Exact Nodal Location |
 
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Exact Nodal Location |
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As mentioned, there are two exceptions where nodal locations are exact rather than approximate. These are:
i.where a particular node is a boundary node, specifically a node at which a translational boundary condition is applied
ii.where a node is also on an element which is not part of a cable (for convenience, an element which is not on a cable is denoted a rigid element, regardless of its actual stiffness - in fact these elements are often hinges)
In these situations, the specified nodal coordinates are exact locations. These two exceptions are illustrated in the figures below.
The first figure shows a simple catenary riser. To specify this system you would input the coordinates of the end nodes, define a cable between them, and apply restraints at the two ends. Obviously in this situation the locations of the end nodes are exact rather than approximate.
Boundary Nodes
The second case is illustrated by the figure below, which shows a hybrid riser system comprising an underwater rigid riser supported by a subsea buoy, and a flexible riser descending to the buoy from the Mean Water Line (MWL). The flexible and buoy are connected by a hinge. In modelling this system, the flexible is identified as a cable stretching from the hinge to the MWL. For the purposes of the cable pre-static step, the location of the node at which the hinge and the flexible meet is considered to be exact, and in fact the cable module applies a boundary condition here and finds the cable catenary between this fixed point and the vessel connection point at the MWL. Of course in the subsequent full finite element analysis this node is again free to displace (unless restrained by the user).
Intersection of Cable and Non-Cable Elements
It is important to remember that cables you define connect the specified end points for the purposes of the cable step only. For the purposes of the complete finite element analysis with bending and torsion, the full connectivity of all elements comprising each cable must be input using either of the methods discussed in Elements.