Mechanical Performance Analysis And Optimization Design Study Of Deepwater Composite Riser

The marine riser is one of the key equipment of the deep-water oil andgas resource development. With offshore exploration activities are extendedto increasing water depths, offshore structures are becoming more and moreweight-critical. The material applying in deepwater risers must have thecharacteristics of lightweight, high strength and corrosion-resistance, sotraditional steel risers are difficult to meet design requirements. Asalternative materials, continuous fiber reinforced polymer matrix compositeshave many advantages, such as: high specific strength, tailoring ability ofstrength and stiffness, high corrosion resistance, thermal conductivity andexcellent fatigue. As a result, composite risers have a better prospect in thedeep-sea oil and gas development.The transition from metallic risers to composite risers has becomeinevitable due to the cost savings on the system components and operations.Reduced riser weight directly affects required top tension whichconsequently lessens mooring tension and platform size. The significance ofweight savings will be amplified for deeper water sites.However, since designing offshore applications with composite materialsis less mature than that with the metallic materials, it is necessary todemonstrate that a composite riser meets the same requirements as thetraditional steel risers at the onset. Due to the huge differences in materialand construction, analysis of a composite riser may require differentapproaches than conventional riser analysis, and it is necessary to identify the particulars to which attention must be paid when designing a compositeriser. So, using of composites for offshore risers introduces challenges andadds complexities in the design process.First of all, the anisotropic property increases the difficulty of design. Forisotropic materials, the stiffness and strength can be expressed with theYoung’s modulus and Poisson’s ratio and combined stress intensity only needone strength data. But for composite materials, one ply has four stiffnessparameters and five strength parameters, the number of laminate’s stiffnessconstants can be reached to21and the strength parameters has as five timesas the number of ply number. So the failure criterion is unable to simplify asthe scalar form.Secondly, considering the effect of environmental loads, such as currentsand vessel motions, the riser global analysis has to be carried. At the globallevel, the structure is analyzed using a simplified finite beam element model.However, for composite material structures, due to the anisotropy and thechangeable stacking sequence in designing composite structure, it is hard tocombine the equivalent mechanical properties of laminated tubes withoptimization design based on global-local approach.Finally, as the composite riser involves numerous design variables, thecriterion of stiffness matrix and the strength is complex. As a result, theoptimization design leads a huge computational cost. Moreover, marinerisers in sea environment have uncertainties in several aspects, e.g. materialproperties, dimensions, and loadings. The effects of random design variableson the pipe behavior are quantified by probabilistic analysis.This research performs a detailed discussion on the fiber reinforcedcomposite riser macro mechanical characteristics, strength and localbuckling failure mode, reliability assessment and the optimization for globaland local model is also studied. The main work of the research is as follows:1. Local model analysis of composite riser and reliability assessment based on buckling pressureEstablish a rigid fiber reinforced composite laminated riser localnumerical model and give FEM analysis with the considerations of theinternal pressure, external pressure, and tensile load according to thespecification. The coupling effect of the fiber and the matrix is considered inthe strength failure criterion. Take sensitivity analysis of model parameters(material, geometry, layer parameters) based on mechanical response and dostochastic analysis of the buckling loads of composite pipes. The resultshows that the thickness of the layer is the principal factor influencing thescatter of buckling pressure.2. Composite catenary riser dynamic response analysisThe equivalent mechanical properties of laminated composite risers arecomputed using a classical lamination theory approach. These properties areused in the dynamic analysis of a composite catenary riser usingtwo-dimensional beam elements to obtain the displacements and internalforces (axial and bending moments). After the global analysis, the internalforces are used to provide loads and boundary conditions for the localoptimization model.3. The study on design optimization of the composite rigid riserThe genetic algorithm is successfully applied to the design optimizationof laminated composite tubes subjected to internal and external pressure,axial load and bending. Due to the large number of design variables,constraints and constraint functions are complex; the optimization process ofcomposite structure converges slowly. The use of approximation techniquecombined with the experimental design method can largely improve theoptimization efficiency apparently. Multi-objective optimization problemsare often encountered in the choosing winding angle of composite pipelines.This method was shown to be able to capture the Pareto trade-off curve forthat multi-objective problem, providing the designer with a very helpful tool for decision-making.The innovative research works are primarily represented as theestablishment of deep-water composite catenary riser global analysis method,the integration of local model and global analysis in composite riseroptimization design, probabilistic analysis of the buckling property ofcomposite pipes with random design variables, and combining advancedoptimization method with the composite riser design.Classical lamination theory is used to obtain the equivalent mechanicalproperties of composite laminated pipes and the global beam model isestablished according to these properties. Through the global analysis for thecomposite catenary riser, this research definitely shows that the joints nearthe top hanging zone and touchdown point are the critical points of the riserstructure design. Then subtract the section internal forces and constrainswhile apply to the local FEM model. Finally, optimize the thickness and plyangles with considering both the strength and stability. Stochastic analysis isused to do the reliability assessment on the buckling analysis, theuncertainties related to the pipe wall thickness and the winding angles arethe most important variables for the collapse property of composite risers.The method is helpful to practical engineering design.