| Floating fans have been exposed to harsh environmental loads such as wind,waves,and currents for a long time,and the significant slow drift motion has always been a key issue in marine engineering research.In moored floating fans,the horizontal restoring force is provided by the mooring system,and second-order differential wave loads can stimulate the large slow drift motion of moored floating fans.To study the motion characteristics of floating fans and ensure mooring safety,attention should be paid to their second-order differential wave forces.In the industrial design of floating fans,most of the potential flow commercial software for numerical simulation of hydrodynamic characteristics of floating bodies is based on panel method.The time required for simulation is related to the square or third power of the number of grids,and the calculation of the second order differential frequency excitation force is more complex than the first order wave excitation force.In the early stages of engineering design,structural parameter optimization makes the design process more cumbersome and timeconsuming.Therefore,predicting the differential excitation force of floating fans more quickly has become an important research topic.At present,the foundation of floating fans can be simplified into the splicing of geometric structures such as cylindrical and square shapes.This article quickly solves the hydrodynamic characteristics of floating foundations using geometric approximation methods within the framework of potential flow theory.This method divides floating foundations into two categories: cylindrical arrays and submerged rectangular pontoons.For cylindrical arrays,Linton’s linear theory of cylindrical arrays and its second-order extended theory are used for hydrodynamic analytical calculation,and Green’s formula and Haskin’s relationship are used to transform the second-order diffraction potential into integral on the body surface and free surface,so as to solve the second-order problem indirectly.For rectangular pontoons,analytical solutions and relevant empirical formulas are used for solving.Obtain the hydrodynamic results of the overall fan foundation through linear superposition.This dissertation takes the Spar column and three column semi submersible fan foundations as the research object,and uses the Fortran program Swim developed by MIT to achieve geometric approximation calculation.Random wave simulation and differential excitation force time history are solved through frequency bisection method.Finally,coupled with the Moordyn program developed by Matthew Hall,the mooring recovery force is calculated to complete an efficient and fast solver for slow drift motion frequency domain and time domain analysis.The primary transfer function(RAO),the quadratic transfer function(QTF),the time history of large and slow drift motion under irregular waves,the time history of cable tension,as well as their statistical analysis and short-term extreme value prediction results are analyzed by using geometric approximation method,and compared with the frequency and time domain results of commercial software Hydro D and Aqwa to verify the rapidity and applicability of the geometric approximation analysis method in this dissertation. |
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