In this paper an efficient small-scale, detailed finite-element modelling method for flexible risers is presented. The method can be effectively implemented in a fully-nested (FE2) multiscale analysis based on computational homogenization, based on the theoretical framework provided in [1] for cases where different structural models are used at different scales.

The motivation for this work is that previous models in the context of sequential multi-scale analysis [2-3] are too computationally expensive for a fully nested analysis. Therefore, to reduce the computational cost only a small fraction of a flexible pipe is used for a detailed nonlinear finite-element analysis at the small scale by exploiting the cyclic symmetry of the model and applying periodic boundary conditions. In this model, using three-dimensional elements, all layer components are individually modelled and a surface-to-surface frictional contact model is used to simulate their interaction.

The approach is applied on a 5-layered pipe made of inner, outer and intermediate polymer layers and two intermediate armour layers, each made of 40 steel tendons. The capability of the method in capturing the detailed nonlinear effects and the great advantage in terms of significant CPU time saving are demonstrated by comparing the results obtained on elements of pipe of different lengths.

Details of how the small-scale model is implemented in a fully nested multiscale analysis where a beam model is used at the large scale are also discussed.

References

• B.D. Edmans, G. Alfano, H. Bahai (2013). Nonlinear multi-scale homogenization with different structural models at different scales. International Journal for Numerical Methods in Engineering, 94(4): 355-373.

• Bahtui, G. Alfano, H. Bahai, S.A. Hosseini-Kordkheili (2010). On the multi-scale computation of un-bonded flexible risers. Engineering Structures, 32(8): 2287-2299.

• Bahtui, H. Bahai, G. Alfano (2009). Numerical and analytical modeling of unbonded flexible risers. Journal of Offshore Mechanics and Arctic Engineering, 131(2).

• Edmans, B., Alfano, G., & Bahai, H. (2012). Large-scale analysis and local stress assessment of flexible unbonded pipes using FEA. In Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE Vol. 3 (pp. 947-953).