Research On The Hydrodynamics Of Flat Plates And Their Mechanisms In Heave Motion Suppression Of Deepwater Platforms

Plate-type structures are widely used in engineering applications. Due to the particular geom-etry, specially designed plate structures often play an irreplaceable role, such as the aircraft wingsand the fin stabilizers. The heave plates installed in deepwater Truss Spar platforms play an impor-tant role in improving the heave motion performance of the platforms. Heave motion performanceof offshore platforms is always highly focused by the engineers, as it is closely related to the se-lection of equipments for oil/gas drilling and exploitation, and also to the safety of the personalsand equipments. Due to the complex hydrodynamic mechanisms of the plate-type structures, thedesign and prediction of the hydrodynamics of heave plates have been challenges for many years.Over the past two decades, many studies have been conducted about the hydrodynamic per-formance of heave plates. These studies were based on various techniques and have made animportant contribution in better understanding the interaction mechanisms between the heave plateand the fluid. Due to the limitation of the previous technical means, the hydrodynamic performanceof heave plate is still not thoroughly understood. The majority of the previous studies were limitedto discuss the hydrodynamic force characteristics (i.e. added mass and damping), rather than thefluid mechanism of the heave plate. Most of the previous numerical simulations were carried outbased on too much hypothesis, such as the two-dimensional assumption and the potential assump-tion. In this case, further studies about the mechanisms of the plate-fluid interactions are required.The studies were carried out about the numerical techniques, hydrodynamics of plates in variousflow configurations, the effects of flow parameters and plate geometries, as well as the simulationof the heave motion of platforms under various sea conditions. All the numerical simulations werecarried out with the open source code OpenFOAM.The limitations of the two-dimensional numerical simulations for calculating the flow aroundthe plates with an infinite span are revealed based on the numerical simulations with the k-ωSST (Shear-Stress Transport) turbulence model. Specifically, for a plate with the aspect ratio(streamwise dimension/crossstream dimension) less than0.6, the drag force acting on the platewould be overpredicted significantly by the two-dimensional numerical simulations. Three-dimensional numerical simulations are carried out for the high Reynolds number flowaround a plate with an infinite span or a circular plate. The characteristics of the forces actingon the plates, the Reynolds-averaged flow field, the frequency characteristics and the relationshipsbetween the force and the flow features have been discussed in details.The flow around a forced oscillating circular disk at low Reynolds numbers have been calcu-lated with direct numeral simulations. Three flow patterns were revealed: the axisymmstric flow,the plane symmetric flow with a fixed symmetry plane and the plane symmetric flow with a rotatingsymmetry plane. This is of significance for better understanding the basic flow mechanisms.The scale effect (i.e. the effect of Reynolds number) on the hydrodynamics of the circularheave plate (or the circular disk) have been studied based on large-eddy simulations. The effects ofReynolds number on the added mass coefficients and the damping coefficients of the heave platesare not significant in a wide Reynolds number region (Re3o=10–108). This study provides abasis for the offshore basin model test which is based on the Froude similarity law.The effects of the in-plane current on the hydrodynamics of the heave plate at low Reynoldsnumbers have been investigated based on direct numerical simulations. As the reduced currentvelocity increases, the added mass of the heave plate decreases and the damping of the heaveplates increases. The effect of the current is more significant at higher reduced current velocities.The influences of the geometry of the heave plate on its hydrodynamics have been studiedwith a self-invented one-dimensional motion mechanism. Various geometric factors, such as thethickness ratio, plate shape, edge corner radius, porosity (opening area/plate area) and the holesize, as well as the distances between multiple plates have been considered.A simplified method for calculating the time domain heave motion response of the platformsequipped with heave plates in various wave conditions is introduced. The idea of this method is tocalculate the wave force on the floating body with potential theory and calculate the hydrodynamicforce acting the heave plates with the CFD tools. First, the potential added mass and damping ofthe floating body is calculated using the commercial software HydroD, and then the hydrodynam-ic drag force acting on the heave plates are calculated based on OpenFOAM. Finally, the heavemotion equation is solved in OpenFOAM using the fourth order Runge-Kutta method. This workprovides a new strategy for predicting the motion performance of the platforms with heave plates.In summary, based on the applications of heave plate in offshore platforms, this thesis focuseson the hydrodynamics of plate-type structures and presents the studies on the relevant numericalmethods and fluid mechanics. This thesis provides some useful references for revealing the hydro-dynamics of plates and optimizing and predicting the hydrodynamic performance of heave plates,and some of the new findings and results have filled the blank in the field of fluid mechanics.