Vessel Motion

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Vessel Motion

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overview

The Vessel Motion component is used to specify the dynamic motions of a vessel, including first order (RAO) motions and second order (drift) motions.

 

Vessel Motion Component

Vessel Motion Component

 

 

Note

Only RAO motions can be specified in PipeLay Starter Edition

 

There are two main options available in PipeLay for the specification of vessel motions. With the first option, the vessel motions are defined by separately specifying the vessel RAOs and optional drift motions, while the second option allows the vessel motions to be read from a file containing a time history of the combined (first and second order) vessel motions. The component view for a new Vessel Motion component is shown in the figure above.

At the top right corner of the component view is the Motion Type drop-down list, which has two options RAO + Drift and Timetrace, corresponding to the options mentioned above. It defaults to the former, but whichever option you select, only the inputs relevant to that motion type are enabled.

Pressing the RAO File button allows you to select an ASCII data file containing your RAO data. This is identified via a standard Windows Open dialog, which allows you to browse for the file on your computer or local network. The layout of the RAO data in the file depends on the option you select in the RAO Layout drop-down list. Two options are available – MCS Kenny, where the RAOs are laid out in the standard Wood Group layout, and Line, a more general-purpose line-type layout.

The RAO Format drop-down list allows you to specify the format of the input RAO data. Format in this context refers to the conventions used  in the definition of the RAO data (heading definitions, phase angles etc.). Two options are available – MCS Kenny and Custom – the former being the default whereby the standard Wood Group format is assumed. If you choose Custom to specify that the RAOs are defined using different conventions, then you must define these conventions using a RAO Conversion component. You use the RAO Conversion drop-down box in the figure above to select the component appropriate to your data; as always you can have many RAO Conversion components in your project file, and the drop-down box list of the components currently in the file. PipeLay uses to convert your input RAOs to the standard Wood Group format.

By default, PipeLay generates plots of vessel RAOs when an analysis is run, although this facility can be disabled by selecting No in the Plot RAOs drop-down list.

 

drift

If your Motion Type is RAO + Drift, you can specify that second order vessel drift motions are to be including using the Drift Type drop-down list. Three options are provided – No Drift (the default), Sinusoidal and Timetrace. Sinusoidal drift motions are specified in terms of a drift amplitude and period for each particular degree of freedom. You specify this data using the Drift dialog, which is enabled when you select Sinusoidal or Timetrace on the Drift Type drop-down list. Timetrace drift motions are read from an ASCII file containing a time history of vessel drift. Pressing the Drift button in this case allows you to select the file via a standard Windows Open dialog.

 

timetrace

If you select Timetrace in the Motion Type drop-down list at the top right corner of the Vessel Motion component, then only the Motion drop-down list and the Timetrace File button at the bottom of the above figure are enabled. In this case the combined (first and second order) motions of the vessel are read from a file containing the time history of vessel motions, so all options relating to separate RAO and drift motions are inappropriate. In many respects, the operation of this facility is similar to the operation of the drift timetrace facility, with the exception that vessel RAOs may not be specified in addition to the vessel timetrace – the vessel timetrace completely defines both the first and second order motions of the vessel. Pressing the Timetrace File button allows you to select the file containing the motion timetrace data, via a standard Windows Open dialog as before.

Two options are provided in the Motion drop-down list related to vessel timetrace motions in the translational degree of freedom. If Global (the default) is selected, then the DOF 1, 2 and 3 motions specified in the timetrace file correspond to the global X-, Y- and Z-directions respectively. If on the other hand Local is selected, then the DOF 1, 2 and 3 motions specified in the timetrace file correspond to the initial local vessel x-axis, y-axis and z-axis, respectively. In other words, the DOF 2 and 3 motions specified in the timetrace file are in the direction of the initial vessel surge and sway axes (that is, the direction of the vessel surge and sway axes at the start of the analysis), respectively. DOF 1 motions specified in the timetrace file are always in the vertical direction as the initial vessel heave axis is assumed to be vertical. Note that this option only affects the specification of translational motions, not the rotational degrees of freedom.

 

Two additional articles outline Vessel Motion Data Inputs and Vessel Motion Best Practice.