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The exploration of oceans and development of offshore industries through coring, drilling, pipelaying and ocean observation have been making increasing use of dynamic positioning (DP) systems for vessels in waves, wind and current environments. The DP systems usually include the following subsystems as acoustic positioning system, control system, thrust and power system. Generally, a dynamically positioned vessel with DP systems will produce thrust to counteract the environment forces acting on it for the purpose of maintaining the position and heading as closely as required to some desired position. However, the DP systems usually make errors due to the inaccuracy of sensors and the unexpected underwater sound speed. Therefore, this work presents a method to improve it and raise its accuracy.
In earlier days, the dynamic positioning of offshore vessel used the sensor system, such as Short baseline system ( SBL ) and Long baseline system ( LBL ), to estimate the relative position of vessel with respect to the seabed sensor. Unfortunately, its accuracy of sensor system is less and much difficult to hold according to uncertain and complex sound speed which is a function of temperature, salinity and depth. This paper then presents a corrected mode, which use Euclidean geometry, adding to the SBL and LBL for a precise position on surface vessels and submersible vehicles.
The conventional control system uses many sensors, such as acceleration sensors, velocity sensors, environment sensors, and filters in estimating the thrust. Those sensors are too many and may have electric errors. To reduce the quantity of sensors and decrease the errors, we develop a Work and Impulse-Momentum principles (WIM) control system, which is based upon Work and Impulse-Moment principles, to estimate thrust and moment commands.
The purpose of the thrust system is to maintain the vessel within a given distance of a desired position in the thrust-producing mechanism as quickly and accurately as possible. It usually assumes the angles of some specific azimuth thrusters are fixed, and all the thrust contributes equally to deliver the required force in one direction. However, there would be generated a counter moment, which may not satisfy the required moment according to the above assumption. Therefore, this work presents an optimum thrust control system; thus term constrained sequential quadratic programming (SQP) which can automatically adjust the azimuth thrust angles and thrust to reduce the wear and tear, minimize the required thrust and power, and allow the enabling and disabling of thrusters.
This work performed experiments to verify the accuracy of the correct mode of SBL system. And, Donha & Brinati’s example is followed to verify the feasibility of the WIM control system, which performed semi-submersible platform positioning using the LQG and Adaptive controller, and the results are feasible and economical. Furthermore, Simulation of a 17,500 tons coring vessel positioning and a 2.5 tons explorative submersible vehicle navigating in a southern Taiwan shallow ocean region has shown that the dynamic acoustic positioning system can provide good positioning. The results provide highly accuracy and valuable contribution for the dynamic acoustic positioning of offshore vessels.
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