Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2001-07-26
2003-06-17
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S162000, C405S171000, C405S211000, C405S224400, C166S350000, C166S367000, C175S008000
Reexamination Certificate
active
06579040
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is generally related to risers for floating offshore oil and gas production structures and more particularly to air-can tensioning devices for the risers.
In the production of oil and gas at offshore locations, it is necessary to support the risers used in production and drilling operations. Air-can tensioning devices are commonly used to provide such support. The air-cans use buoyant forces to support and over tension the risers that extend from the structure down to the sea floor. Referring now to
FIGS. 1 and 2
, a conventional air-can
7
is seen located around stem
32
. Lower sections have open ports
30
a
-
30
c
and are pressurized via air-lines
5
a
-
5
c
. The segments
10
,
15
,
20
, and
25
are sealed from each other and have independent air-lines
5
a
-
5
c
attached to segments
15
,
20
and
25
respectively to provide for redundancies. The upper-most segment
10
is not only sealed from the other segments
15
through
25
but is also sealed from fluid contact with any surrounding water, having no open port. The lower segments
15
,
20
, and
25
have open ports
30
a
-
30
c
to compensate for the high pressures the lower segments see at depth. Without an open port, the deeper a segment is submerged, the greater its wall thickness has to be to avoid collapse, reducing the segments buoyancy. With an open port and pressure from above, thin segment walls are available.
FIG. 2
shows the cross-sectional view of
FIG. 1
where air-lines
5
a
-
5
c
are located around stem
32
. The conventional stem
32
is sized to have an inner diameter that is larger than the outer diameter of a riser such that the stem
32
is readily received around a riser.
Segments with open ports are commonly called “soft tanks” or “variable buoyancy tanks.” Those that are closed are called “hard tanks.” Although
FIG. 1
shows one hard tank and multiple soft tanks, in practice, multiple hard tanks are used at the top and multiple soft tanks are used at the bottom in a given air-can arrangement. It is also noted that it is not necessary that the tanks be connected to one another in a series arrangement where the air-lines pass through the upper tanks to reach the lower tanks as shown in FIG.
1
. One alternative tank arrangement is described by Davies in U.S. Pat. No. 5,758,990, incorporated herein by reference, where a stem having an inner diameter larger than the outer diameter of the riser is positioned around the riser and is fastened in position at the wellhead of the riser on the offshore structure; a yoke attached to the stem supports a number of sleeves around the stem; each sleeve receives a variable buoyancy air can; and the sleeves and air cans are provided with a retainer that retains the air cans in the sleeves and transfers the vertical loads of the air cans to the sleeve.
There are problems, however, with the tanks described above. First, in practice, one cannot pump out all the fluid through the open port in a soft can.
FIG. 3
illustrates this problem showing a soft tank
45
including air-line
35
for introducing gases
40
, typically air, into soft tank
45
and water
55
, indicated by hash marks, below the open port
60
. There is a level below which the soft can cannot be evacuated due to the conventional placement and design of the open port. Additionally, in practice, the soft tanks see upward and downward motion. When the tank is moved up, during heave, the water level in the soft tank will drop and the air will escape causing the need to pump more air into the soft tank. To avoid this during normal operations, the water level is left above the open port. Thus, not only the volume below the port is lost for buoyancy, but also some volume above the port is lost. Further, when there is pitch due to wave action at the surface and other forces, the water surface level in the soft tank can drop below the open port, causing air to escape. So, the water level is kept even higher than what would be needed without pitch. Again, volume of the tank is lost for buoyancy.
Downward motion can be caused by forces at the surface or other forces. For a “spar” structure, as described in U.S. Pat. No. 4,702,321, incorporated herein by reference, as the spar moves laterally, the spar is “offset” from its nominal position. The risers pull the tanks lower in the water, causing the water level in the soft tanks to rise, due to the increase in pressure, again causing a decrease in the available volume for buoyancy, at least without pumping more gases into the soft tank or designing for the offset position, leaving an overcapacity in the soft tank when the spar is in the nominal position.
There is a need therefore, to address the above-mentioned problems.
SUMMARY OF THE INVENTION
A riser tensioning device according to the invention comprises a first tank having a central axis; a first passage having a diameter less than the inner diameter of the first tank; the first passage providing a fluid contact between the interior of the first tank and the exterior of the first tank; and the first passage having a portion extending outside the first tank at an angle less than 90 degrees from parallel to the central axis. In one embodiment of the first tank, the first passage is attached in fluid communication with the interior of the first tank at the bottom of the first tank. In one embodiment of the first tank, the first passage is attached in fluid communication with the interior of the first tank at the side of the first tank. In still another embodiment of the first tank, a gas line is in fluid contact with the interior of the first tank.
In a particular embodiment of the invention, the riser tensioning device comprises a second tank having a central axis; a stem connected to the first tank; a second passage having a diameter less than the inner diameter of the second tank; the second passage providing a fluid contact between the interior of the second tank and the exterior of the second tank with the water and the second passage having a portion extending outside the second tank at an angle less than 90 degrees from parallel to said central axis. In one embodiment of the second tank, the passage is attached in fluid communication with the interior of the second tank at the bottom of the second tank. In one embodiment of the second tank, the second passage is attached in fluid communication with the interior of the second tank on the side of the second tank. In one embodiment of the second tank, the second tank is attached to the stem. In still another embodiment of the second tank, the second tank is attached to the first tank.
In a particular embodiment of the attached tanks, the first passage is providing a fluid contact between the interior of the first tank and the exterior of the first tank while passing through the second tank. In one embodiment of the attached tanks, the second tank is attached to the first tank by a stem. In one embodiment of the attached tanks, a gas line is in fluid connection with the interior of the second tank. A particular embodiment of the invention includes the gas line in fluid connection with the interior of the second tank where the gas line passes through the first tank.
In still another embodiment of the invention, the first tank comprises an interior surface having a first corrosion resistance and the first passage has an interior surface having a second corrosion resistance where the second corrosion resistance is greater than the first corrosion resistance. In one particular embodiment, the interior surface having a second corrosion resistance is selected from a group consisting essentially of stainless steel, fiber reinforced pipe, or rubber. In one particular embodiment, the interior surface having a second corrosion resistance is selected from a group consisting essentially of rust inhibiting paint, epoxy, electroplated metals, or thermal sprayed aluminum.
A method of manufacturing a riser tensioning device comprising providing a first tank having an interior surface of a first material; connecting to t
CSO Aker Maritime, Inc.
Klein O'Neill & Singh, LLP
Lee Jong-Suk (James)
Shackelford Heather
LandOfFree
Method and apparatus for air can vent systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for air can vent systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for air can vent systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3159385