This Week

20 March 1999

Guiding light

A LEAKING pipeline carrying chemicals or oil can lead to catastrophe unless
the hole is quickly detected and repaired. But when pipelines are thousands of
kilometres long, problems can be difficult to spot. Now a British engineer says
he has the answer, in the form of a super-sensitive optical fibre that can
detect and locate spills.

At present, petroleum and chemicals companies usually examine long pipelines
visually, from a helicopter or a land vehicle. Chemicals companies are beginning
to try out sensors based on electrochemical or semiconductor cells, which detect
electrical or temperature changes associated with the presence of a particular
substance and trigger an alarm.

But any network of sensors, no matter how extensive, always leaves gaps, and
in windy conditions it can be hard to tell which alarm is nearest a gas leak. So
Frank Kvasnik of the University of Manchester Institute of Science and
Technology has designed a chemically sensitive optical fibre that can be laid
alongside a pipeline, or throughout a chemical plant or ship. If a leak occurs,
the fibre undergoes a chemical change which alters its optical properties. This
allows the leak to be detected and pinpointed almost instantly.

In any optical fibre there is an inner glass core surrounded by a layer of
transparent polymer “cladding”. The key is the difference in refractive index
between the two: if they are correctly matched, no light can escape from the
fibre. To keep the cladding layer intact, and maintain the necessary difference
in refractive indices, normal optical fibres are protected by a tough outer
sheath. “Optical fibres used in telecommunications are designed to keep the
environment out,” says Kvasnik.


The fibres Kvasnik proposes using as detectors turn this idea on its head.
“If you design them to let something in you can make a chemical sensor,” he
says. His fibre uses a permeable silicon-based cladding containing chemicals
that react with the substance being monitored. This reaction changes the
refractive index of the cladding, altering the optical properties of the fibre.
In effect, the presence of the chemical causes a fault in the fibre, allowing
light to leak out.

But merely detecting a fault in the fibre is not enough. “There is no way of
knowing whether you are seeing a small change in the optical properties of the
whole fibre or a larger change in a single place,” Kvasnik says. And pinpointing
the location of a leak is as important as knowing that a leak has occurred.

So Kvasnik has borrowed a trick from the telecoms industry. Called optical
time domain reflectometry, it works by regularly sending short pulses of light
down the fibre. Any fault or break reflects a portion of this light back to the
source. The time delay between the initial pulse and the arrival of the
reflected light is a measure of how far the fault is along the fibre, with an
accuracy of about half a metre.

Kvasnik has already built a 100-metre stretch of fibre capable of detecting
ammonia leaks. Next, he hopes to develop a fibre sensitive to methane, for use
with gas pipelines. “We’re now looking for commercial partners,” he says. He
hopes the principle could be used to detect a range of environmentally damaging
chemicals, including petroleum.