OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 152
6.1.5 In vibration analysis one may use a coarse structural model if only the first few 'eigenvalues' are of
interest, see [6.2.2]. Nevertheless, a reasonable representation of structural mass and stiffness is crucial.
6.1.6 If a large number of 'eigenfrequencies' are required, one shall apply a detailed description of the structure.
6.1.7 Due account should be taken of fluid-structure interaction effects where these are significant. These may
include resonance between structural response and wave excitation frequencies, or more complex, highfrequency
vibration phenomena (ringing and springing) caused by non-linear wave loads. In some cases of
fluid-structure interaction it may be necessary to perform a dynamic analysis of the coupled fluid-structure
system.
6.1.8 In case of accidental loads, such as explosions, dynamic effects should be considered carefully.
6.1.9 The dependence of the material properties on strain rate should be taken into account, see Sec.4 [3.10].
Guidance note:
Although static material properties may yield conservative predictions of displacements, a strength assessment based
on static properties is not necessarily conservative since both the material strength and the material stiffness may be
enhanced at high strain rates. The higher stiffness may increase the induced stress so that the benefit of the increase
in the material strength may be lost. Furthermore, ductile materials often become brittle at high rates. Thus, the extra
margin provided by ductile behaviour may be destroyed.
There is a lack of sophisticated material models taking the rate dependent behaviour into consideration.
6.2 Dynamics and finite element analysis
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6.2.1 For analysis including dynamic loads with frequencies of interest up to ω cr. , the mesh shall be able to
accurately represent modes associated with frequencies up to 3ω cr. , and a mode superposition analysis shall
include frequencies up to about 3ω cr. .
6.2.2 For 'eigenvalue' analysis, there should be 4 or more times as many degrees of freedom as 'eigenvalues'
to be calculated.
6.2.3 For direct integration methods, the following points should be ensured:
— the time step Dt should be approximately 0.3/ω cr. or less, and should provide numerical stability if the
integration method is conditionally stable
— there should be a match between the type of algorithm and the mass matrix
— abrupt changes in element size should be avoided, thereby avoiding spurious wave reflection and numerical
noise.
7 Impact response
7.1 Testing
7.1.1 Impact test requirements shall be defined since there are no well-established calculation methods today.
7.1.2 Component testing (see Sec.10) should be carried out in order to evaluate the impact characteristics of
the structure/component.
8 Thermal stresses
8.1 General
8.1.1 Changes in temperature from the environment resulting in dimensional changes of the body shall be
taken in account. The general thermal strains, e i , can be expressed as:
e = α ∆T
where α i is the thermal expansion coefficients. Temperature is denoted by T.
i
8.1.2 Residual strains shall be calculated against the reference temperature for which α i was determined. It is
usually the curing temperature.
8.1.3 Accordingly, the stress-strain relations shall be modified to account for the stress free environmentally
induced expansion strains as follows:
{ ε}
= [ S ]{ σ } + { e}
i
DET NORSKE VERITAS AS