Short- and Long-Term Property Behaviour of Polyester Rope

Abstract

Abstract This paper reports on the results of an investigation into the mechanical property behaviour and fatigue performance of polyester rope. Dynamic modulus under different mean loads and strain amplitudes has been examined from ropes in the as-new to fully-worked conditions of 200 000 and 2 million cycles. Under sinusoidal and stochastic loading conditions, both mean load and strain amplitude is shown to affect modulus. Equations allowing these variables to be quantified are presented. During high cycle fatigue testing, for a given mean load/load range combination, modulus has been shown (after bedding-in) to remain effectively constant. Resulting from this investigation into long-term property behaviour, possibly the most comprehensive fatigue test programme ever to be performed on a batch of polyester rope has been produced. A T-N fatigue line based on these results is presented. Introduction A mooring system must be designed so that it has enough axial stiffness to stop the platform from drifting too far off station while having sufficient compliance to avoid overload in the lines resulting from the motions of the platform under the influence of first and second order wave forces. It is important therefore that the expected behaviour of rope axial stiffness (or modulus) is known under loading conditions applicable to offshore mooring conditions. To this end a number of investigators (Refs. 1 through to 11) have studied the modulus behaviour of polyester rope with a view to mooring applications. The purpose of this paper is to advance further the knowledge on the dynamic modulus behaviour of polyester rope with a view to these ropes being used within the mooring line systems of offshore platforms. Modulus data was generated under sinusoidal and stochastic conditions over a range of mean loads and strain amplitudes. Equations linking modulus to mean load and strain amplitude have been generated under both as-new and bedded-in conditions. The coefficients provided should allow the reader to evaluate dynamic modulus behaviour for the combinations of mean load and strain amplitude that may be expected in service, particularly storm conditions. A still unknown factor in the application of polyester ropes for deepwater moorings is their long term fatigue performance under the realistic loading levels which are to be experienced in service. Most of the fatigue data in the literature has been generated from tests performed using higher loads in order to generate failures below one million cycles, with this figure often being taken to represent a runout. This was the case in Fibre Tethers 20003. In addition to studying modulus behaviour therefore, this investigation has also addressed the high cycle fatigue performance of polyester ropes under lower loading conditions. Nine fatigue tests, with a total number of applied cycles of around 80 million, have been performed. Test Samples The tests were conducted using 855 TN polyester 3-strand sub-rope samples of 2 m nominal pin to pin length. The samples were un-jacketed and supplied with spliced terminations and soft eyes protected with Dyneema cloth.