Strength Assessment For Deepwater Drilling Riser And Engineering Application

This dissertation is part of the project"Deepwater Drilling and Completion Key Technologies"supported by national 863 hi-tech projects of China under Grant No.2006AA09A106. According to the harsh environment of South China Sea, systematic study on approach and application of strength assessment for deepwater drilling riser has been carried out. The research covers some hot study issues of deepwater drilling riser including floater/mooring/riser coupled system analysis and wave-loading fatigue characteristic assessment, numerical simulation and fatigue assessment as well as suppression technique of VIV, hangoff dynamic analysis and typhoon-avoidance management, and riser/wellhead coupled analysis and its applications. The research findings provide effective technical supports for the development of deepwater drilling and completion.1 Floater/mooring/riser coupled system analysis and riser wave-loading fatigue characteristic assessmentFinite element model of a moored deep water drilling system including floater, riser and mooring lines is built. Time-domain non-linear analysis of the coupled system is carried out. The floater motions and riser responses are obtained based on dynamic equilibrium between the forces acting on the floater and slender structure response at each time instant. Riser dynamic analyses based on both coupled and traditional approaches are performed, and their results are compared. As the results indicate, the extreme response of the riser is dominated by the average floater offset and the low-frequency floater motions. The traditional approach, which considering the low-frequency floater motions as a quasi-static offset, produces inaccurate results. The wave-loading fatigue characteristics of the riser are studied based on coupled system analysis. The effects of wave loads and floater motions on fatigue of the riser are analyzed and compared. The contributions of low-frequency loads and wave-frequency loads to the wave-loading fatigue are compared and discussed. Critical fatigue damage positions are identified. The research shows that the floater motions are the primary contributor to the wave-loading fatigue of riser. The low-frequency fatigue has significant contribution to wave-loading fatigue and is influenced largely by the coupling effects between the floater and slender structures.2 Study on numerical simulation and fatigue assessment as well as suppression technique of VIV of riserAccording to prototypal Reynolds numbers of riser in deep water, two-dimension numerical simulation of VIV of riser is carried out based on RNG k -εturbulence model and dynamic mesh technique. The response branches of the cylinder, the vortex shedding formations and the frequency response characteristics are studied. The results of the simulation match the classic experiments well. A new response branch is found which is named'Super-lower Branch'by the author. Higher harmonics is found in lift forces of Lower Branch, which is the main cause of the conversion of cylinder response from Upper Branch to Lower Branch.VIV fatigue assessment of a deepwater drilling riser is carried out using enhanced risk based fatigue criterion. By considering the implicit uncertainty and bias associated with the VIV analysis model, limitations of the traditional fatigue criterion is avoided. Standard VIV fatigue analysis of the riser is performed with respect to the long term current statistics in order to obtain the basic fatigue damage. Stochastic variables governing the fatigue damage are identified. Standard fatigue sensitivity studies for these stochastic variables are performed, therefore uncertainty of fatigue damage are evaluated. VIV fatigue safety factors are calculated using enhanced risk based criterion.VIV suppression technologies of deepwater drilling riser are studied. Numerical simulation of VIV of fairings is carried out. Study on VIV suppression effects of fairings and their principles, as well as fairing tail angle optimization have been performed. Optimization analysis of buoyancy distributions is performed for the purpose of VIV suppression. According to different coverage ranges, recommendation of buoyancy distribution patterns and summary of the corresponding guidelines are provided.3 Riser hangoff dynamic analysis and typhoon-avoidance managementDeployment and retrieval analysis of an ultradeepwater drilling riser is carried out with the purpose of determining reasonable buoyancy deployment and ensuring the operation safety. The tension fluctuation characteristics of the riser in different buoyancy configurations are studied. The range of buoyancy demand is determined, and optimization analysis of the buoyancy deployment position is performed. Axial dynamic characteristics of risers in hard hang-off and soft hang-off configurations are investigated respectively; and the corresponding service performances of risers are compared and discussed. The axial response and tension fluctuation characteristics of risers as well as the operation risks in the two hang-off configurations are compared and analyzed. Hangoff analysis of an ultradeepwater drilling riser under typhoon conditions is performed. The performance characteristics of the suspended riser are studied, and the possibility of hanging the riser off the vessel under typhoon conditions is discussed. The operation windows of hang-off operation are established, and the applicable lengths of suspended riser in different environments are determined. Typhoon evacuation analysis of an ultradeepwater drilling riser is carried out; optimization analysis of the vessel speed and heading is performed so as to ensure the safety of the suspended riser. The stresses of the riser top end and the rotations of the upper ball joint under different vessel speeds and headings are studied. According to different environments and varied risers of different lengths, the practicable speed ranges for the vessel are determined.4 Study on riser/wellhead coupled analysis and applicationFinite element model of a complete drilling riser and the subsea wellhead system is built. The p -y curve is used to model the interactions between the conductor and soils in consideration of cementing quality. The riser response and wellhead system strength analyses based on both coupled model and de-coupled model are performed; the results have been compared and discussed. The influences of cementing quality, riser top tension, BOP and conductor on wellhead system strength are studied. Specific measures for improving the strength of the wellhead system are recommended. Weak point analysis of the riser and wellhead system is carried out to determine the appropriate breakpoint of the system. The analysis can contribute to the structural design or component selection. The critical weak points are identified, and reasonable breakdown positions are recommended. VIV of the riser and wellhead system is studied; and the fatigue problem of wellhead system is given a special attention. Measures for improving fatigue performance, such as ensuring the cementing quality, using light and short BOPs, increasing the bending strength of conductor and reducing the height of wellhead, have been suggested. As the research indicates, the increased riser top tension will aggravate the fatigue damage of wellhead system while reduce the fatigue damage of riser. When the fatigue of wellhead becomes an influencing factor in system design, the riser top tension should be controlled cautiously.