Numerical investigation of dry bulk cargo load during ship vertical motion

Abstract

The accurate estimation of cargo load is important for designing double bottom structures. However, dry bulk cargo load due to ship acceleration has rarely been investigated in detail. In this study, DEM (Discrete Element Method)-FEM (Finite Element Method) coupling analyses with simplified vessel models were used to evaluate the dry bulk cargo load acting on the bottom plate; the dry bulk cargo and deformation of the bottom plate were expressed by discrete and finite elements, respectively. The analyses for the first time showed the occurrence of a large force near the edge of the bottom plate and small force around the center when the bottom plate was elastic. These forces resulted in the reduction of the bending stress on the bottom plate compared to a condition assuming uniform distribution of the dry bulk cargo load. This result firstly explained that the reduction of the bottom plate bending stress in previous measurements of double bottom strain of a bulk carrier during ocean going. Above non-uniformity of the force distribution was not observed when the bottom plate was rigid. This change in the distribution was caused by the arching effect, which occurred due to interlocking between discrete elements. The amplitude of force redistribution depended on the ratio of the width of the vessels to the height of the aggregate of the discrete elements. The DEM-FEM coupling analyses also showed that an appropriate design of the hopper plate can support the inertia of the discrete element aggregate located above the flat bottom plate, and thereby reduce the force acting on the flat bottom plate.