Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers, P.O. Box 228, The Hague, The Netherlands. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
After a short description of the general principles of dynamic-stationing systems, a method principles of dynamic-stationing systems, a method of optimising the controller is described. In the past, dynamic stationing has been regarded as a past, dynamic stationing has been regarded as a solution only for deep-water operations. Complete simulation of the behaviour of the vessel, the controller and the thruster units under the influence of current, wind and irregular waves now enables us to optimise the system in such a way that stationing is even possible in as little as 200 ft of water.
A description and some results of such simulations, which can be performed equally well on digital and hybrid computers, are given. As a result of these studies, the need for two further improvements became apparent. The first is a form of filtering technique to remove the cyclic component from the steering signal of the thrusters, so allowing improved control, higher efficiency and less wear and tear on the thrusters. The requirements for such filters are given. The second improvement is the feeding forward into the control system of a signal proportional to the wind force to enable quick response to wind gusts, so reducing Use deviations from location that would otherwise result.
Very few dynamically stationed vessels have been built to date, although the principles involved are very simple. The vessel is kept on station with the aid of a servo system, rather than by mechanical means. To accomplish this, the deviation from Be origin is measured and translated into command signals, which activate propulsion units in such a way as to compensate for the deviation.
Thus this is in fact a normal control system, of a type used in many engineering fields. The difference between this and normal control applications, however, is the size of the mass to be controlled and the magnitude of the outside disturbing forces. These forces are delivered by wind, waves and current; the first two of these are irregular in character and only the third one remains stable over a restricted period of time. The sum of these forces may be larger than the restoring forces that can be delivered by the propulsion units, and it is only because of the cyclic nature of (especially) the wave forces that control is still possible. It will be clear that the nature of these possible. It will be clear that the nature of these forces will influence the design of the control system substantially.