Ancrage dynamique des navires, plates-formes ou autres supports flottants

Dynamic positioning of ships, platforms and floating vehicles in general

Division des Techniques de forage et de production Institut Français du Pétrole

Abstract

The basic object of dynamic positioning is to maintain a ship, barge or platform at a fixed station near a reference point (e.g. above an oil-well). The system equipment is designed to maintain position of the vessel within specified limits by permanent opposition to disturbing forces due to wind, current, waves or other factors with the reaction of propulsion units or other active systems. Deviations are detected and transmitted to a computer, which sends optimized correcting force signals out to the propulsion units concerned. This type of system comprises the following components : a) A set of sensors b) A propulsion system c) Control and monitoring facilities The components are interlinked in the form of three control loops (Fig. 1), which are assumed to be completely unconnected. The type of vessel required almost invariably depends on geographical location and the type of operation involved, e.g. geological prospection, core-sampling, drilling, general underwater work, etc. Dynamic positioning systems are used on ships and semi-submersible platforms and will be on pipe-laying barges very shortly. Size of vessel is no object : the first applications were for 1 000 tons, but the semi-submersible SEDCO 709 platform recently equipped for the Shell Company is about 25000 tons. Position of the vessel is monitored by gyro-compass, inclinometer, acoutic or radio-electric sensors and increasingly efficient station-keeping ensured by anemometers, sonar, etc. At the design stage, the size of propulsion units required to whithstand outside forces is determined by calculation and from results of hydrodynamic and aerodynamic model tests. The computer ensures the following : a) Coordination of conversion and filtering of signals from sensors. b) Distribution of thrust requirements among available propulsion units. c) Optimization of instantaneous power consumption. d) Efficient information of the position, sub-unit deficiencies and other relevant factors. e) Switchover to manual positioning control at any required time. f) As a navigation aid or in approaching a site. g) Location of a diving bell, underwater equipment or auxiliary vessel. Station-keeping accuracy is usually to within a few per cent of the water depth. For depths between 300 m and 1 000 m for example, it can be assumed to improve from 6 % to 3 % of the depth (Fig. 8). Careful design of vessels with dynamic positioning equipment has resulted in excellent availability at operating locations. Autumn and winter rates for the "Pelican" in the North Sea, for example, were 81 % and 64 % of total time respectively. Dynamic positioning systems ensure better station-keeping than conventional "funicular" systems down to a present practicable depth limit of 1 000 m. Their relative positioning accuracy increases with water depth (contrary to anchoring systems). Their really outstanding feature, however, is that they are much more available and reliable in use. Major factors are that icebergs can be dodged without hauling-in cables, chains or anchors, headings can be maintained in most kinds of weather (and therefore, outside effects and roll reduced to a minimum), movements from station to station can be effected at high speed, and there are no connections between the seabed and surface (a tremendous advantage for comprehensively-equipped undersea fields). These factors frequently add up to a c1ear advantage for dynamic systems, despite their higher cost.