Study On Structural Multi-Objective Optimization Of Independent Type B LNG Tank

With the rapid development of liquefied natural gas(LNG)industry,the market demand for LNG carriers is also expanding.Membrane-type LNG carriers are prone to violent sloshing and impact pressures during transportation,resulting in damage of the tank structures.However,the independent type-B LNG carriers have tanks designed with internal bulkheads and a variety of girders to reduce liquid sloshing resonances in tanks effectively,thus ensuring the structural safety of the tank at partial load during transportation.It is necessary to carry out structural optimization design of internal girders in independent type-B LNG tanks from the aspect of reducing sloshing pressures.At the same time,the internal temperature in LNG tanks is minus 163 degrees Celsius,so there are special requirements for the material of tank structures.From the aspect of reducing the cost,it is also necessary to optimize the structures of independent type-B LNG tanks and carry out lightweight design.There are contradictions between the two objectives of reducing sloshing pressures and production costs,so it is of great engineering significance to carry out structural multi-objective optimization design and study of independent type-B LNG tanks.First of all,the methods of sloshing assessment in tanks based on rules of different classification societies were introduced and compared mainly from three respects,including criteria for the evaluation of the risk of resonance,calculation formulas of non-resonant sloshing pressures and resonant sloshing pressures.Secondly,the cargo hold finite element model of independent type-B LNG carrier was established by using the finite element software MSC.Patran/Nastran.Then the structural adequacy of primary members of the cargo tank,tank supports and associated hull structure were analyzed and assessed under a variety of load cases.According to the results of yield strength assessment,local structures were strengthened to meet safety requirements.Thirdly,the sloshing coefficient of independent type-B LNG tank was defined to show the complicated effects of internal girders on sloshing in tanks.Then the structural multi-objective optimization model of independent type-B LNG tank was established in ISIGHT with two objectives including the sloshing coefficient and the structural weight of tank.To save the time costs of FE analysis process in optimization,a high-accurate approximation model was established with the methods of DOE(design of experiments)and RBF(Radial Basis Functions).Then the structural optimization of LNG tank was performed by three kinds of multi-objective genetic algorithms including NSGA-II,NCGA and AMGA.Finally,an improved scantling design of independent type-B LNG tank was achieved and verified.Numerical results showed that AMGA was the best algorithm for this problem compared with others.Furthermore,the sensitivity analysis of design variables to the objectives was carried out,which provided a reference for the structural design of independent type-B LNG tank.