In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.
A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the OPB mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.
This paper summarizes the full scale fatigue tests on chains and also the tests on small samples addressing the environmental influence on fatigue initiation and crack propagation stages. This paper also addresses the major step that was achieved: the implementation of a multiaxial fatigue criterion to address OPB hotspots as a standard practice in offshore industry. Moreover, the paper presents the first Industry OPB based S-N curves and its comparison to the existing industry fatigue S-N curve. Lastly, this paper provides a summary of the main steps in a framework for OPB fatigue calculation guidelines.