The Elastic Modulus Characteristics of Polyester Mooring Ropes
Authors: Bosman R.L.M. , Hooker J. 
Published: October 2008

One of the most important design factors for FPS mooring systems with polyester ropes is the maximum peak load and excursion during benign-and storm weather conditions. The main rope characteristic which determines the peak load and influences the maximum offset is the dynamic modulus. Several people and institutes (1: Del Vecchio 1998, 2: TTI /NEL JIP 97-98) have made efforts to define the dynamic modulus and to measure relevant data. Del Vecchio was the first attempt to define a formula which covers a lot of test parameters. In the present paper this formula serves as a starting point, while the test data has been obtained from tests with a random loading mode (as was already suggested by Del Vecchio). It will be demonstrated that the mean load is the main parameter determining the dynamic modulus. The main test to be satisfied in order to arrive this conclusion is that a sufficient number of cycles have to be conducted (&gt; 10,000 cycles). A second conclusion is that under these test conditions good predictions (error &lt; 10%) can be made from small-scale tests to full-size tests.

Introduction

Polyester ropes are increasingly being considered for use in FPS mooring systems. The many advantages of this system compared to steel system, have already led to a variety of installed systems at Petrobras (Lit.3). Other oil companies and contractors are busy testing their first systems. The major difference between a steel and a polyester mooring system is the taut configuration of the polyester mooring system. The restoring force is induced by the stretching of the polyester, whereas the restoring force with the steel system is mainly determined by the weight increase resulting from the chain being lifted off the sea bottom. It is therefore important to have good insight into the parameter responsible for the restoring action of the polyester rope: the (specific) dynamic modulus. The specific dynamic modulus (E) is defined as the reaction of a rope (? elongation) to dynamic loading (? Force), corrected for the rope weight per meter and specific weight of the polymer used. The specific modulus is expressed in GPa.

Formula 1 shows the definition of the dynamic modulus.(Mathematical equation) (Available in full paper)

The specific modulus can also be expressed in N/tex, which is commonly used in the textile industry. 1 ktex is equal to 1gr/m. The specific modulus in N/tex is obtained simply by dividing the modulus in GPa by the specific weight ? in kg/dm3. In this paper the specific modulus is expressed in GPa (=E) only.

The dynamic modulus is usually observed to be much higher than the modulus measured in a quasi-static break test. The dynamic modulus is not a constant parameter because of the viscous elastic properties of polyester. The main test parameters which influence the dynamic modulus are loading history, mean load, strain amplitude, and temperature.

Del Vecchio (Lit.1) proposes the following formula (2) for determining the rope modulus at constant temperature as a function of the test parameters. (The formula has been rewritten for the specific modulus in GPa).