Mooring Chains Out of Plane Bending Fatigue Analysis Clarifications

Out of Plane Bending (OPB) fatigue of mooring chains has been the subject of several studies, of which the most comprehensive was the “Chain Out of Plane Bending” JIP completed in 2013. The currently applied fatigue damage calculation procedures are derived from the mentioned JIP. Following practical use of those procedures, it has been found that alternative procedures and updates can be considered. This paper describes alternative OPB fatigue damage calculation procedures, and the work performed to demonstrate their applicability and safety level with respect the JIP results most common applications. A detailed implementation of the JIP test results with focus on interlink stiffness for wide angle amplitude cycles (cycles with sliding events) is presented. The resultant safety levels based on this methodology compared with a simplified interpretation of JIP test results is presented. Alternative simplified procedures, for OPB fatigue damage estimation for mooring systems using OPB mitigation devices or for which very low OPB fatigue damage is expected, are presented and compared with the currently accepted procedures. In these systems it is demonstrated that the complex mooring analysis dedicated to OPB fatigue estimations, requiring mooring line angle estimations, may be waived and an indication of the conservativeness of the calculation results is presented. Comparative analysis of the different interlink stiffness and fatigue damage calculation procedures were performed on several representative environmental states, demonstrating for different levels of procedural simplification the increasing level of safety. It is demonstrated that different OPB fatigue damage calculation procedures can be used with different levels of required calculation effort, and therefore cost, according to the system optimization objective level. Simplified and easier to perform procedures for OPB fatigue calculation are presented and verified.