Dynamic Response of Flexible Pipes Considering Different Damping Models
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- Institutt for marin teknikk 
Flexible pipe is a layered structure composed of plastic and steel materials. Under a large bending moment, the pipe layers may slide relative to each other due to internal friction. The moment curvature relationship for the flexible pipe is a tri-linear curve. Under cyclic bending moment, a hysteresis loop will be formed in the moment curvature curve. The area of the loop is the energy loss due to the internal friction. This thesis is aimed to study the effects of hysteresis damping on the global analysis of the flexible riser. To begin with, a review on the flexible pipe technology and nonlinear finite element method is performed. Then a local analysis is carried out in BFLEX to obtain the cross sectional characteristics. Then the global analysis is conducted to study the responses of the flexible riser in terms of the curvature, moment and axial force. From the study, slip behavior only occurs at the hang off part of the riser. For the rest part, pipe layers stay in the stick regime, meaning there is no energy loss due to the internal friction. Therefore for the global analysis of the flexible riser, there is no need to further study the equivalent linear damping models. In addition, the influence of linear and nonlinear bending models on the global response of the riser is investigated. It is found that the current standard industrial practice, namely applying the linear bending model with the full slip bending stiffness, gives an over conservative response prediction. It is therefore recommended to use the physically correct nonlinear moment curvature relationship for the global analysis of flexible riser.