Residual Strength Assessment Of Damaged Warships

Residual strength assessment is an important content of warship's structures survivability assessment. In some other countries, residual strength assessment has been included in the naval ship rules and been regarded as the same important with other strength assessments. However, the studies on this aspect are just being considered as a special problem in our country, there is no responding regulation about residual strength in our naval ship rules. Based on above reasons, it's necessary to build a residual strength assessment method for damaged warships. Some basic problems of residual strength assessment of damaged warships are studied in this thesis, and the major work is as follows,1. Combined with the characteristics of damaged warship, a three-dimensional wave loads calculation method for damaged warship is presented based on three-dimensional hydrodynamic theory, where the damaged warship's floating condition is determined based on the non-uniform rational B-spline (NURBS) representation of ship hull surface and the hydrodynamic coefficients of damaged cabin region is modified by the Reynolds Averaged Navier-Stokes Equations(RANSE) considering the water exchange and viscous flow. According to the practical sea condition damaged warship in, a forecast method for design load of damaged warship during stationary period is presented which is different from the shot and long term analysis of integrated ship. Based on this, the building method of simplified formula of design load is researched.2. Based on Smith method, considering the asymmetry of damaged warship's resisting section, a new simple progressive collapse approach method is presented to assess the residual bearing capacity of the damaged warship. The probabilistic statistics of the sizes of broken holes along the ship are analyzed, and the broken holes'size and the initial flexibility is defined. The stress-strain relationship is established by elastic large deflection analysis and plastic analysis with initial deflection considered. The unsymmetrical resisting section's neutral axis of damaged ship is derived by unsymmetrical beam bend theory.3. Based on nonlinear finite element analysis method, combined with ship damage mechanism and the method dealing with the initial defect, considering the impact of lateral loads, such as cargo pressure, water pressure and hydrodynamic pressure, a finite element method for residual bearing capacity of damaged warship is established, which thinks of impact of lateral loads.4. Based on incremental plastic collapse principle, considering the integral damage of hull girder by many times of ultimate loads, a method to calculate the ultimate longitudinal strength of the ship hull under cyclic load following the Smith Method is established. And the average stress-strain relationship when the element under cyclic load is studied, the corresponding assessment philosophy and treatment method are derived. According to theoretical analysis and numerical calculation, the relationship of incremental plastic collapse with progressive collapse and low cycle fatigue failure is analyzed.5. Starting with analyzing of the limit state and failure mode of damaged warship, based on the reliability method, a method for analyzing the damaged warship's residual strength is presented by considering the uncertainties of residual capability and loads. Furthermore, a simplified assessment method (certainty method) for damaged warship's residual strength is developed on this basis.Under above basis, a warship is taken as an example to compute its residual strength by using above method. The results showed that the reliability method presented in this paper is worth made thorough study which can reflect the residual condition of damaged warship more clearly through calculating its failure probability and this method makes quantitative analysis about the effects of sea conditions, damaged degree, sea direction and ship speed etc. The certainty method is an easy calculation, however, the selection of safety factor needs massive theoretical calculation, test verification and/or statistical data of damaged warship.