45. Barge mooring

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

45.1. Tutorial overview

This tutorial covers:

  • Initialization of a four point barge mooring
  • Defining multiple cable states and connections
  • DCable payout tension controllers
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Fig. 45.1 Moored barge

45.2. Setting up a barge RigidBody DObject

  • Create a new project.
  • Create a new RigidBody DObject that will act as the barge.
  • Create the RigidBodyCuboid feature below and add it to the barge input file.
// Added Mass Coefficients
$CAx 1
$CAy 1
$CAz 1

// Dimensions
$LengthX 25
$LengthY 9
$LengthZ 1.5

// Drag Coefficients
$CDt 0.01
$CDx 1.1
$CDy 1.1
$CDz 1.1

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

// Mesh
$SegmentsX 15
$SegmentsY 8
$SegmentsZ 5

// Soil loading
$SoilLoading 1
  • Ensure that wind loading, hydrodynamic loading and Froude-Krylov buoyancy are being applied to the barge cuboid feature.
  • In order to have a barge draft of 0.6 m, give the barge a mass of 154 000 kg.
  • To provide accurate barge movement, set the moments of inertia to the following: $Ix 10.6E5, $Iy 80.5E5, $Iz 90.6E5.
  • Set the barge to remain stationary while the cables are pretensioned by setting the $Kinematic flag to 1.

45.3. Setting the segmented cable properties

Note

  • The barge will be moored to the seabed by 4 mooring lines. Each line will be segmented into 15 m of chain and 85 m of rope.
  • All 4 mooring cables will be comprised of the same materials and therefore will share a the same cable segment features.
  • The cables are all going to be anchored to the seabed at node 0. Enable the property $Node0Static.
  • Create a DCableSegment feature named chain with the following properties:
// Axial Rigidity
$AxialRigidityMode 0
$EA 1.1e8

// Fluid loading
$CDc 2.4
$CDt 0.1
$CAc 0.5

// Mechanical
$EI1 1
$EI2 1
$GJ 0
$Diameter 0.116
$Density 7850
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 0

// Strain Limit
$ElongationLimitMode 0
  • Create a DCableSegment feature named rope with the following properties:
// Axial Rigidity
$AxialRigidityMode 0
$EA 4.3e6

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

// Mechanical
$EI1 1
$EI2 1
$GJ 0
$Diameter 0.052
$Density 980
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 1

// Strain Limit
$ElongationLimitMode 0
  • Add the chain and rope cable segment features to each cable input file.
  • Create a 15 m chain section and an 85 m rope section. The rope feature declaration must be below the chain feature declaration to position the rope after the chain along the span of the cable.

45.4. Placing the four cables around the barge

  • Set $WaterDepth to 50 m.
  • Set the barge RigidBody state to be at the global origin (0,0,0) m.

Note

  • Each cable’s node N will be connected to a corner of the barge. The initial location of each node N must be located at the corresponding location in the global reference frame.
  • The 4 connection points represented in the global reference frame are (12,4.5,-1) m, (12,-4.5,-1) m, (-12,4.5,-1) m, and (-12,-4.5,-1) m.
  • Each node 0 will be located 70 m away from the barge connection point along the X and Y axis and at the seabed (50 m).
  • Define the initial states of the cables to be a straight line from the anchor position to the corner of the barge.
  • The position of the first cable, in this case defined as the north-east cable (+X, +Y quadrant) will have a Node 0 position of (70,70,50) m, and a node N position of (12,4.5,-1) m. Use the straight line length as the cable length with 20 elements.
  • Repeat this process with the correct coordinates for the remaining 3 cables.
  • Create a point connection for each cable node N to the barge. The connection location with respect to the rigid body frame will be the same as the global position of the node N because the barge frame is coincident to the global frame at the start of the simulation.

45.5. Tensioning the cables using the tension controller

Note

  • A tension controller can be used to create realistic initial loads for the cables.
  • Add the property $NodeNPayoutMode to each cable input file and set the property to 2 in order to set a tension controller at node N in pretension mode.

Note

  • In pretension mode, a desired cable tension is specified and the tension controller will either pay out or pull in cable in an attempt to reach the desired tension.
  • Set the desired cable tension to 10,000 N using the property $NodeNPretension.
  • Set the maximum pay out/pull in speed to 0.5 m/s using the property $NodeNPretensionPayoutSpeed.

Note

  • It is important to set a reasonable maximum payout speed. If the maximum payout speed is too high, the controller will become unstable and may introduce large oscillations in the cable.
  • After a tension controller is added to all 4 cables, save the project and run the simulation for 5 seconds.
  • Visualize the results in PostPDS. Plot the cable tension and note the tension controller pays out cable to reduce the tension.

Note

  • The tension controller will stop paying out once the desired tension value is reached.

45.6. Simulate the barge mooring in storm conditions

Note

  • At the end of the simulation, the cables are now in steady state and the tension controllers have achieved an acceptable tension.
  • Select the Export button to use the final state from the completed simulation as the initial state in a new project. Open the new project when prompted.
  • Remove the tension controller properties (i.e. set $NodeNPayoutMode to 0 and resolve the follower properties) from the Cable input files
  • Remove the $Kinematic flag from the RigidBody input file.
  • Add some current, wind or waves into the simulation to introduce some environmental loading.
  • Run the simulation and then visualize the results in PostPDS.
  • Plot the cable tensions.