|
Abstract:
|
Warships are generally sleek, slender with V shaped sections and block
coefficient below 0.5, compared to fuller forms and higher values for commercial
ships. They normally operate in the higher Froude number regime, and the
hydrodynamic design is primarily aimed at achieving higher speeds with the
minimum power. Therefore the structural design and analysis methods are
different from those for commercial ships. Certain design guidelines have been
given in documents like Naval Engineering Standards and one of the new
developments in this regard is the introduction of classification society rules for
the design of warships.The marine environment imposes subjective and objective uncertainties
on ship structure. The uncertainties in loads, material properties etc.,. make
reliable predictions of ship structural response a difficult task. Strength, stiffness
and durability criteria for warship structures can be established by investigations
on elastic analysis, ultimate strength analysis and reliability analysis. For analysis
of complicated warship structures, special means and valid approximations are
required.Preliminary structural design of a frigate size ship has been carried out . A
finite element model of the hold model, representative of the complexities in the
geometric configuration has been created using the finite element software NISA.
Two other models representing the geometry to a limited extent also have been
created —- one with two transverse frames and the attached plating alongwith the
longitudinal members and the other representing the plating and longitudinal
stiffeners between two transverse frames. Linear static analysis of the three models have been carried out and each one with three different boundary
conditions. The structural responses have been checked for deflections and
stresses against the permissible values. The structure has been found adequate
in all the cases. The stresses and deflections predicted by the frame model are
comparable with those of the hold model. But no such comparison has been
realized for the interstiffener plating model with the other two models.Progressive collapse analyses of the models have been conducted for the
three boundary conditions, considering geometric nonlinearity and then
combined geometric and material nonlinearity for the hold and the frame models.
von Mises — lllyushin yield criteria with elastic-perfectly plastic stress-strain curve
has been chosen. ln each case, P-Delta curves have been generated and the
ultimate load causing failure (ultimate load factor) has been identified as a
multiple of the design load specified by NES.Reliability analysis of the hull module under combined geometric and
material nonlinearities have been conducted. The Young's Modulus and the shell
thickness have been chosen as the variables. Randomly generated values have
been used in the analysis. First Order Second Moment has been used to predict
the reliability index and thereafter, the probability of failure. The values have
been compared against standard values published in literature. |