Multiscale Finite Element Analysis of Unbonded Flexible Risers

What is it about?

Unbonded flexible risers remain a key technology in existing and proposed deepwater developments. The development of deepwater fields has led to increasing demands on risers, increasing hydrostatic and tension loads and risks of failure due to collapse, fatigue and buckling of tensile armour wires. In addition, many existing subsea production systems are approaching the end of their design life. To enable benefits from deepwater production and life extension projects to be realized while minimizing risks to life, property and the environment, accurate modelling and analysis tools are required. To improve the prediction of failure modes and develop better understanding of the conditions leading to progressive failure of flexible risers, models are required to be able to predict stresses and displacements in pipe components that arise as a result of large-scale riser dynamics.

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Why is it important?

We propose a multi-scale approach in which new models for both global and local analyses are developed, together with a physically realistic method for linking the models. Firstly, we describe a nonlinear constitutive model for use in large-scale dynamic analysis of flexible risers based on an analytical homogenization of composite cylinders and the analogy between slip between pipe layers and plastic flow in continua. The model is able to reproduce the bending hysteresis behaviour observed in flexible pipes and its dependence on internal and external pressure. Secondly, we show a procedure for obtaining equivalent material parameters for this model from finite element local analyses of a flexible pipe. Finally, we show the implementation of the constitutive model in a dynamic analysis of a riser system using corotational hybrid beam finite elements.