ssc-367 - Ship Structure Committee

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The effect of modeling complexity on analysis schedule and
cost.
The current state-of-knowledgeprovides us with the tools necessary
to develop and analyze models. The desirable level of modeling
sophistication, different for each structure, is thus determined
based on tradeoffs among some of the variables given above.
The goal of a modeling strategy should always be to achieve
realistically accurate results consistently and without excessive
complexity. The analysis assumptions and the modeling strategy is
very important in minimizing modeling accuracy/error. Most
engineers rely on previouswork and engineeringjudgement to reduce
the level of modeling errors, typically defined as the ratio of
actual-to-predicted results. Such a subjective approach can be
supplemented by statistical methods to define the modeling
uncertainty. The mean value of the modeling error, Xme, is defined
as the “bias.”While the modeling uncertainty is referred to as the
random component of the modeling error. The modeling uncertainty,
given by its coefficient of variation, (C.o.v.)x is meaningful
me
only if sufficient data is available.
5.1.2
Comparison of Structures
A discrete system composed of numerous members and joints (such as
an offshore platform) is modeled as a 3-D space frame. Individual
members of the system are modeled as stick elements, with correct
dimensions (diameter, net length) and hydrodynamic coefficients.
The two basic premises affectingthe accuracy of wave loadings are:
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The hydrodynamicforces are typically computed based on water
particle kinematics along each member centerline. When the
wave length-to-cylinderdiameter ratio is less than about 10,
the wave force computed based on a stick model centerline is
too conservative.
5-2