Some Further Studies on the Axial–Torsional Behavior of Flexible Risers

Abstract

Flexible risers are complex structures composed of several concentric polymeric and steel armor layers that withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. Determining the response of these structures when subjected to axisymmetric loadings (i.e., any combination of traction, torsion, and internal or external pressures) is an important task for the local structural analysis since it provides probable values for the loading distribution along the layers and, thus, allowing estimating the expected life of a riser using fatigue tools. Although finite element models have been increasingly used to accomplish this task in the last years, the simplicity and the reasonable accuracy provided by analytical models can be seen as reasons that justify their continued use, at least in the initial cycles of the design. However, any analytical model proposed for such a task must be checked with well-conducted experimental results in order to be considered as an acceptable analysis tool. The aims of this article are twofold: (i) to present the main results of experimental tests involving both internal pressure and traction loadings on a 63.5 mm (2.5 in.) flexible riser, carried out at the Institute for Technological Research of Sao Paulo (IPT), which can be used as a means of checking finite element or analytical models proposed by other researchers, and (ii) to compare some results obtained experimentally with those predicted by an analytical model which can also include any combination of axisymmetric loadings. Besides presenting full data concerning the internal structure of the riser, the experimental procedures used to perform the tests and the main results (e.g.. Force x Displacement curves) are also presented. A brief discussion about the validity of some hypotheses that are usually assumed by analytical models found in the technical literature is made.

Review of the Development of an Unbonded Flexible Riser: New Material, Types of Layers, and Cross-Sectional Mechanical Properties

“…In addition, Bahtui [44], based on Kebadze's theoretical model, simplified the helical layer structure with complex cross-sectional geometrical properties, such as the carcass layer and pressure armor layer, to an orthotropic anisotropic cylindrical shell. Ramos Jr. et al [63,64] derived equilibrium equations for unbonded flexible risers under axisymmetric loading by a method similar to that of Kebadze, and presented in detail the assumptions of the theoretical approach:…”

Flexible Pipes

“…Eqs. ( 11), ( 13) and ( 14) together form a close equation set (15), which leads to the equilibrium of the steel wires with tensions considering the compressible deformation of the internal core.…”

“…Flexible pipes are primarily designed to resist torque in the clockwise/counterclockwise direction through the double tension armor layers [15]. The basic theory and assumptions are the same as those in the previous section, and this section focuses on the relationship among the torque, the radial pressure and the axial stress along the wire helical.…”

Axial mechanical experiments of unbonded flexible pipes

“…However, quite a few nonlinear properties caused by geometric, material, contact conditions make it arduous. Both numerical and analytical studies have to make lots of assumptions and put forward a slice of interesting results in the past 40 years (Feret and Bournazel 1987;Witz and Tan 1992;Kebadze 2000;Custódio and Vaz 2002;Pesce et al 2010;Saevik 2011;Ramos et al 2014;Ramos and Kawano 2015;Ebrahimi et al 2018). Although some advances like viscoelastic considered in model (Guedes 2010;Liu and Vaz 2016a;Santos et al 2017, Liang et al 2018, 2019, there is…”