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  • Rotary TechniquesOpen or Close

    A comparison between the water chamber and contact shoe techniques
    for full body tube inspection using rotary probes

    Water Chamber Technique

    Most manufacturers of ultrasonic rotating systems offer heads using the water chamber technique. The transducers are located within a rotating water chamber through which the tube is made to pass as centrally as possible. With small diameter tubes close tolerance guides can be used either side of the rotating chamber to ensure the tube is maintained as concentric as possible with the chamber during testing. Depending on the O.D. tolerance of the tubes to be inspected there may have to be larger gaps between the guides and the tube which nearly always leads to off centre conditions particularly at the tube ends; variation in the concentricity between the tube and rotating chamber gives rise to ultrasonic incident angle variation with resulting sensitivity variations and the repeatability statistics of the overall system deteriorates.

    The main advantages of the water chamber is (i) higher rotational speeds can be achieved and (ii) the ability to get more transducers in a shorter space in terms of length; this point is important when trying to achieve minimum untested end lengths and also achieving optimised testing conditions at the tube ends.

    Typically water chamber rotating heads are very good for small diameter precision tubes with surfaces free from scale. If these type of heads are used with hot finished (black) tubes then problems can occur with the scale dust collecting in the chamber during production testing which ultimately attenuates the ultrasound thereby affecting sensitivity.

    Although manufacturers claim to be able to flush the scale away this is only partially successful and generally to achieve a good test the tubes need to be clean and free from scale.

    water chamber technique

    Contact Shoe Technique

    In this technique the transducers are housed in a water column block which actually rides on the tube surface by means of a contact wear shoe. The main advantage of this method is the ability to test hot finished material even with rough surfaces and the ability of the probe assemblies to test with typical tube off-centre conditions of ± 10mm without affecting the test sensitivity.

    Excellent repeatability figures can be achieved with rotary heads using this technique and, since the ultrasonic angle remains fixed, this inevitably leads to less spurious marking and prove up requirements. Typical seamless tubes can suffer from some out of straightness particularly at the pipe ends and the contact shoe design with its tube following capacity is by far the better system for testing this type of product. The overall length of the contact shoes containing the transducers must be kept a short as possible to ensure testing can be carried out as close as possible to the tube ends. The only disadvantages of this technique are the slightly lower rotational speeds that can be used and the minimum tube O.D. size that can be tested. Unicorn are probably the only supplier which offer both types of rotating head; the water chamber technique can be used for tube diameters up to 120mm maximum O.D. and above this the contact shoe technique is recommended; the minimum diameter which can be tested by the contact shoe range of rotary heads is about 25mm O.D.

    contact shoe technique
  • S.A.W. Pipe TestingOpen or Close

    Basic principles and guidance in ultrasonic testing of submerged arc welded pipe.

    Imperfections

    Imperfections occurring in SAW pipe seams can broadly be classified into two categories i.e.

    (a) ‘Longitudinal’ imperfections which are generally parallel to the weld seam e.g. lack of side wall fusion, toe cracks, centre line cracks, lack of penetration etc, and

    (b) ‘Transverse’ imperfections e.g. transverse cracking and volumetric type imperfections, vertical gas holes, porosity etc.

    Note - Chevron cracking does not fall into any of these two categories and is a special problem requiring a special approach to detection, and is not included in the proposals for auto equipment. Many of the above imperfections are readily detectable by radiographic testing techniques especially those listed under ‘transverse’ imperfections and those ‘longitudinal’ imperfections which are not close to the parent metal/weld metal interface.

    These imperfections which, however, are close to the parent metal/weld interface can be difficult to detect using radiography and ultrasonic is the only reliable technique to detect e.g. toe cracks lack of side wall fusion etc., which are generally accepted as being ‘critical imperfections’ in terms weld seam integrity.

    API 5L

    API 5L requires mandatory ultrasonic inspection of the weld seam using the following reference standard (targets):

    (a) Internal/External notches of a depth equal to 5% of the nominal pipe thickness or a 1.6mm () thro’ hole – trigger/alarm at 100% of signal height, or

    (b) Internal/External notches of a depth equal to 10% of the nominal pipe thickness or a 3.2mm (⅛") thro’ hole-trigger/alarm at 33% of signal height.


    World-wide practice in using the variety of allowable reference standards varies considerably, e.g.

    (i) In Japan, either the thro’ hole is used (normal method) or notches, located at the weld seam centre, are used.

    (ii) In the USA, holes and/or notches are used, sometimes on the weld centre and sometimes at the weld cap edges, dependent on the particular pipe manufacturer more importantly on the type of ultrasonic test equipment in use.

    (iii) In Europe, the situation is somewhat different, though not universally so. Basic European practice is to use notches on the weld edges (in the parent material on both sides of the weld seam for the detection of ‘longitudinal’ imperfections, and a single thro’ hole in the weld centre for the detection of ‘transverse’ imperfections.

    In SAW pipe mills in the UK for example, 5% deep notches are used, internal and external on both sides of the weld, i.e. 4 notches (2 int/2 ext) for ‘longitudinal’ detection and the 1.6mm hole drilled radically thro’ the weld at weld centre is used for ‘transverse’ detection. It should however be appreciated that many of the oil/gas majors impose supplementary NDT requirements to API 5L on pipe manufacturers, where apart from the normal reference standards (targets) used for establishing the test sensitivity, other targets have to be introduced into a test piece to establish the equipment’s capability of detection of such targets (and checked on a regular basis during normal pipe production).


    Moving Pipe vs Moving Probe Head

    This question of whether to move the pipe past a fixed inspection point or moving the transducer assembly along the weld seam with a the pipe stationary is a difficult question to answer.

    Most pipe manufacturers prefer to move the pipe as the u/s test facility is then within the ‘flow’ of pipe thro’ the mill, and is good practice in terms of flow logistics. When this approach is adopted it is normal practice to load each pipe in turn onto a bogie or chain conveyor and, with the weld at TDC, translate the pipe past the inspection point.

    Translation of the pipe along a roller conveyor is to be avoided, due to the inability of the probe head/tracking system to follow instantaneous movement of weld position imparted to the pipe as the pipe-end engages each roller in turn. The primary advantage of moving the pipe, apart from flow logistics, is that the electronics is installed at a fixed location, usually in a controlled environment within a cabin from where the equipment operator executes the test.

    In many cases, it is more practical to move the transducer assembly along the weld seam with the pipe stationary (weld at TDC), either with the operator/electronics on the traversing carriage. The latter has the advantage that the operator is in close attendance during the test to monitor weld seam tracking and carry-out confirmation checks on auto indications as they occur etc, etc.

    The choice of which system, moving pipe or moving test head, to adopt is highly dependant on the particular circumstances within the pipe mill, the degree of automation of the ultrasonic equipment, the degree of operator involvement in the test etc., etc, and the cost to implement.

    Ultrasonic Transducer Configurations

    There are a number of different ultrasonic transducer configurations possible in SAW pipe weld seam testing, the most common being the X, K and IX configurations (see figure 1). The X configuration is the most common but is not popular with many of the oil majors and others, as it suffers from a number of technical limitations. Most of the major pipe makers, e.g. use the K, modified K or IX configurations. These configurations use separate longitudinal and transverse imperfection seeking transducer pairs, as opposed to the X system which uses the same transducers for both longitudinal and transverse detection.

    The choice of whether to use the K or IX configuration is dependant on the maximum pipe thickness to be tested, and to some extent on the prevalence of particular imperfections in a specific process route. The simple K – configuration is suitable for pipe thicknesses up to circa 18mm, while the modified K increases the longitudinal detection up to about 26mm thickness, and the IX configuration full detection up to 26mm thick pipe.

    Pipe/Weld Geometry

    The pipe shape and weld seam geometry are critically important to a satisfactory ultrasonic test when using automatic/semi-automatic equipment (incorporating fixed transducers). UOE pipe mills provide a high degree of pipe shape and weld geometry control, whereas other SAW pipe processes may have limitations in this area. The following pipe shape/weld geometry conditions can cause limitations to a satisfactory ultrasonic test of the weld seam:


    (a) Apple/Pear Shape (including ‘flats’ close to the weld)

    (b) Scale on the parent pipe external surface close to the weld

    (c) Weld spatter on the external pipe surface

    (d) Poor weld shape, both inside and outside welds

    (e) Edge misalignment

    (f) Weld misalignment

    (g) Flat/peaked welds

    (h) Presence of edge laminations/inclusion clusters in plate

    (i) et al

    It cannot be stressed too highly that good pipe shape and weld seam geometry is essential to a satisfactory ultrasonic test of the weld seam.


    saw pipe option 1 - k configuration

    Cycle 1 – transducers 1 & 2 transmit longitudinal signals received on 1, coupling check on 3 trans. signals received on 2 & 4


    Cycle 2 – transducers 3 & 4 transmit longitudinal signals received on 3, coupling check on 1 trans. signals received on 4 & 2


    Electronics – no. of test channels = 4, plus 2 coupling channels.


    saw pipe option 2 - modified k configuration

    Cycle 1 – transducers 1 & 5 transmit longitudinal signals received on 1 & 2, coupling check on 4 trans. signals received on 5 & 6


    Cycle 2 – transducers 2 & 5 transmit longitudinal signals received on 2 & 1, coupling check on 3 trans. signals received on 5 & 6


    Cycle 3 – transducers 4 & 6 transmit longitudinal signals received on 3 & 4, coupling check on 1 trans. signals received on 5 & 6


    Cycle 4 – transducers 3 & 6 transmit longitudinal signals received on 4 & 3, coupling check on 2 trans. signals received on 5 & 6


    Electronics – no. of test channels = 6, plus 4 coupling channels.

    saw pipe IX and X configurations

    Cycle 1 – transducers 1 & 5 transmit longitudinal signals received on 1 & 2, coupling check on 4 trans. signals received on 5 & 7, coupling check on 8


    Cycle 2 – transducers 2 & 6 transmit longitudinal signals received on 2 & 1, coupling check on 3 trans. signals received on 6 & 8, coupling check on 7


    Cycle 3 – transducers 4 & 7 transmit longitudinal signals received on 4 & 3, coupling check on 1 trans. signals received on 7 & 5, coupling check on 6


    Cycle 4 – transducers 3 & 8 transmit longitudinal signals received on 3 & 4, coupling check on 2 trans. signals received on 8 & 6, coupling check on 5


    Electronics – no. of test channels = 8, plus 8 coupling channels.

  • Welded TubeOpen or Close

    From Steel to Tube. An Overview of Welded Tube Manufacture.

    Steelmaking

    High quality strip is produced by the Basic Oxygen Steelmaking process from high grade materials with low residual elements. The steel is continuously cast under carefully controlled conditions and rolled on a wide strip mill to tight dimensional control limit.

    The cleanness of the steel is very important in strip for EW pipes particularly the avoidance of inclusion stringers mainly present as oxides and sulphides which can divert near the weld line. Inclusions as small as 200 microns can give rise to ultrasonic signals. Process routes have therefore been developed to minimise the incidence of non metallic inclusions and to eliminate stringer types by controlling the shape of the inclusions in the rolled product.

    The volume fraction of inclusions present is influenced by the overall steelmaking operation. Vessel practice is designed to minimise the oxygen content of the bath when tapped. When the steel is in the ladle special steps are taken to remove a high proportion of solid deoxidation products and to prevent the formation of new inclusions formed by re-oxidation or erosion/chemical attack on refractories.

    Casting

    Alternatively, oxygen may be removed as a gas by use of vacuum treatment. In spite of these precautions some oxide inclusions will remain in the steel. These are rendered harmless by deep injection of suitable modifying agents such as calcium silicide into the steel ladle to produce non-clustering inclusions, which do not deform into stringers during hot rolling.

    Low sulphur steels down to .002% can be supplied which improve weld and body impact values, through thickness ductility and resistance to hydrogen cracking.

    calcium silicide injection station

    The steel is transferred to the tundishes on the casting machine through a refractory shroud tube, which prevent reoxidation occurring. The liquid steel is continuously cast through two tundishes into moulds producing slab sizes of 229mm thick x 1.1 to 1.8 metres wide referred to as concast strands. A diagram of the casting operation is shown below.

    The large tundishes provide a further opportunity for inclusions to float out and the liquid steel is covered with an insulating powder to preserve temperature and prevent contact with a the atmosphere. The steel passing from the tundishes to the moulds is again shrouded with refractory tubes to prevent reoxidation. The moulds are fitted with automatic mould level control to prevent sub surface entrapment of casting powder which could lead to surface and internal defects.

    steel tube casting

    The casting conditions are very carefully controlled to minimise segregation and the formation of surface cracks. Segregation is reduced by using a low superheat and a slow casting speed adjusted to suit the grade of steel and slab dimensions. Machine geometry of the concast strand is very important, and roll gaps have to be adjusted to within fine limits.

    Surface slivers on the strip are the result of surface cracks on the concast slab which form when the concast curved strand is straightened out. Control of the cooling water flow reduces the tendency for cracks to form. Sulphur prints on transverse sections flame cut from the concast strand are taken regularly to ensure freedom from severe segregation and inclusion clusters.


    The continuous cast strand is flame cut into slabs 7.5 metres long which are cooled in stacks. When cool, the surfaces of the slab are inspected and any unacceptable defects observed dressed out by hot scarfing. The slabs are then ready for rolling to strip.

    welded tube cast strand

    Strip Rolling

    The slabs enter a reheat furnace where a computer calculates the heat content of each slab in the furnace, and compares this with the desired heat content of the slab in that position. Positive or negative errors are summed for the whole furnace and used to control the speed of slabs through the furnace. On leaving the furnace the slabs enter a reversing mill, which reduces the thickness in 7.9 passes from 229mm to 30mm.

    The slabs then enter a 6 strand-finishing mill fitted with automatic set up and automatic gauge control. A computer system adjusts the roll gaps to produce the correct gauge at the particular finishing temperature taking into account all the slab variables. Information from each strand is fed on to the next one to ensure the desired finishing conditions are obtained.

    strip rolling

    Gauge performance is measured by an x-ray technique along the centre line of the strip and is consistently within ±. 10mm. Information from the width meter is fed back to the reversing mill where the width can be adjusted by altering the amount of side spread of the slab. Laminar cooling is used to control coiling temperatures.

    Slitting

    Each coil is labelled with steel grade, dimensions and cast and coil numbers. In addition a computer printout giving the details of all coils including the cast analysis is produced. From these records it is possible to trace back an individual coil to the position of its slab on the concast strand and hence its precise steelmaking and casting details. The grade of the coils is regularly checked with a portable optical emission spectrometer prior to slitting. The leading end of each coil is cropped to present a square end to the slitting blades and the coil edges are slit or milled off to obtain a precise and consistent width. The coils are generally slit into three narrower coils of a width suitable for feeding directly on to EW Mills to make pipes 139.7 – 193.7mm diameter. Pipes of smaller diameter and thin walled 139.7mm diameter are made by stretch reducing 168.3mm o.d. pipes. Pipes of larger diameter up to 508mm are made from strip rolled either from slit or full width concast slab.

    Making The Pipe

    The coils are received at the Electric Weld (EW) Mill from the slitting line in widths which are governed by the wall thickness and the outside diameter of the finished pipe. Figure 6 diagrammatically shows the various stages through the EW Mill from the coil to the welded pipe. The coil is first uncoiled and fed through a strip leveller, which flattens the strip and prepares it for forming. The front and back of each coil is sheared by the strip end shears to present a clean square edge to the flash welder. At the flash welder, the ends of the coil are joined by flash butt welding and the upset cleaned off. This allows the mill to run continuously. The flash welded section is cut out at a later stage and discarded. The looping pits form a strip accumulator between the flash welder and the Forming Mill which allows the EW Mill to continue welding, and producing pipes while the flash welding operation between coils is taking place.

    On EW Mills which produce quality welded pipes, it is essential to present consistent edge conditions to the mill to ensure that optimum welding conditions can be maintained and this is achieved using the Edge Scarfing Unit. The Edge Scarfing Unit removes the shear plain edge and presents a square machine edged surface to the mill. The forming mill progressively shapes the strip from the flat ingoing material to the closed oval. Careful design of this section avoids the creation of unnecessary residual stresses within the final product. Regular checks are made on mill alignment and precise setting of the mill is carried out to ensure that the strip travels up the centre of the roll train so that on reaching the end of the forming mill, there is no tendency to twist thus avoiding uneven working of strip edges. The last stands within the forming mill are generally referred to as the fin passes which work the edge of the strip to present a consistent edge profile to the welding vee. The forming mill is adjusted to standard settings and the actual values recorded for each size and gauge. Discipline in the control of the mill set up contributes to the assurance of consistent product quality and weld integrity.

    The strip edges are heated to a welding temperature by the high frequency welder. The welder is a large radio frequency oscillator producing and alternating current of 200.400 kHz which resistance heats the strip edges. Due to the two phenomena associated with radio frequency electric current (i.e. The skin effect and the proximity effect) this current concentrates in the surface of the strip edge.

    At the weld head the two heated edges are brought together and pressure applied to form a forged weld. All previously liquid metal is expelled together with any oxides and the plastic areas behind the heated edges upset. The geometry of the weld area is very important and the ingoing and outgoing circumferences are measured to assess the amount of metal, which has been pushed out. The symmetry and dimensions of the heat pattern are regularly checked by cutting a sample from the tube, polishing and etching a cross section of the weld and examining the microstructure under a microscope.

    Subsequent heat treatment completely removes the heat pattern resulting in a uniform structure in the weld region. After welding, the internal and external weld flash or bead is planed from the tube. The strip edges are thickened in the fin passes before welding which allows the internal and external bead planning equipment to marginally cut into the parent metal without reducing the tube wall below the nominal thickness, (Figure 14). The weld bead is regularly checked for uniformity and integrity.

    The weld line is then water cooled to lower the temperature of the pipe as it enters the sizing mill. The sizing mill rounds up the pipe and marginally reduces its diameter to give the required finished dimensions. Within the sizing mill the weld line is inspected continuously using ultrasonic shear wave and surface wave techniques together with eddy current testing. The sizing mill is also used to produce a straight pipe by adjusting the final restraining roll pass at the end of the sizing section. The rotary cut-off is used to cut the continuous pipe into the required lengths unless stretch reduction is required when the pipes are left in lengths of up to 122 metres.

    Making The Pipe

    making the pipe

    The coils are received at the Electric Weld (EW) Mill from the slitting line in widths which are governed by the wall thickness and the outside diameter of the finished pipe. The diagram above shows the various stages through the EW Mill from the coil to the welded pipe. The coil is first uncoiled and fed through a strip leveller, which flattens the strip and prepares it for forming. The front and back of each coil is sheared by the strip end shears to present a clean square edge to the flash welder. At the flash welder, the ends of the coil are joined by flash butt welding and the upset cleaned off. This allows the mill to run continuously. The flash welded section is cut out at a later stage and discarded. The looping pits form a strip accumulator between the flash welder and the Forming Mill which allows the EW Mill to continue welding, and producing pipes while the flash welding operation between coils is taking place.

    On EW Mills which produce quality welded pipes, it is essential to present consistent edge conditions to the mill to ensure that optimum welding conditions can be maintained and this is achieved using the Edge Scarfing Unit. The Edge Scarfing Unit removes the shear plain edge and presents a square machine edged surface to the mill. The forming mill progressively shapes the strip from the flat ingoing material to the closed oval. Careful design of this section avoids the creation of unnecessary residual stresses within the final product.

    Regular checks are made on mill alignment and precise setting of the mill is carried out to ensure that the strip travels up the centre of the roll train so that on reaching the end of the forming mill, there is no tendency to twist thus avoiding uneven working of strip edges. The last stands within the forming mill are generally referred to as the fin passes which work the edge of the strip to present a consistent edge profile to the welding vee. The forming mill is adjusted to standard settings and the actual values recorded for each size and gauge. Discipline in the control of the mill set up contributes to the assurance of consistent product quality and weld integrity.

    mill alignment

    The strip edges are heated to a welding temperature by the high frequency welder. The welder is a large radio frequency oscillator producing and alternating current of 200.400 kHz which resistance heats the strip edges. Due to the two phenomena associated with radio frequency electric current (i.e. The skin effect and the proximity effect) this current concentrates in the surface of the strip edge.

    At the weld head the two heated edges are brought together and pressure applied to form a forged weld. All previously liquid metal is expelled together with any oxides and the plastic areas behind the heated edges upset. The geometry of the weld area is very important and the ingoing and outgoing circumferences are measured to assess the amount of metal, which has been pushed out. The symmetry and dimensions of the heat pattern are regularly checked by cutting a sample from the tube, polishing and etching a cross section of the weld and examining the microstructure under a microscope.

    Subsequent heat treatment completely removes the heat pattern resulting in a uniform structure in the weld region. After welding, the internal and external weld flash or bead is planed from the tube. The strip edges are thickened in the fin passes before welding which allows the internal and external bead planning equipment to marginally cut into the parent metal without reducing the tube wall below the nominal thickness, (Figure 14). The weld bead is regularly checked for uniformity and integrity.

    The weld line is then water cooled to lower the temperature of the pipe as it enters the sizing mill. The sizing mill rounds up the pipe and marginally reduces its diameter to give the required finished dimensions. Within the sizing mill the weld line is inspected continuously using ultrasonic shear wave and surface wave techniques together with eddy current testing. The sizing mill is also used to produce a straight pipe by adjusting the final restraining roll pass at the end of the sizing section. The rotary cut-off is used to cut the continuous pipe into the required lengths unless stretch reduction is required when the pipes are left in lengths of up to 122 metres.

    Making The Tube (continued)

    The strip edges are heated to a welding temperature by the high frequency welder. The welder is a large radio frequency oscillator producing and alternating current of 200.400 kHz which resistance heats the strip edges. Due to the two phenomena associated with radio frequency electric current (i.e. The skin effect and the proximity effect) this current concentrates in the surface of the strip edge.

    making the tube

    At the weld head the two heated edges are brought together and pressure applied to form a forged weld. All previously liquid metal is expelled together with any oxides and the plastic areas behind the heated edges upset. The geometry of the weld area is very important and the ingoing and outgoing circumferences are measured to assess the amount of metal, which has been pushed out. The symmetry and dimensions of the heat pattern are regularly checked by cutting a sample from the tube, polishing and etching a cross section of the weld and examining the microstructure under a microscope. Subsequent heat treatment completely removes the heat pattern resulting in a uniform structure in the weld region.

    After welding, the internal and external weld flash or bead is planed from the tube. The strip edges are thickened in the fin passes before welding which allows the internal and external bead planning equipment to marginally cut into the parent metal without reducing the tube wall below the nominal thickness, (Figure 14). The weld bead is regularly checked for uniformity and integrity. The weld line is then water cooled to lower the temperature of the pipe as it enters the sizing mill. The sizing mill rounds up the pipe and marginally reduces its diameter to give the required finished dimensions. Within the sizing mill the weld line is inspected continuously using ultrasonic shear wave and surface wave techniques together with eddy current testing.


    The sizing mill is also used to produce a straight pipe by adjusting the final restraining roll pass at the end of the sizing section. The rotary cut-off is used to cut the continuous pipe into the required lengths unless stretch reduction is required when the pipes are left in lengths of up to 122 metres.

    the sizing mill

    Control of Pipe Quality at the Wedling Mill

    The quality of EW weld is assessed by standard destructive tests such as cone expansion and flattening and by inspecting the microstructure of the weld area. This testing augments the non-destructive testing and provides a regular feedback to the mill on weld quality.

    The objective is to ensure that the weld area is as strong, if not stronger than the body of the pipe. The microstructure is inspected to ensure that the heat affected zone is symmetrical and satisfactory with regard to width and structure, that there is adequate diversion and that all the previously liquid metal and oxides have been removed from the weld region. Having established that the welding conditions are correct and that the weld is satisfactory it is then essential to ensure that the conditions under which the sample was taken do not change.

    There are many aspects of the pipe making process, which affect weld quality. These include setting of the forming mill, strip presentation to the forming mill, consistent width dimensions of the ingoing strip and control of the welding temperature.

    The two main variables, which can affect the weld temperature are the strip speed and thickness of the strip edge. The welding power is proportional to strip speed and strip thickness and the welding power requirement is measured using a constant called the energy factor. The energy factor is the welding power in watts per mm thickness per metre, per minute speed (watt min/mm²).

    Strip speed variation is dealt with by measuring the strip speed with a wheel driving a measuring instrument, which runs on the strip and is used to control the welder power output. Any variation in strip speed is therefore directly compensated for.

    Strip edge thickness is governed by the thickness of the slit strip plus thickening occurring in the fin pass rolls, which is impractical to measure.

    The most effective way of compensating for edge thickness and other variations is by keeping the temperature of the weld region constant. This is measured immediately after the formation of the weld with a two colour pyrometer which is unaffected by steam and water. The temperature measurement is used in a computerised control loop to adjust the speed power system to maintain a constant weld temperature.

    Stretch Reduction and Heat Treatment

    For pipe diameters of 139.7mm or less and wall thicknesses of 7.9mm or less, a 168.3mm diameter pipe is produced at the EW Mill and stretch reduced to the required finished size.The 168.3mm as welded pipe is heated in a 122m long gas fired barrel furnace followed by a 10m in line 10 megawatt induction furnace to about 1000°C, and stretch reduced through a series of water cooled rolls to the required size. After cooling, the pipes are cut to the required length and either sent to the visual inspection tables or for quenching and tempering. All pipes of diameter greater than 139.7mm or thickness greater than 7.9mm are formed to dimensions close to the finished size on the welding mill and heat treated using one of the following three methods. The weld line only may be heated to the normalising temperature using in line induction heaters, the complete pipes may be normalised in a separate furnace or for higher strengths, the pipes may be quenched and tempered.

    The pipes passing on to the quench unit are heated rapidly in a gas fired barrel furnace to 850-950°C and are water quenched on the outside surface by passing through 20 in-line spray rings as soon as they leave the furnace. To aid even heating and cooling, pipes are rotated on passing through the unit and a pyrometer is used to monitor the temperature at the furnace exit prior to quenching to ensure temperatures within the required range are obtained. Hardness’s are measured on the transverse faces of rings cut from each quenched batch to ensure sufficient through wall hardening has occurred. Normalising and tempering is done in gas fired walking beam or electric roller hearth furnaces where the pipes are maintained at an accurately controlled temperature for about 10 minutes and are then removed from the furnace to cool in air.

    tube inspection

    Inspection

    The bore and the outside surfaces of each pipe are visually inspected and the reasons for rejection recorded and analysed. The diameter, thickness and degree of ovality of each end of each pipe are checked. The pipes are bevelled and hydrostatically tested using the conditions laid down in the API specifications. If the pipes are to be subsequently threaded this test may be done at a later stage. The pipes are then sent for rotary probe ultrasonic testing.

  • Rotary Probe HistoryOpen or Close

    Ultrasonic Contact Shoe Rotary Heads A History of Development

    Prior to the 1960’s most of the full body ultrasonic tube testing systems for tubes above 4½" OD relied on the technique of either rotating or spiralling the product under test. This type of approach was, and still is limited in the testing speeds that could be achieved and even with multiple banks of transducers, the testing speeds could not match the production requirements of the typical tube mills. For this reason together with the aim of simplifying tube handling in a tube production/flow line the then Department of Research and Technical Development (DR/&TD) of the Stewarts and Lloyds company based at Corby in the UK embarked on the development of ultrasonic rotating heads.

    For the smaller diameter tubes rotating heads were already beginning to emerge from other commercial N.D.T. companies; these heads used conventional bearings and water chambers and were generally restricted to diameters less than 100mm. The first larger diameter heads developed by the DR and TD.

    Corby used hydrodynamic bearings and carried water jet transducers; heads to test diameters up to 6⅝" O.D. were produced and installed in various plants within the Stewart and Lloyds group, later to become British Steel Tubes Division. These initial designs under the trade name of Heliscan were also supplied to various companies around the world, under licence by the Davey Instruments Company.

    This type of rotary head with its water jet non-contact probe system proved highly successful for welded and cold drawn tube type of product, although testing speeds were relatively slow. For seamless pipe however, it soon became apparent that out of straightness conditions particularly at the pipe ends meant that a more sophisticated probe mechanism would be required.

    The solution was to develop a probe block/shoe containing an increased number of transducers which actually rides on the surface of the tube and follows the pipe bends during testing.

    Increasing numbers of transducers together with the requirement to test the larger diameters in the OCTG range and the increase in weight due to the more complex probe assembly mechanisms meant that a bearing with a higher load carrying capability would be required.

    This ultimately led to the development of the world’s first hydrostatic bearing to be used for a rotary device and by 1970 rotating heads using this type of bearing began to be produced by the then Research Centre of the British Steel Corporations Tube Division based at Corby. The major benefits which were realised when using hydrostatic bearings for ultrasonic rotating heads were (i) high load carrying capabilities (ii) virtually no bearing wear since there is no metal to metal contact between the static and rotating parts and (iii) water is used for both bearing and the ultrasonic coupling medium, which avoids the need for seals and simplifies the mechanical design.

    The first hydrostatic bearing head to be produced was known as the RP18 with the capability to test pipe diameters in the range 4½" to 18" O.D.

    Rotary Head 6 Rotary Head 18

    At the same time the first contact shoe design probe assemblies also began to emerge; these designs relied on a pneumatic application system with spring loaded probe blocks. The early designs proved to be unreliable and this finally lead to the development of the self-applying system whereby the probe block was kept in contact with the tube by means of the centrifugal force generated from a counterweight. This simplified design formed the basis of the designs in use today and a succession of rotating heads (RP20's) have been produced throughout the 1970’s and 1980’s and installed at various locations around the world.

    Designs for smaller RP200 and RP350 rotary heads, again using hydrostatic bearings and self applying probe assemblies were also produced by the British Steel Technical Centre at Corby prior to its closure in 1991.

    unicorn urp350s

    At that time some of the Engineers involved with the development of rotary heads formed the company known today as Unicorn Automation (NDT) Ltd; this has ensured the continuation of the developments, which now stretch over five decades since 1991 Unicorn recognised the need for even faster testing speeds, and have further developed the rotary heads and probe application mechanisms to incorporate many more channels, higher rotational speeds and the ability to now test tubes with uncut or upset ends.

    Over the last decade Unicorn have installed their rotary heads in major tube producer plants in various locations around the world. Even today, the development program is on-going with the advent of the next generation of rotary heads which will achieve faster testing speeds, improved maintenance features and even reduced costs.

    Unicorn now offer a range of contact shoe rotating heads covering tube diameters from 38mm to 700mm o.d.

  • UT-Large EW PipeOpen or Close

    High Integrity In-Line Ultrasonic Inspection of Large Diameter Electric Welded Pipe

    Introduction

    Several ultrasonic testing systems have been installed in an Electric Weld mill in the United Kingdom, specialising in large diameter pipes, for both in-house quality control and customer acceptance purposes.

    These systems were originally to provide the pipe mill with modern ultrasonic testing facilities to meet the ever increasing demand for quality assurance of large diameter electric welded pipe in the gas, oil and associated industries. Automatic ultrasonic inspection has been almost universally adopted at various strategic locations within the process route to ensure product integrity, strongly influenced by stringent customer acceptance criteria, coupled with excellent test performance and reliability necessary for a high volume, modern welded pipe mill.

    Description of Test Systems

    The automatic ultrasonic test systems at the mill are sited at three separate locations within the process route. Each of the three test systems incorporate a weld line tracking facility which relies on the detection of a white line edge by means of a line scan video camera. Both the camera and ultrasonic probe assemblies are mounted on a common rotating ring and a weld seam tracking accuracy of ± 2mm is readily achieved. All pipes produced are automatically marked with a narrow white line which is accurately reference to the weld line centre and is applied just prior to the actual welding point.

    The first test system in the process route is located immediately after welding and includes an ultrasonic shear wave weld seam inspection. The ultrasonic shear wave probes are similar to those used on the sizing mill exit and are as described in the following paragraph.The weld trim profile monitoring system uses a laser viewing system located on the I.D. trimmer bar and has been recently developed by the plant engineers. These two systems therefore provide a rapid feedback to the welding mill operator on the weld and bead trimming quality.

    large UT EW Pipe

    Further downstream a second system is located at the exit of the sizing mill and comprises of a composite weld seam/weld edge inspection unit. This system provides a high sensitivity weld seam integrity check together with an ultrasonic probe system for the detection of laminar imperfections immediately adjacent to and on either side of the weld line.

    The weld seam inspection equipment at this location consists of two pairs of specially developed 45° shear wave probes. The probes have been designed to enable reliable detection of very short imperfections whilst still maintaining a tolerance of probe to weld position of ± 6mm. An additional feature of the probe design also ensures that, in addition to detection surface breaking imperfections, totally enclosed imperfections are also detected. The electronic equipment associated with the shear wave probes provides for twin trigger levels on each channel and allows for enhanced sensitivity to be used for those imperfections (purely radial in nature) giving rise to similar reflected signal amplitudes from both sides of the weld. The weld edge inspection unit, when required, utilises four compression wave probes, providing an inspection for laminations over a 25mm wide band either side of the weld seam. This composite inspection system provides a means of segregating acceptable and suspect pipes prior to further pipe finishing operations.

    Subsequent to pipe-end bevelling and hydrostatic testing, pipes pass to the third location, which is a sophisticated automatic ultrasonic testing acceptance facility comprising of a final weld seam inspection unit immediately followed by a full body rotary probe examination. The weld seam inspection unit, similar to that on the sizing mill incorporates automatic weld line tracking, 45° probe angle inspection and twin sensitivity levels. The probe mechanics are designed such that self-application of the probes, together with automatic pipe speed control permit testing very close to the pipe ends, i.e. less than 25mm at the nose end and less than 12mm at tail end. During calibration adjustment of the probes circumferential position relative to the calibration notch is controlled remotely from inside the operator’s cabin; in addition the longitudinal position of the probes can be altered by up to 50mm to allow signal optimisation from the calibration holes or notches. The rotary probe unit RP20 provides full body inspection for the detection of laminar imperfections, longitudinal imperfections and thickness measurement.

    The rotary probe unit can test pipes in the diameter range 114mm – 508mm at rotational speeds up to 450rpm. Normally four probe assemblies are used, each carrying one compression wave and two shear wave probes. The design of the self applying probe assemblies, which utilise contact wear shoes, allow for the testing of pipes which can be positionally offset by ± 10mm. The electronic equipment associated with the two test systems produces automatic test result graphic displays and pass/fail data which can be used for process/quality control purposes and customer documentation if required. In addition the results of the test are automatically stored and transferred to a ‘Hand Probe’ confirmation site to facilitate operator assessment of detected imperfections.

    Additional pipe-end testing facilities using MPI and/or ultrasonic methods are available further downstream in the finishing line.

    Consistent with the high level of quality assurance throughout the plant, the ultrasonic systems described represent a total inspection capability to ensure the integrity of large diameter welded pipe used in many critical applications as required by the current needs of the gas, oil and associated industries.

  • Thickness AnalysisOpen or Close

    Thickness Variation in Seamless Tube

    Seamless tube is formed by piercing a hole along the axis of an ingot or bloom followed by one or more processes of elongation. Precise measurement and control of the outside dimension of solid rolled steel sections poses a severe problem, but for the seamless tube maker, controlling the shape and concentricity of the inside of the tube is an even more challenging problem. To help solve this problem, ultrasonic equipment has been developed which will measure the wall thickness of tubes over their entire surface at a sufficiently high speed to enable the whole output of a plant to be monitored. The tube thickness variations can also be analysed to provide information highlighting areas where adjustment of manufacturing equipment or process techniques may give even better thickness control.

    thickness analysis figure 1

    Manufacture of seamless tube above about 150mm diameter, uses the rotary forge process shown above. Ingots which have been specially cast, carefully inspected and dressed to remove surface imperfections are heated in a furnace and a hole is punched along the centre of the ingot by a hydraulic press. A rotary elongator spins the tube over a mandrel to give a first stage of elongation. The tube is then processed in a rotary forge mill where specially shaped rolls forge the tube to its final length and wall thickness. The pierced ingot or bloom is threaded on to a mandrel and pushed into the rolls.

    An eccentric lobe on the rolls rotates to meet the incoming bloom and bites into it. The rolls rotate further, rolling the bloom back out of the mill and swaging the steel in the bite over the mandrel. Further rotation of the rolls brings an open portion or gap round to the bloom enabling it to be pushed back into the mill ready for the next bite. This process continues with the bloom advancing through the roll gap on each cycle. Typically, the mill rolls rotate at about 60 r.p.m. causing the forging cycle to occur once per second.

    If the various machines used during each stage of tube manufacture are not set up and maintained correctly, or if the ingot is not uniformly heated, variations in wall thickness will occur. For example, if the ingot passes through the furnace too quickly, the centre will still be hard and the punch in the piercer may wander off centre giving an eccentric hole in the back end of the bloom. The subsequent process may not entirely remove this eccentricity. For this reason, the thickness of every pipe is measured over the entire surface of the tube by automatic equipment.


    The data obtained are processed in two ways:

    1. By checking against maximum and minimum tolerance limits to ensure that no tube delivered to customers has a wall thickness outside the range of the specification in any part of the tube (Inspection).

    2. By analysis of wall thickness variations within the limits of tolerance to determine patterns of variation, which can be related to the process so that corrective action can be taken to trim process adjustments (Quality Control).

    thickness analysis figure 2

    Automatic Measurement

    An ultrasonic compression wave technique is used to measure the tube wall thickness as shown in Fig.2. A transmitter (TX) energises a probe containing a piezo electric element, 1000 times per second, to generate a burst of high frequency ultrasound (5 MHz) which then travels down a column of water (a) As it reaches the tube wall, part of the energy is transmitted into the steel (b) and part is reflected (c).

    The transmitted pulse (b) is reflected from the tube inner surface (backwall) and returns to the outside of the tube where part of the energy passes back into the water column (d), and part is reflected back to bounce once more off the backwall. Each time the pulse returns to the outside of the tube, part of it is passed back into the water column, where it travels back to the transducer and is converted into an electrical signal. These echo signals are amplified in a receiver (RX) to produce a train of pulses (c, d, e, f, etc). The time between each, pulse if proportional to the tube thickness. The third and sixth echoes (f & g) are selected electronically and the time between the echoes is measured; a check is made to ensure that this period is approximately equivalent to three times the period between the third and fourth echoes. This technique gives an accurate and reliable thickness measurement.

    To enable thickness measurement to be made over the entire tube, four probes are mounted on a rotor, which rotates at approx. 600 r.p.m. around the tube whilst the tube moves slowly through the rotor. Each probe thus trances a helical path along the tube. Using four probes, the entire surface of a tube, 500mm diameter and 10m long, can be examined in about two minutes, resulting in approximately half a million individual thickness measurements. The same rotor also carries probes which use different ultrasonic techniques to search for small cracks and other imperfections.

    For inspection purposes, the thickness data are checked electronically against preset minimum and maximum tolerance limits and any area of the tube which is outside these limits, can be automatically marked using a paint spray. A chart showing the location and severity of any thick or thin section and any imperfections is also produced.

    For quality control purposes, techniques have been developed whereby thickness data are recorded on disc for analysis by computer. This enables powerful mathematical analysis techniques to be used to identify trends and repetitive patterns in the thickness variations.

    Analysis of Thickness Variations

    For the detailed analysis of thickness variations, the recording from a single transducer on the rotary probe is selected and the measurements obtained for each revolution of the transducer analysed as a data set. The effect of eccentricity may be considered by taking an eccentric tube and plotting the wall thickness against angle of rotation (Fig.3a); the variation obtained is very close to sinusoidal at a frequency of one cycle peer transducer rotation.

    The action of rotary forging tends to thin the tube wall effectively at the top and bottom (deep in roll groove) but leaves it thicker at the sides; this produces ovality (Fig.3b) which when plotted approximates to a sinusoidal graph with two cycles per probe rotation.

    To eliminate ovality, the bloom is turned by about 90° between each blow from the mill rolls. The small residual variation takes the form of a sinusoidal thickness variation having four cycles per probe, rotation which is termed ‘squareness’ (see diagram to the right)

    The above components, together with mean thickness, can be extracted from each set of thickness data using Fourier series analysis. This technique calculates an amplitude and phase angle for each component. The amplitude is a measure of the size or importance of the component, and the phase angle is the angle of rotation around the tube to the thickest portion of the tube due to that component.

    thickness analysis figure 3

    Using Data Analysis to Improve Performance

    The use of the microprocessor-based unit enables the components of variation to be plotted to a base of distance along the tube. In the example illustrated in Fig. 4, mean thickness variation (Fig. 4a) is shown to be well under control except at the very front of the pipe. Fig. 4b and 4c show the amplitude and phase angle of the eccentricity component respectively. The amplitude is fairly constant but the phase angle moves steadily through 180° along the tube. A possible explanation of this variation is that the ingot was eccentrically pierced along its full length and that the rotary elongator has twisted the eccentrically pierced bloom by 180° along its length.

    The sensitivity of the data analysis technique is such that even a small and quite acceptable squareness effect can be detected and monitored to ensure that it does not become excessive; Figs. Ed and 4c are examples of the amplitude and phase angle of this effect. Variation in phase angle shows that the twist given to the bloom between blows of the rolls is not 90° but somewhat higher; this causes the pattern to spiral down the tube. (The vertical breaks in phase angle from + 45° to - 45° are caused by the four-corner pattern shown in Fig.3c which causes an ‘artificial’ jump as the next corner comes nearest to zero angle of reference). From the rate at which this spiral progresses down the tube, the twist per blow can be calculated. This angle of twist, typically 110°, is carefully and deliberately chosen and set up on the mill to minimise the squareness effect.

    Analysis of thickness variations of seamless tubes by the technique described in this article has to be combined with measurements and observations made on the rotary forge mill itself. Further developments are in hand to enable the full potential of the rotary forge method of seamless tube manufacture to be realised and to provide a closer dimensional tolerance tube to cater for future market trends.

    thickness analysis figure 4
  • Seamless Pipe TestingOpen or Close

    Development of High Speed Rotary Probe Ultrasonic Testing System of Large Diameter Seamless Pipes for the Oil and Gas Industries

    Introduction

    Modern tube mills now produce large diameter seamless pipes at relatively higher production rates and as such the demands made on testing systems are also that much greater in terms of testing speeds. To meet this ever increasing demand and also to accommodate the need for more inspection directions than just longitudinal and thickness/lamination checking, a new generation of Ultrasonic Rotary Heads has been developed. The following summary descriptions provide details of the capabilities of these new rotary test systems:

    Rotary Head Mechanics

    The original operation of ultrasonic rotary heads were generally designed to test tubes with square cut ends but now more recent designs incorporate a unique probe arm lift mechanism which enables tubes with a variety of end conditions e.g. un-cut or upset ends, to be tested. The contact shoe principle has always proved to be the best technique to use when testing hot finished seamless pipe, and this technique has been retained with improvements to probe assemblies in terms of maintainability. A range of water column probe blocks have been designed to carry novel transducer arrays and inspections for thickness, laminations, transverse and oblique (variety of angles) defects can now be carried out simultaneously and at much higher testing speeds than ever before. The rotary heads can be equipped with many more transducers and can run at faster rotational speeds; the ability to cope with off-centre pipes (± 10mm) due to hooked ends, has always been an excellent feature of the contact shoe design and this aspect has been improved still further. All the new range of rotary heads have retained the tried and tested hydrostatic bearing designs which have proved very reliable in a production environment over many years of operation. One area which is vital to the reliable operation of the system is the signal transfer system via slip rings; here the brush assembly has been simplified and re-developed in recent times to provide easier maintenance.

    The range of pipe diameters that can be tested with the contact shoe design has been extended, and the rotary heads that currently can be supplied are:


    URP200 40mm to 200mm o.d

    URP350 101mm to 350mm o.d

    URP425 114mm to 425mm o.d

    URP500 139mm to 508mm o.d

    URP700 350mm to 700mm o.d

    Processing Electronics

    As with the rotary head mechanics the multichannel electronics for generating and processing the ultrasonic data has recently been developed to take advantage of the advances of the new data processing technology that is now available. The state of the art electronics used in conjunction with the rotary head, can generate and process ultrasonic data at P.R.F. rates greater than 20KHz per channel and from 128 channels or more without any compromises being made or loss of data from any individual channel. The electronics is fully programmable to cater for any customisation requirements and standard software is available for a whole array of facilities and analysis; the following list (although not exhaustive) provides an idea of the new systems capabilities:

    Thickness Measurement

    Thickness Analysis – Comparison of back wall signal amplitudes to ascertain presence of bore drop out type defect.


    Thickness Measurement Validity checking by means of division of various back wall counts; ideal for high-speed rotary tube testing.


    Interface and back wall following to maximise through thickness inspection depth for laminations.


    Signal amplitude comparison between lamination signal and back wall.


    Min Max Eccentricity chart displays; C Scan display showing thickness trends along the pipe; useful feedback for setup information at the production mill.


    Automatic computation for approximate tube weight and tube length.

    Defect Detection

    Selectable successive shots and proportional shots.


    Coincidence detection from opposite pairs of probes. Useful for testing with increased sensitivity for certain types of defect.


    Auto calibration – gates and gains.


    Gain step profile gates – particularly useful for transverse inspection.


    Defect Length Discrimination.


    Defect Signal Profile capture; look at comparison of capture from opposite side – defect signature analysis!


    Pitch and Catch technique concurrent with pulse echo technique with same probes; useful for transverse probes for detection of bore drop-outs.


    Priority list for prove up operation. Defect signal amplitude compared with thickness.


    Multiple gates and sequence correlation to achieve minimum un-inspected end length for transverse inspection (end testers).


    Automatic diagnostics including recognition of probe output deterioration.

    Ancillary Equipment

    Very often, even with the best test mechanics and electronics a good reliable production test also relies on the ancillary equipment used in conjunction with the test system. Reliable operation is crucial and Unicorn have also paid particular attention to ensuring the reliable and efficient operation of items such as the tracking and paint gun marking of detected defects, water Recirculating systems, operator safety features, conveyor sensors and encoder driving mechanisms and automatic fault diagnosis for ease of maintenance.

    Good tube restraint is also a very important aspect in performing a good test, and Unicorn recommend to customers that they use two pinch rolls on both to inlet and outlet side of the rotary head.

    Advantages of the New High Speed Rotary Systems

    The new generation of ultrasonic rotary test systems available today provide the OCTG pipe manufacturer with more flexibility in terms of location of the ultrasonic test systems within the production plant. The ability to test at faster throughput speeds and to test prior to end-cutting operations allows the option of siting the test system much earlier in the process route; this provides the obvious benefits of having a completely tested pipe after the pipe ends are eventually cut back. In addition the quality of the pipe produced is determined much earlier in the process route, thereby allowing possible corrective action and/or savings been made. The detailed results information for each pipe, together with the ability to carry out a defect signal profile analysis on each defect detected, can lead to genuine savings in terms of production efficiency, as well as providing a wealth of quality control information.

    With all these facilities and proven ability for the systems to work in a production environment for 24 hours 7 days a week the modern ultrasonic Rotary Testing Systems now available should be considered as an integral part of a Pipe Production Plant and not merely be viewed as a production line bottleneck which should only be used when absolutely necessary.

  • Q and T PipesOpen or Close

    The increasingly arduous conditions encountered in oil well drilling and production operations demand oil country tubular with improved high yield strength and toughness. One way of achieving these properties is by quench and temper processing. While Q&T plants are primarily designed for the production of casing and tubing with minimum yield strengths of up to 1034 N/mm², special property line pipe and structural hollow sections can also be produced.

    The salient features of the process requiring close technical control are careful selection of steel chemistry, control of austenitising conditions, efficiency of quenching and control of tempering. All of these influence the through thickness properties of the product and must be designed to ensure that variations in physical structure and properties within the pipe are kept to the absolute minimum.

    Process

    The layout of the Q & T unit, including heat treatment and finishing facilities, is shown in Fig.1. Natural gas-fired walking beam furnaces, capable of taking pipe lengths up to 15.2m (50ft), are used for heating and the quench system is of the external water quench type. The austenitising furnace, pressure and temperature control has a multiple burner arrangement split into eight zones each with air/gas ratio control. The walking beam system has thirteen holding stations provided with facilities for constant pipe rotation to ensure uniform heat distribution within the pipe and the development of a homogeneous austenitic structure for quenching. Furnace charging and discharging are synchronised and a series of roller ways is used to impart high speed rotation to the pipe on discharge into the quench system.

    The quench system incorporates an initial air ring to confine the quench water, and the seven quench rings following this provide the drastic quenching needed for through hardening. The quench rings have three rows of uniformly spaced holes angled to allow water to impinge on the pipe along the direction of travel. The flow rate is 7000 litres per minute at pressures of up to 10 kN/m². After the quench section, a 11 m spray unit gives a further water delivery of 5000 litres per minute, ensuring no recalescence in the in the quenched pipe. Reservation is made in the plant layout for other forms of quench systems should these be required for specific applications.

    The tempering furnace incorporates control facilities similar to those used for austenitising, but is provided with twenty-one pipe holding stations to give the longer heating times necessary in tempering.

    Heat treatment schedules are determined for each pipe size and the furnace walking beam mechanisms operated on pre-set cycles to provide consistent austenitising and tempering conditions throughout the size range. Control of heat abstraction through the quench unit is varied by adjusting the rotations speed of the furnace discharge rollers to ensure that the full cross-section of the pipe is below the Martensite finish (Mf) temperature on completion of the quench operation. The positioning of the pipe centrally within the quench unit by adjusting the ring assembly through a worm-gear mechanism and capping the pipe ends with thin metal discs to prevent water ingress to the pipe bore, ensures uniform quench conditions along individual pipe lengths and eliminates distortion problems.

    After heat treatment, the pipe are hot sized in a 3-stand sizing mill. This ensures close compliance with the dimensional tolerances required, particular for threading. The pipes are then cooled on transfer racks before straightening in a rotary straightener.

    Quality control is maintained by hardness testing and in-line checking of chemical composition. Each pipe is non-destructively tested by an ultrasonic rotating probe system, utilising compression and shear wave ultrasonic techniques. The information so obtained on thickness measurement and defect detection is electronically processed to provide a permanent record, and facilities are available for automatic marking of the longitudinal and quadrant positions of any defects. External surface imperfections are precisely located by magnetic particle inspection and removed by grinding, after which all ground areas are ultrasonically checked for compliance with the thickness specification. The mechanical properties of the finished pipe are determined as required by specification.

    Metallurgical Considerations of Production Control

    The optimum combination of strength and toughness is obtained from tempered martensite. The basic requirement on quenching is transformation of the pipe thickness to a high percentage of martensite, subject to freedom from quench cracking arising from internal strain induced by the volume changes of occurring during transformation. This is effected by limiting the carbon content to o.34%, controlling austenitising to minimise grain coarsening, and quenching to give a slight through-thickness gradation in the degree of martensite transformation. The criterion for quench-hardening is based on maintaining an across-thickness hardness variation after tempering of less than 30 Hv.

    For API grades, C-75 to P-110, the strength properties are obtained from steels at 0.28 to 0.34% carbon, by varying the manganese content from 1.0 to 1.6% and the tempering temperature from 560°C to 685°C. The effects of these variables on the hardness of the steel are illustrated in Fig.2, which indicates the hardness ranges corresponding to the strength limits of grades C-75, N-80, C-95 and P-110. In addition to enabling these grades to be processed from one type of steel, the effect of tempering temperature is used to control the yield strength within close limits, particularly for restricted yield strength and high collapse strength grades. For these special grades it may be necessary to apply cast selection within restricted composition ranges for tempering at intervals of 20°C.

    The major factor limiting the use of carbon steels is pipe thickness, which influences the attainment of the required martensitic structure on quenching. The Jominy hardenability characteristics of the steel are used to define the limiting conditions and determine the level of alloy additions required to meet the through hardening criterion. Hardenability concepts are also used for chemical composition control to ensure adequate hardenability from steel supplied with differing amounts of residual elements. This control, exercised in relation to chromium and molybdenum contents, is particularly important in avoiding quench cracking. For unalloyed carbon steel products, acceptable and upper limits are specified for chromium and molybdenum and the carbon and manganese contents adjusted to compensate for additional hardenability induced by residual element contents above the acceptable limit. Grades with yield strengths above P-110 require the use of alloy steels in which yield strengthening is derived mainly from the greater resistance of alloy carbides to tempering. The alloy additions, used either singly or in combination dependent on thickness and strength requirements, include the carbide forming elements, chromium, molybdenum and vanadium. Molybdenum, which is also a ferrite strengthener and gives the best combination of strength and low temperature toughness, is generally preferred. For these grades, in which the alloy additions increase the steel hardenability, the carbon content is limited to 0.32% to minimise any tendency towards quench cracking.

    Special types of casing require the pipe ends to be thickened for enhanced ‘pull-out’ or high pressure sealing properties, and the steel chemistry and tempering parameters must therefore be selected to take account of the thickness difference between the ‘upset’ and pipe body. The steel composition is designed for adequate through hardening at the upset region, and the tempering treatment controlled to give pipe body strength properties above the lower quartile of specification.

    Mechanical and Performance Properties

    Tensile properties are the essential criteria of process and metallurgical control for standard API casing grades. The compositions and tempering treatments used for the C-75 to P-110 grades are given in Table 1. The wider tempering ranges for the restricted yield strength grades C-75 and C-95 are required to maintain close control of the yield strength. This is also essential for high collapse strength grades with a minimum yield strength of 655 N/mm². The Charpy V-notch impact strength properties of the carbon-manganese steels are dependent to some extent on the steelmaking deoxidation practice. The transition data obtained are typical of N-80 and P-110 grades processed from silicon killed steels. For both grades a Fracture Appearance Transition Temperature (F.A.T.T.) of around –45°C can be maintained at energy absorption levels of 40-45 J for N-80 grade and 30-35 J for P-110 grade. Improved notch ductility results from the use of aluminium treated fine-grained steels and a further enhancement from the use of lower carbon alloyed steels. Control parameters have been established enabling the ‘F.A.T.T.)’ criterion to be maintained down to – 90°C.

    Mechanical tests do not, by themselves, give a realistic assessment of the performance of high grade pipe under the required operating conditions, and it may be necessary to apply fracture toughness studies particularly where other environmental effects such as susceptibility to sulphide stress corrosion cracking may exist. In casing string design, control of crack initiation rather than propagation may be more important, since the complex stress system of high axial and circumferential stress, combined with mechanical coupling between casing lengths will lead to crack arrest. Crack-opening displacement and other fracture toughness tests have shown that quenched and tempered products maintain high-energy ductile initiation characteristics down to lower temperatures than are obtained for normalised and tempered material. For quenched and tempered N-80 casing, for example, brittle fracture initiation will not take place at temperatures above –100°C, confirming its suitability for service temperatures down to – 80°C. Resistance to sulphide stress corrosion cracking has been studied in considerable detail. Quenched and tempered steels are accepted as being more resistant to this type of cracking than normalised and tempered steels, and the suitability of grades C-75, N-80 and C-95, as produced by quenching and tempering, has been indicated. With higher ‘bottom hole’ pressures and more aggressive environments being envisaged with deep well drilling, this research activity is now receiving considerably more detailed attention.

  • Large OD LinepipeOpen or Close

    Linepipe design has advanced due to demands for thicker pipes in a wider range of specifications, particularly for the sub-marine pipeline market. Enhancements to manufacturing techniques are required to meet these stringent demands. These enhancements are described in this article.

    Linepipes are formed by the conventional ‘U’ and ‘O’ press method followed by submerged arc welding (SAW) and cold hydraulic expansion. Incoming plates are shot blasted on the top and bottom longitudinal edges over a distance of 76mm from the edges before machining to the required width. The process provides the required weld profiles which are usually a single VEE for thicknesses of 6mm and a double VEE for thicker plate; the ends of the plates are then machined square. Shaping of the plates comprises edge crimping, ‘U’ pressing and ‘O’ forming. It is important to ensure that the variation in gap of the pipe leaving the ‘O’ press is kept to a minimum and to achieve this the press settings are carefully controlled according to the required pipe dimensions.

    Before the pipes are submerged arc welded, ‘run on/run off’ plates are welded onto the ends of the pipes after they leave the ‘O’ press. Longitudinal seam welding is carried out internally and externally and, following inspection, the pipes are mechanically expanded to obtain the desired dimensional accuracy and to develop the correct physical properties. The pipes are then hydrostatically pressure tested prior to ultrasonic inspection of the weld seam, end bevelling and magnetic particle inspection. The five principal areas of enhancement are described in the following sections.

    Edge Preparation of Plate

    In order to machine the edges of thicker high-grade plate at a fast enough speed, a new edge planing machine was installed in the pipe mill. The machine chosen offered the most economic method of preparation consistent with the achievement of a good surface finish. Plate is positioned accurately and is held rigidly while both edges are planed simultaneously Thus ensuring that edges are parallel to within ± 0.5mm. Specially developed, gauged carbide-tipped tools mounted in a series of indexable cassettes are used to prepare each plate edge to pre-determined profiles ready for welding. A great deal of trial work has been carried out on varying styles and grades of cutting tips to ascertain the best tool geometry and ‘set-up’ in order to give the required high quality finish for weld preparation, with acceptable production rates and tool life. As a result of this work, tool life has been increased to an average of over 2250m per edge with a maximum of 8000m per edge at cutting speeds of up to 65m/min. the ends of the plate are machined square to the length of the plate by the use of large milling heads equipped with carbide tipped cutters.

    'U' Pressing

    During ‘U’ pressing, allowances have to be made for ‘spring back’ which is a function of steel grade, plate thickness and plate production process route. All the forming operations in the traditional pressing operations were mechanically linked and although adjustment to the product shape was possible, difficulties would be experienced in catering for ‘spring back’ particularly with higher grade materials. The vertical punch beam, the side beams and the pipe support beam are now hydraulically operated from a central control panel which gives three forms of control mode: ‘setting’, ‘manual’ and ‘auto’. Depending upon the mode selected, accurate positioning relative to a known datum is possible for all individual hydraulic rams but once in production the press will operate in the ‘auto’ mode. This allows a greater degree of operational flexibility; it permits a more controlled method of forming and thus caters for the wider range of materials and thicknesses being processed. Achievement of a high degree of uniformity is essential in the ‘U’ press so that, during the subsequent ‘O’ pressing operations, high levels of consistency and accuracy can be obtained.

    Welding

    The achievement of a metallurgically sound seam weld is of paramount importance in linepipe manufacture. Uniform weld preparation is especially important and, as material thickness increases, it becomes more difficult to maintain a constant weld root gap. To overcome this problem, an automatic argon-oxygen tack welder is used; this multi-head unit, with ten of the heads operating simultaneously, lays down an intermittent, single bead tack weld. Seam welds in high strength linepipes are required to achieve weld metal toughness values commensurate with those stipulated for the parent steels; these requirements are more stringent for the higher grade steels. With the consumables presently employed for the welding of X60 and X65 pipe qualities, i.e. CMnMo wires and acidic or semi-basic fluxes, the modest toughness requirements of these grades can usually be achieved without too much difficulty. At the more demanding X70 level, however, the viability of this approach becomes questionable. Simple extrapolation up the alloying range, in relation to the choice of welding wires in order to meet the strength requirements, involves complications with excessive hardness. This hinders the achievement of the toughness properties and is possibly unacceptable where sulphide stress cracking is a danger. The development of a new lean-alloy welding wire, marketed by Oerlikon Ltd as TIBOR22, based upon the well documented MoBTi approach is one solution to this problem. Even when used with conventional fluxes, some improvement can be obtained but when TIBOR 22 is employed with one of the new generation of fully basic fluxes suitable for high current multi-arc welding then an excellent level of seam weld-metal toughness is achieved (Fig.1). This is the result of a very clean, fully acicular ferrite microstructure, with the added bonus of limited solid-solution hardening restricting the weld hardness to an acceptable level.

    Pipe Expansion

    International specifications make increasing demands on strict dimensional control and accuracy both in diameter and wall thickness, and call for a high degree of roundness and straightness. Also, in most specifications, the internal diameter of the pipe is the prime dimension. Because of these demands and the up-grading of materials, the hydraulic method of expansion has been superseded by a mechanical expander. This type of machine has the capability of expanding pipes to the specified internal diameters and of the equal distribution of pipe wall stress and ensures straight pipe lengths. The average ‘out-of roundness’ and deviation from straightness achieved by this type of expander is less than half that of the tolerance requirements in the API specifications.

    Inspection

    The type of market supplied by the pipe mill demands the highest degree of inspection during manufacture. This is carried out by using NDT and X-ray equipment to meet the most stringent international specifications. The ultrasonic equipment for weld-seam inspection in current use, four transducers in a K-configuration, is limited to a maximum pipe thickness of 19mm, above which it is not possible to obtain adequate ultrasonic ‘flooding’ of the weld zone to ensure satisfactory detection of imperfections. As a result of the increase in the pipe thickness range, it has been necessary to develop improved ultrasonic weld-seam inspection equipment to permit full through-the-thickness weld zone coverage at the upper end of the thickness range. This equipment incorporates up to eight ultrasonic transducers arranged in an X1-configuration intended for the detection of both longitudinally and transversely oriented weld imperfections.for this development, it was necessary to determine the optimum transducer arrangement geometry, transducer test frequency and ultrasound refraction angle in steel. The mechanical design of the X1-configuration is now well advanced and certain assembly items are being manufactured including mechanical modifications to the K-configuration (in current use). These modifications will allow the performance of the new configuration to be pre-determined under plant operating conditions prior to final installation and commissioning of the X1 system. The eight transducers in the X1-configuration are connected to a 12-channel electronics unit of modular construction which makes use of recent developments in electronics technology and allows maximum flexibility of operation during production testing; calibration to recognised international specifications is also possible. The equipment has been commissioned in the plant ready for the installation of the final mechanical assembly which will allow weld-seam ultrasonic inspection up to the maximum pipe thickness capability.

    Steel Supply

    In recent years, continuous casting of the slab from which the plate is rolled has been developed and all the material currently being processed by the pipe mill is produced by this route. The quality of the pipe has been shown to be as good as or better than that produced through the conventional ingot route.For the high grades of pipe, the use of alternative compositions of the CMnNbV and CMnMoNb types have been evaluated; it has been found that, with the former steel, a decrease in yield strength occurs during the pipe processing, but with the latter steel the opposite is true and the yield strength is increased (Fig.2). For a given pipe strength, this enables a somewhat lower yield strength plate to be used in the CMnMoNb compositions and therefore a thicker pipe can be produced at the higher strength levels using the molybdenum containing steel. Modern technology permits greater production control and customers are quick to seize new opportunities of demanding more rigorously manufactured products. This is particularly true in the market for submarine linepipes where improvements have been made.