13. Simulate a QuasiStaticCable

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

13.1. Tutorial overview

This tutorial covers:

  • Creating a QuasiStaticCable DObject
  • Defining the QuasiStaticCable initial state
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Fig. 13.1 QuasiStaticCable layout

IMPORTANT NOTE: The functionality present in the QuasiStaticCable DObject as used in this tutorial is under active development. Care should be taken to ensure the values obtained from the use of this DObject are reviewed prior to use.

13.2. Introduction to QuasiStaticCable

The QuasiStaticCable is an alternative to the Cable DObject. QuasiStaticCable ignores dynamics and resolves the steady state line curvature and tension profile in response to steady forces of drag, weight, and buoyancy. It can be used to resolve a static profile or a starting condition to explore a dynamic response in waves for a Cable. This model should be used when dynamics are not of interest, and the steady state response of a line is desired.

The main benefit to using QuasiStaticCable is the reduction in simulation time required. In most cases, a QuasiStaticCable can simulate to a steady state configuration almost instantaneously.

The QuasiStaticCable has several limitations when compared to Cable.

  1. Connections are not supported.
  2. The line must always be in a state of positive tension. The QuasiStaticCable cannot achieve a state of compression.
  3. There are a fixed number of iterations attempted before convergence is considered to fail. Depending on the computational complexity and computer speed, this may take around 5 minutes.
  4. Regardless of $FluidLoadingMode value, FluidLoadingMode 0 is forced for both the cable and any extmasses.

13.3. Create a QuasiStaticCable

  • Create a new project.
  • Add a new QuasiStaticCable by selecting the Create button in the DObject section of the top Project Ribbon of PST. Select QuasiStaticCable as the DObject type and use the default name QuasiStaticCable_1.
  • Create a DCableSegment feature in the library 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

13.4. Add an Extmass

Note

  • The free end of a QuasiStaticCable requires an extmass.
  • In the Library, create a new ExtMass feature.
  • Set the ExtMass density to 1525kg/m^3 and its diameter to 0.5m.
  • In QuasiStaticCable_1’s input file, add the newly created ExtMass at an arc length of 20m (the free end).

13.5. Set the QuasiStaticCable initial state

  • Define the QuasiStaticCable initial state by selecting QuasiStaticCable_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.
  • Reference the wire rope feature in the cable input file by adding $CableSegment wire_rope_2in 20.

This specifies that the line is entirely composed of wire rope with a length of 20 m. If only one material is indicated, it is used for the entire line 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.

After the state of QuasiStaticCable_1 has been set, ensure the input file looks like the following:

// Boundary constraints
$Node0Static 1
$NodeNStatic 0

// Fluid loading
$FluidLoadingMode 0

// Mechanical
$CableSegmentMode 0
$CableSegment wire_rope_2in 20

$ExtMass extMass 20

// Static solver parameters
$MaximumIterations 50000

13.6. Set the integrator settings

Since this is a simplified QuasiStaticCable simulation, only two output stages need to be defined: initial, and steady state. This is accomplished by setting output interval to 1 second and the simulation end time to 1 second.

  • In the simulation input file, set $IntervalOutput 1 and $EndTime 1.

Note

  • The initial state will be represented at t = 0, and the steady state solution will be represented at t = 1 second.

13.7. Run the simulation

  • Save the simulation and select Run or press F5.

Note

  • QuasiStaticCable completes the majority of iterations to resolve the static profile during initialization.
  • When running the solver with a QuasiStaticCable, the initialization phase may take longer than simulations with Cables or other DObjects, even though the overall simulation execution time is much shorter.

The results can be viewed in PostPDS. The QuasiStaticCable will is be displayed as its initial condition at t = 0, and its final steady state at t = 1. The QuasiStaticCable with will be hanging vertically in the water column at its final state.

Note

  • The QuasiStaticCable final state can be used for an initial state in a new simulation or a quick check on general static behavior of a mooring.