| To satisfy the increasing demand of oil resource, a trend of exploitation of offshore oil and gas resource has appeared, especially in deep water. As the increase of water depth, many new challenges, which might never be met in shallow water, have appeared. In this situation, many new innovative floating structures are being proposed to perform well motion responses in the atrocious sea states, while save the cost in the meantime. The spar platform has been regarded as a competitive floating structure for deepsea oil and exploitations, since the first spar was installed in 1996.Spar platform concept was developed from the Tensioned Leg Platform (TLP). Main feature of the classic spar is its deep-draft vertical cylinder hull, which presents excellent motion characteristics even in severe sea states. In the lowest compartments there is provision for variable ballast so that the draft and trim can be maintained under varying topside loading conditions. The arrangement provides a large positive GM and hence particularly good stability. The spar is kept on station by lateral catenary anchor lines. In comparison with the motions of other floating structures, those of the spar are relatively benign. Its deep draught, combined with the large mass of water entrapped by the skirt, lead to natural periods well away from those in which wave energy is commonly concentrated. From the first generation of classic spar, spar platform has evolved into the second generation of truss spar and the third generation of cell spar.Domestic studies on the spar platform mainly focused on parts of spar performances, such as dynamics of mooring system. And the analysis method was basically limited to numerical simulation.In this situation, this thesis carried out the concept design and numerical and experimental study on spar platform. It is financially supported by the National Natural Science Foundation of China and Technology Commission of Shanghai Municipality. The thesis mainly consists of three parts.Part I: New Spar Concept and Preliminary DesignFirstly, based on the former three generations of spar platforms, the thesis put forward a new spar concept– the CELL-TRUSS SPAR platform. Features of truss spar and cell spar were both taken into account in the design process of the new spar concept, aiming to take advantage of the heave plate damping feature of the truss spar to obtain satisfactory heave motion performance, while reduce manufacture and installation difficulty by means of cell concept. Besides, the cell configuration is supposed to reduce the vortex induced motion of the spar hull.Secondly, to satisfy different ocean environment and design conditions, three cell-truss spar platforms were designed, including: the Cell-Truss Spar-I, Cell-Truss Spar-II-A and II-B platforms. Type I platform is in application with wet trees. Function of the hard tank is only to offer the buoyancy of the platform and no centerwell is arranged. The platform has a comparatively small diameter and lower cost. Type II platforms are designed to arrange buoyancy cans and top tensioned risers. Thus their diameters are bigger and they have larger exploitation abilities. Besides, center cell of the type II-A platform extends to the soft tank of spar hull to offer protection to the risers. While in the sea area where the current is comparatively strong, type II-B platform is more suitable as its center cell is shortened.Thereafter, a preliminary design of both the spar hull and mooring system, as well as simplified theoretical and numerical analysis of its hydrodynamics and mooring line tensions, have been carried out on the cell-truss spar-I platform. Generally speaking, two aspects are involved during the concept design: one deals with the design method and structure form, and the other concerns the hydrodynamic behavior. Many effects may have to be taken into account during the design process, which could be described as an interactive flow. The thesis also accomplished the concept design of the cell-truss spar-II-A and II-B platforms, and offered their main particulars.Part II: Hydrodynamics of Spar PlatformThis part comes down to four spar platforms. Besides the three cell-truss spar platforms mentioned before, as a pro-study on the hydrodynamics, the thesis presents the results of an experimental investigation on the hydrodynamic behavior of a joint-studied spar concept which is called Geometric Spar (G-spar) and its complemented new Integrated Buoyancy Can (IBC) concept.For the study on the G-spar platforms, result shows that the natural periods of surge, heave and pitch motion of the G-spar platform are far from the wave period in the ocean environment conditions, thus the wave frequency motion are not obvious. Different numbers of heave plates have significant effects on the heave motion. With more heave plates, heave motion would have a distinct reduction. Wave frequency motions are sensitive to the variation of ocean environment conditions. Horizontal motions mainly present as low frequency drift motions with large amplitude in 100-year Storm in Gulf of Mexico. Accordingly, forces on the mooring line converge in the low frequency domain with large peak values. Although the swell usually has small amplitudes, resonant heave motion with large amplitude could still be aroused.In the following chapter, hydrodynamic performances of the cell-truss spar-I were studied both numerically and experimentally. The numerical simulation was conducted by means of nonlinear time-domain fully coupled analysis, and its results were compared to the experimental data. It is obtained that, extreme values of horizontal and vertical motions and rotations of the cell-truss spar-I were less than 10% water depth, 3 m and 12°, respectively. Horizontal motion and rotation of spar platform are mainly dominated by second-order low frequency drift force, while the resonance between heave and swell condition with long wave period is another respect need pay special attention to. For the low frequency surge motions, the smallest responses always happen near the fairlead position (z≈-70m), which could be due to the effect of mooring system. However, when total responses are concerned, surge motions at the position near keel turn to be the smallest in all three ocean environments.Cell-truss spar-II platforms were coupled simulated in time-domain. Hydrodynamics of the platform, as well as performances of mooring lines were analyzed in different sea states. Results show that, motion responses of the cell-truss spar-II platforms could be availably restrained in a satisfactory region and dry trees and TTRs could be applied to the spar concept. As may be noted, responses in no-collinear environmental components were special analyzed. Besides, the thesis also calculated hydrodynamics of the cell-truss spar-II-B in 100-year typhoon in east area of South China Sea. Satisfying global performances were also obtained.Basically speaking, the cell-truss spar platform concept successfully inherits the excellent motion performances of the former generations of spar platforms, while processing some improvements to cut down the fabricating difficulty and cost. And it should be competent for the oil and gas production in deepsea area.Part III: Effects of coupling and heave plates on the hydrodynamics of the spar platformFor the floating structures in application in deepwater, coupling effects of the mooring lines and risers on the motion responses of the structures become increasingly significant. The thesis presented the comparison of results from three calculation methods, including frequency-domain analysis, time-domain semi-coupled and fully-coupled analyses, and the experimental data to obtain the applicability of different analysis approaches. It's obtained that, despite the time-consumption, time-domain fully-coupled analysis is a reliable method to simulate the motion responses and line tensions of floating structures in deep water. Proper hydrodynamic coefficients should still be calibrated from model tests to improve the reliability. For the responses of which the wave-frequency portion are dominant, such as heave response and tensions of semi-taut mooring line with large pre-tension, as well as other motions in wave extremes condition in West Africa, frequency-domain and time-domain semi-coupled analyses are more efficient to get relevant statistics, especially during the preliminary conceptual design.As the efficiency of heave plates is crucial to the heave performance, different types of heave plates are fitted to the truss structure, varying from the number and spacing to the form, such as perforated plates, to investigate various design aspects of the plates. Experimental results for the responses of the spar to the wave loading, as well as loads on the heave plate and added mass and damping coefficients are presented. Main conclusions includes: heave plates efficiently reduce the heave response of the spar fitted with truss section and increasing the number of heave plate could result in an obvious decline of heave motion; in some cases, perforated plates present more effectively than solid plates; by using heave plates with a proper'hole-area ratio', satisfactory heave motion performance could be obtained. For a perforated heave plate, to get a maximum edge length while avoiding loss too much area, the holes should be relatively small. Though the added mass plays a significant role in reducing spar heave response, the damping in the platform system could not be ignored. As no detail numerical simulation has been carried out, this problem needs much deeper study. Conclusions from this thesis could lead to a more comprehensive understanding of characteristics of spar platforms and contribute a lot to the development of offshore exploitations in deep sea areas in China. |
