Engineers within subsea projects are often unsure of the pros and cons of steel armoured cable or aramid fibre armoured cable. How can you know the most beneficial armouring for your subsea project as an engineer?
Evolving cable design:
A cable design is usually an iterative process, and it will probably need to evolve several times as the system development progresses.
This is usually because the parameters of the overall system are still unknown. As they become more explicit, the expected loads, equipment limitations, and environmental operating parameters need to be adjusted in accordance. Once the deployment locations, equipment weight and configuration, operating procedures, and handling system limitations are all defined, a complete dynamic analysis can be carried out.
The calculated loads and fatigue profiles can then be fed back into new design iterations until the solutions align with the analysis.
One of the critical design choices when designing ROV cables is the strength member. Clients often consult DeRegt on whether a steel or aramid fibre armoured cable would work better for their cable solution. Knowing the advantages and disadvantages of both materials will help you make a more informed decision.
Benefits of steel armoured cable
Steel armoured cables have several advantages when used for ROV deployment, which is why nearly all conventional work-class ROVs use a steel wire armoured main lift cable and tether management system.
Steel has both high strength and high modulus (UTS ≈ 2000N/mm², E ≈ 200,000N/mm²), meaning a cable will have a small overall diameter for the break strength. Moreover, steel is relatively cheap and offers good value for strength.
Steel performs as strongly in compression as it does in tension strength, which means that steel armoured cables have good crush resistance and can withstand the compression loads when reeled onto a winch.
In addition to this, deformation is limited and provided the winch core has a suitable grooved shell, such as Lebus grooves, good winding can be achieved when reasonable care is taken during reeling.
Steel also has good fatigue characteristics with a relatively high endurance limit compared to its tensile strength.
Disadvantages of steel armoured cable
The main disadvantage of steel is its high density at 7,890kg/m³. The weight of a steel armoured cable in water is therefore relatively high. Short lengths of cable increase the tensile load on an ROV cable only slightly, but as the length increases, the weight of the cable in water becomes more significant until the cable weight itself actually becomes the dominating load.
Adding steel to strengthen the cable will only contribute further to the weight and provides no more than an incremental increase in load capacity. Steel ropes have a high load-bearing capacity as the rope is limited by the maximum permissible stress of the steel wire.
Armoured electro-optic cables, however, are limited by the stretch in the cable, which will impact the flex life of the copper conductors or optical fibre.
Benefits of aramid fibre armoured cable
Aramid fibre has very high strength (≈ 3,000N/mm²) and a low density (≈ 1,450kg/m³) so when used in water, adding strength to the cable means adding significant load bearing capacity.
While more expensive than steel, the large quantities produced per year make aramid fibre relatively cheap compared to other high-strength fibres.
Disadvantages of aramid fibre armoured cable
Aramid fibre has several disadvantages when used for electro-optic cables. Unlike steel, the material is only strong in tension, and the fibre kinks and buckles when it is in compression. The radial strength is also low, making the fibres susceptible to abrasion.
Aramid fibre has a lower tensile modulus than steel (≈ 100,000N/mm²), so as electro-optic cables are designed for stretch, twice as much cross-sectional area of fibre is needed in comparison to a steel cable.
Aramid fibre also needs to be protected from external damage and UV radiation, meaning an overall sheath is required. As a result, an aramid cable with the same working load as a steel cable has a much larger diameter.
Additional differences between steel and aramid fibre
Yield - Steel has a defined yield stage, meaning nearly all steel wires reach their maximum tensile strength before failure. This means that steel has high actual strength versus theoretical strength. Aramid fibre has no yield phase, so cables break with a cascade failure profile. Additionally, the higher level of friction and testable break strengths are between 60% and 80% of theoretical break strengths. This means that aramid fibre cables have significantly lower terminated strength than theoretical.
Crush Resistance - Another issue with all (aramid) fibres is the limited crush resistance. In fact, any crush resistance is usually a function of the underlying copper conductors and the inner and outer thermoplastic sheaths. This results in a lot of deformation when reeled under tension onto the winch.
Spooling - When spooling the cable onto a winch, the cable does not come fully flush at the second flange where it turns around into the second layer. This can leave partial gaps that must be packed so the cable does not get pulled into the gap. Steel cables can also get pulled into these gaps but are less susceptible than aramid cables.
Flexing - When an aramid cable is flexed, the aramid fibre layers abrade each other, so unlike steel, the flex life of aramid fibre is governed by internal friction and wear. Larger bend radii are usually necessary to compensate for this.
The best option for your subsea project
Above, the most critical differences between aramid fibre and steel armouring are mentioned. You probably wonder what this means for your specific project.
To discuss your design specifications with one of our experts, you can always schedule a Feasibility Check (in which our cable expert helps assess your specs' feasibility and discusses the fitting materials with you.
Another option is to request a Custom Cable Design. Our experts would love to make a design based on your specs, showing where improvements might be.