15. Simulate a surface mooring with QuasiStaticCable

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

15.1. Tutorial overview

This tutorial covers:

  • Creating a simple surface mooring using a QuasiStaticCable DObject
  • Custom current profile
  • Anchoring a line to the seabed

IMPORTANT NOTE: The functionality present in the QuasiStaticCable DObject as used in this tutorial is under active development. Care should be taken to ensure the values obtained from the use of this DObject are reviewed prior to use.

15.2. Create a QuasiStaticCable simulation from a template

An introduction to QuasiStaticCable can be found in the tutorial Simulate a QuasiStaticCable.

  • Create a new project in PST.
  • Save the project to a new folder.

A simulation template will be loaded into the new project. The surface mooring template provides a starting point to build a surface mooring using QuasiStaticCable. A QuasiStaticCable DObject requires specific environment and integrator settings, which are provided in the template.

  • Click Project, mouse over New Project From Template, and select Surface Mooring Template.

A new QuasiStaticCable called SURFACE_MOORING will be created. The input file should appear like the following:

// Boundary constraints
$Node0Static 0
$NodeNStatic 1
$NodeNAnchor 1

// Fluid loading
$FluidLoadingMode 0

// Mechanical
$CableSegmentMode 1

$ExtMass BUOY 0 

//two attached at same location
$ExtMass SUBSURFACE_CLAMPON_FLOAT 128
$ExtMass SUBSURFACE_CLAMPON_FLOAT 128

$ExtMassCylinder INLINE_SENSOR_OR_RELEASE 147.5

$SubmergenceProbe 0

The QuasiStaticCable is anchored to the seabed using $NodeNStatic 1. Additional detailed file output related to anchors is indicated using $NodeNAnchor 1. The QuasiStaticCable has multiple material properties using different DCableSegment Features. An instrument is also modeled with ExtMass Features attached along the mooring.

15.3. Mooring components

The surface mooring in the surface mooing template is modeled using a series of cable material properties. Attachments include a top (Node 0) surface buoy, two clamp-on floats, and an in-line sensor.

  • Open the QuasiStaticCable state generator by selecting SURFACE_MOORING in the left project tree and clicking State.

Notice the mooring segments and their associated unstretched lengths, elements, and element lengths. The mooring begins at the seabed with a 2 m section of Chain14mm, followed by a NULL_SEGMENT, followed by 19 m of WireRope0.25in, finishing with a 50 m section of AmsteelBlue8mm. The NULL_SEGMENT is used as a placeholder material for the in-line sensor to be attached to. The NULL_SEGMENT is neutrally buoyant and does not contribute drag or added mass forces to the system.

15.4. Integrator settings

A typical surface QuasiStaticCable simulation requires three output stages to be defined: initial condition, no current condition, and steady current condition. This is accomplished by setting the output interval to 1 second and the simulation end time to 2 seconds.

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Fig. 15.1 Output stages of QuasiStaticCable subsurface mooring

Note

  • The initial condition is defined at 0 seconds, the no current condition is solved for at 1 second, and the steady current condition is solved for at 2 seconds.
  • View the simulation input file. It should look like the following:
// Instrumentation
$IntervalOutput 1

// Integration
$StartTime 0
$EndTime 2
$Integrator integrator

Note

The integrator type ($IntegratorType) property in the integrator library feature is ignored when simulating a QuasiStaticCable. A quasi-static integrator is automatically invoked upon execution of the simulation.

15.5. Environment settings

The three individual output stages for the QuasiStaticCable simulation are created using environmental ramping, allowing steady environmental loading to be introduced after the first two stages have been solved (initial condition and no current condition). Environmental transition mode is used to offset the start of steady environmental loading from the start of the simulation.

  • Click on the environment input file.

The current profile and environmental ramping section of the environment input file should look like the following:


// CURRENT PROFILE
$CurrentProfile 5
$CurrentCustomProfileData 0.0	1.00	0 // 1 m/s current at surface
$CurrentCustomProfileData 50.0	0.50	0 // 0.5 m/s current at 50m
$CurrentCustomProfileData 100.0	0.0	0 // 0.25 m/s current at 100m

//ADDITIONAL CURRENT PROFILES POINTS CAN BE ADDED AS NEEDED:
//$CurrentCustomProfileData 200.0	0.00	0 // 0.0 m/s current at 200m
//$CurrentCustomProfileData 250.0	0.00	0 // 0.0 m/s current at 200m
// .
// .
// .

// Environment Transition
$EnvironmentTransitionMode 1
$CurrentTransitionMode 1
$CurrentStartTime 1.0 // at 1.0 second, the current will turn on - this will produce the static results at the 2.0 second output interval 
$CurrentRampDuration 0

$WindTransitionMode 0
$WaveTransitionMode 1
$WaveStartTime 0
$WaveRampDuration 10

$CurrentStartTime is set to 1.0 with a $CurrentRampDuration of 0. This will allow the third output stage at 2.0 s to be fully settled in current.

15.6. Run the simulation

  • Save the simulation and click Run.

Notice the simulation execution time. Generally execution time of QuasiStaticCable simulations are substantially lower than simulations with Cables, as the iterative process is only solving for a steady state solution rather than a time-domain transient solution.

  • View the results in PostPDS to visualize the three distinct output stages.
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Fig. 15.2 Surface mooring at steady current condition