22. Mooring seabed interaction

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

22.1. Tutorial overview

This tutorial covers:

  • PointMass as a surface float
  • Extmass components
  • Cable seabed interaction

22.2. Creating a Cable

Two different mooring materials will be used. The first being a generic synthetic material and the second being a relatively large and heavy chain.

  • Create the DCableSegment features for the two materials using the following properties.
// Axial Rigidity
$AxialRigidityMode 0
$EA 8.5E5

// Fluid loading
$CDc 1.5
$CDt 0.01
$CAc 1

// Mechanical
$EI1 4
$EI2 4
$GJ 8
$Diameter 0.008
$Density 850
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 0

// Strain Limit
$ElongationLimitMode 0

//Grade R3
//Dia (mm) 5.000E+01
//Proof load (tonne) 1.509E+02
//MBL (tonne) 2.273E+02
//Weight (kg/m) 5.475E+01
//Fluid loading 
$CDc 2.400E+00
$CDt 1.400E+00
$CAc 1.000E+00
//Mechanical 
$EA 2.042E+08
$EI1 0.000E+00
$EI2 0.000E+00
$GJ 0.000E+00
$FluidDiameter 5.000E-02
$Diameter 9.454E-02
$Density 7.800E+03
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0.000E+00
$TCID 0.000E+00
$CE 0
[$MBS 2.228E+06]
[$Cost 4.887E+02]
  • Create a Cable with node 0 at (10,0,0) m and node N at (0,0,50) m.
  • Set the Cable length to be 70 m.
alternate text

Fig. 22.1 Cable state

Note

  • The extra cable length will create a catenary.
  • Set the water depth to 50 m and anchor the cable at the seabed using $NodeNStatic 1.
  • Create a 0.25 m/s uniform current with a heading of 0 degrees.

22.3. Seabed contact dynamics

A seabed contact model is used to apply forces on Cables, RigidBodies, and PointMasses to prevent them from passing through the seabed. The seabed is represented with a contact stiffness, damping, and tangential motion friction that are scaled by the area of interaction between the objects. The soil properties are constant regardless of the object interacting with it. A figure demonstrating the forces acting on a PointMass beneath the polygonal seabed can be seen in Fig. 22.2.

When the cable is in contact with the seabed, restoring forces from the seabed are computed. All the forces combine to a normal and tangential contact force. More detail on the seabed contact model can be found in the ProteusDS manual.

In this case, the anchor chain is heavy enough to penetrate the soil.

Note

  • By using heavy chain at the anchor position, compliance is added to the system in the form of the chain mass being lifted off the seabed and placed back down.
alternate text

Fig. 22.2 Seabed contact model

22.4. Adding ExtMass floats and weight

To create a lazy-s shape or false bottom in the cable, additional flotation and weight must be added to the cable.

  • Create 2 ExtMass features. One representing the float and the other representing the clump weight.
  • Give the float ExtMass an added mass and drag coefficient of 1, a diameter of 0.28 m, and a density of 450 kg/m3. $FluidLoadingMode can be set to 0.
  • Give the weight ExtMass the same added mass and drag coefficients used for the float but use a diameter of 0.125 m and the density of 7000 kg/m3. $FluidLoadingMode can be set to 0.
  • Add 3 $ExtMass properties to the cable input file to define the floats and weight on the cable. Set one float ExtMass at a cable arc length of 25 m, and the the second at 30 m. Set the weight ExtMass at a cable arc length of 20 m.

The floats will create upward load to lift the lower portion of the cable and to generate a slack region in the cable. The amount of flotation required can be calculated by taking the net wet weight of the lower cable and matching that to the total ExtMass wet weight.

22.5. Creating a PointMass buoy

A point mass will be created to model a surface buoy.

  • Create a PointMass. The PointMass state should be set at (10,0,0) with zero velocity.
  • Connect the PointMass to node 0 of the cable.
  • Set $PointMassType 2 and resolve follower properties.
  • The PointMass can be defined in terms of an ExtMass feature. For the $ExtMass property in in the PointMass input file, create a new ExtMass feature (e.g. right click on the $ExtMass property and select Go To Feature Definition).
  • Supply the newly created ExtMass feature with a 1 m diameter and a density of 250 kg/m3.
  • Ensure $FluidLoadingMode is set to 1 in the ExtMass feature, since the buoy will be at the water surface.

Note

  • The ExtMass feature will act as a surface buoy designed to follow the surface of the water but be embedded in the PointMass DObject.

22.6. Benefits of false bottom

The false bottom decouples the surface movement from the lower portion of the cable. In a strong sea state, the surface buoy will follow the heave of the waves but the lower portion of the cable will have reduced motion due to the displacement of the cable being absorbed by the false bottom. The false bottom has a similar effect on high frequency tensions. The tensions occurring at the top of the cable will be absorbed by displacing the false bottom, therefore the tensions will not be transferred down the cable.

22.7. Running the solver and viewing the results

  • Set the length of simulation to be 60 seconds and run the solver.
  • View the results in PostPDS.