Damping Identification From Subsea Logger Axial Riser Response Data

For decades, it has been known that, as drilling riser deployment depths increase, the potential for excessive hookload response will also increase. Using data collected from a drilling riser deployed to a record-setting water depth, nearly 12,000 ft, this paper provides insight that significantly reduces uncertainty about the severity of this resonant response.The typical drilling riser and blowout preventer (BOP) stack, disconnected from the wellhead, has its first axial resonant period at approximately 1 second for every 2,000 ft of deployed length, thus 5 seconds for 10,000 ft, 6 seconds for 12,000 ft, and so on. Therefore, vessel heave response can incite a significant, adverse axial resonant condition in very deep water. Damping reduces resonant response. Historically, the true amount of damping has been uncertain, and damping has been applied in the form of hydrodynamic drag. This typically produces a predicted response with total damping that is well under 1% of critical. This can lead to the prediction of a large dynamic hookload that can produce significant restrictions on riser configuration (running weight) and seastate for BOP stack deployment as well as storm hang -off of the riser and lower marine riser package (LMRP).A recent drilling riser deployment to the record-setting water depth of 11,903 ft produced a unique opportunity to collect high-quality data that reduces damping uncertainty. This paper describes damping ratio and natural frequency identification for the first few axial riser modes for this deployment. The data were collected during deployment and retrieval using subsea vibration data loggers (SVDLs) installed on the BOP stack, drillship, and riser. These measurements reveal damping that is between 2% and 3% of critical. This result can be used to provide more accurate predictions of dynamic hookload response.