Direct OPB Fatigue Calculation Method From Floater Motion

Abstract Damage to chains has been identified as the most common type of mooring failures, and among the causes of chain failures, fatigue is reported as the primary factor. Out of Plane Bending (OPB) is considered as a potentially relevant cause of fatigue damages, and various research has been conducted to figure out the mechanism of OPB. Whereas the previous research has focused on calculating the OPB moment using Interlink angle and Stiffness, methods for calculating the interlink angle caused by floater motion are undeveloped, and measurement of the interlink angle under dynamic motion is challenging. In addition, the interlink stiffness is not defined in number on the design guideline yet. To address these gaps, the authors conducted a scaled model test to investigate the relationship between floater motion and OPB stress. The result indicated the correlations between both acceleration and displacement of the mooring connection point, and the measured bending strain. Based on the model test results, a calculation model is proposed to estimate the OPB moment. In this model, OPB moment is calculated as a sum of two components: I) OPB generated by floater acceleration, and II) OPB resulting from floater displacement. Regarding the component I), the floater motion provides the acceleration to the connection point between the floater and the mooring chain. This acceleration imposes an inertia force on the connection point where the mooring load is also applied. This inertia force induces the force in the normal direction to the chain, constrained by the friction between the links and the downstream mooring catenary, that causes OPB stress on the chain. Therefore, the calculation is structured as a function of the mooring tension, the departing angle of the mooring line, the acceleration of the mooring connection point, and the dimension of the chain. Regarding the component II), the floater displacement alters mooring catenary shape. Until the chain link starts to slide, the chain link deforms by bending and absorbs the change in shape of the mooring catenary. Hence, the bending moment is calculated by multiplying a bending coefficient derived from the model test result and the floater displacement from the simulation result with Orcaflex. By calculating and summing these components, the proposed model calculates the stress from the OPB moment. A fatigue calculation was carried out using the proposed OPB calculation and Orcaflex simulation. Floater motion and mooring behavior were simulated with a 15MW semi-submersible Floating Offshore Wind (FOW) model distributed by Orcina Ltd. Using the simulation result as the input to the OPB calculation model, the time series OPB stress was calculated. The calculated OPB stress is combined with the mooring tension stress time history to calculate the fatigue life of the mooring chain by Rainflow stress counting. This fatigue calculation quantified the contribution of OPB stress to the fatigue life of the mooring chain. This paper presents the details of the tank test result, the OPB calculation model and the fatigue calculation result with the calculated OPB moment.