Explicit and implicit three dimensional Finite Element (FE) models of a pipe-in-pipe (PIP) system are developed. The models represent the casing pipe and the carrier pipe using shell elements. Frictional contact ensures kinematic compliance between the pipes while allowing unlimited axial relative movement. Steel for both pipes is modeled using a kinematic hardening material model capturing plastic deformations. The model allows large deformations and rotations. The developed models are used to study structural response of a PIP system under typical loadings experienced by pipelines. It is shown that a PIP system can be designed to avoid upheaval buckling without any external support. It is also shown that a PIP system has significantly superior structural response when measured in terms of strains experienced by the carrier pipe against the same size single pipe under some extreme axial and lateral loadings. Currently most PIP applications are motivated by the insulation properties of PIP systems or for ease of construction. The results presented here open opportunities for the application of PIP systems. Examples include offshore pipelines designed to protect against ice scour and onshore pipelines against unstable slopes. It is shown that explicit FE can be used to determine closely spaced chaotic solutions for the carrier pipe. It is known that the elastica problem has many closely spaced solutions. The results obtained compared to known chaotic solutions of elastica. Behavior of a PIP system presents interesting real life examples for bifurcations of elastica.

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