Current subsea wellhead fatigue monitoring systems typically measure subsea BOP stack response and convert accelerations directly to the stress on various critical wellhead components using transfer functions. The veracity of this process relies on the accuracy of input data and the numerical modelling of the riser, subsea stack and wellhead conductor system. Poor representation of a real system could potentially yield an inaccurate calculation of transfer functions and consequently, imprecise estimation of the stress levels and predicted fatigue damage. The transfer function is strongly influenced by subsea stack system stiffness, which depends on dynamic soil response, stack hydrodynamic added mass and drag, location of the subsea stack fixity point, and stack-conductor system characteristic frequency. The latter two can be measured in the field and compared with predictions from numerical models. This paper evaluates the subsea wellhead fatigue monitoring algorithm and accuracy using verification and calibration techniques with field measurements. Important considerations for verification and calibration (e.g. soil property) are also discussed.
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