U.S. patent application number 09/739915 was filed with the patent office on 2001-08-16 for floating spar for supporting production risers.
This patent application is currently assigned to FMC CORPORATION. Invention is credited to Fitzgerald, John A., Skeels, Harold B..
Application Number | 20010013414 09/739915 |
Document ID | / |
Family ID | 22232827 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013414 |
Kind Code |
A1 |
Fitzgerald, John A. ; et
al. |
August 16, 2001 |
Floating spar for supporting production risers
Abstract
A subsea production system is provided for producing a number of
subsea wells which may be arranged in groups. Each of the groups of
subsea wellheads is connected to deliver production flow to a
subsea manifold and each subsea manifold is connected to deliver
production flow to a production riser. A plurality of production
risers each being connected to receive production flow from one of
said subsea manifolds extend from the subsea manifolds for groups
of wells. A deep draft floating spar is located generally above the
subsea wellheads with mooring lines and has a production platform
located above the sea surface and has buoyancy and ballast chambers
to control floatation. The spar structure defines a riser bore
receiving the production risers extending from the subsea wellheads
to the production platform. The spar is also capable of being
shifted laterally by its mooring lines for positioning above a
selected well to thus permit well intervention activities as
needed. The subsea wells are each provided with wellheads having a
removable cap to permit ROV actuated cap removal and replacement as
needed to permit well intervention.
Inventors: |
Fitzgerald, John A.;
(Houston, TX) ; Skeels, Harold B.; (Kingwood,
TX) |
Correspondence
Address: |
Mayor, Day, Caldwell & Keeton, L.L.P.
Suite 1900
700 Louisiana
Houston
TX
77002
US
|
Assignee: |
FMC CORPORATION
|
Family ID: |
22232827 |
Appl. No.: |
09/739915 |
Filed: |
March 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09739915 |
Mar 27, 2001 |
|
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09350332 |
Jul 9, 1999 |
|
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60092354 |
Jul 10, 1998 |
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Current U.S.
Class: |
166/366 ;
166/346; 166/350 |
Current CPC
Class: |
E21B 19/004 20130101;
E21B 33/076 20130101; E21B 33/035 20130101; E21B 41/04 20130101;
E21B 43/01 20130101; B63B 35/44 20130101; E21B 17/015 20130101;
B63B 35/4406 20130101; E21B 43/017 20130101 |
Class at
Publication: |
166/366 ;
166/350; 166/346 |
International
Class: |
E21B 043/017 |
Claims
We claim:
1. A method for selectively producing and conducting intervention
operations on a plurality of subsea wells having subsea wellheads
located at the sea bed, comprising: (a) mooring a deep draft
floating spar generally above the subsea wellheads with mooring
lines, said deep draft floating spar having a production platform
located above the sea surface, having buoyancy and ballast chambers
and defining a riser bore receiving at least one production riser
extending from the subsea wellheads to said production platform;
(b) producing the subsea wells through said at least one production
riser while permitting a range of lateral movement of said floating
spar responsive to external forces of water current, wind and the
like; (c) for intervention with respect to a selected well,
shifting said floating spar to a station above the selected subsea
wellhead; and (d) conducting well intervention operations on the
selected well.
2. The method of claim 1, wherein each of said subsea wellheads has
a removable wellhead cap for permitting well intervention, said
method comprising: (a) prior to well intervention, removing said
removable wellhead cap; (b) conducting said well intervention
operations; and (c) replacing said removable wellhead cup after
completion of said well intervention operations.
3. The method of claim 2, wherein a remote operated vehicle (ROV)
is provided for removal and replacement of removable wellhead caps,
said method comprising: (a) actuating said ROV for removal of said
removable wellhead cap from the selected wellhead; and (b) after
completing said well intervention operation, actuating said ROV for
replacing said removable wellhead cap to permit resumption of well
production.
4. The method of claim 1, wherein said subsea wellheads are
arranged in groups, with each of said groups having a subsea
manifold connected to receive production flow from each of the
wellheads of said group and said subsea manifold having a
production riser extending through said riser bore, said method
comprising: (a) with said production riser producing from at least
one of the wellheads of said group of wellheads through at least
one of said subsea manifolds, shifting said deep draft floating
spar laterally to a station above a selected wellhead; and (b)
conducting well intervention operations through the selected
wellhead while continuing said producing from at least one of the
wellheads of said group of wellheads through at least one of said
subsea manifolds.
5. The method of claim 1, wherein said subsea wellheads are
arranged in groups, with each of said groups having a subsea
manifold connected to receive production flow from each of the
wellheads of said group and each of said subsea manifolds of said
groups having a production riser extending through said riser bore,
said method comprising: (a) with said production risers producing
from at least one of the wellheads of each of said group of
wellheads through said subsea manifold of said group, shifting said
deep draft floating spar laterally to a station above a selected
wellhead designated for intervention; and (b) conducting well
intervention operations through the selected wellhead while
continuing said producing from the wellheads of said group of
wellheads through said subsea manifolds.
6. A subsea production system for a plurality of subsea wells each
having subsea wellheads located at the sea floor, comprising: (a) a
deep draft floating spar adapted for location generally above the
subsea wellheads and having a production platform located above the
sea surface, having buoyancy and ballast chambers and defining a
riser bore; (b) mooring lines for mooring said deep draft floating
spar and for controlling lateral positioning of said deep draft
floating spar for stationing thereof above a selected wellhead
intended for intervention; (c) at least one subsea production
manifold connected to receive production from a plurality of said
wellheads; and (d) at least one production riser being connected to
said at least one subsea production manifold and extending upwardly
from said at least one subsea production manifold through said
riser bore to said production platform.
7. The subsea production system of claim 6, comprising: (a) said
subsea wells being arranged in groups; (b) said subsea production
manifolds each being connected to receive production flow from the
wellheads of one of said groups of wellheads; and (c) said at least
one production riser being a plurality of production risers each
being connected to receive production flow from one of said subsea
manifolds and extending from said subsea manifold through said
riser bore and to said production platform.
8. The subsea production system of claim 7, comprising: (a) said
plurality of subsea wells each having a removable cap, being
removable to permit well intervention activities; and (b) said
removable cap being removable and replaceable by ROV controlled
servicing activities.
9. The subsea production system of claim 7, comprising: (a) said
plurality of subsea wells defining groups of wells, each group
having two or more wells each having a wellhead; and (b) a subsea
manifold being connected in production flow receiving relation with
said wellheads of a group of wells and having one of said
production risers connected in flow receiving relation
therewith.
10. The subsea production system of claim 9, comprising: said
subsea manifolds being dual bore subsea manifolds.
11. The subsea production system of claim 9, comprising: (a) said
subsea manifolds being dual bore subsea manifolds; and (b) said
plurality of wellheads having production and annulus conduits for
production and which are connected for delivery of production fluid
to the dual bore subsea manifold for the group of wells.
12. The subsea production system of claim 6, comprising: (a) said
plurality of subsea wells being located over a defined area of the
seabed; and (b) said deep draft floating spar having a diameter
less than said defined area of said seabed and adapted to be
laterally shifted for positioning directly above any selected one
of said plurality of subsea wells.
13. A subsea production system comprising: (a) a plurality of
subsea wells each having subsea wellheads located at the sea floor
and being located on a defined area of the sea floor (b) a deep
draft floating spar adapted for location generally above the subsea
wellheads and having a production platform located above the sea
surface, having buoyancy and ballast chambers and defining a riser
bore, said deep draft floating spar having a diameter less than
said defined area of the sea floor; (c) a plurality of mooring
lines for mooring said deep draft floating spar and for controlling
lateral positioning of said deep draft floating spar for stationing
thereof above a selected wellhead intended for intervention; (d) a
plurality of subsea production manifolds each being connected to
receive production from a group of said plurality of wellheads; and
(e) a plurality of production risers each being connected to one of
said subsea production manifolds and extending upwardly through
said riser bore to said production platform.
14. The subsea production system of claim 13, comprising: (a) said
subsea wellheads being arranged in groups; (b) said subsea
production manifolds each being connected to receive production
flow from the wellheads of one of said groups of wellheads; and (c)
said at least one production riser being a plurality of production
risers each being connected to receive production flow from one of
said subsea manifolds and extending from said subsea manifold
through said riser bore and to said production platform.
15. The subsea production system of claim 13, comprising: (a) said
plurality of subsea wells each having a removable cap, being
removable to permit well intervention activities; and (b) said
removable cap being removable and replaceable by ROV controlled
servicing activities.
16. The subsea production system of claim 13, comprising: (a) said
plurality of subsea wells defining groups of wells, each group
having two or more wells each having a wellhead; and (b) a subsea
manifold being connected in production flow receiving relation with
said wellheads of a group of wells and having one of said
production risers connected in flow receiving relation
therewith
17. The subsea production system of claim 16, comprising: said
subsea manifolds being dual bore subsea manifolds.
18. The subsea production system of claim 16, comprising: (a) said
subsea manifolds being dual bore subsea manifolds; and (b) said
plurality of wellheads having production and annulus conduits for
production and which are connected for delivery of production fluid
to the dual bore subsea manifold for the group of wells.
19. The subsea production system of claim 13, comprising: (a) said
plurality of subsea wells being located over a defined area of the
seabed; and (b) said deep draft floating spar having a diameter
less than said defined area of said seabed and adapted to be
laterally shifted for positioning directly above any selected one
of said plurality of subsea wells.
Description
[0001] Applicants hereby claim the benefit of United States
Provisional application Ser. No. 60/092,354 which was filed on Jul.
10, 1998 by John A. Fitzgerald and Harold B. Skeels and entitled
Floating Spar For Supporting Production Risers, which Provisional
Application is incorporated herein by reference for all
purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a floating spar for supporting a
production platform, and more particularly to such a floating spar
for supporting production risers extending from subsea manifolds to
the production platform in deep water offshore wells.
[0004] 2. Description of the Prior Art
[0005] Oil and gas production spars currently utilize a number of
subsea wells placed a given lateral distance on the sea floor and
connected to surface facilities via individual risers where a
Christmas tree is attached for well control. Wells for deepwater
typically are very heavy given their extended length and in some
cases multiple barriers where multiple concentric casing riser
joints exist. Since a production spar is a floating vessel, each
riser must be vertically tensioned to maintain its structural
integrity. Hydraulic piston assemblies, electromechanical devices,
and dashpots are some of the mechanisms used to maintain a constant
tension while the spar is heaving or moving laterally (due to the
ocean environmental forces). Buoyancy devices attached to riser
strings have also been used to allow the risers to free stand
independently of the spar's hull. This method is the most
advantageous with respect to the spar since the tension created by
the buoyancy devices are not transferred to the spar hull, thereby
freeing up the displacement of the spar's hull to support the
weight of the spar and the facilities placed on top.
[0006] The drawback to this method is size. To make an offshore
production spar economically viable, several wells must be tied
back to the surface facility, each requiring a certain amount of
space in the center of the spar for the riser and its buoyancy
devices. As water depth increases, riser weight increases. As riser
weight increases, space for buoyancy to hold up the riser
increases. As the space increases, so does the spar's hull diameter
to accommodate the need for added space. If the spar's hull is
larger, it is more costly to build and install, requiring more
wells. Therefore a spar may reach an economic limit, simply because
the water depth and number of wells create a spar hull so large as
to make it uneconomical. Another aspect that may increase riser
weight or size is the concept of "barriers". If a well's fluid
control devices (tree and manifolds) are at the surface, there may
be a requirement for extra conduits in the riser design for both
structural protection and pressure containment. Added conduits will
increase both size and weight to the riser.
[0007] U.S. Pat. No. 5,706,897 dated Jan. 13, 1998 is directed to a
floating spar which is a deep-draft floating caisson of a hollow
cylindrical construction and utilized primarily for deep water
offshore well operations at depths of 2,000 feet or more. The
floating spar is anchored by mooring lines to the sea floor and may
extend seven hundred feet, for example, below the surface of the
water. The spar or caisson shown in the '897 patent is directed
primarily to a caisson for drilling risers for supporting a high
pressure drilling riser and a low pressure drilling riser extending
from a subsea wellhead. FIGS. 9 and 10, however, are directed to
production risers in which a subsea tree is added to provide a
mechanical safety barrier at the sea floor. Above the subsea tree
is the vertical riser extending to a production manifold at the
surface. An additional surface tree is provided for fluid control
purposes. Thus, a production riser extends from each subsea
wellhead to the surface location via a subsea tree, riser conduit,
surface tree, and surface manifold.
[0008] The utilization of individual production risers extending
from each subsea wellhead through the spar to a surface manifold
and surface tree results in a substantial weight exerted on the
spar particularly when multiple subsea wellheads, such as ten or
more, are being utilized for product supply. Also, a substantial
space within the spar or caisson is required for the multiple lines
extending through the space to the surface platform or deck.
Floatation tanks within the spar are utilized for tensioning the
risers. In some instances, the risers and wellhead connector are
deployed and recovered through the internal diameter of the buoys.
The buoys must therefore be sized to permit the passage of the
large diameter wellhead connector which normally controls the
internal diameter of the spar and contributes to the overall size
of the spar.
[0009] It is desired that a spar be of a minimal size and weight
for minimizing costs and simplifying construction, installation and
operation.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to an offshore production
system utilizing a spar or caisson anchored to the sea floor by
mooring lines and supporting a production platform above the sea
level. A plurality of subsea wellheads each has a subsea tree
mounted thereon with a removable tree cap to permit access to the
subsea tree and subsea wellhead. Production conduits from the
annulus and production bores of each subsea tree extend to either:
a production riser to the spar or a subsea manifold which receives
conduits from multiple subsea trees, such as five or ten subsea
trees, for example. Subsea manifolds are normally provided,
particularly when a plurality of the sub sea wells are located
nearby each other to reduce the number of conduits extending to a
surface location. Production risers from subsea trees and/or
manifolds extend from the sea floor through the spar to the
production platform on top of the spar. Also, test lines and
umbilical lines may extend from the subsea trees and manifolds
through the spar to the production platform for flow control, test
or maintenance work. The production risers from the subsea tree and
manifolds may be flexible cables or vertical centenary risers and
formed of various materials.
[0011] To intervene or provide access to the subsea tree, such as
the tubing string, the spar may be positioned over the designated
well with the intervention riser system over the tree. The tree cap
is then removed and the intervention system is then landed and
locked onto the top of the tree thereby permitting intervention in
the well. To minimize intervention hardware weight and the number
of trips that equipment has to travel between the surface and the
sea floor, the subsea trees may utilize a light weight tree cap
which may be deployed and recovered by a remotely operated vehicle
(ROV).
[0012] Utilizing subsea technology, the costs of deepwater spars
are reduced by reducing the number of risers between the sea floor
and the spar. Instead of individual risers for each well, the wells
are completed in a standard subsea configuration which are
subsequently sent to the surface individually via a light weight
minimal barrier riser, or co-mingled together via manifolding on
the sea floor and sent to the surface by a single larger bore riser
to the spar facility. The production riser(s) may be vertically
supported in the same manner as individual well risers. The
production riser itself may be larger in diameter than the
individual well riser, requiring bigger buoyancy to support its
weight. Other risers for pipeline pigging, well testing, and
control (electrical/hydraulic line) cables to operate the subsea
wells may also be needed, but the overall number of suspended
conduits from the spar is drastically reduced for the same number
of wells. The fewer number of conduits required results in a
smaller space and spar hull size requirement; leading to lower spar
hull fabrication costs. Subsea multi-well technology also does not
limit the number of wells needed, nor the structural and geometric
problems of a riser associated with the lateral reach out to
outlying wells. In addition, single subsea wells with a subsea tree
leading to a production pipeline/riser conduit act as both the
safety barrier and flow control are a simpler design and a more
cost effective approach to the subsea safety tree and surface tree
on either end of the spar riser configuration.
[0013] The reduced area for risers also lets the spar better
utilize its deck space and displacement capacity for drilling and
workover derricks, subsea risers and subsea blowout preventers.
With fewer risers, the spar may move about on its anchor mooring
spread to position itself over any well for subsea drilling
completion or workover operations permitting tubing intervention
into individual subsea wells.
[0014] It is an object of this invention to provide a deep-draft
floating spar of minimum size and weight for supporting production
risers extending from subsea manifolds to a production platform on
the spar.
[0015] A further object of this invention is to provide such a
subsea production system utilizing subsea trees which have a
removable tree cap for intervention and access to the subsea well
without necessarily going through the production riser. Small
intervention well control hardware can be run and suspended from
the spar for periodic maintenance and workovers.
[0016] Another object of the invention is the provision of such a
spar subsea production system in which subsea trees have production
pipelines extending to subsea manifolds which, in turn, have
production risers extending from the manifolds through the spar to
the production platform thereby eliminating surface trees and
minimizing any surface manifolds for the production platform.
[0017] Other objects, features, and advantages of the invention
will be more apparent from the following specification and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic view of a floating spar production
system including a production platform supported on a buoyant spar
with product risers extending from subsea manifolds (or subsea
trees) through a deep-draft caisson spar to the production
platform; and
[0019] FIG. 2 is a schematic view of a subsea tree connected to a
subsea wellhead and having a removable tree cap for removal by a
remotely operated vehicle (ROV) to permit access to the subsea tree
and subsea wellhead such as may be required for workover operations
or the like using lightweight intervention techniques.
DESCRIPTION OF THE INVENTION
[0020] Referring to the drawings a floating spar or caisson is
generally indicated at 10 having a production platform 12 with a
plurality of decks mounted thereon above the sea level 11. Spar 10,
for example, may be about 700 feet in length and about 75 feet in
diameter, with the water depth over about 2000 feet. Mooring lines
14 are secured to anchor piles (not shown) on sea floor 16 for
anchoring of spar 10. Six (6) or eight (8) mooring lines 14 are
preferably utilized for mooring of spar 10. Buoys which comprise
buoyancy tanks or chambers 18 are mounted within spar 10 along with
ballast chambers 20. An axial bore or slot 22 is provided in spar
10 through buoyancy tanks 18 and ballast chambers 20 to receive a
plurality of production risers 24, 26, 28. Test and umbilical lines
may also be provided within spar 10. Suitable support members 30 on
spar 10 within riser bore 22 support production risers 24, 26 and
28.
[0021] Mounted on sea floor 16 are a plurality of subsea wellheads
36. Each subsea wellhead 36 has a subsea tree 38 connected thereto
with a suitable connector and an upper removable tree cap 40 is
provided on each subsea tree 38. A horizontal subsea tree having a
removable tree cap which is satisfactory may be purchased from the
FMC Corporation, Petroleum Equipment and Systems Division, of
Houston, Tex. Subsea tree 38 is preferable of a dual bore type.
Production and annulus conduits 42, 44 extend from each subsea tree
38 to an associated dual bore subsea manifold 46, 48 or 50 on sea
floor 16. Riser 42 extends from the tubing string of the well,
while riser 44 extends from the annulus of the well. Production
risers 24, 26 and 28 from respective subsea manifolds 46, 48 and 50
extend upwardly through riser slot 22 in spar 10 to a surface
manifold 52 on production platform 12. Suitable riser supports 30
in slot 22 support production risers 24, 26 and 28. Suitable test
lines and electrical/hydraulic umbilicial lines (not shown) may
extend to the subsea manifolds and subsea trees for testing and
control as needed.
[0022] Spar 10 may be moved as much as about 250 feet in any
direction without disconnecting mooring lines 14 from spar 10. Each
subsea wellhead 36 and subsea tree 38 having a removable tree cap
40 thereon is arranged so that full vertical access and workovers
may be obtained by removal of the tree cap 40 without removing the
subsea tree. It is necessary for various reasons to intervene into
the tubing string of a subsea well from time to time, such as might
be required for shifting sleeves, wax cutting, bottom hole pressure
surveys, and bailing sand, for example. Wire line or coiled tubing
may be utilized in an intervention riser system for intervening
into the subsea well. The particular type of intervention riser
system depends on various factors, such as water depth, well
pressure, currents, spar length, and may be constructed of a
composite material or coiled tubing.
[0023] The spar 10 is first positioned vertically over the subsea
tree 38 as shown in FIG. 2. A remotely operated vehicle (ROV)
illustrated generally at 54 is normally utilized with the
intervention riser system. Subsea tree cap 40 is first removed
utilizing the ROV. An intervention system (not shown) is landed and
locked onto the top of tree 38. The tree cap 40 is normally
provided with a space for positioning of ROV 54 over cap 40 in an
aligned position for removal of cap 40 and landing and locking of
the intervention system onto tree 38. After the completion of the
workover or other operation, ROV 54 picks up and reinstalls tree
cap 40 and tests the connection to insure pressure integrity.
[0024] The production risers 24, 26, 28 (FIG. 1) extending through
spar 10 may be tensioned, if needed, by buoys 18 within spar 10 or
by piston type tensioners as well known. For further details of
spar 10, the entire disclosure of U.S. Pat. No. 5,706,897 is
incorporated by reference. ROV 54 may be controlled from platform
12 or a separate dive support vessel.
[0025] While three manifolds 46, 48 and 50 are illustrated with
each manifold having a separate production riser extending to
platform 12, it may be desirable to have only a single manifold
with a single production riser extending to surface platform 12.
Also, it may be desirable to combine production risers 24, 26 and
28 into a single riser extending to surface platform 12 through
spar 10 as less space in spar 10 could be utilized.
[0026] In the present invention, a floating spar production system
utilizes subsea trees having ROV removable tree caps and connected
by risers to subsea manifolds which, in turn, have production
risers extending from the subsea manifolds through the spar to the
production platform. Such a system results in a spar of minimal
size and weight and each subsea tree having a removable tree cap
thereon is adapted for vertical access for workover or other
operations.
[0027] In view of the foregoing it is evident that the present
invention is one well adapted to attain all of the objects and
features hereinabove set forth, together with other objects and
features which are inherent in the apparatus disclosed herein.
[0028] As will be readily apparent to those skilled in the art, the
present invention may easily be produced in other specific forms
without departing from its spirit or essential characteristics. The
present embodiment is, therefore, to be considered as merely
illustrative and not restrictive, the scope of the invention being
indicated by the claims rather than the foregoing description, and
all changes which come within the meaning and range of equivalence
of the claims are therefore intended to be embraced therein.
* * * * *