Computational modeling of synthetic-fiber ropes
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The objective of this research is to develop a computational model to predict the response of synthetic-fiber ropes under both monotonic and cyclic loads. These types of ropes are believed to offer a better alternative to more traditional mooring systems for deepwater applications. Of particular interest for this study are the degradation of rope properties as a function of loading history and the effect of rope element failure on overall rope response. A computational tool developed specifically for this research accounts for the change in rope properties as it deforms and the change in configuration of a rope cross-section due to the failure of individual rope components. The software includes both geometric and material nonlinearities, and it incorporates a damage index so the strength and stiffness degradation of the rope elements can be modeled. Following the failure of rope elements, the software considers the possibility that the failed rope elements can resume carrying their proportionate share of axial load as a result of frictional effects. vii Using the computational model developed under this research, several rope geometries are studied. Virgin (i.e., undamaged) ropes and initially damaged ropes are considered. In all cases, experimental data for a monotonically increasing load are available for comparison with the analytical predictions. For most of the cases analyzed, the proposed numerical model accurately predicts the capacity of the damaged ropes, but the model overestimates the rope failure axial strain. A three-dimensional finite element analysis is performed for a particular rope geometry to validate some of the assumptions made to develop the proposed computational model. If failed rope elements resume carrying their proportionate share of axial load, numerical simulations demonstrate the existence of strain localization around the failure region. Based on the damage model, damage localization occurs as well. This damage localization can cause the premature failure of rope elements and reduce the load capacity and failure axial strain of a damaged rope. Possible extensions to the computational model include the treatment of variability in rope properties and a lack of symmetry of the cross-section. Such enhancements can improve the accuracy with which damaged rope response is predicted. With the availability of validated software, engineers can reliably estimate the performance of synthetic-fiber moorings so that the use of these systems can be used with confidence in deepwater applications.