| There exist large amount of oil and gas reserves in South China Sea, and someone calls it The Second Persian Gulf. China has started the projects of drilling and exploiting the oil and gas reserves in that area. Current geologic material demonstrates that most of the oil and gas reserves locate at the site with water depth more than 500 meters. However, present drilling units owned by Chinese enterprises can only operate at the site with water depth less than 500 meters. Cooperating with the famous domestic design institutes, the shipyards and the universities, CNOOC is now organizing a process of designing and building the first semi-submersible drilling unit, which can operate at the site with water depth of 3,000 meters. The government provides a powerful support. A National High Technology Research and Development Program of China (Grant No.2006AA09A103) has been set up, and its aim is to support the research on the techniques for the semi-submersible drilling unit operating in 3,000 meter water depth. The research included in this dissertation is supported by one of its sub-programs, whose aim is to develop techniques in the research for global performance, position capability and model test. Besides, part of the content in the dissertation is also supported by a project founded by Science and Technology Commission of Shanghai Municipality (Grant No.05DJ14001).It is of great importance to master the design skill for semi-submersible drilling unit. Presently, Chinese design facilities are lagged behind the foreign design enterprises, on the capability of designing advanced ocean engineering units. Although purchasing ready-made drawings or fundamental design projects directly from foreign enterprises can shorten the design phase, it would cost large amount of capital. Furthermore, it will be difficult to obtain the core techniques from the foreign enterprises, and the purchase deeds will tamper the innovative tendency of Chinese design facilities. The prevailing design process in China is the series process, based on the purpose to improve the parent unit. This design process has the shortages of relying too much on the parent unit, high design cost and long design period. It is not suitable well for designing the advanced ocean engineering units. New design concept and design technique should be invented to meet the requirements for the operation in deep water depth area. The semi-submersible drilling unit is a high complicated system with many disciplines involved, and many coupling effects exist between the disciplines. The design work is a challenge job. Multidisciplinary Design Optimization, simplified as MDO, is one of the feasible techniques to solve this problem. The research in this dissertation is focused on the application of MDO technique to the design of semi-submersible drilling unit operating in large water depth, and following research work has been finished.(1) The development history of MDO technique and its current research condition are introduced in the dissertation. Several key techniques of MDO are then described thoroughly, including mathematic modeling, design of experiment and polynomial approximation model technique. The fundamental principle and flow chart of Collaborative Optimization are introduced, and its feasibility of application in the marine and ocean engineering is verified by a soft bow design example. It is indicated that the Collaborative Optimization can hand with the marine and ocean engineering operation.(2) The characters of semi-submersible are described in detail, and the disciplines in the conceptual design and their analyzing techniques are decided. An object semi-submersible drilling unit is chosen. Six design variables are selected out and their varying ranges are determined. The varying levels of design variables are made by the theory of Design of Experiment, and 22 loadcases are determined by Uniform Tables.The two key analysis ways, model test and numerical analysis, are described in detail. Accurate hydrodynamic characters of the drilling unit can be obtained by model test technique. Some key points of using model test technique are introduced in the dissertation, including facilities and instruments, law of similarity, coordinate system, environment modeling, and test results. The results include wind-force, current-force, decay result, irregular wave test results, and the results of heave, roll and air-gap. The test results would be used as the correction coefficients to the numerical simulation results. Numerical simulation is another important discipline analyzing technique. The characters of hydrodynamics, stability, positioning capability, structural performance, and engineering operation are obtained with numerical simulation techniques.(3) The hydrodynamic character of the drilling unit is one of the characters concerned mostly by the operators. The hydrodynamic discipline analysis is finished with the hydrodynamic model and the correction coefficients. The characters of heave motion, roll motion and air-gap of the 22 loadcases are obtained, and the approximation models of the three characters are built up.The stability character of the drilling unit is one of the most important characters in the design. Most classification societies and the international organizations all make strict requirements on the stability check. Coefficient K is used here to check the stability of the drilling unit. The stability in beam-sea is selected out to be checked in the research. The overturning moment is calculated by the simplified method, and the righting moment is carried out by numerical simulation method. According to the ABS rule, the coefficient K is obtained, and the approximation model of stability is built up.The positioning capability is one of the important factors influencing the drilling operation. The positioning capability consists of the dynamic positioning and the mooring positioning. The numerical simulation method and the model test technique are used together to finish the discipline analysis of the dynamic positioning. The cause of mean wave force is introduced, and the fundamental theories of Maruo method and Newman method are described. The environmental resistance figures are drawn, based on the conclusion of the mean wave force, the wind force and the current force, and it is a key factor in the determination of dynamic positioning operation machines. Time domain analysis method is used to finish the discipline analysis of the mooring positioning. The maximal horizontal displacement of the drilling unit is carried out. The approximation models are built based on the discipline analysis results of the dynamic positioning and the mooring positioning.Concerning the structural condition of the drilling unit, some defects in the conventional structure design flow are discussed. It is pointed out that the requirement of the global strength of the drilling unit can be shown at the beginning of the detail structural design, with the employment of MDO technique. The design wave method is used to do the global strength analysis, and four typical design load conditions are chosen. The structural components with the highest stress level at the intersection area between the box-deck and the column is selected out to represent the structural condition. The mean stress level of the four design loadcases is chosen to be the key factor, and the approximation model is built up with it.Polynomial fit method is used to acquire the approximation calculation formula for the payload of the drilling unit. The time-domain analysis method is applied to calculate the maximal mooring force of the mooring lines. The building cost of the drilling unit is calculated out by the estimation formulae for the semi-submersible drilling unit weight. All of the three discipline analyses are finished to reflect the engineering operation character and the economic character.(4) The optimization process for the semi-submersible drilling unit is accomplished by Collaborative Optimization method, on the basis of discipline analysis, model test results and MDO technique. The conceptual design plan is optimized and then obtained. The optimization process runs for seven times, according to different restriction variables. The results of the seven optimization processes are concluded. A character of MDO technique can be summarized as a trade-off process.(5) An innovative process, called MDOEXT process, is brought forward, based on the summary of utilizing MDO technique in the conceptual design of semi-submersible drilling unit. The new process can help the designers to obtain a better design plan. The working flow of the new process and the way how it can adjust the work between the experts and the average engineers are described in detail. The new way is suitable for the conceptual design for ocean engineering units.In summary, three innovative accomplishments have been finished in the dissertation.(1) It is the first time of introducing the Multidisciplinary Design Optimization technique into the floating platform design field in China. The feasibility of applying MDO technique in the ocean engineering field is verified, through the process of conceptual design for a semi-submersible drilling unit. Furthermore, a new way of mastering the global characters of the semi-submersible drilling unit is promoted.(2) A new way of guiding the numerical simulation process to acquire better analysis results with the help of model test technique in the conceptual design phase is promoted.(3) An innovative process, called MDOEXT, is brought forward. The new process has the power of adjust the work between the experts and the average design engineers, and it can help the designer to obtain a better conceptual design plan. This is a prosperous design concept. It acquires more designers joining the conceptual design phase, and it will increase the proportion of conceptual design in the whole design phase, reduce the contradictions in the following design process and save the total design cost. |
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