Plastic theory and numerical simulations are used to discuss the deformation patterns and the collapse mechanism of longitudinal web girder attached stiffeners in a shoal grounding scenario of a double bottom tanker. In the study, numerical simulation code LS_DYNA is used to calculate the deformation patterns and energy dissipation, and a mathematical model of the progressively deformed stiffener is built. With the application of plastic mechanism theory and numerical simulation method, the distortion energy and mean grounding resistances are obtained, and the results were verified. The analytical expressions obtained can be used to calculate and assess the crashworthiness of a ship during its design phase.