An experimental investigation is conducted to evaluate the effects of windward strakes on the autorotational characteristics of a typical noncircular cylinder. The results indicate that autorotational speeds are very sensitive to strake height and, more particularly, to their location. Reductions in autorotational speeds by as much as 75% were obtained for a strake height of h/b$ = 0.3 located at r/b Q = 0.2. Exploratory two-dimensional pressure measurements indicated that the strakes themselves develop significant, pro-or antiautorotational side forces depending on their location. A strip theory analysis showed that a reduction in ^c/ is mainly responsible for this phenomenon. -yO ys2 h t *i 'i N P r V a. P Nomenclature = width of the model = moment coefficient about axis of rotation, = moment/ Vip V 2 b 0 f = pressure coefficient, = (p -p^ )/ l /2p V 2 -sectional side force coefficient, = side force per unit length/ VtpV^bQ = sectional side force coefficient of the basic crosssectional shape only = maximum positive value of C y -net side force on strake assembly, C ysl + C ys2 = side force coefficient of left strake = side force coefficient of right strake = strake height = length of the autorotation model = axial distance between the leading edge of the autorotation model and its center of gravity =v/ = axial distance between the center of gravity and trailing edge of the autorotation model = V* = moment about axis of rotation = pressure = distance measured along the bottom surface from corner to strake location (Fig. 3) = coordinate measured from center of gravity along autorotation model axis (Fig. Al) = freestream velocity = angle of attack = density of air = cross-flow angle = value of
This paper discusses computational fluid dynamic (CFD) analysis that has been applied to a blowout preventor (BOP) stack undergoing vertical oscillation during deployment from a floating drilling rig. CFD analysis has been used to determine added mass and to show that the drag on an oscillating BOP stack can be much more favorable than the calculations based on steady flow would suggest. The mechanism responsible for this improved drag and its importance to offshore drilling operations are discussed. Exploration drilling in deeper water coupled with the need for larger, heavier BOP stacks is causing increased utilization of hook load capacity on many drilling rigs, including the current generation of newbuilds. Thus, accurate prediction of dynamic hookload fluctuation is of very significant practical importance and it cannot be done conservatively without damping assumptions that have solid justification. For deployments to 10,000-12,000 feet, the resonant period of most drilling risers (characterized by vertical motion of the BOP stack) is 5-8 seconds. In otherwise benign sea states (low wave heights with short wave periods), even small amounts of vessel heave can induce resonant dynamic variation in hook load. Predictions of this resonant response are very sensitive to assumptions about damping. The use of drag coefficients based on steady flow can significantly underestimate the amount of damping that occurs as the BOP stack oscillates vertically, especially for small-amplitude oscillations. Previous work has shown that this effect occurs in other offshore structures as well. This convergence of resonant period and wave period is more pronounced for drilling risers run to the current water depth limit of offshore exploration (10000-12000 feet) than in shallower water, where most of the industry's experience has been gained. For deep deployments, this type of analysis can be used to substantiate more favorable assumptions about drag, when more conservative assumptions may show that riser deployment to deep wells is not practical and when arbitrary assumptions about damping may lead to very significant errors, conservative or otherwise, in the predicted dynamic load. This insight can also be applied to other large payloads deployed on long strings. Introduction For deployments to sites in water depths of 10,000 to 12,000 feet, the prediction of dynamic variation in hook load during running of a drilling riser and BOP stack requires accurate calculation of drag and added mass. Although the drag and inertial forces that act on the BOP stack are relatively small in comparison to the dynamic hook load at the top of the riser, accurate assumptions about them are important. Added mass influences the resonant period of the system (where increasing mass leads to longer resonant periods which tend to increase the dynamic response). Drag is an important source of damping. Since damping reduces resonant response, analysis that reveals larger drag forces can be used to demonstrate that deployment of a BOP stack to extreme depths is more feasible than typical drag assumptions would indicate. This work shows that the use of drag coefficients based on steady flow significantly underestimates the amount of damping that occurs as the BOP stack or riser oscillates vertically, especially for small-amplitude oscillations. This finding is of significant practical application for predicting dynamic response of drilling risers to deep depths.
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.
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