US2940265A - Offshore platform and method of erecting the same - Google Patents

Description

June 14, 1960 P. J. DOODY ET AL OFFSHORE PLATFORM AND METHOD OF ERECTING THE. SAME Filed Jan. 30, 1956 2 Sheets-Sheet 1 llllllllllllll INVENTORS F! J. DOODY B.L. GOEPFERT BY fift THEIR AGENT June 14, 1960 P. J. DOODY ETAL 2,940,265

OFFSHORE PLATFORM AND METHOD OF ERECTING THE SAME Filed Jan. 30, 1956 2 Sheets-Sheet 2 INVENTORS P. J. DOODY B.L. GOEPFERT THEIR AGENT OFFSHORE PLATFORM AND METHOD OF ERECTING THE SAME Patrick J. Doody, New Orleans, La., and Benjamin L.

Geopfert, Houston, Tex., assignors to Shell Oil Company, a corporation of Delaware Filed Jan. 30, 1956, Ser. No. 562,169

6 Claims. (Cl. 61-46) This invention relates to offshore platforms of the type used in well drilling operations, such as drilling platforms which may be positioned at a desired location for drilling a well, or production platforms which may be positioned around a wellhead and its casing after the well has been drilled.

During the past several years considerable interest has been shown in drilling oil wells in oflfshore locations, particularly in the Continental Shelf off the shores of Texas and Louisiana. The drilling and maintaining of offshore oil wells is a difficult and expensive operation necessitating the use of suitable operating platforms for positioning the required drilling or servicing equipment at a safe level above the surface of the waters in which drilling operations are being carried out.

At some locations oif the coast of Louisiana, there is no solid bottom on which a platform may be set, or into which the legs (normally piles) of the platform may be driven. This is especially true at locations off the mouth of the Mississippi River where the water may be only 30 feet deep While the soft, fluid or unconsolidated mud underlying the water may be several hundred feet deep.

An offshore platform to be used in a muddy location of this type, for example, a small platform erected around a wellhead after the well has been drilled, may comprise a reinforced foundation member or template having a plurality of vertical tubular legs through which piling may be driven deep (200 to 300 feet) into the mud. Generally, there is sulficient skin friction between the mud and piles of this length to support the platform. However, a storm of considerable violence or duration can cause waves or a movement of the water tending to flex these long piles from side to side. This can result in a reduction in the skin friction effect between the mud and the piles so that the piles and platforms supported thereon sink deeper into the mud.

Various remedies have been tried to prevent piling and platforms supported thereon from sinking into the muddy ocean bottom. For example, at one wellhead a 15 ft. diameter steel caisson 45 feet high was positioned around the piling in a Water depth of 25 feet so that ten feet of the caisson penetrated the mud and ten feet of it extended above the water level. Due to the extremely poor soil conditions at this well site, the caisson and piling sank 13 feet during a storm which produced 8 to 10 foot waves. The failure was caused by the impact loading from the waves which broke the skin friction on the piling to a degree sufiicient to cause the structure to sink into the mud.

It is therefore a primary object of the present invention to provide an offshore platform having substantial lateral stability whereby its lateral movement during a storm is greatly minimized or entirely eliminated.

Another object of the present invention is to provide an offshore platform structure having an open framework above the ocean floor permitting water to pass readily through, while at the same time having additional bearing area in contact with the mud below the ocean floor ted rates Fatent O to absorb all or a portion of the lateral force caused by wind and wave and thereby lessen the otherwise required pile reaction.

A further object of this invention is to provide a method of erecting offshore platforms of the pile driven type having the characteristics outlined above.

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

Figure 1 is an elevation of an offshore platform constructed in accordance with the present invention and shown as forming a well protector jacket or servicing platform around a wellhead.

Figure 2 is an elevation of an offshore platform provided with aligning means for driving batter piles therethrough.

Figure 3 is an isometric view of the reinforced substructure of Figure 1 which is provided with means for rendering the substructure substantially stable to lateral movement in muddy sub-soil.

Figure 4 is an elevation of the sub-structure of an offshore platform provided with removable stabilizing means.

Figure 5 is a plan view of a portion of the structure illustrated in Figure 4.

Figure 6 is an isometric view of removable stabilizing means adapted to be used with a batter pile sub-structure of the type illustrated in Figure 2.

Referring to Figure 1 of the drawing, the sub-structure in Figure 3, at least four vertical columns are arranged in a rectangle with the structural cross bracing members 13 so disposed that an open space is provided down through the center of the template to accommodate the necessary well casing 14 (Figure l) rising out of the water. The top of the well casing 14 is closed by the necessary valves contained in the casing head assembly or Christmas tree 15.

While a rectangular template 11 is preferred, it is realized that it may take any shape such as triangular, octagonal or even round, said template being composed of a minimum of at least three vertical legs. The internal diameter of the tubular legs or columns 12 is large enough to accommodate piles 16 that are driven through the columns 12 and into the ocean floor.

These piles 16 may be from 200 to 300 feet in length so as to support the sub-structure 11 and the platform 17 mounted thereupon. The weight of the sub-structure 11 and platform 17 is supported mainly by the skin friction between the mud and the outer surface of the piles 16 over its entire embedded length.

To provide lateral stability for the entire oifshore foundation, the sub-structure 11 is provided with one or more plates of substantial area that are fixedly secured in a vertical position as by welding or bolting, between preferably at least two legs of the sub-structure. The plates 18 are secured to the lower portion of the sub-structure 11 but their exact position thereon depends entirely upon the depth to which the sub-structure 11 is to be sunk into the mud. It is essential that ture.

- to the inclined column 24, as shown.

'The plates 33 may be fixedly secured either to the v'ertical columns 12 or many of the cross-bracing members 13 which may be positioned below the mud line.

' Although itis; possible to use only one plate 18 provided 7 it were positioned in a direction normal or transverse to the prevailing wave and wind forces, it is preferred that at least two plates 18 be 'aflixed to the sub-structure 11' at an angle to eachother, preferably at about a 90 angle, since there are few .ofishore locations where prevailing wind'and wave forces act always from the same direct-ion.

The exact size of the plates18 depends on many factors, such as, for example, .(I) the number of theplates employed,l (2) the weight of the entire .ofisho're platfor-m structure to be stabilized (3) the area of the substructure and platform exposed'to the wind and wave forces, and (4) the viscosity or solids content of the mud in which the installation is positioned; Thus, the

'thinnerthe mud the greaterrthe tendency of the substructure to be moved by winds and Waves thus necessitating the use of larger area plates. In a like manner,

the greater the area of the sub-structure 11 that is exposed to the wind and wave elements, the greater the V tendency for it m be tilted by the wind and wavesand therefore the greater the need for larger plates to prevent thisaction. 1 a

Another form of the offshore structure is shown in Figure 2, wherein the template 21 comprisese a plurality of vertical columns 22 and structural cross-bracing members 23 similar to those employed in the structure of Figure 1.- In addition, it is provided with a plurality of tubular columns 24 sloping outwardly at an angle away from the center of the template 21 so as to give a broader base to the entire offshore structure. 'As illustrated in Figure 2, batter piles 25 are driven through the inclined tubular columns 24 down into the muddy ocean floor. If desired, piles may also be driven through the vertical columns 22. .After. the piles are driven in, a platform 27 may be positioned on top of the sub-structure 21. The stabilizingplates 28 may be either fixedly secured to the vertical columns 22 vor As an example, it may be stated that inone offshore well location where piles were sunk about 225 'feetinto the mud, a .15 foot'square template 11 was employed wuic h was aboutoo feet .high and had'four vertical plates mounted in a box-like arrangement as shown in Figure 3, the plates being 30 feet high. On the other hand, at another well location, a template 11 having 'four vertical piles and eight inclined or batter'piles whichc template was. 16 feet square and about 60 feet high only'need ed plates 25 ft. high becauseof the added resistanceprovided'by the eight batter piles.

In positioning a-sub-structure' or template 11 (Figures l and 3) at a well site at an offshore location, the

casing 1-4. 'Pressure is exerted in any suitable manner,

as :by placing weights on'the'top of the vertical columns 12, to force the lower portion of :the'template and the. stabilizing, plates 13 fixedly secured.thereto into the-mud 7 s0 'that the plates are entirely below the mud line,

7 template 11 is carried to the location on the barge where 7 'it is hoisted in the air andlowered down aroundthe well surface of the vertical columns 34 'sothat a plate 38 could be dropped down in the tracks along the length of the sub-structure and then forced into the muddy ocean floor. The closed-bottoms of the'tracks 32 and 33 prevents the plate 38 from dropping further.

Instead of fixedly securing the plates 28 to the vertical or inclined columns '22 and 28 respectively. ,(Figure 2), the plates 28 can be'arrangedin a skirt-like Structure 4 i u wh h an. be dropp down over the'outside .of the'vertical columns 22 and 24 prior to positioning theplatform '27 on the substructure 21. Due to the inclined position of the columns 24, the skirtshaped stabilizing means would be prevented from slipping ofi the bottom of the substructure. When this same type of a skirt-shaped stabilizing means is used on a sub-structure having only vertical legs, as shown in Figure 1, it is necessary to provide the necessary stop means at the bottom of the vertical columns 12 .to prevent the stabilizing plate'assernbly from slipping oh? the bottom of the structure. ,7

When a template in accordance with the present invention is positioned on the top of a muddy ocean bottom, there is normally suflicient body to the mud to equally as well in stabilizing offshore platforms that are set on very long flexible piles that have a tendency to sway even though the bottoms thereof areanchored in the ocean floor.

We claim as our invention:

1. The method of constructing a permanent offshore foundation in an oifshore location where the 'oceanfloor underlying the water is soft and unconsolidated, said method comprising assembling at least three vertical .tubular columns, connecting said columns in spaced relationship at'points'intermediate the ends thereof-by crossbracing to form a rigid'structure, positioning plate means on said structure near one end thereof in a plane substantially parallel to the axis of said columns and fixedly secured against movement normal thereto, lowering the structure into the water with the columns positioned vertically and the plate means at the bottom portion of said structure, forcing said structure into the 'rnuddy ,ocean bottom until thelplate means penetrate the mud line, inserting piles through said columns, forcing said piles into the muddy, oceanbottomto a depth sufficient to support said foundation, vand subsequently connecting saidfstructureflin supporting engagement with said piles.

2. The method of constructing a permanent offshore foundationin an offshore location where the ocean floor underlying the water is soft and unconsolidated, said method comprising assemblying at'least three vertical b l olumn n ct ng aid. c lum i paced lationship at points intermediate the ends. thereof by shown inFigure l, whereilthey are not exposed to wind and wave forces; lEilesifi are then driventhrough"-the1 hollowvertical columns- 12 deep into the-mudand the template and piles are;fiXedly secured together in -any. suitable manner well known -to the art, as by welding i (not shown) Afplatform l7- may be attached after v thetops of the piles' have been cutoff.

7 While the plates l .(Ei-guregl) and plates 2:8 (Figure 2)- are-preferably fixedly attached to the vertical col-E umps pf the sub-structur an therL- b-st neture 31 (Figures 4 and 5) may be provided with guides or'traclgs 32 and 33 fixedly secured, .as by welding, to the outer cross bracing to form. a rigid set-ructur e, lowering the structure into the water with the columns positioned vertically, forcing the lower portionof .said structure into the muddy ocean bottom, inserting piles throughsaid columns, forcing said. piles into thelmuddy ocean'bottom V to a depth sufiicient .to support said foundation, connecting said columns in support engagement .with said piles, positioning platemeans between atleast two of said :columns insubstantially vertically movable engagenaent ,therewith and fixedly secured ;-against movement rma theretaa dorflnamidplat me nsdownwar 1y to a point penetrating the mud line.

3. The method of constructing a permanent offshore foundation in an offshore location where the ocean floor underlying the water is soft and unconsolidated, said method comprising assembling at least three vertical tubular columns, connecting said columns in spaced relationship at points intermediate the ends thereof by cross bracing to form a rigid structure, lowering the structure into the water with the columns positioned vertically, forcing said structure into the muddy ocean bottom until the lower portion thereof is below the mud line, inserting piles through said columns, and forcing said piles into the muddy ocean bottom to a depth sufficient to support said foundation, connecting said columns in supporting engagement with said piles, surrounding said columns with substantially vertically-positioned plate means, and forcing said plate means downwardly to a point penetrating the mud line.

4. A permanent marine foundation having a high degree of lateral stability for use in offshore locations where the ocean floor underlying the water is soft and unconsolidated, said foundation comprising a substructure of at least three vertical tubular columns, said columns being disposed in horizontal spaced relationship to define a geometrical figure with a closed perimeter, cross-bracing means rigidly interconnecting adjacent columns, piles positioned within said columns and extending into the soft and unconsolidated ocean floor to a depth sufficient to support said foundation, said piles extending into the ocean floor below said substructure at least a length equal to the height of said substructure, means interconnecting said piles to said substructure, and substantially vertical plate means of substantial area secured to said substructure in the upper portion of said foundation, said plate means being afiixed'to said substructure so that said plate means extend continuously along at least one side of said substructure and are submerged in the muddy ocean bottom, whereby the substantial area of the plate means in the mud serves to anchor the foundation against lateral movement and absorb a substantial portion of the lateral forces caused by wind and wave forces.

5. A permanent marine foundation having a high degree of lateral stability for use in ofishore locations where the ocean floor underlying the water is soft and unconsolidated, said foundation comprising a substructure of at least four batter tubular columns, said columns being disposed in horizontal spaced relationship to define a geometrical figure with a closed perimeter, cross-bracing means rigidly interconnecting adjacent columns, piles positioned within said columns and extending into the soft and unconsolidated ocean floor to a depth sufiicient to support said foundation, said piles extending into the ocean floor below said substructure at least a length equal to the height of said substructure, means connecting said piles to said substructure, and plate means of substantial area secured to said substructure in the upper portion of said foundation, said plate means being aflixed to said substructure so that they extend continuously along at least two sides of said substructure and are submerged in the muddy ocean bottom, whereby the substantial area of the plate means in the mud serves to anchor the foundation against lateral movement and absorb a substantial portion of the lateral forces caused by wind and wave forces.

6. A permanent marine foundation having a high degree of lateral stability for use in oifshort locations where the ocean floor underlying the water is soft and unconsolidated, said foundation comprising a substructure of at least four vertical tubular columns, said columns being disposed in horizontal spaced relationship to define a geometrical figure with a closed perimeter, cross-bracing means rigidly interconnecting adjacent columns, piles positioned within said columns and extending into the soft and unconsolidated ocean floor to a depth sufiicient to support said foundation, said piles extending into the ocean floor below said substructure at least a length equal to the height of said substructure, means connecting said piles to said substructure, and substantially vertical plate means of substantial area secured to and surrounding said substructure in the upper portion of said foundation, in

the form of a skirt, said plate means being aflixed to saidsubstructure so that they are submerged in the muddy ocean bottom, whereby the substantial area of the plate means in the mud serves to anchor the foundation against the lateral movement and absorb a substantial portion of the lateral forces caused by wind and wave forces.

References Cited in the file of this patent UNITED STATES PATENTS 904,156 Slick Nov. 17, 1908 2,429,952 Willey Oct. 28, 1947 2,598,088 Wilson May 27, 1952 2,637,172 Howard May 5, 1953 2,637,978 Evans et al May 12, 1953 FOREIGN PATENTS 40,413 Norway Dec. 1, 1924 551,449 Great Britain Feb. 23, 1943 69,135 Denmark Dec. 15, 1951 730,835 Great Britain June 1, 1955

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