Governing factors and locations of fatigue damage on mooring lines of floating structures

In most literature and offshore industry practices, mooring line fatigue analyses typically focus on local locations, such as mooring top chains at fairleads. In this paper, an analytical analysis for low frequency (LF) and wave frequency (WF) tension range along a homogeneous-material catenary mooring line is conducted. It is found that the most critical LF fatigue damage occurs at the lower section of a mooring line near the touch-down position. For WF fatigue, there are several factors affecting the location of the critical position, and it can occur at the fairlead location or at a lower section of a mooring line. Through numerical analyses of fatigue damage along the mooring lines for semi-submersibles in the deepwater Gulf of Mexico and Offshore West Africa, we identify the most critical fatigue damage locations for different mooring systems. The factors affecting the critical location are discussed. These factors include mooring patterns, pretensions, chain lengths, water depths, viscous damping, as well as those that are typically neglected in early design analysis based on common industry practices, such as mass concentration components and chain/seabed friction. The paper provides further recommendations to those contained in current industry practices for mooring fatigue design of floating structures.