To meet this challenge the demand of seamless pipes/tubes or for that matter oil country tubular goods will increase manifold in the coming years and with the current high level of activity in the oil and gas industries, a rigid quality assurance is called for. This has necessitated in the development of an automatic ultrasonic rotary probe test system. The manufacturing process for these pipes consists of Rotary Forge Process and other variety of manufacturing processes. The defects that are encountered are longitudinally oriented discontinuities and similar radial imperfections. The basic idea behind the development of rotary probe system is to defect all these imperfections and subsequent recording of the same. And to achieve these, the system utilizes both the compression wave and shear wave systems simultaneously.

In this system the pipes/tubes are fed to the rotary test head and the results of the test are processed by the electronic unit placed in a nearby control cabin. A series of guide rolls are used in conjunction with two large pinch rolls for accurate pipe centering in the rotary test head. The paint gun assembly is located downstream of the outlet pinch roll at a suitable distance after the inspection point. An electronic delay memory unit ensures that the approximate defect position is paint marked. The rotary test head consists of twelve ultrasonic transducers arranged in four groups of three. Each group is mounted at 90° intervals on a rotor supported by a hydrostatic bearing. Each group of three transducers is mounted in a probe unit which is self applied to the pipe surface. The probes are rotated round the pipe at 450 rpm. The pipe under test is passed through the rotary head at a constant linear speed dependent upon the requirement of the test. The twelve transducers are connected via slip rings and brush gear to two computing units which process the relevant information from the transducers. The sound waves generated by the ultrasonic transducers are water coupled to the pipe surface.

Each probe unit contains one ultrasonic transducer oriented to produce compression waves and the remaining two transducers are used for the generation of shear wave, they being arranged to scan in opposite direction round the tube surface. The four transducers for compression wave are 15 mm diameter with a resonant frequency of 5 MHz and are arranged at normal incidence to the pipe surface. The water column which couples the sound from the transducer to the pipe surface is 50 mm in length. Each transducer is electrically pulsed 1000 turns per second. Each impulse causing a short burst of high frequency sound waves propagate down the water column to the pipe surface. At the interface, part of the incident sound wave enters the pipe wall and continues in a radial direction to the bore of the pipe. The short pulse of sound wave which enters the pipe wall continues to reverberate between the inter and the outer surface of the pipe. This successive reverberation of the incident, sound wave within the pipe wall results in a series of short sound wave pulses arriving back at the ultrasonic transducer at the top of the water column. The time interval between the successive pulse is dependent on the pipe wall thickness. A measurement of this time interval can thus enable the pipe wall thickness to be measured. Any changes in the form of series of pulses can be interpreted electronically to identify the existence of laminations or other pipe conditions like thickness variation.

The shear wave generating system consists of eight transducers, two in each probe unit. One being arranged in the leading and the other in the lagging direction. The transducers are 15 mm in diameter and operate at the frequency of 2 MHz. The transducers are located at the top of the water columns 45mm in length and are inclined to the surface of the pipe at an angle of approximately 17° (degree) to the normal incidence.The transducers, in this case, are energised simultaneously 8000 turns per second and the short pulse of ultrasonic wave is obtained in the same manner as described during compression wave generation. The sound wave being incident at an angle of 17° enters the pipe wall and is refracted away from the normal at an angle of approx. 45° following a zigzag path through the wall. If the pipe wall is free from discontinuity, some of the energies are reflected back along the same path to the transducer. The information is processed electronically to identify the presence of longitudinally oriented discontinuities and similar radial imperfections. It is possible to determine whether a discontinuity is located on the inner surface or the outer surface by observing the turns at which reflected sound wave pulses return to the transmitting transducer and physically being paint marked by two different colours by delay memory system.

The processing of the information received by all the transducers and their interpretation is performed by two electronic processing unit.

The units are synchronised to prevent interaction between the two systems. The processed information relating to pipe quality is recorded by means of an 800 pen electrostatic recorder, 8 channel in put with 100 pens per channel. The extremely rapid of this recorder enables measurement obtained from a single ultrasonic pulse to be displayed on the chart. The rotary head is arranged to generate 200 pulses per revolution and these pulses are used for the synchronisation purposes to determine the instantaneous position of each of the 12 transducers during rotation around the tube circumference. For the purpose of paint marking the tube circumference is divided into for quadrants, by two St. of paint gun units, each set comprising 4 guns spaced at 90° intervals around the tube. One set is used for indicating the approximate longitudinal ands quadrant position of discontinuities on the external pipe surface and the second set to indicate the position of internal discontinuities. The recorder chart is divided into 4 main sections on which the information are recorder for various test parameters. They are: -

1. Average thickness assessment matrix
2. Shear Wave record
3. Thickness record
4. Compression wave or Geographic record

The information printed out on section 1, 3 and 4 of the chart are obtained from the compression wave system while the information on section 2 are obtained from shear wave system.

Compression Wave System: -

a) Thickness Record:

It is the third section of the recorder chart and displays the absolute wall thickness measurement obtained from the thickness measurement transducers during their helical scan along the length of the tube. Reading from left to right of this portion of the chart, the absolute thickness represented, is from Zero to 25mm (1 inch). 200 pens of the recorder are used, each pen corresponding to a (. 005 inch) 0.012mm change in thickness. The absolute values of the upper and lower thickness tolerance as permitted on a given pipe thickness are present into the electronic unit and these tolerances are indicated on the thickness record chart by broken lines. The exact nominal thickness of the pipe under test is also preset into the electronic unit and the pen corresponding to this particular thickness is rendered inoperable. The results in a continuous blank space appearing down the center of the thickness record which clearly shows the nominal thickness of the pipe wall. The information obtained by the thickness measuring transducers consists of a series of short ultrasonic pulses separated by a time interval directly proportional to the measured thickness of pipe wall. The time interval between selected pulses is measured and the wall thickness corresponding to this interval is indicated on the recorder chart. If a lamination or laminar type discontinuity is present in the pipe wall, it is possible that the series of ultrasonic pulses within the area of the lamination is obtained between the tube outer surface and the lamination as opposed to the tube outer surface and the tube inner surface. Under these circumstances the depth of the lamination below the tube outer surface is measured and this appear on the thickness record chart as a section of thin tube wall.

b) Geographic Record:

The fourth section of the chart is the geographic record. It is essentially a development of the pipe surface, the width of the record representing the circumference. The thickness record shows where the tube wall has gone outside the permitted tolerances. This record correlated with the geographic record, whereby the latter indicates the area over which the out of tolerance condition persisted. Thus the geographic record indicates the area of thick wall, thin wall and the laminated area.

c) Average thickness Assessment Matrix:

It is the first section of the chart and is also the development of the tube surface which is shown as a matrix with each point on the matrix, representing an area of pipe surface. The ultrasonic thickness measurement obtained, from one of the four rotating transducers (for the area corresponding to each point from the matrix) are averaged. This average thickness is then compared against the shows at each point on the matrix a different symbol corresponding to the measured average thickness being within tolerances, above tolerances or below the tolerance.

d)Shear Wave Record:

The shear wave electronic unit monitors the times at which the ultrasonic pulses reflected by discontinuity in the tube wall return to the corresponding ultrasonic transducer. Each of the eight shear wave transducers is connected to its own twin gate unit in the shear wave electronic unit which enables the ultrasonic pulses to be segregated into those arriving from the outer tube surface and those from inner tube surface. The amplitude of each electronic ultrasonic pulse detected by the shear wave transducers is measured. Ultrasonic signals originating from the tube outer surface are shown on the left hand side of the chart with increasing amplitude from left to right whereas ultrasonic signals originating from the internal surface are shown on the right hand side of the chart with increasing amplitude from right to let.

e) Calibration:

The thickness measuring and associated electronic unit are calibrated using machined stranded thickness blocks. There is no absolute standard used for setting the sensitivities of the four transducers during production testing. However, the overall sensitivity of the compression wave equipment is generally set at a high level in order to ensure that the required series of ultrasonic pulses can be consistently obtained from pipes with finished as rolled surfaces.

The sensitivity setting of the shear wave transducers is obtained by calibrating the system on standard notches. Typically these notches are 58mm in length, up to 1mm in width and the depth corresponding to 5% of the wall thickness. Notches are machined on both the inner and outer surface of the tube. During calibration the sensitivity of the shear wave transducers is adjusted to obtain a signal amplitude corresponding to 5 quantiseds levels on the recorder chart. The equipment is then set to respond to any signal amplitudes in excess of this pre-set trigger level.The trigger levels are indicated on the shear wave record for both internal and external positions by a broken line, when a signal is obtained with an amplitude in excess of the trigger level, the quadrant of the pipe circumference in which the defect was detected is paint marked, showing the longitudinal and the quadrant position of the defect.

The above system is generally integrated into the pipe manufacturing processes. After the pipes have been cooled in the mills, preliminary pipe inspection is made by visual inspection and any gross external surface defects found during the inspection are removed by surface grinding. Following this initial grinding the pipes are forwarded to the Rotary test system for ultrasonic inspection. Tube lengths from 5 m to 14 m can be accommodated by the system. The system requires the pipe ovality to be within +(or)- 1%, the pipe straightness to be better than 1 in 500 and pipe ends be free from burrs. The location of any surface by means of paint sprays. Magnetic particle inspection used for the accurate location of the external defects, following which the defects are ground out. A final manually wall thickness check is made at these locations on the tube surface where surface grinding has occurred. This ensures that the remaining wall after the removal of external defects are within the tolerances. Internal surface defects, marked by a different colour, can not be removed by grinding. This necessitates that either the whole tube is rejected or if the defect occurs near the end, the end containing the defect is cut off.