Note: Descriptions are shown in the official language in which they were submitted.
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OFFSHORE HYDROCARBON PROCESSING FACILITY
AND METHOD OF OPERATION
The present invention relates to an offshore hydrocarbon processing facility.
A main source of hydrocarbons is offshore oil wells located on the sea bed.
Sea
bed installations have risers through which hydrocarbons can flow towards a
surface
installation, which is either a fixed or a floating installation at or near
the surface of the
sea.
At the surface installation, the hydrocarbons are processed, for example, by
separating natural gas from liquid hydrocarbons and often compressing the
natural gas
into a liquefied state. These products can then be transferred from the
surface
installation to tankers or other floating vessels, for subsequent transport to
the shore.
Another possibility is that some products are processed at the sea bed and may
be piped via a pipeline to shore, without the need for flowing through a riser
to the
surface of the sea. One problem with this set-up is that the sea bed is often
2-3km
below the surface of the water. This means that it is complicated to carry out
maintenance work at the sea bed installation, particularly maintenance of the
seabed
processing equipment.
Another problem is that surface conditions offshore are more severe in terms
of
weather, water and ice. For example, high winds, precipitation, hurricanes,
monsoons,
icebergs, waves and ice floes and other adverse conditions can cause
weathering and
damage to the riser near the surface of the sea.
According to a first aspect of the present invention, there is provided an
offshore
hydrocarbon processing facility comprising: an offshore floating structure; a
submerged
floating riser deck operatively connected to a subsea hydrocarbon riser; and
hydrocarbon processing equipment disposed on the submerged floating riser
deck.
With this arrangement, hydrocarbons can flow from the sea bed to the
submerged floating riser deck for processing there. This reduces or eliminates
the need
for processing equipment at the sea bed and so less maintenance will be
required there.
Moreover, since the submerged riser deck is floating, it is not as far
underwater as the
sea bed and so the processing equipment on the submerged floating riser deck
which
needs to be maintained is much easier to maintain. Since hydrocarbon
processing can
take place at the submerged floating riser deck, the risers, hydrocarbons and
processing
equipment are unaffected by sea surface conditions such as wind, waves,
precipitation
and ice floes.
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The submerged floating riser deck may be located at a depth of between 180m
and 500m below the surface of the sea. The sea bed may be between 200m and
4000m deeper than the floating riser deck. Thus the effective level for
maintenance can
be raised from the sea bed to the more easily accessible location of the
submerged riser
deck. The riser may be a riser of the conventional type used in offshore
hydrocarbon
installations.
The submerged floating riser deck is supported in the water in a freely
floating
fashion. ft may float suspended due to its own buoyancy or it may be coupled
to positive
buoyancy elements. It may hang suspended from the offshore floating structure,
and
thus may float together with the offshore floating structure. This can be
aided by
buoyancy of the offshore floating structure and/or by positive buoyancy
elements
connected to the offshore floating structure to aid floating. Thus, the
submerged floating
riser deck in example embodiments is not tethered to the sea bed using tension
cables.
Instead it may be anchored to the sea bed in a way that permits free floating,
such as via
a catenary mooring system, which may include catenary anchor cables or
catenary
risers. The catenary mooring system may be coupled to the submerged floating
riser
deck and/or to the offshore floating structure. As is known in the art it is
possible to hold
an offshore floating structure in a generally fixed location whilst permitting
movement
with tide and wave motion by using a catenary mooring of various types.
Movement
may also be permitted to avoid drifting ice, as discussed below. The presence
of a
catenary mooring arrangement as opposed to tension cables forming a taut
mooring
arrangement can provide one way to distinguish a freely floating structure
from a tautly
moored structure with positive buoyancy. The latter has a tendency to float
but is not
freely floating due to the moorings used.
The offshore hydrocarbon processing facility may comprise a riser arranged for
disconnectable connection between the offshore floating plafform and the
submerged
floating riser deck. This may be a catenary riser as mentioned above.
With this arrangement, when the surface conditions are not damaging, a riser
may carry processed and/or unprocessed hydrocarbons from the submerged
floating
riser deck up to the offshore floating platform, for processing and/or
transfer to a tanker,
floating vessel or the like. When the surface conditions are deemed to be
potentially
damaging, the riser can be disconnected from the offshore floating platform,
to avoid
damage.
The floating structure (e.g. the platform or spar buoy) may have a mooring
system enabling safe use of the structure where drifting ice may be present,
such as in
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artic/sub-arctic regions. In one arrangement this may be based on an
adjustable
mooring system or an active mooring system that enables the platform to move
horizontally to avoid drifting ice without disconnecting from the risers. In a
second
possibility a partly disconnected mooring solution may be used for the
floating structure.
In some examples, depending on the nature of the possible horizontal movement
of the
floating structure it may be advantageous for the floating structure to
partially or fully
disconnect from the submerged riser deck. For example, this may involve the
use of
disconnectable risers between the submerged riser deck and the floating
structure,
and/or a disconnectable mooring system coupling the submerged riser deck to
the
floating structure. In the case where full disconnection is required then the
submerged
riser deck should be able to float independent of the floating structure.
Thus, as noted
above the submerged riser deck may have its own inherent buoyancy or it may be
supported by additional positive buoyancy elements, such as floats tethered to
the
submerged deck.
The offshore hydrocarbon processing facility may be an unmanned hydrocarbon
processing facility. This may be unmanned in that it has at least one of: no
permanent
personnel; no provision of facilities for personnel to stay on the offshore
hydrocarbon
processing facility, for example there may be no shelters for personnel, no
toilet
facilities, no drinking water, no personnel operated communications equipment
and/or
no lifeboat; and/or a requirement that personnel be present for fewer than
10,000
maintenance hours per year.
The offshore floating structure may be any structure suitable for use in an
offshore hydrocarbon processing facility. It may be an offshore floating
platform, which
in this sense is taken to include structures such as spar buoys as well as
platform
structures with decks. The offshore floating structure can float with a
proportion of the
structure above sea level and a proportion of the structure below sea level.
As
discussed above the offshore floating structure may provide buoyancy for
supporting the
submerged floating riser deck.
The offshore floating structure may be an unmanned structure such as an
unmanned platform. In some examples it is an unmanned production platform. The
structure may be unmanned in that it has at least one of: no permanent
personnel; no
provision of facilities for personnel to stay on the platform, for example
there may be no
shelters for personnel, no toilet facilities, no drinking water, no personnel
operated
communications equipment and/or no lifeboat; and/or a requirement that
personnel be
present for fewer than 10,000 maintenance hours per year.
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An unmanned offshore floating structure has no permanent personnel and may
only be occupied for particular operations such as maintenance and/or
installation of
equipment. The unmanned offshore floating structure may be a structure where
no
personnel are required to be present for the structure to carry out its normal
function, for
example day-to-day functions relating to handling of oil and/or gas products
at the
structure.
An unmanned offshore floating structure may be a structure with no provision
of
facilities for personnel to stay on the platform, for example there may be no
shelters for
personnel, no toilet facilities, no drinking water and/or no personnel
operated
communications equipment. The unmanned offshore floating structure may also
include
no heli-deck and/or no lifeboat, and advantageously may be accessed in normal
use
solely by a gangway or bridge connected to a service vessel or to another
offshore
floating structure. For example, the unmanned offshore floating structure may
be
accessed in normal use solely via a Walk to Work (W2W) system.
An unmanned offshore floating structure may alternatively or additionally be
defined as unmanned based on the relative amount of time that personnel are
needed to
be present on the offshore floating structure during operation. This relative
amount of
time may be defined as maintenance hours needed per annum, for example, and an
unmanned structure may be a structure requiring fewer than 10,000 maintenance
hours
per year, optionally fewer than 5000 maintenance hours per year, perhaps fewer
than
3000 maintenance hours per year.
All of the hydrocarbon processing equipment for the offshore hydrocarbon
processing facility may be entirely disposed on the submerged floating riser
deck. in this
arrangement, the processing can all take place in a submerged location
unaffected by
surface conditions such as waves, wind, precipitation or ice floes. The
processed
products are then ready for transfer to floating vessels, tankers or the like
whenever
surface conditions are appropriate, and/or for piping along pipelines to
shore.
Alternatively, the offshore floating structure may comprise topside
hydrocarbon
processing equipment located on the offshore floating structure in addition to
the subsea
hydrocarbon processing equipment. The hydrocarbon processing facility may be
configured such that a hydrocarbon fluid to be processed is processed first by
the
subsea hydrocarbon processing equipment and then by the topside hydrocarbon
processing equipment. This may involve just a part of the hydrocarbon fluid
being
passed to the offshore floating structure from the subsea riser deck, with
another part of
the hydrocarbon fluid, which may be a product of the processing at the subsea
riser
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deck, being retained at the subsea riser deck or directed to a destination
other than the
offshore platform. This other destination may for example be a buoy as
discussed
below.
In this arrangement, all of or a part of the processing can take place at the
subsea riser deck before the hydrocarbons are transferred to the offshore
floating
structure, allowing processing to carry on even during adverse surface
conditions.
The hydrocarbon processing equipment at the subsea riser deck and/or at the
floating structure (where present) may be arranged to produce at least one of
a
hydrocarbon liquid product, a hydrocarbon semi-stable liquid product and a
liquefied gas
product.
A semi-stable liquid product may consist of any or all of water, oil,
condensate,
Liquid Petroleum Gas, LPG, and Natural Gas Liquids, NGL. A liquefied gas
product
may be stored under pressure or may be exported via a pipeline to the shore or
a host
such as a floating vessel or tanker.
The processing equipment at the submerged floating riser deck may be arranged
to separate liquid hydrocarbons, flash the separated liquid hydrocarbons and
store the
flashed hydrocarbons at the submerged floating riser deck. In this
arrangement, this
processing step can be done underwater and so avoid the need for transferring
all of the
hydrocarbons to the offshore floating structure.
The submerged floating riser deck may comprise a tank for storage of
hydrocarbons, for example for storage of unprocessed or processed
hydrocarbons, i.e.
hydrocarbons before or after processing with the processing equipment at the
=
submerged floating riser deck. In some examples there is a tank for storage of
the
flashed hydrocarbons.
The offshore floating structure may be horizontally offset from the submerged
floating riser deck, such that the two are not vertically aligned.
With this arrangement, an intervention vessel, such as a tanker, floating
vessel
or the like, may position itself directly above the submerged floating riser
deck, for direct
communication with the submerged floating riser deck. This allows more direct
communication between the submerged floating riser deck and an intervention
vessel,
without the need to communicate all fluids via the offshore floating platform.
The offshore hydrocarbon processing facility may comprise a buoy in addition
to
the offshore floating structure, wherein the submerged floating riser deck may
be
operatively connected to the buoy for offload of hydrocarbon products to a
floating
vessel.
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With this arrangement, hydrocarbons do not need to be transferred via the
offshore floating structure before being transferred to a floating vessel,
tanker or the like,
The processing equipment at the submerged floating riser deck may comprise a
separator. The separator may also act as a drain for the hydrocarbon
processing
equipment and piping, and hence may be at a low point in relation to other
parts of the
processing equipment.
Electric power for the submerged riser deck may all be provided by submerged
and topside units, i.e. units above the surface of the sea on the offshore
floating
platform. Alternatively, there may be local Hydraulic Power Units, HPU, subsea
for
providing the power. The power system may include Variable Speed Drive, VSD,
for
subsea pumps.
According to a second aspect of the invention, there is provided a method of
operating the offshore hydrocarbon processing facility of the first aspect,
comprising:
receiving hydrocarbons at the submerged floating riser deck from the subsea
hydrocarbon riser; and processing the hydrocarbons using the hydrocarbon
processing
equipment.
This method may include the use of any or all of the features described above
in
relation to the first aspect, thus achieving the same advantages.
The method may include disconnectably connecting the submerged floating riser
deck to the offshore floating structure using a disconnectable riser for
transfer of
hydrocarbons between them.
This allows a riser to be disconnected from the offshore floating structure
when
the surface conditions such as wind, precipitation, waves, ice floes and the
like are
deemed to be potentially damaging. When the surface conditions are not deemed
to be
damaging, the riser can carry up processed and/or unprocessed hydrocarbons
from the
submerged floating riser deck to the offshore floating structure, for
processing and/or
transfer to a tanker, floating vessel or the like.
The method may comprise fluidly connecting the submerged floating riser deck
to a floating vessel for transfer of hydrocarbons from the submerged floating
riser deck
to the floating vessel; and/or fluidly connecting the submerged floating riser
deck to a
pipeline for transfer of hydrocarbons from the submerged floating riser deck
via the
pipeline. The pipeline may be for transporting the hydrocarbons to shore.
With this method, hydrocarbons can be transported either directly from the
submerged floating riser deck to shore, or via a floating vessel, tanker or
the like to
shore, or both. There is no necessity to transfer the hydrocarbons via the
offshore
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floating structure. In this manner, the hydrocarbon risers and the like need
only
minimally emerge, or not emerge at all from the surface of the sea and thus
can reduce
or avoid damage due to surface conditions.
The method may comprise fluidly connecting the submerged floating riser deck
to a buoy; and transferring hydrocarbons from the buoy to a floating vessel.
In some instances, a permanent buoy can be used for floating vessels to
connect
to for transfer of hydrocarbons. This arrangement allows the hydrocarbons to
remain
almost entirely submerged until the buoy and thus significantly avoids damage
due to
adverse surface conditions.
The method may comprise: separating and flashing liquid hydrocarbons at the
submerged floating riser deck; and storing the flashed hydrocarbons in storage
tanks at
the submerged floating riser deck. This processing step can be done underwater
and so
avoid the need for transferring all of the hydrocarbons to the offshore
floating structure.
The method may further comprise drying and pressuring the gas obtained from
the received hydrocarbons, for example as set out in W02016192813 Al.
Optionally, a
water stream obtained from the drying process may be reinjected into a well,
for
example via the riser.
Certain preferred embodiments of the present invention will now be described
in
greater detail by way of example only and with reference to the accompanying
drawings,
in which:
Figure 1 shows an offshore hydrocarbon processing facility where an offshore
floating structure includes a platform with decks; and
Figure 2 shows another offshore hydrocarbon processing facility where an
offshore floating structure is a spar buoy.
Figure 1 shows an offshore hydrocarbon processing facility 2, located
offshore,
comprising an offshore floating structure 8 (designed to be unmanned as
discussed
above) and a submerged floating riser deck 10. The offshore floating structure
8 is a
platform with decks that may hold equipment as required for the intended use
of the
offshore facility 2. The submerged floating riser deck 10 is operatively
connected to a
number of subsea hydrocarbon risers 12, through which hydrocarbons can flow
from the
seabed 4, The hydrocarbon risers 12 may be steel catenary risers, supported by
buoyancy aids 14. The submerged floating riser deck 10 comprises hydrocarbon
processing equipment 26 for processing hydrocarbons received via the
hydrocarbon
risers 12. The submerged floating riser deck 10 is submerged, i.e. fully below
sea level
6, but remains floating through the use of buoyancy aids 18 and/or through
suspension
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from the offshore floating structure 8. The platform 8 can be secured to the
seabed via a
catenary mooring system (not shown).
Floating near the surface of the sea (sea level 6) and at least partially
protruding
above the surface of the sea level 6 is the offshore floating platform 8,
which has
hydrocarbon processing equipment 24. The offshore floating platform 8 is
connected to
the submerged floating riser deck 10 via several risers 16. The risers 16 can
be
disconnected from the offshore floating platform 8 in case of inclement
surface
conditions.
Figures 2, 3 and 4 show another hydrocarbon processing facility where the
offshore floating structure 8 is a spar buoy 8. In this example the spar buoy
8 floats due
to its own buoyancy and the submerged floating riser deck 10 is suspended from
the
spar buoy 8 via a cable suspension system 32. The submerged floating riser
deck 10 is
fully supported by the cable suspension system 32 and does not need any
additional
positive buoyancy elements. The spar buoy 8 is moored via catenary mooring
cables 30
and hydrocarbons are transported between the seabed 4, the submerged floating
riser
deck 10 and the spar buoy (offshore floating structure) 8 via catenary
hydrocarbon risers
12 and disconnectable risers 16 in a similar way to Figure 1. The catenary
risers 12 are
supported by buoyancy aids 14.
Figures 2 and 3 show an example of this spar buoy system at a realistic scale
and also include vessels servicing the offshore facility 2, along with subsea
structures to
which the offshore facility is connected. Figure 4 shows a similar system in a
more
schematic arrangement similar to Figure 1, with the scale distorted and the
catenary
mooring cables 30 omitted from the drawing.
In relation to the offshore facilities 2 of both of Figure 1 and Figure 2 the
hydrocarbon processing facility 2 has a buoy 20 connected to the offshore
floating
platform 8 for transferring gas or pressured and cooled gas therefrom to a
tanker,
floating vessel or the like which can connect to the buoy 20.
The hydrocarbon processing facility 2 also has a buoy 22 connected to the
submerged floating riser deck 10 for transferring semi-stable liquid therefrom
to a tanker,
floating vessel 34 or the like, as shown in Figure 2, which can connect to the
buoy 22.
In operation, hydrocarbons flow from the seabed 4 through the risers 12 to the
submerged floating riser deck 10. Here, the hydrocarbons may be fully
processed,
partially processed or not processed at all. Any processing is carried out by
the
hydrocarbon processing equipment 26. One or more storage tanks at the
submerged
floating riser deck 10 can be used for storage of fully processed, part
processed or
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unprocessed hydrocarbons. In the exemplary diagram shown in the Figures, the
hydrocarbons can be processed out by the hydrocarbon processing equipment 26
at the
subsea riser deck 10 to produce two products, namely a gaseous product and a
semi-
stable liquid product. The former is transferred to the offshore floating
structure 8 via the
disconnectable risers 16, while the latter is transferred to the semi-stable
liquid buoy 22.
The semi-stable liquid product may comprise any or all of water, oil
condensate, LPG
and NGL.
At the offshore floating structure 8, the received gas product is optionally
processed using hydrocarbon processing equipment 24. The processing may
include
drying and/or cooling and pressurising. The final product, which may be
liquefied, is
transferred to buoy 20. Alternatively, the offshore facility 2 may be arranged
so that all
of the required processing is done at the submerged riser deck 10 with no
processing
equipment on the offshore floating structure 8. This can allow the offshore
floating
structure 8 to be a smaller floating platform such as the spar buoy 8 of
Figure 2.
Tankers, other floating vessels and the like can connect to buoys 20 and 22 to
be
loaded with the gas and semi-stable liquid products respectively.
In the case of inclement weather conditions at sea level 6, such as wind,
waves,
hurricanes, tropical monsoons, precipitation, icebergs, ice floes or other
conditions
deemed to be harmful to the riser 16, the riser 16 can be disconnected from
the offshore
floating structure 8 for the duration of the harmful conditions, and can be
reconnected
once these conditions pass.
Any maintenance work which needs to be carried out on the submerged
hydrocarbon processing equipment 26 can be done more easily than in the prior
art
where the equipment is at the sea bed, since the hydrocarbon processing
equipment 26
is not as deep underwater as the sea bed 4.
In some embodiments the offshore floating structure does not have any role in
processing the hydrocarbons that are processed at the submerged floating riser
deck.
For example, the spar buoy Thus, hydrocarbons may be processed at the
submerged
riser deck and transferred elsewhere, via the offshore floating structure or
via other
means such as the buoys, without any processing aboard the floating structure.
