Estimation of BOP Stack Drag and Added Mass Using Computational Fluid Dynamics

Abstract

The hydrodynamic drag and added mass of a blowout preventer (BOP stack) influences the resonant amplitudes and frequencies of a drilling riser system during connected (low amplitude oscillations) and disconnected (high amplitude oscillation) conditions. The prediction of hydrodynamic loads on a BOP stack at resonant frequencies is of importance for analyzing wellhead and casing fatigue and ensuring well integrity. Accurate prediction of dynamic hook load fluctuation is also of significant importance, particularly in determining feasibility for deploying and hanging-off drilling riser systems in ultra-deep water. A predictive technique based on computational fluid dynamics (CFD) methods is developed to estimate hydrodynamic forces exerted on a BOP stack. This method is applied to analyze the flow behavior during steady-state flow, connected oscillation, and disconnected hang-off oscillation to characterize the stack added mass and drag properties. This study considers four scenarios: Steady-state drag over the BOP stack, Lateral oscillations of the BOP stack in stationary water, Lateral oscillations of the BOP stack under nominal current conditions and Coupled axial and lateral oscillations in stationary water. This paper describes the use of CFD coupled with analytical methods to obtain key characteristic parameters associated with an oscillating BOP stack. The analysis shows that the steady-state drag coefficient significantly under predicts the drag coefficient for an oscillating BOP stack. The drag coefficient of an oscillating BOP stack in stationary water is significantly higher than the corresponding steady-state drag coefficient. The added mass coefficient shows dependence on oscillation amplitude and frequency; however, the dependence is not as significant as that observed for the drag coefficient. The combined lateral and axial oscillations show similar values of drag coefficient and added mass as the uncoupled lateral or axial oscillations. A study of BOP stack added mass and drag under nominal background current shows changes to the drag and added mass computed with stationary water conditions for an oscillating BOP stack. The viability of using computational methods for determining drag and added mass coefficients for a BOP stack under various conditions is established. Hydrodynamic coefficients that have been determined by this approach can be used to improve the accuracy of dynamic global drilling riser and wellhead fatigue analysis.