Fatigue is one of the most critical design and operational issues for a permanent offshore mooring system. Most recent literature and a lot of offshore industry practices for mooring fatigue design are often limited in local locations such as the mooring top chain at the fairlead.
In this paper, an analytical solution for LF tension range along a homogeneous-material catenary mooring line is derived. It is found that the most critical LF fatigue damage occurs at the lower section of a mooring line near touch down position. For WF fatigue, there are several factors affecting the location of the critical position, and it can occur at fairlead location or at lower section of a mooring line. It is thus necessary for industry community to further investigate the governing locations of mooring line fatigue.
As a case study, comprehensive frequency-domain analysis of fatigue along mooring lines is conducted for a semi-submersible in the deepwater Gulf of Mexico. The mooring system consists of chain-polyester-chain sections, as well as connecting components. Two-slope static-dynamic model is used for polyester rope stiffness. Wave scatter diagram and loop current fatigue bins in the central Gulf of Mexico are applied. The combined spectrum method with dual narrow-banded correction factor is applied for fatigue evaluation due to low frequency and wave frequency tensions. Fatigue of top chain, bottom chain and rope are calculated in accordance with API RP-2SK. Taut and semi-taut mooring systems with various pretensions are investigated. Tension ranges and fatigue damages along the mooring lines are investigated for different mooring configurations. Locations subject to the most critical damage are identified.
It is found that the critical fatigue damage locations on a mooring line are governed by several factors including mooring line pattern of taut, semi-taut or catenary, motion pattern of wave frequency or low frequency governing, length of bottom chain, and friction force on the ground. Recommendations are provided for mooring design of floating structures in terms of combined fatigue due to wave and Vortex Induced Motion (VIM). For LF motion governed mooring system, it is shown both analytically and numerically that the maximum tension range occur at lower section of mooring lines. The exact location varies between the top of bottom chain or seabed interaction point, depending on friction coefficient and chain length at seabed. For a semi-taut or catenary mooring system, most critical fatigue damage for WF can occur at fairlead. For taut mooring system, most critical fatigue damage for WF occurs at the top of bottom chain. Therefore, considering various mooring components, it is recommended to assess the fatigue life at each and every transitional location of the mooring system such as top chain at the fairlead and chain stopper, top of bottom chain and chain/seabed interaction point since all of these are of special interests for mooring system fatigue design.