To ensure
continuity of supply, E&P companies have to consider opportunities in ever
increasing water depths. Assisting this are new technological advances,
including pipeline manufacture and design that increase the technical
feasibility of deepwater developments.
Deepwater
pipeline challenges
Conventional
pipeline design, although concerned with many factors, is dominated generally
by the need to withstand an internal pressure. The higher the pressure that
products can be passed down the line, the higher the flow rate and greater the
revenue potential. However, factors critical for deepwater pipelines become
dominated by the need to resist external pressure, particularly during
installation.
Local infield
lines, such as subsea umbilicals, risers, and flowlines (SURF) usually are
modest challenges as they are small in diameter and inherently resistant to
hydrostatic collapse. In smaller sizes, these lines generally are produced as
seamless pipe which is readily available and generally economical.
However, deepwater
trunklines and long-distance tiebacks present a greater challenge. To increase
subsea production these lines tend to be larger in diameter with a thicker pipe
wall to withstand the hydrostatic pressure and bending as it is laid to the
seabed.
Typically
these lines are often 16 in. to 20 in. (40 cm to 50 cm) in diameter, which
presents a further complication as the pipe sizes lie at the top end of
economical production for seamless (Pilger) pipes. The Pilger process can
produce the thick walled pipe required for these developments but often the
manufacturing process is slow, the cost of material high, and the pipe lengths
short. As a result, the most economical method to manufacture these lines is
the UOE process. The increasingly stringent industry demands have driven this
design toward its practical limits of manufacture and installation.
Corus Tubes
has responded by manufacturing UOE double submerged arc welded (DSAW) linepipe
to the deepest pipelines in the world. This pipe overcomes significant challenges
associated with deepwater developments and facilitated a number of pioneering
projects such as Bluestream and Perdido.
In the UOE
process, steel plate is pressed into a “U” and then into an “O” shape and then
is expanded circumferentially. Wall thickness and diameter requirements for
deepwater trunkline pipe continue to be challenging for manufacturing economics
and installation capabilities.
 |
|
Distribution curve depicting ovality of Perdido pipe
(457 mm x 20.62 mm thick).
|
While few
producers manufacture UOE pipes at 16- to 20-in. outside diameter, this
manufacturing method is quicker to market and more cost-effective than seamless
alternatives. Corus Tubes’ process seeks to optimize the design of the material
and minimize the wall thickness to:
o
Reduce material cost
o
Reduce welding cost
o
Reduce installation time
o
Reduce pipe weight for logistics and submerged pipe weight considerations
o
Increase design scope enabling a wider range of deepwater developments.
Det Norske
Veritas (DNV) says the acceptability of a pipeline design for a given water
depth is determined by means of standard equations that measure the
relationship between OD, wall thickness, pipe shape, and material compressive
strength.
Pipe shape
Finished pipe
shape is optimized by balancing the manufacturing parameters, pipe compression,
and expansion. The crimp, U-press, and O-press combination ensures that the
pipe size is controlled, often beyond most offshore specifications. Enhanced
pipe “roundness”, wall thickness, and diameter tolerance removes uncertainty in
the design and production stages and allows pipe wall thickness optimization.
Compressive
strength
Pipe
manufactured by the UOE process undergoes various strain cycles, both tensile
and compressive. The combination of these cycles affects the overall behavior
of the material in compression. This is indicated in the equation given in the
offshore design standard DNV OS F101by the presence of the Fabrication Factor αfab.
For standard UOE processes, the term represents a de-rating of 15% in the
compressive strength as a result of the material response to the strain cycles
during forming, known as the Bauschinger Effect.
When material
is first placed in tension such that it is deformed plastically, the yield
stress in compression is reduced. This originally was reported by Bauschinger
in 1881. It is relevant to pipe making because during the forming process the
material is placed in tension during expansion. Following this, the material is
dispatched for installation, where the pipe sees compressive stress from the
pressure of the seawater. Conventionally, the 15% reduction in compressive
strength compensates for the Bauschinger Effect.
Since the
early 1990s, Corus Tubes has observed that the results it obtained from the
forming process often yielded higher compressive strengths than those obtained
from the standard equations. Research and process development leads to a greater
understanding of the metallurgical transformations during pipe forming. It is
possible to reverse the Bauschinger Effect to deliver pipe with compressive
strengths higher than conventionally expected.
Three things
influence the final pipe mechanical properties in compression:
1. Choice of
plate feedstock. The strength of the final pipe is a function of the chemistry
and grain structure of the mother plate from which it is fabricated. All
aspects of plate manufacture, the chemistry, rolling schedule as well as
cooling rates ensure that the final plate properties change to give the
required pipe characteristics.
2. Choice of
mill compression and expansion parameters. By optimizing the various
compression and expansion cycles, a set of manufacturing conditions can be
determined to enhance collapse performance to potentially reduce pipe wall
thickness in future deepwater applications.
3. Controlled
low temperature heat treatment. With the correct plate chemistry it is possible
to deliver a lift in compression strength through the application of a low
temperature heat treatment. This final part of the process can be measured and
assured only if the correct attention has been paid to the previous
manufacturing stages.
A number of
groundbreaking projects have pushed the boundaries of deepwater exploration and
production, and enhanced understanding of pipeline capabilities and limits. In
2000, ExxonMobil used 64 km (40 mi) of line pipe for the Hoover/Diana project
which reached depths of 1,450 m (4,800 ft). This also was the first time that
small diameter pipe from Corus Tubes’ UOE mill in Hartlepool, UK, was supplied
to the deepwater Gulf of Mexico market.
In 2001, Corus
Tubes supplied 94 km (45,000 metric tons [49,604 tons]) of three-layer
polypropylene coated, high grade, sour service linepipe and bends for the
technically challenging Bluestream project which supplies gas from Russia to
Turkey under the Black Sea. Corus also was selected to provide pipe for the
deepest section of the pipeline at 2,150 m (7,054 ft) water depth.
Corus Tubes
recently supplied line pipe to the Perdido Norte project in the Gulf of Mexico.
Williams commissioned the production of small diameter UOE pipe and
approximately 312 km (194 mi) of uncoated steel line pipe for ultra deepwater
depths from 3,500-8,300 ft (1,067-2,530 m) with a rugged seabed terrain. The
pipe, manufactured to withstand a service rating equivalent to ANSI 1500, is
one of the deepest pipelines in the world.
One section of
the pipeline transfers hydrocarbons from the FPS host in Alaminos Canyon block
857 and terminates in East Breaks block 994 (78 mi [126 km]). The gas pipeline
terminates at Williams Seahawk pipeline in East Breaks block 599 (106 mi [171
km]). The 18-in. (46-cm) diameter pipe was manufactured in wall thicknesses
ranging from 19.1 mm to 27.0 mm (¾ in. to 1 in.).
Further to the
experiences on Perdido, Corus has produced a thicker pipe at 18-in. diameter
for the Petrobras Tupi project. The pipe has a wall thickness of 31.75 mm (1 ¼
in.) and lies in a water depth of 2,200 m (7,218 ft) offshore Brazil. While
this project is not the deepest, it represents a milestone in pipe forming.
This is the thickest UOE pipe ever manufactured at 18-in. diameter (note as the
diameter of a pipe reduces and thickness increases, the levels of strain and
power required to forming it increases).
Tupi is a
testimony to the complexity of deepwater pipe design. While collapse at these
water depths is a critical design state, there also were concerns about
corrosion, since the Tupi production has some small amounts of contaminants in
the exportation gas (about 5% CO2 and a very small amount of H2S). Even though
the exported gas should be dehydrated, the CO2 raises concerns about pipe
corrosion and is managed by increasing the nominal wall thickness to account
for loss of material during life. At the end of the pipe life it still must
withstand the pressure at the seabed even with a reduced wall thickness.
The H2S,
although not expected in the exported gas, could cause cracking to occur in steels
where the grain structure and cleanliness is not optimized. In addition, high
levels of forming strain can exacerbate the situation. Corus Tubes applied its
knowledge of steel production and pipe forming to ensure that the plate it
procured from Dillinger Hutte and Voest Alpine provided ultimate resistance to
H2S corrosion.
Pipelines in
deepwater require the tightest dimensional tolerances to maximize resistance to
collapse and to maximize girth weld fatigue resistance. Furthermore, pipelines
from 16-in. to 28-in. (71-cm) are seen as the future for deepwater export
pipeline systems.
No comments:
Post a Comment