Many clients are unsure of the pros and cons of steel armoured cable or aramid fibre armoured cable. The design of a cable 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, and as they become clearer, 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 full 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 whether a steel or aramid fibre armoured cable would work better for their cable solution. The answer is usually not straightforward due to the unknown parameters of the overall system, but 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 a high strength and high modulus (UTS ≈ 2000N/mm², E ≈ 200,000N/mm²), which means a cable will have a small overall diameter for a given 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 are more able to 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 make the cable stronger will only contribute further to the weight and provides no more than an incremental increase in load capability. Steel ropes have 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 now produced per year make aramid fibre relatively cheap when compared with other high-strength fibres.
Disadvantages of aramid fibre armoured cable
Aramid fibre has a number of disadvantages when used for electro-optic cables. Unlike steel, the material is only strong in tension and when it is in compression the fibre kinks and buckles. 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.
Steel has a defined yield stage which means that nearly all the 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. Add to this 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 compared to theoretical strength.
Another issue with all fibres is the limited crush resistance the fibre provides. 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.
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 which need to be packed so that 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.
When an aramid cable is flexed the aramid fibre layers abrade each other, so unlike with steel, the flex life of aramid fibre is governed by internal friction and wear. Larger bend radii are usually necessary to compensate for this.
If you would like to discuss your particular situation, please don’t hesitate to contact us. We are always keen to discuss cable solutions, whether we have worked together before or not.