Point Masses and Point Buoys |
 
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Point Masses and Point Buoys |
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A point mass is used to model a concentrated mass or rotational inertia. While a point buoy is a reasonably similar entity, it is more complex in a number of respects:
(i)The buoy is characterised by a weight in air and a total buoyancy force, and the buoyancy force is activated/deactivated depending on the instantaneous location of the buoy with respect to the ambient water surface.
(ii) Different rotational inertias can be specified about the individual local buoy x-, y- and z-axes.
(iii) Hydrodynamic loading on the buoy can be modelled, and comprehensive specification options (similar to that of the Buoy Structure facility) are provided.
The point buoy facility might typically be used to model the effects of a subsea buoy in a flexible riser model. It is intended as an alternative to modelling the buoy explicitly using beam elements – a point buoy is specified at a node of the discretisation, but is not an integral part of the structure model.
Typically the hydrodynamic properties of a subsea buoy are very different in the vertical and horizontal directions. For this reason the buoy hydrodynamic coefficients are normally specified separately for each of three directions (one vertical, two horizontal). Furthermore, for each direction the data is often specified as three products, namely i) CdAd (drag coefficient by frontal area), for the hydrodynamic loading due to relative fluid/buoy velocity; ii) CmVin (inertia coefficient by reference volume), for the hydrodynamic loading due to water particle acceleration; and iii) CaVin (added mass coefficient by reference volume), for the hydrodynamic loading due to buoy acceleration. This means a subsea buoy may be characterised by nine hydrodynamic coefficients or products.
Additionally you can also define rotational hydrodynamic coefficients for the point buoy. Similar to the translational terms, coefficients for each direction are specified as three products, namely i) CdDMA (product of drag coefficient and drag moment of area); ii) CmHI (product of inertia coefficient and hydrodynamic inertia); and iii) CaHI (product of added mass coefficient and hydrodynamic inertia). Rotational hydrodynamics are applied with respect to the global axis system by default, but a facility is also provided to define a local axis system and use this as a reference for the rotational hydrodynamics.
•*MASS is used to specify a point mass or a point rotary inertia.
•*POINT BUOY is used to define point buoys and their associated hydrodynamic properties.
If you would like to see an example of how these keywords are used in practice, refer to A01 - Deepwater Drilling Riser (*MASS) and A02 - Spar Production Riser (*POINT BUOY).