| Type C independent tank-type LPG carrier is one of the mainstream types of LPG carrier at present,and its structure is different from the general cargo ship’s structure:The liquid cargo is stored in the liquid storage tank,and the tank body is connected to the hull structure through saddles,the laminated wood is cushioned between the saddle.In order to ensure the thermal expansion and contraction of the tank body and the sufficient displacement of the tank body in the ship’s length direction when the ship is moving,generally,each tank is supported by a fixed saddle and a sliding saddle.In this dissertation,the structural direct strength analysis（DSA）method and the key technologies of the hold section and the whole ship of the C-type independent tank-type LPG ship are studied.A 5 000 m~3 Type C independent tank-type LPG carrier is being taken as a research object,in strict accordance with the requirements of the new specification,Specification for Construction and Equipment of Ships Carrying Liquefied Gases in Bulk（2018）,establishing the models of the‘1/2+1+1/2’hold section for the fore cargo hold and the stern cargo hold including the hull structure,saddle,laminated wood,and tank is established;the direct strength calculation of hold section structure is carried out according to the calculation conditions,design loads and boundary conditions specified in the specification;the laminated wood between the liquid storage tank and the saddle is simulated by the rod elements and the iterative calculations is performed;performing strength assessment of the hull and saddle structure of the target ship using the strength standards specified in the specification and locally strengthening the components that do not meet the strength requirements until the requirements of the specification are met.Structure design based on the structural direct strength analysis of hold section model shows that for a target ship with only two cargo tanks,the deck（corner）structure at the forward bulkhead of the fore cargo tank and the rear bulkhead of the stern cargo tank and its associated side structures,the size of the components obtained according to the conventional specification design dimensions is difficult to meet the strength requirements（many additional enhancements have been made compared to the parent ship）of direct strength analysis,and they are the critical areas of this ship’s structural design.The first reason is that the two critical areas fall far away from the midship,and the bending moment of the hull girder applied according to the requirements of the specification（the largest value in the range of the model）is over-conservative,at the same time,the external pressure（combinations of load condition,draught,dynamic pressure,and inertia force）required by the specification is also over-conservative;the second reason is that the two critical areas are closer to the boundary constraints of the tank model,and the stress results are greatly affected by the constraints,especially the fore cargo tank model,considering the fact that the side structure at the forward bulkhead of the fore cargo tank changes from a double shell to a single shell structure and corner,etc.,the superposition of several stress concentrations has caused them to require unconventional strengthening to meet the strength requirements of direct strength analysis.The specification has unreasonable requirements and regulations for the direct structural strength analysis of such LPG ship.In order to further prove that the value of the applied load in the hold section model of the code is too conservative rather than the question of structural form,and in order to more reasonably simulate the true force state of the target ship,a whole ship model of the target ship is established,wave load in running working condition is forecasted,then the direct calculation and strength check of the whole ship structure are carried out.The results show that the hull and saddle structure strength of the target ship can meet the standard design requirements.Finally,analyzing the structural strength of the head cargo hold section model again,keeping the load in the head cargo hold section model consistent with the load in the whole ship model,comparing the hull girder shear curve and hull girder bending moment curve of the whole ship model and the hold section model within the range of the hold section,and analyzing the stress of shell elements within the range of the head cargo hold,the results show that the results of the two are basically the same,and the real state of the ship can be reasonably calculated by using the hold section model and determining the load through the auxiliary software.Through the research in this dissertation,the applicability of the new specification to the C-type independent tank-type LPG carrier is evaluated,proved the rationality of the hold section model combined with auxiliary software to analyze the true state of the ship and it also provides a reference for the direct strength calculation and evaluation of hull structure for similar types of LPG carriers. |
