Fatigue Curves for Polyester Moorings - A State-of-the-Art Review

Abstract

Abstract The gathering momentum for the use of polyester ropes for long term production system moorings in deep water has instigated a thorough review of their fatigue performance. The traditional view of designers has been to use spiral strand steel wire rope T-N curves to assess the fatigue performance of polyester ropes on the basis that available evidence indicates that the latter's fatigue performance is superior to spiral strand wire rope. The authors have examined all the available fatigue testing data and the proposals for polyester specific T-N curves and have identified a number of issues pertinent to the derivation of fatigue design data for fibre ropes and propose an improved design curve. This paper systematically identifies the various issues and proposes a rational way of including the available data (including runouts) for the development of a T-N curve for polyester ropes. The new data was used to re-visit the fatigue life and reliability for an example West of Shetland FPSO moored using polyester ropes (Ref.1). Introduction Synthetic fibre ropes have undergone a rapid testing and development phase over the last five years through a number of Joint Industry Projects (Ref. 2,3,4 and 5) and other studies. Ref. 2 is a recently launched JIP which utilises the latest materials with marine finish, terminations and rope designs. Due to their good behaviour in fatigue, testing fibre ropes to quantify their fatigue life can be an expensive process and individual tests at loads representative of FPS conditions are predicted to run for man-years even when accelerated several fold. Thus, most fatigue testing has been conducted at high load levels and accelerated rates to produce data within reasonable time and budget. This paper will review the data to show the true potential of polyester fatigue life and to provide a realistic design T-N curve. Terminated Rope Breaking Strength Rope strength is defined as Minimum Breaking Strength (MBS) with terminations and is guaranteed by the manufacturer. The average measured breaking strength will typically be 10% higher based on the most recent technology. As shown in Table 1, a material, rope and termination review of the public domain data shows considerable scatter in the margin of ABS (Actual Breaking Strength) over MBS between -7% to +36%. This illustrates that care needs to be taken in analysis of data when selecting the basis of rope strength and that a common basis for strength can reveal new information. The column titled theoretical ABS is calculated from the yarn content to give the theoretical strength of the rope to allow for terminations, variability (manufacture of rope, not terminations) and obliquity (effect of the lay angle). This gives a strength expected using a well made, designed and terminated rope with a high quality fibre. This now gives a margin well within 10% for the spliced ropes, with the exception of number 5 for which the terminations may not be reflecting the true potential of the rope.