Thruster

Thrusters can be added to a body and will, when the thruster is rotating, provide a force on the body.
The model includes a simplified method of modelling the dynamics of the propeller which means that when
the thruster receives a thrust signal it will take some time for the rotation speed to rise or fall to the
demanded level.

Rotatable thrusters may, on demand, change direction during the simulation. A simple servo controller model
is used so that the thruster direction will not be changed instantaneously.
The servo controller is configured with the parameters Max revolving speed and Tc Azimuth.

1. Thruster Control Sequence

Thrusters are usually controlled by the DP system, however an option to manually send control signals to the
thruster is also available.
A sequence of control signals may be specified where each signal has an activation time, a demanded thrust and
a demanded thrust force direction.
Due to the dynamics in the thruster model the thruster will not necessarily reach the demanded thrust and direction
instantaneously.
The control signal will be active until the activation time of a new control signal is reached.

The control signals specified in the control sequence will override signals from the DP system as long as the
checkbox Accept DP System Input is left unchecked.
When Accept DP System Input is checked control will be handed over to the DP system and the demanded thrust
and direction for that control signal will be ignored.

1.1. Example: Modelling Drive Off

Drive off is fault scenario where a hardware of software fault causes one or more thrusters to give thrust
in an adverse direction.
The crew on board the vessel would typically discover the fault after some time and initiate actions to
counteract the effect.
The magnitude and direction of the thruster drive off, the reaction time of the crew and the actions of the
crew will all influence the resulting motion of the vessel.

One way of modelling such a scenario is with the following thruster control sequence:

Time [s]	Thrust [N]	Direction [deg]	Accept DP Input
4000.0	4.0e5	90.0	No
4060.0	0.0	0.0	Yes

Here, we have specified that the thruster will start to give a large thrust force after 4000 seconds.
After 60 seconds the crew have successfully initiated counter actions and the thruster once again accepts
signals from the DP system.