Subsurface Mooring

Overview

Subsurface moorings are typically made up of rope and with attached instruments, weights, floats, and a dead weight anchor, with a buoy that remains under the water surface. An example of a typical subsurface mooring design is shown in Fig. 1.

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Fig. 1 A typical subsurface mooring design. (Design by S.Worrilow) [1]

These types of moorings can be simulated in ProteusDS to determine vertical displacements (knockdown), horizontal displacements (excursion), and inclination angles of attached instruments, floats, and weights. Line tensions and anchor forces are also determined in a subsurface mooring simulation.

Line models

A subsurface mooring can be simulated in ProteusDS using two different DObject models: Cable and QuasiStaticCable.

The Cable DObject is a fully dynamic finite element line model which solves for the line curvature and tension profile in the time domain due to dynamic drag, weight, buoyancy, added mass and wave forces. This model should be used when the dynamic response of the subsurface mooring in current and/or waves is of interest.

  • An introduction to the Cable DObject is found in the tutorial Simulate a Cable.

The QuasiStaticCable DObject is an alternative to the dynamic Cable DObject. The QuasiStaticCable model ignores dynamics and resolves the steady state line curvature and tension profile in response to steady drag, weight, and buoyancy forces. The QuasiStaticCable model can be used to resolve a static profile or a starting condition to explore a dynamic response in waves of a Cable. This model should be used when dynamics are not of interest, and the steady state response of the subsurface mooring is desired.

Single leg mooring fundamentals

There are several recommended tutorials that will help build the required knowledge to effectively build a subsurface mooring model in ProteusDS.

  1. The ExtMass and ExtMassCylinder features are used to represent weights, floats, instruments, or buoys that are attached to a mooring.

  2. Applying multiple cable segments to lines is used to represent moorings that contain multiple materials, such as wire rope, synthetic rope, and chain.

  3. Null segments are used as cable materials to occupy the length in the mooring where inline instruments will be placed. Null segments do not contribute any mass or external forces to the mooring.

Dynamic subsurface mooring using Cable

If the dynamic response of the subsurface mooring is desired, the Cable DObject must be used.

Steady state subsurface mooring using QuasiStaticCable

If the steady state response of the subsurface mooring in current is desired, the QuasiStaticCable DObject should be used as simulation time is reduced substantially in comparison with the Cable DObject, and output is limited to the final steady state.

Once the subsurface mooring has simulated to a steady state, it may be desired to simulate the mooring in dynamic wave conditions. A QuasiStaticCable DObject is not able to simulate in dynamic conditions, therefore it must be converted to a Cable DObject.

  • The final state of a QuasiStaticCable must first be exported to a new simulation. The tutorial Export simulation results outlines how to export a simulation.
  • The QuasiStaticCable DObject must then be converted to a Cable DObject for a dynamic simulation. The tutorial Convert a QuasiStaticCable to Cable outlines the required steps to convert a QuasiStaticCable to a Cable.

Reporting

After a simulation has been completed, a simulation summary report can be generated to summarize key results of the simulation.

  • The tutorial Create a simulation summary report outlines the steps to generate a summary report for a subsurface mooring, specifically with the QuasiStaticCable DObject.

References

[1]Trask, R. P and R. A. Weller, “Moorings”, Woods Hole Oceanographic Institution, Woods Hole, MA, USA Copyright 2001 Academic Press.