Research On Dynamic Problems In Concept Design Of A New Offshore Equipment

With the word wide increasing demand for energy, to exploite oil and gas in deep water is becoming an irresistible trend in the offshore industry. China has a vast maritime area, and in the bottom of the southeast China Sea has been proved containing a wealth of oil and gas resources. At present, our country has basically mastered the exploitation technology in shallow water. But for the deep-sea exploitation which is more than500m, there are still many chelleges which need to be research. Therefore make breakthrough in key technologies of deepwater oil equipments is become a decisive factor for deep-sea development. Floating Drilling Production Storage and Offloading unit (FDPSO) is a new type of equipment, which adapted to deep-sea and marginal oil fields development, having a great promising future in application, and becoming a hot topic in recent years.FDPSO is evolved on the basis of the existing development equipments, which includes the functions of two systems, one is typical floating drilling unit (such as TLP, Spar and semi-submersible platform, etc.), and the other is producing unit as FPSO. As a multipurpose floater, FDPSO is more efficiency and economy. In fact, different working functions have different requirements of hydrodynamic performance. As drilling operation has relatively rigour requirement of motion performance, the structures of drilling platforms are mainly in form as cylindrical, tubular truss and caission, which aimed to reduce the surface subjected wave load and maintain its motion in a small range. While to storage and offloading crude oil needs large space, so the FPSO is usually a ship-shaped vessel. Because the ship-shaped vessel has very large wave-force area, its motion performance is poor and does not support drilling capabilities. Therefore, to meet all operational requirements, FDPSO must to be designed with proper motion performance, and is not a simple combination of two existing units. At present, FDPSO is still in the concept design stage, which has no specification to guide the design, and also in lack of construction experience. Research institutions at home and abroad are carrying out research works on FDPSO, to make it applicated in engineering in the near future. In this paper, the main research works have be done about key issues in concept design of FDPSO, such as hydrodynamic analysis and dynamic analysis of Tension Leg Deck (TLD), which can be summarized as follows:(1) Expatiate on the basic principle of concept design of FDPSO-TLD, given the layout of hull and TLD system. Based on potential flow theory, the hydrodynamic performance of the hll in frequency domain is studied. The influence of the moon pool size of the hull to the dydrodynamic coefficients is discussed. The effect of baseplate which installed at the bottom of the hull has been studied with the aim to improve the hydrodynamic performance.(2)Through the establishment of coupling heave motion model of hull and TLD system in frequency domain, the couping characteristics is anlysised, and the result show that the TLD system have little effect on the motion of hull, thus the system can be decoupled. Using the semi physical simulation experiment technology, the mesoscale indoor model of TLD system simulation platform is designed, and the experimental model results coincide with the results of theoretical model. Based on the theoretical model, numberical simulation analysis was carried out for the oceanic condition of South China Sea, which indicated that the TLD system can automatic compensate the hull’s heave motion.(3) The Euler-Bernoulli beam is employed to modeling the riser in TLD system, and the modal anlysis is studied for both fixed and free hanging state. When considering the physical characters of the section and the axial tension as spacial variations, the governing equation is in form as a fourth order partial differential equation with variable coefficients, which is difficult to get an analytic solution. Therefore, the differential transformation method (DTM) is used to dealing with the eigenvalue problem efficiently. The numerical simulation is shown that the nature frequencies obtained by DTM are agreed well with other method. Since the proposed method in essence is a numerical prodcedure, convergence rate of the solution is examined. The result shows that the convergence rate is linearly increase with the growth of the riser length. The effect of parameters of riser to the frequencies and mode shapes are investigated. From a pratical viewport, the resonance region of the riser is estimated during the down and up operations in splash zone.(4) A frequency domain method is proposed to analyse the lateral response of a riser in TLD system, which subjected to a combination of parametric exciation and random wave forces. When considering the parametric excitation in the axial, the riser is regarded as a periodic time-varying system suffering random load, and the traditional method could not given a solution in frequency domain. By employing the pseudo-excitation method, the frequency solution is obtained efficiently. Moreover, the Floquet-Liapunov theorem is used to estimate the stability of the vibration system in the space of parametric excitation. The proposed method is verified by comparing with Monte-Carlo simulation in time domain. The response character of the parametric excited riser under random wave is studied.(5) The wake oscillator model is established to predict vortex-induced vibration of riser caused by sea flow, and the influence of parametric excitation to vortex-induced vibration is studied. The numerical results show that the parametric excitation has important influence on the vortex-induced vibration, when the heave amplitude and frequency of parametric excitation is small, the parametric excitation can increase the response of vortex-induced vibration, and affects the response frequency components, where sum frequency and difference frequency of the parametric excitation frequency and vortex frequency is arised; When the heave amplitude and frequency of the parametric excitation is becoming larger, the response of the riser is mainly determined by parametric excitation, and the vortex shedding is disturbed, as consequence the response of riser increases several times than the condition that only vortex-induced vibration occurs, which could reduce the fatigue life significantly.(6) As the fluctuation of tension in the riser which caused by the heave motion of weight may lead to parametric resonance, the active control of winch is proposed to installed on the hull, which can reduce the amplitude of fluctuating tension. Based on the dynamic mechanical model of TLD system, Hoo controller and Hoo observer are designed for active control of axial dynamic stress of the riser, and the solution in both frequency and time domain is developed. The simulation results in frequency domain and time domain show that the proposed control method is effective and feasible if the parameter of Hoo controller is appropriately selected. The stress standard deviation of riser is reduced significantly under active control. simulation in time domain indicates that the riser tension at bottom is always positive under control, which reduces the possiblity of buckling failure due to compression.