66. Floating breakwater mooring

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

66.1. Tutorial overview

This tutorial covers:

Setting up a pontoon mooring containing three floating breakwaters
Using the Duplicate Selected DObject(s)… option
Process for setup of a moored floating RigidBody
Improving understanding of how to setup more complex simulations in ProteusDS (i.e. multiple cables and rigid bodies)

66.2. Setting up the Breakwater 3300 RigidBody

Note

Consider the Breakwater 3300 shown in Fig. 66.1. In this tutorial, a mooring will be setup with 3x Breakwater 3300 pontoons and moored with chain.

Fig. 66.1 Breakwater 3300

Create a new RigidBody called Breakwater3300. This body will be used to model the breakwater pontoon.
Add a Cuboid hydrodynamic feature to the breakwater RigidBody: type $Cuboid in the RigidBody input file then select enter. The default properties will be auto-filled. Replace the rigidBodyCuboid text with Breakwater3300Cuboid, then press F12. Select Create from the Create New Library Feature dialogue. This will create a new RigidBodyCuboid feature in the library called Breakwater3300Cuboid.
Change the dimensions of the cuboid feature to match those given in Fig. 66.1 for model M3320BRS. The length of the pontoon is chosen in the feature frame X direction (19.9 m), while the width (3.0 m) is in the Y direction. The height is 1.0 m. The listing on page 273 below gives the completed feature properties.
Change the mesh resolution by editing the $SegmentsX, $SegmentsY and $SegmentsZ to have the values: 10, 5, and 11, respectively.

Note

The cuboid feature should have the following properties:

// Added Mass Coefficients
$CAx 1
$CAy 1
$CAz 1

// Dimensions
$LengthX 19.9
$LengthY 3.0
$LengthZ 1.0

// Drag Coefficients
$CDt 0
$CDx 1
$CDy 1
$CDz 1

// Fluid loading
$WindLoading 1
$HydroLoading 1
$HydrostaticFroudeKrylov 1

// Mesh
$SegmentsX 10
$SegmentsY 5
$SegmentsZ 11

// Soil loading
$SoilLoading 1

Press F4 to open the visualizer and zoom in to view the RigidBody with its cuboid feature.

Note

The Breakwater spec sheet indicates a draft of 0.45 m (1 m height less 0.55 m freeboard). There are a many choices for the RigidBody state and the cuboid feature location that would result in this freeboard.

Set the X offset of the cuboid feature to be half the length of the pontoon, 9.95 m. The complete RigidBody input file is given in the listing below.
Set the Z offset of the cuboid feature to be -0.5 m relative to the local coordinate frame in the RigidBody input file.

Note

These offsets will place the RigidBody coordinate frame at the pontoon center line, at the stern, and at the baseline.

The CG will be assumed to be at the volume centroid of the pontoon. Set the CG position in the RigidBody input file such that it is at the center of the pontoon.
Set state of the RigidBody so that its reference frame position is (0,0,0.45) m. This produces a draft of 0.45 m.

Note

With a 0.45 m draft, the displacement in sea water is about 27.5 tonne. Note the simplified hull shape with a similar draft used results in a slight difference from 25.9 tonne listed on the data sheet.

The mass moment of inertia properties are computed assuming uniformly distributed mass in the hull. This results in the inertia values shown in the Breakwater3300 RigidBody input file listing below:

// Mechanical
$Ix 2.29E+004
$Iy 9.10E+005
$Iz 9.28E+005
$Ixy 0
$Ixz 0
$Iyz 0
$DefineInertiaAboutCG 1
$CGPosition 9.95 0 -0.5
$Mass 27.5e3

// Numerical
$Kinematic 0

$Cuboid Breakwater3300Cuboid 9.95 0 -0.5 0 0 0

66.3. Setting up chain catenary mooring system

Fig. 66.2 Breakwater 3300 Side view schematic (Not to Scale)

Note

Set the water depth to 5 m in the environment input file.
Four moorings with a scope of 3:1 will be created and attached to the breakwater.
The mooring chains are connected to the fairleads at the Breakwater 3300 baseline 1 m towards the center of the pontoon.

Create two cable DObjects - one called Mooring1 and the other called Mooring2.
Define the state of the two cable DObjects according to Fig. 66.2; the global coordinates of the end nodes of the cables are shown. The length of the cable can be the straight line distance of 15.6749. Use 8 elements in each cable.
To create the 3rd and 4th mooring lines, select both moorings in the PST project explorer, select the option Duplicate Selected DObject(s) from the Configuration drop down menu. The state of the new cable can be translated by a specified amount. The mooring lines will be translated in the global X direction a distance of 17.9 m (i.e. 19.9 m less 2 m), towards the other end of the pontoon - 1 m from the end.
Rename the duplicated lines to Mooring3 and Mooring4.

Note

Your mooring system should look like the one in Fig. 66.3.

Fig. 66.3 Breakwater 3300 4-point mooring system before pre-tensioning

To make the fairlead connections between each mooring line, select the Connection button in PST. Select the Breakwater 3300 RigidBody object as the master DObject, and select a mooring line as the follower object. Select the point type connection, then create the connection. Repeat this for each mooring line.
The fairlead connection location must be entered in each of the connections.

Note

The connection properties between Breakwater3300 and Mooring1 are:

// Mechanical
$DCableFollowerNodeN 0
$DCableFollowerLocation 1 1.5 0

Note

The connection properties between Breakwater3300 and Mooring2 are:

// Mechanical
$DCableFollowerNodeN 0
$DCableFollowerLocation 1 -1.5 0

Note

The connection properties between Breakwater3300 and Mooring3 are:

// Mechanical
$DCableFollowerNodeN 0
$DCableFollowerLocation 18.9 1.5 0

Note

The connection properties between Breakwater3300 and Mooring4 are:

// Mechanical
$DCableFollowerNodeN 0
$DCableFollowerLocation 18.9 -1.5 0

Set node N of each mooring line to be static, as this will be the anchor.
Add the $NodeNAnchor flag to each cable input file and set its value to 1.
This mooring will have a single cable segment with the properties of 40 mm chain. The properties for the 40 mm chain are given in the listing below (see the Chain properties tutorial for determining these properties).

// Axial Rigidity
$AxialRigidityMode 0
$EA 1.32E+008

// Fluid loading
$CDc 2.2
$CDt 0.4
$CAc 1

// Mechanical
$EI1 0
$EI2 0
$GJ 0
$FluidDiameter 0.04
$Diameter 7.26E-002
$Density 7800
$CID 1.00E+004
$BCID 0
$TCID 0
$CE 0

// Strain Limit
$ElongationLimitMode 0

To pretension the mooring, consider we want the mooring angle at the fairlead to be 90◦ . The weight in water of 40 mm chain is approximately 28 kg/m, so 4.5 m of cable suspended vertically would result in a load of 1234.8 N. As an initial guess of a good mooring pretension for this breakwater’s moorings, select a pretension of 2500 N.

Note

The completed cable input files should look like the listing given below.

// Boundary constraints
$Node0Static 0
$NodeNStatic 1
$Node0Fairlead 1
$NodeNAnchor 1

// Mechanical
$CableSegment chain40mm 15.6749

$Node0Pretension 2500
$Node0PayoutMode 2
$Node0PretensionPayoutSpeed 0.1

66.4. Running an initial simulation to get an settled initial state

In the integrator settings, change the $TruncationError to 1e-4. This tells the integrator to use more aggressive time-stepping to increase execution speed, but may result in less accurate results.
Set the simulation end time to 10 seconds, and run the simulation.
The simulation will take several minutes to complete as the cable tension is being set by the controller.
When the simulation is complete view the results in PostPDS, at the end of the simulation, the breakwater should look like Fig. 66.4.

Fig. 66.4 Breakwater 3300 4-point mooring system before pre-tensioning

Export the simulation results and open the new project when prompted.

66.5. Creating multiple breakwaters using Duplicate functionality

Note

In this section, the Duplicate Selected DObject(s) command will be reused on multiple DObjects that have connections between each other.

Using the exported simulation input files, remove the pre-tensioning properties from the input file, such that your cable input file looks like the listing below.

// Boundary constraints
$Node0Static 0
$NodeNStatic 1
$NodeNFairlead 1
$NodeNAnchor 1

// Fluid loading
$FluidLoadingMode 0

// Mechanical
$CableSegment chain40mm 10

To create multiple breakwaters that are side by side, one can use the Duplicate Selected DObject(s) option in the Configuration drop down menu in PST. First, press the Ctrl button and then select all of the DObjects in PST. Next, select Duplicate Selected DObject(s). When prompted, enter 20 in the X translate box, and change the text to append to the DObject names to 1.

Note

A second breakwater will be created which is offset in the X direction by 20 m. Note: connections are also duplicated between each floater and its mooring lines.

To create the third breakwater and its mooring lines, again Ctrl-select the first DObjects that were created (omitting those with 1 appended to their name). Next, select Duplicate Selected DObject(s). When prompted, enter 40 in the X translate box, and change the text to append to the DObject names to 2.
Look at the visualizer to see something like Fig. 66.5.

Fig. 66.5 3X Breakwater 3300 mooring

Add 0.5 m wave height 4 second beam waves to the simulation, by changing $WaveType to 1 in the environment input file and resolving and editing the follower properties.
Run the simulation for about 20 seconds and visualize the results in PostPDS.
To view a summary of the results of the simulation, select Report from PST. Select the RigidBody Mooring report. Select Generate Report when prompted. When the simulation report is finished generating, select View Report to examine the html report.