38. Surface mooring with rigid body floater

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

38.1. Tutorial overview

This tutorial covers:

  • Anchoring a cable to the seabed
  • Cable and soil interaction
  • Connecting a cable to a rigid body
  • Rigid body hydrodynamic loading
  • Rigid body coordinate frame
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Fig. 38.1 Surface Mooring

38.2. Anchoring a cable to the seabed

  • Create a new project.
  • Create a cable DObject.
  • Specify the initial cable state to span from node 0 at global frame location (0,0,0.5) m to node N at a location of (5,0,50) m with 10 elements and a length of 55 m.
  • Node N of the cable, at the seabed, can be fixed in place by enabling the $NodeNStatic property.

Note

  • If the catenary checkbox is selected in the state window some of the cable nodes may be initially penetrated in the soil (view Visualizer). Due to the soil stiffness, defined in the default $EnvironmentSoil feature, a seabed contact force will push the cable out of the seabed at the start of the simulation.

38.3. Creating a floating RigidBody

  • Create a RigidBody DObject.
  • By design, the cylinder is 50% submerged in calm conditions. Set the $Mass to 800 kg and $Ix, $Iy, and $Iz arbitrarily to 1e2.
  • Create a RigidBody cylinder feature called float with a diameter of 1 m and a length of 2 m.
  • The RigidBody cylinder feature should be placed at the DObject local origin but lie horizontal on the water. This can be realized by rotating the feature by 90 degrees about the y axis. Do this by adding the following to the RigidBody input file: $Cylinder float 0 0 0 0 90 0.
  • Use the default state of the RigidBody at the global frame.

38.4. Connecting a cable to a RigidBody

  • Create a new connection and select the RigidBody as the master and the cable as the follower. Select point connection.

Note

  • Just as a PointMass, a RigidBody is the master because it sets the position and velocity state of the boundary node of the cable. In turn, the cable automatically passes reaction forces to the RigidBody during simulation.
  • Once a connection between the cable and cylinder is created, the connection location relative to the rigid body coordinate frame must be defined. This connection location in terms of the rigid body frame is constant throughout any simulation.
  • In the connection properties section, $DCableFollowerLocation is a vector that locates the cable boundary node attachment location relative to the rigid body frame.
  • Set the $DCableFollowerLocation to (0,0,0.5) m so the cable attachment point is on the outer surface of the cylinder on the bottom.
  • Use $DCableFollowerNodeN 0 to specify the connection applies to the end of the cable at node 0.
  • Be sure that the $DCableFollowerLocation on the RigidBody frame matches the cable state defined in global space. In this case the location corresponds to (0,0,0.5) m in the global coordinate frame and therefore no destabilization from excessive cable boundary element strain will occur.

38.5. Running the solver

  • Specify an environment with 50 m water depth and a 0.5 m/s uniform current with a heading of 90 degrees.
  • Set the length of simulation to 60 seconds and run the solver.
  • View the results in PostPDS.

38.6. Changing the connection location

  • Connect the cable to one of the ends of the cylinder by changing the $DCableFollowerLocation to (1,0,0) m.
  • Rerun the simulation.

Note

  • Notice the change in the connection location. View the RigidBody DObject local origin in PostPDS to determine where the location of (1,0,0) m is in relation to the RigidBody origin.