A drilling campaign was recently undertaken by Shell Oil Company in a region with high surface andsubmerged currents. The water depth ranged from 5500–7000 ft at the various well sites in the region.Strong surface currents with maximum speeds of 4.5–5.0 knots were measured using an Acoustic DopplerCurrent Profiler (ADCP). In addition, submerged currents with maximum speed of around 1.5 knots wererecorded. High fidelity Subsea Vibration Data Loggers (SVDLs) were used to monitor the in-situ riser andBOP stack vibrations due to the arduous current environment, as well as wave and vessel-driven motions.A semi-analytical method was developed to estimate wellhead fatigue damage directly using themeasured BOP stack motion data. High quality vibration data from the SVDLs were used in conjunctionwith analytical transfer functions to directly compute stress time histories and S-N fatigue damage at anylocation of interest in the conductor/wellhead/BOP system. The method was utilized in a larger fatiguereconstruction scheme that was applied to subsea wellhead and riser fatigue monitoring activities duringdrilling operations in the region. ADCP data was correlated to the SVDL data to determine the source ofvibrations at low and high frequencies.Simultaneous ADCP and SVDL data were also used to calibrate SHEAR7 v4.2 parameters. In betweenSVDL deployments, wellhead and riser stress and fatigue values were determined using the calibratedSHEAR7 models, driven by the measured current profiles. Wellhead motions were tabulated from ROVvideo and used to validate vibration reconstruction from the SVDL data and predictions from SHEAR7simulation. Using these methods, stress and fatigue life consumption estimates are robust to unavailabilityof ADCP data and/or ROV video and/or data from one or more SVDLs.Normalized vibration, stress and fatigue consumption are presented over the riser deployment period.It was found that moderate speed submerged currents, which extend over a broad range below typicalfairing depths, lead to significantly higher wellhead stress and fatigue life consumption rate comparedhigher speed surface currents. The sensitivity of a typical wellhead and BOP stack to lower-frequencyvibrations was examined. It is shown that because the submerged currents are of a lower speed, they excitemodes that are closer in frequency to the “flagpole” mode of the casing/wellhead/BOP subsystem, leadingto higher wellhead motion and stress.The methods introduced herein provide rapid turn-around of raw data to fatigue consumption, enablinginformed decisions to be made in adverse conditions. The methodology is easily extendable to real-timefatigue monitoring using a cabled system or acoustic modem to transmit data to the surface. In addition,the significance of regional submerged currents for wellhead stress and fatigue is highlighted, as well asconsiderations for vibration mitigation.
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