Challenges to
manufacture of pipe for deepwater, corrosive hydrocarbons
Richard Freeman -
Corus Tubes Energy
Gas is increasingly
important in a historically oil-driven world economy. Its increased value is a
driver of pipeline technology developments. To meet the demand for gas
transportation through more onerous environments, there are factors the pipe
and plate makers need to consider to ensure the finished product meets the
standards required especially for sour service and deepwater applications.
One trend driving
pipeline demand is the gas production from deepwater fields. Traditionally this
gas would either be flared or re-injected into the well for enhanced recovery.
However, operators now are keen to capture this production and trade it as
either liquefied natural gas (LNG) or domestic gas. These gas-gathering
prospects present challenging combinations of deepwater installation, corrosive
well fluids, and difficult shore approach conditions. These all combine for
demanding pipe specifications for manufacturers to meet.
These requirements
can be met only with a holistic technical approach from plate procurement to
pipe dispatch. The foundation of this approach is to use the highest quality
sour plate, which is delivered using state-of-the-art primary and secondary
steel making, continuous casting, and proper plate rolling practices. During
UOE (U-ing, O-ing, and Expanding) pipe manufacture, the forming process is
optimized so strain is managed to minimize any reduction in sour resistance.
For these demanding applications, low-temperature toughness in the heat
affected zone, demanding hardness, and Battelle drop weight tear test
requirements commonly are specified. In combination with forming, welding using
optimum consumables and design parameters ensures the mechanical properties and
integrity of the pipe.
Gas-gathering in
West Africa
Corus recently completed
a series of gas-gathering development projects in West Africa to link fields
and to transport the gas for export as LNG. In total, the company supplied
81,000 metric tons (89,287 tons) of thick-walled, sour-resistant steel linepipe
to three projects
The pipe, ranging
from 457 mm (18 in.) to 610 mm (24 in.) in diameter and up to 33.5 mm (1.3 in.)
in wall thickness, is to transport gas in water depths of up to 1,500 m (4,921
ft) over difficult seabed bathymetry and also with critical shore approach areas.
Corus exported the pipe from its Hartlepool 42 in. capacity mill in the UK to
West Africa where the project is being completed with first gas scheduled for
2012.
Thick-walled sour
service pipe manufacturing
Gas lines typically
are larger diameter and generally constructed from welded linepipe – the most
economical production method. However, for deepwater prospects, the parameters
for gas transmission are restricted by the following:
- The offshore lay process and the need for speedy, reliable welding restricts the line chemistry to strength grades at X65 or below
- Seabed stability restricts the diameter of the line that can be installed – larger diameter pipe is more buoyant and less stable
- Larger diameter pipe is more vulnerable to hydrostatic collapse, meaning wall thickness needs to be increased
- Wall thickness also needs to be increased because of corrosion concerns and fatigue life considerations.
Corus supplied 81,000 metric tons (89,287 short tons) of
thick-walled, sour resistant steel
linepipe to three projects offshore West
Africa.
|
These reasons drive
a need for thicker pipe wall with higher induced strain during forming, but
pipe which also conforms to international standards such as DNV, ISO, and API.
Successful manufacture of these pipes needs not only an expert understanding of
steel and pipe making but also an appreciation of the service demands.
Challenges of
pipe forming
During service, the
pipe bore is exposed to a wet, sour (H2S) environment. Atomic hydrogen is
generated at the pipe surface via a cathodic reaction, and enters the steel.
Migrating hydrogen atoms move through the structure, gather and combine with
each other at discontinuities, voids, and susceptible zones in the
microstructure to produce molecular hydrogen (H2). The increasing quantity of
H2 at the initiation site creates a high hydrogen pressure, which can be
magnified by the shape of the site, leading to a stress concentration that
ultimately “cracks” the microstructure.
Strain compromises
sour service phenomena such as hydrogen induced cracking (HIC), and with the
industry looking for more stringent sour resistant ratios, pipe milling
influence on these factors need to be understood.
Total micro-strain
from forming could contribute to an increase in the number of available sites
for molecular hydrogen formation throughout the microstructure. Therefore, the
effects of compression and expansion may have to be considered as cumulative.
Control of these features within the microstructure is essential to ensure the
pipe’s sour performance is achieved.
The sour resistance
of the plate is imparted via the chemistry and microstructure. Most modern
steelmakers agree that to balance the mechanical properties needed with sour
resistance, the required microstructure is a very clean, fine-grained,
equiaxed/polygonal, or acicular ferrite structure with limited volumes of
secondary phases such as an artensite/austenite (M/A) phase.
|
|
|
Fine grained equiaxed/acicularferrite structure.
|
To deliver optimum
sour properties in the final pipe, attention needs to be paid to each stage of
the process from steel making to final pipe fabrication. During steel making,
the process must be monitored where the material is treated prior to casting
with the correct composition, homogeneity, and temperature suitable for HIC
resistant quality.
Casting is integral
to ensuring sufficient quality for plate rolling to HIC grade. This includes
controlling macro-segregation, which occurs as steel transitions from the
liquid to the solid phase, achieved through soft reduction, Statistical Process
Control, and Caster configuration processes.
In terms of plate
rolling, single-phase austenitic rolling is favored to meet the sour service
and drop weight tear test (DWTT) requirements of a thick wall for offshore
projects. However, recent experience shows that material with a higher
proportion of acicular ferrite in the microstructure can be susceptible to a
phenomenon known as “inverse fracture” with associated low shear values, which
has not been seen previously in bainitic/acicular ferrite structures. A program
is under way to understand this behavior and to determine whether DWTT is a
viable evaluation of the resistance to long running brittle fracture for these
steels.
Pipe making
While the amount of
strain imparted to form the pipe is set by dimensions, there are key parameters
to consider, specifically strain management when forming and welding.
Control of shape and
formability is required to ensure a consistent product; poorly controlled
forming leads to variable strain effects within each pipe. The forming in the
crimp, U- and O-press, and subsequent expansion must be accurate and consistent
to ensure each pipe produced is representative of the pipeline as a whole.
Suitable welding
consumables are selected to achieve the weld hardness and toughness
requirements, and to deliver good HIC performance across the weld. For
toughness, a moderate manganese wire is used with alloying additions of silicon
and molybdenum; titanium and boron also can be used, depending on the toughness
required. The wire is combined with a high-performance, semi-basic and fully
agglomerated flux, which combine to promote formation of acicular ferrite in
the weld bead, and confer good Charpy and crack tip opening displacement (CTOD)
toughness at low testing temperatures while maintaining a stable welding
performance.
In addition to the
mechanical performance of the weld, a high level of integrity must be
maintained through production. This means low levels of slag entrapment and gas
defects, for example, as well as cracks to ensure a clean seam is presented to
the welding machine to avoid gas defects. The weld arc and flux burden must be
sufficiently stable to minimize slag entrapment.
Future trends
The question remains
whether these pipelines will continue to be required as technology offers other
methods to transport gas such as FLNG. However, the diversity of the offshore
industry almost certainly means a variety of technologies both old and new will
be used in the future.
Deploying an FLNG
liquefaction vessel directly to a field similar to an FPSO for oil, may remove
the need in some instances for gas export pipeline projects, but infield subsea
connections still will be needed.
Additionally,
regassification and liquefaction are being considered for some applications
offshore, opening further pipeline prospects for product transfer from ship to
shore. These offshore pipelines are likely to have demanding specifications,
crossing high-risk shore approach areas and shallows. Additionally, the
increasing trend towards deepwater production means the linepipe must
counteract higher concentrations of impurities, driving the need for products
to meet severe sour conditions.
03/01/2010
Tidak ada komentar:
Posting Komentar