Vessel types: RAO data checklist

RAO data are notoriously error-prone and difficult to check. It is essential that you know what conventions apply to your data; these may be documented with the data, but sometimes you may have to deduce what they are. You should have answers to all the following questions:

To what point on the vessel do the data apply?

This is the RAO origin and is often the vessel centre of mass, but you need to be sure. If it is not specified check with your data supplier.

To what point on the vessel are the phases relative?

This is the RAO phase origin and is usually, but not always, the same as the RAO origin.

Are the responses in dimensional or RAO form?

RAO form (i.e. for unit wave amplitude) is the most common; data giving dimensional form would have to also give the associated wave amplitudes/heights. OrcaFlex will only accept RAO form.

In what form are the rotational roll, pitch and yaw RAOs?

Units such as degrees/m or radians/m (displacement RAOs), or kN.m/m (wave load RAOs), almost always mean the rotational motions are relative to waves of unit amplitude.

Very rarely, rotational RAO amplitudes are given per unit wave height (i.e. double amplitude) – check your data source. In this case you will have to multiply the RAOs by 2 manually, before entry to OrcaFlex.

Units such as degrees/degree, radians/radian, or no units (displacement RAOs), or kN.m/radian (wave load RAOs), imply rotational RAOs relative to waves of unit steepness or maximum slope.

For long wave periods in deep water, the rotational displacement RAOs in the wave plane (e.g. pitch in head or stern seas) should tend to 1 for RAOs relative to unit maximum slope, or to $\pi$ for RAOs relative to unit steepness.

Are the phases in degrees or radians?

Unless you only have a small amount of data, this should be obvious from the range of phase values.

What directions are positive for surge, sway, heave, roll, pitch and yaw?

Often they are surge positive forward, sway positive to port, heave positive up, but some authors use heave positive downwards. Roll, pitch and yaw are usually positive when clockwise about the positive surge, sway and heave directions.

Most data sources use right-handed axes, but not all. OrcaFlex allows complete generality in its data input, but you must find out how your data are defined.

To what phase time origin are the phases relative?

OrcaFlex allows you to specify that the phases are relative to the time the wave crest, trough, zero up-crossing or zero down-crossing passes the phase origin. The passage of the crest past the RAO origin is the most common phase time origin, but you need to check and tell OrcaFlex – see also phase leads/lags below.

Are the phases leads or lags?

Phase conventions are sometimes documented by giving the formula used to represent the harmonic motion. Commonly used ones are:

• $A\,\cos(\omega t - \phi)$ or $A\,\cos(\phi - \omega t)$ imply that phase $\phi$ is a lag.
• $A\,\cos(\omega t + \phi)$ implies that phase $\phi$ is a lead.

Sine rather than cosine may be used in these formulae, with no effect on whether the phases are leads or lags.