On the model tests of double bottom strength for stranding

Abstract

This paper deals with model tests and their analysis conducted to evaluate the structural strength of double bottom for two types of stranding. Striking : The bow of a ship strikes a rock, which horizontally penetrates the bottom structure. Grounding : The ship grounds on a rock, which vertically penetrates the bottom structure depending on sea condition. For the striking test, 1/4 scaled double bottom models of steel plates were made representing the bottom of usual cargo holds and more stiffened bottom of engine rooms, etc. The rock model is a rigid square piller with leading wedge of angle 90°. For the grounding test, 1/8 scaled double bottom model was made representing the bottom of engine room, etc. The rock model is a rigid cone with apex angle of 90°. The rock model was set on the head of a 600 ton Amsler universal testing machine, and forced to penetrate the bottom model set on the moving bed.Following conclusions are obtained from analysis of test results.(1) In striking situation, crushed (buckled or torn) members such as bottom plate, inner bottom plate, longitudinals and girders would have the collapse strength of 80% yielding of their material, while deflected and ruptured members such as floors would induce membrane force in proportion to rock's penetration and subsequently break at yielding of their material with 20% elongation (in case of mild steel). This assumption gives very good estimation for the relation between load and penetration of the bottom structure.(2) In grounding situation, same assumption is also valid. In this case, crushed members are floors and girders, while ruptured members are bottom plate panels supported by floors and girders.(3) The damage of an oil carrier of about 100 thousand tons with double bottom is estimated using calculation formulas acertained by the model test. On striking a rock at the bow during fully loaded running, rock's penetration would reach O.20.5 of ship length. On grounding on a locally projecting rock, the bottom plate would be ruptured at rock's reaction of 900 ton, and the rock would reach the inner bottom plate at that of 3, 600 ton.