Designing large-diameter
pipelines for deepwater installation
Upcoming South
Stream project in Black Sea calls for 560 mi of 32-in. pipe in depths to 7,200
ft
Martijn van Driel
Alex Mayants
Intecsea BV
Alexey Serebryakov
OAO Gazprom
Andrey Sergienko
OAO Giprospetsgaz
Alex Mayants
Intecsea BV
Alexey Serebryakov
OAO Gazprom
Andrey Sergienko
OAO Giprospetsgaz
Gazprom has
successfully realized some of the world's largest offshore gas transportation
systems, with pipelines in the 24-in. (61-cm) diameter range traversing water
depths of more than 2,100 m (6,889 ft) with the Blue Stream I and II projects.
Now, with South
Stream, project planners are considering the challenges of installing 32-in.
(81-cm) diameter pipeline in depths that will exceed 2,200 m (7,200 ft). The
900-km (560-mi) pipeline will extend from the Russian coast to a western
landfall on either the Bulgarian or Romanian coastline. Some of the key
challenges include:
- Water depths exceeding 2,200 m (7,200 ft)
- Relatively large pipeline diameter for given water depth
- Difficult seabed conditions with steep slopes and geohazards
- Potentially aggressive/corrosive subsea environments.
The complexity of an
offshore pipeline typically is expressed in terms of the water depth and
diameter. While these are not the only drivers for a project's complexity, this
expression does provide a good insight in the position of a project in relation
to the current status of the industry.
While a 24-in.
pipeline in 2,150 m (7,053 ft) as installed for Blue Stream in 2003 was a major
challenge at the time, that project did lead to the development of technology
that is now considered proven, and similar projects have been realized in
various regions in the world. With projects like South Stream, the industry is
now exploring a new frontier and preparing for the next step.
Seabed conditions
Pipelines across the
Black Sea need to traverse a deep abyssal plain bordered by steep and sometimes
rugged continental slopes. While the deepwater of the abyssal plain leads to a
high external pressure, which is important for the wall thickness requirement,
the continental slope crossings also can be challenging, often with high risk
of pipeline spanning and geohazards.
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Offshore section of the South Stream project.
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In deepwater, the
current and wave effects are limited, causing little dynamic loading. Allowable
pipeline spans are typically longer than in shallow water and governed by local
buckling criteria. Excessive spans can be corrected either by shoulder shaving,
support placements, or combination thereof; the tooling for both seabed
intervention methods has been developed and is available.
Geohazards are
defined as features of the natural seabed that threaten the integrity of
submarine pipeline systems. Such features include submarine channels, faulting,
unstable slopes, landslides, mud volcanoes, seabed hydrates, pockmarks, debris,
and turbidity flows.
Historically, the
risk posed by such features has been eliminated often simply by routing around
them. However, for pipelines crossing a continental slope into deepwater, it
becomes less likely that all such potential hazards can be avoided. Hence,
engineering solutions must take into account the underlying geological and/or
sediment movement processes.
Geohazards can lead
to significant loads on or displacements of a pipeline. In the Black Sea, the
most relevant geohazards include:
- Faults
- Unstable slopes resulting in slumps or slides
- Mudflows / mass gravity flows
- Earthquake or wave induced liquefaction in the shore approach area
- Mud volcanoes
- Gas-expulsion features.
All of the above
features have been identified in the project area, and need to be addressed
through rigorous survey and engineering. Earthquake-induced slope stability and
mass gravity flows could pose a significant risk to the integrity of the
pipeline at the Russian continental slope, and a similar situation exists for
the western continental margin. An extensive feasibility survey has been
performed to identify these risks and to develop preliminary route options. To
further quantify these risks, it is important to perform a comprehensive design
survey campaign to capture and analyze these geohazards. This can save a
significant amount of time/costs on subsequent detailed surveys, studies, and
construction.
It is one of the
best-known Black Sea properties: deeper than approximately 150 to 200 m (490 to
656 ft), Black Sea water does not contain oxygen, but does contain dissolved
sulfuric hydride. Water mixing (driven by currents and waves) is needed for the
oxygen captured from air and generated by algae at the sea surface to reach
lower layers of the sea. In the Black Sea, there is extremely little vertical
water mixing, resulting in the world's largest stratified water body.
For the Blue Stream
project, the environment of the Black Sea was classified as sour (or “H2S
containing”) based on extensive measurement campaigns and supported by
historical research data that showed accelerated corrosion rates in parts of
the Black Sea environment. The likely cause of the corrosion was identified as
a combination of H2S and sulphate reducing bacteria (SRB). Detailed water and
soil tests are being performed for the South Stream project to establish the
chemistry of the Black Sea environment over the vertical water column, as well
as the top soil to a depth of 4 to 6 m (13 to 19.7 ft) below the seabed
surface.
Contrary to normal
sour service pipelines in which sour medium is introduced inside of the pipe,
the Black Sea environment may cause H2S exposure to the outer surface of the
pipe. This service condition applies over the system lifetime. It is difficult
to quantify, since it depends on highly localized soil conditions and
pipe/soil/water chemical interactions over the complete length and lifetime of
the system. When present, high H2S concentration is typically found at a depth
of 2 to 4 m (6.5 to 13 ft) below the seabed. Its effects on the pipe steel and
welds are being investigated.
Since there are no
concepts readily available to mitigate an external H2S-containing environment
after pipeline operation, it is essential to correctly assess the associated
risks and costs. For South Stream, this issue is being investigated in detail
through an extensive geochemical survey and analysis program, as well as a
detailed material testing and development program.
Hydraulic
performance
For a project like
South Stream, the investment involved is considerable and the ability to
transport significantly more gas at limited additional cost improves the
commercial performance of the project. Hence, an increase in diameter has
significant benefits for the project economics, enabling more gas to be
transported over longer distances. As part of project analysis, planners have
examined the typical relationship between inlet pressure and outside diameter
for different throughputs for a 900-km (560-mi) pipeline. The research showed
that a diameter increase from 24 to 32-in. allows twice the volume of gas to be
transported. While the friction loss increases exponentially for smaller
diameters, it also increases with the higher velocities required to transport
the same volume through a smaller pipe. While this figure only relates to a
typical pipeline length, the same considerations apply for shorter distance
pipelines, justifying the desire to implement larger diameter pipelines for
deep water application. For inlet pressure requirements up to 30 MPa (4,350
psi), the application of existing and field proven technologies is available.
No technology gap is foreseen.
For pipelines as
long as South Stream, the minimum allowable arrival temperature requirement can
become the governing factor rather than the pressure loss. The gas cools when
ascending the continental slope and passing through the buried shore approach
section on the receiving end. Good knowledge of pipeline settlement (and
therefore soil conditions) and concrete coating becomes important to accurately
predict the hydraulic performance of the system. In case that the in-situ
sediment at the downstream shore approach is found to be susceptible to frost
heave, it would be wise to consider engineered backfill.
The parameter that
strongly influences the system's thermo-hydraulic performance is the embedment
on the continental shelf at the receiving end. Overall, embedment in the soft,
often liquid clay of the Black Sea can easily be 50 to 100% or more of the
diameter. Thermo-hydraulic performance is verified against existing operational
information to provide additional certainty; given the importance of pipe
burial, the hydraulic analyses will be revisited after geotechnical survey
results are obtained and pipe burial has been calculated.
Another parameter
influencing the receiving temperature is the application of concrete coating.
Concrete coating provides a thermal insulation in comparison to an uncoated
pipe. One option being considered is to continue the deepwater wall thickness
up to the receiving landfall, thereby reducing the extent of concrete coated
pipe. While this would most likely result in a higher capex, the overall
throughput capacity could be improved.
Steel grade
selection
It is generally
practical to apply the highest possible line pipe grade to minimize the wall
thickness, weight, and cost of the pipeline. For deepwater offshore
applications, DNV SAWL 450 has been used in numerous sour and non-sour
conditions. DNV SAWL 485 grade has been produced almost exclusively for
non-sour service, although recent developments and trials in sour service
conditions have been initiated for small-diameter pipelines. Nevertheless,
additional qualifications for H2S-resistant application are required to ensure
the performance of DNV SAWL 485.
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Full-scale collapse test rig.
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Installability
The combination of
pipeline diameter and maximum water depth for South Stream exceeds that
previously achieved in the worldwide pipeline industry. The first issue to be
addressed in terms of overall construction feasibility is, therefore, the
ability to install the selected pipeline dimensions in the deepwater segment of
the route.
Furthermore, the
significant route length introduces additional challenges to maximize
installation efficiency. Installation of the pipeline will require extension of
the existing global pipelay installation capacity. In doing so, the success
factors and experiences from previous record-setting pipeline projects such as
Blue Stream and Nord Stream must be evaluated and applied where appropriate.
The feasibility of
the installation of the deepwater section of the route governs the overall
system construction feasibility. As part of this process, the capabilities of
the existing deepwater pipeline installation vessels are being assessed against
the deepwater installation requirements on this project. The three existing
deepwater pipeline installation vessels usually considered suitable for a
project like South Stream are the Saipem S7000, Allseas Solitaire,
and HMC Balder. Furthermore, the deepwater installation capacity
will increase in the future if several newbuild vessels are completed on
schedule. These include the Saipem FDS-2 and Castorone;
the Allseas Pieter Schelte, and a new vessel being
developed by Hereema Marine Contractors (HMC). In general, it has been
concluded that installation is feasible using the existing deepwater
installation vessel fleet. However, the assessment of the existing three
deepwater pipeline installation vessels shows that all three vessels will
require some modifications/upgrades to install the South Stream system safely
and efficiently.
Wall thickness
Core to the
capability to develop large diameter projects in deepwater is the wall thickness
design in combination with the manufacturability of the linepipe.
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Full-scale collapse test pipe.
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For the pipe
diameter and wall thickness under discussion, only two pipe manufacturing
processes are feasible: JCOE and UOE.
In the JCOE process,
the plate is formed to a J-shape using a pressed module, step-by-step at a
fixed width interval. Then using a similar method, the plate is formed to a
C-shape until it obtains an O-shape. The pipe is subjected to cold expansion
after tack weld and submerged arc welded at the inside and outside parts.
The UOE process
consists of forming the plate into U-shape and O-shape using a pressed module,
followed by tack weld and longitudinal weld of the pipe. As opposed to the JCOE
process, both the U-shape and O-shape are obtained using one-step forming.
Thereafter the pipe is cold expanded to obtain the required dimension. For both
pipe manufacturing methods, the current DNV code formulation results in a
reduction of the compressive strength after the manufacturing process, with 15%
compared with tensile strength.
The wall thickness
required for South Stream is at the limit of the leading mills' capability. One
limitation for some mills is the capacity of the pipe-forming process (such as
the capacity of the O-press). While this restriction may be avoided through a
considerable investment in upgrade of the mill, the control of pipe properties
in the weld area for such thick-walled pipes remains a major issue (in
particular parameters such as ductility and toughness). For deepwater
application, these pipe properties are critical to the pipe performance.
Achieving the desired material parameters for the wall thickness required using
standard calculation methods is on the edge of what can be produced. A small
reduction in wall thickness can result in a major improvement in
manufacturability, and thereby drive the actual feasibility of the project for
a specific throughput and OD combination.
For the deepwater
section of the pipeline, the design is governed by the local buckling criterion.
This condition occurs during installation at the pipeline sagbend where the
pipeline will experience the most extreme combination of external pressure and
bending. In the calculation of the required wall thickness for this design
limit state, the following critical technological advances can be applied:
- Recovery of collapse resistance through thermal aging
- Tighter dimensional control on line pipe manufacture
- Tight control on bending strain during installation
- A partly displacement-controlled condition is applied in the design for the sagbend.
The largest
contribution to wall thickness optimization is from the recovery of collapse
resistance through thermal aging. Pipe collapse resistance is linked to the
pipe hoop compressive strength. Many studies including small-scale and
full-scale tests have been performed in the past 20 years (for example
Oman-India, Blue Stream, and Mardi Gras), evidencing that a significant
recovery in collapse strength can be gained for DNV SAWL 450 steel (in the
order of 30%). In fact, test results suggest the collapse resistance is
recovered even beyond the original value.
Using the current
DNV F101 formulation, most mills, nowadays, indicate that they are able to
produce pipe with a significantly improved fabrication factor, incorporating
strength recovery through thermal aging. Thermal aging effect is the ability of
steel to recover its strength due to strain aging. It is possible to take
advantage of thermal aging through application of external coating, which
usually takes place at the same temperature range as where the thermal aging
process occurs.
For a deepwater,
large-diameter pipeline such as South Stream, using a thinner wall without
compromising system reliability is desirable not only for the obvious economics
in steel saving but also out of necessity, as blind compliance to the current
international design codes would result in a wall thickness that is beyond
manufacturability.
To give the owner,
designer, and manufacturer sufficient confidence, Gazprom has commissioned a
full testing program, which is currently ongoing. This testing program includes
full scale testing of as-received and thermally treated pipe joints, subjected
to combined loading of external pressure and bending.
Deepwater repair
contingencies
In the past, even
though the probability of failure of a properly planned deepwater pipeline is
small, the risk associated has been a concern because of the difficulties in
making repairs. While the effort required remains considerable, current
deepwater technology provides the tooling that allows repairs large-diameter,
deepwater pipelines. Even within the region, repair systems are available for
the water depth (Blue Stream) or diameter (Green Stream) under discussion. To
combine these into a new application is relatively straightforward, with little
technology gap.
Conclusions
A 24-in. pipeline in
2,150-m water depth or 32-in. pipelines in 1,400-m water depth are accepted by
the offshore industry as proven technologies. The South Stream project is now
investigating the feasibility of using larger diameters (such as 32-in.) in
2,200-m-plus water depths, and its successful construction will be another
step-change for the offshore industry. The use of a larger diameter will
provide obvious benefits for the project economics, allowing a considerably
higher throughput; but this requires an advance application of existing
technologies.
For the present
installation fleet, the installability of such a pipeline is complex but not
governing. This capability will be further improved if the currently scheduled
deepwater installation vessels are completed on schedule. Still, rigorous
design is essential, regardless of the selected diameter.
Key to the success
of such projects is the manufacturability of the line pipe with the requisite
wall thickness. The wall thickness required for large-diameter pipelines is on
the edge of leading mills' capabilities. Several technology advances need to be
applied to achieve feasibility, and a rigorous development program is ongoing
for successful implementation.
Acknowledgment
Based on a paper
presented at the Deep Offshore Technology International Conference and
Exhibition held on Nov. 30-Dec. 2, 2010, in Amsterdam.
08/01/2011
http://www.offshore-mag.com/articles/print/volume-71/issue-8/flowlines-__pipelines/designing-large-diameter-pipelines-for-deepwater-installation.html
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