Axial mechanical experiments of unbonded flexible pipes
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ORIGINAL PAPER
Axial mechanical experiments of unbonded flexible pipes Jun‑Peng Liu1 · Murilo Augusto Vaz2 · Rong‑Qi Chen3 · Meng‑Lan Duan1 · Irving Hernandez2 Received: 26 March 2020 / Published online: 9 September 2020 © The Author(s) 2020
Abstract Axial structural damping behavior induced by internal friction and viscoelastic properties of polymeric layers may have an inevitable influence on the global analysis of flexible pipes. In order to characterize this phenomenon and axial mechanical responses, a full-scale axial tensile experiment on a complex flexible pipe is conducted at room temperature, in which oscillation forces at different frequencies are applied on the sample. The parameters to be identified are axial strains which are measured by three kinds of instrumentations: linear variable differential transformer, strain gauge and camera united particle-tracking technology. The corresponding plots of axial force versus axial elongation exhibit obvious nonlinear hysteretic relationship. Consequently, the loss factor related to the axial structural damping behavior is found, which increases as the oscillation loading frequency grows. The axial strains from the three measurement systems in the mechanical experiment indicate good agreement, as well as the values of the equivalent axial stiffness. The damping generated by polymeric layers is relatively smaller than that caused by friction forces. Therefore, it can be concluded that friction forces maybe dominate the axial structural damping, especially on the conditions of high frequency. Keywords Flexible pipes · Hysteresis behavior · Structural damping · Mechanical experiment
1 Introduction Unbonded flexible pipes are widely used in offshore oil and gas fields development, which consist of several functional layers made by different materials, as shown in Fig. 1. The fact that axial structural damping generated by friction between adjacent layers and within specific layers, viscoelastic properties of polymeric layers on condition of vessel stochastic oscillator is usually disregarded in global analysis may cause conservative results in deep-water development. Accordingly, waste of resources leads to rising costs (Silveira et al. 2011). Therefore, identification and quantification of the axial structural damping are necessary and important,
Edited by Xiu-Qiu Peng * Jun‑Peng Liu [email protected] 1
College of Safety and Ocean Engineering, China University of Petroleum, Beijing 102249, China
2
Ocean Engineering Program, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil
3
Engineering and Construction Department, China National Offshore Oil Corporation, Beijing 100010, China
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which can be characterized by the loss factor 𝜂 defined as ratio of dissipated energy loss per radian Wd ∕2𝜋 divided by the peak potential or strain energy U (Ungar and Kerwin 1962). The expression of loss factor can be written as:
𝜂=
Wd 2𝜋U
(1)
Figure 2 shows a general result for the mechanical behavior of a flexible pipe, in which the e
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