Lifetime prediction of aramid yarns applied to offshore mooring due to purely hydrolytic degradation

Abstract

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.

Post-impact mechanical characterization of HMPE yarns

“…For example, polyester has an LT of 0.733 N/tex, whereas HMPE has an LT of ∼ 3 N/tex. However, a similar behaviour is observed for aramid fibres [1,14,15]. HMPE is rigid because of its high macromolecular orientation and crystallinity [3].…”

“…Synthetic ropes have been widely used in the oil and gas industry to replace the traditional twisted steel wires, because they offer better flexibility, lower friction coefficient, lighter weight, easier handling, and no potential corrosion-related structural damage. The main application of these ropes is for offshore mooring systems [1]. However, they can be applied in several other fields, such as mountain climbing, fire rescue, robotics, artificial muscles, and general load-lifting operations.…”

Experimental study on the behavior of polyamide multifilament subject to impact loads under different soaking conditions

“…It should be noted that hydrolysis is the reaction of the chemical group in the polymer chain with water molecules; acidic or basic environments and with temperatures that favor hydrolysis, which leads to chain breakage and the binding of oxygen and hydroxyl to separate groups (De Paoli, 2009). The relationship of polyamide hydrolysis (and other synthetic fibers) with acidic environments and high temperatures is addressed in the literature (Liu & Reineke, 2010;Hocker et al, 2014;Duarte et al, 2019). In this study, as the immersions were made at room temperature, the hydrolysis effect is not the deleterious effect of this loss of strength.…”

Evaluation of residual strength of polymeric yarns subjected to previous impact loads

“…Apart from the mechanical loads originating from the movement of the floating unit, such anchoring systems may be subjected to some degree of environmental damage caused, for example, by ultraviolet incidence and hydrolysis, depending on the fibre group [5,6]. Yarn-on-yarn abrasion is another (now a mechanical) degradation mechanism, even more relevant than the previous ones, that can affect the material's mechanical behaviour [7].…”

“…Polyester yarns have a high mechanical resistance, good tenacity and abrasion resistance [7,11,12]. When exposed to the environment under typical mooring conditions, they do not degrade considerably and are resistant to hydrolysis and ultraviolet incidence [5,13]. Polyester is also not biodegradable, has a negligible creep behaviour at room temperature and, when exposed to high temperatures, it contracts instead of expanding [14,15].…”