| As offshore petroleum exploration proceeds into deep sea (500-1500m) and ultra-deep sea (>1500m), various innovative offshore structures are proposed. Currently, the design studies of novel offshore structures mainly focus on the floating ocean platform concept, and the main floating ocean platform types in the world are Tension Leg Platform, Spar platform, Semi-submersible platform, deepwater drillship, FPSO and FDPSO. Although the floating ocean platforms are widely applied for offshore petroleum production activities in ultra-deep sea in the world, it truly has some certain shortages by itself, such as huge platform size, high requirement of station keeping, complex design and construction technology as well as the lack of effective risk avoiding technology, when it faces severe ocean environmental elements characterized as strong wind and huge wave. Therefore, it is necessary and has great significance to develop innovative offshore supporting systems which are adaptable to severe ocean environmental elements in ultra-deep sea.The dissertation mainly consists of the following five parts:(1) Based on the awareness of severe ocean environmental elements as well as the technical challenges encountered during production activities in ultra-deep sea, an innovative Subsurface Tension Leg Production (STLP) system concept is proposed, which provides a totally new solution for offshore petroleum production in ultra-deep sea. A concept design of STLP system is conducted after the design basis and principle are determined. The conceptual design contents include the determination of the subsurface pontoon configuration, the general arrangement of the subsurface pontoon, estimation of the main dimension, the subsurface pontoon weight control, mooring system design, rigid riser system design, manifold design and so on.(2) An innovative Truss Seastar Pontoon (T-SSP) is proposed. Hydrodynamic coefficients of the T-SSP are derived by means of computational fluid dynamics (CFD) approach. Parametric sensitivity studies of hydrodynamic coefficients are investigated. It is found that horizontal drag coefficients are of vital importance in determining the behavior of the global system while the effects of added mass coefficients and vertical drag coefficient are small.(3) A coupled numerical model of subsurface plarform (including T-SSP, oil production equipment, mooring system and rigid riser system) is established. Then, a detailed parametric study on mooring system is carried out to study the effect of various parameters, such as top inclined angle, initial pretension, water depth and current velocity, on the behavior of T-SSP. It turns out that in order to meet the requirements of stability of T-SSP as well as optimize the seabed layout, the mooring system in relation to subsurface pontoon consisting of vertically loaded tethers needs to be selected preferentially. After that, extensive parametric studies on the global behavior of rigid risers, such as global dynamic analysis, riser interference analysis, riser strength analysis, riser buckling analysis, riser fatigue damage calculation, etc., are conducted. The results clearly indicate that the pre-installed T-SSP provides a stable and reliable shallow-water rated working platform.(4) Criteria using catenary concept is developed to define the critical length for the optimum design of flexible jumpers. Further, the fully coupled numerical model of the STLP system as well as FPSO (including T-SSP, oil production equipment, mooring system, rigid riser system, flexible jumper system and FPSO) is established. Based on the STLP system model, the effects of first-order wave motions and FPSO excursions on the subsurface platform are studied. The results obtained confirm the quasi-static nature of STLP system and also clearly indicate that STLP is well adaptive to the severe ocean environmental elements in ultra-deep sea.(5) The theoretical framework of quantitative risk assessment (QRA) for the potential hazardous events which may result in releases to sea with regarding to STLP system is established. On the basis of the established risk acceptance criteria, hazard identification is conducted and the overall risk model is established. Further, the probabilities and consequences associated with each hazardous event are calculated by means of analytical approach based on the failure rate data dossier and expert evaluation. In addition, uncertainty assessment is also systematically carried out. After that, the calculated risks are evaluated against the risk acceptance criteria, and thus further risk reducing measures are proposed. The QRA results indicate that only the environmental risk level for the subsea wellhead's large leak lies within the As Low As Reasonable Practicable (ALARP) area while other risk levels are tolerable. The results clearly show that STLP system has obvious technical safety advantages for the offshore petroleum production activities in ultra-deep sea. |
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