Key Mechanical Problems In Design Of Flexible Risers And Auxiliaries Used In Deep Water

An unbonded flexible riser is the key equipment in oil and gas production in deep water. The flexible riser used in deep water requires high resistance to hydrostatic pressure, internal pressure, tension and fatigue as well as small minimum bend radius (MBR), therefore there are many challenging mechanical problems in design, analysis, test, manufacture and even installation. Currently, the design and manufacturing techniques are mastered by only a few international coMPanies, which leads to high price and long procurement cycle and restrict oil and gas production in deep water of our country.Typical flexible riser system comprises the flexible pipe and bend stiffener, and each of them involves complicate mechanic problems. It should be specially pointed out that for the flexible riser design, it is the key point to control failure modes, such as buckling collapse under external pressure and fatigue under dynamic environment, by designing reinforced armour layers. In order to design the fatigue life, the complicated mechanical behavior of the flexible riser loaded by axial tension due to self-weight and floater heave and curvature caused by environmental loads and floater motion should first be predicted correctly. For the bend stiffener with high performance, the fatigue resistance and economy as well as protecting requirement of the maximum curvature are needed. Therefore, an advanced flexible riser involves multiple leading topics from many fields of structural mechanics.Based on the "Key techniques of offshore flexible pipes" project supported by National High Technology Research and Development Program of China (863) in the twelfth five-year plan, this thesis investigated systematically the key problems in design and analysis of flexible risers. The analytical models are proposed for critical pressure, tensile stiffness, MBR and fatigue life of flexible risers. The finite element models and prototype tests are also developed relatively to verify the theoretical models. In addition, the multi-objective optimization model of the bend stiffener shape is developed. The main research contents, methodologies and conclusions of the thesis are summarized as below:(1) Strain energy equivalence of flexible risers to resist external pressure and analysis of buckling behavior considering manufacturing imperfections.In order to increase radial stiffness rather than bend stiffness, the innermost carcass layer with interlocked profile is used in flexible riser structure. Buckling-resistance design and analysis involve many challenging problems about modeling and computing because of the complex gemetric profile. A buckling-resistant design method of the carcass layer is proposed based on the strain energy equivalence, and verified by the radial compression tests of three different test pieces. Additionally, a three-dimensional finite element model is developed to predict the buckling behavior of the carcass layer with different initial imperfections under uniform external pressure. While the cold deforming process of the carcass layer is simulated with three-dimensional numerical model and then the residual stress caused by roll bending and interlocking of the metal ribbon is obtained. Moreover, the critical pressure of a carcass layer with 4 inches inner diameter is designed and the buckling behavior is analyzed with proposed methods. And the influence of different imperfections such as initial ovality, material nonlinearity and residual stress on the critical pressure is discussed. The design and analysis models of the critical pressure of carcass layers may provide effecitve tools to design effectively a buckling-resistant flexible riser.(2) Design and verification of tensile stiffness of flexible risers.Multiple helical armour wires wound round the cylinder are used in flexible risers for enhancing the axial tensile stiffness rather than bend stiffness. Considering the inner cylindrical component compressible radially, an analytical model of helical wires wound around a cylindrical shell is proposed. Then the 3D finite element model for the tensile sitffness of flexible riser is developed and the prototype test method is proposed to verify the analytical and numerical models. The tensile stiffness test of a flexible pipe with 4 inches inner diameter used in shallow water is performed. Both the analytical and numerical results agree well with the experimental results, which verifies the validity of the proposed analytical model. Additionally, taking a flexible riser with 8 inches inner diameter used in deep water as an example, the influence of different design parameters on the tensile stiffness of the flexible pipe is studied and discussed. The model proposed in this chapter may help engineers design effectively the tensile stiffness of flexible riser and calculate fatigue loads and stress of flexible riser.(3) MBR design and verification of flexible risers.In order to reduce bend stiffness, the helically wound structure is adapted by all the resinforced layers in flexible riser. Considering the limitation of existing analytical models for the bending stress of a helical wire, an analytical model based on the Spring theory is proposed in this chapter. A three-dimensional finite element model of a helical armour wire wound around a cylindrical shell is established and the local stress of the wire is calculated. Then the validity and limitation of analytical models for different length-width ratios of the wire section and helical angles are investigated considering the numerical results as references, and the proposed analytical model is verified. In addition, the influence of different wire parameters on the MBR is studied, which provides effective tool to rapidly design the MBR of flexible riser. And the proposed model for the bending stress of a helical wire also supports the analysis of fatigue stress of flexible riser.(4) Fatigue-resistant design of flexible risers with interlayered friction and verification by fatigue test.The fatigue-resistant design and anlysis involve many key mechanical problems due to the high nonlinearity caused by contact and friction between the metal armour layers, which is one of the hot topics in the feild of ocean engineering. Considering the possible fatigue failure of flexible riser, the analysis method and process are proposed for the prediction of fatigue life of flexible riser. The analytical and numerical models for nonlinear bending stiffness of flexible risers are established and verified by the tests of bending stiffness, and then the calculation method of the global fatigue loads based on nonlinear bending stiffness is proposed. The theoretical model for nonlinear fatigue stress including the local stress caused by tension, bending and interlayered friction is also developed. Taking a flexible riser with 8 inches inner diameter as an example, the fatigue life is calculated and the influence of various nonlinear factors and mean stress on the fatigue property is discussed. In addition, a fatigue test conception based on the alternative pure bending loading is proposed. The fatigue strength test of a flexible pipe with 4 inches inner diameter is performed with the proposed method, and then the influence of the uncertain factors like interlayered wear on the fatigue life is studied quantificationally and considered by modifying material S-N curve. Therefore, the fatigue test method proposed in this chapter can be used to verify the fatigue life design of flexible riser, which powerfully complements the lack of the classical fatigue test simulating actual cases.(5) Structural design and optimization of bend stiffeners with high performance for flexible risers.Considering material nonlinearity and large geometrical deformation of bend stiffener, the analytical model to calculate the curvature distribution of the upper flexible riser is established. Then a 3D finite element model of the riser with the bend stiffener is developed and used to verify the proposed analytical model. Taking the bend stiffener for a flexible riser with 8 inches inner diameter as an example, the curvature distributions with different material behaviors of the bend stiffener are analyzed and the sensitivity of the curvature results to different shape parameters is studied. Additionally, considering the fatigue-resistant property and economy as the objectives, the multi-objective shape optimization of the bend stiffener is performed using the optimal method based on a surrogate model. The calculation accuracy and efficiency with different optimal algorithms are discussed. Moreover, the Pareto frontier of the optimization case is developed and can be used to guide the engineering application of the optimal results. A new conception of bend stiffener based on hyperbolic type profile is proposed, which advantages about improving curvature distribution of flexible riser are verified by coMParing with the results from other typical or optimal bend stiffeners. The proposed configuration may provide a new strategy in high-performance design of bend stiffener.The design methods, analysis models and relative conclusions proposed in this thesis provide useful tools to efficiently design the safe structures of flexible risers and auxiliaries.