Research On Preliminary Design Of Deepwater Semisubmersible

Wide oceans contain abundant resources. With increasing worldwide demand for oil and gas, exploitation of ocean oil and gas switch to deep water and ultra-deep water fields with water depth of 450~1500 meters and more than 1500 meters respectively. Conventional platforms like jack-up platform and Gravity platform are not suitable for deep water given their substantial increase of weight and cost. Then research on drilling and production system for deep water has become hot item.At present, TLP (Tension leg platform), Spar and semisubmersible are three main platforms used for deepwater oil and gas exploitation. Semisubmersible has priority of less total investment compared with the other two platforms. More deck space, deck variable load, production ability and wider range of water depth are achieved in semisubmersible. Semisubmersible has multiple functions of drilling, well repairing, production and so on. It could also be easily transformed according to different requirements, and has good performance in both operation condition and survival condition. Semisubmersible is the better choice for deep water operation based on factors like cost, range of available water depth, number of well, service life, zone and so on. In fact, semisubmersible development comes to a new era as offshore oil and gas exploitation extends to deep water. Lots of companies and academic institutes invest much in a sixth-generation development of semisubmersible.Given the rapidly development of semisubmersible, trends of this platform are discussed here. Researches on preliminary design of deepwater semisubmersible are developed and presented in this dissertation.This dissertation mainly investigates the following items:1. History of Semisubmersible, development trends and the new generation's characterization are described here. Present researches on preliminary design of semisubmersible are reviewed. 2. Four common types of deepwater semisubmersible are designed based on the prototype of a sixth-generation semisubmersible. Comparative analysis of heave in waves, intact stability, towing resistance and hull steel weight is carried out, which assists the selection of the proper platform.3. A multi-objective optimization model of the deepwater semisubmersible is established in this dissertation. The model is determined by hydrodynamic coefficients of the platform with economic factor and performance in wave in mind. Here, a program based on the two-dimensional strip theory is developed for hydrodynamic coefficients calculation. The effect of design variables on the optimization object is also studied by design of experiment. Considering the nonlinear of motion equation and hydrodynamic computation, a multi-objective optimization genetic algorithm, non-dominated sorting genetic algorithm, is adopted in the optimization design, and Pareto front is achieved. Three representative results are selected for advanced analysis, showing that all the semisubmersibles after optimization design have good motion in wave and their natural periods are all away from the common wave period. According to comparative analysis, it is believed that semisubmersible's motion performance will be better after optimization design. The cost increases excessively beyond some extent, while less optimization in motion performance can be expected. Finally, one intermediate optimization result is taken as preliminary design result.4. Motion responses and wave forces of the adopted platform are thoroughly analyzed by three-dimensional potential theory. Short term response and long term response are achieved with P-M wave spectrum. All the results are in frequency domain, and will be used in further research.5. Heave RAOs of tow semisubmersible are achieved by model test, and are compared with results computed by potential theory, which verify the reliability of the theory. Airgap in regular waves is also achieved.6. Taut mooring system now steadily replaces catenary mooring system in deep water. Also researches on synthetic mooring lines are performed with the purpose to reduce the weight of mooring line, and to increase the variable load of semisubmersible. Various synthetic mooring lines, such as nylon and polyester rope, have been widely used in deep water mooring now. Research and design on taut mooring system are reviewed and a wire-polyester rope-wire mooring line is designed in this dissertation. Static analysis of mooring system with nine different line patterns is carried out to study the effect of line patterns on restoring force and the tension of lines under design condition.7. More exact results are gained when applying time domain analysis that turns motion responses of semisubmersible from frequency domain to time domain, coupled with all nonlinear and dynamic effect analysis. With the interaction between semisubmersible and mooring system in mind, time history results of wave loads, motion of semisubmersible, tensions of mooring lines under both operation condition and survival condition are achieved. These results can be turned into terms of spectral density and Weibull probability distribution. All the analysis results can be used in design and analysis of deepwater mooring system.Studies on platform type selection, optimization design, hydrodynamic computation, model test, short and long term response, preliminary design of mooring system, and coupled analysis of mooring semisubmersible in time domain are carried out in this dissertation. The results are possessed of important value for preliminary design of deepwater semisubmersible.