Defining, Measuring, and Calculating the Properties of Fiber Rope Deepwater Mooring Lines

Abstract

This paper explains and defines the synthetic fiber rope stretch and stiffness change-in-length properties which are important for deepwater platform mooring system design. It introduces the concept of accumulated elastic stretch, which causes temporary stretching and stiffening of the rope during tension cycling. It proposes how these properties can be determined by rope testing and by computer modeling. It proposes how the change-in-length properties should be used in mooring system design and analysis. Background The offshore industry and the fiber rope manufacturers have now had some experience in using fiber ropes in deepwater platform moorings. Petrobras pioneered this in the mid 1990s. In the late 1990s several class societies 1 2 3 and then API 4 published guidelines for fiber rope deepwater moorings. Tension Technology International with others also published fiber rope design guidelines.5 Those guidelines were based on what was then perceived to be the best design methods and the best test methods for the rope properties needed to carry out those methods. Since those guides were published, several fiber rope mooring systems - Mad Dog 6 7 and Red Hawk - have been designed for the Gulf of Mexico. A number of large fiber ropes have been tested and analyzed for use in those and other projects. Also, findings are now available from several significant fiber rope research programs which were conducted while and after those guides were written. Introduction The unique stretch and stiffness characteristics of synthetic fiber ropes are important in the design and analysis of deepwater platform mooring systems. Mooring system designers are experienced in designing with wire rope and chain catenary mooring lines. The stiffness characteristics of wire rope and chain are essentially linear. These components do not experience permanent stretch until high tension. With these components, the principal mooring system restoring force is produced by the catenary effect. As mooring water depths increase, there are incentives for using fiber ropes, as discussed in other papers. But with fiber rope, the principal mooring system restoring force is produced by the tension vs. stretch (stiffness) characteristics of the rope instead of by the catenary effect. Mooring system designers are still trying to understand the unique stretch and stiffness characteristics of fiber ropes and to incorporate them into traditional mooring design procedures. The stretch and stiffness characteristics of typical synthetic fiber ropes are time dependent and non-linear. Fiber ropes experience both elastic and permanent stretch. Conflicting and confusing terminologies are sometimes used. Some important rope properties have only recently been adequately understood and explained. Here the term stretch refers to any change in rope length hich is produced by tension. Stiffness is the relationship between stretch and applied tension. Other terminology is introduced in the text and summarized in Nomenclature at the end of this paper.

Synthetic mooring ropes for marine renewable energy applications

“…Synthetic materials display timedependent viscoelastic and viscoplastic behaviour (e.g. Figure 1) which is dependent on previous load history as well as the applied mean load, load amplitude (and to a lesser extent) load rate [18,19]. Comparatively these materials display greater compliance or lower axial stiness than steel components with non-linear, viscoelastic load-extension behaviour.…”

Synthetic rope responses in the context of load history: Operational performance

“…During mild-storm conditions (H s ≈ 2.67m, T p ≈ 7.5s, average water depth 31.9-32m) much larger loads were measured, with an average tension of 5.97kN. In the plotted timeseries the majority of loads are low amplitude and in the range of 0-4% of the rope's MBL, in which the sti ness of the rope is highly non-linear [9]. A notable snatch load of 0-52.2kN (0-11.2% MBL) occurred during the mild-storm interval for Line 1 at 02:09:55, providing insight into the survivability of the system during the application of a short duration, high magnitude load.…”

Synthetic rope responses in the context of load history: The influence of aging

“…The standardised approach of rope testing involving many hundreds (or thousands) of harmonic load cycles [42,37] may not be directly applicable for small oshore equipment deployed in highly dynamic environments (i.e. MRE devices) due to the temporal response behaviour of synthetic ropes resulting from non-linear material and structural processes [22]. Given that these processes are poorly understood in the context of the highly dynamic mooring loads, the rope samples were also subjected For wet testing at the University of Exeter, the aged and new samples were submerged in tap water (initially measured to be 20°C) for 31 and 32 days respectively prior to fully submerged testing in the DMaC test facility.…”