Numerical modelling of a steel catenary riser section in the touchdown zone under cyclic loading

• Published in: Ocean engineering 2018-09, Vol.164, p.168-180
• Main Authors: Dutta, Sujan, Hawlader, Bipul, Phillips, Ryan
• Format: Article
• Language: English
• Subjects: Cyclic loading; Finite volume method; Soft clay; Steel catenary riser; Strength degradation; Suction
• ISSN: 0029-8018; 1873-5258
• DOI: 10.1016/j.oceaneng.2018.06.040

The fatigue life of steel catenary risers is significantly influenced by cyclic riser–seabed–water interaction in the touchdown zone. In this study, the penetration and extraction of a shallow embedded riser section, subjected to cyclic vertical loading, are simulated using a computational fluid dynamics approach with ANSYS CFX. An empirical strength degradation model is proposed for soil softening due to undrained remoulding and clay–water interaction in the highly sheared interface, termed ‘shear wetting.’ The combined effects of strain rate and softening on the mobilized shear strength of deepwater offshore clay are implemented in CFX. A sufficiently large number of loading cycles is simulated using this computationally efficient numerical technique to achieve a stable response. A significantly large reduction of vertical resistance of a shallowly embedded riser section due to cyclic loading, as observed in physical model tests, is obtained using the proposed strength degradation model with shear wetting, which cannot be explained simply by undrained remoulding. •Riser–seabed–water interaction is modelled using a computational fluid dynamics approach.•A method to implement shear strength degradation of soft clay in ANSYS CFX is presented.•The role of trench formation and suction under the riser during uplift is examined.•The present numerical approach is computationally efficient.