·Table of Contents
·Methods and Instrumentation
New Generation Crawler's Design With Safe Ecology Signaling SystemV.V.Kliouev, V.P.Kourozaev, A.S.Bakounov, V.F.Moujitski, M.A.Kuzin
MSIA "Spectrum", Russia
JME ltd., UK
Special equipments named crawlers are very popular for NDT of welds on pipelines during their construction. They use radiographic method. The most part of crawlers have panoramic X-ray tube that is based on motor chassis. The using of panoramic X-ray tube lets to have high test productivity. All crawlers also have their own power source, electronic unit and signalling detector that gets signals from operator and sends them to electronic unit. Electronic unit decodes the signals from detector and works out operating signals for motors and X-ray equipment. The operator must send signals of commands to crawler from the outside of the pipeline. Traditionally there are next commands to crawler that is necessary to signal: "go forward", "go reverse", "go slowly", "stop" and "switch on X-ray". Besides "stop" signal must ensure high accuracy of stop position that is necessary for getting qualitative film of the weld.
Traditional method of operating foresees the using of isotopic signalling detectors. It lets for operator to send the signals to crawler by isotope through a pipe wall. This method of operating is very reliable and guarantees good accuracy of crawler's stop near the weld that is necessary to get a qualitative film of this weld. But at the same time isotopes involve some problems. The first one is the influence of isotope's radiation on operator's healthy. Another one problem is bind with isotopes' export. There are many countries that prohibit to import any radioactive materials. All these problems make compel crawlers' producers to find alternative to isotope method of operating.
There are many companies that are crawlers' producers all over the world. John Macleod Electronics ltd. (UK) is one of the most famous companies among them. JME was found in 1986 by John Macleod and crawlers are the main products of this company. JME produces three types of crawlers that let to include all sizes of pipelines that are necessary to test. These types are 6''JME Crawler for 135-356 mm pipe's diameter, 10/60''JME Crawler for 240-1500 mm pipe's diameter and 22''JME Crawler for 558-1829 mm pipe's diameter. All JME Crawlers can be supplied with different X-ray panoramic tubes with high operating voltages till 160, 180, 200, 225 or 300 kV. Crawler can test till 120 welds and return to the start point without battery recharge.
But JME Crawlers have the same demerit as other crawlers. They need isotopes for operating. It forces JME and other crawler's producers to find alternative methods of operating.
There were attempts to use radio signals for operating. But in this case the operator must have at least one open end of the pipeline. There is a big problem to send radio signal for a long distance. And the main problem is: how to stop crawler under the weld because the operator can't see crawler's position inside of pipeline. So this method isn't good for operating but, as a result of these attempts, JME Crawlers have radio recovery system that lets to return Crawler back if there is something wrong with it in a pipeline, for example, it can't go further because of any obstacle inside of the pipeline. The maximum distance from the end of a pipeline to Crawler when recovery system works reliably is two kilometres.
Approximately three years ago JME and MSIA "Spectrum" that are old partners in business started joint research. It's goal was to get safe ecology signalling system for crawlers. Analyse of different methods of operating was made and magnetic method was chosen.
Theoretical and experimental researches were made. The main question was how to get compact high energy source of magnetic field that is possible to form magnetic filed inside of pipeline when the source itself is outside of that pipeline. And another important question was: what kind of magnetic field configuration must be when operator signals crawler to stop, because stop accuracy depends of it.
As a result Õ-form magnetic system was designed. It consists of low carbon steel bridge piece and poles made from high magnetic energy NdFeB permanent magnets. The final magnetic system lets to obtain magnetic field strength at least 3 A/sm. inside of a pipe when pipe's wall thickness is 16.5 mm, protective covering on a pipe is 5 mm and the gap between inside pipe surface and the point of measurement is 25 mm (1 inch). A signalling magnet with this magnetic system has adjustable wheels for its movement along the pipe and up/down unit that lets to put the magnet on a pipe surface and take it off without essential efforts of operator.
Magnetic detector is placed on crawler on the same place where isotopic detector was before. It consists of magnetic sensors' unit, logic unit and decoder. In turn magnetic sensors' unit consists of some Hall effect sensors that fix magnetic field's configuration inside of pipeline. Increased Hall effect sensors' output signals come into the logic unit that forms different logic signals depending on magnetic field configuration. Thus logic unit works out signals of crawler's commands: "go forward", "go reverse", "go slowly", "stop" and "switch on X-ray". Then decoder forms the signals for crawler in its standard format. It lets to change isotopic signalling system on magnetic signalling system without any crawler's reconstruction.
Operator's manipulations with magnetic signalling system are close to isotopic signalling system that makes a process of operator's adaptation to magnetic signalling system very easy. But sometimes there isn't easy to move the signalling magnet along the pipeline to form signals of commands for crawler because of the edge of protective covering of the pipe near the weld. In this case it is possible to form this signals by moving the signalling magnet up and down.
Good JME Crawler's construction provides its motion along a pipeline without any distortion at rectilinear part of the pipeline. But if the pipeline turns crawler can rotate on any angle. This problem has very simple decision for isotopic signalling system when they form wide beam of radiation. Really the width of this beam is 900. Of course it's an advantage of isotopic signalling system from the one side, but it increases an irradiation of operator from the other side.
For magnetic signalling system this problem has another decision when there are some parallel groups of Hall effect sensors that are carried in space. For example, this decision lets 22''JME Crawler to rotate on the angle till ±100. An additional decision of this problem is the using of two signalling magnets that are putted on a pipe in parallel. In this case it is possible to operate with Crawler even if the angle of its rotation is about 250.
Traditionally JME uses isotopes not only for operating by crawler but for tracking system too. In this case the isotope is putting into special box on crawler so it directs the radiation from the place where crawler is to the outside of the pipeline. So if the operator lost crawler anywhere in the pipeline he can take any radiation detector, then he must go along the pipeline and find the place where radioactivity has maximum value that means crawler is here. The tracking system is very helpful if operator wants to control real crawler's position inside of a pipeline. But if consumer uses isotopic signalling system together with tracking system all problems that relate to isotopes must be multiplied on two.
That is why design of ecology safety tracking system is necessary to do. This problem is easier then design of signalling system and there can be different solutions of it. One of them is magnetic tracking system. It consists of Õ-form permanent magnet with wheels that is moving by crawler inside of a pipeline and magnetic field detector that operator has.
One feature of ferromagnetic materials is their property that is called magnetic memory. It means that if you put a magnet on article made from ferromagnetic material you magnetize it in any direction. After you take the magnet off it its material has residual magnetization in opposite direction.
This property is used in magnetic tracking system. When crawler goes through a pipeline together with tracking magnet this magnet stay a trace of residual magnetization on a pipeline. And as the Õ-form magnet has two opposite poles near pipe surface the sign of trace's magnetic field will depend of direction where crawler goes. If the operator needs to find the placement where crawler is he must measure magnetic field induction on the place of magnetic trace and depend the direction of crawler's motion through this place. If operator can't find the magnetic trace it means that crawler never been hear before.
The property of magnetic memory makes the process of searching for crawler more easy then using of isotopic tracking system because the operator doesn't need to pass all pipeline trying to find the placement of crawler. He can determine the direction where to go easily.
Magnetic signalling and tracking systems were designed, manufactured and tested in laboratory and there was proved that they can be used. Magnetic signalling system provides very high accuracy of crawler's stop in given point. The error of stop position isn't more then ±2 mm and it guarantees the possibility of getting high quality X-ray film of a weld.
The main attention was given to magnetic signalling system. There was made large volume of tests in field conditions. This tests were made in Russia (Moscow and Rjazan regions) and in Great Britain (East Anglia, Scotland). Now it is possible to say that magnetic signalling system can operate reliably on pipelines having wall thickness till 25 mm and protective coating till 10 mm. It's possible to use magnetic signalling system for all types of nonmagnetic pipelines.
Caspian Pipeline Consortium bought six 22''JME Crawlers at the end of 1999. They use Crawlers in construction of new pipeline from Caspian Sea to Black Sea. Two Crawlers are completed with magnetic signalling systems. The operators of these Crawlers have got one day training course in operating with magnetic signalling system. Now these Crawlers are operated successfully. Maximum daily load is till 100 welds for every Crawler (there are 44 welds per 1 kilometre).
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