Flexible pipes, which can be divided into bonded and unbonded types, have been applied in the oil industry for years. An unbonded structure, the so-called flexible steel pipe, can drastically hoist the resistance of the pipe to pressure, tension and torsion and in the meanwhile, could be manufactured in a rather lengthy configuration and therefore, can be served as flowline, jetting line, kill and choke line or production riser, which presents a large interest in offshore production because of the realization of a simple and convenient connection between the seafloor and the surface. In the present paper, the nonlinear hysteresis characteristics of an unbonded flexible pipe under loading has been investigated via finite element (FE) method in which eight layers of the unbonded flexible pipe have been established with the introduction of solid and shell elements to layer-simulation. All the layers are modeled respectively with contact and friction interfaces between adjacent layers. The numerical results, compared with the other literature's results, exhibits good agreement with the other existing results, which verifies the effectiveness and the validity of the given model and furthermore, might provide practical and technical support for the application of flexible steel pipes.
Flexible pipes, comprising of several different layers, are slender marine structures which are extensively used in offshore engineering. From the most inner layer to the outmost layer, there are a carcass, an internal pressure sheath made of polymeric material, an interlocked pressure armour layer, an anti-wear layer, two tensile armour layers and an outer sheath with each layer having a particular function arranged sequently (Kagoura, et al., 2003), seen in Fig. 1. All layers are free to slide with respect to each other. The external plastic sheath layer protects the riser from surrounding seawater intrusion, external damages during handling and corrosion. The internal plastic sheath layer, made of polymer, ensures internal fluid integrity.
The flexible pipes are grouped into two types: bonded and unbonded pipes. Unbonded flexible pipes are being increasingly used in the oil industry. However, their behavior, particularly in deep water applications, has not yet well understood, due to their complex structure which makes their response extremely difficult to analyze.
