Research On The Interaction Between Riser And Broken Ice Based On Improved SPH Method With GPU Acceleration

The Polar Regions’ nature resources like oil and gas are rich.As the ice in the polar regions is gradually melting away due to global warming,the human activities for marine oil and gas development have reached the high latitude polar regions,and with more and more ocean structures constructed.In the polar seas,marine structures are not only affected by the environmental loads of wind,waves,and currents,but also affected by ice loads.Compared with the other environmental loading,the hazards of ice loads are more prominent,and disasters and accidents caused by sea ice have also occurred many times in history.In order to ensure the safety and stability of marine structures in polar seas,it is necessary to carry out the ice load research.As the drilling riser is an important facility of the drilling platform,the study of the interaction between broken ices and the riser has important theoretical and practical significance.Ice-water-structure interaction(IWSI)is a kind of more complex fluid-structure interaction problems involving multi-media and multi-interfaces.To solve this problem,the traditional numerical mesh methods are usually troubled by the interface tracking difficulties and large mesh deformation problems in the process of the simulation.However,as a representative of the meshless method,the Smoothed Particle Hydrodynamics(SPH)has inherent advantages of solving such problems.Additionally,the SPH can easily implement the parallel large-scale computing by using GPU’s strong parallel computing ability because of the concurrent intensive computing characteristics of SPH method in enormous data.In this paper,a SPH algorithm with GPU Acceleration is developed for the vibration characteristics numerical simulation of the riser in the ice-water flow based on the groundworks of the algorithm of SPH method and the CPU+GPU heterogeneous computing.The algorithm has been validated to be an effective IWSI prediction tools and can provide technological supports for the design of the riser in the ice area.The main research contents and results are as follows.In order to enhance the accuracy and efficiency of the SPH method,an improved GPU algorithm with GPU Acceleration is proposed by combining the CPU+GPU heterogeneous computing architecture based on traditional δ-SPH algorithm.The SPH benchmark problems with free surface flow such as two-dimensional dam breaking and three-dimensional dam breaking with obstacle are simulated and the SPH with GPU Acceleration algorithm is verified by comparing with the experimental data and related literature’s results.The method in this paper can accurately predict the position of the free liquid surface and the change of the pressure value of the flow field while capturing the strong nonlinear phenomena such as blasting,wave rolling,breaking and splashing.The results show that the SPH algorithm with GPU Acceleration is suitable in dealing with the fluid dynamic problems with large free surface deformation.In addition,a particle reorder technology is proposed to solve the problem of GPU memory access conflicts caused by the disorder of particles,which can ensure a stable single-step running time and enhance the computation efficiency considerably.By running the algorithm program on different hardware devices,it is found that the SPH algorithm with GPU Acceleration has obvious algorithm efficiency advantages compared with the CPU method.For the number of million-level particles,the efficiency of the GPU parallel algorithm in this paper has the increasement of more than a thousand times compared to the CPU serial algorithm.And the robust of the SPH algorithm with GPU Acceleration is ensured by adapting the particle reorder technology.All the results have demonstrated that the SPH algorithm with GPU Acceleration developed in this paper is efficient and stable.The simulation accuracy and calculation efficiency of the SPH method is further improved by using technical means to optimize the traditional SPH method.First,this paper improved adaptive particle refinement(APR)technology by optimizing the interpolation method of the particle parameters in buffer area and correcting the kernel gradient of particles troubled with boundary truncation to eliminate the truncation error.The improved APR effectively solves the coupling instability at the particles coarse/fine interfaces in the particle refinement process,and the algorithm efficiency is also significantly improved by reducing large number particles.Second,the artificial viscosity was normally used instead of physical viscosity in the traditional SPH method,that would result the Reynolds number cannot be accurately defined during the simulation of flow around a bluff body.Aiming at this problem,the SPS-SPH algorithm is established by combing the SPS turbulence mode that takes into account the effects of turbulence viscosity.The effectiveness of the algorithm is validated by comparing with the experiment’s results in the simulation of the flow around circular cylinder.Accurate simulation of the flow field around the cylinder is the basis for the study of the flow around the riser in the ice area.Base on the above improvements,the numerical simulation of the two-dimensional flow around a cylinder and three-dimensional flow around a surface piercing cylinder under different Reynolds numbers and Froude numbers were respectively carried out by the self-developed SPH algorithm with GPU Acceleration program.The wake flow pattern of the 2D flow around a cylinder under different Reynolds numbers was analyzed and the isolated secondary vortex,α phenomenon and β phenomenon in the corresponding experiment were accurately captured.For the simulation results of the 3D flow around a cylinder,the uplift values of the free liquid surface at the front stagnation point of the cylinder were accurately predicted,as well as the wake flow velocity near the free liquid surface.It is also found that the resistance coefficient of each section of the cylinder in the vertical direction is decreases apparently which is a characteristic of a typical 3D distribution.All the simulation results approve that theconstructed method is effective for simulating the viscous flow around a bluff body that make a strong foundation for further research in this paper.In order to realize the simulation of ice flow field,the SPH algorithm of water-ice two-phase flow is established based on the basic principles of the pseudo-fluid model.The broken ice is regarded as a single rigid floating body,and the breaking of ice and its internal stress are not considered.On this basis,the SPH description method of the broken ice based on the particle bonding model is proposed,which successfully simulates the broken ice load and systematically analyzes the hydrodynamic characteristics of the riser under the action of ice-water two-phase flow.Based on the principles of the controlled variable method,the influences of the parameters on the broken ice load working on the riser are studied in,such as ice shape,size,thickness,density and the diameter of the riser.The influence of the relevant parameters on the load of the broken ice is summarized for reference to the riser design.Aiming at the fluid-solid coupling problem of the riser in the ice area,in this paper,the riser motion control equation is established by combining the two-way fluid-structure interaction method and the water-ice two-phase flow of the SPH method.With the resultant force of water and ice on the riser as the total fluid force,the Newmark-β method is used to realize the riser motion control Equation solving.The water-ice two-phase fluid-solid coupling SPH method is proposed and extended to the field of IWSI.It is worth to noting that this is the first time to model the riser vibration in the broken ice area by SPH method.The numerical simulations of the Vortex-induced vibration(VIV)in cross-flow direction and the 2 dof VIV are carried out respectively.By analyzing the simulation results,the following conclusions can be found.Firstly,the ice load is still the main component of the riser’s resistance and lift force when the riser is vibrating in the broken ice.Secondly,affected by the randomness of ice load,the riser no longer exhibits regular motion,and a coupling characteristic is shown between the force and movement of the riser.Thirdly,the peak of the ice load working on the vibrated riser is reduced to a certain extent compared with that on the still riser.In this paper,an improved SPH algorithm with GPU Acceleration is developed based on the GPU parallel algorithm and the basic principles of the SPH method,and the effectiveness of the algorithm is verified by comparison with the results of related experiments and numerical simulations.On this basis,the study of the influence of relevant parameters on the load of broken ice and the vibration of the riser in the broken ice are carried out,which is beneficial to better understanding of the flow around the riser in the broken ice and provide a reference for further improving the simulation method of the ice-water-riser interaction.