US3224402A

US3224402A - Stabilized floating drilling platform

Info

Publication number: US3224402A
Application number: US359110A
Authority: US
Inventors: Lawrence C Kobus
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1964-04-13
Filing date: 1964-04-13
Publication date: 1965-12-21
Anticipated expiration: 1982-12-21
Also published as: NL6504622A; GB1048775A

Description

Dec. 21, 1965 c. KOBUS 3,224,402

STABILIZED FLOATING DRILLING PLATFORM Filed April 15, 1964 n I I 27 Q 26 I2 25 2:22., 30 25:2

j I 33 l6 I5l4 34-- F|G. l

1 1 i g 20 Z j/ 1 l6 v 1 FIG. 2

INVENTORI L. C. KOBUS United States Patent Ofi ice 3,224,492 Patented Dec. 21, 1965 3,224,402 STABELEZED FLQATING DRILLENG PLATFQRM Lawrence C Kobus, Houston, Tex, assignor to Shell Gil Company, New York, NFL, a corporation of Delaware Filed Apr. 13, 1964, Ser. No. 359,110 4 Claims. (Cl. 114.5)

This invention relates to apparatus for use in drilling offshore wells and pertains more particularly to a floating drilling platform which may be anchored in deep water and from which an underwater well may be drilled and/ or produced.

In an attempt to locate new oil fields, an increasing amount of well drilling has been conducted at offshore locations, such for example, as off the coasts of Louisiana, Texas and California. Nell drilling operations are being carried out further and further from shore and in increasingly deeper water. At present, however, substantially all offshore well drilling is being conducted from a platform having legs affixed to the ocean floor, as by piling, or from mobile drilling platforms or barges having legs which may be extended downwardly through the water and in contact with the ocean floor in order to fixedly support the barge thereon. To date, using either of the two above-described platforms, well drilling operations have been limited to water depths of about 125 feet. Most deep water drilling that has been conducted to date has taken place from a converted ship in which a drilling well has been provided. Since ships are prone to roll fairly easily, deep water drilling operations carried out from ships can only take place when wind and wave forces are fairly small.

it is therefore an object of the present invention to provide a floating drilling platform for use in carrying out drilling operations in waters that are too deep to have bottom supporting legs extending down through the water to the ocean floor and to drill wells during sea conditions not possible with converted ships or too severe for converted ships.

A further object of the present invention is to provide a floating drilling platform of the semi-submersible type (one-relying on freeboard for stability, but having much of its mass underwater) from which drilling operations may be carried out economically and safely in deep waters.

Another object of the present invention is to provide a floating drilling platform which is designed with a natural period of movement different from the wave periods so that resonance is avoided.

Still another object of the present invention is to provide a floating drilling platform designed to have a natural period of roll larger than that of the wave periods to be encountered.

A further object of the present invention is to provide a floating platform having stabilizing columns of a design to prevent motion amplification in resonant oscillation.

A further object of the present invention is to provide a floating drilling platform having a discontinuous natural period of roll, so that when the platform or vessel rolls more than, say 3 degrees, which occurs only in extremely high waves, means are provided for changing the natural period of roll, thus destroying any resonance effect between the waves and the platform.

It is also an object of the present invention to provide a hull of unique design for the floating platform so that the amplitude of the angular motion (pitch or roll) of the platform is substantially reduced when the platform is anchored at a selected operating draft depth.

A further object of the present invention is to provide a floating drilling platform having a hull designed with suflicient buoyancy to support the load and sufficient reserve buoyancy to resist overturning, while at the same time the hull has sufficient area in the water plane zone to resist large changes in draft of the platform with changes in deck loads.

Still another object of the present invention is to provide a floating drilling platform with a hull of a design which is virtually independent of direction of wave approach whereby motions at for head or beam seas would be the same.

These and other objects of this invention will be understood from the following description taken with reference to the drawing, wherein:

FIGURE 1 is a diagrammatic view showing the outboard profile of the floating platform of the present invention; and

FIGURE 2 is a longitudinal view taken in enlarged detail and partially in cross-section illustrating the platform stabilizing means of the present invention in the form of open and floodable chambers in the hull of the platform above its operating water line.

Referring to FIGURE 1 of the drawing, the hull of the floating drilling or production platform may comprise three or more vertically-extending support or stabilizing columns 11 and 12 which are interconnected in a close geometric configuration by

cross-bracing hull members

14, 15 and 16 which extend laterally between the stabilizing columns near the lower ends thereof. The cross-bracing grid-type hull members l4, l5 and 16 are preferably hollow, fluid tight members divided into a plurality of buoyancy tanks by means of watertight bulkheads.-

The four corner support or stabilizing columns ll and 12 (only two being shown for ease of illustration) are preferably in the form of cylindrical tanks, as illustrated, which may be selectively flooded in order to lower the entire drilling platform in the water, thus stabilizing it by lowering its center of gravity. The stabilizing columns 11 and 12 may be of the same diameter throughout their height, say for example 25 feet in diameter. However, the upper portions of the stabilizing columns 11 and 12 are provided with a series of open ports 13 a short distance above the water line when the floating platform is in its semi-submerged position or operating depth as illustrated in FIGURE 1. The area of the fluid ports 13 from each chamber 17 is equal to at least 35 percent of the area of the side of said chamber. The diameter of the stabilizing columns 11 and 12 is large in order to reduce the amount of immersion which takes place when abnormally high hook loads are imposed on the floating platform through the fall lines and derrick. As shown in FIGURE 2, the ports 13 in the outer surface of the bull or columns 11 are in communication with a readily floodable and drainable annular chamber 17 within the hull which may be divided for re-inforcing purposes into a series of smaller chambers by vertical plates 18. Preferably, the plates 13 are provided with fluid flow ports 19 therethrough allowing water pressure to equalize on opposite sides of the plates. The vertical plates 13 may extend between horizontal column-divider plates 26 and 2.1 which are reinforced by

gussets

22 and 23. The central portion between

plates

29 and 21 may be filled with plastic buoyant material 24 to keep water out.

The provision of floodable open chambers 17 (FIG- URE 2) in each of the support columns Ill and 12 also provides the floating platform with a discontinuous natural period of roll or pitch. When the floating platform eX- periences angular motion more than a predetermined amount, in excess of 3", which occurs only in extremely high waves, the corner stabilizing column diameter suddenly changes from 25 feet to 20 feet if the chamber 1'7 is 2.5 feet wide and the natural period of the angular motion increases while the amplitude of the angular motion is substantially reduced. Therefore, if the floating platform were subjected to large swells or hurricane waves of an undesirable period so that a resonance oscillation build-up occurs, the sudden increase in the natural period of angular motion to a higher value for angles above 3 would destroy any further resonant affects. Since the natural period of the angular motion of the floating platform is a function of the water plane area of the hull or stabilizing columns, a significant change in the natural period of angular motion of the platform takes place when a wave comes along to dip one supporting column below the surface of the water so that the range of reduced diameter on one or more of the chambers 17 in the columns become flooded. Since the period of the platform and that of the waves are then different, resonant oscillation is destroyed and the amplitude of pitch and roll motion of the vessel immediately decreases.

The stabilizing columns 11 and 12 may serve as the primary ballast tanks with the tanks being filled substantially to the water level 25 when the platform is positioned as shown in FIGURE 1. Since the Water level inside the stabilizing columns is nearly the same as that outside, a leak caused by a work boat rupturing the wall of the column would not be serious. Since the buoyancy tanks in the

lower hull members

14, 15 and 16 are deeply submerged and support the weight of the platform there is little likelihood of their being damaged. Damage to the stabilizing columns would not affect the intact buoyancy tanks so the platform would neither sink nor heel. This feature provides a safer floating platform than can be achieved by converting ships. The space in the columns 11 and 12 above the plate 29 form emergency buoyancy tanks.

When the floating drilling platform of the present invention is in its semi-submerged state, as illustrated in FIGURE 1, the section of the large diameter portion of the stabilizing columns 11 and 12 above the water line provides reserve buoyancy to absorb heavy hook loads, increase the stability of the platform, and afford a sharp change in the natural period of roll and pitch as previously discussed. These tanks are further designed to have the correct size for the ballast water required to bring the platform down to the predetermined drilling draft, the size being such that the ballast water inside the tanks will be at or near the water level outside the tank. The ports 13 in the wall of the column by which the annular chambers 17 are flooded may be positioned from 2 to 8 feet above the water level 25 as desired.

Extending upwardly from the

horizontal buoyancy tanks

14, 1S and 16, as shown in FIGURE 1, are a plurality of

vertical support legs

26 and 27 for supporting with or without the aid of the stabilizing columns, any suitable type of operational platform thereon. The buoyant force in

tanks

14, 15 and 16 is designed to equal the platform is achieved. The particular platform illustrated is a multi-deck platform having an operating deck 29 and a main deck 28 as well as a partial or spider deck 30 suspended between the legs 2-6 and 27 and which is preferably movable horizontally toward and away from the center hole of the well extending down through the decks and hull. A derrick 31 is positioned over the center of the hull structure on the operating deck 29. Suitable hoist means 32 and cable reeling means may be provided. If desired the operation platform may be entirely supported on the stabilizing columns. As shown in FIG- URE l, the platform is provided with a pair of

mooring lines

33 and 34 running from each of the stabilizing columns 11 and 12 as well as using anchor lines (not shown) on the other side. If desired other suitable vesselpositioning means may be employed instead of anchor lines.

I claim as my invention:

1. A semi-submersible floating platform for carrying out at offshore locations operations such as drilling a well, said platform comprising a buoyant hull having a plurality of vertically-extending columns,

cross-bracing hull members extending between said columns at least near the lower ends thereof to form with said columns a closed geometric configuration, and

a plurality of said vertically-extending columns having chamber means formed therein in open fluid communication with the space outside said columns, said chamber means being positioned in each of said columns at a level adjacent and above the surface of the water in which the platform is positioned when carrying out operations therefrom.

2. The apparatus of claim 1 wherein each of said chamber means is in the form of an annular chamber formed with a column and extending radially inwardly from the outer vertical surface thereof.

3. The apparatus of claim 2 wherein each chamber is in communication with the space outside each column through a plurality of large area fluid port means.

4. The apparatus of claim 3 wherein the area of the fluid port means from each chamber is equal to at least 35 percent of the area of the side of the chamber in which they are provided.

No references cited.

MILTON BUCHLER, Primary Examiner.

Claims

1. A SEMI-SUBMERSIBLE FLOATING PLATFORM FOR CARRYING OUT AT OFFSHORE LOCATIONS OPERATIONS SUCH AS DRILLING A WELL, SAID PLATFORM COMPRISING A BUOYANT HULL HAVING A PLURALITY OF VERTICALLY-EXTENDING COLUMNS, CROSS-BRACING HULL MEMBERS EXTENDING BETWEEN SAID COLUMNS AT LEAST NEAR THE LOWER ENDS THEREOF TO FORM WITH SAID COLUMNS A CLOSED GEOMETRIC CONFIGURATION, AND A PLURALITY OF SAID VERTICALLY-EXTENDING COLUMNS HAVING CHAMBER MEANS FORMED THEREIN IN OPEN FLUID COMMUNICATION WITH THE SPACE OUTSIDE SAID COLUMNS, SAID CHAMBER MEANS BEING POSITIONED IN EACH OF SAID COLUMNS AT A LEVEL ADJACENT AND ABOVE THE SURFACE OF THE WATER IN WHICH THE PLATFORM IS POSITIONED WHEN CARRYING OUT OPERATIONS THEREFROM.