Page 353 - Practical Design Ships and Floating Structures

328

The stress components to be combined are the notch stresses, i.e. stresses including stress
concentration factor of a structural detail depend on weld geometry, structural geometry and type of
loading. Geometric stress is defined as a linear extrapolation of surface stresses at a distance 0.5t and
1.5t from the weld toe in case the thickness of parent metal being t. Transverse direction stress is
similar to the principal stress in hopper structure. So, transverse direction stress is applied to evaluate
fatigue life of hopper structure. The local stress resulted from hull girder bending moment is added to
the local stress calculated from the sub-model analysis. Normal axial stress calculated by beam theory
using hull girder bending moment is modified by considering geometry stress concentration induced
by local geometry configuration of the sub model.
As a result, hopper knuckle of the new VLCC has sufficient fatigue life though the fatigue life is a
little bit short in comparison with that of original VLCC.

5 EVALUATION OF COLLISION STRENGTH
Double hull tankers should be designed to have suficient energy absorption capacity to reduce oil
spillage in case of collision accident [Jang, et al., 19991. Therefore, the hull resistance of subject VLCC
against collision is investigated in the viewpoint of energy absorption capacity and resulting damage.
Contribution of each structural component such as side shell, side longitudinal bulkhead, stringer, web
frame is investigated and energy absorption capacity and the amount of resulting damage are
investigated according to the variation of ship speed with various plastic strain rate.
In this study, two types of VLCC are considered as struck ship and a 156,000 DWT oil tanker is
considered as striking ship. The first struck ship is the new VLCC with wide web frame space and the
other is original VLCC.
In each scenario, striking ship with ballast condition, moving ahead at a speed of 10 knots, collides
with the struck ship. The struck ship is stationary and in full load condition. The position of collision is
the middle of two successive web frames and struck ship’s longitudinal center line is normal to the
direction of motion of striking ship. The rolling, yawing and swaying of the striking ship are neglected.

5.1 Numerical Analysis
The explicit method to integrate the governing dynamic equations of a system with respect to time is
used to simulate ship hull structural behavior in collision. Lagrange finite element method is also used
by using a computer program MSCDYTRAN [MSC, 19961. The central difference method is used
to perform this integration. As lumped mass is used, the mass matrix becomes a diagonal matrix and
the equation of motion of each degree of freedom becomes independent and no matrix decomposition
is necessary to obtain accelerations.
Plating and webs of web h e and longitudinal are modeled using Belytschko - Tsai shell elements
and flange and small stiffeners are modeled using rod elements. The struck ship is modeled as
deformable structures and striking ship is modeled as rigid bodies. In collision analysis, tearing of
welding lines is important in areas where large damage occurs. Therefore, idealization of weld lines
considering failure may be necessary in order to accurately predict collision damage. In this study,
breakable joints are used to idealize welded connections.
Initially, as loads are applied, the two nodal points of each joint move together. When stresses acting
on the weld satisfy a predefined failure condition (weld ultimate strength), the joint breaks up and
internal forces are unloaded to the surrounding structure. Then each of two nodes may move
independently. The weld failure of striking ship bow structure is not modeled because primary interest
is focused on the behavior of the side structure of the struck ship.
5.2 Results

Collision force with penetration is shown in Fig. 4. The penetration is defined as the change of the