12. Insert inline mooring instruments in Cables

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

12.1. Tutorial overview

This tutorial covers:

  • Attaching Extmass cylinders inline onto moorings
  • Creating a null cable segment

12.2. Add the null segment

  • Load the simulation files from the tutorial Segmented Cable. If the tutorial has not been completed, the input files can be downloaded here.

A NULL_SEGMENT will be used on the mooring line. A null segment is a cable segment that does not contribute any forces to the model. They are commonly used to model a break in the cable material for an instrument or float to be inserted. When an Extmass is attached to the cable, the forces from both the cable and Extmass are counted. In this case, a device is attached by its ends to the mooring and there is no mooring material along the device.

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Fig. 12.1 Examples of attached instruments connected to and inline with a mooring.

  • Create a DCableSegement feature called NULL_SEGMENT with the following properties.
// Axial Rigidity
$AxialRigidityMode 0
$EA 1.6e7

// Damping
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$FlexuralDampingMode 1
$FlexuralReferenceDampingRatio 0.5
$TCID 0

// Fluid loading
$CDc 0
$CDt 0
$CAc 0

// Mechanical
$EI1 0
$EI2 0
$GJ 0
$Diameter 0.01
$Density 1025
$CE 1

// Strain Limit
$ElongationLimitMode 0
  • Open the cable state generator for the mooring.
  • Insert a new cable segment after the Wire_rope_6x19_1in segment.
  • Define its segment type to NULL_SEGMENT with an unstretched length of 1 m and a single element.
  • In order to maintain the same mooring line length, remove 1 m of unstretched length from the Wire_rope_6x19_1in segment.
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Fig. 12.2 Inserting a null segment into the mooring

12.3. Add a ExtmassCylinder to the Cable

The ExtmassCylinder that we will be attaching to the mooring line already exists in the project library.

// Fluid loading
$CD 0.78
$CA 0.5
$CDAxial 0.85
$FluidLoadingMode 1

// Mechanical
$Diameter 0.13
$Length 0.946

// Type
$ExtMassType 1
$WeightInWaterDensity 1025
$WeightInWater 28
$Mass 36
  • Add the ExtMass cylinder 19.5 m along the mooring line arc length using $ExtMassCylinder AcousticRelease 19.5.

The null segment is placed at 19 m arc length along the cable and the instrument is approximately 1 m in length. The center of the ExtmassCylinder is placed at the arc length specified in the cable input file.

12.4. Run the simulation

  • Save and run the simulation.