9. Simulate a subsurface mooring with a Cable

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

9.1. Tutorial overview

This tutorial covers:

Creating a simple Subsurface mooring using an ExtMass and an ExtMassCylinder
Anchoring a cable to the seabed

9.2. Setting up the Cable

Create a new project in PST.
Save the project to a new folder.
In the environment input file, set $WaterDepth 50.
Create a new Cable DObject with the default name Cable_1.
Click the State button on the top project ribbon to define the state of Cable_1.
Place Node0 at (0,0,25).
Place NodeN at (0,0,50).
Set the number of elements to 10.
Set the length of cable to 25 m. Click Generate.
Pin NodeN to the seabed by setting $NodeNStatic 1 in the cable input file.
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

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

9.3. Add a mid-line acoustic release

To add an acoustic release to the mooring line, an ExtMassCylinder will be used.

Add the following line to the cable input file: $ExtMassCylinder AcousticRelease 10
With the cursor on the ExtMassCylinder line in the input file, press F12. A prompt appears to create an extMassCylinder feature called AcousticRelease in the Library. Select Create.
Set $ExtMassType 1.

By setting $ExtMassType 1, there are now some additional follower properties that need to be defined.

Right click on $ExtMassType 1 and select Resolve follower properties.

This property flag defines how the weight of the ExtMass or ExtMassCylinder is calculated. In this case, the weight will be calculated with the $Mass and $WeightInWater properties instead of $Density.

Set the mass of the ExtMassCylinder to 36 kg with $Mass 36.
Set the weight in water to 28 kg with $WeightInWater 28.
Set the diameter of the cylinder to be 0.13 m.
Set the length of the cylinder to be 0.946 m.
Set the normal drag coefficient to 0.78 ($CD 0.78) and the axial drag coefficient to 0.85 ($CDAxial 0.93).

9.4. Add a buoy

To represent a spherical float at the end of Cable_1, an ExtMass will be used.

Add the following line to the Cable_1 input file: $ExtMass buoy 0
With the cursor on the ExtMass line in the input file, press F12. A prompt appears to create an ExtMass feature called float in the feature library. Select Create.

The default density of the ExtMass is 1025 kg/m³. This is approximately the density of seawater, so the buoy would be neutrally buoyant. A buoyant float can be represented by using an ExtMass with a density lower than the surrounding fluid. This is common practice to model a surface buoy or mid-line floats.

Change the density to 300 kg/m³ to create a positively buoyant float.
Set the diameter of the weight to 1 m.
Look at the mooring in the Visualizer by pressing F4.

9.5. Run the simulation

Specify a 2.0 m/s uniform current with a heading of 90 degrees.
Set the length of simulation to 60 seconds and run the simulation.
View the results in PostPDS.

Fig. 9.1 Subsurface mooring visualized in PostPDS using report mode.

Fig. 9.2 Acoustic release and buoy visualized in PostPDS using report mode.