Page 311 - Marine Structural Design

Chapter 14 Offshore Structures Under Impact Loads 287

The characteristic length L, , is a function of the outer diameter, the length of the tube, and the
shape of indenture. In order to get an empirical equation, linear finite shell element analysis
results from Ueda et a1 (1989) and indentation tests conducted by Smith (1983) are analyzed.
A mean value is found to be:
L, = 1.9D (14.3)
More experimental or numerical data are necessary to get a more rational value of the
characteristic length L, .

When the load P is larger than the critical value Po. A permanent indentation will take place,
and the critical value can be determined using a rigid-plastic analysis for a pinch loaded ring
with length L, . The result obtained is:

P, = 20,T2L,/D (14.4)
where o, is the yield stress of the material.
The permanent indentation 6, can be calculated using a semi-empirical equation. Through
energy considerations and curve fitting of experimental data, Ellinas and Walker, 1983
obtained:
6, =D( 37.5~~ )’ (14.5)
T2

The unloading linear deformation 6; can be obtained by multiplying the linear elastic
solution by a coefficient a:
(:I
6; =0.111& - - (14.6)
:,

The coefficient a will be less than or equal to 1.0 depending on the deformation at the
unloading point.
Finally, the local displacement at the load point for a load larger than the Po is calculated as:
s=s; +6, (14.7)

14.2.3 Beam-Column Element for Modeling of the Struck Structure
A finite beam-column element as described in Chapter 12 of this Part is adopted to model the
affected structure.

14.2.4 Computational Procedure
The procedure described above has been implemented into the computer program SANDY
(Bai, 1991), three types of loads can be applied, to the simulated model, to obtain a collision
analysis of the affected structure.
Impact loads are applied at the node points and/or spatially distributed over the finite elements.
The time variation of these loads is given as input data before initiating a calculation. This
type of loading is used in Examples 14.1-14.3.