GB2184402A

GB2184402A - Semi-submersible offshore structure

Info

Publication number: GB2184402A
Application number: GB08627481A
Authority: GB
Inventors: Hiroshi Isshiki
Original assignee: Sumitomo Heavy Industries Ltd
Current assignee: Sumitomo Heavy Industries Ltd
Priority date: 1985-11-18
Filing date: 1986-11-18
Publication date: 1987-06-24
Also published as: GB8627481D0; KR870004876A; JPS62116390A; GB2184402B

Abstract

A semi-submersible offshore structure such as an oil ring has a pair of submerged floats 1, 2 arranged side-by-side at the port and starboard sides, a plurality of supporting columns 3A, 3B extending upright from respective submerged floats 1, 2, the submerged portion of each supporting column 3A, 3B producing a certain level of buoyancy, and an upper structure 5 carried by the supporting columns 3A, 3B. The semi-submersible offshore structure further has a plurality of columnar floats 7 disposed between the supporting columns 3A, 3B on both submerged floats 1, 2 at the fore side of the upper structure 5 and between the supporting columns 3A, 3B on both submerged floats 1, 2 at the aft side of the upper structure 5. The columnar floats 7 are suspended from substantially breadthwise mid portions of the upper structure 5 at the fore and aft sides of the upper structure 5. The lower ends of the columnar floats 7 are positioned at a level which is below the water level but above the level of the lower ends of the submerged floats 1, 2. <IMAGE>

Description

SPECIFICATION Semi-submersible offshore structure BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a semi-submersible offshore structure such as an oil ring used in offshore oil-well drilling.

Description of Prior Art: In general, offshore structures of semi-submersible type exhibit good matron characteristics against waves and many of such structures have been built and put into operation.

Figs. 7 and 8 show typical examples of known semi-submersible offshore structures.

These known semi-submersible offshore structures have a pair of submerged floats 1, 2 which support upper structure 5 through a plurality of columns 3, 4 which also have buoyancy. In order to obtain high strength and rigidity, it is a common measure to connect the columns through bracings 6.

Fig. 9 shows another known semi-submersible offshore structure in which, in order to reduce the heave motion of the structure due to wave, a vertical columnar float 7 is provided on the center of the structure. This central float 7, however, does not contribute to improvement in the stability because it is located at the center of the structure and does not produce any force which would act to keep the structure upright.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a semi-submersible offshore structure which is improved in such a way as to reduce the heave motion due to wave and to improve the stability.

To this end, according to the invention, there is provided a semi-submersible offshore structure comprising: a pair of submerged floats arranged side-by-side; a plurality of supporting columns extending upright from respective submerged floats, the submerged portion of each supporting column producing a certain level of buoyancy; an upper structure carried by the supporting columns; and a plurality of columnar floats disposed between the supporting columns on both submerged floats at the fore side of the upper structure and between the supporting columns on both submerged floats at the aft side of the upper structure, the columnar floats being suspended from substantially breadthwise mid portions of the upper structure at the fore and aft sides of the upper structure, the lower ends of the columnar floats being positioned at a level which is below the water level but above the level of the lower ends of the submerged floats.

These and other objects, features and advantages of the invention will become clear from the following description of the preferred embodiment when the same is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of an embodiment of a semi-submersible offshore structure in accordance with the present invention; Figure 2 is a cross-sectional view of the embodiment shown in Fig. 1; Figure 3 is a plan view of the embodiment shown in Fig. 1; Figure 4 is a side elevational view of the embodiment shown in Fig. 1; Figure 5 is a front elevational view of the embodiment shown in Fig. 1; Figure 6 is a graph showing the relationship between frequency of wave and Froude-Krylov force diffraction force; Figures 7A, 7B, 7C and 7D are a perspective view, a side elevational view, a front elevational view, and a cross-sectional view of an example of known semi-submersible offshore structures;; Figures 8A, 8B, 8C and 8D are a perspective view, a side elevational view, a front elevational view, and a cross-sectional view of another example of known semi-submersible offshore structures; and Figures 9A, 9B and 9C are a perspective view, a side elevational view, and a crosssectional view of still another example of known semi-submersible offshore structure.

DESCRIPTION OF THE PREFERRED EMBODI MENT A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings.

Referring to Figs. 1 to 5, a semi-submersible offshore structure of the present invention has a pair of submerged floats 1 and 2, one on the starboard side and one on the port side. A plurality of supporting columns 3A and 3B, each having buoyancy, are provided on the floats 1 and 2 so as to stand upright therefrom. Usually, the number of the supporting columns range between 4 and 8. In the illustrated embodiment, four supporting columns are used: two on the float 1 and two on the float 2. The offshore structure further has columnar floats 7 and 7 which are disposed between supporting columns 3A and 3A on the fore and between the supporting columns 3B and 3B on the aft of the structure and suspended from substantially breadthwise mid portions of the fore and aft of the upper structure 5.In addition, a suitable number of bracings 6 are employed for the purpose of providing high strength and rigidity.

As will be best seen from Fig. 5, the lower surfaces of the columnar floats 7 is at a level I which is below the water level but is above the level il of the bottom of the submerged floats 1 and 2.

The semi-submersible offshore structure is moved up and down by the force of waves.

Theoretically, the wave force is decomposed into Froude-Krylov force component and diffraction force component.

The Froude-Krylov force component is the force which is obtained by integrating, over the surface of an imaginary hull, the pressures which would be obtained if there were no structure. On the other hand, the diffraction force component is the force obtained by integrating, over the surface of the hull, the pressure caused by diffraction of wave due to the presence of the structure. Fig. 6 shows how the Froude-Krylov force component indicated as upward force and the diffraction force component indicated as downward force are changed in relation to the frequency of the wave.It will be seen that, if the Froude-Krylov force is increased to a level which is high enough to negate the diffraction force, the wave force as the composite force of the Froude-Krylov force component and the diffraction force component is nullified or reduced, thus reducing the amount of heave motion of the offshore structure due to waves.

It is known that, in high-frequency region of waves, i.e., when the wavelengths of waves are short, a high level of Froude-Krylov force is produced at small depth but the Froude Krylov force is drastically decreased as the depth becomes greater.

In the described embodiment, the lower surfaces of the columnar floats 7 are positioned at a small depth, i.e., at the level I, which is above the level II of the bottoms of the submerged floats 1 and 2, so that a large Froude-Krylov force component is applied to the columnar floats 7 and 7 particuiarly when the frequencies of waves are high. This in turn increases the Froude-Krylov force component which acts on the whoie structure.

It is, therefore, possible to create such a condition that the Froude-Krylov force component acting on the whole structure and the diffraction force component on the same are negated or substantially negated by each other, thus nullifying or reduing the wave force and, hence, reducing the amount of the heave motion of the offshore structure, over a considerably wide range of frequency or wavelength of waves. The buoyancy of the columnar floats 7 and 7 which are disposed on the fore and aft end of the offshore structure produce moments which act about the center of the offshore structure, so as to produce righting force when the offshore structure is inclined, thereby enhacing the stability of the structure.

As has been described, according to the present invention, the vertical wave force acting on the semi-submersible offshore structure is reduced to reduce the amount of heave motion of the structure, by a comparatively sim ple construction which includes columnar floats provided on the fore and aft ends of the structure. The columnar floats also serve to enhance the stability of the semi-submersible offshore structure to increase the tendency to keep the structure upright. The reduced heave motion and the improved stability in combination provide improved conditions for various works which are to be conducted on the semi-submersible offshore structrure of the kind described.

Although the invention has been described through its specific form, it is to be understood that the described embodiment is only illustrative and various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claims.

Claims

1. A semi-submersible offshore structure comprising: a pair of submerged floats arranged side-byside; a plurality of supporting columns extending upright from respective submerged floats, the submerged portion of each supporting column producing a certain level of buoyancy; an upper structure carried by said supporting columns; and a plurality of columnar floats disposed between said supporting columns on both submerged floats at the fore side of said upper structure and between said supporting columns on both submerged floats at the aft side of said upper structure, said columnar floats being suspended from substantially breadthwise mid portions of said upper structure at the fore and aft sides of said upper structure, the lower ends of said columnar floats being positioned at a level which is below the water level but above the level of the lower ends of said submerged floats.

A semi-submersible offshore structure substantially as hereinbefore described with reference to and as shown in Figs. 1 to 6 of the accompanying drawings.