Lightweight Research Of Ship Structures Based On Response Surface Method

With the rapid development of Chinese economy, environmental problems have becomeincreasingly prominent. In recent years, the serious fog and haze even make a breath of fresh airbecome a luxury. The ship is one of the major means of transport, so its lightweight can bringabout reduction of fuel consumption and pollutant emissions, and play a role of protecting theenvironment. Therefore, more and more attention is paid to the lightweight research of shipstructures.At present, there are two main methods in the lightweight research of ship structures:specification-based method and direct calculation method. In the specification-based methodship structures are optimized just based on the specification, so this method has low calculationprecision. In direct calculation method based on integrated software, finite element analysismust be carried out at each iteration calculation, so the computational efficiency is low. Thedirect calculation based on constructor can improve the accuracy and efficiency, but theexisting studies only considered the static characteristics of the ship and did not take thedynamics characteristics into account. Therefore, the response surface method is introducedinto the lightweight research of ship structures considering both statics and dynamicscharacteristics of the ship in this paper. The lightweight of cargo ship is studied based on thefinite element method, response surface method, specifications and optimization method. Themain works accomplished are as follows:(1) On the basis of summarizing the research of the ship structures lightweight both athome and abroad, the finite element method, response surface method, specifications andoptimization method are applied to lightweight research of ship structures and then the specificresearch plan of ship structures lightweight is made.(2) The end face moments, outboard water pressure and cargo load of the cabin in the mostdangerous conditions are calculated after establishing three-dimensional finite element modelof50m cargo ship. The static analysis of cabin is carried out and the values of maximum vonmises stress and shear stress of each cabin component are obtained. At the same time, thedistributions of cabin von mises stress and shear stress are also obtained. The results show thatboth the maximum stress von mises stress and the maximum shear stress occur at hatch corners and do not exceed the allowable stress. So there is a certain space-optimized.(3) The added water mass of ship is calculated with Lewis theory. The first order addedwater mass of vertical vibration, the second order added water mass of vertical vibration and thefirst order adder water mass of horizontal vibration are obtained together. The cabin modalanalysis is carried out by using ship hybrid finite element model after applying added watermass and boundary constraints. The first three natural frequencies, vibration modes anddistribution of modal stress are obtained. The results show that the first three vibration modesare vertical vibration mode of two nodes, torsional vibration mode of two nodes and verticalvibration mode of three nodes respectively. The first three natural frequencies of cabin are faraway from operating frequencies of propellers and main engine, which means that the ship canavoid the occurrence of resonance.(4) Sensitivities of maximum von mises stress, maximum shear stress, first orderfrequency(vertical vibration), second order frequency(torsional vibration) and third orderfrequency(vertical vibration) of cabin relative to15kinds of components are carried out bymeans of parameter sensitivity analysis method.7kinds of components which correspond toseveral large sensitivities obtained are selected as design variables for constructing responsesurface function.(5) Box-Behnken experimental design method is adopted to arrange experiment and thenexperiment data is obtained by finite element analysis according to the experiment program.The response surface functions of maximum von mises stress, maximum shear stress, first orderfrequency, second order frequency and third order frequency are constructed based on theexperiment data. The correlation coefficient, correction coefficient and coefficient of variationare used to test the5kinds of response surface function. The results show that the responsesurface models are very close to the physical models. At the same time, random samples areselected to test the response surface functions. The values calculated by response surfacefunctions are compared with the values calculated by finite element analysis, and the resultsshow that the5kinds of response surface models have high precision and can reflect the testvalues.(6) The minimum sizes of the ship structures limited by specification are used as the sizeconstraints; the stress and frequencies required by specification are used as the ship performance constraints at the same time; the ship minimum mass is taken as objective function.The cabin structure optimization is carried out with sequential quadratic programming methodby calling the5kinds of response surface functions to calculate the stress and frequency. Afterthe optimization, cabin mass reduces6.01%. The finite element analysis is carried out foroptimized ship structure. The results show that the stress of optimized ship structures meets thespecification requirements and the first three order natural frequencies of cabin are far awayfrom operating frequencies of propellers and main engine. So the ship mass is reduced in thecase of meeting both ship static and dynamic characteristics, and the purpose of ship structureslightweight reserch is achieved.