US 20030099516 A1 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.
Claims(4) 1. A floating platform comprising:
a superstructure; a minimized wave-zone buoyancy structure; and a substructure; with said minimized wave-zone buoyancy structure having a low cross sectional area; with said substructure effective in providing buoyancy and stability; and with the minimized wave-zone buoyancy structure effective in transmitting said superstructure's weight to the substructure. 2. A floating platform according to 3. A floating platform according to one or more stabilizers; and limited-free-movement means; with said stabilizer or stabilizers effective in providing buoyancy lift; and with the stabilizer or stabilizers attached to said floating platform from 4. A floating platform according to Description [0001] Minimized Wave-zone Buoyancy (hereinafter MWB) offers a new approach to platform design that has superior construction and performance characteristics compared to current state-of-art off-shore oil and gas drilling and production platforms. 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. MWB platforms can be constructed at lower costs compared to similar off-shore structures in used or being designed today. [0002] The following is claimed as objectives and advantages of the current invention: to improve floating platform performance, to reduce frequency of vertical oscillation, to lower amplitude of vertical oscillations, to minimized wave-induced vertical forces on platform, to provide steady platform for drilling operations, to improve fatigue lives, to lower materials costs, to reduce construction costs, and to permit accelerated schedules for implementing new platforms. [0003] 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. Recent installation of a deep-draught platform represents the latest mega-structure to advance production frontiers in waters thousands of feet deep. The structure floats in deep waters and relies on its mass and deep draught for stability and for a low natural frequency of vertical oscillation. [0004] 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 component attached to the platforms. [0005] Dynamics of motion is governed by a commonly known differential equation [0006] 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. [0007] 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, ω, for the solution to the stated differential equation is defined by the following equation ω=( [0008] 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
[0009] 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 ω that is proportional to the commonly known formula of (gravity/delta static) [0010] It would be obvious at this time to those knowledgeable of the art that a reduction of the distance-proportional K in the ω 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. [0011] For benefit of readers not familiar with dynamics or differential equations, the implication of the ω 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 ω equation with the larger M in the denominator produces a lower frequency and supports our intuition that loaded trucks bounce slower than empty trucks. [0012] 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. [0013]FIG. 1 shows a Minimized Wave-zone Buoyancy platform. [0014]FIG. 2 shows a Minimized Wave-zone Buoyancy platform held to the ocean floor by tension cables or chains. [0015] 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 ω 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. [0016]FIG. 1 shows an example of MWB platform floating at water level [0017] An MWB structure [0018] Since the objective is to minimize wave-zone buoyancy, the cross section of MWB structure [0019] Substructure [0020] It should be obvious to those knowledgeable of the art that substructure possibilities are vast as is 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. [0021] As MWB structure [0022] For live loads with mass changes beyond the displacement capacity of live load stabilizer [0023] Live load stabilizer [0024] In the foregoing discussion of stabilizers [0025] 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. [0026]FIG. 2 shows an MWB tension cable platform with floating stabilizers [0027] Low stiffness cables [0028] 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 [0029] 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. [0030] 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. [0031] 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. [0032] 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. [0033] 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. [0034] 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. Compared to current designs, MWB offers an attractive alternative with improved platform stability, fatigue considerations, lower construction and installation costs, and shorter implementation schedule. Referenced by
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