Torqueing cables

Laying today’s subsea fibre optic telecommunications cables requires close control of torque and feed control. An integrated system of PLCs and electric drives provides the solution.

For many years the machines for laying the cables on the seabed have been hydraulically driven. Old cables were heavy and rugged and hydraulic systems proved to be the most reliable, suiting the arduous conditions on the exposed decks of vessels at sea.

In its history, Fraser Hydraulic Power, a family owned company based in Newcastle Upon Tyne, has overcome some of the problems associated with the laying of the latest generation of subsea telecommunication cables. But now Fraser is faced with new challenges as the introduction of fibre-optic cables begins to replace old armoured copper based cables.

During laying operations a large plough is towed behind a ship at approximately six knots, typically at depths of 1000m and up to 2000m behind the ship. The plough opens up a furrow and the cable is fed into the trench. The same plough then back fills the trench to cover the cable. All of this happens as a continuous operation.

It is essential that the tension in the cable is maintained within predetermined limits to suit the laying conditions and the type of cable being used. Failure to maintain tension can cause the cable to foul up on the plough or on the towing vessel and if there is too much tension the cable can break. With typical repair costs for cable damage being around £100,000, it is imperative that the cable is not damaged.

As fibre-optic cables are not as robust as copper cables, it has become more important to be able to achieve improved control over the new criteria of lower tensions.

Linear cable engines manufactured by Fraser are based on a series of tyred wheels, set along the axis of the ship with the cable being fed through the tyres. The wheels are squeezed together and hydraulic motors on each wheel generate the power to drive or apply a braking affect on the cable as it leaves the vessel for the seabed. The new generation of Fraser machines have been designed to meet the changing needs of the market and are designed as Plug and Play units.

They are compact, self-contained and modular to enable them to be installed with minimum re-engineering on smaller vessels or vessels of opportunity. Being skid mounted, they can be transported from vessel to vessel and for many lay operations the small flexible mount design removes the need for expensive dedicated cable laying vessels.

The laying machine is based on a series of load cells, which monitor the tension in the cable. The measurement values of the load cells then cause the driving wheels to speed up or slow down to maintain the pre-set tension.

The need for finer control over tension has resulted in a new generation of machines that use a Mitsubishi Electric A2S series PLC system as the electronic brains behind the controls and Mitsubishi’s AC inverter drives on the motors to control the wheels.

The PLC calculates and compensates for change in cable tension then feeds information to the processor that adjusts the drive wheel speeds, to meet the prevailing conditions.

Each dispensing wheel on the machine is connected to an FR-A540 variable speed drive (VSD), with all the drives connected onto the system torque-sharing the load and working together to control torque, which is measured in the drive controllers.

Each of the drives on the platform is connected to the PLC system via a CC-Link Fieldbus system. The drives send constant information of their torque status via CC-Link to a PID loop control in the PLC, which then outputs speed commands to the Drives, to ensure that all drives are matched and constant torque maintained. The PLC also monitors via an absolute encoder input, exactly how much cable has been laid, allowing the calculation of dispensing rates and other important data.

All information regarding the entire control and monitoring of the cable laying machine is done with an E300 human machine interface, which allows the user to see at any time, any torque on any of the drives and check cable dispensed and monitor any equipment alarms.

Torque matching is critical in preventing cable breaks as each cable break costs around £100,000 to repair. The monitoring of torque on all dispensing wheels is also very important, if a tyre punctures or bursts, torque is seriously affected. This could cause catastrophic effects if not reacted to quickly enough.

A major problem Fraser overcame was when the cable dispenser was rendering (free spooling) cable onto a seabed; when this occurs the motor becomes a generator. Fraser’s overcame this by using the Mitsubishi Electric FR-A540 VSDs and its regenerative breaking function on each dispensing wheel set.

This approach removed the need to have huge break resistors ensuring the machine remained compact. The regenerating element of the VSDs simply connects to a heating element. This then cooled using sea water which is readily available, allowing them to use the VSD as a simple four quadrant controller.