Advanced Design Methodology for Synthetic Moorings

ABSTRACT Synthetic mooring ropes are among the several alternatives being developed for positioning deepwater floating platforms. These ropes are constructed from polymer fibers such as polyester, nylon, or aramid. Individual fibers are grouped in yarns and strands, and overlayed to yield ropes of different diameters. Synthetic mooring lines have shown to provide numerous advantages over steel cables, enabling deepwater exploration and production. Widespread utilization, however, entails addressing technical challenges in rope material characterization, dynamic response and mooring system design, installation, design life prediction, and reliability assessment. In this paper, a polymer mechanics based methodology is presented to address the long-term performance of mooring lines. The methodology provides an insight into the timetemperature dependent nature of synthetic rope deformation. A procedure for estimating damage evolution and the residual strength of a synthetic mooring line is also discussed. Failure of mooring terminations is beyond the scope of this study. However, outlined deformation and failure models can be used to design terminations, which are subjected to localized multiaxial stresses. INTRODUCTION In addition to obvious weight savings, characteristics such as neutral buoyancy, lower pretension requirements, improved fatigue performance, and better corrosion resistance make synthetic ropes an attractive replacement for steel cables as mooring lines for deepwater floating vessels. The potential advantages of polyester mooring lines have been successfully demonstrated in several feasibility studies [1,2] and pilot [3,4] projects. These projects clearly illustrate the operational and economic benefits of synthetic moorings and outlined the following technical challenges:special requirements during installation,complex loading history and associated time-dependent deformation during service,a multitude of long-term failure mechanisms due to combined creep and fatigue, andpotential strength degradation, in part, due to hysteresis effects, seawater exposure, and fiber abrasion. These challenges may be overcome with long-term testing and physical deformation and failure prediction model developments, and further field demonstration projects. In this paper, a polymeric mechanics based method is outlined to address deformation and failure of synthetic moorings. First, a review of mechanical characterization models of synthetic ropes is presented. Subsequently, a deformation model for synthetic moorings is described. Failure prediction methodologies for synthetic mooring ropes subjected to a marine environment are then discussed. LITERATURE REVIEW Single Fibers, Yarns, and Small Ropes A number of studies [5-12] have been reported in the open literature on mechanical properties of synthetic fibers, yarns, and small ropes. Cyclic and creep fatigue data were obtained [6,7] for Nylon 66 and single polyester fibers, yarns and ropes in air and sea water for load frequencies ranging from 0.1 Hz (typical in a marine environment) to 20 Hz. It was observed that the strain at failure under different loading (monotonic loading, static fatigue and cyclic fatigue) is the same (Fig. 1). It was postulated that the failure mode of fibers was mainly creep rupture, regardless of the loading cases. Using the cumulative time under load as a failure parameter, creep rupture models for fibers may predict the failure in cyclic tests at different frequencies in wet and dry conditions. The fatigue resistance of Nylon 66 and single polyester fibers, yarns, and small ropes are of similar nature at all frequencies, when the S-N curves are plotted in the S-time