Direction and Heading Conventions |
 
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Direction and Heading Conventions |
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The loads are calculated in a vessel axis system defined with its origin at the vessel centre of gravity, G, as shown in the figure below. The vessel y axis defines the local surge axis, and the z axis the local sway axis. The instantaneous vessel orientation, θ, is measured positive anticlockwise from the global Y axis.
Definition of Wind and Current Directions
Vessels and Vessel Motions provided the Flexcom definition of wave heading appropriate to the input of first order vessel motion (or displacement) RAOs. As outlined in Calculating Vessel RAO Response, heading in this context is defined as the angle (positive anticlockwise) from the negative direction of the vessel local surge axis to the wave direction drawn at the vessel reference point. This is shown in the first figure below. In defining floating body vessel data, this definition of wave heading also applies to the input of first order force RAOs.
The definition of wave heading (or strictly speaking the relative floating body/wave heading) used in conjunction with QTFs is slightly different. In this case, heading is measured anti-clockwise from the positive direction of the local surge axis to the wave. This definition is contrasted with the definition for first order motions and forces in the second figure below.
Definition of Wave Heading for RAO Data
Definition of Wave Heading for QTF Data
The definition of heading for the input of coefficients for current (Relative floating body/current heading) and wind (Relative floating body/wind heading) calculations is consistent with the definition used for QTFs, as it was for moored vessel data. See the Vessel Heading section for full details.