The Anti-bending Technology Of Deep—water Steel Risers

Deep-water riser system is one of the weakest components in deep-water oil and gas development system. The strength fracture, buckling failure and fatigue damage will be generated at riser ends and local bending position, especially under the combined loads of wind, wave, current and top floating body movement. This paper is funded by national high technology research and development program (863program), and focused on the key technical programs of deep-water riser bending restraint system. It includes functional joint design, hydrostatic collapse, buckling and buckling propagation and local bending restraint design.Deep-water riser mainly reflects the characteristics of flexible structures. When they are directly connected with rigid structures, high stress concentration will be generated at he connection structure. In this case, the Stress Joint (SJ) is developed to reduce the bending stress concentration at the ends of riser. At present, most research work of Stress Joint (SJ) is focused on new materials application and structural analysis, and the SJ design mainly depends on the actual engineering experience of designers. Hence, the design work is subject to loads enviroment. To resolve this problem, a SJ design method is proposed in the paper, and the parameters control methods are recommended for actual projects in consideration of various aspects of influencing factors. According to this new design method, the SJ design dimensions which satisfy practical engineering demands can be rapidly and accurately obtained, and the related mechanical performance can be assessed accurately. In the meantime, taking the100-year extreme sea state conditions into account, the accuracy and rationality of design results are verified based on the finite element analysis, and the bending stress reduction effects and bending control ability are evaluated for the designed SJ. Then based on the actual engineering project, the dynamic response analysis and fatigue analysis are carried out for a riser system with SJ, and the analysis results further validate the feasibility and effectiveness of this design method in practical engineering.In practice, in addition to design of SJ, the analysis and evaluation of engineering problems caused by excessive bending are also very important. The buckling and buckling propagation, especially in the defective structures, could occur in the large bending deformation area. The existing riser buckling assessments are based on submarine pipelines of large diameter-thickness ratio, which leads to enormous calculation error due to the different structural characteristics and load environments. So using the cylindrical shell buckling theory, the collapse buckling characteristics of deep water risers with different diameter-thickness ratio are investigated. The collapse buckling and buckling propagation analysis are implemented for riser models with various diameter-thickness ratio and ovality. A critical pressure formula for the collapse buckling analysis of small diameter-thickness risers is proposed for combined loads. And the riser depression coefficient of local buckling is proposed with the complicatd loading conditions and buckling modes. In addition, the numerical simulation method of buckling propagation is established under local buckling. The above researches have great significance to the development of deep-water riser buckling assessment theory.Furthermore, the local excessive bending, due to structural characteristics and loading conditons, could be generated beyond the end of riser. For example, at the near end of Steel Catenary Riser (SCR), the touch-down zone often has strength failure and buckling failure due to excessive bending. The near-end becomes a high risk area of fatigue failure because of the cyclic bending motions. At present, to deal with these problems is mainly achieved by increasing the wall thickness, which causes stress concentration and fatigue problems. Therefore, a structural optimization method is proposed for the excessive bending location based on SJ. Then, buckling and buckling propagation analysis and assessment are studied for a SCR touch-down zone according with the proposed methods in this thesis. The dynamic response analysis and fatigue analysis are carried out by finite element analysis. These analysis results useful for the SCR touch-down zone restraint design in the engineering application.To summarize, in order to resolve the excessive bending of deep-water riser system in design and service, a new design method for bending restraint structures is proposed. The radial buckling and buckling propagation theory is developed with recommendations for bending suppression system in the actual engineering application.