GB2135262A

GB2135262A - A semi-submersible drilling platform

Info

Publication number: GB2135262A
Application number: GB08404167A
Authority: GB
Inventors: Hans Langenberg
Original assignee: Blohm and Voss GmbH
Current assignee: Blohm and Voss GmbH
Priority date: 1983-02-22
Filing date: 1984-02-17
Publication date: 1984-08-30
Also published as: DK78484A; DK161627C; GB2135262B; SE457524B; DK161627B; KR940009261B1; SE8400951L; NO163275B; NO163275C; KR840007692A; DE3306057A1; JPS59160694A; DK78484D0; NO840620L; JPH02136797U; SE8400951D0; GB8404167D0

Abstract

A semi-submersible platform comprises two or more elongate floats 1, at least one working and/or accommodation deck 3, and at least three columns 2 extending between said deck and said floats. The semi-submersible platform is provided with means for reducing and/or limiting the inclined floating position comprising buoyancy volumes 4 provided on the floats outside of the columns 2 in particular at the ends of the floats 1, with the buoyancy volumes each having a centre of volume which lies above the centre of volume of the associated float. In the normal semi-submersed position of the semi-submersible platform the buoyancy volumes lie substantially below the waterline whereas, in an abnormal inclined floating position, at least one of the buoyancy chambers lies at least substantially above the waterline thus resulting in a displacement of the net centre of buoyancy of the semi-submersible platform relative to the centre of buoyancy in the normal semi-submersed position. <IMAGE>

Description

SPECIFICATION A semi-submersible platform with means for reduc ing and/or limiting an inclined floating position thereof The present invention relates to a semi-submersible platform having means for reducing and/or limiting an inclined floating position thereof.

Recent studies concerning the cases of damage to semi-submersible drilling platforms have caused some authorities to reconsider and to amend their safety regulations for such semi-submersible plat forms. Such regulations include, by way of example, the limitation of 15 of the angle of inclination which may result from leakage due to external or internal damage.

There are a number of known ways of achieving this limitation.

One possibility is to choose an appropriate narrow sub-division of the buoyancy generating bodies by watertight bulkheads and decks. Another possibility is to increase the stability of the semi-submersible platform in the normal intact state. It is also possible to consider suitable additional ballast arrangements and to fill watertight compartments with foam material (foam filling) which prevents water entering these compartments. It is also known to improve existing semi-submersible platforms by the retros pective attachment of additional columns.

The above named technical measures are howev er all disadvantageous in some way or another for the economical operation of a semi-submersible platform. By way of example increasing the number of water-tight bulkheads and decks substantially increases the cost of the semi-submersible platform.

Increasing the stability in the intact state deleterious ly affects the movement behaviour of the platform in the semi-submersed floating position. The attachment of additional columns or ballast and other measures which substantially increase the weight of the semi-submersible platform generally result in a reduction of the transit freeboard.

The principal object underlying the invention is to provide means for reducing and/or limiting an inclined floating position of a semi-submersible platform which avoids the above named disadvan tages and in particular does not lead to a consider able increase in weight or to a deterioration of the movement behaviour of the platform in the semi submersed state.In order to satisfy this object there is provided, in accordance with the invention, a semi-submersible platform comprising two or more elongate floats, at least one working and/or accom modation deck disposed above the floats, at least three columns extending between said deck and said floats and means for reducing and/or limiting an inclined floating position of said semi-submersible platform, characterised in that said means compris es buoyancy volumes provided on said floats out side of said columns, in particular at the ends of said floats, said buoyancy volumes each having a centre of volume which lies above the centre of volume of the associated float; and in that, in the normal semi-submersed position of the semi-submersible platform said buoyancy volumes lie substantially completely below the waterline whereas, in an abnormal inclined floating position, at least one of said buoyancy volumes lies at least substantially above the waterline, thus resulting in a displacement of the net centre of buoyancy of the semisubmersible platform relative to the centre of buoyancy in said normal semi-submersed position, which reduces and/or limits the angle of inclination of the semi-submersible platform.

Accordingly the present invention resides in the arrangement of water-tight buoyancy volumes on or at the float ends (typically 4 in number) with the centre of volume of the buoyancy volumes preferably lying significantly above the centre of gravity of the floats, which are normally of rectangular or cylindrical shape. If the semi-submersible platform should move towards an inclined or heeled over position then one or more of the buoyancy volumes will move out of the water which will lead to a shift of the centre of buoyancy which tends to reduce and/or limit the angle of heel.

For the sake of completeness it is pointed out that United States patent No.3874314 shows a semisubmersible platform of the type set forth in the precharacterising part of the claim. In the arrangement of US-PS 3 874314 the semi-submersible platform consists of four floats, a working platform and columns extending between the floats and the working platform. The semi-submersible platform does not however have the auxiliary means for reducing and/or limiting an inclined floating position thereof as envisaged by the present invention.

The Journal "Schiff und Hafen" vol. 2, 1981 discloses semi-submersible platforms for use in icy waters, the floats of which are raised at the front and have nose-like added structures. These floats are however intended to be thrust onto the ice in order to break it in a manner similar to the operation of an ice breaker. The journals "Ship & Boat International", July/August 1977, pages 79 and 85, and "New Ships/Die Neubauten" vol. 2, pages 42,43 and 46 disclose a semi-submersible platform in which the columns shown at the ends of the floats project in the semi-submersed symmetrical state substantially out of the water and thus serve to ensure adequate stability in the upright floating position (i.e. in the intact state).

US-Patent No. 3771 481 and Continuation-in-part application 27478 disclose a semi-submersible platform having lateral half columns which are intended to produce sufficient stability in the intact state at small to medium depths, i.e. until the state at which the centre of buoyancy wanders over the centre of gravity. The arrangement disclosed is not effective to reduce and/or limit the inclined floating position of the semi-submersible platform because the half columns have not surfaced at an inclination of 15". The prevailing buoyancy conditions in this prior art arrangement are illustrated in Figure 1 of the accompanying drawings.

In one preferred form of the invention the buoyancy volumes comprise buoyancy chambers mounted on said floats, and indeed preferably at the ends of said floats. Using buoyancy chambers of this kind existing semi-submersible platforms can be retrospectively modified to incorporate the inclination limiting means of the present invention.

In an alternative arrangement the buoyancy volumes are defined by integral wall means of said floats. This arrangement may be convenient when constructing a semi-submersible platform to include the inclination limiting means of the invention from the very outset.

The buoyancy chambers are in either case case preferably filled with foam which ensures that the buoyancy of these chambers is not reduced even if they should develop a leak; the presence of the foam prevents the entry of water into the buoyancy chambers.

An alternative definition of the invention is set forth in claim 5.

In a practical arrangement it is convenient if the centre of volume of each end of each float lies above the centre of volume of the associated float by an amount equal to at least 30 % of the depth of the centre of volume of the whole float below the water line.

The invention will now be described in more detail in the following, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows the buoyancy conditions prevailing in the prior art arrangement of US patent 3771 481, Figure 2 shows the manner in which the inclination limiting means of the invention affects the stability curve of a semi-submersible platform, and Figures 3a and 3b show an embodiment of a platform equipped with the inclination limiting means of the invention, Figure 3a being a section on the line illa-llla of Figure 3b and Figure 3b being a section on the line Illb-lllb of Figure 3a.

Referring firstly to Figure 3 there can be seen a semi-submersible platform of the kind in which a working platform 3 which supports at least one working and/or accommodation deck is supported on two floats 1 via three columns 2, with two of said columns extending between the working platform 3 and one of the floats 1 and the third one of the columns 2 extending between the working platform and the other one of the floats. In the usual way cross braces are provided between the floats so as to stabilise the structure against mechanical loads.

As can be seen in particular from the section I-I of Figure 3 the two floats 1 are of generally rectangular cross-section with upwardly sloped hull surfaces at their two ends 1'.

Buoyancy chambers 4 are attached to the extreme outer ends 1' of the floats 1 outside of the area defined within the columns.

As the buoyancy chambers 4 are mounted on the upper surfaces of the floats 1 the centres of volume of the end portions 1' of each float 1 will lie at the points designated S1'+4, whereas the centre of volume of the whole float will lie at the point designated Sol'+4 which is a distance h below the points So'+4 As can be seen from Figure 3b the buoyancy chambers 4 are broadened at their top surfaces so that the centre of volume of each of the buoyancy chambers lies closer to the top surface of the buoyancy chambers than to the top surfaces of the associated floats 1.

The semi-submersible platform is normally towed out to sea and then partially submersed by partly flooding compartments provided in the floats 1 or in the columns 2. In the normal semi-submersed position in which the working platform 3 is desirably horizontal the waterline 5 can be imagined as lying on the broken line 5. a leak should occur, for example into a watertight compartment 7 in the rear column 2 of the semi-submersible platform, the semi-submersible platform will tend to heel over so thatthewaterline can be imagined along the line 6a.

In this position the forward buoyancy chamber 4 is located above the surface of the water and therefore no longer contributes to the lift exerted on the platform by the buoyancy chambers. As a result of this the net centre of buoyancy will move away from the central vertical axis of the platform and will reduce the angle of heel or at least be able to limit it to the 15 angle shown in the drawing. The size of the reduction depends of course on the precise layout of the buoyancy chambers and the heeling moment caused by the leak.

If the leak should occur in the front column 2 then the waterline will tend to follow the line 6b in which case the rear buoyancy chamber 4 will surface and the buoyancy provided by the buoyancy chamber(s) at the front of the semi-submersible platform will tend to compensate the heeling moment due to the leak.

If byway of example, the leak should occur in the central column 2 then the two buoyancy chambers on the float to which the remaining two columns 2 are connected will tend to surface so that they no longer contribute to the buoyancy of the platform, whereupon the buoyancy of the two submersed buoyancy chambers 4 of the float 1 to which the central column 2 is attached will generate a moment tending to counteract the heeling moment due to the leak in the central column 2.

The effects of the buoyancy chambers can be seen graphically from Figure 2. This curve clearly shows the shift in the variation in the uprighting moment with attachments at the float ends relative to the uprighting moment without such attachment as a function of the angle of heel. It will be seen that the change in the angle of heel due to the attachments is several degrees and this can be critical in ensuring the safety of the platform.

Claims

1. A semi-submersible platform comprising two or more elongate floats, at least one working and/or accommodation deck disposed above the floats, at least three columns extending between said deck and said floats, and means for reducing and/or limiting an inclined floating position of said semisubmersible platform, characterised in that said means comprises buoyancy volumes provided on said floats outside of said columns, in particular at the ends of said floats, said buoyancy volumes each having a centre of volume which lies above the centre of volume of the associated float; and in that, in the normal semi-submersed position of the semisubmersible platform, said buoyancy volumes lie substantially completely below the waterline whereas, in an abnormal inclined floating position, at least one of said buoyancy volumes lies at least substantially above the waterline, thus resulting in a displacement of the net centre of buoyancy of the semi-submersible platform relative to the centre of buoyancy in said normal semi-submersed position, which reduces and/or limits the angle of inclination of the semi-submersible platform.

2. A semi-submersibie platform in accordance with claim 1, characterised in that said buoyancy volumes comprise buoyancy chambers mounted on said floats.

3. A semi-submersible platform in accordance with claim 1, characterised in that said buoyancy volumes are defined by integral wall means of said floats.

4. A semi-submersible platform in accordance with either of the preceding claims 2 and 3 wherein said buoyancy chambers are filled with foam.

5. A semi-submersible platform comprising two or more elongate floats, at least one working and/or accommodation deck disposed above the floats, at least three columns extending between said deck and said floats, and means for reducing and/or limiting an inclined floating position of said semisubmersible platform, characterised in that said means comprises the end portions of the floats being shaped so that the centre of volume of each end of a float lies above the centre of volume of the whole float, with the shaped end portions of the floats being completely below the waterline in the normal semi-submersed position of the semisubmersible platform.

6. A semi-submersible platform in accordance with claim 5 wherein the centre of volume of each end of each float lies above the centre of volume of the associated float by an amount equal to at least 30 % of the depth of the centre of volume of the whole float below the waterline.

7. A semi-submersible platform substantially as herein described and as illustrated with reference to the accompanying drawings.