Longitudinal natural vibration of ultra-long drill string during offshore drilling

Offshore drilling with ultra-long drill strings (i.e., longer than 10,000 m) creates a new class of technical challenges, such as dynamic variation of weight on bit (WOB). This article investigates observational data obtained from a drilling program conducted by the scientific drilling vessel Chikyu using an 8000 m drill string, which is first of their kind to be recorded. The data exhibited longitudinal oscillation of the drill pipe at frequencies higher than that of heave motion. To identify the cause of such oscillation, one-dimensional dynamic simulation to model drill string's behavior was carried out. The key component of this model is the detailed modeling of the heave compensator. Numerical experiments revealed that longitudinal natural vibration was excited in the ultra-long drill pipe, and the excitation modes corresponded to the behavior of a string with both ends fixed, indicating the possible influence of friction in the heave compensator. We named this class of phenomena compensator-induced vibration (CIV). In addition, longitudinal elasticity leads to divergence of the tension variation near surface and downhole WOB, which calls for the need to directly measure the downhole WOB behavior.