Subsea Technology and Operations
Part 2: Subsea Production Systems
Appendix E
● Subsea Processing - “The Subsea Factory” (Copied from an article in Teknisk
Ukeblad (a Norwegian technical periodical, sources are Statoil, Aker Solutions and
Shell. Internet reference to the original article in Norwegian: http://www.tu.no/olje-
gass/2012/12/08/slik-vil-en-havbunnsfabrikk-kunne-fungere).
● Total amount of pages: 7.
Subsea technology and operations
by Jens Chr. Lindaas translated by Kjell Eivind Helgesen
revised August 2013 July 2016
Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
Figure 1: The subsea factory is the wet dream of the offshore oil and gas companies. They
now look at the possibilities of combining standalone modules into a complete subsea
production system. Photo: Statoil.
The Subsea Factory
This is how a subsea factory will work: module by module,
process by process.
By: Tale Sundlisæter
Published: December 8th 2012 17:35 - Updated: December 8th 2012 18:28
A subsea factory is a collection of processing modules placed on the seafloor. These have
been developed for years and now the possibility of combining them to a complete system is
taking shape.
With the subsea factory it is possible to optimize production from smaller fields,
contribute to developing fields at greater depths and in areas devoid of existing
infrastructure, and make it possible to extract oil and gas from fields in eco-sensitive
areas.
The facility is modular to reduce maintenance complexity. Although it is desirable to be able
to do maintenance on the entire system subsea, it might at times be necessary to replace
parts of the system and having it divided into modules will be much more efficient than the
alternative of a fully integrated system. A modular design also simplifies the installation
process.
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Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
Module A: Seawater injection with pump
Pumps on the seafloor to inject seawater into the reservoir will maintain pressure in the
reservoir and increase the available reserves.
Water injection is an efficient way of increasing production.
A separate pump module is already installed on the Tyrihans field, where the pumps inject
untreated seawater into the reservoir. The Tyrihans pumps are capable of injecting 14 000
m3 seawater every day at a pressure of 225 bar.
Not all reservoirs tolerate injection of untreated seawater. The water then has to be treated
or purified before injection.
Figure 2: Tyrihans consists of five frames where four of the frames handle production and
gas injection, and one frame handles injection of untreated seawater (Module A). Photo:
Statoil.
Module B: Separation of oil, gas and water
Separates water from the production flow. Water and sand is injected back into the reservoir,
while oil and gas is pumped along for further processing. Current separation modules are
capable of producing water with less than 1000 ppm (parts per million) of oil. This module
increases production and the extraction factor, and reduces energy costs with regards to
transporting water. In addition, the volume of water with oil content released to the ocean is
reduced.
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Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
Figure 3: Module B separataes oil and water from the wells. A full scale separation module
on the seafloor increases the extraction factor of the Tordis field with 55 %. The separation
module is 19 meters tall, 40 meters long and 25 meters wide. It weighs 1250 metric tonnes.
Photo: Statoil.
Module C: Oil pump
Pumping oil from the seafloor will increase production from reservoirs with low pressure. The
module can utilize single phase or multiphase pumping, dependent on the amount of gas in
the production flow. Single phase pumping is used when the production flow mainly consists
of liquid. Multiphase pumping can be used when there is a mixture of gas and liquid in the
production flow.
Module D: Power distribution and control
Electric power is delivered to the subsea installation through high voltage, subsea cables.
This module acts as a distribution unit, and ensure proper distribution of power to the
different modules and components in the system.
The module also ensures that the power is transformed and delivered as specified.
The control system supervises the entire subsea factory, ensuring that the maintenance
schedules are adhered to, and gathering data for analysis.
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Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
Figure 4: Module D receives electric power through high voltage cables. This module
distributes power to the different consumers. Photo: Statoil.
Module E: Gas compression
With subsea gas compression, it is possible to compress the gas on the seafloor and then
send it directly to a land based facility. By placing gas compressor on the seafloor as close
to the wellhead as possible it is possible to increase the extraction rate and the production of
gas from the well. Because the reservoir pressure decreases over time while producing, gas
compression is necessary to maintain the extraction rate from a field. Subsea compression
also reduces the total investment and operation costs. There are mainly two different types
of gas compressors used in a subsea factory:
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Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
Figure 5: Module E: The subsea compression in the Åsgard field is the very first of its kind,
and will be completed in 2015. This technology is one of the most important breakthroughs
for production from existing fields on the Norwegian Continental Shelf. Photo: Aker
Solutions.
By using dry gas compressors, gas and liquid will be separated before compression. The
pressure in the liquid is increased by use of a pump, while the pressure in the gas is
increased by use of a compressor. Afterwards, gas and liquid is mixed and sent into the
same pipeline for transportation. The dry gas compressor system that Statoil uses on the
Åsgard field will be able to increase the pressure with 65 bars, using 11.5 MW to increase
the gas pressure, and 0.6 MW to increase the liquid pressure.
The other, already existing alternative, is wet gas compression, where the pressure increase
of gas and liquid is combined. The gas and liquid should be well mixed before increasing the
pressure. Wet gas compression was developed by Statoil for the Gullfaks field, and is
capable of increase the pressure with 32 bars, using about 5 MW, and with a maximum flow
rate of 6000 m3/hour.
Module F: Remotely operated and unmanned vehicles
Remotely operated vehicles are the eyes and hands of the subsea factory. An ROV is used
to install, retrieve and inspect equipment. ROVs are connected to a surface vessel through a
cable.
Next generation ROVs will be more autonomous. They will be able to remain
submerged for a longer period, and receive power (charging) from the power
distribution and control module (module D).
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Subsea Technology, part 2, appendix E: Subsea Processing - “The Subsea Factory”
An AUV (Autonomous Underwater Vehicle) will be able to move freely along the subsea
structures, without any physical connection to the surface.
Figure 6: Module F: An ROV (Remotely Operated Vehicle) will be used during installation,
retrieval and inspection of equipment. The ROV is connected to the pilot on the surface
vessel through a cable. Photo: Statoil.
Many challenges
Before it is possible to build a subsea factory, several technological challenges have to be
solved:
A reliable power supply will be crucial to run the system, with high voltages and very long
distances.
It is necessary to ensure that all rotating equipment is reliable in full scale, and capable of
operating continuously at great depths over a long period. It is therefore advantageous to
limit the amount of moving parts. As an example the Åsgard field has to have an availability
(operational time) of 96 % through 20 to 25 years, which is quite a challenge even for a land
based compression facility.
A regular rig usually employs several stages of separation to refine the product, something
that is yet to be developed for the subsea factory. With subsea technology it is, however,
enough to separate the products into transportation quality, so that further separation and
refining can be done on land.
According to a study done on Åsgard, the CO2 emissions can be halved by using a
subsea solution. Because it is autonomous and unmanned, human transportation and
housing will be removed from the equation. The subsea factory is therefore more
economical, safe and ecological than a regular offshore installation.
Sources: Statoil, Aker Solutions and Shell.
http://www.tu.no/olje-gass/2012/12/08/slik-vil-en-havbunnsfabrikk-kunne-fungere
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