Angle between an Element and an Axis System |
 
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Angle between an Element and an Axis System |
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The third category of Angles output provided by Flexcom is the angle between an element you identify and an axis system you specify. This axis system can be either (i) the global axes, or (ii) a local axis system which you define. In this latter case you again associate a name or label with the axes, and input the vector components, in the global axes, of the local x- and y-axes of the system you are defining. The local z-axis is automatically found by Flexcom using the right-hand rule. When inputting a local axis system, you must again identify the node at which the axes are located, which can again be either a node of the structure finite element discretisation or a so-called auxiliary node. The significance of this input is as follows. The local x- and y-axes you specify give the orientation of the axis system at the undisplaced or initial position of the node. As the node displaces subsequently (either statically or dynamically), the rotations at the node are also applied to the local axes, so that the orientation changes with the motion of the node. This is similar to the corresponding facility available when requesting the angle between an element and a vector. The global axes of course are fixed throughout all analyses.
Four output options are available in this case of the angle between an element and an axis system. These are denoted (i) the Actual angle, (ii) the Projected-xy angle, (iii) the Projected-xz angle and (iv) the Projected-yz angle. The significance of these is now discussed, with reference to the two figures below.
Dealing first with the case of the angle between an element and the global axes, as shown in the figure below, then Actual angle is the true or actual angle (using the same definition as previously) between the element in its instantaneous position and the global X-axis. This angle will be in the range 0° to 180°. The Projected-xy angle is the angle between the global X-axis and the projection of the element onto the global XY-plane. The Projected-yz angle is the angle between the global Y-axis and the projection of the element onto the global YZ-plane. Finally, the Projected-xz angle is the angle between the global Z-axis and the projection of the element onto the global XZ-plane. These projected angles will all be in the range –180° to 180°.
Angles between an Element and the Global Axes
For the case of the angle between an element and a local axis system, as shown in the figure below, then Actual angle is the true or actual angle (using the same definition as previously) between the element in its instantaneous position and the local x-axis, in its instantaneous position. The Projected-xy angle is the angle between the local x-axis and the projection of the element onto the local xy-plane. The Projected-yz angle is the angle between the local y-axis and the projection of the element onto the local yz-plane. Finally, the Projected-xz angle is the angle between the local z-axis and the projection of the element onto the local xz-plane. As before, the Actual angle will be in the range 0° to 180°, while the Projected angles will all be in the range –180° to 180°.
Angles between an Element and a Local Axis System
Finding angles between an element and a local axis system allows you, for example, to find the true angle between the last element on a riser connected to a subsea buoy and the buoy structure itself. Alternatively, using Projected angles allows you to find the azimuth and declination angle of the riser relative to the principal axes of the buoy.
•*TIMETRACE is used to request the creation of timetrace plots. Specifically, TYPE=ELEMENT_AXIS_ANGLE is used to request a timetrace of the angle between an element and an axis system.
•*PARA ANGLE AXIS is used to request summary output of the angle between an element and either a vector or an axis system.