The Indirect Monitoring Method Of Typical Loads Exerted On Ship And Marine Structures

There are a lot of uncertain factors in the actual operation of ship and offshore engineering structure,which lead to the prominent problem of structural safety,which puts forward the practical demand for online monitoring of structural safety.The on-line monitoring of structural safety can be carried out in many ways:(1)directly monitoring the stress of structural key points,and evaluating the structural safety on-line through the stress;(2)monitoring the typical loads of the structure(local load and overall load),and evaluating the structural safety on-line through the typical loads.This paper focuses on load inversion method of load monitoring.There are many kinds of loads encountered by ships and offshore engineering structures,in which,the local load is mainly distributed pressure,and the total load is mainly bending moment and torque.Therefore,a series of typical indirect load monitoring methods are studied by taking local ice load,wave bending moment and torque of hull girder as examples,and the main contents are as follows:(1)The conventional and hybrid methods for indirect monitoring of local ice loads on ship and offshore structures are studied.In the conventional method,the ice load monitoring area is divided into several individual cells and the ice load in each cell is assumed to be uniform.Then,a complete mapping relationship between ice load and structural response is established through numerical simulation,and the mapping relationship is expressed as a linear system of equations.At last,several methods are used to solve the linear system of equations to determine the optimal one.In the hybrid method,multiple mathematical models need to be constructed and solved by multiple methods,and the final solution is output by genetic algorithm.The numerical simulations and engineering examples of distributed ice load indirect monitoring of plate and frame structure were used to prove the practicability and superiority of the hybrid method.In addition,considering the uncertainty of the mathematical model obtained from numerical simulation,this paper introduces the D-optimal design method to optimize the mathematical model.Based on the optimized mathematical model and Monte Carlo method,the influence of the uncertainty of the mathematical model on the results of ice load inversion is studied.(3)In the indirect monitoring of typical loads,the ill-posedness of mathematical model has a great influence on the accuracy of identification loads.In this study,the D-optimal design method was used to reduce or overcome the ill-posedness of mathematical models by optimising strain gauge locations.The advantages and disadvantages of D-optimal design method are obvious,i.e.the computational speed is fast and the ill-posedness of mathematical model cannot be overcome entirely when the number of candidate strain gauge locations is large.Therefore,a C-optimal design method was developed to reduce or overcome the ill-posedness of mathematical models also by optimising strain gauge locations.The computational speed of C-optimal design method is lower than that of D-optimal design method.However,the ill-posedness reduction effect of C-optimal design method is better than that of D-optimal design method.Thereafter,the block C-optimal design method was developed to reduce or overcome the ill-posedness of mathematical models with similar ill-posedness reduction effect and faster computational speed compared with C-optimal design method.The numerical simulation and experimental results were used to demonstrate the advantages of the C-optimal and block C-optimal optimal design methods.(3)The c-optimal design method and D-optimal design method are introduced into the indirect monitoring of wave bending moment,and the ill-posedness of the mathematical model is eliminated by optimizing the position of the sensor.Based on that,an improved method of indirect monitoring of wave bending moment is proposed.In the improved method,the mathematical model is obtained by numerical simulation.The simulation results of indirect monitoring of wave bending moment show that the improved method is better than the conventional one.In addition,the real ship monitoring was used to prove the practicability of the improved method to a certain extent.(4)A double section monitoring method of wave bending moment and torque of hull structure with large openings was developed based on basic theory of hull torsion.In double section monitoring method,two adjacent hull sections need to be monitored simultaneously,and at least 4 long base strain gauges need to be arranged on every hull section along X-direction.Then the wave bending moment and torque of middle section are approximated through the indirect monitoring results of the adjacent hull sections.Finally,the numerical simulations of wave bending moment and torque indirect monitoring were used to indicate the applicability of the double section monitoring method.