The weight advantage of copper in submarine cables

The higher wind speeds available offshore compared to land are leading to the expansion of the offshore wind power market, with the use of larger turbines and installations farther from shore. This requires increasingly long connections to shore with subsea MV and HV cables. However, the hostile deep sea environment leads to a multitude of challenges for submarine cables. The selection of the correct material can have a significant effect on how these challenges are overcome.

There are two main methods of installing a submarine cable: burying it underneath the seabed, or laying it on top of the seabed.

Burying underneath the seabed

A traditional way of burying cables is to use high-pressure water jets from nozzles. The water causes the seabed to become fluidized and the cable sinks down in the slurry. Empirical evidence from a cable manufacturer indicates that lighter cables do not sink sufficiently fast and therefore the cable does not reach the required laying depth, resulting in the necessity to repeat the laying procedure.

Laying on the seabed

This is a faster and less expensive method, but leaves the submarine cable subject to movement across the seabed due to tidal currents. This raises the likelihood of premature cable failure due to abrasion damage, or due to damage caused by fishing gear as the cable is no longer protected.

The weight advantage of copper conductor cables

To avoid the potential drawbacks inherent in both methods, the solution is to select a material with sufficient weight. In the first scenario, a heavier cable will sink correctly and more quickly into position in the fluidized seabed. In the second scenario, a heavier cable will mean that when laid on the seabed it can resist the maximum tidal seabed currents expected, even under storm conditions*.

In both respects, the significantly higher specific weight of copper compared to aluminium (by a factor of three times; see table below) gives it an advantage, leading to a more efficient cable-laying process and greater stability of cables, whether buried or not**.

Specific weights of copper and aluminium:

• Copper: 8900 kg/m3
• Aluminium: 2760 kg/m3

References

• Electric Cables Handbook/BICC Cbales; edited by G.F.Moore, 3rd edition, 1997.
• Mechanical aspects of submarine cable armour, E. Zacone, Spring 2012, ICC submarine cables meeting in Seattle, USA.
• DNV-RP-F109, On-bottom stability design of submarine pipelines, October 2007.
• Stability of submarine pipelines on liquefied seabeds, T.C. The a.o. Journal of waterway, port, coastal and ocean engineering, July/August 2006.

Notes

* The resistance to movement is proportional to the square root of the coefficient of friction of the cable with respect to the seabed multiplied by the W/D ratio, where W is the submerged weight of the cable and D is the overall diameter.

** Next to the specific weight, other aspects give advantages to copper conductors compared to aluminium conductors in submarine cables. These are higher current rating, smaller size, and higher resistivity against corrosion.