Drill String Dynamics in Deepwater Open-Loop Drilling of an Ultra-Shallow High-Build-Rate Horizontal Well

Abstract

In deepwater open-loop drilling of ultra-shallow high-build-rate horizontal wells, the drill string spans the exposed seawater section, the mudline transition, and the constrained downhole section. Existing studies have not yet established a unified coupled dynamic model covering the full string, and the response patterns and parameter-control mechanisms remain insufficiently understood. This study examines a surface-hole operation in a block of the South China Sea. A six-degree-of-freedom spatial Euler–Bernoulli beam finite element model was established from the measured wellbore trajectory. Structural and fluid inertia, geometric stiffness due to axially varying force, Morison-type current loading, a wake-oscillator model for vortex-induced vibration (VIV), wellbore contact and friction, and bit loads were coupled within one framework. The generalized-alpha method was used for time discretization, and the nonlinear dynamic equilibrium equations were solved by within-step iteration for multiple operating conditions. Under the baseline condition, the peak in-line displacement was 6.3–6.4 m and occurred in the middle of the seawater section; the peak cross-flow displacement was approximately 0.43 m, the maximum bending moment was approximately 36 kN m, and the maximum equivalent stress was approximately 230 MPa. The high internal-force values were concentrated in the mudline transition and the upper BHA, demonstrating spatial separation between deformation and internal force. Surface current velocity controlled the in-line response amplitude: increasing the velocity from 0.4 to 0.8 m/s increased the peak in-line displacement by approximately 230%. WOB had a limited effect on global deformation and acted mainly on the local near-bit section. These results provide numerical reference for mechanistic analysis and parameter control of drill-string dynamics in comparable deepwater open-loop drilling operations.