57. Cylindrical net cage array

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

57.1. Tutorial overview

This tutorial covers:

Creating a cylindrical net
Creating a disk net
Attaching cables to nets edges
Duplicating objects and connections

Fig. 57.1 Cylindrical net cage with disk net bottom

57.2. Cylinder nets

Create a new project.
Create a new net DObject.
Define the initial state for the net using the the Cylindrical option.

Note

This state definition option allows specification of the center point, radius and length of a cylinder.

Set the radius of the cylinder to be 15 m and the length to be 15 m, and accept the default center point.
Set the number of elements along the circumference to be 12 and the number of elements along the length to be 6.
Set $Edge2Ring to be 1 and $Edge0Static to be 1 for the newly created net. The $Edge2Ring connects edge 2 and 3 together. Edge 0 is at the surface.
Run a short simulation to visualize the cylindrical net.

57.3. Create a disk net

To create a net bottom create a net DObject and define the state using the Disk option.
Center the disk at the center of the cylindrical net, and set the depth to 15 m. The disk net has a hole at the center of it. It is recommended that the hole be a reasonable size to ensure that the element lengths used for the elements on the inner radius are not too small relative to the other elements. Small elements may result in very slow simulations.
Choose an inner radius of 2.5 m and and an outer radius of 15 m. Select 12 elements around the circumference and 5 elements along the length (radial direction).
The edge locations created with the disk net option when defining the state are shown in Fig. 57.2. To prevent edge 2 and edge 3 from becoming disconnected, set $Edge2Ring to 1.

Fig. 57.2 Disk net schematic

Note

Nets can be connected to each other along edges. A master net and follower net is specified in a net connection. The master net edge sets the position and velocity of the nodes along specified edges of a follower net.

Connect net edge 1 of the cylinder net (master) to net edge 0 of the bottom net (follower) using the connection properties:

// Mechanical
$DNetFollowerEdge 1 0
$DNetFollowerArcSpan 0 94.247778

Note

When specifying the the circumference of the net using the $DNetFollowerArcSpan property, specify the arc span to a minimum of 6 decimal places. If this is not done, with a polar cable and cylindrical net, the finite-element cable and net models will become unstable at the beginning of a simulation. Attachment of bridle lines to the floating collar (polar cable) may hide the problem.

Run a simulation and with a steady current and watch the net cage deflect.
To prevent the net from deflecting a clump weight can be added to the cylindrical net. Add an ExtMass feature called clump to the library that has a diameter of 0.5 m and a density of 2400 kg/m³ , then add the following text to the net input file:

$ExtMass clump 0 15
$ExtMass clump 15 15
$ExtMass clump 30 15
$ExtMass clump 45 15
$ExtMass clump 60 15
$ExtMass clump 75 15

Run a simulation and observe the cylindrical net pen.

57.4. Creating a circular floating collar

Create a cable DObject
Define the initial state for the cable using the Polar option.
Set the circle radius to be 15 m, and the number of elements to be 12.
Ensure that the collar is centered with the cylinder net in the state definition.
Set the floating collar to be a ring using $NodeNRing.
Set the cable (floating collar) properties in the appropriate library feature:

// Axial Rigidity
$AxialRigidityMode 0
$EA 4.2e7

// Fluid loading
$CDc 1
$CDt 0.1
$CAc 1

// Mechanical
$EI1 4.24e6
$EI2 4.24e6
$GJ 0
$Diameter 0.445
$Density 317
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 1e4
$TCID 1e4
$CE 1

// Strain Limit
$ElongationLimitMode 0

To accurately model buoyancy forces on the floating collar, ensure the following advanced buoyancy properties in the cable input file are defined:

$FluidLoadingMode 1
$AngularDiscretization 12
$AxialDiscretization 30

To create a connection between the floating collar and the net, create a DCableDNetEdgeConnection.
Set the connected arc length using the $DNetFollowerSpan property. The property specifies the start arc length and end arc length on the cable.

// Mechanical
$DNetFollowerEdge 0
$DNetFollowerSpan 0 94.247778

Note

By connecting the collar to the top of the cylinder net, edge 0 of the cylinder net is now being constrained by the collar.

Since the cylinder net is now constrained by the collar, set $Edge0Static to 0 on the cylinder net.
Run a simulation of the net cage in waves to observe how the net responds to waves.

57.5. Duplicating the net cage

Note

In order to create additional instances of multi-object systems, the duplicate object tool can be used.

Select each object in the object tree using the Ctrl button.
Under the Configuration menu option, select Duplicate Selected DObject(s).
Leave the appended text and translate the nets and cable 35 m in the x direction.

Note

The duplication tool will automatically recreate the applicable connections and features.

Run the simulation of the two net cages together.