Synthetic Wire Rope

26.2.2. Synthetic Wire Rope#

Synthetic wire ropes have gained increasing popularity in offshore mooring due to their low submerged weight and favorable cost (~$25/m/MBL). They are also easier to handle during installation compared to steel wires.

26.2.2.1. Main Types#

  • Polyester: Moderate stiffness, good fatigue resistance, higher stretch; often used for compliance in moorings.

  • Nylon: High elasticity, excellent energy absorption, but strength decreases when wet; suitable for shock loads.

  • High Modulus Polyethylene (HMPE): Very high strength-to-weight ratio, stiffness comparable to steel, low stretch.

  • Aromatic Polyamide (Aramid): High stiffness and strength, good thermal stability, but sensitive to UV and bending fatigue.

26.2.2.2. General Characteristics#

  • Strength: Can match or exceed steel in tensile capacity.

  • Weight: Much lower than steel; near-neutral in water.

  • Handling: Requires careful protection to avoid abrasion or fiber damage.

  • Cost: Significantly lower than steel wire rope.

  • Fatigue: Better resistance to tension cycling compared to chain or steel wire.

  • 3-T Endurance: Life expectancy depends on the combination of Tension, Time, and Temperature; higher sustained tension or elevated temperatures reduce endurance.

  • Visco-elastic Behavior: Stiffness changes with load history; typical stiffness range is 12–30 × MBL.

Next to the chain and steel wire rope, synthetic wire rope has shown increased use in the industry. In the last part of this session we will focus on this material in particular because it is expected to be used more and more. There are various types of synthetic wire rope, the main types are: Polyester Nylon High modulus polyethylene (HMPE) Aromatic polyamide (Aramid) The figure shows the tenacity of the wire rope in strength per unit weight or TEX versus the elasticity. Polyester and Nylon have a lower strength modulus and stretch more, which can have a purpose in mooring applications. On the other hand, the newer HMPE has steel equivalent strength and a lot more stiffness. But still these materials are a lot lighter and are near neutral in water. Before considering a design using synthetic wire ropes, it is good to understand the differences as compared to chain or steel wire rope; As indicated some materials have very high strength, comparable to steel. A low weight, in particular when submerged. This needs to be considered for permanent but also for installation conditions. This would indicate easier installation but the material needs to be well protected and carefully handled because the fibres can be easily damaged. Synthetic wires are generally much cheaper than steel wire. As an indication \( 25/m/MBL versus \) 75/m/MBL. The synthetic wires generally have a greater tension fatigue resistance than steel wire or chain. The key aspect to consider when designing with synthetic wires is what is called 3-T endurance: The Tension/Time/Temperature consideration. If the mean tension increases it means that the endurance will reduce. Similarly when the temperature increases the endurance will decrease. This in combination with the time this tension or temperature is retained. Another key aspect to understand is the visco-elastic behaviour which we will discuss now.

When synthetic wire rope is first slowly brought under tension, the rope elongates and when the tension is reduced the rope will retain a permanent stretch. Once the rope is loaded slowly again it will move to the original strength/strain curve and slowly elongate further. If at a mean tension the rope experiences fast dynamic load variation it will not follow the working curve but experience a much higher dynamic stiffness. The difference between the slower working curve stiffness (12 x MBL) and the dynamic stiffness can be (30 x MBL) can be substantial.
The reason is that the polymer materials typically contain crystalline parts and non-crystalline (amorphous) parts. Static stiffness is the stiffness of a tension member when it is loaded slowly, leaving time for both the amorphous and crystalline part to react to the load. The resulting fiber stiffness is an average of the stiffness of both parts. Dynamic stiffness is the stiffness response of a tension member when it is under (faster) cyclic loading. As the amorphous part does not react fast enough to the quickly changing loading regime, it is the stiffer crystalline part that takes on the load, resulting in a more ridged response of the whole fiber. For polyester rope, this behavior results in dynamic stiffness being 2 to 3 times the quasi-static stiffness. Failure to account for this behavior will inevitably yield inaccurate line tension and vessel offset predictions, and the inaccuracy for vessel offset can be particularly large.

This is the reason that it is advised to test samples of the ropes which are going to be used to confirm the stiffness characteristics. The advised code is the DNV-RP-E305, Design, testing and analysis of offshore fibre ropes. ABS uses an upper and lower bound approach for analysis.

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