PRELIMINARY ASSESSMENT OF THE CHANGE IN THE MECHANICAL BEHAVIOR OF SYNTHETIC YARNS SUBMITTED TO CONSECUTIVE STIFFNESS TESTS

Felipe Tempel Stumpf,Carlos Eduardo Marcos Guilherme,Fulvio Enrico Giacomo Chimisso

doi:10.14311/app.2016.3.0075

Abstract

Anchoring systems of offshore platforms are designed to work uninterruptedly for 20 to 30 years, and despite the increasing number of studies concerning these systems’ mechanical behavior in working conditions, most of them are conducted in speeding environments, due to the necessity of results in the short term. There are standardized stiffness tests, such as ISO 18692, based on quantities like Dynamic Stiffness and Quasi-static Stiffness, through which one can compare, quantitatively, two different ropes. However, there is a lack of research aiming to assess the change in the mechanical behavior of these ropes after certain number of consecutive loading cycles (or stiffness tests). This study aims to assess, at the yarn level, the change in the quasi-static and dynamic stiffness parameters of two materials when submitted to repeated stiffness tests. Results in terms of qualitative assessment of the mechanical behavior, as well as quantitative changes in the parameters are presented.

Highlights

Mooring lines used as anchoring systems of oil platforms must be designed to work uninterruptedly over more than 20 years, which means that, from a mechanical design point of view, their material’s mechanical behaviour must be well known at each moment over the system’s life span
One of the main disadvantages of PET, is that it tends to show considerable elongation in tension, what might present a problem for long mooring lines in terms of station-keeping [1, 2]
There is a lack of research aiming to assess the change in the mechanical behavior of these materials after a certain number of consecutive loading cycles

Summary

Introduction

Mooring lines used as anchoring systems of oil platforms must be designed to work uninterruptedly over more than 20 years, which means that, from a mechanical design point of view, their material’s mechanical behaviour must be well known (or, at least, accurately predictable) at each moment over the system’s life span. Polyester (PET) was considered the most suitable material for such applications [1], especially due to its low cost and its reliability in terms of small changes in the mechanical behaviour over time (very low creep rate, for instance). Aiming to find a more suitable option for these systems, the oil companies are testing several other synthetic materials, such as aramid, polyamide, high modulus polyethylene (HMPE), liquid crystal polyethylene, etc. All of those show a considerable lower elongation in comparison to PET [3, 4], but sometimes the price to be paid is a significant increase of their creep rate, or even higher production costs [5, 6]. For example, has the advantage to have a lower-than-water density [2], which means that it does not add weight to the platform when installed

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Results