4. Simulate a Cable

The identification tag for this tutorial is PDS-AAD. Input files for this tutorial are found in the folder named PDS-AAD in the provided tutorial input files.

4.1. Tutorial overview

This tutorial covers:

  • Creating a Cable DObject
  • Defining the Cable initial state
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Fig. 4.1 Cable layout

4.2. Create a Cable

  • Create a new project.
  • Add a new Cable DObject by selecting the Cable button in PST and use the default name Cable_1.

The Cable DObject is a cubic finite-element model for simulation of moorings, pipelines, or other flexible structures. Fig. 4.2 shows how a cable is defined in terms of node positions. The ends of the cable are referred to as node 0 and node N. The cable section between two nodes is defined as an element. Each element is defined in terms of its material properties that includes mechanical (e.g. rigidity, diameter, etc.) and hydrodynamic (e.g. drag coefficient) properties and its unstretched length.

A cable can have elements with different material properties. A segment is a group (typically 1 or more) of consecutive elements with the same properties. For example, in Fig. 4.2, cable segment segment0 could be fibre rope, segment1 could be chain, and segment2 could be wire rope. The targeted cable segment length is specified in the input file. The ProteusDS solver interprets this and applies these properties to the cable elements when a simulation is executed. You may receive a warning if you have a mismatch between your specified cable segment lengths and the cable element transitions that occur due to the defined cable element lengths. This is further discussed in the tutorials.

In this example, node 0 is at the top of the pendulum. This node is fixed in place. This is done by setting $Node0Static 1 in the Cable input file.

A pinned cable node can freely rotate about the pinned location. As an alternative, the boundary of the cable may be clamped. If clamped, the principle vectors P1 and P2 must be defined, which define the plane of the cross section at the boundary. More information regarding pinned and clamped constraints can be found in the ProteusDS User Manual.

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Fig. 4.2 Cable terminology

4.3. Set Cable initial state

  • Define the Cable initial state by selecting Cable_1 in the Project Explorer and clicking the State button or pressing Ctrl-K.
  • Place Node 0 at (0,0,0).
  • Place Node N at (20,0,0).
  • Set the length of the cable to 20 m.
  • Create a DCableSegment feature called wire_rope_2in.
  • Change the properties to the values shown below. The properties listed represent generic 2 inch wire rope.
// Axial Rigidity
$AxialRigidityMode 0
$EA 2e8

// Fluid loading
$CDc 1.5
$CDt 0.01
$CAc 1

// Mechanical
$EI1 1e3
$EI2 1e3
$GJ 1e3
$Diameter 0.05
$Density 5000
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 1

// Strain Limit
$ElongationLimitMode 0

The cable feature property definitions can be found in the Property Description Panel when the property is selected and in the ProteusDS manual.

  • Reference the new wire rope feature in the cable input file by adding $CableSegment wire_rope_2in 20.

This specifies that the cable is entirely composed of wire rope with a length of 20 m. If only one material is indicated, it is used for the entire Cable length regardless of the cable segment length indicated. The Segmented Cable tutorial demonstrates the use of multiple materials in a single Cable.

  • Set $NodeOStatic 1 to pin the cable at the water surface.

4.4. Run the simulation

  • Extend the simulation run time to 30 s in order to capture the cable swing and settle.
  • Select Run or press F5.
  • Post processing using PostPDS will be completed in the PostPDS and plotting tutorial.