GB2153885A - Stabilised floating platform - Google Patents

Abstract

A stabilised floating platform for oil and, gas exploration in deep water is disclosed, the platform having a superstructure supported on at least one floatation column 3 having a counterweight 9 at its lower end. A stabilising envelope 1 having a width considerably in excess of its depth is located between the ends of the column so that in use it is at a depth below the water surface such that waves have little effect on the platform. The stabilising envelope has a volume which is at least half of the total submerged volume of the platform and may incorporate internal water-tight compartments. Similar compartments may also be provided in the column or counterweight. Machinery may also or alternatively be located in the envelope. It is preferred that the column be surrounded by a plurality of columns which can be used either for storing oil or for water ballast. <IMAGE>

Description

SPECIFICATION Stabilised floating platform This invention relates to a stabilised floating platform.

Today's tendancy with offshore oil and gas technology, research, exploration and production is to go into deeper and deeper waters due to the impoverishment of existing oil and gas fields. Recent discoveries of new oil and gas fields at great water depths (350m) and the estimation of the world's oil and gas reserves which are, in most cases, at a depth of more than 200m of water, all show that in order to develop and exploit these huge wealths, it is necessary to develop new structures or platforms to support the drilling and production equipment. The adaption of the existing platforms, usually of the jacket-type with tubular supports resting on the bottom of the sea and emerging 10-20m above sea level, leads to enormous structures with regard to the steel weight and total cost.

Semi-submersible platforms are limited as well, because of the small amount of weight they can support (typically a maximum of 1 0,000 tons); this is small compared with their steel weight.

There are some existing floating structures based on the "Froud Perch" principle and comprising a long steel cylinder of which the submerged lower part is weighted and the upper part emerges far enough above the sea level to keep the deck and its equipment above the waves.

A characteristic of existing structures is that they are very stable but the maximum superstructure loads are limited because, in order to compensate for the overturning moment due to those loads, it is necessary to extend the cylindrical submerged part in order to obtain the restoring loads necessary for stability; consequently those structures very quickly reach a limit of draft because the effect of the waves produces too much shear force and bending moment on the extended submerged part.

The present invention seeks to overcome the problems associated with known platforms or structures and allows, by its simplicity of function, a reduction of the steel weight, compared to presently developed equipment, and can be designed and built to support any kind of superstructure (possibly over 40,000 tons). Therefore, one aim of the invention is to limit the submerged length and keep the same stability as per the "Froud Perch" principle. This is achieved in that the equivalent volume of a lower part of the cylinder which has been taken off, is placed just below the sea level, but at a certain depth in order to decrease the effect of the waves.

According to the present invention, we provide a stabilised floating platform consisting of a superstructure supported upon at least one floatation column and having a counterweight at or adjacent its lower end wherein a stabilising envelope, the width of which is greater than its depth, is located between the ends of the column and situated in use below sea level in order to provide a stabilising effect, but is located at a depth so that the effect of the waves on the platform is limited so as to increase the stability of and reduce the motion of the structure.

In the device herein described, the shape of this equivalent volume is flattened to provide the envelope and act as a heave compensator and to offer a minimum section to the wave action. This important punctual volume, below sea level, raises the centre of buoyancy, as compared to other units, and is the reason why it is possible to decrease a part at the dead weight compared with that necessary for the "Froud type" structure having the same stability and the same superstructure load capacity.

It is also possible to add a series of oil storage compartments in the stabilising envelope or below this envelope, while keeping the benefit of the above stabilising device, then the structure has two functions: Production and Storage.

With the wave action applied on this structure being smaller than the "Froud Perch" type structure, it is possible to reduce the relative steel weight of the whole structure.

The structure may be made of a plurality of metallic columns which may be made cylindrical and which give a longish shape to the whole structure. Preferably, the structure has a casing for the counterweight at its bottom end. The preferably metallic envelope is of large capacity, with a wide and flat shape, is a part of or is fixed to the structure, and is situated just below sea level in order to produce a stabilising effect at all times but at a level of depth where the effect of the waves is limited in order to gain in stability.

The envelope preferably represents a volume at least equal to or greater than half the total submerged volume. This is a necessary condition to give the maximum efficiency for stability improvement, steel weight economy and dead weight economy. The envelope is preferably hollow and is normally partially empty. Preferably, it has a large available capacity so that it can be used as an equipment or machinery space.This envelope preferably also has enough watertight void compartments to be filled up with sea water or any other liquid coming from the outside or from the casing containing the dead weight material in order to allow (a) total immersion of the vertical structure built apart as a package; this operation, being shortened in time, is an economical method of completing the total structure, and (b) transportation in vertical or inclined position or horizontal position depending on the water depth during a tow ing operation.

The lower columns supporting the dead weight can be limited to one unit column around which a series of oil storage compartments can be assembled; these compartments, always full either of oil or sea water or oil and sea water, have a neutral buoyancy and do not affect the stability. The unit column contains an important ballast reserve large enough to compensate the weight variation of the storage compartment when they are either full of oil or full of sea water.

The storage compartment can store a large amount of oil corresponding to 3 to 4 weeks of field production.

The structure allows the support of any kind of superstructure from a small unitto a large unit, for example from 100 tons to 40,000 tons or larger.

The invention is now described by way of example with reference to the accompanying drawings, in which: FIGURE 1 is a schematic elevation view of the structure as it is moored "in site" when functioning; FIGURE 2 is a plan view of the superstructure including an intermediate level deck with accommodation facilities and also showing a helideck on its top with drilling or other items of equipment; FIGURE 3 is a plan view of an envelope which supports the columns supporting the superstructure, etc; FIGURE 4 is a schematic elevation view of the structure with additional oil storage compartments around the lower column. and FIGURE 5 is a plan view of the stabilising envelope with the upper and lower columns around which is assembled the additional oil storage compartments.

Referring to Figures 1-3 of the drawings, a stabilising floating platform is illustrated which has a hollow stabilising envelope 1 which would normally be metallic and has a wide but flat shape, which is located between one or more upper floatation columns 2 and one or more lower floatation columns 3. The upper columns 2 support a superstructure 4 including living quarters 5 with a helideck on top and machinery decks with drilling or other production equipment and/or other items and equipment 6 and 7.

Four lower columns 3 are illustrated, which converge and continue as a single column 3' at the lower end of which a casing 8 containing a dead weight 9 is supported. The columns 3 and 3' can be filled and one or more mooring devices 10, which may include a chain, fixes the structure in position in its operating area.

The hollow envelope 1 can contain production equipment, such as generators, electrical equipment, pumps, etc. It is located sufficiently far beneath the water surface so that it is hardly, if at all, influenced by wave action. Due to its large transverse dimensions (i.e. in a horizontal plane) relative to its depth (in a vertical plane as illustrated) it acts as a baffle to resist any tendancy of the platform to move vertically and/or to rock, due to the wave action and surface currents. The precise shape of the envelope is not important.

The counterweight at the bottom of the structure need not be enclosed in the casing 8. It could be distributed, for example, along the length of the column 3'. Furthermore, more than one envelope 1 could be provided, and these would then be spaced vertically.

The lower or lowest one could contain the counterweight.

Referring now to Figures 4 and 5 of the drawings, a stabilised storage floating platform is illustrated which is similar to the platform of Figures 1-3 but has a single central lower support column 3" extending downwards from the envelope 1, there also being a series of oil storage compartments 11 assembled around the central column 3".

These compartments could be individual columns, or a compartmented sleeve as illustrated.

This structure has approximately the same stability characteristics with or without the oil storage compartments 11 because of their neutral buoyancy.

Suitable apparatus with sea water ballast pumps and oil pumps situated in the envelope 1 allows the following operations: a) Filling the storage compartments 11 with oil by means of a flexible line 1 2 by pumping out the equivalent amount of sea water already in place in the compartments 11 and ballasting the central column 3" with an amount of sea water corresponding to the losses and weight due to difference in density between water and oil. In this respect oil can be displaced by injecting water into the compartments; likewise water can be expelled by displacing it with oil.

b) Loading a tanker 1 3 by pumping out the oil through a flexible pipeline 14 and, at the same time, ballasting the storage compartments 11 with the equivalent volume of sea water, while deballasting the central colum 3 with the amount of sea water corresponding to the additional weight due to the difference of density of water and oil.

A metal crown device 1 5 allows the loading arm 1 6 to rotate through 3604 to supply, through the flexible hose 14, the tanker 1 3 moored to the structure by a mooring line 17.

It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope and spirit of the invention.

Claims (11)

1. A stabilised floating platform consisting of a superstructure supported upon at least one floatation column and having a counterweight at or adjacent its lower end wherein a stabilising envelope, the width of which is greater than its depth, is located between the ends of the column and situated in use below sea level in order to provide a stabilising effect, but is located at a depth so that the effect of the waves on the platform is limited so as to increase the stability of and reduce the motion of the structure.

2. A stabilised floating platform according to claim 1 wherein the stabilising envelope is at least equal to half of the total submerged volume of the platform when in use.

3. A stabilised floating platform according to claim 1 or 2 wherein a part of the envelope is empty in the normal operating condition and wherein the envelope has at least one internal watertight compartment which can be filled with sea water or any other liquid.

4. A stabilised floating platform according to claim 3 wherein there is at the lower end of the column a casing for the counterweight.

5. A stabilised floating platform according to claim 4 wherein said casing communicates with the envelope to allow material from the casing to be transferred to the envelope.

6. A stabilised floating platform according to claim 3, 4 or 5 wherein the column can be submerged by filling of the at least one watertight compartment until the upper end of the column extends a few metres above sea level in order to permit installing the superstructure in one single operation or by installing it in parts as required or desired.

7. A stabilised floating platform according to any one of the preceding claims wherein the envelope contains machinery and/or other items of equipment and/or supplies.

8. A stablised floating platform according to any one of the preceding claims which can be towed in a vertical position with or without its superstructure or can be towed in an inclined position and which can be up-ended from its vertical position and back to its first position when necessary.

9. A stabilised floating platform according to any one of the preceding claims which can be designed and built to support any superstructure for example from 100 tons to over 40,000 tons, and without any limit and for any kind of use.

10. A stabilised floating platform according to any one of the preceding claims and including a series of oil storage compartments arranged in use below sea level.

11. A stabilised floating platform according to claim 10 wherein the oil storage compartments are located around a single lower support column extending between the envelope and the counterweight.

1 2. A stabilised floating platform substantially as hereinbefore described with reference to the accompanying drawings.