Inertial Effects |
 
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Inertial Effects |
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An option is provided to specify that the internal fluid contribution to the structure axial inertia is to be omitted in a dynamic analysis. This modelling capability is specifically intended for the analysis of drilling risers in emergency disconnect mode (riser hangoff analysis). By default, when such a riser is specified as being full of internal fluid (typically seawater), Flexcom automatically includes the contribution of the internal fluid to the riser inertia (mass matrix) in both transverse and axial directions. This assumes that the internal fluid was entrapped in the riser and forced to move with it axially as well as laterally. This may not always be a reasonable assumption; in many cases the disconnected riser is open at its base so that fluid can move in and out of the riser instead of being constrained to move axially with it. The axial inertia omission option is intended for this application, to exclude internal fluid from the riser axial inertia in an emergency disconnect analysis. This option can also be considered for use in rigid riser installation analyses, where the riser is allowed to free flood as it is being run.
In a similar fashion, an option is provided to exclude the internal fluid contribution from the structural lateral inertia. Although the option is of very limited use, it may be desirable in certain circumstances. For instance, in an emergency disconnect analysis, it may be argued that the internal fluid does not contribute to the lateral inertia, and this option allows you to suppress this contribution, and consequently to quantify its effect on the overall solution.
•*INTERNAL FLUID is used to define the properties of an internal fluid. Fluid inertial effects are included by default, but the AXIAL_INERTIA= and LATERAL_INERTIA= options allow you to suppress the effects of fluid inertia in the axial and lateral directions, respectively.
If you would like to see an example of how this keyword is used in practice, refer to the emergency disconnect simulation in A01 - Deepwater Drilling Riser.