With the help of a computer and an ultrasound apparatus, simpler than those used by doctors to see the baby in the belly of pregnant women, researchers from the Nuclear Engineering Institute (IEN), in Rio de Janeiro, have brought to Brazil a quick, cheap and non-destructive method for measuring the tensions present in different kinds of metallic structures like the walls of a nuclear power station or the tubes used in oil or gas pipelines.
Denominated acoustic birefringence, the new technique implemented in Brazil, still experimentally, measures and relates the speed of the propagation of ultrasonic waves in two perpendicular directions of the material under study. In possession of this data, and making use of some equations, the engineers believe they have the conditions for assessing the risk that this structure is running of bursting and causing an accident.
“Compared with other procedures, one of the advantages of acoustic birefringence is that it causes no damage to the structure analyzed”, says engineer Marcelo Bittencourt, from the IEN, a research unit connected with the National Nuclear Energy Commission (CNEN). “Nor does it offer any risk to the health of the person using it. These two positive points are not mere details. They can favor the dissemination of the new technique – implanted in Brazil with a few novelties, compared with the one existing in other countries -, which is protected by three patents.
This is because one of the most popular methods for recording the tension in constructions and artifacts, the so-called hole technique, requires a small orifice to be opened in the object being analyzed. Doing that in the walls of a nuclear power station is obviously impossible. And another method employed to measure the tensions in metals that uses the diffraction of X-rays exposes the people doing the analysis to a kind of radiation that calls for some precautions. No wonder, then, that the International Atomic Energy Agency decided to fund the studies about the new techniques between 1999 and 2002.
Tension in the chamber
The first tests with acoustic birefringence tried to measure the tensions close to the solder joints present in a hyperbaric chamber of 5 meters in length and 90 tons in weight. Made out of high resistance steel, with a thickness that varies from 17 to 25 centimeters, the hyperbaric chamber, built by Nuclep, a company that makes some components used in nuclear power stations, is equipment that simulates pressures of a column of water of up to 3,000 meters. The good results achieved in the initial tests lead the researchers to cogitate using the method in monitoring the tensions at work in the walls of atomic power stations, although this kind of measurement had not yet been done in a nuclear plant.
Even before the experiments with the new technique make headway in the nuclear area, Transpetro, a Petrobras subsidiary, became interested in the subject and went to see the engineers from the IEN. The initiative taken by the company, which looks after the Brazilian state-sector company?s network of oil and gas pipelines, resulted in a partnership entered into in the second half of last year.
In the course of two years, Transpetro, by means of the Duct Technology Program, is going to invest R$ 1 million for the investors to test the ultrasonic method in samples of seven different families of steel tubes used in the oil company’s oil pipelines. “Each family has characteristics that are a little different from the others”, Bittencourt explains. Some kind of tube have an internal seam, others do not. This is not to mention the differences in diameter and thickness of the steels used in the making of the oil pipelines. Tubes with different times of use will also be analyzed.
On land and at sea
At the moment, the first task for the researchers from the IEN is to create a database with the necessary information on these materials, to make it possible to measure tensions in the field. These parameters are important for knowing how much internal and external pressure a tube, which is often to be found underground or at the bottom of the sea, can stand without bursting or showing fissures.
Assessment by Ultrasound of Tensions in Ducts and in Nuclear Power Stations; Coordinator Marcelo Bittencourt – IEN; Investment US$ 200,000 (International Atomic Energy Agency) and R$ 1 million (Petrobras)