RAO Conversion

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RAO Conversion

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The RAO Conversion component is used to define the various conventions that are used in the specification of vessel RAO data.


This component is used in conjunction with the Vessel Motion component. PipeLay uses the conventions specified in the RAO Conversion component to convert the data in a third-party RAO file to the standard Wood Group RAO format. Note that it is not necessary to use an RAO Conversion component if the data in the RAO file is already presented in the standard Wood Group RAO format. The component view for a new RAO Conversion component is shown in the figure above. The default selections for each of the drop-down lists here correspond to the standard Wood Group RAO Convention.

You define the required RAO conversion using a series of drop-down lists, and the inputs are largely self-explanatory. The inputs are divided into four main categories, as follows:

1.The Zero Degree Wave Heading section allows you to define the conventions used with regard to wave heading. Specifically, you specify how the zero degree incident wave heading is oriented relative to the vessel, and whether or not the specified RAO values are to be mirrored around one or more of the vessel axes. You do this by clicking the Specify dialog button.

2.The Units section allows you to associate units with the RAO data. Specifically, you may specify units for the incident wave (frequency, period or circular frequency), incident wave headings (degrees or radians), phase angles (degrees or radians) and rotational RAO amplitudes (degrees or radians).

3.The Sign Conventions section allows you to define various sign conventions used in the specification of the RAO data. Specifically, you may specify whether positive phase angles represent a phase lag or a phase lead, whether positive wave headings are measured clockwise or anti-clockwise from the zero degree heading, and whether a positive wave elevation is up or down.

4.The Positive Directions section allows you to define the sign conventions regarding the responses in each degree of freedom. For example, you can specify whether a positive heave represents vessel motion upwards or downwards, and so on.


Validation of Converted RAOs

As mentioned in the ‘Vessel Component’ article, there are several ways by which RAOs may be inspected and validated. Specifically, PipeLay generates plots of the converted RAOs in the standard Wood Group RAO format if you do either of the following:

1.You select Yes in the Plot RAOs drop-down list on the Vessel Motion component, in which case the plots are generated when an analysis is run; the plotted RAOs are then accessible via the Analysis component Results tab.

2.You choose Plot RAOs from the Vessel Motion section of the Home tab, in which case the plots are generated and presented immediately.

It is recommended that the plots of converted RAOs be carefully scrutinised to ensure that the RAO conversion parameters have been specified correctly. It is especially advisable to use the second option above, so that any errors can be identified and corrected before an entire analysis is run.

PipeLay includes a graphical tool called the RAO Checking Module for quickly validating RAOs. This tool allows you to view the vessel response to a nominal unit amplitude wave.


Wood Group RAO ConveNTion

The default Wood Group RAO units and sign conventions are as follows. Note that these are provided as default input on newly created RAO Conversion components.

RAO Units Convention

Units of Wave Incidence/Heading


Units of Frequency


RAO Rotational DOF Units


RAO Phase Units


RAO Sign Convention

Zero Wave Incidence/Heading

Head Sea

Positive Wave Incidence/Heading

Anti-Clockwise in Plan from Bow

Positive Water Surface Elevation


Positive Phase


Heave Positive


Surge Positive


Sway Positive


Yaw Positive

Bow to Port

Roll Positive

Port Up

Pitch Positive

Bow Down


Two additional articles describe the RAO Conversion Data Inputs and RAO Conversion Best Practice.