Research On The Preliminary Design Method Of Flexible Marine Riser Configurations

A flexible marine pipe is typically hung from a floating platform or vessel,serving as a riser or being laid vertically.In that case,the flexible pipe is not only subject to functional loads,such as the mass and buoyancy of the pipe,contents and attachments;but also subject to environmental loads,such as waves,currents and floater motions and so on.And failures may occur,including tensile failure,overbending,fatigue,interference with other objects,and so on.Therefore,the configuration of a flexible riser should be designed,and some ancillary components should be added to control the global and local deformations.The configuration design of flexible risers in China has relied on foreign companies for a long time,which made the design costly and restricts the development of the related industry.This dissertation is based on the project of "key technology research of subsea flexible pipes(2012AA09A212)",which was supported by National High Technology Research and Development Program of China(863 project).In that project,a systematic research is conducted on the related theories and methodologies of the design and analysis of flexible riser configurations,ancillary components and so on.The task of the preliminary design of a flexible riser configuration is to determine the key parameters of its global layout and geometry,and give the basic requirements on ancillary components.Nowadays,the widely used technique for the preliminary design mainly follows the "trial and error" procedure,which is conduced as follows:the key parameters of the riser configuration are firstly given based on engineering experience;next,global analysis is performed to check all the possible load cases and failure modes;and then,the key parameters will be modified if any failure mode occurs;finally,a feasible design could be obtained by using the above iteration design procedure.As flexible risers being developed both to ultra-deep and extremely shallow waters,the above "trial and error" design method might be no longer satisfied because of its low efficiency.More specifically,the numerous load cases,various failure modes,and the expensive global analysis,make the design of flexible riser configuration with "trial and error" method very long period and high cost,and sometimes can not even be achieved.In addition,the above disadvantages also bring big challenges to introduce some new methodologies,for example,coupled design,optimization design and performance-based design.This dissertation therefore conducts some research on the preliminary design methodologies of flexible-riser-configuration in-place and laying,and the main contents are summarized as follows:(1)An integrated design methodology of flexible riser configurations and ancillary components is proposed,to overcome the problems caused by the traditional separated design procedure,such as many design cycles and inconveniences for global performance optimization.Based on mechanical principles of the preliminary designs of riser configurations,bend stiffeners and buoyancy modules,simplified theoretical equations are provided to conduct and integrate the above designs efficiently.And then,the designs of the configuration and ancillary components can be verified simultaneously with state-of-the-art global analysis software.Therefore,benefits for the design efficiency and performance optimization are obtained by means of the above synchronous design and verification procedure.A configuration design case of an ultra-deep water(1500 m)flexible riser is performed following above method,and its fatigue life is improved significantly using the method of integrated design of the configuration and ancillary components.(2)Surrogate-model-based-optimization method is introduced to cope with problems of the flexible riser configuration design for extreme extremely shallow waters.The design problems include the need of nonlinear time domain analysis for highly structural nonlinearities,significant dynamic amplifications;and several critical failure modes making the feasibility domain too narrow to find a feasible design.A surrogate model is constructed to replace the time-consuming nonlinear time domain analysis;and then optimization formulation is constructed algorithms are used instead of the traditional artificial "trial and error" design.An extremely-shallow-water design case is conducted as follows:firstly,the design problem is converted into a single-objective optimization model aiming at the maximum dynamic curvature,using its length parameters as design variables and design criteria as constraints;then,surrogate models are constructed separately with the Kriging model and radial basis function(RBF)networks,using the samples obtained by optimal Latin hyper cubic sampling(oLHS)and responses from the time domain analysis.Finally,A hybrid optimization strategy composed of multi-island genetic algorithms and NLPQL algorithms is applied for fast seeking of the optimum design based on the RBF model.An optimized design is found to meet all of the design criteria with high accuracy and efficiency,even though all of the samples fail to meet the curvature criterion.(3)An innovative design method,called "angular design",is developed specifically for the preliminary design of lazy wave configurations of flexible risers,using the end angles of the three catenary sections as design variables.In this way,the angular design method obtains some advantages such as independent and non-dimensional variables,and ease of control of the geometric shape.And some disadvantages of the traditional length-design are effectively overcome,especially the problem of generating geometrical infeasible configurations and the waste of calculating resources.This dissertation discusses the idea and background theories of angular design method firstly.And then,parametric equations are provided to realize the angular design,by solving lazy wave configurations and calling existing state-of-the-art global analysis software to verify the design.Since the simplification and highly efficiency design method for lazy wave configurations have been obtained,a further study on "compliancy-performance-based" design is then conducted.The conception of compliancy of flexible riser configuration is therefore discussed and the measurable index is given.More specifically,the compliancy curve is simplified as a circle,using the mean position as the center and the minimum allowable floater offset as the radius;so,the allowable offset can be taken as the measurable index of compliancy performance.Based on the angular design method,several lazy wave configurations with various compliancy-performances are obtained by means of parametric studies in a design case.(4)During the flexible pipe laying,the configurations are more complicated than those in-service cases and should be designed too.Particularly,the local contact forces between a flexible pipe and an over-boarding chute(over-bend section)has stress concentration phenomenon and dynamic coupling effects,making it is difficult to predict.This dissertation firstly studies the simulation method of a flexible pipe laying procedure,using nonlinear dynamic analysis technique,to obtain the important information including dynamic laying tensions and departure angles of the sag-bend for pipe laying design.Then,an analytical deformation-coupling model is established for quick prediction of the over-bend contact forces in engineering design,based on the Euler beam theory and Hertz contact assumptions.According to the sag-bend force analysis,an extra boundary condition is obtained for above contact analytical model,so that it can be solved.Finally,a laboratory semi-physical simulation test system is established to analyze the dynamic coupling problem of overbend section,and verify the overbend forces predicted by analytical or numerical simulation.The test system includes a six-degree-of-freedom motion platform,large scale(1:6)models of a pipe section and an over-boarding chute,and a clump weight to simulate the cut-off pipe sections.The results show that the semi-physical simulation test is an effective method to predict and verify the load effects on the over-bend region.