Mooring chains are critical components of off-shore installations. The fatigue assessment of these components often requires complex calculations to determine the loadings in the mooring chains. Traditionally the loadings can be converted into fatigue lives using S-N curves such as the DnV Posmoor curve, or the API RP2 SK curve. Deep water SPARs undergoing vortex-induced-motion (VIM) in loop current conditions may be subject to higher mean/cyclic loadings with considerably lower fatigue life estimates — compared with earlier installations in which fatigue life estimates were so large that fine tuning fatigue prediction methodologies was only of academic interest. In this case a more accurate evaluation of the fatigue performance of mooring chains is needed. In this study the stress concentration factors (SCFs) of a studless 5.25” (133mm) mooring chain were examined in a seven-pocket fairlead. The chain-fairlead system analyzed in this study had a very tight fit (i.e., was not designed for passing connector links), and the results of this study will, in general, not be applicable to other chain-fairlead combinations without additional study. The computed SCFs of the stud-less link interacting with the fairlead pocket were compared to the corresponding SCFs in a chain link away from the fairlead. The study shows that the maximum SCF ratio is 1.15, significantly less than the upper bound 2.5 value recommended by Det Norske Veritas in OS-E301 in lieu of detailed analyses. This has a significant impact, nearly an order of magnitude, on fatigue life prediction of the chain, justifying the analytical effort. The study also found that the SCF of the chain link in the fairlead is a function of the geometry of the chain and the fairlead. As a result some guidance is provided in this paper with respect to the implications of minimum SCFs on other link and fairlead geometries. This study combines computational efforts from NEL and ChevronTexaco in a two-pronged approach where: 1) NEL provided calculations addressing parametric variations of the chain link angles of the mooring line leaving the fairlead and the chain tension levels, and 2) ChevronTexaco validated simplified modelling assumptions done by NEL to make the parametric problem tractable.

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Attachment lugs are frequently used in aerospace applications to connect major structural components. It is important to assess the structural integrity of such lugs with cracks present. These assessments are necessary to prevent abrupt failure of the lugs before their intended service lives are reached. The estimation of the associated fatigue life of a lug, which consists of crack initiation and crack growth periods, demands stress and fracture analyses of the lug. In this paper, the results of two-dimensional stress and fracture analyses of tapered attachment lugs subjected to symmetric and off-axis loadings are presented. The effects of various parameters such as the outer-to-inner diameter ratio, crack length, and crack location on the tangential stress distribution, pin contact pressure distribution, stress concentration factor, fatigue critical location, and stress intensity factor are investigated. The finite element method utilizing a crack-tip singularity element is used in the analyses.

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