Experimental Study of Creep Behavior at High Temperature in Different HMPE Fibers Used for Offshore Mooring

Abstract

Offshore moorings have been extensively researched in recent decades, and since the introduction of Taut-Leg systems made with synthetic polyester fibers as an alternative to steel catenaries, several fibers have gained significant prominence. High modulus polyethylene (HMPE) stands out due to its superior mechanical resistance and linear tenacity compared to many other fibers. However, it shows challenging behavior in creep, showing high strain rates and low creep resistance. Consequently, manufacturers have focused on developing HMPE fibers typified as "Low Creep," aiming to achieve satisfactory creep behavior and enable offshore mooring systems made entirely of HMPE. Such advancements could lead to the use of HMPE in offshore installations in ultra-deep waters, given its low elongation. The goal of this study is to evaluate the creep behavior of 6 HMPE fibers, including both European and Asian manufactures, with 3 fibers designated as "Low Creep." The evaluation encompasses different load temperature conditions. The experimental approach uses HMPE multifilaments, which serve as the base material for making mooring ropes. The results are compiled in tables, providing initial characterization results and average creep results, including creep strain rates. Creep graphs are drawn to facilitate understanding and comparison of the behaviors, along with a generalized statistical modeling of the creep-rupture surface for each fiber. The results show that the fibers designated as "Low Creep" indeed exhibit significantly better behavior than others, with lower strain rates. Among them, Fibers A and D emerge as the most promising for offshore mooring, showing greater resistance to creep. From a commercial standpoint, Fiber D offers the lowest cost per kg and presents favorable constitutive behaviors in specific operational contexts, as well as better stability with increasing temperature. In terms of future directions, other methodologies linked to the acquired experimental database should be explored, including analytical models and numerical simulations. Additionally, an investigation into sub-ropes and a detailed study of thermal degradation decoupling in the studied phenomenon should be considered for further research.