GB2142596A

GB2142596A - Stabilizing semi-submersible structures

Info

Publication number: GB2142596A
Application number: GB08415165A
Authority: GB
Inventors: Hans Langenberg
Original assignee: Blohm and Voss GmbH
Current assignee: Blohm and Voss GmbH
Priority date: 1983-06-15
Filing date: 1984-06-14
Publication date: 1985-01-23
Also published as: NO165333C; SE8403141L; FI842430A7; NL8401868A; DE3321618A1; FI842430A0; DK161132C; NO842356L; DK161132B; FI79990B; FI79990C; DK292084D0; FR2548130B1; FR2548130A1; SE8403141D0; NO165333B; GB2142596B; GB8415165D0; DK292084A; SE457872B

Abstract

A semi-submersible structure for use in open water comprises a working platform 7; a plurality of columns 5 on which the working platform is mounted and buoyancy generating means for supporting the working platform above the surface of the water. The buoyancy generating means can either comprise the columns themselves or float means disposed, at least in the semi-submersed state of the device under water. Alternatively the buoyancy generating means can comprise a combination of buoyancy generating columns and float means. Stabilising chambers are formed on or attached to either the columns or the float means outside of the central area staked out by the columns. Each stabilising chamber has a vent 3 at its upper end and a remotely controllable device 2 at its lower end, and the stabilising chambers are freely flooded with water in the semi-submersed position of the structure. In this way, in the event of an undesired leakage in the semi-submersible structure leading to a substantial change in the trim angle thereof, one or more of the stabilising chambers surfaces and thereby changes the position of the centre of buoyancy of the structure in the sense of counteracting the change in trim angle resulting from the leakage. <IMAGE>

Description

SPECIFICATION A semi-submersible structure and a stabilising system for use therein The present invention relates to a semi-submersible structure and to a stabilising system for use therein and has particular reference to a semi-submersible structure in which a working platform is mounted on buoyancy generating columns and is provided with stabilising chambers for limiting the trim angle of the structure when a leakage occurs in a semisubmersed position.

Some national authorities, for example NMD (Oslo) require the provision of safety systems which limit the angle of inclination of semi-submersible structures on the occurrence of damage (leakages).

Known ways of satisfying these regulations include, for example, the use of structures such as buoyancy bodies which are fully submerged in the intact semi-submersed floating position of the platform and which surface to an increasing degree as the platform tilts, which generates a resetting moment.

By way of example, in the event of a leakage in a rear column of a semi-submersible structure, the rear column will tend to sink and the front columns will tend to rise out of the water. This may be exploited, by the use of strategically positioned buoyancy chambers, to produce surfacing of the chambers which results in a loss of buoyancy at the front columns (displacement of the centre of buoyancy), and the generation of the aforementioned resetting moment which restricts the angle of tilt of the damaged structure.

It is however a problem with such arrangements thatthe buoyancy chambers have to be relatively heavy because of the water pressure acting on their walls in the submersed state. Furthermore, the attachment of such buoyancy bodies to the on board conduit system is expensive, particularly in a case in which an existing semi-submersible platform is retrospectively equipped with the buoyancy chambers.

Accordingly, the principal object underlying the present invention is to simplify the construction of the stabilising system, in particular for stabilising systems which are retrospectively added to existing platforms, in order to reduce the constructional expense and complexity thereof.

In order to satisfy this object there is provided, in accordance with the present invention, a semisubmersible structure for use in open water, the structure comprising a working platform; a plurality of columns on which said working platform is mounted, said columns being arranged around a central area; buoyancy generating means for supporting said working platform above the surface of the water, said buoyancy generating means comprising either the columns themselves, or float means disposed, at least in the semi-submersed state of the structure, under water, or a combination of buoyancy generating columns and float means; and stabilising chambers formed on or attached to said columns and/or on orto said float means outside of said central area, whereby, in the event of an undesired leakage in the semi-submersible structure leading to a substantial change in the trim angle thereof, one or more of said stabilising chambers surfaces, at least partially, and thereby changes the position of the centre of buoyancy of the semisubmersible structure in the sense of counteracting the change in trim angle resulting from the leakage, characterised in that each stabilising chamber has a vent at its upper end and a remotely controllable closure device at its lower end; and in that the stabilising chambers are freely flooded with water in the semi-submersed position of the structure.

Also according to the present invention there is provided a stabilising system for use in a semisubmersible structure as recited above, with the stabilising system comprising a plurality of stabilising chambers each having an open vent at its upper end and a remotely controllable closure device at its lower end.

Thus the invention is based on the recognition that the required shift in the position of the centre of buoyancy on the occurrence of leakage can be achieved by the use of stabilising structures which do not generate buoyancy in the normal semisubmersed position of the structure but which generate negative buoyancy on surfacing following tilting of the structure as a result of a leak.

Semi-submersible platforms are generally towed in a state of high buoyancy, and thus shallow draft, to an operating site and the buoyancy of the structure is then reduced, by allowing water to flow into certain buoyancy chambers, so that the structure adopts the semi-submersed floating position.

For movement of the semi-submersible structure in the buoyant state between operating sites the stabilising chambers will normally be positioned above the water. By simply opening the closure devices all water contained in the stabilising chambers will drain away under the action of gravity so that the stabilising chambers empty and the weight of the water previously contained therein does not detract from the buoyancy of the structure. On submersion of the platform the closure devices can be left open and the stabilising chambers gradually flood as the structure sinks. In the submersed floating position the closure devices can be left open or can alternatively be closed.If a leakage occurs which results in tilting of the platform then one or more of the stabilising structures will surface and prompt closing of the closure device ensures that the water contained therein is not lost so that the required shift of the centre of buoyancy occurs when the respective stabilising chamber or chambers surfaces.

The advantages of the invention are the omission of the attachmenttotheonboard conduit system and the smaller wall thickness required for the stabilising chambers.

The remotely controllable closure devices could be closed as a result of a command given by the personnel concerned with management of the structure. A particularly advantageous embodiment of the invention however envisages that the semisubmersible structure is provided with trim angle sensing means and that the remotely controllable closure devices are automatically closed in dependence on signals from said trim angle sensing means.

In one particular advantageous embodimentthe stabilising chambers are wholly or partly open at their upper ends. In this arrangement at least some of the stabilising chambers can be used as chain lockers.

In one particularly favoured embodiment the float means comprises two elongate floats arranged at the foot ends of the columns and the stabilising chambers are provided on the floats, preferably adjacent the ends thereof.

The invention will now be described in the following in more detail by way of example only and with reference to accompanying drawings which show: Figure 1 a semi-submersible structure in which the buoyancy generating means comprises essentially the columns on which the working platform is mounted, Figure 2 a semi-submersible structure in which the platform is mounted via four equi-spaced legs on two catamaran-like floats, the structure being seen from the side of one of the floats, Figure 3 a view similar to that of Figure 2 but showing the situation which prevails when the platform has tilted due to a leakage in a compartment of one of the columns, Figure 4 a view similar to that of Figure 2 but showing the structure in the surface transport position, Figure 5a view similarto thatof Figure 2 but showing a modified embodiment in which the stabilising chambers are used as chain lockers.

Referring firstly to Figure 1 there can be seen three columns 5 which are arranged at the apices of an equi-lateral triangle and which serve to support a working platform 7. Respective stabilising chambers 1 illustrated in section are provided on the outer sides ofthe columns 5, i.e. on the sides outside of the central area staked out by the three columns 5. A similar stabilising chamber is provided on the outside of the third column 5, the central one as seen in the drawing, but is of course hidden from view in the illustration of Figure 1.

The stabilising chambers 1 are provided with open vents 3 at their upper ends. Closure devices 2 are provided at the lower ends of the stabilising chambers 1 and can be opened or closed via schematically illustrated remote control devices 4. In this case the stabilising chambers are mounted on the outside of the buoyancy generating columns 5.

An alternative arrangement is shown in Figure 2.

For the sake of simplicity the same reference numerals have been used to designate parts which have counter-parts in the embodiment of Figure 1. In the semi-submersible structure of Figure 2 the working platform 7 is supported on four columns 5 which are arranged at the four corners of a square. Only two of the columns 5 can be seen in the illustration of Figure 2, and indeed the port pair which are mounted on a float 12. The starb rd pair of columns which are hidden from view are similarly mounted on a corresponding float. In this case the stabilising chambers take the form of tanks 1 which are mounted on the extreme outer ends of the floats 12.

Again the stabilising chambers are provided with open vents 3 and closure devices in the form of valves 2 which can be remotely controlled, i.e.

opened or closed by means of schematically illustrated devices 4. Figure 2 illustrates the water level Vs in the submersed condition and also the water level VF in the fully floating condition used when moving the semi-submersible structure from one side to another. In the arrangement of Figure 2 the two floats 12 are connected together by cross braces (not shown) and the columns 5 form part of the buoyancy generating means.

Figure 3 shows the situation which occurs when a leak develops at 5' in one of the columns. In this case the platform tilts so that the water line follows the line VT. I.e. the stabilising chamber 1' which is filled with water (indicated by cross hatching) sinks relative to the position shown in Figure 2, as a result of the leakage 5', but does not affect the overall buoyancy of the platform. The stabilising chamber at the right hand side of Figure 3 however surfaces above the water line VT and the water 8 contained therein, which did not previously affect the buoyancy of the platform, now exerts a downward thrust on the right hand end of the float 12 and thus exerts a resetting moment which partly counteracts the moment caused by the leakage 5', thus restricting the tilt angle ofthe platform.

Figure 4 shows the same platform when in the fully floating position in which the water line adopts the position F In this state the valves 2 are opened (illustrated by the continuous passage 6 from the stabilising chambers 1 to the base of the floats 12) and water present in the stabilising chambers 1 drains away under the action of gravity so that air 9 is present in the stabilising chambers and these no longer detract from the overall buoyancy of the structure in the fully floating state.

Figure 5 shows an alternative embodiment in which one of the stabilising chambers is fully opened at its upper end and is simultaneously used as a chain locker for storing the unused length of anchor chain 11. Although the remote control devices 4 for the valves 2 have only been schematically illustrated in Figures 1 to 5 it will be appreciated that these can readily be controlled by signals from a trim angle sensor provided on the platform.

Claims

1. A semi-submersible structure for use in open water, the structure comprising a working platform; a plurality of columns on which said working platform is mounted, said columns being arranged around a central area; buoyancy generating means for supporting said working platform above the surface of the water, said buoyancy generating means comprising either the columns themselves, or float means disposed, at least in the semisubmersed state of the structure, under water, or a combination of buoyancy generating columns and float means; and stabilising chambers formed on or attached to said columns and/or on or to said float means outside of said central area, whereby, in the event of an undesired leakage in the semi submersible structure leading to a substantial change in the trim angle thereof, one or more of said stabilising chambers surfaces, at least partially, and thereby changes the position of the centre of buoyancy of the semi-submersible structure in the sense of counteracting the change in trim angle resulting from the leakage, characterised in that each stabilising chamber has a vent at its upper end and a remotely controllable closure device at its lower end; and in that the stabilising chambers are freely flooded with water in the semi-submersed position of the structure.

2. A semi-submersible structure in accordance with claim 1, characterised by the provision of trim angle sensing means and wherein said remotely controllable closure devices are closed in dependence on signals from said trim angle sensing means.

3. A semi-submersible structure in accordance with either of the preceding claims, characterised in that the stabilising chambers are wholly or partly open at their upper ends.

4. A semi-submersible structure in accordance with claim 3, characterised in that at least some of said stabilising chambers are used as chain lockers.

5. A semi-submersible structure in accordance with any one of the preceding claims, characterised in that said float means comprises two elongate floats arranged at the foot ends of the columns; and in that the stabilising chambers are provided on the floats.

6. A semi-submersible structure substantially as herein described with reference to and as illustrated in the accompanying drawings.

7. A stabilising system for use in a semisubmersible structure in accordance with any one of the preceding claims, the stabilising system comprising a plurality of stabilising chambers each having an open vent at its upper end and a remotely controllable closure device at its lower end.