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Lookup NU author(s): Dr Arun Dev,
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The mooring chains for offshore floaters are typically designed to withstand only tension loads in an 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 focuses on understanding the underlying mechanism of OPB fatigue failure by addressing the problem from two key aspects which include the assessment of the main excitation source for OPB failures and studying the dominant parameters that influence chain bending inducing OPB fatigue failures. Part A of this paper analyzes the main sources that generate 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. 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 the generalized methodology for simplified assessments using numerical methods.
Author(s): Dev AK, Edward C
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: 14th International Symposium on Practical Design of Ships and Other Floating Structures (PRADS 2019)
Year of Conference: 2019
Online publication date: 22/09/2019
Acceptance date: 01/01/2019