Computational Fluid Dynamics Analysis as Applied to the Prediction of Dynamic Hookload Variation in Deepwater Drilling Risers

摘要

This paper discusses computational fluid dynamic (CFD) analysis that has been applied to a blowout preventor (BOP) stackrnundergoing vertical oscillation during deployment from a floating drilling rig. CFD analysis has been used to determinernadded mass and to show that the drag on an oscillating BOP stack can be much more favorable than the calculations based onrnsteady flow would suggest. The mechanism responsible for this improved drag and its importance to offshore drillingrnoperations are discussed.rnExploration drilling in deeper water coupled with the need for larger, heavier BOP stacks is causing increased utilizationrnof hook load capacity on many drilling rigs, including the current generation of newbuilds. Thus, accurate prediction ofrndynamic hookload fluctuation is of very significant practical importance and it cannot be done conservatively withoutrndamping assumptions that have solid justification.rnFor deployments to 10,000-12,000 feet, the resonant period of most drilling risers (characterized by vertical motion of thernBOP stack) is 5-8 seconds. In otherwise benign sea states (low wave heights with short wave periods), even small amounts ofrnvessel heave can induce resonant dynamic variation in hook load. Predictions of this resonant response are very sensitive tornassumptions about damping. The use of drag coefficients based on steady flow can significantly underestimate the amount ofrndamping that occurs as the BOP stack oscillates vertically, especially for small-amplitude oscillations. Previous work hasrnshown that this effect occurs in other offshore structures as well.rnThis convergence of resonant period and wave period is more pronounced for drilling risers run to the current water depthrnlimit of offshore exploration (10000-12000 feet) than in shallower water, where most of the industry’s experience has beenrngained. For deep deployments, this type of analysis can be used to substantiate more favorable assumptions about drag, whenrnmore conservative assumptions may show that riser deployment to deep wells is not practical and when arbitrary assumptionsrnabout damping may lead to very significant errors, conservative or otherwise, in the predicted dynamic load. This insight canrnalso be applied to other large payloads deployed on long strings.