Title:
METHOD OF AND MEANS FOR MOUNTING EQUIPMENT AT A SUBSEA LOCATION
United States Patent 3589133


Abstract:
A structure for mounting equipment at a subsea location has a platform member and an anchor member, each with independently adjustable buoyancies. The two members are interconnectable and can be nested together for towing to their emplacement site, at which their buoyancies are sequentially adjusted, the anchor positioned, and the platform winched down and secured to the anchor. The platform is retrievable to the surface without an external power assist. The anchor member may also be retrieved if external power is available to blow the anchor ballast.



Inventors:
Lowd, Judson D. (Tulsa, OK)
Hill, Ernest C. (Tulsa, OK)
Burrus, Bill S. (Tulsa, OK)
Application Number:
04/824790
Publication Date:
06/29/1971
Filing Date:
05/15/1969
Assignee:
COMBUSTION ENGINEERING INC.
Primary Class:
Other Classes:
114/331, 405/205
International Classes:
E02B17/02; E21B43/01; E21B43/36; (IPC1-7): B63C11/00
Field of Search:
61/46,46.5,69 114
View Patent Images:
US Patent References:



Primary Examiner:
Bell, Karl J.
Claims:
I claim

1. A subsea structure, including:

2. a frame formed of tubular elements with means to communicate liquid and gas to the interior of the elements to adjust their buoyancy,

3. a plate member secured to the frame in a watertight relationship to form a hull having a hold,

4. and a predetermined amount of solid ballast mounted in the hold;

5. The subsea structure of claim 1, in which,

6. The subsea structure of claim 2, in which,

7. A method of erecting a structure at a subsea emplacement location, the structure having independently buoyant anchor member in the form of a hull with a hold and platform member releasably fastened together to form a buoyant unit adapted to be towed to a surface location above the subsea emplacement location, including,

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to establishing a stable structural support at a subsea location. More particularly, the invention relates to a support of selective buoyancy, including platform and anchor members which can be towed to their location and emplaced sequentially at a subsea location to provide a base for performance of work. Both platform and anchor are retrievable.

2. Description of the Prior Art:

As industry has turned its attention to the sea in recent years to develop and protect its resources, the technology and operating problems have been rapidly subdivided. For example, in the oil industry, the discovery of oil beneath the sea, drilling under water, producing from subsea wellheads, processing the fluids at the wellheads and transporting the products to shore are some of the subdivisions of the problems. In many areas of relatively shallow water depths, platforms have been built up from the bottom and the techniques developed on shore have been merely transferred to the platforms having this damp underfooting. At present, oil fields are being planned to produce from reservoirs which lie beneath the surface of the distinctly hostile environment of the sea.

The casual observer might conclude that it is common practice for a simple run of pipe to be connected to the wellhead to transport well fluids to shore. In many instances, where it is but a short distance to shore, this is the present practice. However, there are peculiar problems inherent in the two-and-three-phase flow of produced fluids which usually limit the distance over which the total produced fluids may be transported without processing. For example, cooling of unseparated phases by the sea, as a heat sink, can cause hydrates and paraffins to form. Slug flow induced by gas separation can damage well equipment and flow lines. Solid materials and viscous slugs can plug lines and otherwise disrupt normal producing and processing operations. With wells located many miles from shore, it is essential that produced fluids be initially separated by processing equipment near the wellhead.

The equipment to separate oil well fluids includes vessels with relatively simple controls to regulate fluid flow rates, pressure and fluid levels within the vessels. A stable base must be provided for this equipment. Obviously, there is a great range of problems in providing the base at a wellhead below a water surface. Many problems also arise in controlling the flow rates, pressure and fluid levels within the vessels mounted on the base.

The discovery and drilling of offshore wells has run far ahead of the development of onsite processing and transporting technologies. For example, even the relatively simple initial separation process discussed has yet to be carried out successfully beneath the surface of the sea. The oil industry seeks solutions to this and the many other problems of subsea operations. It will be readily apparent from the disclosure, in fact, that any industry concerned with the problem of conducting operations with the assistance of equipment which must be furnished a stable base at a subsea site will find the present invention adaptable to its use.

The present invention focuses then upon the specific problem of providing a base upon which equipment may be mounted at a subsurface location. Further, the invention solves the problem of providing a towable base structure which may be emplaced with a minimum of external assistance, and which may be retrieved for maintenance or removal.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an improved base structure upon which equipment may be mounted at a subsea location.

Another principal object of the invention is to provide a towable selectively buoyant base structure comprising a platform member and an anchor member upon which equipment may be mounted.

Another object is to provide independent adjustment for the buoyancies of the anchor and platform followed by sequential positioning of the anchor and platform at the subsea site.

Another object is to provide improved winching of the platform between the surface and the emplaced anchor on the sea floor.

Another object is to provide for emplacement and retrievability of both members with a minimum of external assistance.

Another object is to provide for retrievability of the platform member with no external assistance.

The invention contemplates anchor and platform members in the general form of a tubular frame with means to lock the two frames together as a unit. The hollow frames are independently adjustable in buoyancy. When positively buoyant and locked together, they can be towed to their subsea emplacement site.

The invention further contemplates the anchor being unlatched from the platform and sunk to the sea floor at the emplacement site. The anchor member is guided to its precise position of emplacement on the bottom and further buoyancy adjustment made to characterize it as a stable anchor for the platform. Buoyancy adjustments are made to the platform member and winch lines between the anchor and platform are then reeled in to draw the platform down to the emplaced anchor. A surface craft provides power. The platform and anchor are again locked together. Winches and power source can thereupon be disconnected, removed from the platform and returned to the surface craft.

The invention further contemplates retrievability of the platform to the surface with no external assistance, and retrievability of the anchor with a minimum of external assistance.

Other objects, advantages and features of this invention will become apparent to one skilled in the art upon consideration of the written specification, appended claims and attached drawings, wherein;

FIG. 1 is a cross-sectioned elevation of a base and platform structure embodying the invention and operational in its subsea location, together with representative production process equipment, a capsule housing its control elements, and a diagrammatic showing of the producing wellhead structure;

FIG. 2 is a plan view of the structure of FIG. 1 showing the process equipment, the capsule, and various piping detail;

FIGS. 3--6 are diagrammatic elevation views of the base and platform structures being launched, moved to the surface location above the emplacement site, and erected at the subsea site.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. The Operating Situation:

The environment for the present invention is beneath the surface of the water. The use of the invention is as a base structure for mounting any equipment which can operate exposed under water or within a housing, such as a capsule, mounted on the structure. The specific preferred embodiment of this specification discloses a separation vessel exposed to a subsea environment (with its controls encapsulated) in service to a subsea oil well. A similar installation could also be arranged to serve more than one well, usually from a central location.

Initial processing of oil well production is a complex art even in good conditions. It is apparent that all the usual and well-known problems of dealing with these corrosive, toxic, combustible, two- and three-phase hydrocarbon reservoir fluids are much compounded by moving the operation to the inimical environment beneath the surface of the sea. 2. The Structure:

a. In General

The drawing of FIG. 1 discloses the invention in situ and operational on the ocean floor. The structure consists of two basic components, namely the anchor member 1 and the platform member 2.

FIG. 2 shows that the two components disclosed in the preferred embodiment are rectangular in shape. At this point of generalization, however, it is emphasized that although these components are illustrated in the specific form of a rectangle, this is not necessary to their function. The form of the rectangle has several advantages, for example in manufacture and shipping, but other considerations could dictate some modification in special cases. A circular or toroidal shape, for instance, could be advantageous in providing greater floating stability in a rough sea.

FIG. 1 also depicts the equipment in service at only a single well location. It is noted a single platform similar to the one of FIG. 1 can easily serve a multiple of wells. In such case, the addition to the platform equipment shown of at least a test separator for individual well testing would be made.

The Figures disclose the process equipment mounted on platform 2 as simply an oil and gas separator vessel 3. It must be understood that the equipment shown in the drawings is merely representative of any type of equipment. It is theoretically possible to have any function that is operably and performable under water, either exposed or in a housing such as capsule 4, mounted on the platform deck. It is also likely that certain functions will be carried out at the structure disclosed, others at the water surface, and yet others at a shore location.

It is essential to recognize that the concept of the instant application of a buoyant, two-stage, towable structure of great stability during emplacement and use, and having independently buoyant, interconnectable members which require a minimum of eternal support and site preparation to both emplace and retrieve is not limited or obscured by a showing of particular equipment in the drawings and its description in the specification. The equipment depicted and its work function, as well as its support and control and other systems (for example latches 5, flowline connection couplings 6, and the control system capsule 4), are purposely simple representatives of the large range of equipment and auxiliary systems and their functions adaptable to the inventive structure and concept. The choice of equipment used thereon or its work function is not material to the use or concept of the invention.

b. The Anchor Member

The anchor member 1 in the preferred embodiment has a rectangular tubular frame 1a formed of large steel cylinders. Its bottom is closed by a flat steel plate 7 to form a watertight hull for the anchor member, thereby providing it a displacement considerably larger than that afforded by the cylinders alone. Thus when the hold of this shiplike structure is filled with a predetermined volume of dense ballast 8, such as concrete, a restraining force of the anchor substantially greater than that due to frame 1a displacement alone is realized.

Mounted on the anchor 1 are piping 9 and flange ends 10 positioned to mate with appropriate counterparts on the platform member 2. Four female guides 11 are also mounted thereon to provide half of the necessary locking assembly to secure anchor and platform together as shown in FIG. 1.

c. The platform Member

Fig. 1 also depicts the platform member 2 of the invention. It is shown in place and locked to the anchor member. In the preferred embodiment it also exhibits a rectangular tubular frame 2a formed of large steel cylinders. Its bottom is open, however, and its buoyancy at the surface is obtained from the cylinders of the frame alone. When in position at its subsea site as shown in FIG. 1, however, the controls capsule 4 will provide a buoyant force if gas-filled (but will not if liquid-filled).

The crossbeams 12 will represent both bracing structures for stability and a frame structure for mounting the line connections 6 and the platform piping 13.

Platform deck 14 carries the separator vessel 3, capsule 4, various piping detail, and ancillary elements of the structure. The vessel 3 is connected by lines 13 to a flowline 15 from the wellhead and, via the controls capsule 4, to a fluids flowline 16 leading to downstream process facilities. Gas is gathered or vented from line 17.

Also mounted on the platform member are four vertical vessel structures 18. These vessels, or trim tanks, serve three critical functions. They provide additional ballast; they provide the necessary stability during the movement of the always buoyant platform between the surface and the anchor; and they provide a sensitive trim mechanism. This will be disclosed in detail below in numbered paragraph 4b.

3. Assembly, Launch, and Tow:

Although many variations in this procedure are obvious, in the preferred embodiment the anchor 1 and the platform member 2 are marshalled at a convenient location for handling and pouring concrete, for example, a dock site. Please refer now to FIGS. 3--6. In the preferred method, the anchor is hoisted and placed in the water with all ballast volume of the frame 1a dry. The watertight hold formed by plate 7 is then filled with a predetermined weight of concrete ballast 8 such that the anchor member will still have a positive buoyancy as long as its hold is not also filled with water.

The platform 2 is now hoisted as depicted in FIG. 3, set atop anchor 1, and secured to the anchor. Winch lines 19 are strung between the members. The hoist 20 is swung away, and the full weight of the platform on the anchor submerges it. As water spills over the frame and fills the hold, the anchor becomes negatively buoyant though easily supported by the positive buoyancy of the platform. Please see FIGS. 3 and 4. The two members are then towed to the location.

One advantage of dockside launch is that no crane capacity of a tender ship at the construction location is required. However, it is certainly within the scope of the invention to conduct the concrete fill operation and launching at the location. The "tow-to-location" described is then merely the movement from ship to the surface position above the subsea emplacement site, possibly a distance of only a few feet. 4. Erection at the Subsea Site:

a. The Anchor

When on location, power from a support craft 21 is furnished to the winches 22 on the platform. Since the anchor is already negatively buoyant, sinking the anchor is accomplished by unfastening the latches 5 and paying out winch lines 19. A diver will be on hand on the subsea site to insure the anchor is correctly positioned and, if possible, level on bottom. Flowlines are then connected to the anchor lines 9, and all anchor frame 1a ballast is flooded to give the anchor 1 maximum weight. FIG. 5 discloses the condition at this point.

A minimum of site preparation should be necessary in use of this structure. Levelling legs on the anchor or platform could easily be added and extension of the anchor's latch guides made, if the bottom or a foundation structure were not perfectly level.

b. The Platform

The next step requires mating and securing the platform 2 to the anchor, as shown in FIG. 1 and briefly discussed above in numbered paragraph 2.

First, ballast is taken on in the frame 2a to reduce the buoyancy of the platform member. Next, the platform is trimmed by selective flooding of the independently buoyant vertical trim tanks 18, thus reducing the buoyancy further to near zero. The trim accommodates nonuniform weight distribution on the deck 14, and orients the platform 2 in the same attitude as the anchor 1, in this case, level. Trim stripes 23 provide a ready reference during this operation, and gauge glasses 24 show the water level in the various tanks 18. Valves are provided to control the buoyancy of each tank 18, the tanks being isolated from the frame 2a ballast and from each other. These tanks also act in the manner of outriggers, i.e., to attenuate the motion of the deck due to surface motion of the water.

This arrangement provides stability to the near zero but still positively buoyant platform member by placing the center of buoyancy of the member safely above its center of gravity.

The platform is then drawn down to the anchor by operating winches 22 to take up winch lines 19. See FIG. 6. Winch power is supplied through a relay indicated at 25, wiring 26, and winch gearing units 27. Please see FIG. 2. It is noted that any suitable power supply may be employed to operate winches 22, such as a hydraulic or pneumatic system. It is noted that here the frame 2a is only partly flooded as shown in FIG. 6, so that the capsule 4 volume must also be flooded upon its submergence as shown in FIG. 6. This will prevent the added buoyancy due to its volume when dry from possible disturbance of the anchor position during the drawdown of the platform.

After the platform latch pins 28 are mated with the anchor latch guides 11 and secured, the anchor piping 9 and platform piping 13 are connnected through couplings 6, and process operations may begin. Although not necessary to the controls, it is contemplated that the capsule 4 will be blown dry and necessary buoyancy adjustments made to the platform ballast after the platform is secured to the anchor.

In the preferred embodiment, a slight positive buoyancy is retained in the platform member at all times so the platform member may be retrieved without an external power assist. Of course, current forces, a marginally competent foundation, extreme production surges or "heading," the buoyant effect of vessel plus capsule displacements, and so on, may dictate that the platform frame ballast be flooded.

Power cables 29 may be disconnected and returned on board the tender 21. Winch mechanisms 25, 27 may either be designed to be returned to the surface, or may be integral to the platform.

5. Retrieval:

Retrievability of the structure is of prime importance to perform major inspection and maintenance tasks, make major revisions, or replace certain of the elements. The ease of retrievability of the novel structures is described.

Retrieval is essentiallly a reversal of the setting process. To bring the platform member to the surface when a positive buoyancy has been retained, it is necessary only to unlock latches 5 and pay out winch lines 19. No external power is necessary. When desired to winch the platform down to recouple with the anchor, a small power generator transportable in a small craft is the only surface support required. The near zero platform buoyancy allows power requirements and therefore costs to be minimized. It may be desirable, of course, when power is available, to employ winches 22 which require power to unwind. Such may offer superior control to a simple braking system for the paying out of lines 19.

Occasionally, the platform frame 2a and vessels 18 may be required to be completely flooded when set to provide added weight to the anchor-platform structure, as, for example, when vessel 3 and capsule 4 displacements are large, when production surges are intense, or when current forces are strong. When complete flooding is necessary, a power source or source of compressed gas would be necessary to blow sufficient ballast from the frame 2a structure to provide a slight positive buoyancy to the platform member.

Power will also be required whenever the anchor structure is to be raised, though the requirement may be larger to blow all ballast from the anchor preparatory to winching it up to the previously floated platform than merely to provide a flooded platform with slight buoyancy. Similarly, greater power to the winches will be needed to raise the anchor up to join the platform than to draw the platform down to the anchor, due to the greater anchor mass even when without water ballast.

A simple arrangement will be provided to the frames 1a and 2a to control the taking on and discharging of water ballast. It is quite obvious that two valves will be a suitable and simple means of taking ballast on (water in-air out) and of discharging it (compressed air in-water out). Both frame 1a and frame 2a will carry such means to provide the independently adjustable buoyancies specified. Each valve in this basic arrangement is indicated by the numeral 30; see FIG. 1. Bulkheads may be positioned to cmpartmentalize the two frames. It is generally desirable to segment the ballast in such a manner for better control of nonuniform weight distribution, to avoid ballast shift as the unit rolls and pitches on the surface, to minimize the danger of sinking, and for other recognized reasons. The valve arrangement described would then be provided for each chamber. Since each of the vertical vessels 18 are isolated from the ballast volume of frame 2a, each of them would also utilize such a control valve arrangement.

SUMMARY

The broad concept of invention is clearly developed in the disclosure. The two structural members are independently adjustable in buoyancy and interconnectable. The unit is towable. It is emplaced and retrieved with minimum power assist. The sea floor requires a minimum of preparation to provide a suitable foundation for the anchor, and hence the platform.

The final combination readily supports with great stability equipment necessary to perform a subsea work function. The economics of the unit are very attractive. A pilot model has been built and satisfactorily operated, and an actual unit contained in the final stage of construction.

It is strongly emphasized once again that the scope of the invention permits its use as a support for any type of mechanism capable of operating in a subsea environment, whether exposed to the water itself or contained in a protective housing. The oilfield process equipment described is by way of specific example only, and to lend clarity to the disclosure.

From the foregoing it will further be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and inherent.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the invention.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted in an illustrative and not in a limiting sense.