Research On Ultimate Strength Of Ship Hulls And Residual Strength Of Damaged Ships

The ultimate load bearing capacity of ship hull structure is an important index expressing the safety and reliability of the structure, and it has widely been paid much attention and concern. But because of variety of ship types, complexity of ship structure, and relationship to strongly non-linear factors of material and geometry, the calculation of ship hull's ultimate strength becomes a complicated mechanical problem. After many years of research work and development, it is still a task and goal to acquire some analysis methods and computer programs which are mature in theory, easy in application, and accurate in results.On the other hand, a ship in navigation may happen to accidents of grounding or collision. To avoid disastrous consequences such as totally loss of the ship and subsequent pollution to environment, it is necessary to know and assess the residual strength of the damaged ship. So we can take correct technical measure to salvage, rescue and tow the damaged ship. There are only some commence works on the calculation and assessment method for damaged ships at the moment..The main content and achievements of this Dissertation are as follows:1. It is the first time presenting a calculation method in which takes account of axial force effect dealing with the ultimate strength of ship hull girder. Based on plastic theory, the coupled equations of axial and moment are derived which modifies Rahman's method to consider the influence of axial force on the plastic bending moment of compressed structural element in processing collapse period. The presented method in the Dissertation has successfully applied to the calculation of ultimate strength of ship hull girders.2. It is the first time applying progressive collapse method to calculate the residual strength of damaged ships in different heeling floating conditions. This method solves the difficult problem of the residual strength of damaged ships in the form of asymmetrical bending during progressive collapse.3. Using elastic constraint boundary condition in which is accordance with actual ships, several calculation formulae are derived for the grillage stability of ship structures. These formulae can deal with special grillages with different stiffener size in the same direction, and the ones with sparsely or non-evenly arranged stiffeners. The suggested defection forms of polynomial and B-spline function can properly adapt condition of asymmetric elastic constraint boundary. The suggested method for assessing the effectiveness of boundary condition can easily be applied to design and calculation for actual ship grillages.4. It is the first time carrying out theoretical similarity analysis, design and fabrication of large-scale similitude model for a heavy loading inland river ship on ultimate strength experiment. The results are compared with those from progressive collapse method and nonlinear FEM method.5. Based on theory investigation, the proper failure modes are adopted in calculating stiffened plate and stress-strain curve of the element. Three existing methods for stiffener tripping are discussed on the theory base. Many example calculations are preformed and compared to the ultimate strength of stiffened plate structures.6. The computation program using two kinds of methods are developed for the ultimate strength calculation of ship hull girders. Several factors influencing the ultimate strength are analyzed and discussed. A series of stiffened plates, box girders and actual ships are calculated by the computation program.7. The procedures of nonlinear FEM analysis are discussed in calculating the ultimate strength of ship hull girders. A series of stiffened plates, box girders and actual ships are calculated by using nonlinear FEM.8. The residual strength of series actual damaged ships are analyzed and discussed in detail. The sensitive degree analysis is preformed to several main factors.Through the comparisons among the results from many examples of actual ship structures, it verifies that the adopted methods are correct, the developed programs are effective and the obtained results are accurate.To damaged ship in different floating situation, the calculation method, results and conclusion are useful in drawing up the rules and regulations relating ship safety, assessing the residual strength, taking technical measure to salvage, rescue and tow the damaged ship.