Fully Constraint Platform in Deepwater

Abstract

A method and apparatus for securing a buoyant, surface structure in deepwater, such as those used in offshore oil and gas drilling and producing, is disclosed. The surface structure is water surface piercing or fully submerged. Buoyancy tanks inside the surface structure are used as liquid vibration dampers to enhance the motion performance. All tethers are sufficiently taut and some or all of them are inclined. The attachment points of the tethers on the surface structure are at same or different elevations. An alternative attachment method, named as “single tether-multiple attachment leg”, is also disclosed. The tether system provides stiffness in both vertical and horizontal direction so that all six degrees-of-freedom motions of the structure, namely, surge, sway, heave, roll, pitch, and yaw are constrained with stiffness. This fully constraint platform (FCP) has minimum motion in winds, currents, swells, and surface and internal waves, and therefore has many practical uses, as outlined herein.

Description

BACKGROUND OF THE INVENTION

A platform in the ocean, such as those used in offshore oil and gas production, will respond to winds, waves, and currents. Its motion is characterized by six-degrees of freedom (abbreviated as DOFs): three are translational (surge, sway and heave), and three are rotational (roll, pitch, and yaw). The environmental loads can force the platform to move in one or more DOFs.

A design should ensure minimum motion in these DOFs such that the functional requirements of the platform can be met. For example, if the platform is used for drilling a well through the water column, it should not have large horizontal or vertical motion. Otherwise, the drilling operation cannot be conducted. The motion of a platform can be classified as static or dynamic. One example of the former is that due to steady drag loads from ocean currents. The motion due to surface ocean waves is dynamic. In many cases, there are both static and dynamic motions.

When the water depth is shallow, e.g., on the order of 500 ft or less, the platform can be designed such that sufficient structural stiffness is provided (by steel or steel reinforced concrete) to all six DOFs. As a result, the platform will have minimum motion. This type of structures is termed as a “fixed” platform. Many platforms in shallow water of the Gulf of Mexico, for example, belong to this category.

When the water depth is greater, e.g., over 1,000 ft, much more steel or concrete will be needed to have similar motion characteristics if the same fixed platform design concept is adopted. This is because that the horizontal stiffness of the support structure is inversely in proportion to the cubic of the water depth. The concept of “fixed platform” will no longer be deemed economical in deepwater: the amount of steel or concrete required to provide such stiffness to the platform is simply phenomenal.

New platform concepts for deepwater, such as tension-leg platforms, abbreviated as TLPs (such as that disclosed in U.S. Pat. No. 3,577,946), semi-submersibles, and spar platforms (such as that disclosed in U.S. Pat. No. 4,702,321), therefore emerge.

TLPs are now widely used in deepwater. To date, there are over twenty-five installations in world's ocean. Referring now to FIG. 1 (a), a TLP consists of: (1) a buoyant, surface structure 203 which supports deck(s) and structures and equipment on the deck(s), (2) a tether system 202, and (3) a foundation structure 201 which is secured on the seafloor. The tethers are usually tubular in shape and are made from high strength steel.

The working principle of a TLP is as follows. The buoyancy generated by the buoyant, surface structure is larger than the combined weight of its own and of everything it carries. The excessive lift force is taken by the tether system, which is then sustained by the foundation. The tension in each tether is often on the order of a few thousands kilo-pounds (kips). A TLP can support a large amount of payload, such as the weights of a drilling rig, deck(s), equipment (for initially processing oil and gas) on the deck(s), and man quarters.

The TLP concept uses a tether system to constrain a surface structure vertically. The vertical stiffness of a TLP depends on the Young's modulus of the tether material, the cross section area of the tethers, or the amount of steel, and the length of the tethers. Let's say the tether system consists of five steel tubes, 25-in. in outside diameter and 1-in. wall (the cross-sectional area is therefore 75.4 in²), to support a surface structure in 1,500 ft of water (assuming the length of the tether is 1,500 ft). The vertical stiffness of the tethers would be 5×75.4 in²×30,000 ksi/1,500 ft=7,540 kips / ft.

The horizontal stiffness of a TLP is controlled by the tension in tethers and the length of the tether. For example, if the tension in tethers is 6,000 kips and the length of the tether is 1,500 ft, the stiffness would be 6,000 kips/1,500 ft=4 kips/ft (this is the stiffness at a zero offset. When there is an offset due to environmental loads, this stiffness will change nonlinearly). This means that to move the surface structure horizontally by one foot, a force of 4,000 pound is needed. For a surface structure of typically one to two hundred feet in size and often over 20,000 kips in weight (of its own and those it carries), this is really a very small force. In other words, the horizontal stiffness of a TLP is very small, only 1/1,885 of the vertical stiffness, for the above example. Clearly, the stiffness generated from the axial stretch of the tether is significantly higher than that from the tension.

Because of the difference in stiffness by such a large magnitude, a TLP will behave very differently in its vertical and in its horizontal direction. This platform concept has minimum heave, pitch, and roll motion, but its surge, sway, and yaw can be large, if there is a large force (either static or dynamic) to excite its motion in any of these DOFs. Referring now to FIG. 1 (b), e.g., the surface structure 203 will offset under environmental load 204.

In high winds, large waves, and strong currents, such as those generated by a 100-year storm or a 100-year ocean current or an internal wave (such as that encountered in Southeast Asia), a TLP could have an excursion on the order of hundreds of feet. This type of motion characteristic is

• (a) a challenge to the design and operation of the facilities and structures on a TLP, such as the deck, deck equipment, and risers and flowlines,
• (b) a negativity to human's health for those working on the platform, and
• (c) unacceptable if the TLP concept is intended for other uses, such as those disclosed in this invention.

The present invention solves the large horizontal motion problem of a TLP.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for securing a platform in deepwater. In another aspect, the present invention relates to methods and apparatus for fully constraining a platform in deepwater.

SUMMARY OF THE INVENTION

The present invention provides a fully constraint platform for use in world's ocean. The mechanism of this system is different from other deepwater platforms, such as the TLPs, in that all 6 degrees-of-freedom (abbreviated as DOFs) of the buoyant, surface structure, namely, surge, sway, heave, roll, pitch, and yaw, are constrained with the axial stiffness of the tethers. The tethers are taut and some or all of them are inclined. The attachment points of the tethers on the buoyant structure are at different vertical heights. The inclination can be made such that the horizontal distance is as large as the depth of the water column. This fully constraint platform (FCP), supporting one or more decks, has minimum motions in winds, currents, swells, and surface and internal waves.

Because of its motion characteristic and its capability to carry a significant amount of weight, this platform concept can find a number of applications in the deepwater regions of world's ocean. More specifically, the buoyant, surface structure can be used as:

• (a) a stand alone offshore drilling platform with a tender vessel assisting, or
• (b) an offshore production platform with or without drilling and/or well servicing (work-over) capability, or
• (c) an anchored platform in product offloading and other offshore operations, which involves more than one surface structures adjacent to one another, or
• (d) an offshore work platform, or
• (e) an offshore platform to support a wind or current turbine or a transmission line tower or a lighthouse, or
• (f) an artificial island or a group of artificial islands, or
• (g) a marine terminal or a floating harbor, or
• (h) an ocean airport with multiple runways, or
• (i) a host platform for a nuclear reactor.

These and other aspects of the invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tension leg platform (TLP) and its offset under loads

FIG. 2 illustrates a fully constraint platform (FCP)

FIG. 3 illustrates offset due to current loads

FIG. 4 illustrates surge due to wave loads

FIG. 5 illustrates a minimum FCP

FIG. 6 illustrates attachment points of tethers at different elevations

FIG. 7 illustrates a tether separation angle table

FIG. 8. illustrates a method to distribute tether attachment points vertically

FIG. 9. illustrates various configurations of single tether-multiple attachment leg (STMAL) system

FIG. 10. illustrates a minimum FCP with a STMAL system

FIG. 11. illustrates a fully submerged FCP

DETAILED DESCRIPTION OF THE INVENTION

The present invention, termed as a fully constraint platform and abbreviated as “FCP”, is derived from the TLP concept. It solves the large horizontal motion problem of a TLP. Referring now to FIG. 2, a FCP requires the redistribution of the axial stiffness of the tethers 302, anchored at seafloor 301, by making some or all of them inclined at an angle from the vertical. The inclination can be such that the horizontal distance is as large as the depth of the water column. This way, the stiffness in both the vertical and horizontal direction (in particular) will be dependent upon the Young's modulus of the tether material, the cross-sectional area of the tether, and the length of the tether, rather than the tension in the tether as in the case of a TLP. Since the motion of the buoyant, surface structure 303 in any of the six degrees of freedom (abbreviated as DOFs) will stretch the tether and thus will be resisted by its axial stiffness, the buoyant, surface structure is fully constrained. Furthermore, a TLP can be viewed as a special case of the FCP concept.

In the ocean, there are many disturbances which could offset a buoyant, surface structure, such as the winds, currents, swells, surface and internal waves. Wind, current, swell, and internal wave forces are mainly static or quasi-static in nature (meaning changing at a low rate), while the surface waves are oscillatory. When a structure is subjected to steady loads, its motion is controlled by its stiffness. The greater the stiffness a structure has, the smaller the motion it experiences. Therefore, for a properly sized FCP, the steady motion can be minimum (as will be illustrated later in this section with a numerical example). In contrast, a TLP will have large horizontal offset (due to its low horizontal stiffness).

Raising the stiffness in a system will change its dynamic characteristic, to such a degree that the motion, in each of the six DOFs, is controlled by the stiffness. This is one of the key differences in methodology between the present invention and the priori art. A FCP system can be designed such that the natural frequencies of all six degrees of freedom will be above the surface wave frequencies of significant energy, resulting in motion characteristics similar to those of a fixed platform.

Furthermore, since some or all the tethers of a FCP are inclined, vortex-induced vibration or “VIV” is less likely to occur (compared to vertical tethers), due to smaller normal incident flow speeds. The motion of a TLP is strongly dependent on its payload. If the payload is increased, the tether tension will decrease, which will affect its stiffness and therefore its ability to control the motion in the horizontal direction in particular. For a FCP, as long as the payload does not exceed the buoyancy, its stiffness will remain unchanged.

To further illustrate the merit of the FCP concept, a numerical example from a computer model is provided herein, which compares the offset of a FCP and of a TLP in currents in FIG. 3, and in waves in FIG. 4. In FIG. 3, the horizontal axis is the current speeds in knots (nautical miles per hour), the vertical axis is the platform offset in feet. The offset of a TLP is indicated with “diamond”, and that of a FCP with “square”. In FIG. 4, the horizontal axis is the surface wave height in feet, the vertical axis is the surge motion amplitude (dynamic) in feet. The surge of a TLP is indicated with “diamond”, and that of FCP with “square”. In both cases, the motion of a FCP is significantly less than that of a TLP.

The key of the invention is therefore:

Some or all of the tethers are inclined. In the horizontal plane the tethers can be conveniently distributed that the motion is as omni-directional as possible. The attachment points on the surface structure can be at different elevations. This way, any one of the six DOFs motion will stretch at least one tether, such that its axial stiffness, EA/L, will function. This motion will therefore be constrained by the axial stiffness of the tethers. The tethers have finite axial buckling resistance, but large tensile capacity.

To further enhance the dynamic performance of a FCP in ocean waves, the buoyant, surface structure is partitioned into internal buoyancy (also called ballast) tanks which can be used as liquid vibration dampers (abbreviated as LVDs). The size of the tanks and the level of the fluids (water can conveniently be used) are so determined that a certain excitation frequency range in the surface wave force is targeted. When there is wave energy around this frequency range, part of the input energy will be absorbed via the sloshing of the liquids inside the ballast tanks (Lamb, 1993). This feature of the invention is particularly useful since the natural frequencies of the heave, pitch and roll motion could enter the significant wave energy zone, if the FCP is intended for very deep water. There is a trade off between using the LVDs and more tethers. A threaded-bolt and nut mechanism can also be used to spool and tighten the tethers when needed.

One embodiment of a FCP (FIG. 5) is a minimum structure. It consists of a buoyant, surface structure 403 (water surface piercing) and three tethers 402, each at an angle from vertical, and 120-degree apart (in plan view). This structure can be used as a working platform or to support an offshore wind or current turbine, or a transmission line tower. Three weight or pile anchors 401 can be used to secure the tethers. This structure is intended for use in unmanned cases.

Another embodiment of a FCP (FIG. 6) is that the attachment points of the tethers 502, anchored at seafloor 503, are at different elevations to enhance the rotational rigidity of the surface structure 501, in order to resist its pitch, roll, and yaw motion. In the horizontal plane, the tethers are uniformly spread. This way, more planes of symmetry is achieved so that the motion can be more omni-directional. The method to spread the tethers uniformly is to employ the following formula:

a=360/n

where a is the separation angle between two adjacent tethers in the horizontal plane and n is the number of tethers. Note that the attachment point of each tether can be at different elevation. The number of levels, the number of tethers at each level, the total number of tethers, and the corresponding separation angles are listed in FIG. 7. The tethers can also be grouped together to attach to each corner of a buoyant, surface structure with distinct corners.

The method to determine the vertical elevation of each level of tethers is the following:

• • (a) find the center point of the external force (e.g., if the draft of the surface structure is 100 ft and the current is uniform, the center of the drag force is at 50 ft from the keel)
  • (b) distribute the level of tethers uniformly about this center point (as is illustrated in FIG. 8).
    Each tether can have multiple attachment legs (single tether-multiple attachment leg or abbreviated as STMAL), in order to:
• (a) reduce costly multiple tethers and foundation anchors on the seafloor, and
• (b) increase the motion stability of the surface structure for the same number of tethers.

A number of configurations of this STMAL system are illustrated in FIG. 9 The pivot point (from which the legs are coming out) is indicated by a double circle. This pivot point can be inside or outside of the water. The length of each leg can vary to accommodate the distance from the pivot to the surface structure. Furthermore, this configuration can cascade. An embodiment to further illustrate this method is shown in FIG. 10, where the buoyant, surface structure 601 is constrained by the STMAL system 602, which is secured on the seafloor via anchors 603.

Another embodiment of a FCP is illustrated in FIG. 11, where the buoyant, surface structure 703 is completely submerged. In this case, a truss structure 702 is placed on top of the surface structure 703 to support deck 701 or other functional structures. The buoyant, surface structure 703 is constrained by the tether system to 704, which is secured on the seafloor via anchors 705.

Claims

1. A fully constraint platform in deepwater comprising:

(a) a buoyant, surface structure, consisting of a single structure or a plurality of such structures in various orientations each of which is a hollow column, with the to center of the formed buoyant, surface structure either solid or hollow to serve as a moon pool, and with deck(s) placed on top of the buoyant, surface structure.

(b) a tether system, consisting of a plurality of tethers, to attach to the said surface structure, each of which is sufficiently taut and has an angle of inclination from 0 degree to approximately 45 degrees from vertical.

(c) a foundation system with anchors to secure each of the tethers, comprising driven piles or suction piles or gravity based weights.

2. The system of claim 1(a), further being water surface piercing or completely submerged, wherein in the latter case, a truss structure is placed on top of the buoyant, surface structure, to support deck(s) along with other functional structures and equipment.

3. The system of claim 1(a), wherein each single structure is further partitioned into one or more buoyancy tanks filled with fluids to function as liquid vibration dampers and as means to balance the tension in the tethers.

4. The system of claim 1(b), wherein the attachment points of tethers to the buoyant, surface structure are at one or more elevations.

5. The system of claim 1(b), wherein some or all of the attachment points are either outside of or inside of the water.

6. The system of claim 1(b), wherein each attachment point to the surface structure is a steel block (a porch) which is separated from and welded to the main structure.

7. The system of claim 1(b), wherein the tethers are made from carbon fiber or polyester or steel.

8. The system of claim 1(b), wherein the tethers are a chain or a rope or a tube or a combination of segments made from a chain or a rope or a tube.

9. The system of claim 1(b), wherein when the tethers are entirely tubular, is also a conduit to transfer fluids or contains power, fluid and communication cables.

10. The system of claim 1(b), wherein one or more tethers have single or multiple attachment legs attached to the buoyant, surface structure, which can further cascade.

11. The system of claim 1(b), wherein further accompanying a method to uniformly distribute tethers in the horizontal plane, and a method to distribute the tethers in the vertical plane, as described in FIG. 8 and FIG. 9, respectively.

12. The system of claim 1(b), wherein for the system of 1(a) with distinct corners, the tethers are divided evenly into a number of small tether groups, with the number of groups matching the number of corners of the said structure.

13. The system of claim 1 is used as:

(a) a stand alone offshore drilling platform with a tender vessel assisting, or

(b) an offshore production platform with or without drilling and/or well servicing (work-over) capability, or

(c) an anchored platform in product offloading and other offshore operations, which involves more than one structures adjacent to one another, or

(d) an offshore work platform, or

(e) an offshore platform to support a wind or current turbine or a transmission line tower or a lighthouse, or

(f) an artificial island or a group of artificial islands, or

(g) a marine terminal or a floating harbor, or

(h) an ocean airport with multiple runways, or

(i) a host platform for a nuclear reactor.