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Data: Constraints |
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Data: Constraints |
| Note: | If multiple bodies are defined then constraint data must be specified for each body. |
The external stiffness matrix can be used to apply additional loads to a body in the calculation of displacement RAOs, additional to the hydrostatic and wave loads that OrcaWave calculates automatically. The entry $\Kext_{ij}$ in the external stiffness matrix gives the constant of proportionality between the $i$ component of the load on the body and the $j$ component of its motion. Hence the units of the $3{\times}3$ blocks of the stiffness matrix are \begin{equation} \nonumber \left[ \begin{matrix} \dfrac{F}{L} & \dfrac{F}{\rad} \\ \dfrac{FL}{L} & \dfrac{FL}{\rad} \end{matrix} \right] \end{equation} where $F$ and $L$ denote the units of force and length, respectively.
| Tip: | If you are modelling a system with mooring lines, or other external loads, appropriate data for the OrcaWave external stiffness matrix can be obtained using the vessel mooring stiffness report in OrcaFlex. |
The external stiffness matrix should be given with respect to axes through the specified origin and parallel to the body axes $\Bxyz$. If the origin for the matrix is user specified, the position of that origin must also be given (in body coordinates $\Bxyz$).
| Note: | The body axes $\Bxyz$ differ from the mesh axes $\Mxyz$ if the body has a non-zero heel or trim angle. |
The external damping matrix can similarly be used to apply additional damping to a body in the calculation of displacement RAOs, additional to the wave damping loads that OrcaWave calculates automatically. The entry $D_{ij}$ in the external damping matrix gives the constant of proportionality between the $i$ component of the load on the body and the $j$ component of its motion. Hence the units of the $3{\times}3$ blocks of the damping matrix are \begin{equation} \nonumber \left[ \begin{matrix} \dfrac{F}{L/T} & \dfrac{F}{\rad/T} \\ \dfrac{FL}{L/T} & \dfrac{FL}{\rad/T} \end{matrix} \right] \end{equation} where $F$, $L$ and $T$ denote the units of force, length and time, respectively.
The external damping matrix should be given with respect to axes through the specified origin and parallel to the body axes $\Bxyz$. If the origin for the matrix is user specified, the position of that origin must also be given (in body coordinates $\Bxyz$).
| Note: | The body axes $\Bxyz$ differ from the mesh axes $\Mxyz$ if the body has a non-zero heel or trim angle. |
If a body is free to move in all six rigid body degrees of freedom, these boxes should all be unchecked.
A checked box means that the body cannot move in the corresponding degree of freedom (translation parallel to, or rotation about, an axis of the body coordinates $\Bxyz$). Displacement RAOs will be zero in the component corresponding to the fixed degree of freedom and, further, other components will be modified since all are coupled via the body's equation of motion.
A body can be free or connected to some other body as a child. The body to which a child is connected is termed the parent. By convention, we describe the child as being connected to the parent, but not the other way around. If a connection is made, the child and parent will move as if rigidly attached to each other – this motion is encapsulated in their displacement RAO results.
It is possible to have chains of connections, in which one body is a child to a parent which in turn is child to another parent, and so on. Eventually, the chain will terminate at some ultimate parent which is free. The order in which the children are connected does not affect the solution.
OrcaWave will not allow chains of connections to contain cycles, in which one body is an indirect child of itself. This will trigger a validation error. However, by changing the order of connections, it is always possible to construct an equivalent system that does not contain any such cycles.
| Notes: | OrcaWave automatically accounts for the connection load between parent and child. You do not need to provide any external stiffness matrix data as a result of specifying a connection. |
| If you are modelling a system which combines connected bodies and mooring lines, the OrcaFlex vessel mooring stiffness report will transfer all stiffness effects onto the ultimate parent. Therefore the external stiffness matrix is usually zero for a child body. | |
| You can import connection data into OrcaFlex by opening a results file (.owr) in OrcaFlex. It cannot be imported using the import hydrodynamic data process in OrcaFlex that imports data into a vessel type in an existing OrcaFlex model. |
It is common for potential theory to overestimate the roll motion of ships. This is because viscous effects that are absent from potential theory, such as vortex shedding and skin friction, can contribute significant roll damping. The critical damping coefficient is often used as a convenient measure for the various damping effects.
| Warning: | In the published literature, the use of critical damping as a convenient measure of roll damping is generally restricted to conventional, freely floating, ships. |
If you specify a target percent of critical, the specified external damping matrix will be used and, in addition, OrcaWave will include extra roll damping to meet the target. The extra roll damping is determined such that the total damping, including both radiation damping and all external damping, meets the specified target for the wave period that excites the largest roll amplitude.
| Notes: | Since radiation damping is a function of wave period, the target percentage will be met exactly for only one wave period. |
| If the roll damping provided by wave radiation and the external damping matrix already exceeds the target percentage, the extra roll damping will be zero. OrcaWave will not reduce the roll damping in this circumstance. |