Research On The Truncation Model Test Of Internal Turret Moored FPSO Based On Time-Frequency Transformation

For the deep-water mooring system model test,due to the insufficient size of the existing ocean engineering basin,it cannot meet the test requirements of the mooring system.Passive hybrid model test is currently the most effective method to solve this problem,and the equivalent truncation design of the mooring system is one of the important links.The static characteristics of the mooring system after the truncation design can usually be in good agreement with that before the truncation,but the dynamic characteristics of the mooring system are still difficult to be accurately equivalent.Based on this,a mooring power truncation method based on multi-objective function cuckoo optimization algorithm is proposed,and its reliability is verified by numerical simulation analysis and model tests in the deepwater basin.When setting the objective function of dynamic truncation,within the set time range,the dynamic characteristics of the mooring line need to be consistent at each time point.A large number of points of the dynamic results where the amplitudes change with time to cover the entire time history are generally selected.Although this method can ensure that every time point is considered,the calculation efficiency is low due to too many selected points,and the comparison result is not intuitive enough.After converting the time domain results to those in frequency domain through time-frequency transform,setting a small number of key frequency points to establish the objective function can greatly improve the analysis efficiency.In order to find the optimal solution more intuitively,the related concepts of Pareto solution are introduced,and the global optimal solution can be found directly through the front surface composed of multiple Pareto solutions.In order to verify the feasibility of the multi-objective cuckoo optimized truncation method,a catenary and a taut mooring system are taken as examples respectively to carry out the vertical truncation design.The truncation parameters similar to the static force of the full-water depth mooring line are obtained through optimization calculation.On this basis,single-frequency sinusoidal motion and multi-frequency random motion are applied to the apex of the mooring line,and the forces of the mooring line at typical frequencies before and after truncation are compared.The truncation optimization scheme under multi-frequency motion is more reasonable,and the static and dynamic characteristics of the mooring line before and after truncation are consistent.In order to solve the problem of insufficient horizontal span in the model test,a certain internal turret FPSO mooring system with a water depth of 420 m is designed to be horizontally truncated,and the horizontal span is truncated from 1173 m to 588 m.First,perform numerical calculation and analysis on the FPSO mooring system to obtain the static characteristics and dynamic response of the mooring system;secondly,use the truncation optimization program to design the horizontal truncation of the mooring system,and compare the truncated and untruncated results,which can meet the principle of static and dynamic equivalence.Model tests to the FPSO and its truncated mooring system are performed with a scale of1:42,which includes free decay tests in static water,white noise RAO tests,static stiffness tests and irregular wave tests.Comparing the results of the truncation model tests with those of the numerical calculation,the static and dynamic characteristics of the mooring system is in good agreement,and the error is within the allowable range.This verifies the accuracy of the model test results and further proves the reliability of the proposed dynamic truncation method.