Test Results on The Effect of Cyclic Wear of Polyester Sub Ropes
Authors: Ayers Ray R. , Aksu Saltuk 
Published: October 2011

In this paper, research efforts initiated in 2007 to better understand failure mechanisms of polyester rope are reviewed. A strain-based method is described for predicting the remaining life of polyester mooring lines. The fatigue mechanisms of metals vs. polyester are contrasted, leading to a justification for using strand-on-strand, "20-hurricane" subrope testing of polyester mooring lines rather than traditional (as conducted for steel) fatigue testing of the full-size rope assembly.

This paper provides strand-on-strand, "20-hurricane" test results from over 40 individual rope tests, with the key comparative variable in the form of % reduction in breaking strength (after 20,000 cycles of 30% average break load ±15%). Data are presented for five different commercially available polyester mooring rope designs.

Unexpected results due to subrope unloading and reloading during a test sequence are described and explained. This behavior was observed in certain cyclic wear tests, and was caused by fiber-yarn bedding in, breaking out, and then re-bedding.

Finally, a semi-empirical mathematical model to predict rope remaining life is described that employs the results of the " 20-hurricane?? test program.

Introduction

The work reported here was undertaken as a result of both industry and regulators recognizing the need for deepwater mooring system technology development in the area of synthetic fiber mooring systems. While technology development for polyester moorings began in Europe and Brazil in the late 80s and early 90s, the DeepStar Project (a continuing joint-industry research program) has been developing technology for polyester mooring system applications for deepwater Gulf of Mexico (GoM) applications since 1995. In that year, DeepStar conducted the first single-leg mooring system test in the GoM, and in 1999, another DeepStar project found that there were ample design and test data to support the safe application of polyester taut-line mooring systems for floating production systems and mobile offshore drilling units in the GoM.

In 2007, the concept of the "20-hurricane" method described here was first developed. DeepStar has now sponsored further development of the method. The work on polyester rope fatigue reported here is focused on the cyclic wear of subrope strands with the objective of answering the question of the remaining life of polyester rope mooring systems that have been exposed to cyclic tension loadings from hurricanes.

The fatigue life of deepwater mooring lines is a critical issue in the design of offshore structures. This applies to both metal components and to polyester ropes. Material properties of the polyester along with the construction of the rope (number of strands, strand type (i.e., braided vs. twisted, jacketed or unjacketed, etc.)) define both the minimum break strength and the reduction in break strength due to a fatigue mechanism referred to as cyclic wear.

In this paper, previous efforts to better understand rope failure mechanisms are reviewed and a strain-based method is proposed for predicting the remaining life of polyester mooring lines. Rope failure mechanisms are discussed first. Following that, fatigue mechanism of metals vs. polyester are compared and the differences described. The rationale for the "20-hurricane" subrope testing of polyester mooring lines for determining the fatigue performance is discussed next. Finally, a semi-empirical mathematical model to predict fatigue life is described, and results from "20-hurricane" testing are presented.