Stress Concentration Factors for Stud-Less Mooring Chain Links in Fairleads

Abstract

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.

Computational fatigue assessment of mooring chains under tension loading

“…2. This figure also illustrates the fatigue failure locations of chains under tension loading [13,20,44,45]. At both locations Stress Concentration Factor (defined as the ratio between the maximal principal stress and the nominal stress; the latter computed dividing the total external force applied at the chain by its cross section) take high values, approximatively a value of 4 [44].…”

Anchor Leg System Integrity - From Design Through Service Life

“…Depending on how the chain is supported at the fairlead, the SCF along the chain link can be very different than that assumed in the commonly used tension-tension fatigue curves. For example, when designing a mooring system where the chain passes over a chain wheel under load an additional SCF of the chain on the chain wheel must be included in the chain tension-tension fatigue analysis (Vargas, et al 2004). It is very important for the mooring designer to have knowledge of the type of fairlead being specified so the appropriate SCF can be included when performing the fatigue analysis to confirm the size of the top chain.…”

Multiaxial Fatigue Analysis of Mooring Chain Links Under Tension Loading: Influence of Mean Load and Simplified Assessment

“…The nominal dimensions of the chain are given as a function of the diameter [151], as reported in Figure 5.1. This figure also illustrates the fatigue failure locations of chains under tensile loading [27] [40], [145], [153], [154]. At both locations, the stress concentration factor (defined as the ratio between the maximal principal stress and the nominal stress; the latter computed by dividing the total external force applied at the chain link by its cross section) has a value of about 4 [153].…”

“…This figure also illustrates the fatigue failure locations of chains under tensile loading [27] [40], [145], [153], [154]. At both locations, the stress concentration factor (defined as the ratio between the maximal principal stress and the nominal stress; the latter computed by dividing the total external force applied at the chain link by its cross section) has a value of about 4 [153]. The second quench is performed at a temperature above the recrystallization temperature (Ac3, the temperature at which austenite stops forming), therefore, residual stresses generated during previous stages (hot bending, welding, and first quench) will be relaxed.…”