69. Sliding gravity anchor

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

69.1. Tutorial overview

This tutorial covers:

  • Modeling an anchor using a RigidBody DObject and the seabed soil model
  • Single point mooring
  • Soil contact
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Fig. 69.1 Mooring layout

69.2. Static and dynamic anchors

Note

  • When simulating a mooring system the primary focus tends to be the surface buoy submergence, watch circle, or motions and especially the tension in the lines. This means that the anchor is often modeled as a fixed connection to the seabed with the reaction loads monitored. However, this modeling method ignores potential sliding or hopping of the anchor. A moving gravity anchor will limit the magnitude of tension registered in the mooring line. In the software it is possible to simulate interaction of a gravity anchor with the seabed, but this does add some complexity, and should only be modeled when there is justification.

69.3. Creating two single point moorings

  • Create a new project.
  • Set $WaterDepth to 30 m in the Environment input file.
  • Create two cables.
  • Place the mooring lines 20 m apart along the y-axis using node 0 and node N positions of (0,0,0) m - (0,0,30) m and (0,20,0) m - (0,20,30) m.
  • Set the lengths of both cables to be 30 m and use five elements.
  • Set $NodeNStatic on one of the cables to constrain the end node to the seabed.

Note

  • The same surface buoy will be used for both moorings.
  • Add an ExtMass to Node 0 of both cables with a diameter of 2 m and a density of 200 kg/m3.
  • Create a $CableSegment feature based on 30 mm Amsteel Blue synthetic rope.
// Axial Rigidity
$AxialRigidityMode 0
$EA 2.066E7

// Fluid loading
$CDc 1.5
$CDt 0.01
$CAc 1
 
//Mechanical 
$EI1 1.162E3
$EI2 1.162E3
$GJ 1.162E3
$Diameter 3.000E-2
$BuoyancyDiameter 2.646E-2
$Density 7.625E2
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 0

69.3.1. Creating a concrete gravity anchor

  • Create a single rigid body to be used as the anchor on one of the cables.
  • Set its mass and mass moments of inertia to 2400 kg and 400 kgm2, respectively.
  • Add a cuboid feature that is 1 m x 1 m x 1 m and connect it to node N of the cable that is not constrained to the seabed.
  • Set the position of the anchor block to be coincident with the end of the cable.
  • Connect the anchor to the end of the cable at (0,0,-0.5) m, relative to the anchor reference frame.

69.3.2. Defining the soil contact properties

Note

  • Since there is an object interacting with the seabed, the soil properties need to be considered.
  • Use the following soil properties in the $SoilProperties feature:
// Mechanical
$KNSoil 5e5
$CNASoil 1e6
$CNBSoil 1e6
$MuSoil 0.2
$MuNormalSoil 0
$DeadZoneVel 0.001

69.4. Running the simulation

Note

  • A sea state must be created in order to induce anchor movement.
  • Create a 1 m/s uniform current profile at a heading of 0 degrees.
  • Create an Airy wave with a height of 4 m and a period of 9 seconds with a heading of 0 degrees.
  • Ramp the environment over 5 seconds.
  • Run the simulation for 20 seconds.

Note

  • The simulation may take some time to execute because of the small time steps required to resolve the dynamics of the anchor’s interaction with the stiff soil. To improve execution speed, the integration feature’s $TruncationError limit can be relaxed to a value of 1e-4 from 1e-6. This will allow the integrator to use larger time steps.

69.5. Results

Note

  • When visualizing the two mooring systems, notice the movements of the gravity anchor.
  • The gravity anchor system demonstrates an under specified anchor that moves with current and waves.
  • The motions of the anchor are very sensitive to the seabed composition as well.
  • Careful consideration should be taken when setting the soil stiffness and damping at is will also heavily influence the time it takes to complete the simulation.
  • Plot the tensions of both mooring lines and notice the peak tensions in both cases.

Note

  • The system that is constrained to the seabed exhibited much higher tensions as there was no compliance in the system.
  • The system with the dynamic gravity anchor has inherent compliance with the ability for the anchor to move.
  • Therefore the tension in that system are less than the constrained system.