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Title: Fracture mechanics based offshore mooring fatigue assessment
Authors: Xue, Xutian
Issue Date: 2020
Publisher: Newcastle University
Abstract: Safety is crucially important for permanent offshore floating structures. However, over the past few decades, mooring accidents of permanent floating structures occurred at a high rate, and accident reports show that fatigue failure has become one of the most critical failure modes of mooring systems. There is thus a need to develop rational fatigue assessment approaches for offshore permanent mooring systems during the design phase. In this thesis, a fracture mechanics (FM) based approach for offshore mooring fatigue assessment is developed, in which a mooring chain link is treated as a round bar, an initial surface crack is assumed to propagate at the surface of the chain link. Stress ranges are calculated based on the tension ranges of a mooring line subjected to the combined loading process induced by the motions of low-frequency (LF) and wave-frequency (WF). A comparison between T-N & S-N curves based fatigue assessment approaches and the developed FM based mooring fatigue assessment approach is made, and the results show that fatigue lives predicted by the three approaches are well comparable if the safety factors suggested by API and DNVGL are applied to T-N & S-N curves based approaches. The developed FM based mooring fatigue assessment approach is then applied for mechanism investigation on mooring chain links subjected to out-of-plane bending (OPB), mooring chain links subjected to torque, and mooring chain links tensioned over a curved surface. The results show that the fatigue lives of mooring chain links are in general decreased significantly due to the effects of OPB and curved surfaces. In contrast, the effects of torque induced by the chain twist on fatigue lives of mooring chain links are not evident. Finally, an investigation on the effects of load combination methods on fatigue lives of mooring chain links predicted by the FM based mooring fatigue assessment approach is also conducted. The results show that the FM based approach based on the narrow-band method generally offers the most conservative fatigue life prediction and the differences of fatigue lives of mooring chains estimated based on general wide-band methods and dual narrow-band methods are not obvious.
Description: PhD Thesis
Appears in Collections:School of Engineering

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