Till now we have taken a look at various types of ships such as bulk carriers, container carriers, reefers and so forth. In this article we will study about DP vessels. In the exact sense of the word DP vessels do not refer to a particular type of ship but to a class of ships which are fitted with DP systems, so basically we will be taking a look at the DP systems in this article.
Various types of ships used for commercial trade such as tankers and reefers carry cargo from one port to another. When these ships are at port they are tied (moored) so that they do not change their position and remain stationery during the entire process of loading or unloading of the cargo. When they sail out for their destination, the cargo is safe and secure and the weather conditions which make the ship rock and roll within reasonable limits do not have much effect except for inconvenience to the crew members.
But there are another category of vessels whose operations depend on their staying at one particular position and direction despite changing weather conditions and sea currents. These jobs could be several types such as underwater cable laying, drilling, mine sweeping, surveying the ocean floor to name a few. These operations require the ship to remain relatively stable even when weather and ocean are trying to move it around from a fixed position and direction.
These ships require specialized systems which help them to maintain their status quo in terms of positioning, depending on the analysis and reaction to real time forces acting on the ship and such systems are known and dynamic positioning systems.
How it Works?
In the last section I had mentioned the terms rock and roll but the exact terms given for the six degrees of freedom of a ship are roll, pitch, yaw, surge, sway and heave (so not rocking here!!). I will not explain in detail what all these mean but the degree of freedom concerned with the horizontal plane movement are surge, sway and yaw which basically refer to the movement of the ship along the forward/astern, port/starboard and rotation about the heave axis.
There are different types of sensors such as motion reference units, wind sensors and draught sensors which give feedback to the DP system and the thrusters, controller and propelling machinery is driven accordingly to maintain the ships position. The entire process is quite elaborate and the machineries used for DP control are very versatile and will be covered in a separate article. But for the time being you can take a look at the picture below which shows what are known as azimuth thrusters that basically consist of propellers which rotate in the pods and do away with the user of rudders for direction control. There are several variants of thrusters and these are used extensively in DP systems for direction control along with other machineries.