Data: Spring/dampers

Spring/dampers are simple connections linking two points in the model, for example a body to a point fixed in space, or one body to another. They pull the two points together, or hold them apart, with a force that depends linearly on their relative positions and velocities. They have no mass or hydrodynamic loading and simply apply an equal and opposite force to the two points.

Note:

Spring/dampers can be used to improve the displacement RAOs and results which depend on them, such as QTFs and sea state RAOs. Spring/dampers do not affect results for load RAOs, added mass or damping.

Spring/dampers combine independent spring and damping effects:

The spring can take both compression and tension, and has a linear relationship between length and force.
The damper has a linear relationship between velocity and force.

Spring/damper data

Unstretched length

The unstretched length $l_0$ of the spring/damper.

Stiffness

The spring tension $T$ is given by \begin{equation} T = k(l-l_0) \end{equation} where $k$ is the stiffness, $l$ is the stretched length between the two ends and $l_0$ the unstretched length.

Note:

The spring is not slack if $l < l_0$ but instead it is in compression, corresponding to a negative value of $T$.

Damping

The damper exerts an additional tension of \begin{equation} c\, \ODt{l} \end{equation} where $c$ is the specified damping value.

Pen

Drawing data for the spring/damper in the mesh view.

Connect to object

Each end of the spring/damper can be fixed or connected to a diffraction body.

Position

The position defines the location of each end of the spring/damper:

If the end is fixed, values should be given relative to the global coordinates $\GXYZ$.
If the end is connected to a body, values should be given relative to the owner's mesh coordinates $\Mxyz$.

Note:

You can import the data for spring/dampers into OrcaFlex (as links) by opening the results file (.owr) in OrcaFlex. Spring/damper data cannot be imported using the import hydrodynamic data process in OrcaFlex that imports data into a vessel type in an existing OrcaFlex model.