Research On A Floating Structure And A Slightly Rigid Mooring System Considering Sloshing

This paper is derived from the forefront of offshore oil and gas development in the field of marine engineering.The carrier-loaded FLNG moored by a new type of underwater slightly rigid mooring system is studied.This paper mainly uses numerical methods,combined with theoretical analysis and experimental methods,to systematically carry out the related researches on the viscous numerical wave flume,non-linear sloshing,the coupling between the nonlinear wave and a floating body with the inner tank sloshing,and the coupling between a floating body with inner tank sloshing and an underwater mooring system.Firstly,a viscous numerical wave flume is established by using overlapping grid technology based on the theory of piston wavemaker and the theory of flap wave maker.The numerical model is solved by a PIMPLE algorithm combined with the SIMPLE algorithm and PISO algorithm.The effects of the time step,courant number and mesh size on the numerical precision and computational efficiency of the overlapping mesh model are studied.The numerical model uses the laminar flow model and the turbulence model(including the standard turbulence model and the turbulence model)separately.A comparative study of computational accuracy and computational efficiency is carried out for the fluid model.By comparing the numerical calculation results with the theoretical results and experimental results,the wave-making and active wave-absorbing functions of the viscous numerical wave flume are verified.This study not only verifies the reliability and correctness of overlapping meshes in the two-dimensional and three-dimensional two-phase fluid domains to solve the interaction between moving objects and flow fields,but also provides a basis for solving more complex ocean engineering problems using this viscous numerical wave flume.Then,based on the theory of viscous fluid dynamics,the sloshing problem is studied.In this paper,the turbulence model is used and the free surface is captured by the VOF method.The developed numerical model can correctly simulate the complex nonlinear behavior such as sloshing and viscous dissipation the free surface cracking and wave rolling when the nonlinear sloshing occurs.The numerical convergence test and mesh independence test of forced sloshing of the two-dimensional rectangular tank and three-dimensional cube tank are carried out as the basis for numerical calculation.The two-dimensional linear and nonlinear behaviors of the inner tank sloshing,the three-dimensional linear and nonlinear free surface wave motion,the three-dimensional tank sloshing local pressure and the three-dimensional tank sloshing force are numerically simulated.And the calculation results are calculated to compare with the calculation results and experimental results of other research methods,the correctness of the numerical model and the numerical method are verified.The numerical model developed in this paper is further applied to study and analyze the influence of sensitivity parameters such as excitation frequency,excitation amplitude,filling depth and excitation direction on the sloshing characteristics.In addition,the effects of fluid viscosity,a two-dimensional three-degree-of-freedom sloshing,and three-dimensional six-degree-of-freedom sloshing are analyzed.Furthermore,the dynamic model of the six-degree-of-freedom motion of the floating body with the tank in the wave is established.The program is developed to solve the dynamic model based on the propagation method of the rigid body dynamics.Using the viscous numerical wave flume based on viscous flow theory and the nonlinear tank sloshing model,combined with the ship dynamics model,the flow field inside and outside the floating body is solved simultaneously based on the overlapping grid technology and the viscous fluid theory.The coupling of the inner tank sloshing and the overall motion of the floating body is analyzed.Hydrodynamic analysis of a two-tank liquid-carrying rectangular floating box in a wave flume and a two-tank LNG-FPSO in a wave flume is carried out.Comparing the present numerical results with some other published results from experiment and other numerical methods,the correctness of the numerical method is fully verified,which provides a basis and establishes the foundation for the multi-body coupling analysis of the floating body with tank and the underwater slightly rigid mooring system.Finally,based on the above research work,the propagation method is still used to solve the multi-body system dynamics model composed of the mooring system and the floating object with the tank.A coupling study of inner tank sloshing and ship motion is further developed for a simplified FLNG.Considering the coupling of the underwater soft YOKE mooring system,the experimental and the numerical method are used to analyze the mooring characteristics of FLNG with different filling depth under different wave conditions,and the correctness of the numerical method is further verified.Then the numerical method of this paper is used to study the coupling of underwater slightly rigid mooring system and FLNG in viscous numerical wave flume under transverse wave conditions.Using the numerical method of this paper,the sensitivity analysis of structural parameters(including the stiffness of the mooring leg,the weight of the underwater ballast,the length of the mooring leg,the length of underwater YOKE,and the angle between two mooring legs)and environmental parameters(including wave height and wave direction)are continuously carried out.