Surface Mooring

Overview

Surface moorings are typically made up of rope and with attached instruments, weights, floats, a surface buoy, and a dead weight anchor. An example of a typical surface mooring design is shown in Fig. 2.

Fig. 2 A typical semitaut surface mooring design. (Design by P.Clay) [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. Surface buoy submergence, line tensions and anchor forces are also determined in a surface mooring simulation.

Line models

A surface 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 surface 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. QuasiStaticCable ignores dynamics and resolves the steady state line curvature and tension profile in response to steady drag, weight, and buoyancy forces. It 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 surface mooring is desired.

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

Single leg mooring fundamentals

There are several recommended tutorials that will help build the required knowledge to effectively build a surface 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.

   • The tutorial Use ExtMass and ExtMassCylinder Features with Cables outlines the required steps to create and use ExtMass and ExtMassCylinder features.

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

   • The tutorial Use multiple materials in Cables with DCableSegment Features outlines the required steps to import and apply multiple cable segments to a mooring line.

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.

   • The tutorial Insert inline mooring instruments in Cables outlines the required steps to setup and apply null cable segments and inline instruments to a mooring line.

Dynamic surface mooring using Cable

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

• The tutorial Simulate a surface mooring with Cable outlines the required steps to setting up and running a surface mooring simulation using the Cable DObject.

Steady state surface mooring using QuasiStaticCable

If the steady state response of the surface 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.

• The tutorial Simulate a surface mooring with QuasiStaticCable outlines the required steps to set up and run a surface mooring simulation with the QuasiStaticCable DObject.

Once the surface 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 surface mooring, specifically with the QuasiStaticCable DObject.

Advanced single leg surface mooring

• The tutorial Simulate an advanced single leg mooring steps through replicating an advanced oceanographic mooring from the University of Washington Applied Physics Laboratory in ProteusDS using the Cable DObject. It is suggested that previous surface mooring tutorials are completed before moving to the advanced single leg mooring tutorial.

References

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