US20020067956A1

US20020067956A1 - Offshore platform for hydrocarbon production and storage

Info

Publication number: US20020067956A1
Application number: US09/997,838
Authority: US
Inventors: Roy Cottrell
Original assignee: FMC Technologies Inc
Current assignee: FMC Technologies Inc
Priority date: 2000-12-04
Filing date: 2001-11-29
Publication date: 2002-06-06

Abstract

In a floating structure, a ballast weight is suspended at a truss angle from an upper section of the floating structure using at least three tension members. This ballast weight is suspended at such a depth and in such a manner that the center of gravity of the floating structure is below the center of buoyancy of the floating structure. The suspension of the ballast weight allows the floating structure to maintain greater stability and achieve desirable motions in response to environmental conditions such as wind and waves.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application Serial No. 60/251,184, filed on Nov. 29, 2000.[0001]

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to offshore platforms designed and arranged for oil and gas production and storage operations. More specifically, this invention relates to a method and apparatus to provide stability for a floating structure.

2. Description of the Prior Art

Spars are known structures for providing a platform above the sea surface for conducting oil and gas production and storage operations. One arrangement is illustrated in U.S. Pat. No. 4,702,321 to Horton. That arrangement relies on a long cylindrical structure which presents a small waterplane area to sea surface waves and has a large depth which minimizes heave motions. Anchor legs between the sea bottom and the structure substantially fix the structure to the sea floor. Its stability results from designing the structure such that the center of buoyancy is higher than its center of gravity. The design features additional fixed ballast at a lower position in the hull. Actual designs of such a SPAR provide almost ⅓ the total weight in the form of fixed ballast. This fixed ballast requires additional buoyancy to keep the SPAR afloat. Such additional buoyancy requires more steel in the cylindrical hull which results in increased costs of the structure.

Another approach to keep the SPAR's center of gravity below its center of buoyancy is to suspend the ballast from a truss as illustrated in U.S. Pat. No. 5,558,467 to Horton.

Still another approach is illustrated in U.K. patent application GB 2,339,730 A to Horton, et al. (Deep Oil Technology) which shows a structure with a reduced diameter of the SPAR at mid height to achieve a lower ballast center of gravity without a hull steel weight penalty.

Another way to achieve stability of an offshore platform is illustrated in U.S. Statutory Invention Registration H1246 (Huffaker) and U.S. Pat. No. 5,964,550 (Blandford). Tension legs are provided between the sea floor and a plurality of underwater securement points on the subsea portion of a buoyant body. Such arrangements provide station keeping mooring and platform stability. Nevertheless, because oil and gas fields are found in ultra deep waters, there is a fear that a practical limit to depths at which such tension leg mooring systems can be employed. The limit may have already been reached.

IDENTIFICATION OF OBJECTS OF THE INVENTION

A primary object of the invention is to provide a practical, relatively inexpensive offshore structure to support a platform for hydrocarbon production and storage operations.

Another object of the invention is to provide an improved offshore structure with a low system center of gravity to achieve stability and a desirable motion profile in response to environmental forces on the structure.

SUMMARY OF THE INVENTION

The objects identified above along with other advantages and features are incorporated in a floating structure having a buoyant upper section attached to a ballasted lower section. The upper section includes at least one vertical column. The lower section includes a ballast weight suspended by tension members at a truss angle. Such arrangement efficiently lowers the center of gravity for the floating structure well below the center of buoyancy for the floating structure to achieve stability and desirable motion profile when subjected to sea environmental conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the invention where a broad base of the floating structure is connected by tension members to a single point ballast weight for stabilization; [0012]
FIG. 2 illustrates another embodiment of the invention where a broad base of the floating structure is connected by tension members to a distributed ballast weight which has an open end; and [0013]
FIG. 3 is a cross section across lines [0014] 3-3 of FIG. 2.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of the invention showing a side view of a floating structure [0015] 10. The floating structure 10 includes an upper section 100 and a lower section 110. In this embodiment, the upper section includes a superstructure 70, a column 20, and a broad base 30; and the lower section includes at least three tension members 40 and a ballast weight 50.
The [0016] superstructure 70 is adapted to support and connect to a deck used for hydrocarbon production and storage operations as known in the art. (not shown in FIG. 1). Alternatively, the superstructure 70 can serve as the deck, itself, in these operations.
The [0017] column 20 connects to the bottom of the superstructure 70 and extends downwardly piercing the water surface 90. Preferably, the waterplane area created by this surface piercing is small thereby reducing the effects of surface waves on the floating structure 10. In the embodiment of FIG. 1, a single column 20 is shown with a single waterplane area. The column 20, can be at least partially if not completely buoyant. The column 20 extends down and connects with the broad base 30. Although not shown in the embodiment of FIG. 1, column 20 could be a plurality of columns.
The [0018] broad base 30 is preferably at a depth that is below the effects of waves caused by environmental conditions. The broad base 30 is broad enough to reduce heave motions on the floating structure 10 caused by environmental conditions such as waves and swell. The broad base 30 is preferably buoyant, and if desired, can be ballasted by any means known to those skilled in the art. Alternatively, the broad base 30 can be a non-buoyant framework.
At least three [0019] tension members 40 connect to the broad base 30. The tension members 40 extend from base 30 connecting to and suspending the ballast weight 50. The embodiment of FIG. 1 displays only three tension members 40, but more may be used in practice. These tension members 40 may be made of any material and by any means known to those skilled in the art. The connection between the base 30 and the ballast weight 50 via the tension members 40 is done in such a way that the tension members 40 are not purely vertical. Rather, the connection between the base 30 and the ballast weight 50 via the tension members 40 is sloped creating an angle 45. The angle 45 is arranged and designed to prevent any tension members 40 from going slack due to the rolling and pitching motion of the floating structure 10. In other words, the breadth of base 30 is such that the combination of the upper section 100 and upper lower section 110 move and act as a rigid body.
The [0020] ballast weight 50 is at a distance in which the center of gravity for the floating structure 10 (the upper section 100 and the lower section 110) is below its center of buoyancy. In operation, the buoyant and gravitational forces acting on floating structure 10 can create a coupling force which resists undesired motions acting on the floating structure 10. Such a coupling force allows the floating structure 10 to maintain a greater static stability. The magnitude of the coupling force increases with the increased distance between the center of buoyancy of the floating structure 10 and the center of gravity of the floating structure 10. That distance increases with the increase depth of the ballast weight 50 as long as broad base 30 is wide enough to keep tension members 40 at a desirable angle 45 which makes the floating structure 10 move and act as a rigid body.
The embodiment of FIG. 1 shows the [0021] ballast weight 50 as a single concentrated point. This ballast weight 50 can be made of any type of material desired as long as it provides the “weight” or mass needed to lower the center of gravity for the floating structure 10.
The floating structure [0022] 10 is moored via a plurality of mooring lines 60. In the embodiment of FIG. 1, the mooring lines 60 are connected to the broad base 30. However, the mooring lines 60 could also be connected to the column 20. Mooring can occur through any mooring means known to those skilled in the art. Examples of mooring means include, but are not limited to conventional mooring to the sea floor, dynamic positioning, and the like.
Because the floating structure [0023] 10 in the embodiment of FIG. 1 is capable of being moored through conventional means and/or dynamic positioning, the floating structure 10 can be placed in deeper water environments than traditional tension leg structures. If the tension members 40 are long enough, the floating structure 10, can approach qualities similar to tension leg platforms while not suffering their depth limitations. While the embodiment of FIG. 1 maintains a depth advantage over other systems, this advantage is not intended to be a limitation. The floating structure 10 of FIG. 1 could equally be used in shallow water environments. Furthermore, while “tension legs” are not required by the floating structure 10 of FIG. 1, they could be used to moor the system.
FIG. 2 illustrates another embodiment of the invention showing an alternative arrangement for floating structure [0024] 10. The embodiment of FIG. 2 operates in a similar manner to the embodiment of FIG. 1, except that the ballast weight 80 is now “distributed”. The ballast weight 80 is connected in such a way that at least three of the tension members 40 are not purely vertical. Rather at least three of the tension members 40 are trussed creating an angle 45. The angle 45 is arranged and designed to prevent any of the trussed tension members 40 from going slack. Due to perspective, only two cross-trussed tension members 40 and two vertical tension members 40 can be seen in FIG. 2.
Referring to FIGS. [0025] 3-5, trussing is required in both pitch and roll planes. Trussing can be implemented in a minimum of two planes or in all four planes.
FIG. 3 is a cross section across lines [0026] 3-3 of FIG. 2 showing the distributed ballast weight 80. Cross trussing is done in four planes using eight tension members 40. Additionally, four vertical tension members 40 are included at each of the four corners of the ballast weight. (not seen due to perspective). A passage 150 is in the center of the ballast weight 80. In practice, production equipment such as a drilling or work over rig string and the like can pass along the centerline of the floating structure 10 from the sea floor up through the passage 40 and up to the rest of the floating structure 10. The illustration in FIG. 3 partially shows the tension members 40 and a how these tension members 40 can be cross-trussed.
FIG. 4 is an alternative embodiment of FIG. 3. Cross trussing of [0027] tension members 40 is done in two planes using four tension members 40. Additionally, four vertical tension members 40 are included at each of the four corners of the ballast weight 80 (not seen due to perspective).
FIG. 5 is another embodiment of FIG. 3. In this embodiment, cross trussing is done in three planes using six [0028] tension members 40 for the cross trussing. Additionally, three vertical tension members 40 are included at each of the three corners of the ballast weight 80 (not seen due to perspective).
The embodiments of FIG. 1 and FIG. 2 can be launched by floating the [0029] upper section 100 on its broad base 30 onto the installation site using a barge or the like. Once the upper section 100 is in deep enough water, while floating on the broad waterplane of broad base 30, the ballast weight 50 (FIG. 1) or 80 (FIG. 2) can be suspended from the broad base 30. After installation of the ballast weight 50 (FIG. 1) or 80 (FIG. 2), the upper section 100 of the floating structure 10 can partially be lowered into the water via ballasting of the broad base 30 and/or column 20. Then, the mooring lines 60 can be attached. This method of launching avoids the need to upend the broad base 30.
It should be understood that the invention is not limited to the exact details of construction, operation, or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. For example, the [0030] column 20 instead of being a single column can be a plurality of columns. Also, the column 20 can include any of the typical items used by those skilled in the art for hydrocarbon production and storage including ballast tanks, risers, pumps, equipment and the like. The amount of buoyancy desired in the column 20 will depend on other parameters of the system including how buoyant the broad base 30 is, if it buoyant at all. The broad base 30, while preferably buoyant, need not be buoyant in every embodiment. The broad base 30 could be a non-buoyant framework employed to suspend the ballast weight 50 or 80. In some embodiments, the buoyancy is entirely provided by the column 20 (or columns); in other embodiments, the buoyancy is provided entirely by the broad base 30; and in another embodiment, the buoyancy is provided from a combination of both the column 20 (or columns) and the broad base 30. The ballast weight 50 (FIG. 1) and 80 (FIGS. 2, 3) can be any shape and made of any material and in any manner known by those skilled in the art. Also, while the ballast weight 50 (FIG. 1) and 80 (FIGS. 2, 3) is described as a single ballast weight, it can be a plurality of ballast weights connected to one another. Furthermore, while at least three tension members are required in all embodiments, more can be used in practice. Additionally, during launching of the floating structure 10, the ballast weight 50 (FIG. 1) and 80 (FIGS. 2, 3) could either be pre-ballasted or ballasted at sea. Accordingly, the invention is therefore limited only by the scope of the claims.

Claims

What is claimed:

1. A floating structure, comprising:

a buoyant upper section which includes at least one vertical column arranged and designed at its upper most end to support at least one deck for offshore hydrocarbon operations;

a lower section including a ballast weight suspended beneath said upper section, wherein said ballast weight is suspended at a depth such that a center of gravity of said floating structure is below a center of buoyancy of said floating structure, said lower section further including:

at least three tension members which connect said ballast weight to said upper section, wherein at least three of said tension members in connecting said upper section to said ballast weight are arranged to form truss angles in at least two non-coplanar planes, and wherein said truss angles are arranged and designed to prevent any of said tension members to go slack, allowing said floating structure to act like a rigid body.

2. The floating structure of claim 1, wherein at least one of vertical column of said upper section is buoyant.

3. The floating structure of claim 2, wherein the floating structure includes a broad base at an underwater location.

4. The floating structure of claim 3, wherein said ballast weight is at least one distributed mass with and opening disposed through it, and said floating structure is moored via a mooring means.

5. The floating structure of claim 3, wherein said ballast weight is at least one concentrated mass, wherein the floating structure is moored via a mooring means.

6. The floating structure of claim 1, wherein said floating structure includes a buoyant broad base at an underwater location.

7. The floating structure of claim 6, wherein said ballast weight is at least one distributed mass with and opening disposed through it, and wherein said floating structure is moored via a mooring means.

8. The floating structure of claim 6, wherein said ballast weight is at least one concentrated mass, and wherein said floating structure is moored via a mooring means.

9. A method for providing static stability to a floating structure comprising step of:

suspending a ballast weight below an upper section of said floating structure via at least three tension members, wherein said ballast weight is at a depth such that a center of gravity for said floating structure is below a center of buoyancy for said floating structure; and

trussing at least three of said tension members between the connection of the upper section to the ballast weight in at least two non-coplanar planes, wherein the truss angles are arranged and designed to prevent any of said tension members to go slack, allowing said floating structure to act like a rigid body.

10. The method of claim 9, further comprising the step of:

minimizing heave motions on the floating structure via the use of a broad base as part of said upper section.

11. The method of claim 10, wherein the floating structures has a single, small waterplane area.

12. A method of installing a floating structure comprising the steps of:

towing an upper section of the floating structure on site while floating the upper section on a broad base of the upper section;

deploying a ballast weight from the upper the broad base to lower the center of gravity of the floating structure to a depth below the center of buoyancy of the floating structure; and

ballasting the base to a desired equilibrium of floatation.