A hydroacoustic monitoring system is deployed on a BOP-stack and riser during drilling of production wells to record fatigue accumulation in the subsea wellhead and conductor. Data is relayed to the vessel in real-time so that well operations can be planned to manage fatigue life. The monitoring assures the asset integrity is maintained throughout its lifecycle: construction, work-over operations and abandonment.
Motion measurements of the BOP-stack and riser are analysed in conjunction with a finite-element model of the wellhead-BOP-riser-vessel system to determine stresses at various welds and hotspots. The main drawback of such an approach is that “measured” fatigue still relies on the model accuracy, which cannot be guaranteed due to uncertainty associated with many parameters used in modelling.
This paper describes validation of the modelled subsea stack and its foundation characteristics against measurements so that “measured” fatigue is as accurate as possible. This involves (1) determining the depth of BOP-stack centre of rotation; (2) identifying the BOP-stack characteristic frequency, and (3) matching stress responses derived from measurements taken at different heights on the stack. Model parameters (e.g. soil stiffness and added mass) are tailored to optimise agreement with measurements thereby improving the accuracy of “measured” fatigue estimations.