US3327668A - Marine structure - Google Patents

Description

June 1957 H. u. VON SCHULTZ 3,327,668

MARINE STRUCTURE Filed Feb. -i, 1966 5 Sheets-Sheet 1 FIG. I

HANS UDO VON SCHULTZ INVENTOR BY Q6 ATTORNEY June 27, 1967 H, u. VON SCHULTZ 3,327,668

MARINE STRUCTURE Filed Feb. -1, 1966 3 Sheets-Sheet 2 FIG. 2

HANS UDO VON SCHULTZ INVENTOR BY a ww ATTORNEY June 27, 1967 u. VON SCHULTZ 3,327,668

MARINE STRUCTURE Filed Feb. 4, 1966 3 Sheets-Sheet to o u u HANS UDO VON SCHULTZ INVENTOR ATTORNEY United States Patent 3,327,668 MARINE STRUCTURE Hans Udo von Schultz, Fort Worth, Tex, assignor to Mobil Oil Corporation, a corporation of New York Filed Feb. 4, 1966, Ser. No. 525,071 3 Claims. (Cl. 114--.5)

This invention relates to a versatile marine structure providing a stable above-surface platform at sea. More particularly, the invention relates to a marine, above surface drilling or workover platform of the Wave transparent type utilizing retractable buoyant support columns.

Exploration and production eflorts in the oil and/ or gas industry have been extended to remote areas throughout the world as the more accessible and desirable acreages are leased. In the recent drive to discover and tap hydrocarbon deposits beneath the continental shelves and even the more recent developing ability to extend the exploitable regions to the continental slopes, leases have been obtained for such remote offshore areas as the west coast of Africa and the coasts of Australia. A single lease may contain areas in which the water depth varies two hundred feet or more, due to the sharp incline of the marine bot tom or the size of the lease. In some of the largely untested areas, leases may be obtained for only one or two hundred dollars an acre, resulting in the obtaining of very large tracts.

In water depths of up to three hundred feet, particularly when drilling production wells, the bottom-supported platform has been predominated. Its stable deck, unaffected by wave action, is most desirable. Furthermore, it is less exensive than a comparable fully floating drilling vessel for moderate water depths. In water depths of over fifty feet, especially when drilling wildcat or exploratory wells, the floating drilling vessel has found acceptability; and in water of over three hundred feet, it is almost exclusively used due to the practically exponentially rising costs of the bottom-supported platform as water depth increases. Of the floating drilling vessels, the transparent type has been found to be the most acceptable, particularly in areas noted for frequent storms. (Transparent refers to a vessel designed so that the surface waves can travel almost unobstructed therethrough and which will be relatively unaffected by surface conditions.) The cost of a drilling template of a fixed depth design may exceed $1,000,000 while a jack-up platform may cost $2,000,000 or more. The most expensive type of marine drilling or workover rig is the transparent drilling vessel which may cost $5,000,000.

Since most of the major oil companies do not own their own drilling platforms and rigs and must rent them at a per diem rate of from $2,000 for a jack-up to $6,000 for a floating transparent platform, it is obviously advantageous to keep those, under contract, in use at all times. Furthermore, it is the practice in this competitive industry to obtain the services of these far in advance, by contracts extending for several years, and therefore it is not usually possible to rent a marine drilling platform on short notice or for a short or indeterminate duration. At the abovecited per diem rates, any unproductive time at the drilling site becomes an important economic consideration. During a drilling program, particularly at a remote lease, a specific platform may remain idle for days because of its unadaptability to the water depth at which drilling is being conducted at the moment, or there may be a considerable amount of lost time resulting from moving the drilling vessel from the lease at which it is situated to another at which the water depth is more suitable to its type of construction. Of the transparent drilling vessels, the central spar type, at represented by the experimental flip-ship (Flip-Floating Instrument Platform, Journal of Accoustical Society of America, vol. 35, No. 10, October 1963), has been found to be the most stable; however, the draft of the submerged central spar, as now envisioned, may exceed three hundred feet, and therefore would not be suitable in moderate water depths. Where the water is at least fifty feet in depth, the more usual type of transparent drilling vessel or platform, formed with a plurality of shorter spaced caissons, has proved quite satisfactory. In many of the remote areas now being considered for exploitation, both of the above-enumerated types of transparent drilling vessels would be desirable at a single lease.

A further economic consideration is the ninety to two hundred days that a conventional platform, needed at one of the above-discussed remote areas, will spend under tow from the United States to the remote foreign site and back at the usual towing speed of two to three knots. The cost of transporting so immense a structure across the span of an ocean includes towing charges of $1,000 to $1,000 a day plus towing insurance of $400 to $500 a day in addition to the per diem rate. (The quoted charges are merely representative and are only included to permit the reader to obtain an appreciation of the magnitude of the costs involved in the operation of a marine drilling platform.)

The optimum drilling platform, when only considering the speed at which it can move in the water, is one of a shipshaped design. Examples of such vessels are the Glomar and the Torry used for drilling in the coastal United States waters. These vessels are similar to an ordinary ocean-going freighter with the exception of a moonpool or central well which is formed through the lower hull of the ship from the drilling deck. Of late, the catamaran hulled vessel has received some favorable attention since the lack of cargo space is not as much a disadvantage in a drilling vessel as increased maneuver ability is an advantage. The problem with all shipshaped drilling vessels is that they are greatly affected by any surface wind and wave action and must be completely removed from the drilling site during violent storms. While heave, pitch, roll and the other possible motions occurring on a ship at sea may be compensated for, to some degree, during the drilling operation, there is an acceptable level above which the drilling must be terminated. With shipshaped vessels in the Gulf of Mexico in 1964, over five percent down time was experienced by contractors due to weather.

A transparent drilling platform, however, can remain on the site at all times and drilling may continue through all but the most violent of storms. Due to the depth at which the supporting caissons or central spar hull extends beneath the water surface, the deck motions, resulting from surface agitation, are greatly subdued to a point where they are no longer a major problem. One problem, however, with this type of drilling platform is that the supporting caissons or spar hull, extending deep into the water, results in a tremendous hydrodynamic drag during towing, making a speed near that of a shipshaped drilling vessel impossible. This problem is applicable, in varying degrees, to the bottom-supported platforms. The fixed depth template is usually provided with hollow supporting legs from which water can be expelled to buoy the template and cause it to float on its side in the body of water. While this alleviates the problem, such a structure is still of less than an ideal hydrodynamic shape. For any long distance towing a template is usually set on a barge. With jack-up platforms, the support legs can be raised up above the platform deck so as not to produce a hydrodynamic drag. The jack-up leg, extending far above the platform deck produces a top-heavy unwieldy structure which is easily overturned while the legs themselves are subjected to severe stresses 'from their own unsupported weight as well as wind loading.

One solution to this problem, when utilizing transparent platforms, that has been suggested is the inclusion of shipsh-aped pontoons on the lower ends of each of the caissons of the plural caisson type. While under tow, the drilling vessel would rise out of the water until only the pontoons would break the Water surface. This type of structure can travel at a reasonably rapid rate while under tow, but would be extremely top-heavy and would present a hazard in any type of rough weather. No practical solution has been found that would appreciably lessen the drag of a central spar hull. The flipping operation performed by the experimental flip-ship would not be feasible with an economically competitive drilling vessel.

Accordingly, it is an object of the present invention to provide a marine platform capable of drilling and maintaining subaqueous wells in substantially all water depths.

It is another object of the invention to provide a marine platform capable of being towed, or propelling itself, long distances at speeds substantially equal to those of a shipshaped vessel.

Other objects and advantages will be readily apparent from the following description, when taken in conjunction with the accompanying drawings that illustrate useful embodiments in accordance with this invention, in which:

FIGURE 1 is a side view of the marine platform of the present invention with buoyant corner caisons extending out horizontally and a central spar hull depending vertically to support the drilling platform deck when drilling in {deep water;

FIGURE 2 is a side view of the marine platform of the present invention with its spar hull sections canted outward and its spaced caissons depending vertically to support the drilling platform deck when the structure is drilling in shallow water;

FIGURE 3 is a side view of the marine platform of the present invention with the spar hull sections and the caissons extending horizontally beneath the drilling platform deck when the structure is being towed or self-propelled from one location to another; and

FIGURE 4 is a top plan view of the marine platform of the present invention when under tow or being selfpropelled as shown in FIGURE 3.

The marine platform of the instant invention comprises a drilling platform deck supported above the surface of a body of water on one or more retractable support columns which can be reoriented to extend down beneath the surface of the body of water. The buoyancy of the columns is controllable to permit the platform to float or to be supported on the marine bottom. When the structure functions as a floating transparent platform the columns extend beneath the surface of the body of water deep enough to permit the structure not to be seriously affected by surface wave actions. The support columns consist of a very long central spar hull and a plurality of spaced, relatively short caissons. The retractable central spar hull is formed from a pair of longitudinally elongated spar hull sections, with sufficient buoyancy between them to support the platform, which are movably mounted beneath the center of the drilling platform so that they can be concurrently pivoted together to form the composite central spar hull. The composite spar hull may extend two hundred and fifty to three hundred feet into the water beneath the drilling platform. The actual length of the spar hull would depend on the planned height of the platform deck above the water, it being desirable that no more than ten percent of the length of the spar protrude above the water line to minimize the effect of wind and wave forces on the structure. The plurality of retractable shorter caissons, which double as hottom-supported legs in shallow water, are also pivotally mounted beneath the polygonal platform, preferably one under each corner thereof, to form in intermediate water depths a floating support means extending thirty or forty feet beneath the surface when the caissons are oriented in a spaced, parallel configuration, each perpendicular to the water surface. With a draft of thirty feet, the platform could be safely used for floating drilling or workover operations in fifty feet of water. For yet shallower water the caissons can rest on the marine bottom or on a landing mat set on the bottom to support the platform above the surface.

The central spar hull sections and the corner caissons may be considered as elongated, cylindrical columns for simplicity, although, in actuality, the shapes of the columns may be more complex. All of the columns are pivotable into positions in which their longitudinal axes are in horizontal planes wherein the platform deck is supported above the surface of the body of water floating on the elongated sides of the hull sections and the caissons to form substantially a multiple hull or catamaran type of shipshaped vessel. Attached to the free ends of each of the columns are propulsion units, consisting of electric motors driving marine propellers, which are used for the self-propulsion of the marine platform from one location to another. If the marine platform is to be bottom-supported on the caissons, the propulsion units on the free ends of the caissons can be removed or capped with hollow leg extensions to prevent damage to the propulsion units. Further, propulsion units are mounted along the sides of the columns for the dynamic positioning of the platform while located at a drilling site.

It is seen from the above description that this marine structure can be used for drilling or working over oil and/ or gas wells in practically all water depths by simply pivoting either the caissons or spar hull sections into a vertical platform deck supporting position. In readying the marine platform for use in intermediate or shallow water, buoyancy/ballast tanks within the free ends of the short caissons are flooded so that the longitudinal orientation of each of the caissons will change from a horizontal to a vertical position. The long spar hull sections are also partially flooded such that they cant outward between a horizontal and a vertical position, the draft of both the caissons and the spar hull sections being equalized. The spar hull sections may be locked in the canted position, when the platform is to be used in a floating condition, to obtain an outrigger effect. By changing the ballast in the columns, the elevation of the platform above the surface of the water may be regulated. For

deep water drilling, the long spar hull sections are flooded so that each assumes a vertical position, the caissons remaining in the horizontalposition. The height of the platform above the water is regulated by the ballast within the central spar hull. When the marine operations are completed at a particular site and the platform is to be moved, compressed air is forced into the buoyancy/ ballast tanks of all of the columns to blow out the water and return the columns to their horizontal positions. Releasable locking means must be provided for rigidly fixing the particular columns in the horizontal or vertical positions, and it would even be desirable to fix rigidly the spar hull sections when they are canted outward when the structure functions as a floating platform in intermediate depths of water.

The structure may be held over the well being drilled by using bottom-mounted acoustic transponders around the borehole, with corresponding transducers mounted on the platform. Each of the transponder-transducer units would have a different frequency for polar orientation purposes. Any deflection from the straight-above position or any angular movement of the platform about the axis of the drill pipe will cause a corresponding propulsion unit to be actuated for the readjustment of the platform.

FIGURE 1 shows a marine structure having a drilling rig 12 centrally mounted on the platform deck 14 thereof. A pair of elongated spar hull sections 16 are pivotally mounted beneath the drilling platform deck 14 on parallel pivotal axes 18 spaced equal distances from the center line of the rig 12. Dynamically positioning propulsion units 20 are mounted around each of the spar hull sections 16 near the water line 22 and near the free end thereof, with another propulsion unit 24 being afiixed to the free end. A drill string 26 is lowered into the subaqueous formations beneath the marine bottom 28 through a slot or space between the vertically oriented spar hull sections 16. The spar hull sections 16 are longitudinally compartmentalized to form buoyancy/ballast tanks 30 in the outer ends thereof, the buoyancy/ballast tank 30 in each being flooded to keep the spar hull sections 16 vertically oriented and to hold the drilling deck 14 of the marine structure 10 the desired distance above the water line 22. While the flooding of the buoyancy/ ballast tanks 30 will hold the spar sections 16 substantially vertically oriented, it is necessary to lock these sections 16 in place to prevent crushing of the drill string 26 therebetween and/ or the shifting of the platform deck 14. Caissons 32 are pivotally mounted at points 34 near each corner of the, in this instance, square platform deck and extend horizontally outward in parallel pairs from opposite sides of the platform deck 14. These caissons 32 must be locked into position to keep them in the horizontal position. The caissons 32 also have dynamically positioning propulsion units 36 ringing the free ends thereof, although due to their length they do not need an upper ring of propulsion units as found on the spar hull sections 16. Propulsion units 38, for assisting in selfpropelling the marine structure 10, are also mounted on the free ends of the caissons 32.

For drilling in intermediate depths of water, as shown in FIGURE 2, buoyancy/ballast tanks 40 in the outer ends of the caissons 32 are flooded to pivot the caissons 32 into the vertical position in which they are then locked. Due to the lengths of the spar hull sections 16, it is not feasible to lock them in their horizontal positions; and since the water would not be deep enough for them to be moved into their vertical positions, the buoyancy/ ballast tanks 39 are flooded until the draft of each of the spar hull sections 16 equals that of the caissons 32 to obviate any adverse effects caused by surface wind and waves if the outer free ends of the spar hull sections 16 were allowed to float on the surface of the water. For drilling in shallow water (as a bottom-supported platform) the procedure as outlined above for floating drilling in intermediate depths would be followed except that the spar hull sections 16 would not be canted until the caissons 32 rest on the marine bottom 28.

FIGURES 3 and 4 show the positioning of the various elements of the marine structure 10 when it is under tow or when it is propelling itself through the water by means of the propulsion units 24 and 38 on the spar hull sections 16 and the caissons 32, respectively. While moving through the water, all of the caissons 32 and spar hull sections 16 are held in the horizontal position with the buoyancy of the spar sections 16 and the caissons 32 being adjusted to hold the platform deck 14 spaced a distance above the surface of the water. The spar hull sections 16 and the caissons 32 act as catamaran type hulls to reduce the drag of the vessel 10 to shipshaped limits. Obviously, the longitudinal dimensions of all of the columns are parallel to each other when in the horizontal orientation to obtain the desired results.

Although the present invention has been described in connection with details of a specific embodiment thereof, it is to be understood that such details are not intended to limit the scope of the invention. The structure, although primarily designed for oil and/or gas drilling and maintenance operations, can obviously serve other functions. For example, it can be used as a foundation for a versatile mobile lighthouse or radar tower. The terms and expressions employed are used in a descriptive and not a limiting sense and there is no intention of excluding such equivalents, in the invention described, as fall within the scope of the claims. Now having described the invention herein disclosed, reference should be had to the. claims which follow.

What is claimed is:

1. A marine structure for supporting equipment above the surface of a body of water comprising: a platform deck for mounting equipment thereon; at least a pair of longitudinally elongated spar hull sections for buoyantly supporting said platform deck; means for mounting said elongated spar hull sections to said platform deck to provide adjustability between first and second positions thereof, said first position of said elongated spar hull sections being with the longitudinal dimensions thereof substantially parallel to said platform deck, whereby when said marine structure is to be transported through a body of water, said elongated spar hull sections, in said first position, are parallel to the surface of a body of water, to reduce the hydrodynamic drag of said marine structure, said second position of said elongated spar hull sections being with the longitudinal dimensions thereof substantially perpendicular to said platform deck, whereby when said marine structure is to be located at a marine site, said elongated spar hull sections, in said second position, depend substantially vertically into a body of water to form a substantially single composite vertical spar hull centrally located beneath said platform deck so that said marine structure is relatively unaffected by any wave action; and said means for mounting said elongated spar hull sections to said platform deck being arranged so that said platform deck is substantially above the surface of a body of water when said marine structure is in a body of water in either of said first or second positions.

2. A marine structure for supporting equipment above the surface 'of a body of water comprising: a platform deck for mounting equipment thereon; a plurality of platform-deck-supporting, longitudinally elongated columns; means for mounting said plurality of elongated columns to said platform deck to provide adjustability between first and second positions of said plurality of elongated columns with respect to said platform deck, said plurality of elongated columns comprising at least a pair of relatively long spar hull sections and a plurality of relatively short caissons, said first position of said plurality of elongated columns being with the longitudinal dimensions of all of said plurality of elongated columns substantially parallel to said platform deck, whereby when said marine structure is to be transported through a body of water, said plurality of elongated columns, in said first position, are parallel to the surface of a body of water, to reduce the hydrodynamic drag of said marine structure, said second position of said plurality of elongated columns being with the longitudinal dimensions of said at least some of said plurality of columns substantially perpendicular to said platform deck, whereby when said marine structure is to be located at a marine site in a body of water too shallow for the utilization of said spar hull sections, said plurality of relatively short caissons, in said second position, depend substantially vertically into a body of water to form spaced vertical supports so that said marine structure is relatively unaffected by any wave action, and whereby said marine structure is to be located at a marine site in deep water, said spar hull sections, in said second position, depend substantially vertically into a body of water to form a single vertical spar hull centrally locate-d beneath said platform deck so that said marine structure is relatively unaflected by any wave action; and said means for mounting said plurality of elongated columns to said platform deck being arranged so that said platform deck is substantially above the surface of a body of water when said marine structure is in a body of water in either of said first or second positions.

3. A method for utilizing the marine structure of claim 2 as a floating platform in a body of Water that is too shallow for the utilization of said spar hull sections in said second position, and deep enough for the utilization of the relatively short caissons in said second position, comprising the following steps:

(a) locating said structure over a subaqueous site; (b) moving said caissons into said second position to support said platform deck floating above the surface of the body of water in a Wave transparent state; and (0) moving said spar hull sections to a third position between said first position and said second position wherein the draft of said spar hull sections equals the draft of said caissons.

References Cited UNITED STATES PATENTS MILTON BUCHLER, Primary Examiner.

T. M. BLIX, Assistant Examiner.

Claims (1)

1. A MARINE STRUCTURE FOR SUPPORTING EQUIPMENT ABOVE THE SURFACE OF A BODY OF WATER COMPRISING: A PLATFORM DECK FOR MOUNTING EQUIPMENT THEREON; AT LEAST A PAIR OF LONGITUDINALLY ELONGATED SPAR HULL SECTIONS FOR BUOYANTLY SUPPORTING SAID PLATFORM DECK; MEANS FOR MOUNTING SAID ELONGATED SPAR HULL SECTIONS TO SAID PLATFORM DECK TO PROVIDE ADJUSTABILITY BETWEEN FIRST AND SECOND POSITIONS THEREOF, SAID FIRST POSITION OF SAID ELONGATED SPAR HULL SECTIONS BEING WITH THE LONGITUDINAL DIMENSIONS THEREOF SUBSTANTIALLY PARALLEL TO SAID PLATFORM DECK, WHEREBY WHEN SAID MARINE STRUCTURE IS TO BE TRANSPORTED THROUGH A BODY OF WATER, SAID ELONGATED SPAR HULL SECTIONS, IN SAID FIRST POSITION, ARE PARALLEL TO THE SURFACE OF A BODY OF WATER, TO REDUCE THE HYDRODYNAMIC DRAG OF SAID MARINE STRUCTURE, SAID SECOND POSITION OF SAID ELONGATED SPAR HULL SECTIONS BEING WITH THE LONGITUDINAL DIMENSIONS THEREOF SUBSTANTIALLY PERPENDICULAR TO SAID PLATFORM DECK, WHEREBY WHEN SAID MARINE STRUCTURE IS TO BE LOCATED AT A MARINE SITE, SAID ELONGATED SPAR HULL SECTIONS, IN SAID SECOND POSITION, DEPEND SUBSTANTIALLY VERTICALLY INTO A BODY OF WATER TO FORM A SUBSTANTIALLY SINGLE COMPOSITE VERTICAL SPAR HULL CENTRALLY LOCATED BENEATH SAID PLATFORM DECK SO THAT SAID MARINE STRUCTURE IS RELATIVELY UNAFFECTED BY ANY WAVE ACTION; AND SAID MEANS FOR MOUNTING SAID ELONGATED SPAR HULL SECTIONS TO SAID PLATFORM DECK BEING ARRANGED SO THAT SAID PLATFORM DECK IS SUBSTANTIALLY ABOVE THE SURFACE OF A BODY OF WATER WHEN SAID MARINE STRUCTURE IS IN A BODY OF WATER IN EITHER OF SAID FIRST OR SECOND POSITIONS.

Images