Principle of DP

For ABS, by definition, a Dynamic Positioning system is “a hydro-dynamic system which controls or maintains the position and heading of the vessel by centralized manual control or by automatic response to the variations of the environmental conditions within the specified limits”.

ABS, Rules for Building and Classing Steel Vessels, 2003, Part 4 Chapter 3 Section 5, 15.1.2 (a)

The D.P. is therefore designed to perform this task within a certain range limit, using minimum fuel and with minimum amount of wear and tear of the propulsion equipment.

The D.P. computer calculates the difference between the "wanted" position and the "present" position of the vessel,it then calculates the force that the propellers should produce in order to make the difference as small as possible.

The system controls the vessel's position in three horizontal degrees of freedom, called surge (ahead and astern), sway (sideways) and yaw (turning of the bow).

The computer use a mathematical model to calculate the force of reaction, that is based on a hydrodynamic description involving some of the ship's characteristics such as mass, drag etc.

The purpose of this model is to describe, as much as possible, the vessel's motion and reaction to any external forces.

The model is "driven" by the same forces as the vessel itself. These forces are wind and thrusters, and using algorithms the model is also able to calculate the sea current.

The model of course is never 100% as perfect as the "real world", but by using the so-called Kalman-filter technique the model is able to correct itself continuously in the following way.

The vessel heading and position is measured using the gyro-compass, along with one or more position reference systems that are sent to the computer.

These data are compared to the predicted data produced by the model and the difference are "filtered" by the Kalman-filter and then finally used to "tune" the model to the actual situation.