8. Use ExtMass and ExtMassCylinder Features with Cables

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

8.1. Tutorial overview

This tutorial covers:

Creating and manipulating the ExtMass and ExtMassCylinder features
Creating a simple mooring using a Cable

8.2. Introduction to ExtMass and ExtMassCylinder

The ExtMass (spherical) and ExtMassCylinder (cylindrical) features are point mass models that can be used to represent weights, floats, instruments, or buoys that are attached to a line.

The ExtMass and ExtMassCylinder features use defined geometry and density (or mass and weight in water) to determine the weight of the feature. Drag and added mass forces are also calculated based on drag and added mass coefficients. Both features can be added at any location along the span of a Cable or QuasiStaticCable DObject. Forces from an ExtMass or ExtMassCylinder are applied to the attached line as point forces at the defined attachment location.

8.3. Create a simulation with a Cable

Create a save a new project.
In the environment input file, set the depth to 20 m with $WaterDepth 20.
Add a new Cable to the project 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 (0,0,20).
Place NodeN at (10,0,20).
Set the number of elements to 10.
Set the length of cable to 10 m. Click Generate.
In the feature library, create a new DCableSegment feature named 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

In the Cable_1 input file, apply the wire_rope_2in with $CableSegment wire_rope_2in 10. Delete the default cable segment declaration $CableSegment segment0 10.
Set $Node0Static 1 to pin Node 0 to the seabed.
Ensure the Cable_1 input file looks like the following:

// Boundary constraints
$Node0Static 1
$NodeNStatic 0

// Fluid loading
$FluidLoadingMode 0

// Mechanical
$CableSegmentMode 0
$CableSegment wire_rope_2in 10

8.4. Add an ExtMass to the end of a Cable

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 float 10
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.
In the simulation input file, set the simulation end time to 30 s with $EndTime 30.
Run the simulation, and view the results in PostPDS.

The attached float will pick up the 10 m of wire rope and settle vertically in the water column.

8.5. Add an ExtMassCylinder to a Cable

To represent an acoustic release, an ExtMassCylinder can be used. The ExtMassCylinder feature adds a cylinder to the line that is always aligned with the local line tangent.

Add the following line to the cable input file: $ExtMassCylinder AcousticRelease 2
With the cursor on the ExtMassCylinder line in the input file, press F12. A prompt appears to create extMassCylinder feature called AcousticRelease in the Library. Select Create.
Set the $ExtMassType property to 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 with $Diameter 0.13.
Set the length of the cylinder to be 0.946 m with $Length 0.946.

The normal and tangential drag coefficient must be set to represent a cylinder of the correct aspect ratio. Fig. 8.1 and Fig. 8.2 demonstrates the variation in normal and tangential drag coefficient with aspect ratio.

Fig. 8.1 Variation in cylinder normal drag coefficient with aspect ratio. Taken from: Applied Fluid Dynamics Handbook.

Fig. 8.2 Variation in cylinder tangential drag coefficient with aspect ratio. Taken from: Applied Fluid Dynamics Handbook.

For this ExtMassCylinder, the aspect ratio is 7.3. Set the normal drag coefficient to 0.78 ($CD 0.78) and the axial drag coefficient to 0.85 ($CDAxial 0.93).
Ensure the AcousticRelease feature looks like the following:

// Fluid loading
$CD 0.78
$CA 0.5
$FluidLoadingMode 0
$CDAxial 0.93

// Mechanical
$Diameter 0.13
// $Density 1025
$Length 0.946

// Type
$ExtMassType 1
$WeightInWater 28
$Mass 36

Run the simulation, and view the results in PostPDS.