A Novel Approach to Predict Mooring Chain Fatigue Considering Out-of-Plane and In-Plane Bending Effects

초록

Since out-of-plane bending (OPB) moment- and in-plane bending moment-induced fatigue damage on mooring chain links for an offloading buoy had been reported, OPB and IPB moment-induced fatigue damage of mooring system has been a key item in offshore structure engineering and design. Many efforts have been carried out to establish a fatigue damage prediction procedure and a new guideline published by BV (2014) is believed to be an integrated procedure throughout previous studies. But the previous studies including the guideline are based on a superposed resultant stress of individual stresses at different loading steps. Critical disadvantages of the previous studies are addressed in this paper; the previous studies are not well taking into account stress non-proportionality, non-linearity of stress concentration factors (SCFs), and indistinctive shift from locking mode to sliding mode. A new novel approach is proposed which is able to dispel these apprehensions at a single stroke. Significant differences between the current approach based on the previous studies and the new approaches are suggested by comparing relations of OPB/IPB moment-OPB/IPB interlink angle. Two fatigue damages based on two approaches for three simplified load histories, which are combinations of a random tension process and two different regular OPB and IPB tension angles, reveal that OPB moment-induced fatigue damage has a dominant portion to total fatigue damage, IPB moment-induced fatigue damage has a small contribution to total damage but not negligible, and the current approach predicts fatigue damage more conservatively than the new approach.