Page 275 - Marine Structural Design

Part I1

Ultimate Strength

Chapter 13 Collapse Analysis of Ship Hulls

13.1 Introduction
In carrying out the limit state design of ship hulls, it is necessary to estimate the ultimate
longitudinal strength of hull girders. Furthermore, in order to estimate oil spills due to tanker
collisions and grounding, an investigation of the global dynamic behavior as well as the local
plastic response of the individual ship hulls is required.
The collapse strength of the ship hull is governed by buckling, yielding, tension tearing
rupture, and brittle failure of materials. Moreover, the strength against each failure mode is
influenced by initial deformations, residual stresses, corrosion damages, and fatigue cracks.
The complexity of these problems requires that the collapse response of ship hulls be
investigated by means of numerical procedures such as finite element methods (FEM).
However, traditional FEM requires a considerable amount of computer CPU and manpower to
prepare input data and to interpret output data. Consequently, their applications to hull strength
and collision problems are limited. Besides, the accuracy of these FEM methods is not always
guaranteed (Valsgkd & Steen, 1991).
During the last 35 years, several mathematical models have been applied to longitudinal
strength analysis of ship hulls. First, Caldwell (1965) introduced a plastic design method for
ships. He estimated the longitudinal strength of a ship hull based on the full plastic moment of
a cross-section. The effect of buckling is accounted for by reducing the load-carrying capacity
of compressed members. Mansour & Thayamballi (1980) considered torsional buckling of
stiffeners in their analysis.
Caldwell's method was further modified by Smith (1977) who proposed that the progressive
collapse of stiffened plates due to buckling and yielding can be included as stress-strain
relationships of fibers of the hull cross-section, while also considering post-buckling behavior.
In the Smith method, the hull section is discretized into stiffened panels and comer elements.
The prediction of load-shortening behavior of stiffened panels up to the post collapse region is
very important. Several algorithms for the modified Smith method have been applied based on
different formulas for plating effective width and beam-column.
The above mentioned methods are simple and accurate for prismatic ship hulls subjected to
pure bending. However, they are less accurate when other sectional forces and lateral pressure
present, because plane sections of hull girders are assumed to remain plane in the modeling.
Chen et ai (1 983) presented a general finite element approach for the collapse analysis of ship
hulls. Their approach is applicable to any type of loading and any type of structure, but it is
costly with respect to both computer CPU and manpower. Ueda et a1 (1986) presented a finite
element procedure based on the Idealized Structural Unit Method (ISUM), which has been