Chenfeng Li

Study on Fatigue Strength of Trimaran Cross-Deck Structure Based on Spectral Method

The stress concentration of trimaran’s cross-deck structure is serious and the problem of its fatigue strength appears particularly important. In this paper, the responses of ship motion and hydrodynamic pressure on ship’s surface in regular waves are calculated using 3D linear potential flow theory, then the stress responses of the hot-spots in regular waves with different wave angles are evaluated by finite element analysis of the global trimaran structure. Based on linear cumulative damage theories, the fatigue damage is calculated according to the direct calculate method used spectral analysis. The result can offer the reference for the trimaran’s design and development.

Calculation Method of the Residual Capability of Damaged Warships

The residual capability of damaged warships is one of the important aspects of its residual strength assessment. The calculation method of ultimate longitudinal bending moment was based on the Smith method. Then, a method to calculate the statistical characteristic values of residual capability of damaged warships was presented, in which some uncertainties, such as the size of broken holes caused by weapons, material mechanical properties of the steel and the size of structures, etc, were considered. The ultimate longitudinal bending moment of Reckling No.23 model was calculated and compared with the result from the literature, the result is quite satisfactory. With a warship as an example, the statistical characteristic values of residual capability of the damaged ship hull were calculated, and some factors which may evidently affect the residual capability were investigated.Copyright

Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies

Coupled dynamic analysis between floating marine structures and flexible members such as mooring lines and risers, is a challenging work in the ocean engineering field. Coupled analysis on mooring-buoy interactions has been paid more and more concern for recent years. For floating offshore structures at sea, the motions driven by environmental loads are inevitable. The movement of mooring lines occurs due to the excitation on the top by floating structures. Meanwhile the lines restrict the buoy’s motion by forces acting on the fareleads. Positioning is the main function of mooring system, its orientation effects can’t be ignored for floating structures such as semi-submersible, FPS, and TLP, especially when the buoy’s equilibrium position shifting to another place. Similar as hydrostatic restoring forces, mooring force related with the buoy’s displacement can be transformed into mooring stiffness and can be added in the differential equations of motion, which is calculated at its equilibrium point. For linear hydrodynamic analysis in frequency domain, any physical quantity should be linear or be linearized, however mooring stiffness is nonlinear in essence, so the tangent or differential stiffness is used. Steel chains are widely used in catenary mooring system. An explicit formulation of catenary mooring stiffness is derived in this article, which consists of coupled relations between horizontal and vertical mooring forces. The effects of changing stiffness due to the shift of equilibrium position on the buoy’s hydrodynamic performance are investigated.Copyright

Fatigue Strength Assessment of Aged Floating Production System

Fatigue is one of the most frequent causes of failure for offshore structures subjected to environmental loads, especially aged structures. An effective method for calculating the fatigue damage of aged platform under regular wave is presented. Three-dimensional source distribution theory in the frequency domain is used to calculate fatigue load of FPS in regular wave. The hot spot stress in the welded knuckles is calculated on the basis of a detailed finite element model. Based on the S-N curve and the Miner linear accumulative damage theory, the calculation process of fatigue damage of components was simplified. The calculated results show that this computing method is convenient and accurate. It is suitable for engineering application.Copyright

Fatigue Crack Propagation Rate Test of Q235 Steel in Ship Hull

A series of crack propagation rate tests of compact tension specimen for Q235 steel are conducted. Compliance method and pixel method are introduced to measure the crack length, and the comparison of the two methods is proposed. Incremental polynomial method and least square method are applied to fitting and regressing the test data. By statistical analysis, the statistical characteristics of crack propagation parameters C and m are obtained and the fatigue propagation rate described by Δ CMOD and ΔK is proposed. On the basis of probability fracture mechanics theory, the functions of crack propagation rate under different probability standards are presented.Copyright

Numerical Simulation of Water Impact of 2D Wedge with Different Density

It remains unresolved to study the relation between the density of wedge and its impact. Fluent is applied to simulate its mechanism with 6DOF model. After the comparison of numerical result and the theoretical, it matches good and then the transient free surface scene is observed. Thus Fluent is able to solve this kind of non-linear problem. And the effect of density of wedge on the slamming character is also studied.

Study on Structural Form Design of Trimaran Cross-Deck

As a new high performance shipform, the structural form of trimaran is special and the stress concentration of its cross-deck structure is serious. According to the Rules for Classification of Trimaran Ships developed by Lloyd’s Register, the global finite element model of a trimaran is built. Main factors such as Thickness of bulkhead and wet deck, transitional forms and strengthening forms, which affect the stresses at local details are compared and discussed. Then the best structural form of trimaran cross-deck is given. The result can offer the reference for the trimaran’s design and development.Copyright

Virtual Prototype Based Virtual Test Technique for Fatigue Life of Ship Structure

Virtual test (VT) technique is a new kind of test technique which takes computer simulation analysis as its core. It develops rapidly in aviation, automobile manufacturing as well as in electronic system, but it is at an initial stage in ship field. In this paper, the virtual prototype (VP) based virtual test technique for fatigue life of ship structure was studied, and a virtual test system for fatigue life of ship structure was developed.Copyright

Study on the Standardized Nonlinear Finite Element Analysis of the Ultimate Strength of Ship Hull Girder

The ultimate strength of ship hull girder has been a study hot spot in ship mechanics. Caldwell method, Smith method, idealized structural unit method and nonlinear finite element method are usually used to predict the ultimate strength of ship hull girder. In the paper, a standardized procedure on the nonlinear finite element analysis of the ultimate strength of ship hull girder is presented. Firstly, the finite element modeling for the ultimate strength of ship hull girder is studied, which include material property, element type, mesh size, model length and boundary conditions. Then, the influence of welding residual stress and initial deflection are studied. For the validation of the method in this paper, a MST-3 model from the test of Nishihara is used as an example. Finally, the results from the nonlinear finite element analysis and Smith method are compared for the ultimate strength analysis of a container ship. The study shows the standardized procedure on the nonlinear finite element analysis of the ultimate strength of ship hull girder is satisfactory and suitable for engineering application.Copyright

Safety Assessment of Ship Launching Based on Airbags With the Nonlinear Rigidity of Airbags Being Considered

Launching based on airbags is an important method of ship launching. However, there has not a scientific method for quantitative assessment on the strength of hull structure and safety of airbags at present. With the increases of tonnage of ships launching on airbags, this problem becomes increasingly apparent and important. With the nonlinear rigidity of air-bags and force equilibrium conditions of ship-hull beam being considered during launching, a method based on full ship FEA was put forward to assess the strength of ship structure and the safety of airbags in this paper. The main contents are as follows: Considering nonlinear rigidity of airbags, a calculation method of rigidity of airbags was studied; Based on the elastic launching calculation, a method of calculating ship motion and force on ship hull was studied with the nonlinear rigidity of elastic foundation being considered; Based on the above condition of ship motions and forces, the stress of hull structure and the force on airbags are calculated by full ship FEA; The safety of a real ship launching based on airbags was assessed, based on the comparison of the results with references, the safety of ship launching by airbags and the reliability of the calculation method in this paper are verified.© 2009 ASME