Dynamic analysis of a drilling string for deepwater riserless drilling with vortex-induced vibration considered

Riserless drilling will become the mainstream technology for deepwater drilling in the future owing to its operational characteristics. At present, researchers only analyze the seawater section or formation section but do not analyze the whole drill string to study the dynamics of riserless drilling. In fact, the ocean load affects not only the vortex-induced vibration characteristics of the drill string in seawater but also the load distribution of the drill string in the formation. Ignoring the formation or ocean load will lead to inaccurate prediction results. On this basis, in addition to the traditional drilling string boundary condition of seawater and formation, a new boundary condition of drilling string at mud line for riserless drilling is proposed. Considering vortex-induced vibration, a full-well drilling string dynamic model without riser drilling is established by the finite element method, and the dynamic characteristics of the full-well drilling string under multiple loads are analyzed. In addition, the effects of WOB, surface flow rate, and drilling fluid density on the dynamics of riserless drilling string in seawater and formation sections are discussed. This study provides theoretical guidance for the safety of riserless drilling in complex marine environments.