U.S. patent application number 10/685681 was filed with the patent office on 2005-04-07 for hydro-pneumatic tensioner with stiffness altering secondary accumulator.
Invention is credited to Kelly, Andrew Campbell, McCarty, Jeffery Kirk, Pallini, Joseph William JR..
Application Number | 20050074296 10/685681 |
Document ID | / |
Family ID | 33477236 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050074296 |
Kind Code |
A1 |
McCarty, Jeffery Kirk ; et
al. |
April 7, 2005 |
Hydro-pneumatic tensioner with stiffness altering secondary
accumulator
Abstract
A hydro-pneumatic tensioner includes a barrel having an inner
bore and a pressurized fluid contained within to form at least part
of a primary accumulator having a preset volume of gas at a
preselected pressure. A piston having a piston rod extending from
an aperture in the barrel is slidably carried in the bore of the
barrel and is in communication with the pressurized fluid and
positioned to increase the fluid pressure when the piston strokes
in the direction of the pressurized fluid. A secondary accumulator
also has a preset volume of gas at a preselected pressure. A fluid
separator maintains functional separation of the fluid volumes of
the primary and secondary accumulators when the primary accumulator
pressure is less than a preselected secondary accumulator pressure.
The fluid separator allows functional combining of the fluid
volumes of the primary and secondary accumulators when the primary
accumulator pressure equals or is greater than the preselected
secondary accumulator pressure.
Inventors: |
McCarty, Jeffery Kirk;
(Houston, TX) ; Kelly, Andrew Campbell; (Houston,
TX) ; Pallini, Joseph William JR.; (Tomball,
TX) |
Correspondence
Address: |
James E. Bradley
Bracewell & Patterson LLP
P.O. Box 61389
Houston
TX
77208-1389
US
|
Family ID: |
33477236 |
Appl. No.: |
10/685681 |
Filed: |
October 15, 2003 |
Current U.S.
Class: |
405/224.4 ;
166/367; 405/224.2 |
Current CPC
Class: |
E21B 19/006 20130101;
F16F 9/512 20130101; F16F 9/062 20130101; F16F 9/064 20130101 |
Class at
Publication: |
405/224.4 ;
405/224.2; 166/367 |
International
Class: |
E02D 005/62 |
Claims
What is claimed:
1. A tensioner unit, comprising: a barrel having a bore and an
aperture on one end and having a pressurized fluid contained within
and forming at least part of a primary accumulator, the primary
accumulator having a preset volume of gas Vg1 at pressure P1; a
first piston having a first and second side and slidably carried in
the bore of the barrel, the piston having a piston rod that extends
from the second side of the piston and through the aperture of the
barrel, having one of the sides of the piston in communication with
the pressurized fluid, and positioned to increase the pressure P1
of the primary accumulator when the piston strokes in the direction
of the pressurized fluid; a secondary accumulator housing having a
bore and forming at least part of a secondary accumulator, the
secondary accumulator having a preset volume of gas Vg2 at
preselected pressure P2; a fluid separator having first and second
sides and positioned between the primary and secondary accumulators
to maintain functional separation of fluid volumes of the primary
and secondary accumulators when the primary accumulator pressure P1
is less than the secondary accumulator pressure P2, and to allow
functional combining of the fluid volumes of the primary and
secondary accumulators when the primary accumulator pressure P1 is
greater than or equal to the secondary accumulator pressure P2;
wherein an effective total gas volume VgT available to the
tensioner to maintain tension on a supported riser system
substantially equals the primary accumulator gas volume Vg1 when
the primary accumulator pressure P1 is less than the secondary
accumulator pressure P2; and wherein the effective total gas volume
VgT available to the tensioner to maintain tension on the supported
system substantially equals the sum of the primary accumulator gas
volume Vg1 plus the secondary accumulator gas volume Vg2 when the
pressure P1 is greater than or equal to the secondary accumulator
pressure P2.
2. The tensioner unit of claim 1, further comprising a tubular
housing surrounding the barrel and having pressurized fluid
contained within and forming a part of the primary accumulator, and
wherein: the piston rod includes a bore which forms the secondary
accumulator housing; the barrel has a port on one end in fluid
communication with the tubular housing; the first piston has an
opening in fluid communication with the port; the bore of the
piston rod is in fluid communication with the opening in the
piston; the fluid separator comprises a second piston having first
and second sides and sealingly and slidably carried in the bore of
the piston rod; the bore of the barrel, the bore of the tubular
housing, and the first sides of the first and second piston define
the primary accumulator; and the bore of the piston rod and the
second side of the second piston define the secondary
accumulator.
3. The tensioner unit of claim 1, wherein: the first piston has an
opening in fluid communication with the barrel; the piston rod
includes a bore in fluid communication with the opening in the
piston; the secondary accumulator housing is separate from the
barrel and is in fluid communication with the second side of the
fluid separator; and the first side of the fluid separator is in
fluid communication with an opening in one of the bore of the
piston rod and the bore of the barrel.
4. The tensioner unit of claim 3, wherein: the bore of the barrel,
the bore of the piston rod, the bore of the secondary accumulator
housing, the first side of the first piston, and first side of the
fluid separator define the primary accumulator; the bore of the
secondary accumulator housing and a second side of the fluid
separator define the secondary accumulator; the fluid separator
comprises a second piston sealingly and slidably carried in the
bore of the secondary accumulator housing; and the bore of the
secondary accumulator housing further includes an extension stop
spaced from an end of the secondary accumulator housing which
limits maximum travel of the second piston during piston rod
extension corresponding to a decrease in primary accumulator
pressure P1 below that of the secondary accumulator pressure
P2.
5. The tensioner unit of claim 3, wherein: the bore of the barrel,
the bore of the piston rod, the bore of the secondary accumulator
housing, the first side of the first piston, and first side of the
fluid separator define the primary accumulator; the bore of the
secondary accumulator housing and a second side of the fluid
separator define the secondary accumulator; and the fluid separator
comprises a bladder sealingly engaged in a fixed position within
the bore of the secondary accumulator housing.
6. The tensioner unit of claim 3, wherein: the fluid separator
comprises a valve having a first fluid connection assembly in fluid
communication with the pressurized fluid in one of the bore of the
piston rod and the bore of the barrel, and a second fluid
connection assembly in fluid communication with the pressurized
fluid in the bore of the secondary accumulator housing; the bore of
the barrel, the bore of the piston rod, the first side of the first
piston, and the first fluid connection assembly of the valve define
the primary accumulator; the bore of the tubular housing and the
second fluid connection assembly of the valve define a secondary
accumulator; and wherein the valve connects the pressurized fluid
in the bore of the barrel with the pressurized fluid in the bore of
the secondary accumulator housing when the pressure P1 in the
primary accumulator exceeds the preselected pressure P2 of the
secondary accumulator.
7. The tensioner unit of claim 1, further comprising a tubular
housing surrounding the barrel, the tubular housing having a bore,
a fluid communication opening, and pressurized fluid contained
within, and forming a part of the primary accumulator, and wherein:
the secondary accumulator housing is separate from both the barrel
and the tubular housing and is in fluid communication with the
second side of the fluid separator; the first side of the fluid
separator is in fluid communication with the tubular housing
through the fluid communication opening; and the first side of the
first piston forms a fluid barrier.
8. The tensioner unit of claim 7, wherein: the bore of the barrel,
the bore of the tubular housing, the bore of the secondary
accumulator housing, the first side of the first piston, and first
side of the fluid separator define the primary accumulator; the
bore of the secondary accumulator housing and a second side of the
fluid separator define the secondary accumulator; the fluid
separator comprises a second piston sealingly and slidably carried
in the bore of the secondary accumulator housing; and the bore of
the secondary accumulator housing further includes an extension
stop spaced from an end of the secondary accumulator housing which
limits maximum travel of the second piston during piston rod
extension corresponding to a decrease in primary accumulator
pressure P1 below that of the secondary accumulator pressure
P2.
9. The tensioner unit of claim 7, wherein: the bore of the barrel,
the bore of the piston rod, the first side of the first piston, the
bore of the secondary accumulator housing, and first side of the
fluid separator define the primary accumulator; the bore of the
secondary accumulator housing and a second side of the fluid
separator define the secondary accumulator; and the fluid separator
comprises a bladder sealingly engaged in a fixed position within
the bore of the secondary accumulator housing.
10. The tensioner unit of claim 7, wherein: the fluid separator
comprises a valve having a first fluid connection assembly in fluid
communication with the pressurized fluid in the bore of the tubular
housing, and a second fluid connection assembly in fluid
communication with the pressurized fluid in the bore of the
secondary accumulator housing; the bore of the barrel, the first
side of the first piston, the bore of the tubular housing, and the
first fluid connection assembly of the valve define the primary
accumulator; the bore of the secondary accumulator housing and the
second fluid connection assembly of the valve define the secondary
accumulator; and wherein the valve connects the pressurized fluid
in the bore of the tubular housing with pressurized fluid in the
bore of the secondary accumulator housing when the pressure P1 in
the primary accumulator exceeds the preselected pressure P2 of the
secondary accumulator.
11. The tensioner unit of claim 1, further comprising a primary
accumulator housing having a bore, a plurality of fluid
communication apertures, and having pressurized fluid contained
within and forming a part of the primary accumulator, and wherein:
the first side of the first piston forms a fluid barrier; the
primary accumulator housing is separate from the barrel and is in
fluid communication with the barrel and the first side of the fluid
separator through the fluid communication apertures; the secondary
accumulator housing is separate from the barrel and the primary
accumulator housing and is in fluid communication with the second
side of the fluid separator; and the first side of the fluid
separator is in fluid communication with the bore of the primary
accumulator housing and bore of the barrel through the fluid
communication apertures.
12. The tensioner unit of claim 11, wherein: the bore of the
barrel, the first side of the first piston, the bore of the primary
accumulator housing, the bore of the secondary accumulator housing,
and the first side of the fluid separator define the primary
accumulator; the bore of the secondary accumulator housing and the
second side of the fluid separator define the secondary
accumulator; the fluid separator comprises a second piston
sealingly and slidably carried in the bore of the secondary
accumulator housing; and the bore of the secondary accumulator
housing further includes an extension stop spaced from an end of
the secondary accumulator housing which limits maximum travel of
the second piston during piston rod extension corresponding to a
decrease in primary accumulator pressure P1 below that of the
secondary accumulator pressure P2.
13. The tensioner unit of claim 11, wherein: the bore of the
barrel, the first side of the first piston, the bore of the primary
accumulator housing, the bore of the secondary accumulator housing,
and the first side of the fluid separator define the primary
accumulator; the bore of the secondary accumulator housing and the
second side of the fluid separator define the secondary
accumulator; and the fluid separator comprises a bladder sealingly
engaged in a fixed position within the bore of the secondary
accumulator housing.
14. The tensioner unit of claim 11, wherein: the fluid separator
comprises a valve having a first fluid connection assembly in fluid
communication with the pressurized fluid in the bore of the primary
accumulator housing which is in communication with the bore of the
barrel, and a second fluid connection assembly in fluid
communication with the pressurized fluid in the bore of the
secondary accumulator housing; the bore of the barrel, the first
side of the first piston, the bore of the primary accumulator
housing, and the first fluid connection assembly of the valve
arrangement define the primary accumulator; the bore of the
secondary accumulator housing and the second side of the fluid
separator define the secondary accumulator; and the valve connects
the pressurized fluid in the bore of the primary accumulator
housing and the bore of the barrel with the pressurized fluid in
the bore of the secondary accumulator housing when the pressure P1
in the primary accumulator exceeds the preselected pressure P2 of
the secondary accumulator.
15. A riser tensioning system having a riser extending between
subsea well equipment and a floating vessel having an operational
platform engaged with the riser, a tensioner unit comprising: a
barrel having a port on one end; a first piston slidingly carried
in the barrel and having an opening in fluid communication with the
port; a piston rod extending from the barrel and having a bore in
fluid communication with the opening in the piston; a second piston
sealingly and slidably carried in the bore of the piston rod; a
primary accumulator in fluid communication with the port and having
pressurized fluid therein that communicates with first sides of the
first and second piston; and wherein: the bore of the piston rod
and a second side of the second piston define a secondary
accumulator containing a pressurized fluid; the tensioner unit is
positioned between the riser and the platform; a first section of
the tensioner unit is connected to the riser; and a second section
of the tensioner unit is connected to the platform.
16. The tensioning system of claim 15, wherein the bore of the
piston rod of the tensioner unit includes an extension stop which
limits maximum travel of the second piston during piston rod
extension.
17. The tensioning system of claim 15, wherein the bore of the
piston rod of the tensioner unit includes a retraction stop spaced
from an end of the piston rod which limits maximum travel of the
second piston during piston rod retraction.
18. The tensioning system of claim 15, wherein the primary
accumulator of the tensioner unit comprises a tubular housing
surrounding the barrel.
19. The tensioning system of claim 15, wherein the tensioner unit
further comprises a barrel extension extending from the barrel
which provides for fluid communication between the barrel port and
the primary accumulator.
20. A tensioner unit, comprising: a barrel including a bore; a
first piston slidingly carried in the bore of the barrel and having
an opening in fluid communication with a pressurized fluid; a
piston rod extending from the barrel and having a bore in fluid
communication with the opening in the piston; a secondary
accumulator housing having a bore; a fluid separator sealingly
engaging the bore of the secondary accumulator housing and having a
first and a second side to separate a plurality of volumes of
fluid; a primary accumulator in fluid communication with the bore
of the secondary accumulator housing through an opening in at least
one of the piston rod and barrel and having pressurized fluid
therein that communicates with the first side of the first piston
and first side of the fluid separator; and the bore of the
secondary accumulator housing and a second side of the fluid
separator defining a secondary accumulator containing a pressurized
fluid within.
21. The tensioner unit of claim 20, wherein the fluid separator
comprises a second piston sealingly and slidably carried in the
bore of the secondary accumulator housing.
22. The tensioner unit of claim 21, wherein the bore of the
secondary accumulator housing further includes an extension stop
spaced from an end of the secondary accumulator housing which
limits maximum travel of the second piston during piston rod
extension corresponding to a decrease in primary accumulator
pressure below that of the secondary accumulator pressure.
23. The tensioner unit of claim 20, wherein the fluid separator
comprises a bladder sealingly engaged in a fixed position within
the bore of the secondary accumulator housing.
24. A tensioner unit, comprising: a barrel having a port on one end
and in communication with a pressurized fluid; a tubular housing
surrounding the barrel and having a bore, an opening, and an
aperture on one end; a piston having a first and second side,
slidingly carried in the bore of the barrel, and having a piston
rod extending from the barrel on the second side of the piston
through the aperture; a secondary accumulator housing including a
bore, an opening in the bore in fluid communication with the
opening in the bore of the tubular housing, and a fluid separator
sealingly engaging the bore of the secondary accumulator housing
and having a first and a second side to separate a plurality of
volumes of fluid; a primary accumulator in fluid communication with
the port of the barrel and the opening in the bore of the tubular
housing, and having pressurized fluid therein that communicates
with the first side of the piston and the first side of the fluid
separator; and the bore of the secondary accumulator housing and
the second side of the fluid separator defining a secondary
accumulator containing a pressurized fluid.
25. The tensioner unit of claim 24, wherein the fluid separator
comprises a second piston sealingly and slidably carried in the
bore of the secondary accumulator housing.
26. The tensioner unit of claim 25, wherein the bore of the
secondary accumulator housing further includes an extension stop
spaced from an end of the secondary accumulator housing which
limits maximum travel of the second piston during piston rod
extension corresponding to a decrease in primary accumulator
pressure below that of the secondary accumulator pressure.
27. The tensioner unit of claim 24, wherein the fluid separator
comprises a bladder sealingly engaged in a fixed position within
the bore of the secondary accumulator housing.
28. A tensioner unit, comprising: a barrel having a bore, a fluid
communication opening, an aperture on one end, and having a volume
of pressurized fluid therein; a first piston having a first and a
second side, slidingly carried in the bore of the barrel and having
a piston rod that extends from the piston through the fluid
communication opening of the barrel; a primary accumulator housing
having a bore, a plurality of fluid communication openings, and in
fluid communication with the fluid communication opening of the
barrel; a secondary accumulator housing having a bore, a fluid
communication opening, and having a fluid separator sealingly
engaging the bore of the secondary accumulator housing, the fluid
separator having a first and a second side to separate fluid in the
secondary accumulator housing, the secondary accumulator housing
further being in fluid communication with the primary accumulator
housing through the fluid communication opening of the secondary
accumulator housing and one of the fluid communication openings of
the primary accumulator housing; a primary accumulator in fluid
communication with the fluid communication opening of the barrel
and the fluid communication opening of the secondary accumulator
housing and having pressurized fluid that communicates with the
first side of the first piston and first side of the fluid
separator; and the bore of the secondary accumulator housing and
the second side of the fluid separator defining a secondary
accumulator containing a pressurized fluid.
29. The tensioner unit of claim 28, wherein the fluid separator
comprises a second piston sealingly and slidably carried in the
bore of the secondary accumulator housing.
30. The tensioner unit of claim 29, wherein the bore of the
secondary accumulator housing further includes a lower stop spaced
from an end of the secondary accumulator housing which limits
maximum travel of the second piston during piston rod extension
corresponding to a decrease in pressure of the primary accumulator
below the pressure of the secondary accumulator.
31. The tensioner unit of claim 29, wherein the fluid separator
comprises a bladder sealingly engaged in a fixed position within
the bore of the secondary accumulator housing.
32. A tensioner unit, comprising: a barrel including a bore and an
aperture on one end and having a pressurized fluid contained within
and forming at least part of a primary accumulator; a piston in
communication with a pressurized fluid and slidingly carried in the
bore of the barrel and having a piston rod that extends from the
piston through the aperture; a secondary accumulator housing having
a bore and a pressurized fluid having a preselected pressure
contained within; a valve arrangement having a first fluid
connection assembly in fluid communication with the pressurized
fluid in one of the bore of the piston rod and the bore of the
barrel, and a second fluid connection assembly in fluid
communication with the pressurized fluid in the bore of the
secondary accumulator housing to connect the pressurized fluid in
the bore of the barrel with the pressurized fluid in the bore of
the secondary accumulator housing when the pressure of the
pressurized fluid in the bore of the barrel exceeds the preselected
pressure of the pressurized fluid in the bore of the secondary
accumulator housing; the bore of the barrel, and the first fluid
connection assembly of the valve arrangement defining a primary
accumulator; and the bore of the secondary accumulator housing and
the second fluid connection assembly of the valve arrangement
defining a secondary accumulator.
33. A method of maintaining a selected range of tension on a riser
extending between subsea well equipment and a floating vessel,
comprising the steps of: providing a barrel having a bore and an
aperture on one end, the barrel forming at least part of a primary
accumulator; mounting a piston slidably in the barrel, the piston
having a piston rod extending from the barrel through the aperture;
providing a secondary accumulator housing having a bore and forming
at least part of a secondary accumulator; mounting a fluid
separator between the primary and secondary accumulators;
connecting either the piston rod or the barrel to the vessel and
the other to the riser; applying fluid pressure from the primary
accumulator to a first side of the first piston and a first side of
the fluid separator; applying fluid pressure into the secondary
accumulator housing on a second side of the fluid separator until
pressure on both sides of the fluid separator equal; applying
tension to the riser by the force due to pressure of fluid in the
primary accumulator, urging the piston rod to extend; if vessel
moves closer to the subsea equipment, allowing the piston rod to
extend farther and allowing the fluid separator to functionally
isolate the fluid volume in the secondary accumulator from the
fluid volume in the primary accumulator to maintain tension range;
and if vessel moves farther from the subsea equipment, allowing the
piston rod to retract and allowing the fluid separator to
functionally combine the fluid volume in the secondary accumulator
with the fluid volume in the primary accumulator to maintain
tension range.
34. The method of claim 33, further comprising limiting maximum
travel of the second piston during piston rod extension by
contacting the second piston with an extension stop.
35. The method of claim 33, further comprising limiting maximum
travel of the second piston during piston rod retraction by
contacting the second piston with a retraction stop.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to tensioning of seabed-to-vessel
marine risers. More particularly, this invention relates to
tensioning the marine risers with a plurality of pneumatic or
hydraulic cylinders.
[0003] 2. Brief Description of the Related Art
[0004] A problem presented by offshore hydrocarbon drilling and
producing operations conducted from a floating platform is the need
to establish a sealed fluid pathway between each borehole or well
at the ocean floor and the work deck of the platform at the ocean
surface. A riser typically provides this sealed fluid pathway. In
drilling operations, the drill string extends through a drilling
riser, the drilling riser serving to protect the drill string and
to provide a return pathway outside the drill string for drilling
fluids. In producing operations, a production riser is used to
provide a pathway for the transmission of oil and gas to the work
deck.
[0005] The riser is projected up through an opening referred to as
a moon pool in the vessel to working equipment and connections
proximate an operational floor on the vessel. A riser pipe
operating on the floating vessel in water depths greater than about
200 feet (34.72 meters) can buckle under the influence of its own
weight and the weight of drilling fluid contained within the riser
if it is not partially or completely supported. For floating
platforms, a special piece of equipment known as a "riser
tensioner" is required to maintain each riser within a range of
safe operating tensions as the work deck moves relative to the
upper portion of the riser. If a portion of the riser is permitted
to go into compression, it could be damaged by buckling or by
bending and colliding with adjacent risers. It is also necessary to
ensure that the riser is not over-tensioned when the vessel hull
moves to an extreme lateral position under extreme wave conditions
or when ocean currents exert a significant side loading on the
riser.
[0006] A tension leg platform ("TLP") is a type of marine structure
having a buoyant hull secured to a foundation on the ocean floor by
a set of tethers. The tethers are each attached to the buoyant hull
so that the hull is maintained at a significantly greater draft
than it would assume if free floating. The resultant buoyant force
of the hull exerts an upward loading on the tethers, maintaining
them in tension. The tensioned tethers limit vertical motion of the
hull, thus substantially restraining it from pitch, roll and heave
motions induced by waves, currents and wind. However, unlike
conventional platforms which are rigidly attached to the subsea
floor, TLPs are not designed to resist horizontal forces induced by
waves.
[0007] The marine risers have been tensioned in various manners,
including the use of counterweights and pneumatic spring systems.
The counterweight was the first technique utilized to apply tension
to the top of the marine riser. The weight was hung from a wire
rope which was run up through wire rope sheaves and down to an
upper portion of the riser pipe. The tension was equal to the
counterweight and therefore was practicable only for shallow water
that required low tension.
[0008] The pneumatic spring systems replaced the counterweight
systems as deeper water drilling evolved. The pneumatic tensioning
devices stored energy in the form of compressed air to apply
tension to the top of the riser through wire ropes. The pneumatic
tensioning devices typically involved the use of cylinders from
which a piston rod was extended, the piston rod having a sheave
engaged by the wire rope to be tensioned. The fluid within the
hydraulic cylinder was thereby compressed into an accumulator. The
cylinder and the accumulator were normally supported by support
structures on the floating vessel.
[0009] Many tensioner systems in use today act as oil-damped
pneumatic springs. A large gas supply keeps a nearly constant
pressure above the oil in a gas-oil accumulator. The oil then
provides pressure to the face of the piston. As the vessel heaves,
the piston moves up and down against a relatively constant force
and the tension lines maintain a relatively constant pull.
[0010] Many riser tensioners today utilize hydraulically actuated
cylinders with pneumatic pressure accumulators to provide the force
necessary to maintain the upper portion of the riser within a
preselected range of operating tensions. One implementation is
accomplished by the use of sheaves attached to the buoyant drilling
structure whereby tensioning cables are run over the sheaves and
attached to the riser so that the riser is supported by one end of
the tensioning cables. The other end of each tensioning cable is
connected to a piston of a hydraulic cylinder. The hydraulic
cylinders are connected to a relatively large accumulator which
maintains the load applied by the cylinders at a relatively
constant level over the full range of travel of the pistons. Thus,
as the platform moves vertically, the pistons stroke to maintain a
relatively constant upward loading on the riser.
[0011] Another type of riser tensioner typically used on TLPs also
uses a pneumatically pressurized fluid accumulator but eliminates
the cables and sheaves used in earlier riser tensioners. Gas and
oil accumulators are connected to the cylinders to control the
stroke of pistons. The piston rods are directly attached to a riser
tensioning ring which supports the riser.
[0012] Both classes of riser tensioning systems described generally
require separate and relatively large accumulators to maintain the
load applied by the cylinders with an acceptable range.
Accordingly, it can be appreciated that there still exists a need
for an improved riser tensioner system which provides high-capacity
tension and provides for limiting peak loads while incorporating
high nominal stiffness, and that does not require an excessively
large accumulator.
SUMMARY OF THE INVENTION
[0013] In view of the foregoing, an embodiment of the present
invention advantageously provides a tensioner unit including a
barrel having a bore with pressurized fluid contained within. The
barrel forms at least part of a primary accumulator having a preset
volume of gas Vg1 at pressure P1. The barrel of the tensioner unit
also includes an aperture to allow extension and retraction of a
piston rod. The tensioner unit also includes a piston slidably
carried in the bore of the barrel. The piston has a piston rod that
extends from one side of the piston and through an aperture in the
barrel. The piston has one of its sides in communication with the
pressurized fluid, and is so positioned to increase the pressure P1
of the primary accumulator when the piston strokes in the direction
of the pressurized fluid. The tensioner unit also includes a
secondary accumulator housing having a bore which forms at least
part of a secondary accumulator having a preset volume of gas Vg2
at pressure P2. The tensioner unit also includes a fluid separator
positioned between the primary and secondary accumulators to
maintain functional separation of fluid volumes of the primary and
secondary accumulators when the primary accumulator pressure P1 is
less than the secondary accumulator pressure P2. The fluid
separator also allows functional combining of the fluid volumes of
the primary and secondary accumulators when the primary accumulator
pressure P1 is greater than or equal to the secondary accumulator
pressure P2. The tensioner unit is configured so that when the
primary accumulator pressure P1 is less than the secondary
accumulator pressure P2, the effective total gas volume VgT
available to the tensioner to maintain tension on a supported
system, such as a riser system, is substantially equivalent to the
primary accumulator gas volume Vg1. Correspondingly, when the
pressure P1 is greater than or equal to the secondary accumulator
pressure P2, the effective total gas volume VgT available to the
tensioner to maintain tension on the supported system equals the
sum of the primary accumulator gas volume Vg1 plus the secondary
accumulator gas volume Vg2. This provides for reduced stiffness and
reduced maximum tension applied by the tensioner unit to a
supported system when the system is directing a maximum load on the
tensioner unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] So that the manner in which the features, advantages and
objects of the invention, as well as others which will become
apparent, are attained and can be understood in more detail, more
particular description of the invention briefly summarized above
may be had by reference to the embodiment thereof which is
illustrated in the appended drawings, which drawings form a part of
this specification. It is to be noted, however, that the drawings
illustrate only a preferred embodiment of the invention and is
therefore not to be considered limiting of its scope as the
invention may admit to other equally effective embodiments.
[0015] FIG. 1 is a perspective view of a hydro-pneumatic tensioner
system in accordance with the invention.
[0016] FIG. 2 is an enlarged perspective view of one of the
hydro-pneumatic tensioner units of the hydro-pneumatic tensioner
system of FIG. 1.
[0017] FIG. 3 is a schematic view of the hydro-pneumatic tensioner
unit having a first piston enclosing a second piston shown with the
first piston in an extended position.
[0018] FIG. 4 is a schematic view of the hydro-pneumatic tensioner
unit of FIG. 3, shown a mid-point position.
[0019] FIG. 5 is a schematic view of the hydro-pneumatic tensioner
unit of FIG. 3, shown a retracted position.
[0020] FIG. 6 is a schematic view of an embodiment of a
hydro-pneumatic tensioner unit, including an internal cylinder
barrel connected to an external cylinder having a second
piston.
[0021] FIG. 7 is a schematic view of a variation of FIG. 6.
[0022] FIG. 8 is a schematic view of a variation of FIG. 6 having
no internal cylinder barrel.
[0023] FIG. 9 is a schematic view of an external cylinder for use
with the tensioner unit of Figure 6, having a bladder which forms
an enclosure containing a gas volume Vg2 and a hydraulic volume
Vh1.
[0024] FIG. 10 is a schematic view of an external cylinder for use
with the tensioner unit of FIG. 6, having a pilot valve interface
that forms an enclosure containing a gas volume Vg2 and a hydraulic
volume Vh2.
[0025] FIG. 11 is a schematic view of another embodiment of a
hydro-pneumatic tensioner unit, having a first piston connected to
an external cylinder having a second piston, shown with the first
piston in an extended position.
[0026] FIG. 12 is a schematic view of the hydro-pneumatic tensioner
unit of FIG. 11, shown with the first piston in a mid-point
position.
[0027] FIG. 13 is a schematic view of the hydro-pneumatic tensioner
unit of FIG. 11, shown with the first piston in a retracted
position.
[0028] FIG. 14 is a schematic view of an external cylinder for use
with the tensioner unit of FIG. 11, having a bladder which forms an
enclosure containing a gas volume Vg2.
[0029] FIG. 15 is a schematic view of an external cylinder for use
with the tensioner unit of FIG. 11, having a pilot valve interface
that forms an enclosure containing a gas volume Vg2.
[0030] FIG. 16 is a schematic view of a variation of FIG. 7.
[0031] FIG. 17 is a schematic view of a pull-type hydro-pneumatic
tensioner unit having a first external cylinder hydraulically
connected to the hydraulic side of a first piston.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] The present invention will now be described more fully
hereinafter with reference to the accompanying drawings which
illustrate embodiments of the invention. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the illustrated embodiments set forth
herein. Rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art. Like numbers
refer to like elements throughout, and the prime notation, if used,
indicates similar elements in alternative embodiments.
[0033] Referring to FIG. 1, shown is a hydro-pneumatic riser
tensioner system 20 for maintaining a riser system within a range
of safe operating tensions as a work deck or platform moves
relative to the upper portion of the riser system. The system 20
advantageously provides a methodology for incorporating high
nominal stiffness while limiting peak loads at extreme up and down
strokes. More specifically, the system 20 includes a riser system
22, extending between a floating vessel having an operational
platform 24, 26, and the sea bottom (not shown).
[0034] The riser system 22 is supported in tension at its upper end
to prevent the riser system 22 from buckling due to its own weight.
The upward tension is maintained within acceptable limits over a
typical platform deviation range by the hydro-pneumatic riser
tensioner system 20. The riser tensioner system 20, discussed in
detail below, includes a plurality of hydro-pneumatic tensioner
units 28. The tensioner units 28 include a main body or housing 30,
typically tubular shaped, stationarily located between support
frames 24, 26, of a support module 23. Each tensioner unit 28
includes a hydro-pneumatic ram tensioner piston rod 32 attached at
its upper end to a riser tensional mount or top plate 34, which
includes a riser tensional ring 36. The riser tensional ring 36
engages a tension joint 38 of the riser system 22. The
hydro-pneumatic ram tensioner piston rod 32 telescopes relative to
housing 30 in response to movement of the operational platform.
[0035] Referring now to FIGS. 1 and 2, in a typical implementation
of the hydro-pneumatic tensioner system 20, typically each
hydro-pneumatic tensioner unit upper section is attached to the top
plate 34 and the lower section is connected to the support module
lower support frame 24, with each unit 28 protruding through an
aperture 27 in the upper support frame 26 of the support module 23.
In this implementation, the riser system 22 includes a riser
conductor 42 and roller assemblies 44, on lower and upper support
frames 24, 26, of the support module 23 to provide for axial
movement of the support module 23 with respect to the riser system
22.
[0036] In this embodiment, as the support module 23 shifts position
with respect to the riser system 22, the riser system 22 slides up
and down through the roller assemblies 44 and is engaged with
tension from the hydro-pneumatic riser tensioner system 20 through
the tensioner units 28 applying tension to the top plate 34. The
top plate 34 via the mounting ring 36 translates the tension from
the tensioner units 28 to the tension joint 38 of the riser system
22.
[0037] Referring now to FIGS. 2 and 3, as stated, the
hydro-pneumatic riser tensioner system 20 includes a plurality of
hydro-pneumatic tensioner units 28. Each of the units 28 has an
upper end and a lower end. The upper end can include a rod end cap
46. The rod end cap 46, and thus the upper end of each unit 28 can
be connected to the top plate 34 (FIG. 1) to provide the requisite
tension to the tension ring 36 (FIG. 1) and thus to the riser
system 22. The rod end cap 46 is shown as threadingly attached to a
shoulder or flange 48 formed of or attached to the main body of
piston rod 32, using a plurality of bolts 50, each affixed in a
bolt cavity (not shown), however, any appropriate attachment or
sealing methodology as known by those skilled in the art is
appropriate and within the scope of the invention. In an
embodiment, the lower end of the tension unit 28 is connected to
the operational marine platform (not shown). In another embodiment,
the lower end of the units 28 is connected to a lower platform or
frame 24 of a hydro-pneumatic tensioner support module 23 (FIG.
1).
[0038] Referring to FIG. 3 for illustrative purposes only, generic
to embodiments, described below, the tensioner unit 28 includes a
barrel such as inner cylinder barrel 60 or main body 30 each having
a bore and an aperture on at least one end and having a pressurized
fluid contained within. The barrel forms at least part of a primary
accumulator having a preset volume of gas Vg1 at pressure P1. A
piston such as piston 72 is slidably carried in the bore of the
barrel. The piston has a piston rod that extends from a side of the
piston and through the aperture of the barrel. One side 82 of the
piston 72 is in communication with the pressurized fluid. The
piston 72 is positioned to increase pressure P1 of the primary
accumulator when the piston 72 strokes in the direction of the
pressurized fluid. A secondary accumulator housing, such as the
piston rod 32 with inner bore 76 or the external housings shown in
FIG. 9 or 12-15 (described later) form at least part of a secondary
accumulator having a preset volume of gas Vg2 at pressure P2. A
fluid separator, such as floating piston 92 (FIG. 3), or the
bladder or valve arrangement shown in FIGS. 9-12 (described later),
is positioned between the primary and secondary accumulators to
maintain functional separation of fluid volumes of the primary and
secondary accumulators when the primary accumulator pressure P1 is
less than the secondary accumulator pressure P2, and to allow
functional combining of the fluid volumes of the primary and
secondary accumulators when the primary accumulator pressure P1 is
greater than or equal to the secondary accumulator pressure P2.
This arrangement results in an effective total gas volume VgT
available to the tensioner 28 to maintain tension on a supported
system, such as a riser system, to substantially equal the primary
accumulator gas volume Vg1 when the primary accumulator pressure P1
is less than the secondary accumulator pressure P2, and to
substantially equal the sum of the primary accumulator gas volume
Vg1 plus the secondary accumulator gas volume Vg2 when the pressure
P1 is greater than or equal to the secondary accumulator pressure
P2. The specific details of the various embodiments implementing
the above described are further introduced below.
[0039] Referring to the embodiment shown in FIG. 3, tensioner unit
28 includes a primary accumulator 52. The primary accumulator 52
includes an upper end having a piston seal housing aperture 54, a
lower end preferably including an endcap 56 (FIG. 2), and a main
body 30 having an axial bore 58, therebetween. The combination, as
described, along with internal components, described below, form an
enclosure. The primary accumulator 52 contains a gas volume Vg1 and
a hydraulic volume Vh1. Also in an embodiment, the lower end cap 56
(FIG. 2) is threadingly connected within a lower end bore (not
shown), although any appropriate attachment or seal means as known
by those skilled in the art is acceptable and within the scope of
the invention. The sealed enclosure of the primary accumulator 52
forms a gas and hydraulic fluid reservoir. A charging connection
(not shown) can be interfaced with the main body 30 of the
accumulator 52 to permit intermittent or continuous charging of
both the gas and hydraulic volumes Vg1, Vh1, from an external
pressure source (not shown).
[0040] Referring to FIGS. 3-5, the hydro-pneumatic riser tensioner
system 20 (FIG. 1) also can include a stationery or internal
cylinder barrel 60 located within the primary accumulator 52. The
cylinder barrel 60 is secured sealingly to aperture 54 of
accumulator 52. The cylinder barrel 60 includes an upper end having
an upper piston rod aperture 62, a lower end having a lower fluid
inlet aperture 64, and a main body having an axial inner bore 66,
therebetween. In the preferred embodiment, the cylinder barrel 60
includes a lower fluid aperture channel extension 68 which
protrudes to a position adjacent the lower end of primary
accumulator 52, and provides for fluid communication between the
cylindrical barrel inner bore 66 and primary accumulator hydraulic
volume Vh1. The length of aperture channel extension 68 is
generally formulated for the purpose of ensuring only hydraulic
fluid entry into the cylinder barrel inner bore 66, not gas. The
length of the channel extension 68 may be established depending
upon the operating conditions the unit 28 will be subjected to and
the desired ratio of gas Vg1 to fluid Vh1. The combination, as
described, along with internal components, described below, form a
partial enclosure. The cylinder barrel 60 interfaces and is thus in
fluid communication through its lower aperture 68 with the primary
accumulator hydraulic volume Vh1. The volume of hydraulic fluid
contained within the cylinder barrel 60 is directly dependent upon
the gas pressure P1 within the primary accumulator 52 and stroke
position of the tensioner piston rod 32.
[0041] The tensioner 28 has a first piston assembly 70 that moves
axially within barrel 60. The piston assembly 70 includes a
combination first piston 72 and piston rod 32. The piston assembly
70 also includes an upper end typically interfaced with a rod end
cap 46 (FIG. 2), a lower end having a piston 72 including an
aperture 74, and a main body therebetween including an inner bore
76. The combination, as described above and as to be described
below, forms a channel which forms an enclosure of varying size
depending on primary accumulator gas pressure P1 and stroke
position of the riser tensioner piston rod 32. In this embodiment,
preferably the aperture 74 of piston 72 is not the same diameter as
bore 76. The piston 72 has an outer diameter substantially
equivalent to the inner diameter of the axial inner bore 66 of
cylinder barrel 60. Also, preferably, the outer diameter of piston
rod 32 is substantially equivalent to the diameter of piston rod
aperture 62.
[0042] The first piston assembly 70 is slidably positioned to
telescope from inside the cylinder barrel 60 to provide tension and
stiffness to the riser system 22. Preferably the outer diameter of
piston rod 32 is smaller than the inner diameter of cylinder barrel
inner bore 66. This differential diameter between the first piston
diameter/cylindrical barrel inner bore diameter and the piston rod
main body diameter/piston rod aperture diameter forms a back
annulus 78. The back annulus 78 is preferably vented to the
atmosphere or through vent lines (not shown) following an
appropriate methodology as known by those skilled in the art.
[0043] The telescopic range of the first piston 72 can be limited
by stops, described below, which present an upper and lower limit
for the travel of the first piston 72. In the configuration shown
in FIGS. 3-5, the differential diameter between the piston rod 32
and first piston 72 form a shoulder 79 on the upper surface of the
first piston 72 adjacent its outer circumference. The upper limit
is the position where the first piston upper shoulder 79 contacts
the lower surface 80 of the upper end of the cylinder barrel 60.
The lower limit is a position where the lower surface 82 of the
first piston 72 contacts the upper surface 84 of the bottom of the
cylinder barrel 60.
[0044] The piston assembly inner bore 76 interfaces and is in fluid
communication through its lower aperture 64 with hydraulic fluid in
the cylinder barrel 60, which as previously stated is in
communication with primary accumulator hydraulic volume Vh1. The
volume of hydraulic fluid entering the piston assembly inner bore
76 adjacent its lower side is directly dependent upon gas pressure
P1 within the primary accumulator 52, which is directly dependent
upon stroke position of the tensioner piston rod 32. The first
piston 72 includes seals 90 that slidably engage and seal the bore
66 of barrel 60.
[0045] The first piston 72, particularly, the inner bore 76,
encloses a second piston 92 which form a housing for a secondary
accumulator. The second piston 92, a fluid separator, has an upper
and lower surface 94, 96, and a diameter that is substantially
equal to the diameter of the first piston rod inner bore 76. The
second piston 92 is slidably carried within the piston rod inner
bore 76. In the embodiment shown, the second piston 92 includes
pressure seals that slidingly engage inner bore 76. The second
piston upper surface 94 in combination with the piston rod inner
bore 76 form an enclosure, which along with a gas volume Vg2 and
hydraulic volume Vh2, sealingly contained on the upper side 94 of
the second piston 92, define a secondary accumulator 98 having a
gas pressure P2. In a preferred variation, a charging connection
(not shown) is located on the piston assembly 70 to permit
intermittent or continuous charging of both the gas Vg2 and
hydraulic volumes Vh2 from an external pressure source (not shown),
depending on the embodiment implemented. In an alternative
variation, the secondary accumulator 98 may include an interface
(not shown) with an external accumulator (not shown) to provide
added gas volume Vg2. Although FIGS. 3-5 show use of hydraulic
volumes Vh1 and Vh2, the primary accumulator 52 and secondary
accumulator 98 can be made purely of a gas having volume Vg1 and
Vg2, respectively. The use of hydraulic oil or fluid is, however,
preferred in this embodiment as it reduces the requirements on the
first and second piston high-pressure seals.
[0046] The first piston rod inner bore 76 can also include a set of
stops, 86, 88. Preferably retraction stops 86 are fixedly attached
or formed of a protrusion in a position to define the maximum upper
travel limit of the second piston 92 and ensure secondary
accumulator 98 integrity. Extension stops 88 can be formed within
either the inner bore 76 of the piston rod 72 or within or as part
of the first piston aperture 74. FIGS. 3-5 depict the extension
stop 88 as formed from part of a lower surface 82 of the first
piston 72 which effectively forms the aperture 74 into a two-stage
aperture within the first piston 72.
[0047] Peak loads or tension on the tensioner unit 28 occurring
during a retraction and extension of the piston rod 32 are reduced
due to the implementation of floating piston 92 within the piston
rod 32. While the floating piston 92 is free-floating (not pegged
against a retraction or extension stop, 86, 88), the primary
accumulator pressure P1 equals the same pressure P2 as in the
secondary accumulator 98, and the total effective gas volume VgT of
the unit is effectively increased by the gas volume Vg2 of the
secondary accumulator 98. The increase in gas volume provides a
slower rate of increase in gas pressure P1 due to the added volume
Vg2, and also reduces the peak load or tension of a given
downstroke (retraction of piston rod 32). This advantageously
allows for the use of a smaller tensioner unit 28. The net effect
of the inclusion of the gas volume Vg2 into the total effective gas
volume VgT and its deletion when pressure P1 decreases below the
preselected pressure P2 is to limit the minimum tension applied to
the supported system (FIG. 1) at full extension and to also limit
the maximum tension applied at near full retraction.
[0048] The following, for illustrative purposes only, is a
description of the operation of tensioner unit 28 on riser system
of 20 according to an embodiment of the present invention as
depicted in FIGS. 3-5. In operation, the hydro-pneumatic tensioner
unit 28 maintains the riser system 22 within a range of safe
operating tensions as a work deck or platform (not shown) moves
relative to the upper portion of the riser system 22. The
hydro-pneumatic tensioner system 20 ideally should be in a similar
configuration to that shown in FIG. 3. With the hydro-pneumatic
tensioner system 20 properly installed, normally, the first piston
72 is established in the vicinity of the upper portion of the
cylinder barrel 60, and the second piston 92 is normally
established in a contact position with the lower stop 88. In this
situation, pressure P1 on the lower side 96 of the second piston 92
is normally less than the pressure P2 on the upper side 94 of the
second piston 92.
[0049] In a partial retraction situation, as best shown in FIG. 4,
normally, the first piston 72 is established in some mid-range
position of the cylinder barrel 60, and the second piston 92 is
still normally established in a contact position with the lower
stop 88. In this situation, pressure P1 on the lower side 96 of the
second piston 92 is still normally less than the pressure P2 on the
upper side 94 of the second piston 92 but with a decreased pressure
differential (P2-P1) maintaining that position. The total effective
gas volume VgT is maintained substantially at that volume of the
primary accumulator Vg1. As the piston 72 continues to stroke down
(retract) tension increases at a sufficient rate to provide
sufficient support to the riser system 22.
[0050] In a near full retraction situation, as best shown in FIG.
5, normally, in a situation where the first piston 72 was operating
without the benefit of the added second piston accumulator 98,
tension would increase rapidly. The second piston 92, however,
having its own "charge" moves upward to maintain the two pressures
P1 and P2 in substantial equilibrium unless the upper stop 86 is
contacted. Thus, the implementation of the second piston 92, within
the piston assembly 70, results in the effective amount of gas VgT
being effectively increased over that of Vg1. The increase in gas
volume provides a slower rate of increase in gas pressure P1 due to
the added volume and also reduces the peak load of a given
downstroke (retraction of piston rod 32).
[0051] Beginning now with the piston assembly 70 in a fully
retracted position (not shown), whereby the pressure in the primary
accumulator P1 may exceed that of the secondary accumulator 98
(piston 92 in contact with upper stop 86), as the piston 72 strokes
up (extends), the pressure in the primary accumulator P1 decreases
until it substantially reaches the pressure of the secondary
accumulator P2. At that point, the floating piston 92 becomes
evenly loaded on either side (e.g. FIG. 5). As mentioned above,
while the floating piston 92 is free-floating, the primary
accumulator pressure P1 substantially equals that of the secondary
accumulator P2, which results in a total effective gas volume VgT
increase by the gas volume of the secondary accumulator Vg2.
[0052] Beginning at a near full retraction position, as best shown
in FIG. 5, in an extension situation, without the second piston
accumulator 98, tensions would decrease rapidly. The second piston
92, however, having its own "charge" moves downward, maintaining
the two pressures P1 and P2 in substantial equilibrium until the
lower stop 88 is contacted. Thus, as stated previously, due to the
implementation of a second piston 92, within the piston assembly
70, the effective amount of gas VgT is effectively increased. The
increase in gas volume provides a slower rate of decrease in gas
pressure due to the added volume. Once the primary accumulator
pressure P1 decreases sufficiently, the second piston 92 is again
established in a contact position with the lower stop 88 (e.g. FIG.
4). The increase in stiffness is reduced at near full retraction
unless pressure P1 increases above the charging pressure of the
secondary accumulator 98. The net effect of increasing the gas
volume VgT at the near full retraction positioned to decrease
maximum stiffness and to limit the minimum tension applied to the
riser system 22 (FIG. 1) at full upstroke (extension of piston rod
32).
[0053] In another embodiment of the present invention, as perhaps
best shown in FIG. 6, a tensioner unit 128 similar to the tensioner
unit 28 described with respect to FIG. 3 can be modified to remove
second piston 92 from bore 76 of piston rod 32. In this embodiment,
the piston inner bore 176 also interfaces and is in communication
through its lower aperture 164 with hydraulic fluid in the cylinder
barrel 160. However, with second piston 92 removed, the pressure
and any gas volume within a bore 176 of piston rod 132 is that of
the primary accumulator 152. In this embodiment, the gas volume
contained within main body 130 and any gas volume contained in
piston rod 132 combine to form the primary accumulator gas volume
Vg1. The amount of hydraulic fluid entering the piston assembly
inner bore 176 adjacent its lower side is directly dependent upon
gas pressure P1 within the primary accumulator 152, which is
directly dependent upon stroke position of the piston rod 132.
Note, although FIG. 6 depicts a gas volume in the bore 176 of
piston rod 132, piston rod 132 may be filled entirely with part of
the hydraulic volume of the primary accumulator 152. Additionally,
bore 176, at for example aperture 174, may be capped so that gas
volume Vg1 is contained entirely within main body 130. Also note,
if the structure described in FIG. 3 were used to form this
embodiment, optional features, such as upper and lower stops, 86,
88, described with respect to FIG. 3 are unnecessary. If installed,
however, they do not affect the functional operation of the
embodiment as configured and shown in FIG. 6.
[0054] In this embodiment, the tensioner unit 128 includes a
secondary accumulator 121 having housing 101, separate from the
cylinder barrel 160, piston rod 132, and main body 130, which
interfaces and is in fluid communication through its aperture 103
with primary accumulator hydraulic fluid Vh1 (or gas Vg1) either in
the cylinder barrel 160, piston rod 132, or main body 130 (as
shown). Correspondingly, depending on the configuration, either
cylinder barrel 160, piston rod 132, or main body 130, can have an
aperture 105 and a fluid connection assembly 107, as known and
understood by those skilled in the art, to be connected to the
aperture 103 of the secondary accumulator housing 101. Preferably,
the secondary accumulator housing 101 is in the form of an external
cylinder having an inner bore 109. In the configuration shown in
FIG. 6, the secondary accumulator housing 101 includes a fluid
separator in the form of a floating piston 111. The inner bore 109
encloses the floating piston 111. The floating piston 111 has an
upper and lower surface 113, 115, and a diameter that is
substantially equal to the diameter of the secondary accumulator
housing inner bore 109. The floating piston 111 can include
pressure seals that slidingly engage the inner bore 109. The
floating piston upper surface 113 in combination with the secondary
accumulator housing inner bore 109 form an enclosure, which along
with a gas volume Vg2 and a hydraulic volume Vh2, sealingly
contained on the upper side 113 of the floating piston 111, define
secondary accumulator 121 having a gas pressure P2. The lower side
115 of the floating piston 111 in combination with the secondary
accumulator housing inner bore 109 form an additional portion of
the primary accumulator 152.
[0055] In the preferred configuration, the secondary accumulator
housing inner bore 109 includes a set of stops 123, 125. Retraction
stops 123 are fixedly attached and form a protrusion to define a
maximum upper travel limit of the floating piston 111 and ensure
secondary accumulator 121 integrity. Extension stops 125 are formed
within the secondary accumulator housing inner bore 109. Under
normal conditions, where pressure P2 of the secondary accumulator
121 is greater than the pressure P1 of the primary accumulator 152,
the floating piston 111 rests against extension stops 125. As with
the embodiment depicted in FIGS. 3-5, the loads occurring during a
retraction of the piston rod 32 are reduced due to the
implementation of the floating piston 111 within the secondary
accumulator housing 101. As with the embodiment described in FIG.
3, either the primary accumulator 152 or the secondary accumulator
121 may include charging connections (not shown) to permit
intermittent or continuous charging of both the gas volumes Vg1,
Vg2, and hydraulic volumes Vh1, Vh2, from an external pressure
source (not shown). In another variation, the secondary accumulator
121 may include an interface (not shown) with another external
accumulator (not shown) to provide added gas volume.
[0056] In another embodiment of the present invention, as perhaps
best shown in FIG. 7, a tensioner unit 228 similar to the tensioner
unit 128 described with respect to FIG. 6 can be modified to either
cap lower end 164 of cylinder barrel 160 and/or remove lower fluid
aperture channel extension 168, or, as shown in FIG. 7, cap channel
extension 268 with cap 269. The main body 230, in this embodiment,
if implemented, generally functions to provide protection and to
provide an attachment point to a supported structure. As in the
previous embodiment, described with respect to FIG. 6, the piston
inner bore 276 interfaces and is in communication through its lower
aperture 274 with hydraulic fluid in bore 266 of the cylinder
barrel 260. The amount of hydraulic fluid entering the piston
assembly inner bore 276 adjacent its lower side is directly
dependent upon gas pressure P1 within the primary accumulator 252,
which is directly dependent upon stroke position of piston 272 and
piston rod 232. The gas volume contained within the piston rod 232,
however, in the depicted configuration, solely forms the primary
accumulator gas volume Vg1.
[0057] In this embodiment, the tensioner unit 228 also includes
secondary accumulator 121 having housing 101, separate from the
cylinder barrel 260, piston rod 232, and main body 230 (if so
configured), which interfaces and is in fluid communication through
its aperture 103 with primary accumulator hydraulic fluid Vh1 in
the cylinder barrel 260 (as shown). Correspondingly, cylinder
barrel 260 has aperture 205 fluidly connected to fluid connection
assembly 207 and to aperture 103 of the secondary accumulator
housing 101. As described with respect to the embodiment shown in
FIG. 6, the secondary accumulator housing 101 also includes a fluid
separator in the form of a floating piston 111. The inner bore 109
encloses the floating piston 111. The floating piston 111 has an
upper and lower surface 113, 115, and a diameter that is
substantially equal to the diameter of the secondary accumulator
housing inner bore 109. The floating piston 111 can include
pressure seals that slidingly engage the inner bore 109. The
floating piston upper surface 113 in combination with the secondary
accumulator housing inner bore 109 form an enclosure, which along
with a gas volume Vg2 and a hydraulic volume Vh2, sealingly
contained on the upper side 113 of the floating piston 111, define
a secondary accumulator 121 having a gas pressure P2. The lower
side 115 of the floating piston 111 in combination with the
secondary accumulator housing inner bore 109 form an additional
portion of the primary accumulator 252.
[0058] In still another embodiment of the present invention, as
perhaps best shown in FIG. 8, a tensioner unit 328 similar to the
tensioner unit 128 described with respect to FIG. 6 can be modified
to either remove inner barrel 160 or to cap the lower end 164 of
cylinder barrel 160 and remove the main body (tubular housing) 130
and channel extension 168. If the inner barrel 160 is removed, the
diameter of the first piston 172 can be modified so that the outer
diameter of piston 372 is substantially equivalent to the inner
diameter of the axial inner bore 358 of the main body or barrel
330. Correspondingly, back annulus 378 performs the same function
described above and interfaces with the bore 358 of main body 330
instead of bore 166 (FIG. 6) of cylinder barrel 160.
[0059] Regardless of the configuration selected, as with the
previous embodiment described with respect to FIG. 7, the piston
inner bore 376 interfaces and is in communication through its lower
aperture 374 with hydraulic fluid in the barrel housing the piston
rod 332. The amount of hydraulic fluid entering the piston assembly
inner bore 376 adjacent its lower side is directly dependent upon
gas pressure P1 within the primary accumulator 352, which is
directly dependent upon stroke position of piston 372 and piston
rod 332. The gas volume contained within the piston rod 332,
however, in the depicted configuration, again solely forms the
primary accumulator gas volume Vg1.
[0060] Still referring to FIG. 8, the tensioner unit 328 can
include secondary accumulator 121 having housing 101, separate from
either the main body (bore) 330 and piston rod 332, which
interfaces and is in fluid communication through its aperture 103
with primary accumulator hydraulic fluid Vh1 in the main body 330.
Correspondingly, the main body 330 has aperture 305 fluidly
connected to a fluid connection assembly 307 and to aperture 103 of
the secondary accumulator housing 101. The secondary accumulator
housing 101 also includes a fluid separator in the form of a
floating piston 111, which along with secondary accumulator housing
101 forms an enclosure containing a gas volume Vg2 and hydraulic
volume Vh2, which define a secondary accumulator 121, as previously
described, in detail, with respect to the embodiments shown in
FIGS. 6 and FIG. 7. Note, FIGS. 7 and 8 not only depict a possible
position for a hydraulic Vh1 tap (aperture 305) but also a possible
position of a gas Vg1 tap in the piston rod 332 where the secondary
accumulator housing and is configured in the form of an external
gas accumulator (described later).
[0061] The previous embodiments, as shown in FIGS. 3-8 and
described above, depict the housing for a secondary accumulator in
the form of either a piston rod 32 having inner bore 76 (FIGS. 3-5)
or secondary accumulator housing 101 having inner bore 109 and
containing within a fluid separator in the form of a floating
piston 92, 111, sealingly engaging the inner bore 76, 109,
respectively (FIGS. 6-8). Regarding the embodiments described with
respect to FIGS. 6-8, the floating piston 111 sealingly and
slidingly engages inner bore 109 of secondary accumulator housing
101 and functions in a similar manner as floating piston 92,
described with respect to FIGS. 3-5 (previously described in
detail). Alternative embodiments of the fluid separator in the
secondary accumulator are, however, within the scope of the present
invention.
[0062] For example, as best shown in FIG. 9, the floating piston
111 (FIGS. 6-8) can be replaced by a bladder 211 that sealingly
engages inner bore 209 of secondary accumulator housing 201 in a
fixed position. Similar to the floating piston 111 implementation,
upper surface 213 of the bladder 211 in combination with the
secondary accumulator housing inner bore 209 form an enclosure,
which along with a gas volume Vg2 sealingly contained on the upper
side 213 of the bladder 211, define a secondary accumulator 221
having a gas pressure P2. The lower side 215 of the bladder 211 in
combination with the secondary accumulator housing inner bore 209
form an additional portion of the primary accumulator through
aperture 203.
[0063] In operation, bladder 211 is selected to "balloon" when
pressure P1 of the primary accumulator equals or exceeds pressure
P2 of the secondary accumulator. Hydraulic volume Vh1 having
pressure P1 serves to compress bladder 211 at a preselected
pressure P2, which, in turn, serves to compress the secondary
accumulator gas volume Vg2 to substantially the same extent as that
of the hydraulic volume Vh1 having pressure P1 compresses primary
accumulator gas volume Vg1. This "ballooning effect" results in
functionally combining the gas volume of the primary accumulator
Vg1 and with the gas volume Vg2 of the secondary accumulator when
the pressure P1 of the primary accumulator equals or exceeds
pressure P2 of the secondary accumulator. This results in an
increased total gas volume VgT which serves to reduce maximum
tension and stiffness during a downstroke of the piston rod.
However, as with the configuration described with respect to FIG.
9, secondary hydraulic volume Vh2 would be unnecessary. As stated
above, the secondary hydraulic volume Vh2 in the above described
embodiments and their corresponding configurations is generally
used for the purpose of maintaining hydraulic seals of the floating
piston. As the bladder 211 is fixedly mounted, the second hydraulic
fluid volume is unnecessary. Note also, the second piston 92 (FIGS.
3-5) can be replaced by a bladder similar to bladder 211 fixedly
mounted within bore 76 of piston rod 32.
[0064] As best shown in FIG. 10, the floating piston 111 (FIGS.
6-8) can also be replaced by a fluid separator in the form of a
valve or valve arrangement 311. In this embodiment of the present
invention, valve arrangement 311, typically in the form of a pilot
valve, has a first fluid connection assembly 343 in fluid
communication with pressurized fluid in the primary accumulator
through an aperture 105 (FIG. 6) and a second fluid connection
assembly 345 in fluid communication with the pressurized fluid
contained within secondary accumulator housing 301 through aperture
303 to intermittently connect pressurized fluid in the primary
accumulator with pressurized fluid in the secondary accumulator
housing when pressure P1 in the primary accumulator pressure P1
equals or exceeds the preselected pressure P2 of the secondary
accumulator. Referring to the configurations shown in FIGS. 6-8,
when combined with the fluid separator shown in FIG. 10, the
interface between the fluid separator and the primary accumulator
is with hydraulic volume Vh1. The bore 309 of secondary accumulator
housing 101 contains gas volume Vg2 and hydraulic volume Vh2 which,
in combination with the second fluid connection assembly 345 of the
valve arrangement 311, define a secondary accumulator 321.
[0065] In operation, as primary accumulator pressure P1 equals or
exceeds secondary accumulator pressure P2, the valve arrangement
331 allows a portion of hydraulic volume Vh1 to enter the secondary
accumulator and compress the secondary accumulator gas volume Vg2
to substantially the same extent the hydraulic volume Vh1 having
pressure P1 compresses primary accumulator gas volume Vg1. This
"effect" results to combine the gas volume of the primary
accumulator Vg1 and with the gas volume Vg2 of the secondary
accumulator when the pressure P1 of the primary accumulator equals
or exceeds pressure P2 of the secondary accumulator. This produces
an increased total gas volume VgT which serves to reduce maximum
tension and stiffness during a downstroke of the piston rod. As the
pressure P1 of the primary accumulator decreases to that of the
preselected secondary accumulator pressure P2, the valve
arrangement 311 allows substantially the same amount of hydraulic
fluid from the primary accumulator hydraulic volume Vh1 equivalent
to the amount that entered the secondary accumulator hydraulic
volume Vh2 to be returned through the valve arrangement 311 by the
expanding secondary accumulator gas volume Vg2. When the primary
accumulator pressure P1 decreases below the secondary accumulator
preset pressure P2, the two hydraulic volumes Vh1, Vh2, are
isolated from each other by valve arrangement 311 and the total
equivalent gas volume VgT of the tensioner unit is that of the
primary accumulator gas volume Vg1.
[0066] As stated above, the previous embodiments shown in FIGS. 3-8
and described above in detail, depict a housing for a secondary
accumulator combined with a fluid separator either in the form of a
piston, bladder, or valve arrangement, functionally engaged through
use of a primary accumulator hydraulic volume Vh1. With respect to
the embodiments described that utilize a separate external
secondary accumulator housing, and with reference to FIGS. 6-8,
little modification to those embodiments need be necessary in order
to provide a secondary accumulator which is functionally engaged
entirely through use of a gas such as primary accumulator gas
volume Vg1.
[0067] Referring to FIGS. 11-15, shown is the implementation of a
tensioner unit 428, similar to that described with reference to
FIG. 7, having three variations of a secondary accumulator in the
form of the gas accumulator. The tensioner unit 428 can have
secondary accumulator housing 401 positioned in fluid (gas)
communication with either of the previously described embodiments
primary accumulator gas volume Vg1. For example, aperture 405 can
be located in piston rod 432 to provide primary accumulator
pressure P1 to either of the embodiments of a fluid separator,
described above. Referring to the implementation of a fluid
separator taking the form of a floating piston 411 (FIGS. 11-13),
the floating piston 411 performs the same function as floating
piston 111 as described with respect to FIG. 6-8 or floating piston
92 described with respect to FIGS. 3-5. In this configuration,
however, the gas volume contained within the bore 476 of piston rod
432, lower side 415 of the floating piston 411 in combination with
the secondary accumulator housing inner bore 409, and fluid
connection assembly 407 positioned between apertures 403 and 405
combine to form the primary accumulator having the primary
accumulator gas volume Vg1. The bore 409 of the secondary
accumulator housing 401 in combination with the upper side 413 of
floating piston 411 and containing secondary accumulator gas volume
Vg2 form the secondary accumulator 421. Note, no hydraulic volume
Vh2 is used within the secondary accumulator 421 in this
configuration. Note also, due to the absence of hydraulic oil, the
seals used to seal floating piston 411 in inner bore 409 of the
secondary accumulator housing 401 are gas seals, otherwise the
configuration within secondary accumulator housing 401 can be
substantially the same as that shown in FIGS. 6-8.
[0068] In operation, with a supported system such as the
hydro-pneumatic tensioner system 20 properly installed, normally,
the first piston 472 is established in the vicinity of the upper
portion of the cylinder barrel 460, and the floating piston 411 is
normally established in a contact position with the lower stop 425.
In this situation, pressure P1 on the lower side 415 of the
floating piston 411 is normally less than the pressure P2 on the
upper side 413 of the floating piston 411.
[0069] In a partial retraction situation, as best shown in FIG. 12,
normally, the first piston 472 is established in some mid-range
position of the cylinder barrel 460, and the floating piston 411 is
still normally established in a contact position with the lower
stop 425. In this situation, pressure P1 on the lower side 415 of
the floating piston 411 is still normally less than the pressure P2
on the upper side 413 of the floating piston 411 but with a
decreased pressure differential (P2-P1) maintaining that position.
The total effective gas volume VgT is maintained substantially at
that volume of the primary accumulator Vg1. As the piston 472
continues to stroke down (retract) tension increases at a
sufficient rate to provide sufficient support to the supported
system.
[0070] In a near full retraction situation, as best shown in FIG.
13, normally, in a situation where the first piston 472 was
operating without the benefit of the added second piston
accumulator 421, tension would increase rapidly. The floating
piston 411, however, having its own "charge" moves upward to
maintain the two pressures P1 and P2 in substantial equilibrium
unless the upper stop 423 is contacted. Thus, the implementation of
the floating piston 411 in the external secondary accumulator
housing 401 provides the same function as the second piston 92
(FIGS. 3-5) within the piston rod 32. The implementation of the
floating piston results in the effective amount of gas VgT being
increased over that of Vg1. The increase in gas volume provides a
slower rate of increase in gas pressure P1 due to the added volume
and also reduces the peak load of a given downstroke (retraction of
the piston rod).
[0071] Referring now to FIG. 14, the floating piston 411 (FIGS.
11-13) can be replaced by a bladder 511 that sealingly engages
inner bore 509 of secondary accumulator housing 501 in a fixed
position. In this configuration, the gas volume contained within
the bore 476 of piston rod 432, lower side 515 of the bladder 511
in combination with the secondary accumulator housing inner bore
509, and fluid connection assembly 507 connected to aperture 503
combine to form a primary accumulator having the primary
accumulator gas volume Vg1. As with the floating piston 411, upper
surface 513 of the bladder 511 in combination with the secondary
accumulator housing inner bore 509 form an enclosure, which along
with a gas volume Vg2 sealingly contained on the upper side 513 of
the bladder 511, define a secondary accumulator 521 having a gas
pressure P2. Note, the second piston 92 (FIGS. 3-5) can also be
replaced by a bladder similar to bladder 511 and fixedly mounted
within bore 76 of piston rod 32.
[0072] In operation, bladder 511 is selected to "balloon" when
pressure P1 of the primary accumulator equals or exceeds pressure
P2 of the secondary accumulator. However, in this configuration,
gas volume Vg1, as opposed to hydraulic volume Vh1 having pressure
P1 (FIG. 9), serves to compress bladder 511 at a preselected
pressure P2, which, in turn, serves to compress the secondary
accumulator gas volume Vg2 to relatively the same extent as that
hydraulic volume Vh1 having pressure P1 compresses the portion of
the primary accumulator gas volume Vg1 within bore 476 of piston
rod 432. Where primary accumulator pressure P1 equals or exceeds
the secondary accumulator pressure P2, the gas volumes Vg1, Vg2,
functionally combine to produce an increased total gas volume VgT
which serves to reduce maximum tension and stiffness during a
downstroke of the piston rod 432. When the primary accumulator
pressure P1 is again reduced below the preset secondary accumulator
pressure P2, the total effective gas volume VgT returns to that of
the primary accumulator gas volume Vg1, and stiffnesses to the
supported unit is increased.
[0073] As best shown in FIG. 15, the floating piston 411 (FIGS.
11-13) can also be replaced by a fluid separator in the form of an
entirely gas fluid operated valve or valve arrangement 611. In this
configuration, valve arrangement 611, typically in the form of a
pilot valve, has a first fluid connection assembly 643 in fluid
communication with pressurized fluid in the primary accumulator
through aperture 405 positioned to access primary accumulator gas
volume Vg2, and a second fluid connection assembly 645 in fluid
communication with the pressurized fluid contained within secondary
accumulator housing 601 through aperture 603 to intermittently
connect pressurized fluid in the primary accumulator with
pressurized fluid in the secondary accumulator housing when
pressure P1 in the primary accumulator pressure P1 equals or
exceeds the preselected pressure P2 of the secondary accumulator.
Referring still to the tensioner unit 428 of FIG. 11 combined with
the secondary accumulator 621 shown in FIG. 15, when the interface
between the fluid separator and the primary accumulator is with gas
volume Vg1, the bore 609 of secondary accumulator housing 601
contains gas volume Vg2 only which, in combination with the second
fluid connection assembly 645 of the valve arrangement 611, defines
the secondary accumulator 621.
[0074] In operation, as primary accumulator pressure P1 equals or
exceeds secondary accumulator pressure P2, the valve arrangement
611 allows a portion of gas volume Vg1 to enter the secondary
accumulator and compress the secondary accumulator gas volume Vg2
to substantially the same extent the gas volume Vg1 is compressed.
This "effect" results to combine the gas volume of the primary
accumulator Vg1 and with the gas volume Vg2 of the secondary
accumulator when the pressure P1 of the primary accumulator equals
or exceeds pressure P2 of the secondary accumulator. This produces
an increased total gas volume VgT which serves to reduce maximum
tension and stiffness during a downstroke of the piston rod 432. As
the pressure P1 of the primary accumulator decreases to that of the
preselected secondary accumulator pressure P2, the valve
arrangement 611 allows substantially the same amount of gas volume
that entered the secondary accumulator to be returned through the
valve arrangement 611 by the expanding secondary accumulator gas
volume Vg2. When the primary accumulator pressure P1 decreases
below the secondary accumulator preset pressure P2, the two gas
volumes Vg1, Vg2, are isolated from each other by valve arrangement
611 and the total equivalent gas volume VgT of the tensioner unit
is that of the primary accumulator gas volume Vg1.
[0075] Referring now to FIG. 16, an alternative embodiment, a
tensioner unit 528 similar to the tensioner unit 128 (FIG. 6) is
modified to prevent primary accumulator hydraulic volume Vh1 or
primary accumulator gas volume Vg1 from entering either piston 572,
piston rod 532, or both. This embodiment can interface with either
of the secondary accumulators described with respect to FIGS. 6-15.
In the configuration shown in FIG. 16, the primary accumulator
includes the inner bore of the main body (barrel) 530 and the lower
surface 582 of piston 572 along with the fluid connection assembly
and the primary accumulator side of the fluid separator, according
to the type of secondary accumulator selected.
[0076] In a configuration where the tensioner unit 528 is
implemented with a gas-type secondary accumulator, such as those
described with respect to FIGS. 11, 14, or 15, aperture 505 is
positioned in the main body 530, above the maximum hydraulic volume
fluid line (not shown) corresponding to the level of the primary
accumulator hydraulic volume Vh1 within the main body 530 that is
external to the cylinder barrel 560 and reached when piston 572 is
in full downstroke. Primary accumulator hydraulic volume Vg1
interfaces with the fluid separator of the selected secondary
accumulator to effectively combine the primary accumulator gas
volume Vg1 with the selected secondary accumulator gas volume Vg2.
In a configuration where the tensioner unit 528 is implemented with
a hydraulic-type secondary accumulator, such as those described
with respect to FIGS. 6, 9, and 10, aperture 505 is positioned
below the minimum hydraulic volume fluid line (not shown)
corresponding to the level of the hydraulic volume Vh1 within the
main body 530 when piston 572 is in full up-stroke. The primary
accumulator hydraulic volume Vh1 interfaces with the fluid
separator of the selected secondary accumulator to effectively
combine the primary accumulator gas volume Vg1 with the selected
secondary accumulator gas volume Vg2.
[0077] Referring now to FIG. 17, an alternative embodiment of the
present invention includes a pull-type hydro-pneumatic tensioner
unit 628 having a primary accumulator housing 601 and a secondary
accumulator housing including those described with respect to FIGS.
6, 9-11, 14, or 15. A piston 672 similar to the one described in
FIG. 16 moves axially within main body (barrel) 630 having inner
bore 666. The piston 672 has a piston rod 632 extending from the
lower side 682 of piston 672, the piston rod extends through a
piston rod aperture 662 and through high-pressure seals associated
with the aperture 662. The piston 672 also includes high-pressure
seals 690. The combination, as described above and as to be
described below, forms an enclosure of varying size depending upon
primary accumulator pressure P1 and stroke position of the piston
rod 632.
[0078] In the preferred configuration, the diameter of the piston
672 is substantially equivalent to the diameter of the inner bore
666 of main body 630, and the diameter of the piston rod 632 is
preferably substantially equivalent to the inner diameter of the
diameter of the piston rod aperture 662. The volume formed between
the upper side 684 of the piston 672 and the inner bore 666 of the
main body 630 forms a low-pressure annulus that is preferably
vented to the atmosphere. The volume formed between the lower side
682 of the piston 672 and the main body inner bore 666 forms a
portion of the primary accumulator 652 having a portion of the
primary accumulator hydraulic volume Vh1. The main body 630 also
includes an aperture 605 located adjacent the lower end of the main
body to provide fluid communication between the main body portion
of the primary accumulator and primary accumulator housing 601. The
primary accumulator housing 601 preferably takes the form of an
external cylinder or barrel. The primary accumulator housing
includes aperture 603 in fluid communication with main body 630
through a fluid connection assembly 607 connected between aperture
603 of the primary accumulator housing 601 and aperture 605 of the
main body 630. The bore 609 of the primary accumulator housing 601
includes a portion of the primary accumulator hydraulic volume Vh1.
The bore 609 of the primary accumulator housing 601 also includes
at least a portion of the primary accumulator gas volume Vg1,
depending on the configuration of the selected secondary
accumulator. The amount of primary accumulator hydraulic volume Vh1
is dependent upon primary accumulator pressure P1 and stroke
position of piston 672.
[0079] The tensioner unit 628 according to this embodiment can at
least interface with either of the secondary accumulators described
with respect to FIGS. 6-15. For illustrative purposes, the
following discussion will be with respect to the secondary
accumulator 121 described with respect to FIGS. 6-8. The primary
accumulator housing 601 and secondary accumulator housing 101 are
in fluid communication through a fluid connection assembly 608
connected to aperture 602 of the primary accumulator housing 601
and aperture 103 of the secondary accumulator housing 101. In this
configuration, the primary accumulator 652 includes the inner bore
of the main body (barrel) 630 and the lower surface 682 of piston
672, fluid connection assembly 607, the bore 609 of the primary
accumulator housing 601, the fluid connection assembly 608, and the
bore 109 of secondary accumulator housing 101 in combination with
the primary accumulator side of the fluid separator according to
the type of secondary accumulator configuration selected. The
secondary accumulator 121 includes the inner bore 109 of the
secondary accumulator housing 101 and the secondary accumulator
side of the fluid separator according to the type of secondary
accumulator configuration selected.
[0080] The configuration depicted in FIG. 17 includes a
hydraulic-type secondary accumulator similar to that described with
respect to FIG. 6, whereby primary accumulator hydraulic volume Vh1
provides the impetus to functionally combine the primary
accumulator gas volume Vg1 with the secondary accumulator gas
volume Vg2 to provide the tensioner unit 628 an effective total gas
volume VgT substantially equivalent to that of the sum of the
primary accumulator gas volume Vg1 and secondary accumulator gas
volume. If a gas-type accumulator is used, such as that described
with respect to FIG. 14, the fluid connection assembly 608 would
instead be connected to the upper aperture 602 of primary
accumulator housing 601 in communication with the primary
accumulator gas volume Vg1. Additionally, the maximum amount of
primary accumulator hydraulic volume Vh1 can be configured under
expected extreme operating conditions not to exceed a level within
the bore 609 of the primary accumulator housing 601 to that of
communicating with the upper aperture 602. In this configuration,
the primary accumulator gas volume Vg1 provides the impetus to
functionally combine the primary accumulator gas volume Vg1 with
the secondary accumulator gas volume Vg2 to provide the tensioner
unit 628 an effective total gas volume VgT substantially equivalent
to that of the sum of the primary accumulator gas volume Vg1 and
secondary accumulator gas volume Vg2.
[0081] The invention has several advantages. The hydro-pneumatic
riser tensioner units provide high nominal stiffness while limiting
peak loads at extreme extended and retracted positions. In one
configuration, the telescopic piston rod of the riser tensioner
unit incorporates the bore or annulus of the piston rod as a
secondary accumulator and incorporates a floating piston within the
annulus of the piston rod. In other configurations, a separate
housing having the bore or annulus is used to form the secondary
accumulator. The total effective gas volume of the unit is
increased by that of the secondary accumulator. During piston
compression, the increased effective gas volume results in the load
on the riser system increasing at a much lower rate and a reduced
peak load. During piston decompression, the increased total
effective gas volume results in a slower decrease in pressure and
results in limiting the minimum tension applied to the riser
system. Correspondingly, a smaller primary accumulator may be
utilized.
[0082] In the drawings and specification, there have been disclosed
a typical preferred embodiment of the invention, and although
specific terms are employed, the terms are used in a descriptive
sense only and not for purposes of limitation. The invention has
been described in considerable detail with specific reference to
these illustrated embodiments. It will be apparent, however, that
various modifications and changes can be made within the spirit and
scope of the invention as described in the foregoing specification.
For example, optionally, the system could be entirely pressurized
by gas only, using a separate lubricant for the seals, as known by
those skilled in the art. Also, if full retraction is not required,
the retraction stops used in the floating piston configurations
could be eliminated. Further, the accumulator could be connected to
the riser and the piston rod to the vessel in reverse to what was
described. Still further, the tensioner unit can be attached to the
top plate at an intermediate point along the barrel rather than at
the upper end as shown in figures. Correspondingly the tensioner
unit can be connected to the support module lower support frame at
an intermediate point along the barrel rather than at the lower end
as shown in the figures.
* * * * *