Vessel mooring stiffness calculation

Mooring stiffness reports are available when the model is in statics complete state.

To show the mooring stiffness report, select one or more vessels in the model browser and click report mooring stiffness from the popup menu. The mooring stiffness is reported in a spreadsheet window. For example:

If a single vessel is selected, then the $6{\times}6$ stiffness matrix is reported with respect to both global and vessel axes, as shown above.

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 force and length units respectively.

If $N$ vessels are selected, then the $6N{\times}6N$ stiffness matrix is reported with respect to global axes. The units of each $6{\times}6$ block are given by the expressions above.

The mooring stiffness includes all stiffness effects from all objects connected to a vessel, but it excludes the hydrostatic stiffness of vessels. Specifically, it excludes the hydrostatic stiffness of the selected vessel and that of any connected vessels. The full system stiffness matrix is calculated on this basis and then reduced to $6N{\times}6N$ by the method of Guyan reduction.

This functionality can be used to produce mooring stiffness matrices for input to external diffraction analyses: build an OrcaFlex model which includes the full mooring system and run the statics calculation, report the mooring stiffness, and export the resultant mooring stiffness matrix. This matrix may now be used as input to the diffraction analysis step that generates displacement RAOs. This explains why OrcaFlex excludes vessel hydrostatic stiffness from its mooring stiffness calculation: the hydrostatic contribution is calculated separately in a diffraction analysis and should not be included in the mooring stiffness matrix passed to the diffraction analysis program.