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Compound object properties report |
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Compound object properties report |
The compound object properties report is available from the popup menu on the model browser, and reports properties for the selected objects or groups. In addition, the report is included in the properties report for an individual line.
The reported properties are based on an assumption that the objects are connected together in such a way as to move as a single rigid body. No attempt is made by OrcaFlex to verify whether or not that assumption is valid. It is your responsibility to ensure that the selected objects are appropriately chosen.
A number of object types do not have mass or volume: links, winches and constraints. These objects can be included in the compound object properties report, but contribute nothing to the properties. Shape objects also have no mass, but do have well-defined volume. However, since that volume is never used to generate a buoyancy force, shape objects also contribute nothing to the reported compound properties.
Vessel objects have well-defined mass, but their buoyancy is treated somewhat differently from other object types. Consequently, compound properties are not reported if a vessel is included in the selected objects. Line stiffeners are implemented in a way that makes it difficult to calculate compound properties so, again, compound properties are not reported if a line stiffener is included in the selected objects.
| Notes: | Centre of mass and mass moments of inertia are reported using the instantaneous object positions (in the current model state). These instantaneous positions can change from reset state, to static state, and during dynamics, and the values reported here reflect those changes. |
| Wetted volume properties are also reported using instantaneous positions, and can account for partial submergence. However, other properties that are related to volume and buoyancy are reported assuming complete submergence. | |
| Inertia contributions due to flooding of spar buoy or towed fish hollow cylinders are not accounted for in these values. | |
| Compression due to bulk modulus effects is not accounted for in these values. |
The following properties are reported:
The force due to gravity acting on the mass of the objects.
The weight of water displaced by the volume of the objects, based on a nominal water density.
Weight $-$ buoyancy.
The total mass of the objects.
The mass of water displaced by the volume of the objects, based on a nominal water density.
Mass $-$ displaced mass.
The centre of mass of the objects, calculated as \begin{equation} \frac{\displaystyle \sum_i m_i \vec{p}^\mathrm{m}_i}{\displaystyle \sum_i m_i} \end{equation} where $m_i$ and $\vec{p}^\mathrm{m}_i$ are the masses and centres of mass of the objects, respectively.
The centre of mass is reported with respect to the axes of an object selected on the properties form.
The total rotational inertia of the objects, acting about the compound centre of mass. This is reported as a tensor with respect to the axes of the selected object.
The total volume of the objects.
The centre of volume of the objects, calculated as \begin{equation} \frac{\displaystyle \sum_i v_i \vec{p}^\mathrm{v}_i}{\displaystyle \sum_i v_i} \end{equation} where $v_i$ and $\vec{p}^\mathrm{v}_i$ are the volumes and centres of volume of the objects, respectively.
The centre of volume is reported with respect to the axes of an object selected on the properties form.
The total wetted volume of the objects.
The centre of wetted volume of the objects, calculated as \begin{equation} \frac{\displaystyle \sum_i \bar{v}_i \bar{\vec{p}}^\mathrm{v}_i}{\displaystyle \sum_i \bar{v}_i} \end{equation} where $\bar{v}_i$ and $\bar{\vec{p}}^\mathrm{v}_i$ are the wetted volumes and centres of wetted volume of the objects, respectively.
The centre of wetted volume is reported with respect to the axes of an object selected on the properties form.