U.S. patent application number 11/159089 was filed with the patent office on 2006-08-31 for minimized wave-zone buoyancy platform.
Invention is credited to Andrew W. Chow.
Application Number | 20060191461 11/159089 |
Document ID | / |
Family ID | 25021225 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060191461 |
Kind Code |
A1 |
Chow; Andrew W. |
August 31, 2006 |
MINIMIZED WAVE-ZONE BUOYANCY PLATFORM
Abstract
Minimized Wave-zone Buoyancy is a new approach to oil and gas
platform design with superior construction and performance
characteristics compared to state-of-art off-shore drilling and
production platforms. Minimized Wave-zone Buoyancy platforms
capitalize on low cross sectional area of the portion of the
platform exposed to waves. The low cross sectional area reduces
buoyancy forces that result from vertical platform movement,
enabling the platform to oscillate at a low natural frequency. The
low cross sectional area also minimizes the cyclical vertical
forces induced by waves. Compare to current designs, application of
the Minimized Wave-zone Buoyancy concept will result in a lower
natural frequency of oscillation, lower overall weight of platform,
or both. Minimized Wave-zone Buoyancy offers an attractive
alternative with improved platform stability, fatigue
considerations, lower construction and installation costs, and
shorter implementation schedule.
Inventors: |
Chow; Andrew W.; (Houston,
TX) |
Correspondence
Address: |
Andrew W. Chow
15306 Parkville Drive
Houston
TX
77068
US
|
Family ID: |
25021225 |
Appl. No.: |
11/159089 |
Filed: |
June 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09751264 |
Jan 2, 2001 |
|
|
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11159089 |
Jun 23, 2005 |
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Current U.S.
Class: |
114/264 ;
405/195.1 |
Current CPC
Class: |
B63B 1/048 20130101;
B63B 35/4406 20130101; B63B 39/005 20130101; B63B 2001/044
20130101 |
Class at
Publication: |
114/264 ;
405/195.1 |
International
Class: |
B63B 35/44 20060101
B63B035/44 |
Claims
1-4. (canceled)
5. A floating platform comprising: a floating buoyancy capable
superstructure; a vertical movement damping substructure with
surfaces extending sideways and having substantial width; and a
minimized wave-zone buoyancy structure having cross sectional area
less than platform displacement divided by 300 feet and sufficient
height above and below expected ocean waves; with said minimized
wave-zone buoyancy structure effective in transmitting said
superstructure's weight to said substructure; and with the
substructure capable of overall platform buoyancy and
stability.
6. A floating platform according to claim 5, further comprising one
or more cross sectional area increasing and vertically damping
stabilizers attached to minimized wave-zone buoyancy structure at
specified locations.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] Application Ser. No. 09/751,264 of same title above was
filed on Jan. 2, 2001, with Art Unit 3673 and Examiner Mr.
Frederick L. Lagman. The current application files again the same
invention of the prior application, continued and abandoned due to
office action response lost in mail and lacking proof of timely
submission. While the U.S. Patent and Trademark Office would
consider this application officially as a new filing with a new
date, the inventor in essence continues to, process the same
invention as before.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] not applicable
INCORPORATED BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] not applicable
BACKGROUND OF THE INVENTION
Discussion of Current Deep-Water Floating Design
[0004] As oil and gas operations extend farther and farther out
into deeper ocean areas, new technology has facilitated the
petroleum industry's ability to manage production in more difficult
environments. Installation of deep-draught platform, or structure
with similar mass to wave-zone cross sectional area ratio,
represents latest advancement to produce in deep-water frontiers.
The platform floats and relies on its mass, or deep draught, for
stability and for a low natural frequency of vertical
oscillation.
[0005] The drawbacks of the current technology stem from high
platform wave-zone buoyancy that leads to high forces on the
structure from waves and swells. The negative consequences of not
minimizing wave-zone buoyancy include: excessive ancillary
structures, higher associated costs for materials, construction,
and installation, extended schedule for construction and
installation thus delaying start of oil and gas production,
inferior performance such as less stable platforms and reduced
portability, and shorter fatigue lives for components attached to
the platforms.
BRIEF SUMMARY OF THE INVENTION
[0006] Minimized Wave-zone Buoyancy (hereinafter MWB) capitalizes
on low platform cross sections at the wave zone. With main purpose
of transmitting superstructure weight including those of facilities
and equipment to the substructure which provides buoyancy and
stability, low cross sectional area of the MWB structure enables
low platform natural frequency of oscillation and minimizes
cyclical vertical forces from waves. With physics governed by
spring-mass type motion and dynamics explained by differential
equation, MWB shows the way to steady platforms for improved
drilling operations, with reduced vertical motion to enhance
fatigue consideration for attached production components. Compared
to current designs, MWB offers an attractive alternative with
improved platform stability, fatigue considerations, lower
construction and installation costs, and shorter implementation
schedule for earlier first oil production. MWB platforms can be
constructed at lower costs compared to similar off-shore structures
in used or being designed today.
DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0007] FIG. 1 shows a Minimized Wave-zone Buoyancy platform.
[0008] FIG. 2 shows a Minimized Wave-zone Buoyancy platform held to
the ocean floor by tension cables or chains.
DETAIL DESCRIPTION OF THE INVENTION
Physics of Dynamics of Motion
[0009] Dynamics of motion is governed by a commonly known
differential equation MA+CV+KX=F(t) which basically represents a
balance of forces. In essence the sum of mass times acceleration,
friction forces related to velocity, and distance-proportional
reactive forces must be equal to the forcing function. Engineers
can model complicated structures by developing mass and stiffness
matrices and solve for numerical solutions. In the case of
earthquake analysis, such as for an above-ground petroleum pipeline
like the one in Alaska, the forcing function could be a seismic
event's ground-motion that drives the structure's dynamic response
over time.
[0010] As a floating production platform behaves like a rigid body
bobbing in water, the dynamic equation of motion degenerates to the
most basic one degree of freedom spring mass type system where the
natural frequency of oscillation, .omega., for the solution to the
stated differential equation is defined by the following equation
.omega.=(K/M).sup.1/2/2.pi. For a floating object, the distance
proportional K is the incremental buoyancy force for one unit of
vertical displacement, which is the product of water displacement
change times the density of water for that unit of vertical
movement. Combining this attribute of K with the fact that mass is
equal to weight divided by gravity would yield K/M=A G/DV where A
is the water displacing cross sectional area at the wave zone, G is
gravity, and DV is the water displacement volume of the platform.
Therefore, a floating platform with uniform cross sectional area
will have an .omega. that is proportional to the commonly known
formula of (gravity/delta static).sup.1/2, and in this case, delta
static is the static draught of the floating platform. Without
knowing anything else except for draught, a vessel with a uniform
cross section that sinks 700 feet should have an .omega. of about 2
cycles per minute.
[0011] It would be obvious at this time to those knowledgeable of
the art that a reduction of the distance-proportional K in the
.omega. solution would produce a desired and, not surprisingly,
dramatic result. In other words, reducing the cross sectional area
A of the part of the platform that may be exposed to waves would
enhance platform performance.
[0012] For benefit of readers not familiar with dynamics or
differential equations, the implication of the .omega. solution can
be visualized by the difference in bounce between a fully loaded
truck and the same truck without the load. It would be obvious to a
casual observer that the truck with a full load will bounce up and
down at a slower frequency than the same truck empty. In both cases
the truck has same suspension spring constant K, but the fully
loaded version has more weight and thus a larger mass M. Therefore,
the .omega. equation with the larger M in the denominator produces
a lower frequency and supports our intuition that loaded trucks
bounce slower than empty trucks.
[0013] In short, the frequency of platform vertical oscillation can
be controlled by adjusting the platform's K/M ratio. A low
frequency can be designed by reducing K, increasing M, or a
combination of both, and reducing K means a smaller cross sectional
area A in the wave zone of a platform, or for that matter any
floating object, FSO for example, that may be under
consideration.
Discussion of the Present Invention
[0014] The present invention benefits from reducing the buoyancy
force change that results from a vertical displacement of a
floating platform, in essence to lower the K in the differential
and .omega. equations so as to reduce the platform's natural
frequency of oscillation beyond the frequency range of ocean waves
and to increase the frequency separation between platform resonance
and ocean-wave frequencies. The platform would therefore operate in
the tail end of the ocean waves response spectra.
[0015] FIG. 1 shows an example of MWB platform floating at water
level 10. An offshore platform provides space to house facilities
and equipment required for drilling and production activities, and
the platform has a superstructure 20 which provides space for such
equipment and facilities. Superstructure 20 also provides buoyancy
to keep the platform afloat in the event that water rises to the
level of the superstructure.
[0016] An MWB structure 30 supports superstructure 20 and connects
to substructure 40, 50, and 60. The height of the MWB structure 30
is designed so that the waves expected to impact the platform will
strike the platform at the MWB structure 30. It is solely for
convenience that FIG. 1 displays only one MWB structural unit with
a hollow center for drill pipe access. The MWB structure 30 could
comprise multiple columns or could be made as a braced truss, and
the possibilities for MWB structure are limited only by designer
imagination.
[0017] Since the objective is to minimize wave-zone buoyancy, the
cross section of MWB structure 30 should consist mostly of steel,
or other structural materials; the MWB structure's cross section
should have limited air space to ensure a minimized buoyancy force
change K in the equations previously stated. The shape and design
of the MWB structure 30 do not matter and would not affect the
overall dynamic performance of the platform as long as the water
displaced by the MWB structure 20 is kept to a minimum. The primary
function of the MWB structure 30 is not to provide buoyancy for the
superstructure 20, but to transmit the weight of the superstructure
20 to the substructure 40, 50, and 60.
[0018] Substructure 40, 50, and 60 provides buoyancy for the
platform. FIG. 1 shows an example with a float 40 and a ballast 50.
The Float 40 has substantial width in comparison to height to
enhance exponential damping from the C component of the stated
differential equation. The width also serves the purpose of
elevating the center of lift of the substructure. The Ballast 50
extends downwards and is weighted at the bottom with rocks,
concrete, lead, or other dense material to ensure the center of
gravity of the entire platform is sufficiently below the center of
lift for overall stability. As shown in the example MWB platform,
both float 40 and ballast 50 are cylindrical in shape; conical
sections 60 with positive Gaussian curvature are included to
enhance outer shell strength for the substructure.
[0019] It should be obvious to those knowledgeable of the art that
substructure possibilities are in the designers domain as in the
case previously made for the MWB structure. The principles of
center of gravity and center of lift/buoyancy are well known, and
it is not the purpose of this patent to elaborate on the design of
structures that may be suitable for subsurface floatation. This
patent advances the concept of minimizing water displacement in the
wave zone and the benefits from reducing incremental buoyancy
forces due to waves, swells, and vertical platform movement.
[0020] As MWB structure 30 provides limited additional buoyancy
capacity and to ensure platform stability with variable
superstructure live loads, live load stabilizer 70 increases water
displacement at water level 10. When the platform floats right at
the water level, the natural frequency of oscillation is higher and
corresponds to that of platforms with larger wave-zone cross
sectional area. However, as the MWB platform moves slightly up or
down beyond the height of the stabilizer 70, the benefit of small
cross sectional area kicks in. Mathematically, the K in the
differential equation in this case is no longer a constant; it
varies with vertical distance.
[0021] For live loads with mass changes beyond the displacement
capacity of live load stabilizer 70, an active platform weight
management system could pump water in or out of ballast 50 to
accommodate large changes. While this patent does not teach sensor
usage for active ballast adjustment, live load stabilizer stoppers
80 would restrain large movement resulting from large live-load
changes, to ensure that a weight management system would be
activated to return live load stabilizer 70 to water level 10.
[0022] Live load stabilizer 70 and live load stabilizer stoppers 80
could be made in any shape, size, or material. Their sole purpose
is to displace water. FIG. 1 shows them as plates, and they can be
added or removed to meet operating requirements. For example, if
constant large live load changes are expected, the displacement of
live load stabilizer 70 could be increased. On the other hand,
anticipation of a storm may cause all stabilizers and stoppers to
be lifted out of the water.
[0023] In the foregoing discussion of stabilizers 70 and 80, the
stabilizers are attached to the MWB structure 30. Another
stabilizer example is a float attached to the MWB platform with
loose chains or cables. Loose connections permit the MWB platform
to behave in accordance with the differential equation until the
platform has moved far enough to take up the chain or cable slack
before engaging the floating stabilizer.
[0024] While discussion of this invention has focused on a free
floating platform, the MWB concept applies to tension leg
environment also. Again, it is not the intention of this patent to
dwell on floatation designs, and It should be clear to those
knowledgeable of the art what platform adaptations may be required
for a tension platform.
[0025] FIG. 2 shows an MWB tension cable platform with floating
stabilizers 110 attached by slack cables 120 to the platform. As
discussed above, the float stabilizers 110 have no effect on
dynamic movement until the platform has oscillated or moved far
enough to take up the slack in the slack cables 120. Arrangement
and design of stabilizers 110 are again limited only by designer
imagination. For example, a big donut floating stabilizer could
replace all floating stabilizers shown. A limited-free-movement
means could permit the donut to slide freely up and down the MWB
structure but would prevent the donut from moving beyond certain
heights, for example, by obstructions welded on the MWB structure
to limit movement. Therefore, the donut floating stabilizer would
not provide buoyancy lift until the platform has sunk to a
predetermine depth and would become a downward dead-weight force
when it is lifted out of the water by the rising platform. It would
be obvious that slack cables 120 and the sliding donut are just
specific forms of limited-free-movement means.
[0026] Low stiffness cables 130 hold the platform to the bottom of
the ocean. Related to the foregoing differential equation, the K
for the MWB tension cable platform is the combination of the K from
the cross section of the Minimized Wave Zone structure and the
spring constant of the low stiffness cables 130. FIG. 2 shows high
stiffness slack cables 140 with a slack to illustrate that the high
stiffness slack cables 140 would not engage to inhibit platform
upward movement until the platform has oscillated or risen far
enough to take up the slack in the high stiffness slack cables
140.
[0027] Spring constant of the low stiffness cables can be easily
determined, and actual springs may be added to provide additional
flexibility. Also, the low stiffness cables 130 control natural
frequency over a range of small displacements, and the high
stiffness slack cables 140 provide the strong resistance force to
restrain large vertical platform movement. Low stiffness cables 130
and high stiffness slack cables 140 together produce the effect of
limited-free-movement means as in the previous discussion for
floating stabilizers.
[0028] Compared to traditional tension leg platforms with high
wave-zone cross sectional area and with all cables/chains having
high stiffness and no slack, an MWB tension platform with minimized
wave-zone cross section and low-stiffness cables anchored to the
ocean floor has a lower combined K and will therefore resonate at a
lower natural frequency of oscillation. The lower frequency means
fewer fatigue cycles and thus a longer expected life for the
platform's attached components for production.
[0029] It should be noted that in the limiting case, the K of the
low stiffness cables may be reduced to zero. In other words the low
stiffness cables could be eliminated for vertical dynamic
consideration, and only the high stiffness cables remain to limit
large vertical uplift. Again, it is not the intent of this patent
to discuss ballast management to ensure platform buoyancy at the
desired elevation as it would be obvious to those knowledgeable of
the art. Also, horizontal restraints have been purposely ignored in
the discussion of vertical dynamic response.
[0030] For benefit of readers not accustomed to dynamics and rigors
of mathematics, it may be easier to consider the cyclic buoyancy
forces induced by waves or swells on a traditional tension leg
platform. The same waves or swells will produce lower cyclic
buoyancy forces on an MWB platform due to the minimized wave-zone
cross sectional area. So even if the frequency effects are ignored,
it would still be obvious that MWB designs will have lower induced
cyclic forces and thus longer fatigue lives.
Concluding Technical Remarks and Cost Considerations
[0031] The low wave-zone cross sectional area permits less massive
structure compared to current platform designs while maintaining or
improving the K/M ratio. Less mass translates to a lower
requirement for steel, meaning lower cost and shorter time for
construction. As the floating platform does not depend on deep
draught for stability and for a long period of oscillation, the
shallower draught of MWB platforms permits construction and
assembly in a less hostile environment. For example, without
ballast weight and with MWB tied down and floating high on the
substructure, the entire platform including superstructure
facilities could be constructed in a sheltered and controlled
location. Of course, ballast weights would be added before
deployment.
[0032] Naturally, the favorable characteristics mean that MWB
platforms can be constructed at lower costs and faster schedules,
shortening time of development and accelerating schedules when
deep-sea oil and gas fields can be brought on line.
SEQUENCE LISTING
[0033] not applicable
* * * * *