Mooring and mooring analysis

Mooring and mooring analysis#

The purpose of a mooring is to keep a vessel in position against the environmental or other external forces.

A typical mooring scene contains roughly three parts:

  1. The vessel(s) and cargo

  2. The quay

  3. The mooring, mooring lines and fenders

We will go into these items one by one assuming a typical mooring system.

Note that DAVE does not have limitations concerning the complexity of the mooring. Multiple vessels, mooring beams, spudpiles, lifts, trailered loadouts, anything that can be created with nodes can be analyzed as mooring. It does not even needs to be a mooring. The only requirement is that it is affected by wind.

The vessel#

If you have pre-defined vessel available then these can be used.

  • add a vessel node and select the vessel.

  • Set the draft of the vessel to correctly include the wind and current areas of the hull.

  • Add cargo using the cargo module.

In absence of the option to use pre-defined vessels the vessel can be defined using a Frame or Rigidbody. Wind and current areas can be added using WindArea or CurrentArea nodes.

Degrees of freedom#

For many moorings it is sufficient to consider only motions and rotations in the horizontal plane. For this set x,y and rz to free while leaving the other degrees of freedom fixed. Set z to defined the draft of the vessel.

Reporting#

The quay#

Creating a mooring#

Creating a mooring pattern can be done in the contruction screen. A mooring typically contains mooring-lines and fenders.

Mooring lines#

Cable nodes can be used as mooring lines. Cables are attached to point and/or circle nodes.

Fenders#

The best way to model fenders is by using Contact-Balls and Contact-meshes. By default the contact-mesh is attached to the quay (typically a location element) and the contact-ball to the vessel.

Make sure that the contact-ball knows about the contact-mesh by adding it to the meshes property of the contact-ball.

Defining the limits#

Limits can be applied in the Limits screen

  • To set the maximum capacity of a mooring line (Cable) a limit on tension can be used.

  • For bollards (Point) use force to limit the total force. Use fx or fy to limit the force in specific directions.

  • The capacity of a fender can be set using a limit on its contact_force_magnitude.

Defining what belongs to your mooring system#

Selection-by-tags is used to define the elements that need to be considered in the mooring system

Start-condition#

DAVE analyses everything from static equilibrium. This means that the scene needs to be in static equilibrium before anything can be properly analyzed.

This means the mooring line lengths need to be defined such that the vessel is in its intended position when the system is in equilibrium.

There are two screens to assist with this:

Manual#

In this screen the tensions of the individual mooring lines can be adjusted. Clicking one of the + or - buttons will increase/decrease the length of that line such that the target tension is reached. After that statics is solved which will usually re-distribute the tensions over the lines. This means that increasing a line-tension with (say) 100 kN will firstly shorten that line to set its tension to 100 kN. After that statics is solved which will likely pull the vessel towards that line, lowering the tension. So the net effect of increasing the tension in a line by 100kN may be that the tension in that line increases by 50kN as well as the tension in the line opposite of that line. Also the vessel will move a bit towards the line of which the tension was increased.

Automatic#

Here the line-tensions are solved such that

  1. The value in each line is between its minimum and maximum

  2. The vessel is in equilibrium in the position where it is when stating the automatic line-tensions solver.

  3. The relation between the force in a line and its maximum force is as low as possible. (Ie: the lowest overall UC based on line-tensions). Note this this applies to the tensions in the lines only, other limits (for example bollard) are not considered here.

Finding a automatic solution is not always possible.

Analyzing the capacity#

To analyze the capacity the wind-velocity is increased from BFT1 in steps of 1 bft until a wind-force is found in which the governing unity-check exceeds 1.0 or until BFT-11 is exceeded.

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