Sounding out bubbles in pipes
Pipes and pipelines, these have become the veins of modern society. They transport precious fuels like oil and gas from the ocean floor depths, across country borders, through cities and into homes like yours. But that first step, to bring oil and gas through pipes from the seabed, is a complex procedure fraught with dangers due to the changing pressures involved. An international team led by European researchers has come up with a new method of measuring gas bubbles in pipelines, enabling workers to avoid bubble "blow outs" like that in the Gulf of Mexico in 2010. The study was presented in the Royal Society journal Proceedings of the Royal Society A.
In April 2010 the world was shocked with the environmental damage caused by a methane gas bubble which triggered an oil rig blast, killing 11 people in the Gulf of Mexico. A small methane bubble expanded so big that it shot oil 240 feet (73 metres) into the air. To avoid accidents like this, the ability to measure gas bubbles in pipelines is vital to the manufacturing, power and petrochemical industries. New research from the University of Southampton in the United Kingdom, however, has devised a new method to more accurately measure gas bubbles in pipelines.
From the bottom of the seabed to the water's surface, there is a difference in pressure. Any bubbles present in the oil or gas being brought up through the pipe will naturally expand as the pressure reduces the closer to the surface it gets. These expanding bubbles can cause a blowout, which is the sudden release of oil and/or gas from a well.
At the moment, gas bubble size distribution (BSD) is estimated by sending sound waves through the bubble liquid and comparing the measured attenuation of the sound wave (loss in amplitude as it propagates) with that predicted by theory.
Problems surface when transferring theory into practice. According to theory, the assumption is made that bubbles exist in an infinite body of liquid. This could lead to errors in the estimation of the bubble population.
The European-led research team, headed by Professor Tim Leighton from the Institute of Sound and Vibration Research at the University of Southampton, has devised a new method, which takes into account that bubbles exist in a pipe.
They measured how phase speeds and attenuations in bubbly liquid in a pipe might be inverted to estimate the BSD (which was independently measured using an optical technique). This new technique, appropriate for pipelines such as TTF, gives good BSD estimations if the frequency range is sufficiently broad.
According to Professor Leighton, 'This paper reports on the method we devised half-way through the research contract. It works, but just after we designed it the 2008 global financial crash occurred, and funds were no longer available to build the device into the mercury pipelines of ORNL. A more affordable solution had to be found, which is what we are now working on. The original design has been put on hold for when the world is in a healthier financial state. This has been a fantastic opportunity to work with nuclear scientists and engineers from ORNL and RAL.'
Professor Leighton and his team,were commissioned to undertake the work as part of an on-going programme to devise ways of more accurately estimating the BSD for the mercury-filled steel pipelines of the target test facility (TTF) of the USD 1.4 billion Spallation Source (SNS) at Oak Ridge National Laboratory, Tennessee, United States — one of the most powerful pulsed neutron sources in the world.
This facility is capable of firing a beam of protons using a linear accelerator hundreds of metres long, into 20 tonnes of pumped liquid mercury. Specialised neutron instruments are built in a circle around the source to catch the beams of neutrons and use them to probe the internal structures of materials, such as test aircraft wings, forensic samples and biomedical products.
'The SNS facility was built with the expectation that every so often it would need to be shut down and the now highly radioactive container of the mercury replaced by a new one, because its steel embrittles from radiation damage,' said Professor Leighton. 'However, because the proton beam impacts the mercury and generates shock waves, which cause cavitation bubbles to collapse in the mercury and erode the steel, the replacement may need to be more often than originally planned at full operating power. Indeed, achieving full design power is in jeopardy.'