In order to extract resources or to precisely position sensors and infrastructures on the seabed, dynamic positioning is used. It allows a ship to keep a target at a safe distance for example when following a ROV (remotely operated vehicle – e.g. a scientific mini-submarine).

Dynamic Positioning explained

When combined with other sensors, dynamic positioning data enable us to map the sea bottom on any scale. Furthermore, it benefits a cruise ship or floating research stations by turning the vessel into the wind to take the waves on the bow so that it moves less, using thrusters to stay for hours at a single point. Additionally, when the weather is fine, passengers can avail themselves of scenic views longer, thanks to the dynamic positioning system.

Historically, manually controlled propellers were used to help the ship to stay at a position, the first time these appeared was on a drilling ship in 1961. Then primitive automatic control systems were introduced, they used an anchor and a stiff wire to sense which way the gimble head (which feeds the stiff wire) was pulled. Of course, this system was DP 1 level and therefore did not possess any redundancy to cope with a single point of failure event.

New dynamic positioning systems are now fitted to every GPS unit. Goal achievement depends on the number of sensors and motion controllers under control of the GPS or onboard computer.

Sensors for dynamic positioning

• GPS is used in variety of methods to identify and fix position, but since it does not provide the details that dynamic positioning systems need, more reference points are used in order to determine position.
• Special GPS uses stations with known ground positions to determine the vessel position; this is a more precise method than using sole satellites as a reference. The vessel computer is closely bonded with more than four locations.
• Vessels can use laser-positioning systems to determine their location by shooting a laser at fixed targets (prisms) to calculate distance and angle to them.
• There are various ways of using sensors for DPS, one way is quite similar to an optical system. Sound is sent from a previously sited sound emitter and picked up by the sonar onboard, with this data DPS can correct the movement and position of the vessel.
• It works perfectly for shallow areas, new electronic controls help to make the system relatively low-priced, and it is a simple and easy system for remote deployment.
• Position sensors measure the wind and tide in real time and give feedback to the control system to adjust the ship’s movments.

PID controls

This dynamic positioning system is controlled by a computer using a PID algorithm. This algorithm uses the sensor data to estimate the movement and position of the ship, thus enabling position corrections.

Thrusters

The two different standard types are open thruster (where the propeller is not covered) and ducted thruster. Open thrusters and ducted thrusters are not as safe as ducted pumps – e.g. for divers. Ducted pumps are the third method; they create thrust via a jet stream of water. Currently single thrusters, which are positioned at the center of the vessels, are preferred. For example, they allow circular tugboats to work in shallow water and tight quarters.

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