The Dynamic And Mechanical Performance Analysis For Deep-sea Flexible Risers

As the offshore industry continues to embrace deep water environments,the development of the equipment for deep water oil exploration has beenthe hot issue in the recent years.The unbonded flexible riser is suitable forrough deep-sea environmental conditions because of its characteristics, suchas less bending stiffness, lighter mass, stronger corrosion resistance andanti-fatigue.However, due to the complexity of the structure of the unbondedflexible risers and the deep water environment, the flexible risers will facemore challenges during the design and operation.First of all, the unbonded flexible risers consist of several functionallayers and polymer layers which are not bonded with each other.The frictionand contact conditions between diffenent layers of the unbonded flexiblerisers make the mechanics analysis strong nonlinear and the constitutiverelationship special.Subsequently, the enormous difference between theanti-tension and anti-bending capacity of unbonded flexible risers whichdues to the complex structures results in serious ill-conditional calculation inglobal analysis process.At last, the traditional fatigue-analysis methodscannot solve the fatigue life of the unbonded flexible risers because thefatigue damage in the multi-layer of the flexible risers is not coinstantaneous.Facing these challenges, how to solve these problems is studied in thispaper, the main work is as follow:1、Present a simplified model for the analysis of the local mechanicalperformance of the unbonded flexible risers.Because of the complex structures of the flexible risers, even though thetraditional solid modeling method can evaluate the mechanical performancepreciously, the cost is too much to adapt to the preliminary design.Meanwhile, the traditional simplified method which neglects the frictionbetween the layers cannot only meet the precision demand but also thenonliear consititutive relationship of the unbonded flexible riser. Apply thenew simplified method into the machanics performance of the unbondedflexible risers can evaluate the performance preciously and reduce the costcalculation. 2、Propose a multi-layer stress decomposition method to analyze theconstitutive relationship for unbonded flexible risers.The relationship of the multi-layer unbonded structures is always a hardquestion in the mechanical analysis.This artical proposed the multi-layerstress decomposition method to establish the constitutive equation forevery layer.The contact and the friction between the layers are considered byintroducing the slip/stick model.In order to varify the precious of theconstitutive relationship of the flexible risers, the numerical modeling casewhich is introduced in the chapater two can be used. The constitutiverelationship of the unbonded flexible riser is the key to solve the mechanicalproblem for flexible risers and it has a great significance to solve the fatigueanalysis for flexible risers.3、Application of the "coupled element method" to global dynamicresponse analysis for flexible risers.Aiming at the ill-conditional calculation in global analysis process ofthe unbonded flexible risers, a suitable global dynamic response analysismethod for the unbonded flexible risrs was put up. Considering thecomplexity of the deep sea water, the analysis with time domain method wasinvolved in solving this problem.In order to validate the reliability of thismethod, a typical static case and a benchmark dynamic case were calculatedand the results were compared. The dynamic response analysis case of thefree hanging flexible riser was calculated with this validated method.4、Apply an enhanced multi-layer fatigue-analysis approach to solve thefatigue life of the flexible risers.Aiming at solving the problem of the multi-layer fatigue-life for theflexible riser, this artical combined the multi-layer stress decompositionmethod with the traditional fatigue analysis approach to establish a newmulti-layer fatigue-analysis approach. This approach can distribute theglobal dynamic response to every layer and it also can solve the stress-timehistory of each layer. In this way,the fatigue life of each funcion layer can besolved. What is more, the effects of the design variable on the calculation offatigue life of unbonded flexible risers are also studied.