A Study of Instabilities Observed When Heave Compensating Deepsea Risers

Summary This paper discusses an instability phenomenon sometimes observed on drillships when latching or unlatching a long riser to the well head while the drawworks is operating in the active heave compensation (AHC) mode. The paper starts with presenting field data clearly showing an incident of instability. Next, it focuses on the physical causes or conditions for instabilities to occur. The root cause is identified to be an inherent and poorly dampened bending mode of the entire drillship. This mode has a natural frequency equal to the vibration frequency observed during the instability event. Two additional causes are also discussed in some details: the complex dynamics of the riser and the dynamics of the drawworks and its control system when run in the AHC mode. The dynamics of all 3 systems are coupled to give an analytic formula for the decay rate of the ship vibration mode. This formula is used for predicting the conditions when the decay rate is negative, and instability can occur. The paper also includes results from an advanced simulation model. The results from this model confirm the predictions from the analytic analysis. Finally, various proposals for solving the instability problem are briefly discussed.