Unbonded flexible risers are key components in many offshore oil and gas developments. To evaluate their performance, accurate mechanical models and efficient analysis techniques are required, both for life extension assessments of existing assets and for determining safe operational envelopes of new designs in deepwater environments. Due to the structural complexity of unbonded flexible pipes, predicting stresses and instability failures in these components remains challenging, inviting new approaches. This paper presents developments in the consistent linking of small scale (local nonlinear structural analysis) and large scale (global dynamic analysis). We describe sequential and integrated multiscale formulations aimed at improving modelling accuracy in a computationally feasible form. Principles and procedures for consistent multi-scaling are outlined based on established techniques used for the modelling of composite materials and structures that exhibit periodic symmetry and results are presented using these methods.

The predictive capabilities of the components of the multiscale analysis are explored, including: 1) a nonlinear beam-type finite element for riser representation; 2) a novel nonlinear section response model; and 3) a detailed finite element model computing component interactions. Verification of the integrated multiscale procedure is shown for a simplified case. The utility of multiscale models for investigating the conditions that lead to mechanical instability failures, such as buckling of the tensile armour wires is also discussed. The analysis procedures and model described in this article are proposed as methodologies for assessing fatigue damage, predicting instability failure modes and investigating the consequences of damage, such as breaking of tensile armour wires.

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