Stress Analysis of Bulk Carrier Hatch Corners

One of the most frequent damage cause for bulk carrier ships is represented by high stress concentrations occurring in deck plating close to hatch corners in way of coaming stay. In this area many fractures are found during surveys which are generated mainly by three types of problems: • stress concentration in correspondence of deck large openings; • bad design of stays; • different thickness between transversal and external deck plates. The typical approach followed by IACS classification societies consists in a wide range of qualitative suggestion (transversal stiffening for transversal deck strip, elliptical hatch corner etc.) and a very simple formula providing the extra thickness of the hatch corner compulsory insert. This solution seems to be inadequate to face this phenomenon and a better knowledge is mandatory to understand which parameters are more influent on stress distribution and concentration. The purpose of the analysis herein presented is to identify most important parameters and to study their influence, in order to develop a more sophisticate procedure to be used in preliminary design stage. The investigation has been developed starting from the analysis of five different bulk carriers, selected on the basis of their geometric and structural properties, and modeled with FEM software Leonardo hull 3D v. 2.4.1. The analysis has been divided into two sub-problems: • study of b/B ratio influence (hatch breadth versus ship breadth); • study of lw/b ratio influence (cross deck length versus hatch breadt).. Two different series of FEM models have been realized: Global models of the hull extended to a portion of three cargo holds; Simplified models with increasing refinement degree. All global models have a relatively coarse mesh and are loaded with a wide number of loading condition (homogeneous, heavy and light ballast, alternate ore) corresponding to relevant RINA load cases in order to maximize the effects. For what simplified models are concerned four different types of models have been analysed: a) The first model consists in a simple plate with a rectangular opening and two longitudinal box structures simulating the wing tanks. All the parts of the model have a constant thickness. b) A second series of models has then been created, similar to the first one, adding a coaming around the hatch opening. As in the first case the thickness has been maintained constant. c) The third series is similar to the first one but the thicknesses of the lateral deck strip and of the cross deck have different values, according to the real case. d) The fourth series is similar to the second one. Again deck plates have different thicknesses. For all the models the results have been processed to calculate stress concentration factors “k” (defined as longitudinal stress close to hatch corner versus theoretical hull girder stress) for all selected loading conditions. From the “k” coefficients obtained for every model, a “stress concentration surface”, as a function of b/B and w/b, has been drawn and expressed by a simplified formula.