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*RAO

Purpose

To specify Response Amplitude Operators for a vessel.

Theory

Refer to High Frequency RAO Motions for further information on this feature.

Keyword Format

Block of data consisting of a vessel name followed by RAO data. The data may be explicitly defined, or imported from a file which is generated by an external program.

Explicitly Defined RAO Data

VESSEL=Vessel Name

[FORMAT=Format Name] [, LAYOUT=Layout Name]

Block of RAO Data

or

VESSEL=Vessel Name

[FORMAT=Format Name] [, LAYOUT=Layout Name]

FILE=File Name

The Vessel Name specified must be defined under the *VESSEL keyword. An optional line can be added on the second line, referencing an RAO conversion, and/or defining the layout of the RAO data. Data for any conversion format referenced here must be input under *RAO FORMAT. Format Name cannot be MCS, AQWA, WAMIT, MOSES or ORCAFLEX – these are predefined formats. Layout Name can be either MCS or LINE, defaulting to MCS if omitted. The next line can either specify a File Name (file path of a file containining the RAO data), or is the first of many lines defining RAO data in the keyword file directly.

Flexcom supports two different schemes for laying out RAO data, namely the MCS layout and the Line layout. The MCS layout begins with a single line defining the incident wave heading at which the RAOs are being defined. This is followed by a block of three lines specifying the incident wave frequency and the relevant RAO amplitudes and phases for this heading and wave frequency, as shown below. Note that entries shown below in italics should be replaced with their actual numeric values in the RAO file.

HEADING=Wave Heading

Wave Frequency

Heave RAO, Surge RAO, Sway RAO, Yaw RAO, Roll RAO, Pitch RAO

Heave Phase, Surge Phase, Sway Phase, Yaw Phase, Roll Phase, Pitch Phase

The block of three lines specifying the incident wave frequency, RAOs and phases are repeated as often as necessary until the RAOs are defined over the required range of wave frequencies. To define RAOs at more incident wave headings, you simply repeat the HEADING= line for the new wave heading, and the RAO data for this wave heading is specified as before. If RAOs are being defined for more than one incident wave heading, then they must also be defined for more than one frequency at each wave heading, and the number of frequencies at which the RAOs are defined is the same for each heading. If the RAOs are independent of the incident wave heading, then the HEADING= line should be omitted.

The Line layout is very similar to the standard MCS layout, with the exception that the RAO and phase data for a particular heading and frequency appear on a single line, the format of which is shown below. Note that, for clarity, the data shown below is split over a number of lines, but is in reality specified on a single line of the RAO file.

Wave Heading, Wave Frequency, Heave RAO, Heave Phase, Surge RAO, Surge Phase, 

Sway RAO, Sway Phase, Yaw RAO, Yaw Phase, Roll RAO, Roll Phase, Pitch RAO, 

Pitch Phase

This line is repeated for every wave frequency and every wave heading for which RAOs are being defined. As with the MCS layout, if RAOs are being defined at more than one incident wave heading, then they must also be defined at more than one frequency at each wave heading, and the number of frequencies at which the RAOs are defined is the same for each heading. So, for example, if RAOs were to be defined at three wave headings and ten frequencies per heading, then 30 lines of data would be required to specify the RAOs using the Line layout. If the same wave heading is specified on all lines, then Flexcom assumes that the RAOs are independent of wave heading. Note also that the order in which the lines of RAO data appear is not important. Flexcom automatically sorts the RAO data by heading and frequency.

This concludes the specification for explicitly specified RAO data. The entire block of data can then be repeated to specify force RAO data for subsequent vessels if required.

Imported RAO Data

VESSEL=Vessel Name

PROGRAM=Program Name

FILE=File Name

[UNITS=Unit Type]

The Vessel Name specified here must be defined under the *VESSEL keyword. The next two lines specify the name of the external program from which the RAO data originates and the path of the file containing the RAO data. Program Name may be AQWA, WAMIT, MOSES or ORCAFLEX. This is followed by an optional line which indicates the units used in this RAO file. Unit Type may be METRIC or IMPERIAL.

This concludes the specification for externally imported RAO data. The entire block of data can then be repeated to specify force RAO data for subsequent vessels if required.

Table Input

RAO Data – MCS Layout

RAO Data – Line Layout

RAO Data – External Import

Notes

(a)Flexcom uses linear interpolation to calculate RAOs and phase angles at wave headings and frequencies intermediate to those in the RAO file. Outside of the range of user-specified headings and frequencies, RAOs and phase angles are assumed to be zero, so it is important to ensure you cover the full range of conditions likely to be encountered in an analysis when inputting the RAO data.

(b)Wave heading is defined as the angle between the direction of approach of a wave harmonic incident on the vessel and the local surge axis. However, please note that in strict mathematical terms the incident wave heading is the angle between the wave direction drawn at the vessel reference point and the negative direction of the local surge axis.

(c)Phase angles are specified in degrees, and represent a phase lag or lead relative to the wave at the vessel reference point. A positive phase angle denotes a phase lag relative to the incident wave harmonic.

(d)Flexcom will attempt to ascertain the unit system from the external file, but as this is not always possible, you are advised to explicitly state the unit system in order to avoid any possible ambiguity.