This month a new factory in Dongguan, China, started to produce a device, designed and developed jointly in the UK and China, for a subsea cable network that will be installed in West Africa.
The factory is making a new type of repeater, a device that amplifies the laser signals used to carry data through the miles of fibre optic cable that criss-cross the sea beds between continents and countries to link together their internet and voice communications.
The titanium repeater is a great example of mechanical engineering, built to last 25 years and withstand extremes of pressure and temperature, with many engineering challenges similar to those found in space. It’s also a great example of a Chinese company partnering effectively with a British firm - sharing resources, tapping into engineering expertise and market knowledge.
Huawei Marine Network's latest repeaters and joints are lighter and smaller to make installation easier
Ian Watson is mechanical engineering manager at Huawei Marine Networks (HMN), a joint venture between marine engineering company Global Marine Systems Ltd and Chinese electronics and telecommunications firm Huawei Technologies Co. Ltd that was established in 2008. His mechanical engineering team in Chelmsford has been responsible for the development of the external protection and cable interface of the new repeater. The electronics and optical manufacturing happens in China.
Watson says HMN’s titanium repeater, which is half the weight of other repeaters, is the first time the design of the devices has changed since the first optical repeaters were installed some 30 years ago. “The traditional materials for repeaters are steel and beryllium copper. Most suppliers are reluctant to change because of legacy engineering. We came in as a new player with a blank sheet of paper.”
“The new repeater is lighter, it’s easier to deploy and will last longer. In engineering terms there were corrosion issues and polymer ageing issues to consider. My team also had to think about how the repeater will behave when it passes through the installation equipment.
It’s a kaleidoscope of conflicting problems. There are things that you would not expect to happen which happen because it’s a very difficult environment to work in. Five miles down, 8000m, you don’t want it to go wrong and bring it back.”
At a depth of 8000 m there is 81 mega pascals of pressure (12,000 pounds per sq inch) on the housing. The electronics and optical fibre inside the housing have to be protected from hydrogen ingress, because hydrogen gas can darken the fibre and reduce transmission capability. The repeater is therefore sealed to 1 x 10-9 cc’s per second or better, meaning that only a sugar cube of gas can get inside the housing every 32 years.
The biggest challenge associated with the environment of the repeater is dissipating heat. The devices are tightly sealed to prevent ingress of water and hydrogen, but the energy carried by the lasers and the amplification electronics, up to tens of watts of heat, also needs to be dissipated. The repeater dissipates heat predominantly through conduction, so the materials used are selected for their heat transfer qualities as much as high voltage insulation characteristics. “It’s a massive challenge, especially if the cable is being buried,” says Watson.
HMN have a turnkey capability, installing their own cable systems with cable provided by independent manufacturers. The repeaters for such cable systems are manufactured in China, but integrated into the cable using their mobile facility at the cable factory, prior to ship loading. It costs at least $70 000/day to keep a ship at the quayside. The mechanical engineering team has therefore developed a bespoke suite of integration equipment including a digital x-ray camera and a polyethylene moulding system that has reduced the time it takes to install each repeater into the cable by up to five hours and significantly simplified the process. Repeatability and reliability are the key driving factors behind the design, says Watson. “It's basically slow injection moulding, almost like push moulding,” he says.
A cable integration system used to insert joints and repeaters into a network
Before a cable system is laid a survey is conducted. This reveals the condition of the seabed, the number of cable crossings, if there are any other cables or geographical features that may cause problems, what the sea bed is made of and the water depth. The cable lies on the seabed in deep water and is buried in shallow water.
A plough is used to dig the trench and lay the cable. Trenching is done beforehand if rocks have to be cut through. The cable system is spooled out from the back of the installation vessel and through the plough, which is why light and small are desirable design attributes in the repeaters. Cables are getting smaller to cut costs, so a large and heavy repeater in a cable can cause problems as that portion of the cable will sink first when it is being streamed out. This could lead to loops, knots and plough overruns. Where necessary the installation is checked and assisted with an ROV.
The mechanical engineering team in Chelmsford works very closely with Beijing, visiting every two months and more frequently as the production plant for the new repeater ramped up to commercial operation this month. But effectively the team sits between the electronics, optics and manufacturing side in China and the marine installation side in the UK. The resulting design, says Watson, balances the needs of both the electronics and application demands and the marine environment.
The clean room manufacturing plant in Dongguan, China
The R2 titanium repeater will undergo sea trials later this year, after which its first application will be in a network Huawei Marine is installing off the coast of West Africa. Watson is pleased that their product represents a next step in design for this niche but nevertheless vital area of global communications. “Materials are changing, lead is coming out of solder, so we have to look at how we seal connectors. We have to look at high thermal conductive insulating materials. Cables are getting much thinner. Putting things in the sea is a very expensive business. So you want installation to be simple and fast and we’re achieving high reliability and low FIT rate,” he says.
“We’re pushing the boundaries in terms of speed and compression to get as much data as we can down the cable. Fibre is maximised in terms of its capabilities, there may be more gains to be made in terms of transmission technologies. We had copper to fibre optic, and now everyone is asking what is the next leap?”