Research On Numerical Methods And Applications Of Vortex-Induced Motions Of Column-Stabilized Floating Platforms

As the exploration and drilling of offshore oil and gas move towards deeper water,the deep-sea floating offshore platforms gain much attention from the industry.However,one of the most challenging issues for column-stabilized floating platforms is the vortex-induced motions(VIM).VIM is a complex physical phenomenon.Firstly,VIM involves highReynolds-number flows with massively separation,with contains strongly unsteady characteristics.Secondly,the distinctive geometries of various kinds of floating platforms,such as Spars,semi-submersibles and TLPs,some with helical strakes,make VIM more complicated.Third,even though the floating platforms are moored,it may still suffer large amplitude drift under high speed currents.This problems pose a huge challenge to the accurate prediction of VIM.In view of the above problems,the dissertation uses the open source framework OpenFOAM as the development platform,improves the detached-eddy simulation module and combine it with the dynamic overset grid technique,and modifies the dynamic deforming mesh and mooring system module accordingly,which forms the CFD solver vim-FOAM-SJTU.The solver has a wide range of applications for predicting VIM of floating platforms with complex geometries and moorings.It has been applied to the flow around a cylinder,VIM of a Spar platform with helical strakes,a single mooringline buoy,and multi-column semi-submersible platforms.The solvers main modules consists of,flow field computing module,high Reynolds number module,spring-type mooring system module,six-degrees-of-freedom module,mesh motion and update module,and the overset grid computing module.The flow field computing module uses the PIMPLE algorithm to decoupling velocity and pressure for incompressible fluid,and solves the URANS/DES equations of the high Reynolds number flow to obtain the flow field information.The high Reynolds number module modifies the solving procedure of DES module to include overset information exchange by introducing the overset grid capability provided by naoe-FOAM-os-SJTU.In such way the DES and overset grid is incorporated.The spring-type mooring system consists of several springs to form an integral module,which is crossly called with the six-degrees-of-freedom module to realize the overall solution of spring-type mooring system and platform motion.The former calls the latter to get the platform position and update mooring force,and the latter calls the former to obtain mooring force to solve the motion equation.The mesh motion and update module solves for the mesh node location of the overlapping grid with the platform's position and orientation,and updates the grid with a pure virtual member function in a dynamic mesh class in OpenFOAM.After updating the mesh,the overset grid computing module calls the Suggar++ to compute the overlapping and interpolating data which will be provided to the flow computing module for next time step's computation.Every module of the solver makes full use of OpenFOAM's code and data structure,and adopts C++ objectoriented programming paradigm to realize code reuse and interface unification,which is conductive for future functional extension.The dissertation has conducted a series of test cases to validate the solver.The first part is the simulation of three-dimensional flow around cylinder and VIM of single-column floating structures.A infinitely long cylinder flow at Reynolds number 3900 is studied.The results are compared with the experiment to verify the acurracy of the solver for large separation flow problems.Then numerical simulation of the finite-length single-cylinder flow problem is carried out.The flow field characteristics near the cylinder are analyzed.The flow characteristics and three-dimensional vortex structure of the cylinder with and without helical strakes are compared.After that,VIM simulations of a Spar platform with helical strakes are carried out.The motion responses are compared with model test.Finally,VIM simulations of a single mooring buoyancy can are carried out.Motion responses of the buoyancy can under different velocities are studied.The relationship of frequency between transverse,inline motions and yaw is analyzed.Flow visulizations at typical time steps are depicted.The second part is three-dimensional flow and VIM of a four-column semisubmersible platform.Surface pressure,drag and lift coefficients and vortex shedding frequency of each column are analyzed for flow around the fixed semi-submersible.Flow field visualizations at a period is given in detail.In the VIM study,the spring-type mooring system uses different stiffnesses in the transverse and inline directions.Comparison with the model test shows that the solver can accurately predict the VIM response for anisotropic mooring stiffness platforms.The third part is the three-dimensional flow and vortex motion of a eight-column semi-submersible.In the study of three-dimensional flow problems,flow characteristics under different flow velocity and angle are studied,and drags are compared with model test.In VIM study,VIM characteristics under different reduced velocities and current headings are also studied.The causes of the vortex-induced yaw(VIY)are analyzed and the causes of the “lock-in” phenomenon are revealed.In conclusion,this dissertation implements a CFD solver for VIM of various kinds of deep sea floating offshore platforms,and applies the solver to several floating platforms and structures.The platforms not only include typical Spar and four-column semi-submersible,but also includes single mooring line buoyancy can and conceptional eight-column semisubmersible.Research of VIM characteristics at different conditions are performed.The results have been validated with experimental data and is reliable.The developed solver can help engineers for better understanding the VIM mechanism and motion pattern.It can also build confidence to design VIM suppressor and safety operation for deepsea floating platforms.