Viscous Effect On The Heave Motion Performance Of Deepwater Platform

With the increasing demands of energy,the production of deepwater oil and gas resources under rough sea conditions has become the trend of energy exploitation.To meet the require-ments of deepwater exploitation,a serious of new type deepwater offshore structures,such as,Spar,TLP and FPSO,have been successfully developed for use as deepwater floating drilling and production systems over the past decades.Under the action of waves and wind,during the exploitation duration,offshore platforms are easy to heave violently,which has significant im-pact on the choice of the drilling and exploiting system,the safety of the riser,drill rob and mooring system,and the operation performance of the platforms,and is essential to the con-struction cost and operational safety of the platform.The natural period of heave motion and hydrodynamic damping of structures are the two major factors determine the heave motion per-formance of the deepwater platform.The natural period of heave motion is determined by the mass distribution of the platform and the added mass of the structure during heave motion,while the hydrodynamic damping is consist of wave radiation damping and viscous damping.Wave radiation damping mainly affects the first order motion of the structures,and is mainly obtained through the solve of wave radiation problem.The viscous damping of the structure is compose of friction damping and form damping,and the friction damping results from the viscous shear stress,while form damping is mainly the result of fluid seperation and vortex shedding around the sharp edge of structures.The study and prediction of the heave motion added mass and wave radiation damping are relatively mature in the classic potential theory.However,the study on the viscous damping,the viscous effect on the added mass and radiation damping,the flow field change and the flow field mechanism under viscous effect,and the viscous effect on the heave motion performance of the structures need further study.In this thesis,numerical studies were conducted about the mechanism of the flow field changes,spcifically vortex shedding patterns,and hydrodynamics of the longitudinal oscillat-ing cylinder with a disk attached at the keel under different fluid conditions,baesd on the reviews of the research progress about the hydrodynamic studies of heave plate,TLP springing cylinder,as well as the heave motion of cylinde with a disk attached at the keel.High-order upwinding scheme was adopted to tackle the numerical diffusion from the high Reynolds number fluid field solution,and the muti-level method was applied to speedup the iteration towards convergency during the numerical solving process.Physical mechanism of flow field together with the hy-drodynamic perfermance of the longitudinal oscillating cylinder with a disk attached at the keel were revealed,based on the numerical results and theoretical derivation of the hydrodynamic coefficients.After the comparison between the ship shape FPSO and circular FPSO,numerical studies were conducted about the improvement of damping enhancement device on the heave motion response of the circular FPSO,based on the understanding of the damping enhancement of the disk attached at the keel of the cylinder.The effect of the geometry parameters,diameter ratio D_d/D_c,thickness ratio t_d/D_c and geometry ratio T_d/（D_d-D_c）on the heave motion fluid field together with the added mass coefficient C_aand damping ratio Z were analysed based on the numerical results.Improved solutions were proposed from the engineering perspective.3D numerical results obtained through the self-programming code and 2D numerical re-sults based on axisymmetric assumption were compared to reveal the 3D effect of longitudinal oscillation of the cylinder with a disk attached at the keel.The 3D effect on fluid field is mainly reflected on the significance of three vortex shedding patterns:i.e.,ω_xaffects larger flow field around the sharp edges,compared with the vortices generated in the 2D simulation;in the slice along the axial orientation,ω,the vortex effect along the radial axis is smaller thanω_x,the vortex effect along the circumferential direction,i.e.,the radial effect on the velocity is more pronounced than the circumferential one around the sharp edges of the disk;whileω_z,the rota-tional interaction of the fluid in the horizontal plane during the heave motion is insignificant.The dependence of the hydrodynamic performance,especially those owing to 3D effect on D_d/D_c and t_d/D_c,were analysed.Two different increasing regimes,from the 2D numerical results to its3D counterpart,are revealed:linear increasing regime and（pseudo-）steady increasing regime,which are seperated as the specific KC,referred to as turning point in the thesis.Numerical simulations about the longitudinal oscillation of a cylinder with a disk attached at the keel under current or oscillating flow condition were conducted,and the mechanism of the fluid field change along with the energy exchange were revealed.It is founded that,in the paramter ranges of this study KC=0.1～1.0 andβ=89236,the current speedup the vortex shedding frequency around the disk edges to two-fold,which enlarges the value of C_aand Z.The current enhancement effect smooth C_aover the range of KC,while Z showed an absolute increase.The abnormal variations of the form damping ratio Z₁and constant complement from current effect Z₀₀at D_d/D_c=2.00 compared with the counterparts at D_d/D_c=1.25,1.50,1.75,are interpretted as the resonance between the vortex shedding pattern frequency and heave motion frequency.The dependence of the scale factor k₀₀of Z₀₀on reduced velocity V_rand D_d/D_c is revealed,which is different from former conclusion coming from single model that k₀₀is dependent on V_ronly.Adaptable numerical models were adopted to simulate the longitudinal oscillation of a cylinder with a disk attached at the keel under the oscillating flow effect.Based on the under-standing of the fluid field mechanism,the effect of the oscillating flow on the C_aand Z were studied,together with the analysis of the oscillating flow effect on the damping ratio component from form drag as well as the consistent increase on the total damping ratio.The the formulae of hydrodynamic coefficients of the longitudinal oscillation of a cylinder with a disk attached at the keel under oscillating flow effect,saying adding mass coefficient C_a,osci.,drag coefficient C_d,osci.,linear damping coefficient B_osci.and damping ratio Z_osci.,were deduced from the Mori-son equation and its linearized counterpart using the Fourier integral approach,and mutiple relationships between the circular frequency of oscillating flowω^′and heave motionωwere assumed.Though numerical simulation about the cases n>2 were not conducted owing to limited computing power,methodology and approach are presented for future work.