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Vessel response reports |
The vessel response form can be opened from the results menu or with the report vessel response button on the vessel data form. Generation of vessel response reports can also be automated through the batch script, or from the OrcaFlex programming interface. From this form you may report displacement RAOs and report spectral values of the vessel's response to waves, based on its vessel type's displacement RAOs.
| Note: | The reported RAOs are first order RAOs calculated using small-angle theory to transform the RAOs from the vessel RAO origin to the points specified on the vessel response form. These results are accurate for small-angle rotational motion, but if the vessel undergoes large-angle rotational motion it will exhibit a nonlinear higher-order response that can only be approximated by RAOs. More accurate results can be obtained in this case by running a simulation, which does not use the small-angle approximation, and this can often be done very quickly by removing from the model all other objects that do not affect the motion of the vessel. |
You list one or more vessel-relative wave directions, and one or more output points on the vessel, for which you wish the RAOs and spectral response to be reported.
A direction of ~ means that the direction of the primary wave direction (that of the first wave train), relative to the vessel, is used.
The output points are specified by giving their coordinates relative to vessel axes.
The report RAOs button opens a spreadsheet window with a separate worksheet for each requested direction, containing the RAOs for all the requested output points, derived from the vessel type displacement RAOs.
The reported RAOs allow for the following effects:
RAOs are given for the position, velocity and acceleration of the output points, for all 6 degrees of freedom and for Z above wave.
The RAOs are reported at the original wave periods defining the RAOs (subject to Froude scaling) plus, if appropriate, the regular wave period prevailing in the model.
You can choose the phase origin to use for reporting phases, the options being
The translational RAOs depend on the position of the output point. The rotational RAO amplitudes do not depend on the output point, since the roll, pitch and yaw of a vessel are the same everywhere on it; the rotational RAO phases depend on the output point, but only if the phase origin is each point.
You can choose to have the RAOs reported in the same RAO conventions as defined for this vessel's type or in the default Orcina conventions.
This is the amplitude of the heave motion of the vessel relative to the water surface. It may be useful in evaluating whether an object at some point on the vessel will contact the water surface and, if so, with what relative velocity.
| Warning: | Z above wave does not take account of wave surface disturbance due to the presence of the vessel. |
The spectral response report contains a separate worksheet for each (wavetrain, direction) combination of the given directions and random wave trains. Each worksheet contains tables of spectral values for position, velocity and acceleration, for all 6 degrees of freedom and for Z above wave. Separate tables are given for each output point.
| Note: | The spectral response report is not available if the environment has no random wave trains. |
The following spectral values are reported:
| Note: | The significant and maximum values are reported as single amplitudes, i.e. the motion is +/- the value reported. |
| Warnings: | Wave directional spreading spectra are not taken into account. The calculation assumes that all the spectral energy is in the given wave direction for each wave train. |
| The standard formulae used to calculate the spectral values (see below) are based on the further assumptions that the spectrum is narrow-banded and Gaussian, and that the extremes are Rayleigh distributed. They are also based purely on small-amplitude linear theory. | |
| The storm duration should be short-term, i.e. of the order of hours rather than days or years, since the sea state, as represented by $(\Hs,\Tz)$, is assumed to remain constant over this duration. |
These spectral values are calculated as follows:
The spectrum of the wave train is combined with the vessel response specified by its RAOs, linearly interpolated as necessary, to obtain the response spectrum for each degree of freedom. This requires that the zero period RAO is present and has zero amplitude – OrcaFlex will override the given values if this is not the case. The zeroth and second spectral moments, $m_0$ and $m_2$, of the response spectrum are calculated, and the spectral values are then given by \begin{align} \text{significant amplitude, } A_s &= 2 m_0^{1/2} \\ \text{average period, } \Tz &= (m_0/m_2)^{1/2} \\ \text{max amplitude} &= A_\mathrm{s} \left[\tfrac12 \ln(60^2d/\Tz)\right]^{1/2} \end{align} where $A_\mathrm{s}$ is the significant amplitude, $d$ is the given duration (in hours) and $\Tz$ is the average period (in seconds). See, for example, Faltinsen, pages 24-27, or Ochi, pages 151-152.
| Notes: | For Z above wave, no spectral values can be reported for acceleration, and only the significant amplitude can be reported for velocity. This is because the RAO of Z above wave does not decay to zero as wave period goes to zero, so some of the spectral moment integrals do not converge. For velocity of Z above wave you could perhaps estimate the expected maximum amplitude by assuming that the ratio of maximum amplitude to significant amplitude is roughly the same for velocity as for position. |
| All these reports are based on the vessel type displacement RAOs. Wave load RAO data are not used. |