Dynamic Positioning of Vessels at Sea
The discovery in recent years of vast mineral deposits on and below the floors of the seas and oceans has resulted in a great increase in the number of offshore mining projects. All over the world companies are engaged in the drilling of wells the laying
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ling of wells the laying of submarine pipelines and the recovery of valuable minerals such as diamonds and even gravel and sand from the sea-bed. In deep sea these activities are carried out from surface vessels. The type of vessel used for this work ranges from plain barges to intricate and costly designs as the semisubmersibles. Common to practically all activities concerned with the exploration and exploitation of the sea-bed is the necessity of accurate positioning of the vessel relative to the point of interest on the sea-bed. An
example showing the need for accurate posi-
tioning of the surface vessel in the case of a drilling vessel. (See Fig. 1). In this case the drilling vessel from which the drill string reaches tv the well on the sea-bed, must remain within well specified limits of the point of intersection of the horizontal plane coinciding with the surface of the sea and the vertical line reaching upward from the well. Failure
J. Pinkster, Dynamic Positioning of Vessels at Sea © Springer-Verlag Wien 1972
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Introduction to remain within these limits will, and has in the past, resulted in fracturing of the drill string. It may be noted that the heading angle of the drill-ship is of sec ondary importance. Another example illustrating the need for good station keeping abilities Fig. 1
is that given in Fig. 2.
This shows, schematically, the effect of errors in the co-ordi nates in the horizontal plane and heading angle of a pipe-laying barge on the loading of the pipe. In comparison with the previous example the demands placed on the station keeping abil ities of this ship are more exacting since the heading angle of the barge must also be taken into account.
PIPE
............;;::.......:::::-""oor..___
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COURSE ERROR
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PIPE LAYING VESSEL
These are just two examples illustrating the necessity to maintain the position of the vessel within certain limits. The classical way of accomplishing this is
Fig. 2
by means of a system
of
Conventional Mooring Systems
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mooring lines connecting the vessel in question to anchors placed on the seabed. Such a system is shown schematically in Fig. 3. Provided that the correct number, breaking strength, weight and pretension of these mooring lines is chosen, these systems can provide a reliable and efficient means of positioning a vessel. There are, however, a numberof
Fig. 3
draw backs to such systems, one of these being the fact that in greater water depths the sheer length of the mooring line becomes prohibitive, another being the time lost in laying out and hauling in the anchors when a change in position is desired. Considerations such as these have led to the development of dynamic positioning
sys~ems.
Dynamic positioning systems are
those systems which maintain the position of a vessel
~olely
by means of thrust developed by units attached to the vessel for that purpose. Basically all dynamic positioning systems consist of the following elements:
Measurement of the Position of the Vessel
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- A system to measure the po
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