U.S. patent application number 13/141655 was filed with the patent office on 2012-02-23 for improved subsea riser system.
This patent application is currently assigned to Jacobs Engineering Group, Inc.. Invention is credited to David McNaught, Tor Persson, Brian Phillips, Randall Seehausen, Steven Shu.
Application Number | 20120043090 13/141655 |
Document ID | / |
Family ID | 45593164 |
Filed Date | 2012-02-23 |
United States Patent
Application |
20120043090 |
Kind Code |
A1 |
Persson; Tor ; et
al. |
February 23, 2012 |
IMPROVED SUBSEA RISER SYSTEM
Abstract
An improved riser system which comprises a connector for
connecting conduits and a mooring system for mooring the connector
to the floor of a body of water. The connector may include a
pivoting device. The improved riser system also comprises a buoy
system for supporting the connector. The buoy system is configured
to provide a fixed buoyancy for the connector, the mooring system
and at least a portion of the conduits and for providing variable
buoyancy for placement of the connector at a predetermined water
depth. The improved riser system may also include a flexible
conduit made of titanium.
Inventors: |
Persson; Tor; (Houston,
TX) ; Shu; Steven; (Houston, TX) ; Seehausen;
Randall; (Bellaire, TX) ; McNaught; David;
(Houston, TX) ; Phillips; Brian; (Houston,
TX) |
Assignee: |
Jacobs Engineering Group,
Inc.
Houston
TX
|
Family ID: |
45593164 |
Appl. No.: |
13/141655 |
Filed: |
May 20, 2011 |
PCT Filed: |
May 20, 2011 |
PCT NO: |
PCT/US11/37325 |
371 Date: |
June 22, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12785221 |
May 21, 2010 |
|
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13141655 |
|
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Current U.S.
Class: |
166/344 ;
166/345; 166/367 |
Current CPC
Class: |
F16L 1/24 20130101; F16L
1/15 20130101 |
Class at
Publication: |
166/344 ;
166/345; 166/367 |
International
Class: |
E21B 17/01 20060101
E21B017/01 |
Claims
1-27. (canceled)
28. An improved riser system for use in a body of water, said
system comprising: a first conduit having a first and second end,
said first end interfacing the floor of the body of water; a
connector connected to said second end of said first conduit; a
mooring system for mooring said connector to the floor of the body
of water; a second conduit having a first and second end, wherein
said second conduit's wall comprises titanium, said connector
connected to the first end of said second conduit, said first and
second conduits being coupled together and in fluid communication
with each other; and a buoy system for providing buoyancy support
in said riser system.
29. The system of claim 28 wherein said buoy system is configured
to provide a fixed buoyancy force for said connector, said mooring
system and at least a portion of said first and second conduits,
and for providing a variable buoyancy force for placement of said
connector at a predetermined water depth.
30. The system of claim 28 wherein said second conduit comprises at
least one section of titanium and at least one section of
steel.
31. The system of claim 28 wherein said second conduit and said
first conduit have the same internal diameter.
32. The system of claim 28 wherein said second conduit and said
first conduit have substantially the same internal diameter such
that said first and second conduits may be pigged by a pig designed
for changing diameters.
33. The system of claim 28 wherein said connector comprises a
pivoting device.
34. The system of claim 33 wherein said pivoting device comprises a
hinge.
35. A method of installing a riser system in a body of water, said
method comprising the steps of: assembling a foundation system,
wherein said assembling comprises: attaching a first portion of a
connector to a mooring system above said body of water, said
mooring system comprising a fastening device and a tendon;
attaching said mooring system to the floor of the body of water,
and suspending said connector in said body of water using a
buoyancy device; assembling a riser structure above said body of
water, wherein said assembling comprises: connecting a first
conduit to a second portion of said connector, said second portion
of said connector configured to be coupled with said first portion
of said connector; connecting a second conduit to said first
connector so that said first and second conduits are in fluid
communication; and connecting a buoyancy apparatus to said second
portion of said connector; after said assembling of said foundation
system and said riser structure, deploying said riser structure in
said body of water; and connecting said foundation system to said
riser structure by connecting said first portion of said connector
to said second portion of said connector.
36. The method of claim 35 further comprising: guiding said second
portion of said connector with a guide cone prior to connection of
said second portion of said second connector to said first portion
of said second connector.
37. A system for pigging a riser system in a body of water, said
system comprising: a pig launching station connected to a first
conduit, wherein at least a portion of said first conduit is
located on a floor of said body of water; and a pig receiving
station connected to said first conduit, said pig receiving station
configured to receive a pig and liquid displaced from said first
conduit by said pig, wherein said first conduit is connected to a
second conduit, said second conduit having an internal diameter
different from said first conduit.
38. The system of claim 37 wherein said pig launching station and
said pig receiving station are located on the floor of said body of
water.
39. The system of claim 37 further comprising: a connector that
connects said first conduit to said second conduit, wherein said
pig launching station is located on said connector and said pig
receiving station is located on the floor of said body of
water.
40. The system of claim 37 further comprising: a connector that
connects said first conduit to said second conduit, wherein said
pig receiving station is located on said connector and said pig
launching station is located on the floor of said body of
water.
41. A method for pigging a riser system in a body of water, said
method comprising: launching a pig from a pig launching station
connected to a first conduit of said riser system, wherein at least
a portion of said first conduit is located on the floor of said
body of water and said first conduit is connected to a second
conduit, said second conduit having an internal diameter different
from said first conduit; and receiving, at a pig receiving station,
said pig and liquid displaced from said first conduit by said pig,
wherein said pig receiving station is connected to said first
conduit.
42. The method of claim 41 wherein said pig launching station and
said pig receiving station are located on the floor of said body of
water.
43. The method of claim 41 wherein said pig launching station is
located on a connector that connects said first conduit to said
second conduit and wherein said pig receiving station is located on
the floor of said body of water.
44. The method of claim 41 wherein said pig receiving station is
located on a connector that connects said first conduit to said
second conduit and wherein said pig launching station is located on
the floor of said body of water.
45. An improved riser system for use in a body of water, said
system comprising: a first conduit having a first and second end,
said first end interfacing the floor of the body of water; a
connector that comprises a pivoting device, said connector
connected to said second end of said first conduit; a second
conduit having a first and second end, said second conduit
connected to said connector, said connector connected to the first
end of said second conduit, said first and second conduits being
coupled together and in fluid communication with each other; and a
mooring system for mooring said connector to a floor of the body of
water, said mooring system comprising a tendon connected to said
pivoting device, said pivoting device adapted to allow any one of
or a combination of said first conduit and said second conduit to
pivot about said pivoting device when a load is applied to any one
of said tendon, said first conduit and said second conduit.
46. The system of claim 45 further comprising: a buoy system for
supporting said connector, said buoy system configured to provide a
fixed buoyancy force for said connector, said mooring system and at
least a portion of said first and second conduits, and for
providing a variable buoyancy force for placement of said connector
at a predetermined water depth.
47. The system of claim 45 further comprising: a triangular support
connecting said hinge to said second conduit, said triangular
support adapted for reducing a bending load on said second
conduit.
48. The system of claim 45 wherein said pivoting device comprises a
hinge.
49. The system of claim 45 wherein said pivoting device comprises a
trunnion.
50. The system of claim 45 wherein said second conduit passes
through said trunnion.
51. The system of claim 45 wherein said second conduit comprises
titanium.
52. A method of installing a riser system, said riser system having
a system structure including a first conduit connected to a frame
having a pivoting device and a buoy having a tubular configuration
with a lumen and a lift line passing through said lumen, said
method comprising the steps of: connecting at least one mooring
tendon to said frame; deploying said riser system structure in a
body of water; connecting the at least one mooring tendon to a
floor of said body of water; positioning a second conduit via said
lift line passing through said buoy's lumen; and connecting said
second conduit to said first conduit so that said second and said
first conduit are in fluid communication.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and is a
continuation-in-part of U.S. application Ser. No. 12/785,221, filed
May 21, 2010, entitled "IMPROVED SUBSEA RISER SYSTEM," the
disclosure of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to systems for fluid
transportation in deepwater environments. Specifically, the present
invention relates to a subsea riser system for the transportation
of fluids from, for example, a sea floor to a floating vessel or
from the floating vessel to the seafloor.
BACKGROUND OF THE INVENTION
[0003] Within various industries, pipes are used to transport
fluids from one location to another. In the petroleum industry, for
example, pipes are used to transport crude oil and gas from wells
on the seafloor to the sea surface, and to a distribution network
at least for some distance between the fluid's source and its
destination. Proper design of piping systems is important to ensure
the transportation of fluids in a safe and environmentally friendly
manner. Specifically, a piping system has to be designed so that it
maintains its integrity when put in use in its particular
application. For example, piping systems for use on land have to be
designed to take into account parameters such as the pressure of
the fluid being transported, the corrosiveness of the fluid being
transported, the environment in which the piping system will be
located and seismic activity at the location, to name a few.
Designers of piping systems for use in water must contend with such
parameters and additional parameters such as hydrostatic pressure
(the force exerted by the water due to gravity) and hydrodynamic
forces (forces due to the motion of the water).
[0004] Hydrostatic and hydrodynamic forces become increasingly more
relevant for piping systems as the water depth in which the piping
system is installed increases. In the case of offshore petroleum
production, pipes, known as risers, extend from the seafloor to sea
surface for transporting, for example, oil and gas from a wellhead
on the sea floor to a surface facility. Risers in deepwater systems
are subjected to significant internal and hydrostatic pressure and
hydrodynamic forces. Consequently, designing risers to withstand
the internal pressures, hydrostatic pressures and hydrodynamic
forces of deep water can be challenging. This challenge is
exacerbated when the surface facility to which the riser is
connected is a floating platform because movement of the floating
platform due to the wave, wind and sea currents can transmit
significant stress to the riser. Continuous application of stress
to the riser causes fatigue and eventually could rupture the
riser.
[0005] Close to the surface of a deep body of water, the
hydrostatic pressure is low while the hydrodynamic forces are high
due to the wind, waves and associated currents. Below the surface
currents, there may be submerged currents that cause vortex induced
vibrations. For example, in the Gulf of Mexico, the surface
currents are typically in the first 200 feet of water depth and the
submerged currents can exist in about 1,000 feet of water
depth.
[0006] In the deeper zones of the water, the hydrostatic pressure
is higher and the hydrodynamic forces lower than the zones close to
the surface. Taking into account the different forces existing at
different depths, one type of riser system includes a flexible
conduit in the upper turbulent zone of the body of water. Because
the flexible conduit is limited in its ability to withstand
hydrostatic pressures and axial tension capacity, the flexible
conduit is connected to a catenary riser located in the deeper zone
of the water (the catenary riser normally curves gently upward from
the sea floor). The catenary riser, often made of steel, is able to
withstand the hydrostatic pressures at deeper zones of the body of
water. The connection between the flexible conduit and the catenary
riser is typically located below that zone in the water where the
hydrodynamic forces are high. In some riser systems, a buoy is used
to support the catenary riser by attaching the riser to the buoy.
However, because the flexible conduit is in the upper zone of
water, i.e. the first 200 feet of water depth in the Gulf of
Mexico, it moves with the currents and this movement causes stress
on the catenary riser because the moving flexible conduit is
attached to the catenary riser.
[0007] What is more, the demands on riser systems are changing, in
part, because drilling is increasingly occurring in deeper and more
hostile water depth locations. This development has made it more
challenging to provide cost effective riser systems because of the
corresponding increase in hydrostatic pressure and hydrodynamic
forces as riser systems are deployed in deeper and more hostile
water depth locations. An additional challenge in designing current
riser systems is a need to accommodate subsea systems that permit
the size of gas and oil risers to be on the order of 16 inches in
diameter and larger.
[0008] As noted above, some current riser systems address the
hydrodynamic forces in the turbulent zones close to the surface of
a body of water by connecting one end of a flexible conduit to a
surface vessel. The other end of the flexible conduit is then
connected to a catenary riser made of less flexible material. In
order to make the conduit flexible enough to withstand the
hydrodynamic forces in the turbulent zone, it comprises several
thin layers of steel and elastomeric material (i.e. a composite
flexible conduit). The layers of steel and elastomeric material
imposes limits on the conduit's bore size and the pressure and
temperature it can withstand.
[0009] In view of the bore size limitation, it should be
appreciated that any change in internal diameter between the
catenary riser and one or more flexible conduits connected to the
catenary riser makes pigging a complex operation. Pigging involves
inserting a device (a pig) into a pipeline and using a fluid to
push the pig through the pipeline. As the pig moves through the
pipeline, it performs functions such as cleaning the pipeline and,
for specialized pigs, inspecting the pipeline. Pigging, in some
operations, may need to be done as often as once per week. As the
complexity of the pigging operation increases, so does a riser's
operational costs.
[0010] Though it is possible to pig composite flexible conduits
having an internal diameter less than the catenary riser, such an
operation adds complexity. For instance, if the catenary riser has
an internal diameter of 18 inches and the composite flexible
conduit has an internal diameter of 14 inches, current systems
provide for a pig that will jump in diameter from 14 inches to 18
inches. It should be noted, however, that pigs usually cannot have
a jump in diameter above four inches. What is more, pigs that jump
in diameter usually do not work as efficiently as pigs that
maintain a constant diameter.
[0011] Composite flexible conduits are susceptible to high
temperature production fluids. As such, a composite flexible
conduit is usually the component that limits a riser system's
ability to handle such fluids.
[0012] In addition to hydrodynamic forces due to wind, waves and
associated currents, described above, the conduits in a riser
system are subject to movements caused by a change in the products
that pass through the conduits. For example, a flexible conduit and
a catenary riser will be installed in saltwater. Subsequently, oil
is used to displace the saltwater. In turn, the oil may be later
displaced by natural gas. These different fluids have different
densities. Thus, as the fluid composition in the conduits changes,
the weight of the contents in the conduits and the load on the
conduits change. Indeed, it is possible that submerged conduits
that initially contained a liquid, which is replaced with a gas,
will float up towards the surface of the water. Therefore, as the
contents of the different conduits change, the relative loads
exerted by the conduits against each other change and cause fatigue
of components of the riser system.
[0013] The current technology of suspending an SCR directly from a
host facility is limited due to motions caused by ultra deepwater
host facilities. The use of flexible pipe directly suspended from
the host to the seafloor has different limitations due to its own
weight, collapse pressure and temperature restrictions. Thus, a
need exists to decrease the limitations for fluid conduits
extending the entire length between the seafloor and the host.
There are many different types of host facilities, each having
different associated hull designs and motions. There is a need for
a single system solution that has the versatility to adapt for a
broad range of hosts facilities including Floating Production
Storage and Offloading (FPSO), SPAR, Tension Leg Platforms (TLP),
Semisubmersible (SS), Floating Storage and Offloading (FSO) and any
other type of floating deepwater facility. In sum, a need exists
for an improved riser system that can address the current demands
being placed on riser systems.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention is directed to an improved riser
system and method of installation. Embodiments of the invention
reduce the transmission of forces from one portion of the riser
system to another through a connector and use a buoy system that
provides fixed and variable buoyancy.
[0015] One embodiment of the invention includes an improved riser
system for use in a deep body of water. The riser system includes
two conduits. The first conduit has a first end that is attached
to, or is a continuation of, a pipeline located on the sea floor. A
connector is connected to the second end of the first conduit. The
connector is also connected to a first end of the second conduit
and the first and second conduits are coupled together to permit
fluid communication between the first and second conduits. The
second end of the second conduit is located proximate the surface
of the body of water. The connector is configured to reduce
transmission of forces from one conduit to the other. The improved
riser system includes a mooring system for mooring the connector to
the sea floor and a buoy system for supporting the connector, and
corresponding portions of the first and second conduits. In
embodiments of the invention, the buoy system is attached to the
connector and is configured to provide a fixed buoyancy. The buoy
system also provides variable buoyancy for adjustment of the
buoyancy requirement for the installation method and during the
life of the riser. The buoy system is connected to the connector so
as to provide vertical support and lateral restraint.
[0016] Another embodiment of the invention is a method of
installing a riser system in a body of water. The method includes
preparing a riser assembly above the surface of the body of water.
The preparation of the riser assembly includes connecting a first
conduit and a second conduit to a connector so that the first and
second conduits are in fluid communication with each other. The
preparation of the riser assembly also includes connecting a
mooring line to the connector and connecting a buoy system to the
connector via a flexible member. When the buoy system is connected
to the connector it may be at least partially ballasted. This
embodiment of the invention further includes lowering the riser
assembly into the body of water to a depth below the surface, and
at this point the mooring line is attached to a seabed foundation.
While lowering the riser assembly, the first and second conduits
can be flooded to provide a slight negative buoyancy, and the
mooring line is fixed to the sea floor. After the mooring line is
fixed to the sea floor, at least a portion of the buoy system can
be deballasted, allowing the connector to stabilize at a second
predetermined depth.
[0017] In a further embodiment of the invention, the flexible
conduit is made of titanium. Due to titanium's strength, low
density and elasticity, the flexible conduit may be manufactured
out of titanium instead of several layers of steel and elastomeric
material. Because of the strength and elasticity of titanium, the
wall of a titanium flexible conduit is relatively thin yet strong
enough to meet the pressure rating and withstand the hydrodynamic
forces required for conduits used in turbulent sections of a body
of water.
[0018] Further yet, embodiments of the invention involve a two
stage installation process of a riser system. The two stage
installation process includes assembling two major sections of the
riser system above the surface of the water and installing these
sections at separate times in the body of water. Each of the major
sections includes a buoy apparatus and portion of a connector. The
portions of the connector are connected, under the surface of the
body of water in which they are deployed, to form the riser
system.
[0019] Another embodiment of the invention includes a system for
pigging a riser. The system includes a pig launching station
connected to a first conduit. At least a portion of the first
conduit is located on a floor of the body of water. The system also
includes a pig receiving station connected to the first conduit.
The pig receiving station is configured to receive a pig and liquid
displaced from the first conduit by the pig. The first conduit is
connected to a second conduit and the second conduit has an
internal diameter different from the first conduit.
[0020] A further embodiment of the invention includes a system that
provides components of a riser system to pivot around a certain
point of a connector. For example, embodiments of the invention
include a riser system in a body of water having a first conduit
with first and second ends. The first end of the first conduit
interfaces the seafloor. The riser system also includes a connector
that has a pivoting device. The connector is connected to the
second end of the first conduit. The riser system also includes a
second conduit having first and second ends. The first end of the
second conduit is connected to the connector. The first and second
conduits are coupled together and are in fluid communication with
each other. The riser system also includes a mooring system for
mooring the connector to the seafloor. The mooring system includes
a tendon connected to the pivoting device. The pivoting device is
adapted to allow any one of, or a combination of, the tendon, the
first conduit and the second conduit to pivot about the pivoting
device when a load is applied to any one of the tendons, the first
conduit and the second conduit.
[0021] Further yet, embodiments of the invention include a method
of installing a riser system. The riser system has a system
structure that includes a first conduit connected to a frame, which
has a pivoting device. The system structure also includes a buoy
having a tubular configuration with a lumen and a lift line passing
through the lumen. The method of installing comprises connecting at
least one mooring tendon to the frame and deploying the riser
system structure in a body of water. The method of installing also
includes connecting the at least one mooring tendon to a floor of
the body of water. Further, the method includes positioning a
second conduit via the lift line passing through the buoy's lumen
and connecting the second conduit to the first conduit so that the
second and the first conduit are in fluid communication.
[0022] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It will be appreciated by those skilled in the
art that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present
invention. It will also be realized by those skilled in the art
that such equivalent constructions do not depart from the spirit
and scope of the invention as set forth in the appended claims. The
novel features which are believed to be characteristic of the
invention, both as to its organization and method of operation,
together with further objects and advantages will be better
understood from the following description when considered in
connection with the accompanying figures. It is to be expressly
understood, however, that each of the figures is provided for the
purpose of illustration and description only and is not intended as
a definition of the limits of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0024] FIG. 1 is an illustration of a riser system according to one
embodiment of the invention;
[0025] FIGS. 2A-2C are illustrations of a connector as it is used
in a riser system, according to one embodiment of the
invention;
[0026] FIGS. 3A and 3B illustrate an installation process for a
riser system according to one embodiment of the invention;
[0027] FIGS. 4A-4G illustrate an installation process for a riser
system according to one embodiment of the invention;
[0028] FIG. 5 is an illustration of a riser system according to one
embodiment of the invention;
[0029] FIGS. 6A-6G illustrate an installation process for a riser
system according to one embodiment of the invention;
[0030] FIG. 7 is an illustration of a pigging system according to
one embodiment of the invention;
[0031] FIGS. 8A-8E are illustrations of a connector in a riser
system, according to embodiments of the invention;
[0032] FIGS. 9A-9D illustrate an installation process for a riser
system according to one embodiment of the invention;
[0033] FIG. 10 is an illustration of a buoy according to one
embodiment of the invention; and
[0034] FIG. 11 is an illustration of a buoy according to one
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0035] FIG. 1 is an illustration of a riser system according to one
embodiment of the invention. Riser system 100 may be for the
transportation of oil from a pipeline connected to a wellhead
assembly located on seafloor 103 to a floating production, storage
and offloading vessel (FPSO) 108. It should be noted that in
embodiments of the invention, riser system 100 may be used to
transport other types of fluids, such as water and natural gas, and
to different types of export and surface facilities, such as a
floating LNG facility. Moreover, in addition to the transportation
of fluids from the seafloor to a surface facility, riser system 100
may transfer fluids from the surface facility to the seafloor, for
example, for production enhancement of a seafloor reservoir.
[0036] Referring still to FIG. 1, riser system 100 includes two
conduits, steel catenary riser (SCR) 102 and flexible conduit 106.
In this configuration, SCR 102 is in fluid communication with a
pipeline 109 on seafloor 103 that in turn connects to wellhead
assembly 110. Further, SCR 102 can be coupled to flexible conduit
106 at connector 104 so that SCR 102 and flexible conduit 106 are
in fluid communication. Thus, fluid from the wellhead assembly 110
may flow through pipeline 109, SCR 102, flexible conduit 106 to
FPSO 108. SCR 102 and pipeline 109 are able to withstand the
hydrostatic pressures in the deeper portions of body of water 101
and may be made of material such as carbon steel and other alloys,
the like and combinations thereof.
[0037] Flexible conduit 106 is able to withstand the hydrodynamic
forces of the upper levels of body of water 101 and, in embodiments
of the invention, is designed to be flexible. Flexible conduit 106
may be made of materials such as steel, alloys and synthetic
material the like and combinations thereof.
[0038] Connector 104, in embodiments of the invention, is
configured to reduce the transmission of forces emanating from the
movement of flexible conduit 106 to SCR 102. As such, connector 104
can reduce the overall stress and strain to which SCR 102 is
exposed over time.
[0039] Connector 104 is preferably moored to seafloor 103 by
mooring line 105 and a fastening device 112. Fastening device 112
comprises a suction pile, gravity weight, the like or combinations
thereof. Mooring line 105 comprises a synthetic fiber tendon.
Mooring lines must be able to accommodate high loads. Consequently,
mooring lines have traditionally been made from materials such as
wire ropes and chains. Over time, however, the development of
synthetic fibers has brought about the use of mooring lines made
from synthetic tendons. These synthetic fiber tendons have the
advantage of being lighter than wire ropes and chains but able to
accommodate as high loads as wire ropes and chains do. Therefore,
the use of synthetic fiber tendons as mooring lines allows the
riser system as a whole to be lighter than when other mooring
equipment is used, particularly in the deeper water of current
production activity. Mooring line 105 may comprise materials such
as Polyester, Aramid (aromatic polyamid), LCAP (Liquid Crystal
Aromatic Polyester), the like, and combinations thereof.
[0040] Riser system 100 includes buoy system 107 for vertically
supporting the submerged weight of connector 104, mooring line 105,
flexible conduit 106 and SCR 102. Buoy system 107 can include a
variable buoyancy buoy. As such, buoy system 107 may be unitary or
may comprise two or more buoys. Accordingly, buoy system 107 may
include fixed buoyancy buoy 107A and variable buoyancy buoy 107B.
For example, in one embodiment of the invention, variable buoyancy
buoy 107B may be positioned at a fixed depth of about 150-200 feet
below the surface of body of water 101 and the fixed buoyancy buoy
107A may be positioned at a fixed depth below variable buoyancy
buoy 107B. Because buoy system 107 may be capable of providing
variable buoyancy, buoy system 107 facilitates the placement of
connector 104 at a desired water depth for the attachment of
mooring line 105 to fastening device 112, which fastens mooring
line 105 to seafloor 103. Additionally, buoy system 107, when
connected to connector 104, is preferably configured to provide
only vertical support and thereby lateral restraint to connector
104, mooring line 105, flexible conduit 106 and SCR 102. Therefore,
by reducing the transmission of forces from flexible conduit 106 to
SCR 102, and providing preferably vertical support only to
connector 104 by buoy system 107, the service life of SCR 102 may
be improved. Connector 104 is preferably connected to flexible
member 111 which is connected to fixed buoyancy buoy 107A. In this
manner, connector 104 is suspended from buoy system 107. Thus,
flexible member 111 provides the vertical support for connector 104
and to some extent laterally restrains connector 104. However,
preferably flexible member 111 does not transfer forces from buoy
system 107 to SCR 102 and flexible conduit 106 through the
connector 104.
[0041] Referring to FIG. 2A, the process for connecting flexible
conduit 106 to SCR 102 includes first connecting flexible conduit
106 to connector 104 above the surface of the water. In this
embodiment of the invention, flexible conduit 106 is connected to
connector 104 by placing flexible conduit 106 on curved support
204. Fastener 205, which may be a circular or loop shape, is used
to secure flexible conduit 106 at one end of curved surface 204.
Fastener 205 prevents flexible conduit 106 from being dislodged
from curved surface 204 but has a large enough diameter to allow
flexible conduit 106 to be pulled along curved support 204, as will
be described below. After flexible conduit 106 is secured on curved
surface 204, connector 104 may be placed in the water. While
connector 104 is underwater, SCR 102 may then be pulled into frame
assembly 201 using pull lines. After SCR 102 is pulled into frame
assembly 201, SCR 102 is locked into frame assembly 201 with a
latch mechanism (not shown but well known to those
skilled-in-the-art). At this point, there may be gap 203 between
SCR 102 and flexible conduit 106.
[0042] Referring now to FIG. 2B, to close gap 203 and provide fluid
communication between SCR 102 and flexible conduit 106, flexible
conduit 106 is pulled down onto SCR 102. Mechanisms known in the
art, such as pull lines and hydraulic systems may be used to pull
or push flexible conduit 106 onto SCR 102. Once gap 203 is closed,
flexible conduit 106 and SCR 102 may be coupled together. In
embodiments of the invention, flexible conduit 106 and SCR 102 may
be coupled together by a coupling that comprises a Retlock.RTM.
connector or other such coupling well known to those
skilled-in-the-art. In embodiments of the invention, the coupling
may comprise the pull-in mechanism for pulling flexible conduit 106
onto SCR 102, (in direction x as shown in FIG. 2B). Further, it
should be noted that in a variation of the invention, flexible
conduit 106 may first be locked to frame assembly 201, then SCR 102
may be pulled up to and coupled to flexible conduit 106. SCR 102
may then be secured to frame assembly 201. The top portion of frame
assembly 201 can be connected to flexible member 111 while the
bottom portion of frame assembly 201 is attached to mooring line
105. Referring now to FIG. 2C, this diagram shows how curved
support 204 keeps flexible conduit 106 in a bent configuration.
Because flexible conduit 106 is in a bent configuration, a force
applied to flexible conduit 106 in direction "y," for example,
would bend flexible conduit 106 upwards and pull it away from
curved support 204 but not transmit that force to SCR 102.
Conversely, a force in the opposite direction of "y" may bend
flexible conduit 106 against curved surface 204 but not transmit
that force to SCR 102.
[0043] FIGS. 3A and 3B illustrate an installation process for riser
system 100 according to one embodiment of the invention. FIG. 3A
illustrates an aspect of the installation process that may occur
above the surface of the water, for example, on installation vessel
301. To begin the process, mooring line 105 may be cast from
installation vessel 301 into body of water 101. Mooring line 105
may be connected to connector 104. Additionally, SCR 102 and
flexible conduit 106 may be connected to connector 104 so that SCR
102 and flexible conduit 106 are in fluid communication with each
other.
[0044] Referring now to FIG. 3B, fixed buoyancy buoy 107A may be
connected to connector 104 by flexible member 111. Variable
buoyancy buoy 107B may be connected to fixed buoyancy buoy 107A,
via flexible member 107C, to form buoy system 107. Fixed buoyancy
buoy 107A may be a syntactic foam buoy. Variable buoyancy buoy 107B
may be a buoyancy tank in, and from, which water may be pumped to
vary its buoyancy. Once fixed buoyancy buoy 107A is connected to
connector 104, connector 104, SCR 102, flexible conduit 106 and the
remaining portion of mooring line 105 are lowered into body of
water 101. Because mooring line 105 comprises synthetic fiber,
which is relatively light, fixed buoyancy buoy 107A is able to
handle loads in deeper zones of body of water 101, as compared to
riser systems that use heavier mooring equipment. SCR 102 and
flexible conduit 106 are flooded for the riser system 100 to
achieve negative buoyancy when buoy system 107 is placed in body of
water 101. It should be noted that although buoy system 107 is
shown as having two buoys, in embodiments of the invention, buoy
system 107 may comprise more than two buoys.
[0045] Referring still to FIG. 3B, in embodiments of the invention,
fixed buoyancy buoy 107A is designed to partially support the
weight of mooring line 105, connector 104, SCR 102 and flexible
conduit 106. In this scenario, the riser system 100 is negative
buoyant. To continue the installation operation, flexible member
107C and variable buoyancy buoy 107B may be deployed and allowed to
sink to a predetermined depth in body of water 101. Variable
buoyancy buoy 107B may be deployed in a fully ballasted or
partially ballasted mode so that riser system 100 as a whole still
has a negative buoyancy. That is, riser system 100 continues to
sink but may be supported from a crane located on installation
vessel 301. As riser system 100 sinks, seafloor 103 can support
more of the submerged weight of riser system 100 as more of SCR 102
rests on seafloor 103.
[0046] When the top of variable buoyancy buoy 107B reaches a
desired depth, mooring line 105 may be connected to fastening
device 112 which in turn may be fastened to seafloor 103. The
connection of mooring line 105 to fastening device 112 may be done
with the assistance of a Remote Operated Vehicle (ROV). Indeed, any
of the operations disclosed herein, in particular those that take
place below the surface of body of water 101, may be done with the
assistance of an ROV. In embodiments of the invention, once
variable buoyancy buoy 107B is at the desired depth and mooring
line 105 is connected to fastening device 112, variable buoyancy
buoy 107B is deballasted until it exerts an upward force large
enough to counteract the weight of riser system 100 and thereby
suspend riser system 100 in body of water 101 at a fixed depth. At
this point in this embodiment of the invention, riser system 100 is
installed and variable buoyancy buoy 107B is positioned vertically
above fixed buoyancy buoy 107A so that buoy system 107 provides
only vertical support and lateral restraint to connector 104,
mooring line 105, flexible conduit 106 and SCR 102.
[0047] Typically, riser systems are installed by laying a pipeline
from an end location, such as a wellhead, to the SCR location with
the end of the pipeline furthest from the wellhead forming the SCR.
The SCR is usually located proximate to the expected location of
the FPSO. However, in instances where the FPSO is already moored at
its final location, it may be desirable to install the riser system
so that the installation process begins at the riser location and
proceeds towards the wellhead. Referring now to FIGS. 4A-4G,
embodiments of the invention that may implement this variation of
the installation process may include partially assembling the riser
system, which may comprise connecting the pipeline to a connector.
The preparation of the partial riser assembly includes connecting a
mooring line to the connector with a gravity weight suspended from
the connector. A buoy system is then connected to a connector via a
flexible member, as discussed above in relation to FIGS. 2A-2B. The
gravity weight 112 A may then be placed on the seabed onto a
suction pile. The magnitude of buoyancy provided by the buoy system
may be sufficient to accommodate the weight of the pipeline/SCR
when the pipeline/SCR installation begins.
[0048] Referring to FIG. 4A, some embodiments of the invention may
include a mooring line that comprises synthetic fiber tendons.
Mooring lines made from synthetic fiber tendons usually stretch up
to 30% of their original length when a load is applied. If a
mooring line stretches after it is installed in a riser system,
that stretching may change the whole configuration of the riser
system. To prevent this problem, installation of a riser system
that includes mooring lines made from synthetic fiber tendons
preferably includes stretching mooring line 105 prior to installing
it. The stretching process may begin by attaching gravity weight
112A to lowering line 113 and then lowering gravity weight 112A
into the water with the lowering line 113. In other words, lowering
line 113 may be used to provide support to, and suspend, gravity
weight 112A in body of water 101. One end of mooring line 105 is
attached to gravity weight 112A prior to placing gravity weight
112A in body of water 101 and the other end secured to installation
vessel 301. By increasing the length of lowering line 113 so that
it is longer than mooring line 105 (assuming both lowering line 113
and mooring line 105 are suspended from installation vessel 301 at
the same level), the load of gravity weight 112A may be transferred
from lowering line 113 to mooring line 105. This transference of
load to mooring line 105 may stretch mooring line 105 to a desired
length. If the desired length is not at first achieved, the process
may be repeated to achieve the desired stretching of mooring line
105.
[0049] Referring now to FIG. 4B, after stretching mooring line 105,
lowering line 113 is detached from gravity weight 112A and flexible
conduit 106, buoy system 107 and mooring line 105 are connected to
connector 104 above the surface of the water and then lowered into
the water. It should be noted that though buoy system 107 is shown
as including fixed buoyancy buoy 107A and variable buoyancy buoy
107B, buoy system 107 could be a composite buoy, as discussed
further below.
[0050] Referring now to FIG. 4C, the installation of the riser
system may include the use of a ramp on installation vessel 301 to
assemble pipeline 302 and thus installation vessel 301 may act as a
pipe laying vessel. Pipeline 302 may be connected to the riser
assembly after the riser assembly has been immersed in body of
water 101. In embodiments of the invention, the riser assembly
includes a fastening device 112, which may be gravity weight 112A.
However, in some situations, for example, when seafloor 103 is
sloped, it may be necessary to add suction pile 112B, which
provides gravity weight 112A with horizontal stability. In this
variation of the invention, gravity weight 112A is lowered onto
suction pile 112B. Such devices 112 are well known to those
skilled-in-the-art.
[0051] Referring still to FIG. 4C, after the riser assembly has
been placed in body of water 101 at a final predetermined depth,
pipeline 302 is payed out into body of water 101. As one end of
pipeline 302 reaches the vicinity of connector 104, pipeline 302 is
connected to pull lines 303, which in turn runs through connector
104 to winches located on vessel 305. Referring now to FIG. 4D,
pull lines 303 can be adjusted in length in order for pipeline 302
to conform to a curvature consistent with a permissible stress
level in pipeline 302. Referring now to FIG. 4E, if pull lines 303
are reduced in length by the winches on vessel 305, the end of
pipeline 302 will move upward and give pipeline 302 more of a
curved configuration.
[0052] Referring still to FIG. 4E, installation vessel 301 may
continue assembling and paying out pipeline 302 while vessel 305
continues to shorten pull line 303 and thereby pull pipeline 302
towards connector 104. Pull line 303 pulls pipeline 302 into
connector 104 and then pipeline 302 is locked onto frame assembly
201 of connector 104. Then pipeline 302 may be connected to
flexible conduit 106, similar to the procedure discussed with
respect to FIGS. 2A-2B. After pipeline 302 is locked into connector
104 and connected to flexible conduit 106, pull lines 303 may be
disconnected from vessel 305 and connector 104. As discussed above
with respect to FIGS. 2A-2B, pipeline 302 may be connected to
flexible conduit 106 using a coupling suitable for the purpose and
this coupling may comprise a Retlock.RTM. connector, which is well
known to those skilled-in-the-art. As pipeline 302 is payed out
with one of its end locked onto connector 104, pipeline 302 sinks
and bends into a catenary configuration.
[0053] Referring back to FIG. 1, riser system 100, especially one
where the FPSO is moored prior to installation of pipeline 302, may
require that the section of pipeline 302 extending from connector
104 touches down or intersects with seafloor 103 at a particular
point--a desired touchdown point. To illustrate, this concept, the
touchdown point is labeled T.P. in FIG. 1 and the desired touchdown
point is labeled DTP in FIGS. 4E and 4F. In embodiments of the
invention, a preferred method of achieving the DTP is to use
connecting lines 306 and 307 to establish the touchdown point.
Connecting lines 306 and 307 may be made from wire rope. Referring
to FIGS. 4E-4F, connecting line 306 may be attached to pipeline 302
and connecting line 307 may extend from and run through channel 309
in fastening device 112.
[0054] Referring now to FIG. 4F, as pipeline 302 approaches
seafloor 103, connecting line 306 may be joined to one end of
connecting line 307 using an ROV. The other end of connecting line
307 may then be pulled through fastening device 112 up to vessel
305. Connecting line 307, in this configuration, may be used as a
hauling line by vessel 305 to ensure that the DTP of pipeline 302
is achieved. Specifically, vessel 305 may apply a pulling force on
connecting line 307 in one direction. Connecting line 307 may have
a stopper 308, which is too large to go through channel 309. The
configuration of connecting lines 306 and 307 (including the
position of stopper 308) is such that when connecting lines 306 and
307 are joined and stopper 308 rests against gravity weight 112,
the touchdown point will be the intersection of line 306 with
pipeline 302. In other words, the distance of line 306/307 from
stopper 308 to the end of line 306/307 that intersects with
pipeline 302 determines the desired touchdown point.
[0055] Referring now to FIG. 4G, pipeline 302 may be installed at
one end location, for example, to wellhead assembly 110, and
connecting line 306 and 307 may be severed. In its installed
position, pipeline 302 comprises SCR 302A and sea floor pipeline
302B. In this configuration, pipeline 302B lies on seafloor 103 and
provide fluid communication between wellhead assembly 110 and SCR
302A, which in turn is in fluid communication with flexible conduit
106.
[0056] The installed parameters of riser system 100 may vary
depending on the body of water in which it is installed and the
depth of that body of water. For example, in the Gulf of Mexico,
riser system 100 may be installed so that fixed buoyancy buoy 107A
is located below submerged currents which typically means greater
than 1,000 feet below the surface. Concurrently, the variable
buoyancy buoy 107B is located below upper currents and turbulent
wave action which typically is about 200 feet below the
surface.
[0057] The installation methods described above with respect to
FIGS. 3A and 3B include performing significant portions of the
installation process on an installation vessel. For example, FIGS.
3A and 3B show that connector 104, flexible conduit 106 and SCR 102
are connected together on vessel 301 and then deployed in a body of
water. This type of installation can be complex and requires
concurrent operation of different types of equipment on vessel 301.
Major challenges for installers of riser systems in this type of
operation include (1) concurrently managing major aspects of the
installation process in limited space on an installation vessel;
(2) meeting the time limits set for the installation process; and
(3) reducing safety hazards on the installation vessel.
[0058] To understand these challenges, it should be noted that some
installation processes require at least three different reels on
the installation vessel. A first reel is used to hold tendon 105.
The length of tendon 105 needed depends on the depth of the water.
A second reel is required for holding flexible pipe. A riser system
installation typically requires between several hundred feet to
2,000 feet of flexible pipe. A third reel is required to hold the
SCR 102/pipe 109. In the installation processes described in FIGS.
3A and 3B, tendon 105, flexible conduit 106, connector 104, buoy
107 and SCR 102 are deployed at the same time, which is demanding
on the installation crew and equipment.
[0059] Referring now to FIGS. 6A-6G, a two stage installation
process is shown that involves the consecutive installation of two
major parts of the riser system. The first stage begins on vessel
615 with the assembling of foundation system 600a. The assembling
process includes attaching connector portion 604a to a mooring
system that will be used to moor the riser system to the seafloor
603. The mooring system includes tendon 605, which extends from
connector portion 604a to fastening device 612. The assembling
process also includes connecting buoys 613 to connector portion
604a. Once foundation system 600a is assembled, it is deployed in
water body 601, as shown in FIG. 6B. As foundation system 600a
descends in body of water 601, fastening device 612 is used to
fasten mooring line 605 to seafloor 603 by plugging fastening
device 612 into device 616, as shown in FIG. 6C. Buoys 613 suspends
connector portion 604a in an area in the water where connector
portion 604a will be connected to the other part of the riser
system.
[0060] Once foundation system 600a is installed, the second stage
of the installation of the riser system begins. Referring to FIG.
6D, the second stage includes assembling riser structure 600b on
installation vessel 615. Assembling riser structure 600b includes
connecting connector portion 604b to SCR 602. Connector portion
604b is configured to mate with, and couple to, connector portion
604a forming connector 604 (shown in FIG. 6G). Assembling riser
structure 600b also includes connecting flexible conduit 606 to SCR
602 and connecting buoy 607 to connector 604b.
[0061] Referring now to FIG. 6E, after assembling riser structure
600b, it is deployed in the water. As SCR 602/609 descends in the
water, flexible conduit 606 is connected to FPSO 608, as shown in
FIG. 6F. Further, an ROV may be used to move connector portion 604b
closer to connector portion 604a. As connector portion 604b
approaches connector portion 604a, guide cone 614, which is
attached to connector portion 604a, guides element 615 of connector
portion 604b so that connector portions 604b and 604a are properly
aligned. Once connector portions 604a and 604b are properly
aligned, they are connected.
[0062] Referring now to FIG. 6G, connector portions 604a and 604b
form connector 604 which functions in a manner similar to connector
104 described above with respect to FIGS. 1 and 2A-2C. The
connection of connector portions 604a and 604b may be done by
various means well known in the art such as welding and mechanical
latching. For example, latch sections l.sub.1 and l.sub.3 are
configured to mate and couple l.sub.2 and l.sub.4. Thus, latches
l.sub.1/l.sub.2 and l.sub.3/l.sub.4 connect portions 604a and 604b,
which thereby connect foundation system 600a to riser structure
600b. Referring still to FIG. 6G, an installed riser system 600 is
shown whereby foundation system 600a and riser structure 600b are
connected.
[0063] This two stage installation process has several advantages.
First, the two-stage installation process is less complex as the
crews install the foundation system and the riser structure at
different times.
[0064] Second, the two-stage installation process is more easily
managed on vessels with limited space, thereby creating a safer
working environment. Essentially, the fewer major processes the
installation crew has to perform at any one time, the safer the
working environment.
[0065] Third, the two-stage installation process allows more
installation vessels to install riser system 600. Referring to
FIGS. 6A-6G, installation vessel 615 will be required to have a
lifting capacity sufficient to raise the entire riser system.
However, the two stage installation process described herein
reduces the maximum load that the installation vessel needs to
support at any one time. This reduction in lifting capacity means
more installation vessels are suitable.
[0066] Fourth, the two-stage installation process requires less
space. There has to be enough deck space on a surface vessel to
accommodate the activity. In the two-stage method disclosed herein,
all the components do not have to be handled at the same time.
Thus, the deck space required on the installation vessel is
less.
[0067] Referring now to FIG. 5, a riser system 500 is shown for the
transportation of fluid from a pipeline connected from a wellhead
assembly 510 on seafloor 503 to an FPSO 508. Riser system 500
includes two conduits, SCR 502 and flexible conduit 506. In this
configuration, SCR 502 is in fluid communication with a pipeline
509 on seafloor 503 that in turn connects to wellhead assembly 510.
Further, SCR 502 is coupled to flexible conduit 506 at connector
504 so that SCR 502 and flexible conduit 506 are in fluid
communication. SCR 502 and pipeline 509 are able to withstand the
hydrostatic pressures in the deeper portions of water 501 and may
be made of material such as carbon steel and other alloys, hybrids,
composite materials and combinations thereof.
[0068] As noted above, typical composite flexible conduits usually
have thick walls of steel and elastomeric material. Further,
composite flexible conduits have greater limitations in terms of
combined pressure, temperature and inner diameter relative to
catenary risers to which they are attached. Consequently, riser
systems having composite flexible conduits may require a plurality
of flexible conduits for a single catenary riser to achieve
equivalent flow, require complex and expensive pigging operations
and have fluid temperature limitations based on the temperature
limitation of the elastomeric material.
[0069] To address these issues, the present invention may have
flexible conduits made of titanium. Referring still to FIG. 5,
flexible conduit 506 may be made of titanium. Since titanium is
strong, has low density and is elastic, flexible conduit 506 can be
made entirely of titanium as compared with several layers of
different material used for composite flexible conduits. Since
flexible conduit 506 is made of titanium, it does not have the
limitations composite flexible conduits have with respect to
internal diameter, temperature and pressure. As such, flexible
conduit 506 can be sized to correspond with the design criteria of
the SCR 502. For instance, the need to have multiple smaller
internal diameter flexible conduits connected to a larger internal
diameter catenary riser is eliminated. A catenary riser having
substantially the same internal diameter as the flexible conduit
means that the catenary riser may be pigged using a pig whose
diameter does not need to be changed. Because flexible conduit 506
has the same or substantially the same internal diameter as SCR
502, it is possible to pig both SCR 502 and flexible conduit 506 in
one pigging operation.
[0070] The use of titanium to make flexible conduit 506 presents
further advantages. For instance, with flexible conduit 506 made of
titanium, riser system 500 is able to withstand temperatures higher
than riser systems that use composite flexible conduits. The
temperature limitation of the conduits in a riser system is
becoming increasingly significant as the temperatures of produced
fluids increase. For example, and in general, composite flexible
conduits are not ideal for temperatures above 250.degree. F.
(depending on other design parameters this temperature can be
significantly less), while a flexible conduit 506 made of titanium
can withstand temperatures significantly higher.
[0071] Referring still to FIG. 5, connector 504 is preferably
moored to seafloor 503 by mooring line 505 and fastening device
512. Fastening device 512 may comprise a suction pile, gravity
weight, the like or combinations thereof, all of which are well
known to those skilled-in-the-art. Mooring line 505, in some
embodiments of the invention, may comprise a synthetic fiber
tendon. Riser system 500 includes buoy system 507 for vertically
supporting the submerged weight of connector 504, mooring line 505,
flexible conduit 506 and SCR 502. Connector 504 is preferably
connected to flexible member 511, which is connected to fixed
buoyancy buoy system 507. In this manner, connector 504 is
suspended from buoy system 507. Thus, flexible member 511 provides
the vertical support for connector 504. However, preferably
flexible member 511 does not transfer motions (such as vortex
induced vibrations) from buoy system 507 to SCR 502 and flexible
conduit 506 through the connector 504. Buoy system 507 includes
components 507a -507c which operate similarly to components
107a-107c described above with respect to FIG. 1.
[0072] It should be noted that though the titanium flexible conduit
506 has a greater bend radius relative to composite flexible
conduits, it is still less than that of a steel pipe. Accordingly,
referring to FIGS. 1 and 5, typically distances R5 and D5 of a
riser system 500 made of titanium are greater than distances R1 and
D1 of a system 100 that includes a composite flexible conduit.
[0073] In some embodiments of the invention, flexible conduit 506
is made of both steel and titanium sections. For example, for
sections of flexible conduit 506 that have the most curvature or
exposure to significant stress, titanium may be used. Sections 506a
and 506f, for example, may be tapered stress joints and subjected
to significant loads due to the movement of FPSO 508. As such,
sections 506a and 506f may be made of titanium and typically are
about 30 feet in length. Similarly, since section 506d, known as
the dip or sag bend, has a higher curvature than other sections, it
may be made of titanium. On the other hand, where strength or
elasticity is not critical, steel may be used. Thus, for sections
506b and 506e, which are relatively straight and are not subject to
high stress, steel may be preferable. Another possible reason for
using steel is cost. Different sections of titanium and steel may
be joined by methods known in the art such as via welding,
mechanical flanges and the like.
[0074] The problem described above with respect to the pigging of a
riser system having a SCR and a flexible conduit of different
internal diameters may be solved by other methods. For example, in
the case of a gas export riser in which liquid is periodically
deposited in its pipelines, it is difficult to send a pig through
the catenary riser section of the pipe when the catenary riser and
the flexible conduit have different internal diameters. As
mentioned above, one solution is to use flexible titanium conduits
that have the same diameter as a catenary riser.
[0075] One solution to the problem of pigging different sized
conduits is to locate a pig launching device either on the
connector or on the seabed at the location of the Pipeline End
Termination (PLET). Liquids that are to be displaced in pigging
operations frequently accumulate in the valleys of the pipeline
that rests ion the seabed since the seabed is not flat. As such,
pigging need only be carried out on the section of the pipeline
that is on the seafloor and not through the catenary riser section.
Thus, the problem of pigging through the catenary riser and the
flexible conduit having different diameters is avoided.
[0076] Referring now to FIG. 7, riser system 700 includes
components 701-712 that operate similarly to components 101-112 of
riser system 100. FIG. 7, however, shows pipe 709 having curved
sections 709c and 709e (valleys) and 709a, 709b and 709d (peaks)
due to the unevenness of seafloor 703. Liquid condensate will
accumulate in sections 709c and 709e. To clear the liquid
condensate, riser system 700 has a pigging system that includes pig
launching stations 713 and pig receiving station 714. Pig launching
and receiving stations are known in the art and are available from,
for example, RHINO.RTM. Process Equipment. When displacing liquid
from sections 709c and 709e is necessary, a pig is launched from
pig launching station 713. The pig is pushed by a fluid through
line 709 until it reaches pig receiving station 714 where the pig
and the displaced liquid are removed from line 709. In this system,
it is not necessary to have the pig traverse different diameters of
flexible conduit 706 and pipe 709. It should be appreciated that
riser system 700 could be designed so that pig launching station
713 is located on connector 704 and pig receiving station 714
located on seafloor 703, or vice versa.
[0077] Referring to FIGS. 8A and 8B, a riser system 800 is shown
which includes an SCR 802 in fluid communication with pipeline 809.
Pipeline 809 in turn connects to wellhead assembly 810. SCR 802 is
coupled to a flexible conduit 806, at connector 804 so that SCR 802
and flexible conduit 806 are in fluid communication. Flexible
conduit 806 is also connected to FPSO 808 as shown. Connector 804
functions as a tension frame. At the top end of connector 804 is a
pivoting device 804b.
[0078] Referring to FIGS. 8B-8C and 8E, the pivoting device shown
includes a hinge. However, other pivoting devices may be used such
as a trunnion, as also discussed below. At the bottom end of
connector 804 is crosshead 804a, which is a beam connected to the
sides of frame struts 804c. Crosshead 804a supports SCR 802. To
moor riser system 800, tendon 805 extends from hinge 804b to
fastening device 812. Buoyancy device 807 is connected to connector
804 to provide buoyancy support to riser system 800.
[0079] Referring now to FIG. 8B, a perspective view of connector
804 is shown. Flexible members 811 connects hinge 804b to buoy 807
(See FIG. 8A). Any number of flexible members may be used in
embodiments of the invention. For example, it may be desirable to
have more than one flexible member 811 as a safety feature.
Similarly, the mooring system may include one or more tendons 805.
A plurality of tendons may provide more stability and safety
benefits.
[0080] Referring again to FIG. 8A, angle A is found between frame
strut 804c and flexible conduit 806, angle B between frame strut
804c and tendon 805, and angle C between tendon 805 and flexible
conduit 806. After installation, angles A, B and C tend to change
as a result of, for example, loads resulting from a change in the
density of the contents of flexible conduit 806 and SCR 802.
Replacing the liquid in flexible conduit 806 and SCR 802 with a
less dense gas may cause flexible conduit 806 to move in direction
"x" and SCR 802 to move in direction "y." Movements similar to
those indicated in directions "x" and "y" in a conventional riser
system bend the flexible conduit and the SCR. In contrast, in the
present invention of riser system 800, the forces exerted in
directions "x" and "y" would cause either, or both of, flexible
conduit 806 and SCR 802 to pivot about pivoting device 804b. In
other words, when certain loads are exerted on flexible conduit 806
and SCR 802, riser system 800 utilizes the pivoting device to allow
riser system 800 to move to a new state of equilibrium. Thus, riser
system 800 is configured to have loads pass through the pivoting
device, allowing riser system 800 to adjust automatically to
angular variations between components of the riser system without
inducing and storing bending loads on the components, such as
flexible conduit 806, SCR 802 and tendons 805.
[0081] Referring to FIGS. 8C-8E, different configurations for
attaching flexible conduit 806 to connector 804 are shown. FIG. 8C
shows flexible conduit 806 running above hinge beam 804b. It should
be appreciated that because connector 804 includes a pivoting
device, flexible conduit 806 could be a metal pipe. The pivoting
device would, at least partially compensate for the inflexibility
of the metal pipe when loads are applied.
[0082] FIG. 8D shows flexible conduit 806 passing through the
center of trunnion 804d. In this case, pipe 806a is a part of
flexible conduit 806. It should be noted, however, that instead of
connecting flexible conduit 806 directly to SCR 802, pipe 806a may
be a different pipe from flexible conduit 806. In this instance,
pipe 806a is used to connect flexible conduit 806 and SCR 802 at
point "P." The benefit of the design shown in FIG. 8D is that loads
applied to flexible conduit 806 are transmitted directly to the
center of trunnion 804d.
[0083] FIG. 8E shows a design where flexible conduit 806 is
connected to bent pipe 813 having "a gooseneck shape." Bent pipe
813 is in fluid communication with flexible conduit 806 and SCR
802. Bent pipe 813 is supported by triangular plate 814. In this
configuration, a load applied to flexible conduit 806, in direction
"z," is transferred by triangular plate 814 to hinge beam 804b.
Likewise, because the pivoting device is connected to buoy 807,
upward pull loads from buoy 807 are directed to the pivoting
device.
[0084] In sum, as illustrated in FIGS. 8A-8E, embodiments of the
invention seek to have all the major loads exerted on riser system
800 transmitted through a pivoting device (e.g. hinge beam 804b or
trunnion 804d). The loads transmitted to hinge beam 804b or
trunnion 804d will then cause the components, such as flexible
conduit 806 and SCR 802 to pivot around hinge beam 804b or trunnion
804d.
[0085] Referring to FIGS. 9A-9D, an installation method for a riser
system having a connector 904 with a hinge beam 904b is shown. The
installation begins with vessel 908a transporting buoy 907, tendons
905 and SCR 902/pipe 909 to a desired location in body of water
901. On vessel 908a, buoy 907 and tendons 905 are connected to
hinge beam 904b. Further, SCR 902/pipe 909 is welded to, or
otherwise connected, to crosshead 904a. Once installation vessel
908a is at the desired location, vessel 908a deploys buoy 907,
connector 904 and SCR 902/pipe 909 in the water, as shown in FIG.
9A. As connector 904 sinks in the water, tendons 905 are fastened
to seafloor 903 by inserting plugs 912a into receiving devices
912b, well known to those skilled-in-the-art. Vessel 908a then
unravels SCR 902/pipe 909 from a reel and moves away from connector
904. As SCR 902/pipe 909 descends into the water towards seafloor
903, it pulls connector 904 into a vertical alignment as shown in
FIG. 9B.
[0086] Referring still to FIGS. 9A and 9B, vessel 908a continues to
move away from connector 904 and completes laying SCR 902/pipe 909
onto seafloor 903. The curved portion 902 is the catenary riser and
the portion that lays on seafloor 903 is pipe 909. At this point,
flexible conduit 906 is not yet connected to connector 904. To make
this connection, flexible conduit 906 is lowered into the water
from installation vessel 908b by, for example, wires L1 and L2, as
shown in FIG. 9C. Wires L1 and L2 support flexible conduit 906 up
to the side of connector 904. Another lift wire L3 is lowered to
the center of connector 904 through lumen 907a of buoy 907. An ROV
is then used to take hold of hook 916 located on line L3. The ROV
moves hook 916 to aperture 915 at the top of triangular support
913. L3 is then rolled in to pull flexible conduit 906 towards
connector 904 as shown in FIG. 9D.
[0087] Wires L1 and L2 are then removed from flexible conduit 906
by an ROV, for example. Wire L3 is then used to lower flexible
conduit 906 onto SCR 902 for connection. Supporting flexible
conduit 906 through lumen 907a makes it easier to lower flexible
conduit 906 on top of SCR 902. It should be appreciated that the
installation process shown with respect to FIGS. 9A to 9D uses the
flexible conduit configuration described with respect to FIG. 8C.
However, one skilled-in-the-art may apply the installation process
to different flexible conduit configurations such as flexible
conduit configurations shown in FIGS. 8A and 8B.
[0088] Referring back to FIG. 1, embodiments of the invention may
include a variable buoy buoyancy and a fixed buoy buoyancy as shown
with respect to buoy system 107. The design of buoy system 107 with
fixed and variable buoyancy buoys, for installation in riser
systems, has several advantages. First, this design makes it easier
to install riser systems because it facilitates easy lowering of
the riser system at a desired depth. Specifically, the ability to
vary the buoyancy provides an ability to change the depth of
installation. Second, this variable buoy system provides the
ability to select preferred weight requirements. In other words,
fixed buoyancy buoy 107A may be selected such that it is still at a
slight negative buoyancy at the final operating depth
(approximately 1,500 feet in the Gulf of Mexico). Third, in the
event variable buoyancy buoy 107B looses buoyancy and sinks, fixed
buoyancy buoy 107A may still provide a positive vertical load to
support riser system 100 after it sinks marginally, at which point
it will reach an equilibrium state (remain suspended). Equilibrium
is achieved within body of water 101 because as riser system 100
sinks, some of the weight of SCR 102 will be supported on seafloor
103 rather than by buoy system 107. Fourth, the installation
process as disclosed may be easily reversible and thereby
facilitates repairs that may be performed above the surface of the
water. Specifically, the buoyancy applied to the riser system may
be varied and thus after installation, the upward force from the
buoy system may be increased to allow the riser system to ascend
and be easily removed from the water.
[0089] Referring to FIG. 10, an embodiment of the invention may
include composite buoy 1007. Composite buoy 1007 comprises fixed
buoyancy portion 1007A and variable buoyancy portion 1007B. Fixed
buoyancy portion 1007A may comprise syntactic foam or other
material providing a constant or fixed vertical load. Variable
buoyancy portion 1007B may comprise a tank, to and from, which
water may be pumped or any other configuration for providing
variable buoyancy. The configuration of composite buoy 1007 may be
preferred in selected water depths, particularly if it is desirable
to locate the sources of the fixed buoyancy and the variable
buoyancy below anticipated upper and loop currents.
[0090] Referring now to FIG. 11, an embodiment of the invention may
include buoy 1107. Buoy 1107 comprises housing 1108. Housing 1108
encloses syntactic foam buoy elements 1107A, which are separate
elements and may be separated by voids 1107B. When syntactic buoy
1107 is deployed in water, its buoyancy effect may be increased by
passing a gas, such as air, through, for example, pipe 1109.
Conversely, buoy 1107's buoyancy may be decreased by releasing gas
from housing 1108 through valve 1110.
[0091] In embodiments of the invention, mooring line 105 includes
several tendons, which may include synthetic fiber tendons. If one
or more tendons break, in this configuration, an unbroken tendon
could still maintain the installation in the desired location.
Referring again to FIG. 1, if all the tendons break, fixed buoyancy
buoy 107A will rise to the surface of body of water 101 or rise to
a higher level that is below the surface as it moves into an
equilibrium state when the weight of the SCR 102 and pipeline 109
increases (because they are less supported by seafloor 103) to a
point when the upward force from buoy system 107 equals the
downward force from the weight of SCR 102 and pipeline 109. That
is, riser system 100 becomes suspended closer to the surface. In
sum, failure of the components of riser systems comprising
embodiments of the current disclosure will not cause a catastrophic
failure of the whole riser system.
[0092] Riser systems according to embodiments of the invention may
include several combinations of SCR 102, connector 104 and flexible
conduit 106. For example, a first combination of SCR 102, connector
104 and flexible conduit 106 may connect a first wellhead assembly
to a manifold assembly on FPSO 108. Concurrently, a second
combination of SCR 102, connector 104 and flexible conduit 106 may
connect a second wellhead assembly to the same manifold assembly on
FPSO 108. Other configurations may also include different
combinations of SCR 102, connector 104 and flexible conduit 106
running from the same well head to the manifold on FPSO 108. As one
skilled in the art would recognize, such combinations are within
the scope of the current invention.
[0093] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims. Moreover, the scope of the present application is
not intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
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