"The TIGer is twice as fast"
TIG/GTAW weld overlay cladding and welding for offshore production
systems, the guarantee for Zero-Defects and sustainable safe production
TIG Weld overlay
cladding
TIG Welding
TIGer bi-cathode TIG/GTAW
Hot Wire OD/ID weld overlay
cladding
TIG/GTAW - Narrow gap
Hot Wire welding of clad
pipes
1 Introduction
Tubes and pipes are playing a fundamental role in the field of offshore crude oil exploration
and production activities. As structural parts of platforms, derricks and hoisting devices they
provide high mechanical resistance and lasting stability to the equipment. Sets of drill pipes
are needed to bore the holes in the ocean bed. A particular pipe in vertical direction, which is
called Marine Drilling Riser, encloses the rotating drill pipe and establishes the connection
between the well and the platform above. Auxiliary lines externally attached on the marine
riser allow to control and operate the BOP (Blowout Preventer), i.e. its huge valves must be
closed off if drilling into a gas, oil or water containing geological formation in the ground
causes an impermissible build-up in pressure inside the well.
It is not unusual that the time between the beginning of the first drilling operation and the
production start of the well exceeds three years, whereas production equipment must be
designed to stay operational for decades.
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The initially mounted BOPs on the wellheads are replaced by Subsea Wellhead Systems, more
commonly known as Christmas trees, including a choke for flow adjustment of well fluids,
chemical injection ports, further valves, fittings and equipment for remote control and monitoring
purposes. Production risers may provide a direct connection to pipe the fluids from the “wet”
Christmas tree at the seafloor to the production facility. Wells directly under the platform can be
connected in a straight line by vertically orientated Top Tension Risers.
Production fluids of multiple wells in further away positions are extracted via a network of delivery
lines, which are at last gathered in a manifold centre. A flow line spans the distance between the
manifold and the platform, its end on the seafloor is joined to a Steel Catenary Riser (SCR). This
assembly of pipes allows conducting the fluids from the ground to the surface of the sea.
Suspended in almost vertical direction from the platform, the pipes follow the shape of a catenary
line and arrive horizontally on the seabed.
Subsea production and injection network composed of several hundred kilometres of pipelines,
injection lines and so called umbilicals.
Umbilical lines are duplex stainless steel tubes or bundles of electrical cables, hoses and
conduits inside such a steel tube. Umbilicals supply electric and hydraulic power to maintain
wellhead, Christmas tree and manifold control functions, further they allow to inject chemicals as
fluidity improver or to suppress the formation of scale and hydrates in the production stream.
Umbilicals also permit communication among several platforms or bidirectional data transfer
between platforms and allocated subsea installations.
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Submarine cables, umbilicals made of Super Duplex tubing must withstand great mechanical
stresses during laying and resist against corrosion during operation
2 Oil Country Tubular Goods (OCTG)
Remaining from the early times of prospecting the expression Oil Country Tubular Goods
(OCTG) includes casing and tubing manufactured for oil well drilling operations, furthermore line
pipe, risers, umbilicals and other pipe and tube types which are essential for the production and
transport of crude oil.
Drill pipes and related drilling equipment are designed for frequent removal. “Making a round trip”,
i.e. pulling out the drill string, unscrewing the threaded joints of drill pipe, and, after a repair or
maintenance operation, reassembling and running it back in is a common work procedure while
drilling a well.
Drill pipes made of dissimilar material
Subsea production systems are engineered for ongoing exploitation and mostly expected to offer
a service life of more than 30 years. Upgrades, modifications and repair of those installations
necessitate complex interventions; they cause increased expenses and require the support of
experienced professionals with delicate skills. Operators often prefer to anticipate and invest
initially in best material, sophisticated technology and proofed quality of their production
equipment.
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2.1
Girth welding techniques
2.1.1 Manual welding
Unlike drill pipes with threaded releasable connections, the pipework of subsea installations is
joined together non-detachably by means of highly advanced welding techniques. Above all the
applied welding process must guarantee that a suitable weld quality can be achieved
continuously. At the same time reasonable costs and sufficient productivity are decisive factors.
SMAW: Welding with coated stick electrode
FCAW: Welding with self-shielding flux-core
or GMAW: MIG/MAG welding with solid wire
Due to modest investment expenditure and low requirements for infrastructure, manual welding is
still practised. In particular, even if equipment for automated welding is available, joining of tie-ins
remains often reserved to manual techniques, as the complicated geometry of the weld seam
implies sophisticated clamping, tracking and guiding systems.
If the use of shielding gas is not a concern, high quality levels at fairly good output rates can be
achieved by Gas Metal Arc Welding (GMAW) with solid wire. Pipe welding by means of the
Surface Tension Transfer process (STT), i.e. GMAW with advanced wave control of the welding
current, is extensively applied. Due to proper penetration and reliable fusion of the delicately to
weld root pass, less sputter and smoke release and good control of all positions around the tubes
the process has gained broad acceptance in pipeline welding.
Cold Metal Transfer (CMT) is another modified GMAW process. Originally developed to join
dissimilar materials like steel to aluminium, this technique has proofed its suitability for a large
number of different welding applications. Besides precise digital control of the pulsed welding
current a highly-dynamic wire feeder is mounted directly on the torch. The device initiates single
droplet detachment by periodically applied short intervals of wire retraction. Process stability,
reproducibility, cost-effectiveness and last but not least the acceptable quality of root passes led
to a widespread utilization of the system for girth welding of pipelines.
Gas Tungsten Arc Welding (GTAW), better known as TIG welding (Tungsten Inert Gas), is the
only process where the arc can be maintained completely independent from the addition of filler
metal. A flawless begin of a weld seam, interruptions and continuations of a pass at will,
remelting of an already welded section, tack welding and other ambitious operations can be
carried out in a perfect manner. If manual welding with filler metal is required, a rod of a matching
alloy can be melted by inserting it in the weld puddle. Among the cited processes, TIG welding
allows achieving joints of an unequalled quality level; unfortunately its efficiency remains fairly
low.
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Principles of TIG/GTAW Cold (CW) and Hot Wire (HW) welding with solid wire
2.1.2 Automated welding
Time-saving, increased productivity at lower costs can be achieved if the aforementioned welding
processes are carried out in conjunction with semi or fully automated equipment. Two joints of
pipes, which are manufactured e.g. with a length of 12 m each, can be welded together to get
24 m joints. This is valid for line pipes but also for dissimilar drilling pipes and other workpieces.
As these workpieces can be rotated around their longitudinally axis, economically joining by
automated TIG/GTAW welding with addition of filler material in form of Cold or Hot Wire can be
realized. TIG welding becomes often necessary if high-strength steel pipes have to be joined or
the corrosion-resistance of clad pipes must not be altered.
TIG/GTAW HOT WIRE welding for the joining of line pipes in prefabrication
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TIG/GTAW HOT WIRE welding of dissimilar metal drill pipes
If during pipeline laying from a barge the mainline is successively completed by joining pipe
segments to its end, workpiece rotation is no longer possible. Automated welding, commonly
GMAW or, if necessary, TIG/GTAW is accomplished by welding carriages also called bugs or
welding heads. Bugs and welding heads carry the welding tools, i.e. torches and accessories like
slides for oscillation and arc voltage control, wire spools etc. and trail the cables and hoses for
supply and data transfer. Toothed rails or bands are fixed besides the welding area to guide the
bugs or welding heads when travelling around the tubes. Weld cycle parameters have to be
programmed in advance; appropriate software guarantees that approved welding specifications
are respected rigorously. Based on the enhanced repeatability of the process reliable results are
achieved and allow reducing inspection efforts. Recent automated TIG/GTAW welding
installations are designed to meet specifications of 100% defect-free welds, i.e. the occurrence of
pores of any size or the smallest lack of fusion provokes rejection.
As onshore welding entails significantly lower expenses than welding operations performed on a
barge, as many as possible of the joining activities are carried out on production yards onshore.
Whenever feasible, pipes are joined together in a shop and concurrently coiled on a big spool
which is often directly mounted on a nearby docked barge. Common diameters of the coiled pipe
range from 2″ to 12″ with a total length of up to 80 km. The offshore lay method by unwinding the
pipe from the spool on the barge is known as “Reel Lay”, the important pipe length which can be
laid without interruption ensures high efficiency and short lay time. However, Reel Lay
procedures induce important mechanical loads on the coil and require outstanding quality levels
of pipes and welds.
View of a typical onshore spool base
TIG/GTAW HOT WIRE orbital welding
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S-Lay Installation on a pipelay barge
2.2
TIG/GTAW HOT WIRE orbital welding of
solid Duplex, High Alloy or CRA line pipe
Corrosion resistant alloys (CRA)
A line pipe, i.e. pipes produced to be joined together to a pipeline, is usually made of high
strength low alloy steel (HSLA). Another example is all kind of forged, casted or solid manifolds.
In many applications this type of steel does not meet the requirements of sufficient resistance
against corrosion and abrasion which are caused by the chemical composition of well fluids and
mechanical impacts of rock particles from the well. A number of alloys with a long record of
successful applications are known to provide satisfactory protection against attacks of crude oil
and accompanying constituents. These materials are referred to as Corrosion Resistant Alloys
(CRA). Comprised in this group are austenitic and martensitic stainless steels as well as nickelbase and titanium-base alloys.
TIG/GTAW HOT WIRE rigs made by POLYSOUDE SAS France for vertical weld overlay
cladding of all kind of manifolds
To get lasting wear protection, the selection of a matching CRA must rely on a precise analysis of
the particular corrosion conditions. If the flow line is intended for “Sour Service”, chloride ions,
hydrogen sulphides and carbon dioxide are the most important agents for increased corrosion.
Also temperature, pressure and flow velocity of the production stream have to be thoroughly
taken into account. In many cases it turns out that a member of the nickel base alloy family would
be the best choice. To give an idea of occurring wear, an average loss of wall thickness at the
inside of a properly designed CRA flow line during a service life of 30 years is estimated to
0.15 mm.
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CRA pipes with full length weld overlay
Bimetallic pipes with weld overlay cladded
ends “only”
Despite their excellent wear resistance the mechanical properties of nickel base alloys, if
compared to HSLA, are rather modest. To resist the stress induced during laying operations and
to exclude hydrostatic collapse in deep water, pipes manufactured entirely of this material require
important wall thicknesses. The most economical way to take advantage of nickel base alloys is
to use lined or clad pipe, where the outer wall made of HSLA absorbs the main portion of induced
mechanical stresses, whereas the inner CRA layer with a thickness of only a few millimetres
provides lasting protection against corrosion and abrasion.
2.3
CRA lined or clad steel pipe
Generally, two types of line pipes are distinguished: the pipe is referred to as lined if the outer
host and the inner CRA segment are joined together by a mechanical bond. In case of a clad
pipe, the connection between host and CRA is established by a metallurgical bond.
Clad pipe for subsea applications
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If a pipe with a smaller diameter, a liner, is introduced into another one with a slightly larger
diameter and expanded afterwards, the residual stress causes mechanical bonding. This is the
principle of manufacturing lined pipes. Assemblies composed of a steel casing with a
mechanically bonded internal CRA layer are commonly classified as CRA lined steel pipes.
In case of a metallurgical bond between inner and outer wall the product is referred to as a clad
pipe. Hot-rolled plates with a metallurgical bond between steel and CRA sheet are available on
the market; they are taken as semi-finished material at the beginning of the production process.
Certain combinations of different alloy types cannot be joined by hot rolling, in these cases the
sheets are bonded by explosive plating.
Weld overlay cladding is also widely used to produce clad line pipes. A standard steel pipe of
matching material and dimensions is used as substrate, the CRA layer at the inside is deposited
by automated welding operations.
Metalurgically bonded line pipes produced by full length weld overlay cladding with the
TIGer technology from POLYSOUDE SAS France.
2.3.1 Improving characteristics of CRA lined steel pipe
Joining of a CRA liner with a sleeve of carbon-manganese steel by hydraulic expansion is a
classic manufacturing method for CRA lined steel pipes. As the achievable bond force between
liner and outer pipe is lower than that of comparable metallurgically bonded material pairings, in
the past CRA lined steel pipes have been used exclusively for pipelines positioned on the sea
bed. Reel-laying induces additional mechanical stress to the pipes, whereas pipework of riser
constructions must provide high material fatigue resistance. In further times, for both components
only clad pipes have been admitted.
Modern manufacturing methods allowed a considerable increase of the application properties of
mechanically bonded CRA pipes. During production, the liner ends are not flush with the outer
pipe, the retract difference of about 100 mm at each extremity is covered by a high-quality TIG
weld overlay weld on the inside. Thus, the mechanical properties of the zone where the pipes are
finally to be joined together are virtually the same as that of clad pipes. Furthermore, during the
expansion of liner and sleeve the assembly is placed in a mould with stringent tolerances, so that
the operation provokes an effective calibration of the outer pipe geometry. As a result, the
reduced mismatch between the pipes allows perfect alignment and hence fatigue-resistant girth
welds.
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TIGer, Bi-cathode TIG/GTAW HOT WIRE weld overlay cladding of mechanically bonded
bimetallic pipes developed by POLYSOUDE SAS France
Horizontal TIGer weld overlay cladding station made by POLYSOUDE SAS France
2.3.2 Unique features of CRA clad steel pipes
If clad plates of the required material combination are purchased as semi-finished product,
opposite edges must be prepared to form the future welding gap. Rolling operations are used to
get the plate into a cylindrical shape; the remaining gap is closed by longitudinal welding.
Depending on the pipe dimensions SAW, GMAW including CMT or, if 100% defect-free welds are
required, TIG/GTAW/Plasma welding from the inside and/or from the outside are applied.
Improved corrosion resistance can be achieved by a final TIG overlay weld which covers the
joining zone of the CRA layer. Pipe extremities are calibrated and monitored by Laser End
Measurement Systems (LEMS) and precise protocols of geometry and dimensional tolerances
allow ensuring good alignment to assemble pipework with fatigue-resistant circumferential welds.
For this kind of manufacturing, matching plates of the required material combination must be
available. In case of less common combinations and smaller demanded quantities supply
problems can occur, especially if short delivery times are expected.
Applicable on standard steel pipes, internal CRA cladding can advantageously be carried out by
overlay welding. The horizontally positioned pipes are rotated around their longitudinal axis, the
torches with the attached wire feeding devices are mounted at the end of a welding lance and
guided along the inner wall. A recently by POLYSOUDE SAS France developed bi-cathode
TIG/GTAW cladding process named TIGer stands for a smooth surface of the corrosion-resistant
layer, low dilution rates between deposit and substrate, reliable results and improved productivity.
Based on two separately with current supplied tungsten electrodes, which are situated next to
each other in one welding torch, the resulting combined arc offers unique features concerning
high deposition rates without any losses in quality.
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12m full length horizontal TWIN-TIGer weld overlay cladding rig, made by POLYSOUDE SAS
France
Independently of the particular manufacturing process CRA clad steel pipes can be produced to
meet stringent tolerances, strict regulations and high safety standards. They are suitable for Reel
Lay procedures, construction of top tension or catenary risers and subsea installations with
extended service life.
3 Conclusion
Successful and safe offshore crude oil exploration and production depends for a large extend on
the high quality welding and weld overlay cladding of tubes, pipes and manifolds. Different
welding and weld overlay processes can be applied during the production of tubular goods and if
they have to be joined together. Successful efforts have been made to increase the productivity
of automated TIG/GTAW welding and circumferential welds of unattended quality can be realised
at affordable costs. The recently by POLYSOUDE SAS France developed TIGer technology
allows the production of fatigue and load resistant components by inside cladding of steel pipes
with corrosion resistant alloys. Whenever weld quality has the highest priority, the advantageous
application of automated TIG/GTAW processes and its variants should be taken into account.
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