Damping Identification from Subsea Logger Axial Riser Response Data

Abstract

For decades, it has been known that, as drilling riser deployment depths increase, the potential for excessive hook load 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 blow-out preventer (BOP) stack, disconnected from the well head, has its first axial resonant period at approximately one second for each 2000 feet of deployed length, thus five seconds for 10,000 feet, six seconds for 12,000 feet, etc. 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 therefore conservatively assumed to be less than 1% of critical. The resulting uncertainty in dynamic hook load response can produce significant restrictions on riser configuration (running weight) and sea state for BOP stack deployment as well as storm hang-off of the riser and 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 the damping uncertainty. This paper describes damping ratio and natural frequency identification for the first few axial riser modes for this deployment. The data was collected during deployment and retrieval using Subsea Vibration Data Loggers (SVDLs) installed on the BOP stack, drillship, and riser.