By
Martijn van Driel
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.
Offshore section of the South Stream project.
|
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.
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.
Full-scale collapse test pipe.
|
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.
Source: http://www.offshore-mag.com/articles/print/volume-71/issue-8/flowlines-__pipelines/designing-large-diameter-pipelines-for-deepwater-installation.html.
Acessed by 24-1-2014
Tidak ada komentar:
Posting Komentar