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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. |
