The discovery of oil fields in deeper waters through out the last decades has led the oil industry to the necessity of replacing the mooring systems of offshore platforms from steel to synthetic cables. Consequently, both the industry and the academy started to join forces in order to better understand the mechanical behavior of such materials when subjected to different service conditions. This work aims to assess the change in the fatigue life of polyester (PET) yarns if the material is submitted to an abrupt tension load prior to the application of the fatigue cycling. It was found that the fatigue life of the yarns tested are substantially reduced if the specimen is subjected to this kind of abrupt load in comparison to virgin samples.
Abstract. Mooring ropes used in offshore oil platforms are exposed to a set of extreme environmental conditions that can be crucial to their behaviour in service. Considering the elevated mechanical demands on these ropes imposed by both the undersea environment and the station keeping of the vessel, this paper is focused on the experimental determination of the yarns fatigue behavior. In order to be able to foresee and compare their general wear rate, a diagram that correlates the force to which the specimens are submitted to the number of cycles for failure for each material is achieved. The analyzed fibers are Polyester, Aramid, Polyethylene and Liquid Crystal Polymer (henceforth quoted as PET, AR, PE and LCP, respectively), and this work followed a pattern composed by a fixed test frequency and an established maximum stress for the diagrams.
The present work evaluates the mechanical behaviour of High Modulus Polyethylene (HMPE) yarns after being impacted by sudden axial loads. The influence of loading conditions on the structural integrity of yarns is assessed by tensile, fatigue, and creep tests before and after the impact events. The impact loads were inferred by drop-weight adopting a 300mm height and weights corresponding to 4, 5, and 6% of Yarn Breaking Load (YBL). At 5% YBL, most specimens fail after the impact, and at 6% YBL, all specimens fail. The application of 4% YBL tests results in enhanced creep and fatigue resistances and a decrease in the tensile resistance. Finally, a Scanning Electron Microscopy (SEM) analysis showed that the yarn filaments tend to straighten after the impact, while a decrease in their diameter is noticed due to the longitudinal deformation.
The discovery of oil fields in deepwater over the last decades led the oil and gas industry to the necessity of replacing the steel wire cables of the mooring systems of offshore platforms by polymeric ropes. These systems must be designed to work for at least 20 years without showing substantial loss in tensile strength or in their mechanical behavior along this period. However, some polymers present degradation by seawater through the process of hydrolysis, and the question whether it affects significantly the materials’ ultimate tensile strength arises. Accelerated hydrolysis tests were conducted in yarn samples of aramid at high temperature in order to use the Arrhenius correlation to predict their lifetime under service conditions. In order to decouple the total degradation into a thermal and a purely hydrolytic part, separate aging experiments were performed into a dry chamber and the conclusion was that thermal degradation does not play a significant role in the total degradation of aramid due to the water submersion at the temperatures tested.
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.
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