OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 150
Guidance note:
Some of these difficulties can be detected by error tests in the coding, such as a test for the condition number of the
structure stiffness matrix or a test for diagonal decay during equation solving. Such tests are usually made posterior
rather than prior.
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5.2.11 Need for mesh refinement is usually indicated by visual inspection of stress discontinuities in the stress
bands. Analogous numerical indices are also coded.
5.2.12 For local analysis, a local mesh refinement shall be used. In such an analysis, the original mesh is stiffer
than the refined mesh. When the portion of the mesh that contains the refined mesh is analysed separately, a
correction shall be made so the boundary displacements to be imposed on the local mesh are consistent with
the mesh refinement.
5.2.13 For non-linear problems, the following special considerations shall be taken into account:
— the analyst shall make several trial runs in order to discover and remove any mistake
— solution strategy shall be guided by what is learned from the previous attempts
— the analyst shall start with a simple model, possibly the linear form of the problem, and then add the nonlinearity
one by one.
5.2.14 Computed results shall be checked for self-consistency and compared with, for example, approximate
analytical results, experimental data, text-book and handbook cases, preceding numerical analysis of similar
problems and results predicted for the same problem by another program. If disagreements appear, then the
reason for the discrepancy shall be sought, and the amount of disagreement adequately clarified.
5.2.15 The analyst shall beware the following aspects:
— for vibrations, buckling or non-linear analysis, symmetric geometry and loads shall be used with care since
in such problems symmetric response is not guaranteed. Unless symmetry is known to prevail, it shall not
be imposed by choice of boundary conditions
— for crack analysis, a quarter point element can be too large or too small, thereby possibly making results
from mesh refinement worse
— the wrong choice of elements may display a dependence on Poison’s ratio in problems that shall be
independent of Poisson’s ratio
— if plane elements are warped, so that the nodes of the elements are not co-planar, results may be erratic and
very sensitive to changes in mesh
— imperfections of load, geometry, supports and mesh may be far more important in a buckling problem than
in problems involving only linear response.
5.2.16 In the context of finite element analysis (FEA) of laminate structures (one of) the following element
types should be applied:
— layered shell elements with orthotropic material properties for each layer (for in-plane 2-D analysis, see
[1.3.4])
— solid elements with orthotropic material properties (for 3-D and through thickness 2-D analysis, see
[1.3.4]).
Guidance note:
There are two options for the solid elements: The modelling may be performed with (at least) two solid elements
through the thickness of each ply. Alternatively, one may apply layered solid elements where the thickness of a single
element includes two or more plies.
5.3 Software requirements
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5.3.1 Selection of finite element software package shall be based on the followings:
— software availability
— availability of qualified personnel having experience with the software and type of analysis to be carried out
— necessary model size
— analysis options required
— validated software for intended analysis.
5.3.2 Useful options for the analysis of composite structures include:
— layered solid elements with orthotropic and anisotropic material behaviour
— layered shell elements
— solid elements with suitable core shear deformation models (for analysis of sandwich structures)
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