Analysis of mooring wire rope termination with respect to bending fatigue

Abstract

Mooring lines that are used on floating offshore units are subjected to bending at the end termination. Over-bending in some cases may cause the mooring line to suffer damage from fatigue. A countermeasure for this over-bending has been to implement bend stiffeners at the end terminations. National Oilwell Varco procures wire ropes and two bend stiffeners from 3rd party suppliers. The scope of this thesis is to study the effect of these bend stiffeners and their impact on the fatigue life of the wire rope mooring lines.

Models of spiral wire rope, bend stiffeners and wire sheathing were created in CAD software Inventor™. Finite element analysis was performed using FAE software ANSYS™. The analysis was conducted using two different loads at three different load angles for both types of existing bend stiffeners.

The results indicate that the implementation of a bend stiffener has a positive effect on the fatigue life of the wire ropes investigated. However, there is an insignificant difference between the two types of bend stiffeners currently in production. A third type of bend stiffener was designed and FEA analysis was conducted in the same fashion as for the existing bend stiffeners. The results show a great increase in the service life of the wire ropes in all the load cases investigated when implementing this new design.