Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


load cases can be divided into the following categories for self-propelled container vessels:


1. Upright vessel condition


The hull girder loads are: Vertical still water bending moment and vertical wave bending moment. 2. Incline vessel condition


When the vessel is in oblique waves then the vessel will experience the following hull girder loads: Vertical still water bending moment, Vertical wave bending moment, Horizontal wave bending moment, Still water torsional moment and Wave induced torsional moment.


Figures 6 and 7 illustrate wave induced bending moment in oblique sea conditions for full load and ballast load conditions respectively.


4.6 GLOBAL STRENGTH ANALYSIS


The objective of a global strength analysis is to obtain a reliable description of the overall hull girder stiffness and to assess the global stresses and deformations of all primary hull members for specified load cases resulting from realistic loading conditions and the wave-induced forces and moments. Generally, the purpose of the glob- al analysis is not to judge on local stresses due to stiffener or plate bending, whereas the focus is at realistic stiffness and deformation characteristic of the hull girder. Global strength analysis may be required if the structural response of the hull girder cannot be sufficiently determined by simple beam theory, e.g. for ships: • with large deck openings subjected to overall torsional deformation and stress response, as for container ships


• without or with limited transverse bulkhead structures over the vessel length, as for Ro-Ro vessels and car carriers


• with partly effective superstructure and/ or partly effective upper part of hull girder, as for large Passenger vessels


4.7 BOUNDARY CONDITIONS


Figure 6: Wave bending moment in oblique sea (Full load condition)


If a cantilever beam is considered with a bending moment on one side, the moment will be the same in all sections along the length of the beam. The same concept has been employed in the FE model to study the stresses developed on the hull midship section. Maximum bending moments (Sagging/Hogging) (upright condition) are applied on one side of the FE model and the other side is imposed with fixed constrains (Table 2).


Still water bending moment and wave bending moment is applied in the fore end of the model and aft end is clamped. Rigid elements are created under main deck, which allows transfer of load to various nodes. Rigid element connects free edge nodes and other nodes in the same plane, so that they act together as a single element. Thus, two rigid elements are required to create two boundary conditions: 1. Constraint: a rigid element at the aft of the model with zero DOF to clamp 2. Moment: Bending Moment is applied on a rigid element at the fore part of the model in positive y direction to create hogging/Sagging condition.


Figure 7: Wave bending moment in oblique sea (Ballast load condition)


From Figures 6 and 7 it is observed that wave bending moment is larger for full load condition than ballast load condition. One of the reasons for that is in full load condition the displacement volume is higher than the ballast load condition.


©2019: The Royal Institution of Naval Architects A-453 Table 2: Boundary conditions


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