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Turret-Moored Floaters

Weathervaning systems are often composed of turret-moored systems illustrated by the following figure:

Whatever the selected type of floater, two different parts may be distinguished:

  • The floater itself with all the fairleads directly attached to it;

  • The turret, connected to the floater but free to rotate with respect the floaters Z axis.

The mooring lines and the risers are usually connected to the turret.

Two approaches may be chosen to model a turret-moored floater:

  • A simple modelling: the turret-moored system is considered as a single floater that means that during the calculations, the turret follows the rotation of the floater;

  • A more complex modelling: both parts are defined independently and the connection of the turret to the floater allows a free rotation.

In both cases, DEEPLINES offers some features that make the handling of turret-moored floaters easier.

Simple modelling

Even if the floater with turret is assumed to behave like one single floater during the computation, a turret option has been created in version V5R7 to simplify the model definition.

When the option Turret is selected:

  • A turret centre is automatically defined in the fairlead tab;

  • The FU Reference point positions become greyed since they will be automatically calculated from the turret position and FU heading;

  • An addition box Turret fairleads appears.

In that case, the floater is positioned in the global frame through the turret centre position as illustrated by the figure below:

In this example:

  • The turret center is 50 ahead of the FPSO reference point;

  • Another fairlead FSPO_bot is defined 50m behind the FPSO reference point.

As the FPSO heading is 0 degrees and the turret center is the origin of the global frame, the FPSO reference point is located at -50m and the fairlead FPSO_bot at -100m in the global frame.

Concerning the turret-fairleads, they are simply defined in the associated box Turret fairleads as shown below:

When the FPSO heading is modified:

  • The FU reference point is updated;

  • The fairlead points are updated;

  • The turret centre and its fairleads remain unchanged.

In addition when a floater is a floater with turret, the offsets defined in an EnviromentSet are interpreted differently in the sense that the Yaw motion is not considered as an incremental displacement but as an new initial heading.

That means that the yaw is applied from the start when the LOG file is translated, the change of heading is not part of the computation but a simple change of initial condition (like the draft change option).

So far, to avoid difficulties, the following restrictions shall be complied:

  • No specific heel/trim is imposed to the floater, only the heading shall be changed;
  • The turret orientaiton is fixed and alinged with the gloabl reference frame.

Complex modelling

The more complex modelling consists in defining on one hand a floater (whatever its type) and a rigid body which stands for the turret.

To connect the turret andthe floater, the following process is recommended:

  • On turret side:

    • Define the reference point at the turret top position;

    • Create another turret's fairlead at the turret bottom.

  • On the floater side:

    • Create two specific floater's fairleads on the turret axis corresponding to the two turret points just created.

Then, connect the rigid body reference point to the first floater's fairlead with a pin coneciton and create a spring between the two bottom fairleads with the default stiffness.

This way of doing is usually to most robust modelling to ensure the rotation of the floater around the turret axis in every condition.

Note also, that for coupled analyses, a specific keyword *WVANING has already been introduced in version V5R6.

With this keyword, before static calculations start, the floater and possibly the associated connection lines are oriented in a precomputed equilibrium position considered as an initial position with respect to the envirommental conditions. The precomputed position is based on equilibrium of the forces and moments resulting from the wind and current polar coefficents as well as the wave drift loads acting on the floater.

This reduces the lengthy transitory phase to approach the average position.