Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02544362 2006-04-20
SYSTEM AND METHOD OF INSTALLING AND MAINTAINING AN
OFFSHORE EXPLORATION AND PRODUCTION SYSTEM HAVING
AN ADJUSTABLE BUOYANCY CHAMBER
FIELD OF THE INVENTION
The present invention relates generally to oil and gas exploration and
production, and in a specific, non-limiting embodiment, to a system and method
of
installing and maintaining an offshore exploration and production system
having an
adjustable buoyancy chamber.
BACKGROUND OF THE INVENTION
Innumerable systems and methods have been employed in efforts to find and
recover hydrocarbon reserves around the world. At first, such efforts were
limited to
land operations involving simple but effective drilling methods that
satisfactorily
recovered reserves from large, productive fields. As the number of known
producing
fields dwindled, however, it became necessary to search in ever more remote
locales,
and to move offshore, in the search for new resources. Eventually,
sophisticated
drilling systems and advanced signal processing techniques enabled oiI and gas
companies to search virtually anywhere in the world for recoverable
hydrocarbons.
Initially, deepwater exploration and production efforts consisted of
expensive,
large scale drilling operations supported by tanker storage and transportation
systems,
due primarily to the fact that most offshore drilling sites are associated
with difficult
and hazardous sea conditions, and thus large scale operations provided the
most stable
and cost-effective manner in which to search for and recover hydrocarbon
reserves. A
major drawback to the large-scale paradigm, however, is that explorers and
producers
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have little financial incentive to work smaller reserves, since potential
financial
recovery is generally offset by the lengthy delay between exploration and
production
(approximately 3 to 7 years) and the large capital investment required for
conventional platforms and related drilling and production equipment.
Moreover,
complex regulatory controls and industry-wide risk aversion have led to
standardization, leaving operators with few opportunities to significantly
alter the
prevailing paradigm. As a result, offshore drilling operations have
traditionally been
burdened with long delays between investment and profit, excessive cost
overruns,
and slow, inflexible recovery strategies dictated by the operational
environment.
More recently, deepwater sites have been found in which much of the danger
and instability present in such operations is avoided. For example, off the
coast of
West Africa, Indonesia and Brazil, potential drilling sites have been
identified where
surrounding seas and weather conditions are relatively mild and calm in
comparison
to other, more volatile sites such as the Gulf of Mexico and the North Sea.
These
recently discovered sites tend to have favorable producing characteristics,
yield
positive exploration success rates, and admit to production using simple
drilling
techniques similar to those employed in dry land or near-shore operations.
However, since lognormal distributions of recoverable reserves tend to be
spread over a large number of small fields, each of which yield less than
would
normally be required in order to justify the expense of a conventional large-
scale
operation, these regions have to date been underexplored and underproduced
relative
to its potential. Consequently, many potentially productive smaller fields
have
already been discovered, but remain undeveloped due to economic
considerations. In
response, explorers and producers have adapted their technologies in an
attempt to
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achieve greater profitability by downsizing the scale of operations and
otherwise
reducing expense, so that recovery from smaller fields makes more financial
sense,
and the delay between investment and profitability is reduced.
For example, in published Patent Application No. US 2001/0047869 Al and a
number of related pending applications and patents issued to Hopper et al.,
various
methods of drilling deepwater wells are provided in which adjustments to the
drilling
system can be made so as to ensure a better recovery rate than would otherwise
be
possible with traditional fixed-well technologies. However, the Hopper system
cannot be adjusted during completion, testing and production of the well, and
is
especially ineffective in instances where the well bore starts at a mud line
in a vertical
position. Tl~e Hopper system also fails to support a variety of different
surface loads,
and is therefore self limiting with respect to the flexibility drillers desire
during actual
operations.
In U.S. Letters Patent No. 4,223,737 to O'Reilly, a method is disclosed in
which the problems associated with traditional, vertically oriented operations
are
addressed. The method of O'Reilly involves laying out a number of
interconnected,
horizontally disposed pipes in a string just above the sea floor (along with a
blow out
preventer and other necessary equipment), and then using a drive or a remote
operated
vehicle to force the string horizontally into the drilling medium. The
O'Reilly
system, however, is inflexible in that it fails to admit to practice while the
well is
being completed and tested. Moreover, the method utterly fails to contemplate
functionality during production and workover operations. In short, the
O'Reilly
reference is helpful only during the initial stages of drilling a well, and
would
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therefore not be looked to as a systemic solution for establishing and
maintaining a
deepwater exploration and production operation.
Other offshore operators have attempted to solve the problems associated with
deepwater drilling by effectively "raising the floor" of an underwater well by
disposing a submerged wellhead above a self contained, rigid framework of pipe
casing that is tensioned by means of a gas filled, buoyant chamber. For
example, as
seen in prior U. S. Letters Patent No. 6,196,322 B 1 to Magnussen, the
Atlantis
Deepwater Technology Holding Group has developed an artificial buoyant seabed
(ABS) system, which is essentially a gas filled buoyancy chamber deployed in
conjunction with one or more segments of pipe casing disposed at a depth of
between
600 and 900 feet beneath the surface of a body of water. After the ABS
wellhead is
fitted with a blowout preventer during drilling, or with a production tree
during
production, buoyancy and tension are imparted by the ABS to a lower connecting
member and all internal casings. The BOP and riser (during drilling) and
production
tree (during production), are supported by the lifting force of the buoyancy
chamber.
Offset of the wellhead is reasonably controlled by means of vertical tension
resulting
from the buoyancy of the ABS.
The Atlantis ABS system is deficient, however, in several practical respects.
For example, the '322 Magnussen patent specifically limits deployment of the
buoyancy chamber to environments where the influence of surface waves is
effectively negligible, i.e., at a depth of more than about 500 feet beneath
the surface.
Those of ordinary skill in the art will appreciate that deployment at such
depths is an
expensive and relatively risk-laden solution, given that installation and
maintenance
can only be carried out by deep sea divers or remotely operated vehicles, and
the fact
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that a relatively extensive transport system must still be installed between
the top of
the buoyancy chamber and the bottom of an associated recovery vessel in order
to
initiate production from the well.
The Magnussen system also fails to contemplate multiple anchoring systems,
even in instances where problematic drilling environments are likely to be
encountered. Moreover, the system lacks any control means for controlling
adjustment of either vertical tension or wellhead depth during production and
workover operations, and expressly teaches away from the use of lateral
stabilizers
that could enable the wellhead to be deployed in shallower waters subject to
stronger
tidal and wave forces.
Thus, there is plainly a widespread need for a system and method of disposing
an offshore wellhead in a manner such that drillers can adjust both the depth
of a
wellhead and the vertical tension applied to associated pipe casing throughout
the
duration of exploration and production operations. There is also a need for an
adjustable buoyancy chamber system capable of maintaining approximately
constant
vertical tension on an associated drilling or production string, and adjusting
either the
height of a wellhead at any time during exploration and production by
releasing
additional lengths of tension line from a buoyancy chamber height adjustment
member. There is also a need for an offshore exploration and production system
that
flexibly admits to use in connection with both deepwater and shallow target
horizons,
without necessarily being configured to conform to any particular operational
depth.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side view of an offshore exploration and production system in
which an adjustable buoyancy chamber is employed to adjust the height or depth
of an
associated well terminal member.
Figures 2A and 2B are side views of an offshore exploration and production
system, in which lateral and vertical forces on an adjustable buoyancy chamber
are
held approximately constant while the height of an associated well terminal
member
is adjusted by releasing additional lengths of tension line.
SUMMARY OF THE INVENTION
A system and method of establishing an offshore exploration and production
system is provided, in which a well casing is disposed in communication with
an
adjustable buoyancy chamber and a well hole bored into the floor of a body of
water.
A lower connecting member joins the well casing and the chamber, and an upper
connecting member joins the adjustable buoyancy chamber and a well terminal
member. The chamber's adjustable buoyancy enables an operator to vary the
height
or depth of the well terminal member, and to vary the vertical tension
imparted to
drilling and production strings throughout exploration and production
operations.
Also provided is a system and method of adjusting the height or depth of a
wellhead
while associated vertical and lateral forces remain approximately constant. A
variety
of well isolation members, lateral stabilizers and anchoring means, as well as
several
methods of practicing the invention, are also disclosed.
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DETAILED DESCRIPTION
Referring now to the specific, non-limiting embodiment of the invention
depicted in Figure 1, an offshore exploration and production system is
provided,
comprising a well casing 2 installed in communication with a submerged well 1
and
an adjustable buoyancy chamber 9, wherein a lower connecting member 5 is
disposed
between the well casing and the adjustable buoyancy chamber. In a presently
preferred embodiment, the well 1 is accessed from above by means of a well
hole 3
that has been bored into an associated sea floor surface. In a typical
embodiment, a
well casing 2 is set into the hole in a firm and secure manner, and then
cemented into
place using known downhole technology. In other embodiments, a well casing is
securely set into the well hole 3, and a fluid transport member, such as a
smaller-
diameter pipe or pipe casing, is inserted into well casing 2. Once a desired
fit has
been achieved, the outer surface of the fluid transport member is cemented or
set with
a packer to the inner surface of the well casing. Those of ordinary skill in
the art will
appreciate that while the embodiment described above refers to but a single
well, the
offshore exploration and production system disclosed herein can be readily
adapted to
simultaneously work multiple neighboring wells without departing from the
scope or
spirit of the invention.
According to a one embodiment, a well isolation member 4 is disposed
between well casing 2 and a lower connecting member 5. In some embodiments,
well
isolation member 4 comprises one or more ball valves, which, if lower
connecting
member 5 is removed, can be closed so that the well is effectively shut in. In
further
embodiments, well isolation member 4 comprises a blowout preventer or a shear
ram
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that can be maintained in either an open or closed position in order to
provide access
to, or to instead shut in, the contents of well 1.
In other embodiments, lower connecting member 5 further comprises one or
more receiving members disposed to receive an attachment member disposed on
well
isolation member 4. In an alternative embodiment, lower connecting member 5
comprises an attachment member for attaching said lower connecting member 5 to
a
receiving member disposed on well isolation member 4. Methods and means of
securely fastening lower connecting member 5 to well isolation member 4 are
known
to those of ordinary skill in the art, and may comprise one or more of a wide
variety
of fastening techniques, e.g., hydraulic couplers, various nut and bolt
assemblies,
welded joints, pressure fittings (either with or without gaskets), swaging,
etc., without
departing from the scope or spirit of the present invention.
Likewise, lower connecting member 5 may comprise any known connecting
means appropriate for the specific application contemplated by operators. For
example, in various embodiments, lower connecting member 5 comprises one or
more
of segments of riser, riser pipe, and/or pipe casing. In some embodiments,
lower
connecting member 5 comprises a concentric arrangement, for example, a fluid
transport member having a smaller outer diameter than the inner diameter of a
pipe
casing in which the fluid transport member is housed.
In further embodiments, lower connecting member 5 is disposed in
communication with one or more lateral stabilizers 6, which, when deployed in
conjunction a plurality of tension lines 7, effectively controls horizontal
offset of the
system. By utilizing the buoyant forces of adjustable buoyancy chamber 9,
lower
connecting member 5 is drawn taut and held in a stable position.
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In an alternative embodiment, one or more stabilizers 6 control horizontal
offset of lower connecting member 5, and the height or depth of an associated
well
terminal member 14 is adjusted by varying the length of upper connecting
member
12. In some embodiments, the vertical tension of lower connecting member 5 is
held
approximately constant while the height or depth of well terminal member 14 is
adjusted. In further embodiments, the height or depth of well terminal member
14 is
held approximately constant, while the vertical tension imparted by adjustable
buoyancy chamber 9 on lower connecting member 5 is adjusted. In still further
embodiments, the height or depth of well terminal member 14 and the vertical
tension
applied to lower connecting member 5 are held approximately constant, while
lateral
adjustments are performed using lateral stabilizer 6 and one or more of
tension lines
7.
In certain embodiments, one or more lateral tension lines 7 are individually
adjustable, whereas in other embodiments, the tension lines 7 are collectively
adjustable. In further embodiments, one or more tension lines 7 are both
individually
and collectively adjustable. In still further embodiments, the one or more
lateral
stabilizers 6 are disposed in communication with a tension measuring means, so
that a
fixed or predetermined amount of lateral tension can be applied to lower
connecting
member 5 in order to better control system offset. In some embodiments, the
tension
lines 7 are anchored to the sea floor by means of an anchoring member 8, for
example, a suction type anchor, or alternatively, a mechanical or conventional
deadweight type anchor.
In a presently preferred embodiment, adjustable buoyancy chamber 9 is
approximately annular in shape, so that lower connecting member 5 can be
passed
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through a void longitudinally disposed in a central portion of the device. In
further
embodiments, adjustable buoyancy chamber 9 further comprises a plurality of
inner
chambers. In still further embodiments, each of the chambers is independently
operable, and different amounts of air or gas (or another fluid) are disposed
in the
chambers to provide greater adjustable buoyancy control. In one example
embodiment, adjustable buoyancy chamber 9 further comprises a fluid ballast
that can
be ejected from the chamber, thereby achieving greater chamber buoyancy and
lending additional vertical tension to lower connecting member 5. Those of
ordinary
skill in the art will appreciate that many appropriate fluid ballast can be
used to
increase or retard buoyancy; for example, compressed air is an appropriate
fluid that
is both inexpensive and readily available.
In some embodiments, adjustable buoyancy chamber 9 further comprises a
ballast input valve, so that a fluid ballast can be injected into the chamber
from an
external source, for example, through an umbilical line run to the surface or
a remote
operated vehicle, so that an operator can deliver a supply of compressed gas
to the
chamber via the umbilical, thereby adjusting buoyancy characteristics as
desired. In
other embodiments, the fluid input valve is disposed in communication with one
or
more pumps or compressors, so that the fluid ballast is delivered to the
chamber under
greater pressure, thereby effecting the desired change in buoyancy more
quickly and
reliably.
In other embodiments, adjustable buoyancy chamber 9 further comprises a
ballast output valve, so that ballast can be discharged from the chamber. In
instances
where air or another light fluid is injected into the chamber while water or
another
heavy liquid is discharged, the chamber will become more buoyant and increase
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vertical tension on lower connecting member 5. Conversely, if water or another
heavy liquid is injected into the chamber while air is bled out, the chamber
will lose
buoyancy, thereby lessening vertical tension on lower connecting member 5.
In alternative embodiments, the ballast output valve is disposed in
communication with one or more pumps or compressors, so that ballast is
ejected
from the chamber in a more reliable and controlled manner. In some
embodiments,
the ballast output valve is disposed in communication with an umbilical, so
that
ballast ejected from the chamber can be recovered or recycled at the surface.
In any
event, a principle advantage of the present invention is that adjustments to
the
chamber's buoyancy and tensioning properties, and the ability to control the
height of
the well terminal member 14, can be performed at any time during either
exploration
or production, due to the various ballast input and output control means
disposed
about the body of the chamber.
In further embodiments, adjustable buoyancy chamber 9 is further disposed in
communication with one or more tension lines 10 provided to anchor the
adjustable
buoyancy chamber to the sea floor. As before, tension lines 10 are anchored to
the
sea floor using known anchoring technology, for example, suction anchors or
dead
weight type anchors, etc. The one or more tension lines 10 can also provide
additional lateral stability for the system, especially during operations in
which more
than one well is being worked. In one embodiment, the one or more tension
lines 10
are run from the adjustable buoyancy chamber 9 to the surface, and then moored
to
other buoys or a surface vessel, etc., so that even greater lateral tension
and system
stability are achieved. In further embodiments, the tension lines 10 are
individually
adjustable, whereas in other embodiments, the tension lines 10 are
collectively
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controlled. In still further embodiments, the one or more tension lines 10 are
both
individually and collectively adjustable.
In one example embodiment, adjustable buoyancy chamber 9 is disposed in
communication with a vertical tension receiving member 11. In another
embodiment,
the vertical tension receiving member 11 is equipped with a tension measuring
means
(e.g., a load cell, strain gauge, etc.), so that vertical tension applied to
lower
connecting member 5 is imparted in a more controlled and efficient manner. In
another embodiment, the buoyant force applied to tension receiving member 11
is
adjusted by varying the lengths of tension lines 10, while the buoyancy of
adjustable
buoyancy chamber 9 is held approximately constant. In a further embodiment,
the
buoyancy of adjustable buoyancy chamber 9 is controlled by means of one or
more
individually selectable ballast exhaust ports disposed about the body of the
chamber,
which vent excess ballast fluid to the surrounding sea. In still further
embodiments,
the open or closed state of the ballast exhaust ports are individually
controlled using
port controllers known to those of ordinary skill in the art (e.g., plugs,
seacocks, etc.)
In a presently preferred embodiment, the system is disposed so that a well
terminal member 14 installed above buoyancy chamber 9 is submerged to a depth
at
which maintenance and testing can be carried out by SCUBA divers using
lightweight, flexible diving equipment, for example, at a depth of about 100
to 300
feet beneath the surface. In some embodiments, the well terminal member 14 is
submerged only to the minimum depth necessary to provide topside access to the
hulls of various surface vessels servicing the well, meaning that well
terminal member
14 could also be disposed at a much shallower depth, for example, a depth of
about 50
to 100 feet. In alternative embodiments, well terminal member 14 is disposed
at
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depths of less than 50 feet, or greater than 300 feet, depending upon the
actual
conditions surrounding operations. In still further embodiments, well terminal
member 14 is disposed either at the surface or above the surface of the water,
and a
blowout preventer or a production nee is installed by workers operating aboard
a
service platform or surface vessel. This "damp tree" model avoids the need to
assemble long subsurface riser stacks, as would generally be required during
deepwater operations. Moreover, disposing the well terminal member at or near
the
surface also permits testing and maintenance to be carried out by SCUBA divers
or
surface crews, without the need for expensive and time-consuming remote
operated
vehicle operations.
In some embodiments, well terminal member 14 further comprises either a
blowout preventer or a production tree. In a presently preferred embodiment,
however, well terminal member 14 further comprises a combined blowout
preventer
and production tree assembly configured so as to facilitate simplified well
intervention operations.
In some embodiments, lower connecting member 5 terminates within the void
formed in a center portion of the annular chamber 9, at which point an upper
connecting member 12 becomes the means by which fluids are transported up to
the
wellhead. In other embodiments, lower connecting member 5 does not terminate
within the void formed in a center portion of the annular chamber, but instead
runs
through the void and is subsequently employed as an upper connecting member 12
disposed between the chamber and the wellhead. In other embodiments, a
vertical
tension receiving member 11 is disposed between the buoyancy chamber 9 and
upper
connecting member 12, so that the chamber's buoyant forces are transferred to
the
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vertical tension receiving means 11, thereby applying vertical tension to the
drilling or
production string extended below the chamber.
In further embodiments, upper connecting member 12 further comprises a well
isolation member 13, e.g., one or more ball valves or blowout preventers, used
to halt
fluid flow in the event that well terminal member 14 is either removed or
disabled, for
example, during testing and maintenance operations. Those of ordinary skill in
the art
will appreciate that the precise types and exact locations of isolation valves
13
employed in the system are variable and flexible, the only real requirement
being that
the valves are capable of allowing or preventing fluid flow from the well 1
during
periods in which testing or maintenance, or even an emergency safety
condition, are
present.
For example, well terminal member 14 can be equipped with a production tree
so that a production hose disposed on a surface vessel can be attached to the
system
and production can commence. Alternatively, well terminal member 14 can
terminate
in a blowout preventer, so that the well will not blow out during drilling
operations.
In other embodiments, well terminal member 14 terminates in a combined
production
tree and blowout preventer assembly to facilitate simplified well intervention
operations.
Turning now to the specific, non-limiting embodiments of the invention
depicted in Figures 2A and 2B, a system and method of establishing a height-
variable
well terminal member is provided, comprising a lower fluid transport pipe 21,
an
inner well casing 22, an outer well casing 23, and a wellhead 24. In some
embodiments, a well isolation member 25 is disposed above the wellhead 24, so
that
the well can be closed off or shut in if desired.
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In the example embodiment depicted in Figure 2A, well isolation member 25
further comprises one or more ball valves that can be adjustably opened or
closed as
desired by an operator. A lower connecting member 26 having one or more
interior
seals 27 and an interior polished bore 28 houses a fluid transport member 29
such that
the height of fluid transport member 29 is variably adjustable within a body
portion of
lower connecting member 26 in response to vertical lifting forces imparted by
adjustable buoyancy chamber 30. Various lengths of pipe define the height of
an
upper connecting member disposed between the buoyancy chamber 30 and a well
terminal member 36. In some embodiments, an upper well isolation member 35,
such
as a ball valve or a blowout preventer, is disposed in communication with the
upper
connecting member between buoyancy chamber 30 and well terminal member 36.
In some embodiments, the system is moored to the sea floor using one or more
mooring lines 31 connected to a first vertical tension receiving means 32a,
while
buoyancy chamber 30 is raised or lowered by either spooling-out or reeling-in
lengths
of one or more tension lines 37 disposed between a second vertical tension
receiving
means 32b and a chamber height adjustment means 33. As adjustable buoyancy
chamber 30 rises, vertical tension is applied to vertical tension receiving
member 34,
which in turn lifts well terminal member 36 up toward the surface.
As seen in the example embodiment depicted in Figure 2B, the height of both
the well terminal member 36 and fluid transport member 29 are vertically
adjusted by
increasing the length of tension lines 37 using chamber height adjustment
means 33,
even as vertical and lateral tension on mooring lines 31 and tension lines 37
remains
approximately constant. In one embodiment, vertical tension on lower
connecting
member 26 is also kept approximately constant during this process, since fluid
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transport member 29 is moved vertically within a body portion of lower
connecting
member 26. In another embodiment, a second, lower adjustable buoyancy chamber
is
added to the system to maintain tension on lower connecting member 26, while
the
height of the well terminal member is adjusted as described above.
The foregoing specification is provided for illustrative purposes only, and is
not intended to describe all possible aspects of the present invention.
Moreover, while
the invention has been shown and described in detail with respect to several
exemplary embodiments, those of ordinary skill in the pertinent arts will
appreciate
that minor changes to the description, and various other modifications,
omissions and
additions may also be made without departing from either the spirit or scope
thereof.
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