Methods provided for the inspection of metal products in the oil and gas sector include eddy current, ultrasonic, and flux leakage testing methods. We will discuss solutions for flaw detection and we will focus in on Eddy Current and Ultrasonic inspection methods for Oil and Gas Industry Products. It is important to note that when inspecting to such high standards, both ET and UT methods should be combined for a full body inspection and detection of all flaws and discontinuities that are common to these products. 1
We will now examine the principles of eddy current theory as it relates to eddy current testing.
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Eddy current testing works on the principle of induction. An AC stimulus is applied to the coil as pictured in this figure. (The AC stimulus is typically between 2.5 kHz and 100 kHz). The alternating current in the coil sets up a magnetic field around the coil. The magnetic field from the coil causes a circular current to flow on a metal plate or object to be tested. This circular current is referred to as an “eddy current”. This eddy current opposes the primary field in the coil.
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For both Probes and Encircling coils, there are 2 types:
Absolute encircling coil which consists of a single winding wrapped around a fiber glass or plastic tube. The material under test forms the core as it passes through the core of the coil.
Differential Encircling coil (normally used for flaw inspection) consist of a pair of windings wired in opposition to each other (called NULL probes) and separated by a fraction of an inch., because the signal from each winding cancels the other out to give a NULL balance voltage of 0 volts. Another term for this probe is a self comparison probe. Both windings are examining the metal surface at the same time. These coil pairs are also wrapped around a fiber glass or plastic tube, where the material under test forms the core of the coil. When a defect appears under one of the windings, the flaw will upset the field in that winding creating a balance or impedance difference that is amplified, detected and then reported.
Encircling coil and probe coils are used for inspecting tubing like Subsea Umbilical Tubing.
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The picture in the center shows an encircling coil that has been wound and is now ready to be potted. The coil in the upper right is a tangent coil which is used to inspect the weld zone of welded tubing.
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Rotary probe eddy current inspection is suitable for detecting longitudinal seams, laps and surface cracks in bar products such as Steel Rods. Here you can see that this is a 4 channel system for high speed inspection of bar products with relatively short HPI values. The shorter the HPI value, the shorter the length of a given defect than can be detected reliably.
On the left, we can see the test instrument screen showing the test results simultaneously from both the eddy current probes and the encircling coils.
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Now, moving on, we will discuss the principles of ultrasonic testing.
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Ultrasonic testing frequencies used in industry commonly range from 50 000 cycles per second (50 kHz) to 50 000 000 cycles per second (50 MHz).
All of this is to say that: The old definition still holds — ultrasound begins at 20 000 cycles per second (20 kHz). But our traditional concept of the upper limit has drastically expanded due to exciting developments of new materials and instrumentation. One megahertz may be expressed as 1 x 10 6 cycles/second. It is the PIEZO ELECTRIC EFFECT that is at the heart of the transducer and it is this effect that allows us to send and receive sound waves from within an object.
By definition, it is the electric charge that accumulates on certain solids such as crystals and certain ceramics when stress or deformation is applied, and the effect is reversible such that when a voltage is applied to crystals and certain ceramics, the piezo electric material experiences deformation.
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Contact testing is where the transducer is directly coupled to the surface of the test object via of a couplant.
A couplant is a medium through which the ultrasonic sound waves are transmitted from the lens of the transducer into the test object.
The “coupling medium” is usually an oil or specially formulated gel.
Immersion testing is where both the transducer and the test object are immersed in water. The sound waves travel from the transducer into the test object using water as the Ultrasound couplant. Our UT inspection of Umbilical Tubes, OCTG pipes and Sucker Rods are tested using the immersion test technique.
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The sound beam is projected into the material through a column of flowing water and is directed perpendicularly to the test surface to produce longitudinal waves. It can also be adjusted at an angle to the surface to produce shear waves.
The illustration shows a bubbler system that is used to test sheet metal moving over a fixed transducer station at high speeds. The same system is also adaptable for use in high‐speed scanners that can be attached to large immovable objects such as bridge girders or refining industry pressure vessels.
The Bubbler Technique is employed in the Ultrasonic Carriage, Spin The Tube Tester and UT End Tester that we shall see in the next section.
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The longitudinal wave is used to inspect the internal structure of bar products for cracks and voids, to inspect pipe and tubing for laminations, and to measure wall thickness of tubing and pipe.
The Shear wave inspects bar and rod products for subsurface cracks and defects as well as welded and seamless tubes and pipes for ID and OD defects.
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Note that the same principles for HPI Helical Pitch Inspection for ECT Rotaries applies here. When a proper calculation of beam length, helical pitch, and throughput speed is obtained, the inspection system will provide 100% coverage of the material under test.
The transducer is a line focused transducer which projects a beam on the surface of the bar, pipe or tube as shown above.
the Beam Length (BL) of the transducer paints a spiral helix as shown above. For proper coverage, material throughput speed needs to be controlled such that the bar, tube or pipe does not advance more than the length of the BL of the transducer in one revolution of the test head.
Note here that the pitch (HPI) equals the BL. Complete test coverage is now achieved.
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So now that we’ve covered the basics of Eddy current and ultrasonic inspection methods,
let’s dive in further to discuss the inspection of Oil & Gas products.
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Note: One thing that all Oil & Gas metal products have in common is that they are all best inspected with an Ultrasonic test system in union with some other NDT method, meeting API 5CT specifications. For example:
• Line pipe, drill pipe, casing and other types of ferromagnetic Oil and Gas Tubes such as carbon steel, are best inspected with a combination of Ultrasonic and Flux Leakage test systems.
• Sucker Rods, Umbilicals, and other ferritic metals used in demanding applications are best inspected with an Ultrasonic and Eddy Current multi‐test system; and this will be our focus for today.
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Custom designed systems are the best option for the inspection of oil country tubular goods (OCTG) and other oil and gas products that not only meet individual client specifications, but also that of API 5CT, API 5L, EN, and ASTM and ISO. Typical oil and gas applications include Sour gas, High pressure, Offshore wells, Artic wells, amongst others
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Since we are here to discuss the inspection of Stainless and Alloy Steel products, let’s begin with Umbilical tubing.
The Picture on the left shows the deployment of a cable from the deck of a support ship.
The Picture on the right shows a cutaway view of a stainless steel umbilical cable. The Steel tubing can be either:
•Zinc Clad Lean Duplex 19D
•316L
•Super Duplex 2507
•Zinc Rod
For the purposes of time, we will restrict our attention to the Zinc Clad Lean Duplex 19D alloy for subsea umbilical tubing.
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In this picture, courtesy of RathGibson Tube Company, we can see that sub sea umbilical's range in depths of 600 feet to 2 miles (over 10,000 feet). Distances from the offshore platform to the clustered wells can range from 45 to 60 miles in distance. With that said, the temperature of sea water at 10,000 feet is 1.5 degrees centigrade (35 degrees Fahrenheit) and the pressure is about 300 times the atmospheric pressure at sea level. At sea level, the atmospheric pressure is 14.6 psi. At 10,000 feet, the pressure is more than 4,400 psi. Given that Subsea umbilical’s are required perform for approximately 30 years at these temperatures and pressures, and loss of production can be $500,000 per day and repair costs as much as $20M, it is critical that the components that make up the umbilical receive complete NDT inspection during fabrication.
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Just after the weld box pictured on the left, we use a MAC Hot Probe ®, which has the following features:
• A Water cooled jacket connected to a closed loop weld process cooler.
• Ceramic bobbin and disk designed to withstand temperatures of 1250° F to 1500° F.
• Close proximity to the weld pool allows good early detection of weld defects.
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For example, when inspecting Umbilicals, down stream after in‐line bright annealing, we perform a full body ET test on the tube.
Full body eddy current is useful for detecting ID and OD defects in tubing. Full body encircling coil eddy current inspection is also used to detect short surface defects in bar products.
Our company in particular, offers a variety of testing options. Such options include:
• Dual frequency testing, which incorporates a dual bobbin ET coil designed to optimize sensitivity for both ID and OD type defects.
• Tangent or sector coils that can be used in some welding applications where threading the material through the coil may be an issue.
In both cases, there is a feature that we call Flaw Verify®, in which the eddy current coil tester shown to the right compares a signal response from the first coil with a similar response from the second coil. The second response has to occur within a time‐measured window from the response of the first coil. The timed response is in direct correlation to the speed of the tube passing through the weld mill. This feature is quite advantageous for continuous weld mill applications, since unlike cut tubing, the operator cannot back up the tube to run it again.
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Pictured to the left is a TCP20, meaning a tangent coil platform with built in permanent magnets for saturation of permeability variations in the material.
Pictured to the right is the response from both coils testing the material in tandem to reduce the occurrence of false alarm indications as described in the previous slide. A Tangent Coil inspects just the weld zone of the tube.
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Full body inspection is as the name applies. The encircling coil encompass the entire OD of the Lean Duplex 19D umbilical tube.
Because Lean Duplex 19D is 50% Austenite and 50% Ferrite, the metal grain structure tends to shield or repel the eddy current field. The grain boundaries act like little bar magnets that are scattered in a non‐uniform pattern. In order for eddy current penetration into the metal surface, it is necessary to magnetically “saturate” the tube wall. To saturate the tube wall, we need to use the CP352 pictured above that contains an electromagnet inside the gray bell housing. A welding generator supplies the 200 amps of current necessary to magnetically saturate the tube wall of the Lean Duplex 19D Tube. The energy required to saturate this material also causes significant heating of the magnet structure. The magnet is cooled by a constant flow of cooling water on a closed loop cooling and recirculation unit.
To the lower left, you see pictured a round “can” which is the test coil used with this type of coil platform.
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Shown in the picture above is a 50mm UT Test System, which employs 3 channels of UT inspection in Real Time. The Clockwise Shear, Counter Clockwise Shear and Wall Thickness Tests are performed simultaneously as the tube passes through the UT Rotary.
A water column maintains coupling between the transducers and the tubing as the transducers orbit the OD of the tube. Signal exchange between the transducers and UT electronics is accomplished through a unique rotary transformer and specialized electronics on the rotary drum. The rotary transformer provides an additional benefit of extraneous noise reduction from nearby crane contactors and other production equipment that generate RF disturbances. Such RF disturbances could potentially be picked up by the UT electronics and generate false alarm conditions.
Following the UT inspection, the coils are tested again with air under water 22
The FD5 UT instrumentation shown above is compatible across a variety testing applications utilized by the Oil and Gas Industry. The instrument setup screen displays an A‐
Scan Plot of the returning echoes to the transducer. An A‐scan plot shows a one dimensional plot of amplitude vs. time of the echo response from a defect. The bottom trace is metal path distance or time and the vertical trace is percent screen height or amplitude response. When an echo appears under a gate, the signal response (if great enough amplitude) crosses the defect gate triggering an alarm device or marking system like a painter. The further to the right that a response appears on the screen, the deeper the target is within the product. The electronics also tracks the linear position of a defect relative to the product movement and triggers devices like markers at the proper time down stream of the test plane. In addition to the ability to store setup recipes, the instrument can also store test history reports and strip charts in batch files for archival and easy retrieval, eliminating the need for paper strip charts.
This model of UT electronics can connect to a transducer in a variety of communications mediums such as rotary transformer (current generation rotary), rotary capacitor (older rotaries and also competitors rotary) or directly coupled to the transducer via a signal cable as in a squirter, carriage, bubbler or spin the tube systems. This makes the electronics versatile for a variety of immersion techniques as well as backwards compatible with older NDT test systems and enables a common user interface across multiple test platforms within the same production environment.
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The picture to the left is a small pinhole flaw that was detected with eddy current, but difficult to see with UT. The picture in the center is a seam type defect that can be picked up by either eddy current or ultrasonic inspection.
The picture to the right is a micrograph of a tube wall with a poor ID weld that was picked up with UT, but not with eddy current.
So, we note here that for proper, full body inspection, it is best to use both ultrasonic and eddy current instrumentation since one is best suited to pick up on flaws that the other might not.
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A sucker rod is a steel rod, typically between 25 and 30 feet (7 to 9 meters) in length, and threaded at both ends, used in the oil & gas industry to join together the surface and downhole components of a reciprocating piston pump installed in an oil well. We will refer to sucker rods in a more general sense, as steel rods.
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No one test method or technique is able to detect all of the flaws that can occur when producing sucker rods., it is best therefore to use a multi‐method approach such as an eddy
current coil AND rotary or an with an Ultrasonic rotary. The ECT coil is useful in detecting short surface cracks, chevron cracks and pits on the surface of the bar, since it is this type of defect that passes under each of the coil’s secondarys, one at a time, creating an unbalance condition in the encircling coil that is picked up by the electronics circuitry.
The ECT rotary is useful in detecting longitudinal cracks, laps and seams that run parallel to the length of the rod. This is due to the fact that the probe actually crosses the plane of the defect while orbiting the bar. The longitudinal type defect upsets one secondary winding in the NULL probe at a time producing an unbalance condition in the rotary probe that is detected by the electronics. Neither of these ECT test techniques however is useful for detecting defects internal or below the surface of the rod.
To inspect sucker rods for subsurface and internal defects, we need to employ UT testing, which sends sound waves into the product and returns echo signatures that are detected by transducers which translate the sound waves into electrical pulses that are detected by the UT electronics. Due to the size and width of returning signals from the water/steel interface of the sucker rods, it is impossible to detect defects at or near the surface of the sucker rods using the UT inspection method.
So it can be seen that a multi‐method approach is the best solution for inspecting sucker rods.
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The Steel Rod Testing system includes an inlet and outlet conveyor with safety guarding as well as a rotary mechanism with vertical adjustment that holds a headplate and four ground/polished hardened steel bushings. Also included is a cabinet with a touch screen panel and digital display that houses a power converter, motor controller and PLC’s. The electronics are also kept in a thermally controlled cabinet with in‐built touch screen monitor and pullout keyboard and mouse. 27
Here we see an eddy current rotary tester that is testing cut steel bars. You can see that the bars are being sorted for accept/reject. The steel rods are placed on a rack, where a release mechanism deposits one part at a time onto the inlet conveyor. Stop arms hold back the material on the feed rack. Each steel rod is pushed through the eddy current rotary by a push rod where two fixed probes rotate around the circumference of the rod to detect very small longitudinal surface and subsurface defects.
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The head plate has two fixed rotary probes and is adjusted to air ride with a 0.381mm or 0.015” gap spinning at 3000RPM. 29
All steel rods that exit the rotary are marked with a paint marker and separated automatically by a throw‐off tilt tray into accept and reject pockets.
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When using a multi‐method approach, the electronics have been designed to include up to eight individual channels‐ allowing different types of eddy current inspection to be performed at the same time. This first image is of an encircling coil platform. These include an encircling null coil inspection to detect short surface and some subsurface defects, and an encircling coil absolute inspection to detect more coarse longer continuous type defects. Now above you will see an example rotary probe and below is one of the rotaries. A rotary probe inspection is used to detect shallow longitudinal surface type defects.
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The rotary mechanism consists of a precision sealed bearing assembly with built‐in transformers for signal transfer and a spinning headplate with two probes. Each probe is configured with its own detector channel including a system that suppresses testing of the bar ends with a switching optical sensor that detects the bar entering and leaving the rotary. The rotary headplate is mounted on a rotating drum driven by an integral motor and pulley. The heavy box housing with a cover interlock switch encases the entire rotary mechanism and the variable speed motor controller provides dynamic breaking to quickly stop the rotary when the interlock is broken. The rotary head plate is easily removed for inspection and probe arm adjustment using a machined steel block insert. 32
The steel rods pass through the ECT coil tester first. The ECT detects short pits, longitudinal cracks and transverse cracks.
The rotary ECT tester detects seams, forging laps and longitudinal cracks. Next we will discuss inspecting bar where we will go into further detail of a dual Coil and Rotary eddy current tester.
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As mentioned in the steel rod section, it is most beneficial to use a multi‐method approach for full coverage inspection of rods. The same is said for bars
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INSPECTS STEEL BAR FROM 0.750” ‐ 3.5” • 3 Channels of Eddy Current Electronics (2 Rotary ECT and 1 Encircling Coil ECT)
• Rotary Model 10R 350M with Slide Mechanism
• Coil Platform CP502 with Slide Mechanism (Ferritic grades such as Duplex Stainless bars require magnetic saturation for proper ECT testing)
• Demagnetizer 475 with Air Cooled Coil (Ferritic grades of Stainless Steel Bars require demagnetization for machining processes down stream).
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• The bar first undergoes straightening • The material is lifted onto the feed system
• The material under test first passes through an eddy current rotary to detect long, continuous surface defects
• It then goes through an encircling coil to detect short defects
• Lastly, a demagnetizer removes any residual magnetization
• 2 paint markers spray any detected defects
• The accepted material is then sorted from the rejected material in the outlet conveyor
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Newly designed bench to house the mechanics
Catwalk in the back for ease of access
Slide mechanisms in front for the rotary and coil platform
Protective covers
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• To the left is the Feed table and inlet conveyor
• Up front are the operator controls and eddy current electronics
• To the right is the 2 paint markers
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The operator control panel allows for:
• Ease of use in starting & stopping the system
• Quick size change‐overs
• Changing the speed and direction of the material
• Controlling the bench and its components
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Includes Release Mechanisms, Intermediate Stop Arm Attachments, and E‐Z Drop Channel Section Loading Attachments‐
all within a programmable controller
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When defects are found:
the system is setup to automatically sort the material to the pockets designated for rejected material In this case, it is the reject pockets that are predominantly shown in the picture
Accepted material is sorted to the other side of the outlet conveyor
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Receiving Pockets are used to accept material ejected from the exit conveyor.
The Easy Down Hardware consists of high strength nylon straps that reduce the noise and impact when material is dropped into an empty or partially filled pocket.
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The software provides control of sensitivity, frequency, phase, filters, and threshold outputs through on‐screen menus. The “Multi” screen is currently showing 2 channels in polar and linear view simultaneously
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The material under test first passes through an eddy current rotary with a rotating headplate to detect long, continuous type surface defects. It then goes through an encircling coil to detect short defects
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Eddy current rotary probe technology is used to detect long, continuous surface flaws which may not be detected by the encircling coil.
The Rotary is mounted on a Triple Guide Roll Constant Center test bench with a slide mechanism to position the rotary housing and move the material accurately through the test head.
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The CP502 coil platform uses Direct Current (DC) saturation which is required to inspect ferromagnetic material that contains permeability variations.
The saturation reduces the permeability variations in the test material that would otherwise interfere with the eddy current test.
This DC platform is air cooled and comes with a low voltage, DC power supply for the saturation coil system.
This is ideal surface for surface inspection of bar stock.
A Demagnetizer is then added to the system to remove any residual magnetization.
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When a DC Saturation field is required for Eddy Current Testing, it is often necessary to demagnetize the bar or rod to prevent chips from sticking to tooling in later machining operations.
Demagnetizers permit efficient, continuous elimination of unwanted magnetism in ferrous rod & bar
Demagnetizers use a unique technique which simultaneously applies both alternating and direct current demagnetizing fields.
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This Eddy Current NDT system can ensure surface quality of hot rolled product that meets and exceeds customer expectations.
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Now let’s discuss inspection methods and standards used on more OCTG products
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API5CT lists all dimensions, material grades, mechanical and other requirements, labels on
the tubes, which have to be met by suppliers. These specifications define all kinds of tubes
that are covered by OCTG. Additionally, data collection specifications are also stated.
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To achieve repeatable detection of defects, the testing equipment must be operated at a rotational speed and line speed that ensure a minimum of 100% coverage of the ultrasonic transducer beam length as the product moves through the tester. To achieve a minimum 100% coverage for ultrasonic testing, the effective probe length must be equal to or greater than the pitch as shown in this image here. The calculation for pitch is simply line speed divided by rotational speed.
Note here that the pitch (HPI) equals the BL. Complete test coverage is now achieved.
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(HAS ANIMATION)
Wall thickness verification is done using an ultrasonic rotary. Wall thickness measurements are done by setting up a sufficient number of channels of ultrasonic rotary as a wall thickness gauge. The wall thickness test coverage is calculated in a manner similar to the coverage calculations for detecting defects. However, for wall thickness measurements, it is not always required that 100% coverage be obtained. By incorporating additional channels, the ultrasonic system can be configured to meet the wall thickness measurement requirements of section 10.13.4 of API 5CT. According to API 5CT pipes specified to PSL‐2 and PSL‐3 require 25% and 100% wall thickness coverage respectively. Some tubing customers may have their own wall thickness requirements. Wall thickness capability can be demonstrated easily on reference standards with well documented artificial areas of reduced wall thickness. This Figure shows a typical reference standard used for setting and verifying wall thickness measurements. These tests are normally done during the pre‐acceptance test of the system for customers before it is delivered. It can also later be shown to the customer’s auditors in the tube mill.
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The benefits of an Ultrasonic rotary is that it can provide 100% coverage at high throughput rates. By using a multiplex transducer design, it allows lower noise signals and up to 48 transducer elements. A unique rotary mounted transformer and pulser/receivers provide maximum sensitivity and minimum noise. Multiple transducer elements can simultaneously test in different directions such as CW, CCW, FWD, REV, as well as measure wall thickness and detect laminations. Precision test blocks and transducer holders ensure accurate positioning to detect small defects, even at high throughput speeds. Seal‐less rotaries reduce the likelihood of damage from grit and other mill contaminants. There is also a special designed system for inspecting spinning tube and tube ends that will be discussed in detail later on. This particular rotary product can handle diameters up to 500mm and the spinning tube design can handle even larger diameters.
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To achieve full‐volume testing, the minimum number of transducers in a rotary would include one (1) for the thickness and lamination defects, two (2) for longitudinal notches (clockwise and counter‐clockwise), two (2) for transverse notches (forward and reverse), two (2) for oblique notches (clockwise and counter clockwise). Higher production speed might require that the number of channels and transducers be increased to provide 100% coverage. Lamination detection shares transducers with the thickness channels but use separate processing electronics. It is possible to configure the UT system to detect the standard 6mm flat bottom drilled hole (FBDH) with good repeatability at standard testing speeds The test block image here shows how a rotary test block might be configured with the appropriate transducers for full coverage. 54
This part of the presentation will describe a specific test method designed by MAC for inspecting tube ends as well as spinning tube. The new arrangement of the transducer
carrier, also known as APC Carrier which stands for Automatic Pitch Control rides along the bottom of the tube, thus maintaining constant coupling and allowing for quick and easy diameter change‐overs. First we will discuss the application of testing tube ends followed by the application for inspecting spinning tube.
The spinning tube UT test system gives a nice solution to applications where a UT rotary may be less than ideal.
The Tube End Tester is an excellent solution to untested ends
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An ultrasonic carriage system rotates the TUBE past a set of FIXED transducers at a fixed rate of 3 to 5 RPM. This allows standardization of the Pulse Repetition Frequency for different pipe diameter sizes. For each rotation of the pipe, the pipe is advanced forward at about 50 to 80% of the transducer beam length much like driving a screw into a block of wood. Each rotation moves the screw laterally by the pitch of the threads. A simple Water Tank eliminates the need for tight fitting glands or shoes. The carriage is called and Automatic Pitch Control Carrier or APC carrier that has rolls that automatically adjust to the helical pitch of the pipe. 56
All NDT test methods used to inspect cut length products, have limitations on testing to the very end of the product. These “untested ends” must be cut off and discarded. This results in a substantial loss of product and revenue. Otherwise the manufacturer must develop a method to detect the ends of these tubes. Some manufacturers require a higher level of inspection because it is expanded, threaded, or welded in the field. MAC’s UT End Tester provides a solution to this problem.
Here we show the transducer carrier unit beneath the tube under test, and the blue pinch support arm for secure tube rotation is positioned above the tube. This end tester was a 10 channel system. This particular unit was designed to handle 38 to 170mm diameter tube. Larger sizes up to 450mm can be handled with customized mechanics.
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Typical defects found in OCTG are Longitudinal & Transverse, Rolling Skins, Laminations, Cavities, Cracks, and Laps.
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Demonstrating repeatability is also a part of the pre‐acceptance tests and audits on site. It is important to work with a company that has vast experience in optimizing equipment to satisfy the customer’s needs. In discussions with tube manufacturers, artificial defects in different orientations can be defined to prove the systems capabilities for different defect types. Additionally, samples with natural defects can be used to show the system’s capabilities. The image shown here is a Pre‐acceptance test at MAC’s New York facility involving the customer’s quality control department and MAC’s field engineers, in‐house engineers, district managers, and business development managers
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