Down to the wire: A resistor’s role in laying cables

4th December 2019
Lanna Deamer

While we may associate data with being up in the clouds, it’s very much in the ocean too. Beneath our seas lie thousands of cables, tasked with carrying the world’s internet traffic. These cables form a vital communications network - and the world would seem like a much bigger place without them. What is less often considered is how these cables made it to the sea floor in the first place. Simone Bruckner, Managing Director at Cressall Resistors, explains the challenges of cable laying.

Around the width of a garden hose, these network cables link countries and continents and are responsible for around 99% of global internet activity.

Some cable networks are short, such as the 81-mile CeltixConnect that joins Ireland to the UK. Others, such as the SeaMeWe-3, are as long as 25,000 miles and have dozens of landing points all around the world.

Today, there are around 380 underwater cables in operation, spanning a length of nearly 750,000 miles. While they may carry almost all of our communications, many of us are barely aware of their existence.

Laying it down

A cable layer is a deep-sea vessel used to lay underwater cables on ocean floors for telecommunications and electric power transmission. Cables are stowed on the vessels in a spiral cable carousel and unreeled according to the speed at which the cable laying takes place.

Today, the largest cable layer vessels can store as much as 9,000 tonnes of cables on board. The ships themselves are also of an immense weight, with a tonnage of up to 11,000 tons and the ability to lay several cable lines at once.

With anything up to a mile of cable being laid out over the side during passage, the weight of the cable is so massive that the drive motor has to reverse its function from motor to generator in order to brake the cable reel. 

Braking needs to occur regularly while the cable is lowered onto the sea floor to control the laying speed. This frequent braking, along with atmospheric problems such as salt, water ingress and extreme temperatures creates a pretty tough working environment.

A rugged solution

To safely dissipate the excess energy created by braking, a bank of braking resistors is required. These resistors must be able to withstand harsh, corrosive environments so that the components do not degrade over time.

An additional challenge that these braking resistors face is insulation resistance. As the DC braking voltage of the unit cannot be connected to the vessel, it is not possible to use any of the re-generative capacities of a motor to power the ship. This means that a large resistor is required to dissipate all of the braking energy, as none of it can be fed back into the system.

To combat these challenges and manage excess braking energy onboard cable layers, engineers need to turn to a rugged solution. Most ships have a chilled water system, making it straightforward to incorporate resistors into the cable laying infrastructure.

Cressall’s EV2 water-cooled resistor is a 25kW unit available as a single unit or as a block of multiple units up to MW with a common cable box attached to braking power input, all using a common fresh water supply.

Cooling is achieved in the EV2 by pumping cold water, which comes into one end of the system and then absorbs the heat. It can be pumped through a radiator, which can be located some way from the heat generating equipment.

However, if the end users chooses to cool the resistor using sea water, they would require a different solution. In this case, titanium-sheathed elements in high grade stainless-steel vessels are preferred as they are able to withstand constant harsh environments. This means that the resistor’s voltage capabilities and its durable materials can safely dissipate all of the excess heat created during cable laying braking to make sure the operation is carried out without causing harm to other components on the ship.

Underwater cables are the invisible force driving the internet. These cables are a critical part of modern infrastructure and making sure they reach the ocean floor without any problems is an important task. To successfully manage the frequent braking that is required during this process, a durable and capable resistor should always be found onboard.

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