Research On Nonlinear Hysteresis Behavior Of Unbonded Flexible Riser

Flexible risers are wide applied in offshore oil and gas production due to their excellentmechanical properties. However, there are many problems in the analysis of flexible risersand many unknowns for their mechanical behaviors. It is complex for the structure of anunbonded flexible riser. The nonlinear hysteresis behavior may be introduced by interactionbetween different layers of the riser, which may affect its response to loads. It is beneficial toassess the mechanical behaviors of a flexible riser to develop a theoretical model to describethe nonlinear hysteresis behavior.In practic, many materials and structures may present nonlinear hysteresis behavior withdifferent idiographic forms. Until now, various mathematical models have been developed todescribe nonlinear hysteresis systems. The characteristics and applicability of these modelswere assessed in this thesis. It was found that the Bouc-Wen model was suitable to study thenonlinear hysteresis behavior of an unbonded flexible riser. According to prevenient test data,the parameters of the Bouc-Wen polynomial expressions were identified and the Bouc-Wenmodels were established to describe the hysteresis behavior of an unbonded flexible riserunder different load cases. The agreement between theoretical results and test results indicatedthat the proposed Bouc-Wen model, whose parameters were identified by linear least squaremethod, was an efficient approach to describe the nonlinear hysteresis behavior of anunbonded flexible riser.Due to the comparability between the hysteresis behavior of an unbonded flexible riserand the constitute relationship of an elastoplastic material, it was reasonable to consider theflexible riser being made of elastoplastic material. Then the elastoplastic theory can be used toassess the response of a flexible riser to cycle loads. The generalized stresses and strains of aflexible riser were defined in this thesis. The elastoplastic theory was used to analyze thegeneralized strains and the relationship between the stresses and strains was establishedseparately in their elastic phase and plastic phase. The slippage function can be used to judgethe slippage state between risers layers. A pressure parameter was introduced to allow for theeffect of internal and external pressure on the riser. Due to the reference test data was notenough to identify the parameters of the theoretical model, a series of FE numerical analyseswere conducted to expand data base. The final identified constitute model can predict the hysteresis behavior of an unbonded flexible riser effectively.