Rigs equipped with dynamic positioning systems can maintain their position above a wellhead without using anchors or mooring lines. Instead, thrusters mounted in the hull of the vessel are activated by a computer  contact system. If the rig is pushed off station due to wind or waves etc, the thrusters are automatically started. These then push the vessel back to its original position.
The control computer has to have some method of monitoring the rig’s position. This can be done by using an acoustic sound system.

A sound transmitting device called an acoustic system.
A sound transmitting device called an acoustic transponder is located on or near a well head on the seabed. Hydrophones( sound receivers) which are mounted on the vessel’s hull receives signals sent
out by the transponder.

By comparing the arrival times of the signal and knowing the relative position of the transponder and hydrophones, any offset of the vessel position from the normal will be calculated. This information is fed to the computer which then calculates the forces required to bring the vessel back to its initial position. The correcting force is applied via the thrusters and the vessel main propulsion unit.
If the acoustic monitoring system fails, a back up system is used. This employs a taut wire stretched from the vessel to a heavy weight on the seabed. The angle and inclination of the wire is measured to determine the rigs offset.

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