Dynamic positioning systems provide a highly versatile "anchoring" system for vessels that must remain on a given location out in the deep ocean. While no single system design can be universally used, general guidelines can be established to assist in the selection and the design of the various subsystems.
At the present time a dynamic positioning system has been designed and installed aboard a drilling vessel-The Glomar Challenger--and is currently being used with excellent success in water depths from 3,000 to 18,000 feet.
As the interest and activity in the oceans increase, the need for "working platforms" that have the capability of being used in almost any water depth and at any geographical location has become significant. These working platforms require many peculiar characteristics but two of the more important ones are mobility and stationary stability; that is the vessel which forms the foundation of this working platform should have the capability of being rapidly moved from one location to another and once the desired location has been reached, it should have the capability of being essentially anchored at this location quickly and easily. At the present time, these characteristics cannot in general be obtained to the extent desired but, dynamic positioning systems can satisfy a major and necessary part of the requirements.
A dynamic positioning system uses the energy converted on board the vessel to provide the forces necessary to hold or "anchor" the vessel at a given location; no mechanical connection to the sea floor or other land masses are required. It should be noted that the mechanical connection considered here refers to constraining connections and not small lines that might be used to provide reference signals for the control system input since the necessity of having to set out anchors or make other mechanical connections to the sea floor is eliminated, the relative ease in which the vessel can be positioned at any given location as well as the ease in which the vessel can leave a location having once been maintained on "station" can be readily appreciated. Furthermore, in deep water (for example 18,000 feet) it is the only practical method of maintaining a given geographical position.
A dynamic positioning system is composed of the following general subsystems:
Postion measurement system
The subsystems are related as shown diagramatically in figure I. In essence, the positioning system can be considered as a closed-loop feedback servo or control system. In operation, a reference position (surface coordinates and vessel heading) is selected and provides the input to the control system. This input is compared to the measured position of the vessel and if a difference exists, an error signal is generated. This error signal is processed to provide the proper logic to the controller. The controller moves the vessel so as to reduce the error signal and when the error signal is zero, the measured position of the vessel coincides with the selected reference position.