhile, the increasing rate under high tension ratio is much higher than low tension ratio, and the wellhead tends to roll over. When the drilling vessel’s offsets increase, the tension of the top riser should be reduced to avoid wellhead rollover.