Skip to content

Home > Model Components > Environments Sets > LF Imposed Motion Components

Low frequency motion components

The LF Motion tab defines a list of individual low frequency motion applied to Floaters that may be used to define combined environments from the Combination Matrix tab.

Warning

LF Motion are prescribed dynamic displacements that apply to Floater components only.

Note

LF Motion can be accounted for within both dynamic analyses in the time-domain and within dynamic analyses in the frequency-domain.

The LF Motion tab includes the following items:

  • Number of motions: The number of LF Motion components must be set with the spin box located in the upper right corner of the tab. You may either use the up/down arrows to setup the number of LF motions, or input any value using your keyboard and then pressing the ENTER key. Pressing the ENTER key confirms the number of individual components to be included and generates the main LF Motion data table with appropriate number of rows.

  • Remove selected motion: Pressing this button deletes the selected rows from the LF Motion List data table.

  • LF Motion list table: This table is used to define the list of LF motion components. This table includes 3 columns, as follows:

    • Name: This assigns names that will be used to refer to the LF Motion components in the Combination matrix tab.

    • Object: Used to select the Floater to which the LF Motion will apply. Each cell in this column contains a drop-down list filled in with the names of all floaters components available in the model.

    • Location: Used to select the point on the floater at which the offset will apply. Location are restricted to the COG of the Floaters (actually the centre of motion of the Floater).

Selecting any row in this table enables displaying the LF Motion data table that is associated with the selected LF Motion component.

  • LF Motion data tables: These tables display in the bottom part of the tab. Each table is associated with a single LF motion component in the LF Motion list table. Low frequencies motion are defined as series of periodic motion with specified amplitude and phase for the 6 degrees of freedom. The user has to enter the phase and amplitude for each degree of freedom and can adjust the desired number of periods with the Period tab. Each table includes the following items:

    • Period: This sets the number of harmonic sub-motion components. Each sub-motion component is a sine-like signal.

    • LF Motion data table: The number of rows in the table can be set with the above "Period" item.

    • Periods(s): Defines the period(s) of each sub-motion components

    • Ampl. (m): Defines the amplitudes of each sub-motion components for the 6 degrees of freedom (surge, sway, heave, roll and pitch).

    • Phase (deg): Defines the phases of each sub-motion components for the 6 degrees of freedom (surge, sway, heave, roll and pitch).

    Note

    Amplitudes and phases are not related to waves as per motion RAO.For instance, the surge at time t only depends on the period T, the amplitude \(A_{surge}\), and the phase \(\Phi_{surge}\), as follows: \(Surge(t) = A_{surge} \ldotp \cos(\frac{2\pi}{T}\ldotp t + \phi_{Surge})\)

An example of LF Motion tab is shown below: