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*Force RAO

Purpose

To specify force RAOs for a floating body.

Theory

Refer to First-Order Wave Loads and Force RAOs for further information on this feature.

Keyword Overview

Data may defined explicitly within the keyword file itself, or defined separately in an external file which is simply referenced using a FILE= entry. The latter approach is highly recommended as it reduces unnecessary clutter in the keyword file, and also helps to improve user interface performance and enhanced user experience.

Data Specified in External File

Keyword Format

Line defining floating body name:

FLOATING BODY=Floating Body Name

Line defining name of external file which contains force RAO data.

FILE=File Name

File Name should include the entire path of the file including its extension. If a file name or any part of its path contains spaces then it should be enclosed in double quotation marks.

Table Input

Notes

(a)Refer to the following sections for further information regarding the required format of data within the external file.

Data Specified Explicitly within Keyword File

Keyword Format

Block of data consisting of floating body name followed by force RAO data. An optional line indicates the position of the floating body reference point to be used in calculating the wave forces. If a rolling mean is to be used, this line must be followed by a line specifying the number of time steps. Following this is an optional line defining the layout of the RAO data. This is then followed by blocks of lines defining the actual RAO data. The format in which the RAO data is specified depends on the layout being used. The entire block of data can then be repeated to specify force RAO data for second and subsequent floating bodies.

Line to define floating body name:

FLOATING BODY=Floating Body Name

Line defining the layout of the RAO data.

[LAYOUT=Layout Name]

Optional line defining position of the reference point to be used in calculating wave forces:

[POSITION=ORIGINAL/INSTANTANEOUS/MEAN]

Optional line defining number of time steps, relevant to POSITION=MEAN only:

[TIMESTEPS=Number]

Optional line specifying whether or not a phase shift is to be included due to the position of the RAO reference point away from the global origin:

[WAVELENGTH PHASE SHIFT=INCLUDE/EXCLUDE]

Blocks of lines defining force RAO data. Flexcom supports two different schemes for laying out the data in the RAO file, 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.

Data for any Floating Body Name defined here must be input under *FLOATING BODY. Layout Name can be either MCS or LINE, defaulting to MCS if omitted.

Table Input - MCS Layout

Table Input - Line Layout

Notes

(a)Flexcom supports two different schemes for laying out the data in the RAO file, namely the MCS layout and the Line layout. This former is the standard layout traditionally used by Flexcom. The Line layout is a more recent addition to the software, and it represents a more general layout that, for example, simplifies copying and pasting RAO data from spreadsheet programs.

(b)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.

(c)Wave heading is defined as the angle between the direction of approach of a wave harmonic incident on the floating body and the local surge axis (refer to Definition of Wave Heading used with RAO Data).

(d)The reference point position should be set to Original in the current version of Flexcom.

•Using the instantaneous position can introduce higher order effects which are undesirable and inconsistent with the diffraction-radiation theory. Experience suggests that it can lead to excessive mean surge motions. So using the original position is preferable.  

•One complication to be aware of, however, is that in random sea analyses, the platform may exhibit both high and low frequency movements (assuming it is subject to low frequency second-order forces). Strictly speaking, the first order forces should be based on the instantaneous position based on the second order response only (i.e., excluding first order effects), however it is not trivial to distinguish the instantaneous first and second order response in a time domain analysis (we plan to introduce a low-pass filtering system in a future version). For this reason you are advised to use the original position for the computation of first order forces, except when second order response is likely to be significant in which case a sensitivity to using the instantaneous position should be performed.

(e)When computing the phase angle of the applied forces, Flexcom automatically accounts for the physical separation between the floating body and the global origin. So in addition to the RAO phase angles, an addition ‘ks’ phase shift term is added, where k is the wave number of the regular wave (or wave harmonic in random seas), and s is the horizontal distance from the global X axis to the RAO reference point in the direction of wave propagation. Refer to First Order Wave Loads for further details. If you would like to suppress this additional phase shift term, for example if you believe it has already been accounted for during the derivation of the RAO phase angles, then you can use the Exclude option here.