GB2170451A

GB2170451A - Semi-submersible offshore structure

Info

Publication number: GB2170451A
Application number: GB08524358A
Authority: GB
Inventors: Masaru Mokunaka; Yoshihoro Kudomi; Chiaki Kaishida; Nobuo Murakami; Junichi Tsujitani
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 1985-02-06
Filing date: 1985-10-02
Publication date: 1986-08-06
Also published as: SE8504528D0; NO854340L; GB8524358D0; FI853794A0; NL8502894A; FR2576869A1; SE8504528L; JPS61131396U; FI853794L

Abstract

A semi-submergible offshore structure includes a pair of opposed submergible lower hulls (10), an above-water upper hull (11) and a plurality of columns connecting the lower hulls to the upper hull. Four stabilizing main columns (12) are each arranged at the fore and aft portions of the lower hulls (10) respectively, and two auxiliary columns (13) having a smaller cross sectional area than the main columns (12) are each arranged at an intermediate portion between the fore and aft main columns (12) on each lower hull (10). Each transversely corresponding pair of main columns (12) are interconnected at their lower portions by a submergible horizontal brace (14). <IMAGE>

Description

SPECIFICATION Semi-submergible offshore structure Background on the invention The present invention relates to a semisubmergible offshore structure such as a semisubmergible oil drilling rig.

Various semi-submergible structures have heretofore been developed as offshore structures having satisfactory motion characteristics. Such structures are generally of the twin lower hull type and comprise a pair of submergible lower hulls, an above-water upper hull and a plurality of columns interconnecting these hulls. Conventional offshore structures of this type usually have six to eight stabilizing columns (for example, see U.S. Patent No.

3,490,406). However, many of these stabilizing columns have a large cross section, are generally provided with a large number of braces, therefore encounter great resistance of water and further require much work to build and large amounts of work for fatigue destructive inspection of strength members before navigation. Further if it is possible to position a barge or the like immediately below the upper hull, for example, in the case where the structure is an oil drilling rig, work can be done with ease as when hoisting templates, whereas the conventional semi-submergible offshore structure described, which has many braces, is unable to admit the barge into the space immediately under the upper hull. To eliminate these problems, there is recently a tendency to considerthat it is desirable to minimize the number of columns as well as of braces.In fact, attention has been focused on four-column structures wherein four stabilizing columns are arranged at the fore and aft portions of the lower hulls, one column at each portion (see, for example, U.S. Patent No. 4,436,050). In this case, however, the lower hulls are connected to the upper hull by the four columns only, so that a sufficient space is not available for electric wiring, piping, air channel, passage, elevator, etc., which must therefore be arranged with great limitations. Further when a large-sized semi-submergible offshore structure of the four column type is to be built, the distance between the fore and aft columns on each lower hull becoms too large to support the upper hull free of trouble. Moreover, since it is impossible to space the fore and aft columns apart by a large distance, the structure has the problem of being prone to rolling.

Summary of the invention The main object of the present invention is to overcome all the foregoing problems and to provide a semi-submergible offshore structure having both the advantage of the conventional four-column structure and the advantage of the prior-art structure having at least six stabilizing columns.

The present invention provides a semisubmergible offshore structure which includes a pair of opposed submergible lower hulls, an above-water upper hull and a plurality of columns connecting the lower hulls to the upper hull, the structure being characterized in that the structure comprises four stabilizing main columns each arranged at the fore and aft portions of the lower hulls respectively, two auxiliary columns having a smaller cross sectional area than the main column and each arranged at an intermediate portion between the fore and aft main columns on each lower hull, and a submergible horizontal brace interconnecting each transversely corresponding pair of main columns at their lower portions.

The semi-submergible offshore structure of the present invention is comparable to the conventional four-column structure becasue it has the advantage of being smaller in the resistance of water to be encountered, being reduced in the amount of work required for construction as well as for the fatigue destructive inspection of strength members before navigation and permitting admission of barges into the space immediately below the upper hull. Nevertheless, the present structure also has the same advantage as the known semi-submergible offshore structure provided with at least six stabilizing columns in that the upper hull can be easily fabricated as supported on the columns and that the structure readily affords sufficient spaces for electric wiring, piping, air channels, passages, elevators, etc. and has an increased distance between the fore and aft main columns to mitigate rolling.Thus, the present invention provides an unprecedented excellent semi-submergible offshore structure which has both the advantage of the conventional four-column structure and the advantage of the known structure provided with at least six stabilizing columns.

Brief description of the drawings Figure lisa view in horizontal section showing a semi-submergible offshore structure embodying the invention; Figure 2 is a front view partly broken away and showing the same; and Figure 3 is a side elevation of the same.

Description of the preferred embodiment The present invention will be described below in greater detail with reference to the accompanying drawings.

The semi-submergible offshore structure shown comprises a pair of opposed lower hulls 10 submergible under the surface of the sea L, an upper hull (plafform) 11 positionable above the surface L, and four stabilizing main columns 12 and two auxiliary columns 13 connecting the lower hulls 10 to the upper hull 11.

Each lower hull 10 extends horizontally in front-torear direction and has ballast tanks, etc. as suitably arranged in its interior although not shown.

The upper hull 11 is in the form of horizontal box having a small height. Although not shown, living quarters and various units of work equipment are provided in the interior of the hull 11 and on the deck portion thereof.

The four main columns 12 are hollow cylindrical and are each arranged at the fore and aft portions of the lower hulls 10 respectively. The two fore main columns 12 are interconnected at their lower por tions by a horizontal brace 14 which is submergible under the sea surface L. Similarly, the two aft main columns 12 are interconnected by another brace 14.

The two auxiliary columns 13 are hollow cylindrical and are each disposed at an intermediate portion between the two fore and aft main columns 12 on each lower hull 10. The auxiliary column 13 is smaller than the main column 12 in cross sectional area and has a diameter which is, for example, one half of that of the main column 12. Although not shown, the main column 12 has ballast tanks, etc. as arranged suitably in its interior, while the interior space of the auxiliary column 13 is utilized for providing electric wiring, piping, air channel, passage, elevator, etc.

The semi-submergible offshore structure described above has, for example, the following dimensions. The main column 12 has a diameter of about 14 m, the auxiliary column 13 has a diameter of about 7 m, the center-to-center distance between the fore and aft main columns 12 is about 65 m, the center-to-center distance between the transversely opposed main columns 12 is about 60 m, the lower hull 10 has an overall length of about 97 m, and the structure including the pair of lower hulls 10 has an overall width of about 78 m. These dimensions are variable as required.

For navigation to a work site, the lower hulls 10 are made to float on the surface L by adjusting the ballast. The structure is then broughtto the work site using propelling means (notshown) attached to the lower hulls 10 forturning the structure toward the desired direction. Because the braces 14 are completely above the water surface L during navigation, the propulsion resistance is low. The propelling means may be dispensed with; the structure will then be towed by a boat.

The structure is moored at the work site to conduct the desired work, with the lower hulls 10 and the braces 14 submerged to a predetermined depth below the surface Las seen in Figures 2 and 3. The resistance of water offered at this time is low, because the lower hulls 10 are connected to the upper hull 11 only by the four main columns 12 and the two auxiliary columns 13 which are smaller in cross section and further bcause the braces 14 transversely interconnecting the main columns 12 are submerged under the surface L. With no brace present in the vicinity ofthe water surface Lthereabove ortherebelow, a barge or the like can be admitted into the space between the opposed rows of columns 12, 13 immediately below the upper hull from the front or rear for performing various operations.

With the semi-submergible offshore structure described above, the lower hulls 10 are connected to the upper hull 11 only by the four main columns 12 and two auxiliary columns 13, with each transvrsely opposed pair of main columns 12 interconnected by one brace 14. This reduces the amount of work required for building the structure as well as for the fatigue destructive inspection of strength members before navigation. Further because the auxiliary column 13 is disposed between the fore and aft main columns 12 on each lower hull 10, the upper hull 11 can be installed with greater ease. Moreover, the interior space of the auxiliary column can be utilized for providing electric wiring, piping, air channel, passage, elevator, etc., so that the arrangement within the main column 12 involves no limitation but can be realized efficiently and easily. The presence of the auxiliary column 13 serves to increase the distance between the fore and after main columns 12 to lessen rolling. Since semi-submergible offshore structures are moored usually as oriented toward the direction of main waves, reduced rolling assures efficient advantageous operation.

The construction of the structure of the invention is not limited to the foregoing embodiment but can be varied suitably.

Claims

1. Asemi-submergible offshore structure including a pair of opposed submergible lower hulls, an upper hull to be positioned above water and a plurality of columns connecting the lower hulls to the upper hull, the structure comprising four stabilizing main columns arranged at the fore and aft portions of the lower hulls respectively, two auxiliary columns having a smaller cross sectional area than the main columns and each arranged atan intermediate portion between the fore and aft main columns on a respective one ofthe lower hulls, and a submergible horizontal brace interconnecting each transversely corresponding pair of main columns at their lower portions.

2. A structure according to claim 1, wherein each of the main columns and the auxiliary columns are hollow cylindrical, and each of the auxiliary columns has a diameter substantially one half of the diameter of the main columns.

3. A semi-submergible offshore structure substantially as hereinbefore described wtih reference to the accompanying drawings.