Modelling of trench effect on fatigue performance of steel catenary riser

Shoghi, Rahim (2020) Modelling of trench effect on fatigue performance of steel catenary riser. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The riser-seabed interaction resulting in a trench formed in the touchdown zone (TDZ) of steel catenary risers (SCR) has a significant influence on accumulated fatigue damage. Several studies have used different trench modeling approaches to investigate the trench effect on fatigue performance of SCR. However, contradictory observations have been reported with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue. In this study, the significance of trench geometry in fatigue damage evaluation was investigated. Using boundary-layer methods (BLM) and numerical approaches, a meaningful relationship was observed between the trench geometry in different zones and the peak fatigue damage. A new set of rules was proposed for the qualitative assessment of the overall trend of the trench effect on the variation of fatigue damage. The proposed assessment rules were validated by performing comprehensive numerical fatigue analysis. A comparison with samples of published experimental and numerical studies was also conducted. The developed geometrical model and the set of rules for qualitative assessment of the trench effect on fatigue were used to re-assess the majority of the key published studies. The proposed methodology resulted in a more coherent agreement between the published studies. It was observed that for the near, far, or out of the plane direction of the vessel excursions, the ultimate fatigue damage might be slightly increased or decreased depending on the probability of occurrence in different geographical locations. Instead, the trench effect appeared in the form of significant shifting of the peak damage point towards the opposite direction of the low-frequency vessel excursions. This implied that the case dependency of the trench effect on fatigue response in different geographical locations with various environmental loads was a potential source for the contradictory results reported in previously published studies. Moreover, the study revealed the significance of riser flexural rigidity and its relation with TDP oscillation on the trench surface and the fatigue damage accumulation, consequently. The peak fatigue damage depending on the trench profile was analytically obtained and showed a good agreement with numerical models. The effect of seabed soil stiffness on the fatigue performance of SCR was compared with the contribution of the trench profile in the touchdown zone. The conducted research revealed several significant trench effect on the fatigue performance of SCR and provided an in-depth insight into this challenging problem.

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