Study On Dynamic Positioning Accuracy Of A Deepwater Semi-submersible Platform

As the exploration of ocean resources is extended to the deep ocean, more and more ocean constructions use dynamic positioning system to keep their position and heading in the ocean. As a result, the dynamic positioning technology gets rapid development. The semi-submersible drilling platform has been widely used in the ocean oil and gas exploration field due to its great anti-environmental-load capability, adaptability to a wide range of water depth and great drilling capability. Therefore, it is of great significance to do a systematic research on the positioning accuracy of the dynamic positioning in the deepwater semi-submersible drilling platform, to know the influence of operating environment and thrusting failure mode upon the accuracy of positioning and that of changes in some factors in the designing process upon the dynamic positioning system.With a certain DP-3 dynamic positioning deep water semi-submersible drilling platform as its subject, this thesis is dedicated to explore the positioning accuracy of its self-contained dynamic positioning system and operating environment in different thruster failure modes and thus generate the law of the influence. On the basis of this, a series of extensive researches are done such as the error of the dynamic positioning accuracy caused by the first order motion which is often ignored in the designing process, the influence of the change in the way of placing the thruster, and mooring system assisted DP upon the accuracy of positioning. Therefore, this thesis mainly includes the following parts,(1) Research on the dynamic positioning accuracy of the deep water semi-submersible drill platformFirstly, the loading coefficients of wind, wave and current in the platform is calculated as a preparation for the follow-up research. In this step, the loading coefficients of wind and current is calculated in the building block approach. Secondly, the positioning capability of the system, namely the extreme environment conditions in all directions under which the platform can maintain its position, needs to be evaluated before the formal research on the dynamic positioning accuracy begins. As the result shows, under the designed environment conditions, the dynamic positioning system can meets the positioning requirements in all directions. This system can provide the best and the worst positioning directions, and this can provide guidance to the making of positioning strategy in a construction and thus sets up a basis for the positioning accuracy research in the thesis. Thirdly, it comes to the main body of this thesis, namely the research on the accuracy of the dynamic positioning. In this research, a series of numerical simulations about the movement of this dynamic positioning platform are done in time domain, and the influence law of this platform under different wind, wave and current from different directions and in different thruster failure modes is generated in this way. Finally, the reliability of the positioning accuracy of the dynamic positioning system is examined through monitoring the positioning capability and accuracy of the platform in a model test.(2) The influence of the first order movement of the deep water semi-submersible drill platform upon the accuracy of the dynamic positioning system In the process of designing a dynamic positioning system, the measured data of high frequent movement is usually ignored, and that of low frequent movement is retained for further control and research. However, this thesis renews this high frequent movement and considers the accuracy of the dynamic positioning control in this movement. Firstly, a model of this platform is set up in order to calculate the first order movement response in the designed wave condition. With the help of this model, the average data and the possible maximum one of the platform in three hours'movement are calculated. Then these data are superposed with the result of the time domain modeling of the dynamic positioning of this platform. In this way, the error in dynamic positioning accuracy caused by the first order movement is gained.(3) The influence of the change in the way of placing the thruster of the deep water semi-submersible drill platform upon the accuracy of the dynamic positioning system The platform examined in this thesis is a dynamic positioning semi-submersible drill platform which is equipped with eight thrusters. In a platform of this type, these thrusters are usually placed in two ways. In one way, they are all placed inside while in the other way, they are all placed in the two ends of a floating body. In this thesis, there are four new ways of placing thrusters in addition to these two current ways. In three of these four new ways, every two thrusters are placed along the floating body at a distance of 10 m either inwards or outwards, and in the other one, the thrusters are placed in the two ends of the floating body. Then, the accuracy of dynamic positioning in each way of thruster placing is respectively researched in order to gain the influence of such change in the way of thruster placing upon the accuracy of dynamic positioning.Through numerical calculation and model test, this thesis makes a research on several aspects of the deep water semi-submersible drilling platform in order to find their influence upon the accuracy of dynamic positioning, and the relevant element of such influence and its law has been found. The findings can serve as a reference to making positioning strategy in designing and constructing the dynamic positioning system in deep water semi-submersible drill platforms and this is significant to the development of the technology in designing the dynamic positioning system in deep water semi-submersible drilling platforms.