Research On Deformation Mechanisms Of Stiffeners On Double Bottom Structures In A Shoal Grounding Accident

With the rapid development of the global shipping industry, the totalnumber of ships keeps increasing explosively, accompanied with morefrequent occurrence of grounding accidents. The severe consequences ofsuch grounding accidents may deprive people’s lives, cause greateconomic loss and lead to serious ecological disasters. Examples can beseen as the grounding accidents of Exxon Valdez, Costa Concordia, etc.The severe consequences brought about by such accidents have arousedgreat public attentions. Many researchers worldwide have been focusingon reducing the grounding risk, yet grounding accidents still occur nowand then. Therefore, it is of great significance for engineers to investigatecrashworthiness of ship structures.Ship grounding is a kind of complex nonlinear transient responseprocess under huge impact loads. Materials in the collision area evolvequickly beyond the elastic stage into plastic flow, causing the failure ofship structures. The grounding process contains a variety of nonlinearcharacteristics, as well as the dynamic characteristic with high strain rates.In addition, it also involves the fluid-structure interaction and hydro elasticproblems due to the effects of flow field, which makes it even morecomplex. There are generally four kinds of methods to analyze structuralresponses in grounding actions, namely, experimental method, simplifiedanalytical method, empirical method and nonlinear finite element method(NLFEM). Among all the tools, results of the full scale test are the mostaccurate, but this method is rarely used because the cost is huge. Modeltests are often carried out, but the scaling effects cannot be well interpreted due to strong nonlinearity of grounding actions. The simplified analyticalmethod is a method based on plastic flow theory, providing insight into thegoverning deformation mechanism and good predicting accuracy. Thecalculation is few and highly repeatable. This method is easily applied inemergency decision support systems and in ship preliminary design stages.Moreover, the rapid development of computer technique improves greatlythe efficiency and accuracy of results using NLFEM, known as “virtualexperiment”This paper uses mainly the simplified analytical method and nonlinearfinite element simulation methods to investigate deformation mechanismsof a typical double bottom structure in shoal grounding accidents. Hongand Amdahl’s simplified models of an unstiffened double bottom tanker inshoal grounding scenarios are verified and discussed, and subsequentlydeformation mechanisms of stiffeners on major double bottom structuralcomponents are introduced. The two parts can be incorporated to make fastand reliable predictions of structural performances during shoal grounding.The main work includes:1. A review of the researches on ship grounding is carried out firstly. Then,the research methods in ship grounding area are briefly introduced, andmajor emphasis is laid on the simplified analytical methods and thenonlinear finite element methods (NLFEM).2. Verification and discussion of Hong and Amdahl’s simplified analyticalmodel of the unstiffened double bottom tanker is conducted using thenumerical simulation methods covering a wide range of slope angles andindentations of the indenter when ships slide over the shoal type seabedobstructions3. Deformation mechanisms of stiffeners on the outer bottom plate areinvestigated using simplified analytical methods, and are verified usingnumerical simulations.4. Deformation mechanisms of stiffeners on the transverse floor plating areinvestigated using simplified analytical methods, and are verified using numerical simulations.5. Deformation mechanisms of stiffeners on the longitudinal girders areinvestigated using simplified analytical methods, and are verified usingnumerical simulations.Results and conclusions drawn from this paper may benefit the rapidand reliable assessments of structural performances in shoal groundingaccidents and could be reference to ship crashworthiness in the preliminarydesign stage.