The mooring chains for offshore floaters are typically designed to withstand only tension loads in axial direction and the bending moments on the chain links particularly at the vessel-chain interface connections are generally ignored. However, the unprecedented failure of four mooring lines of the Girassol Buoy mooring system exposed the significant gaps in the existing failure assessment methodology. The root cause investigations of this failure revealed the critical role of out-of-plane (OPB) bending induced fatigue which reduced the fatigue life of the chain links by 95%. However, the methodology to include OPB/IPB fatigue for failure assessments involves a complex process due to numerous parameters required in the formulations and the variability of mooring configurations.
This paper focus on understanding the underlying mechanism of OPB fatigue failure by addressing the problem from two key aspects which includes the assessment of main excitation source for OPB failures and studying the dominant parameters that influence chain bending inducing OPB fatigue failures. The Part A of this paper analyzes the main sources that generates OPB excitation moments and its responses causing OPB failures, by systematically studying the hydrodynamic parameters, skirt plate geometry and second order responses of the CALM Buoy. The Part B of this paper studies the key OPB inducing parameters like chain diameter, types, pre-tension, instantaneous tensions, proof loading, residual stress, material properties, boundary condition etc., and evaluates their inter-dependencies, proportionalities and relative sensitivities to understand their overall contribution to OPB failures.
The findings from these studies shall be used to understand the key design aspects of CALM Buoy that excites OPB loading and identify the dominant OPB sensitive parameters that influence OPB failures. The conclusions will be used for understanding the physics of OPB/IPB mechanism and defining generalized methodology for simplified assessments using numerical methods.
Assessment of CALM Buoys Motion Response and Dominant OPB/IPB Inducing Parameters on Fatigue Failure of Offshore Mooring Chains
Lecture Notes in Civil Engineering
Practical Design of Ships and Other Floating Structures ; 2019 ; Yokohama, Japan September 22, 2019 - September 26, 2019
2020-10-04
32 pages
Aufsatz/Kapitel (Buch)
Elektronische Ressource
Englisch
Mooring of Buoys in Restricted Waters
British Library Conference Proceedings | 1997
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