GB2123883A

GB2123883A - Improvements in and relating to ocean platforms

Info

Publication number: GB2123883A
Application number: GB08319712A
Authority: GB
Inventors: Bernard Vignaux
Original assignee: Compagnie Francaise des Petroles SA
Current assignee: Total Compagnie Francaise des Petroles SA
Priority date: 1982-07-22
Filing date: 1983-07-21
Publication date: 1984-02-08
Also published as: FR2530697A1; BR8303905A; NO832666L; GB8319712D0; ES8502191A1; ES524335A0

Abstract

An oscillating ocean platform, designed especially for use at great depths, comprises base means 16 which rests on or are fixed to the sea bed 18, a rigid upper part 2 which supports the bridge 1 and is associated with a buoyancy reserve 7, and a flexible lower part 3, which comprises a structure of vertical members 9 connected by horizontal members 10, and which is connected to the upper part 2 and to the base means 16 by transition portions 12, 17 designed to ensure transmission of forces to and from the upper part 2 and base means 16. <IMAGE>

Description

SPECIFICATION Improvements in and relating to ocean platforms The invention relates to an ocean platform particu larly but not exclusively for oil installations, and designed particularly but not exclusively for medium and large depthsofwaterforwhich conventional platforms, simply placed on the sea bottom (the "gravitational" type) or held on the sea bottom by piles inserted in tubular legs (the "jacket" type), can not be used or whose use would result in excessively high costs.

In the search for newtypes of platforms designed forthese depths, there was atfirsta trend towards floating platforms ofthe semi-submersible type, in which the loading capacity of the emerged well head was improved, often at the expense of a substantial compiication of the anchoring methods used. However, for oil production, it was then generally considered preferable to use underwater well heads which were much morecomplextoemploythanconventional aerial well heads.

Another line of research involved designing oscillating platforms which would make it possible, in particular, to preservethe conventional system of oil production. The first design along these lines was that ofaguyedtowermaking use of a largenumberof cables which have high mechanical characteristics and the weight of which substantially increases the vertical forces exerted on the platform, thus resulting in the use of a very high total weight of steel. Another design was that of a tower which was articulated at its lower end and the upper part of which could, in particular, incorporate a permanent buoyancy reserve which subjected the tower to a pull which tended to bring it back into a vertical position.The articulations, which can consist of cardan joints, ball - and - socket joints orofdie-castjoints with an interlacing of synthetic fibres, constitute a delicate part of such a platform assembly, the lifetime of which can be limited and the replacement of which requires duplication of the articulation systems and the use of a difficult procedure of disconnection and hoisting and then re-installation.

According to the invention there is provided an oscillating ocean platform comprising at its lower end base means arranged on the sea bottom, a rigid upper partwhich supports a bridge, a buoyancy reserve associated with said upper part, and a lower part which comprises a structure which has a smaller cross-section than said upper part, is flexible and comprises, at its upper and lower ends, upper and lower transition portions for ensuring transmission of forces respectively between said upper part and said lower part and between said lower part and said base means.

By a flexible structure is meant a structure capable ofwithstanding, under operating conditions, bends having low radii of curvature, preferably radii of curvature between 0.2 and 1.5 km.

Thus, the platform being subjected to tension between the buoyancy reserve and the embedding means and the lower part of the platform being flexible, the forces exerted by the environment are absorbed because of the bending ofthe said lower part as a result of the Archimedean lift which is applied to the buoyancy reserve.

The relative length of the said lower part is calculated according to the available data on the environment. Too short a length of this lower part may require an increase in the number of elements forming it, so that each has a sufficiently low inertia to allow it to bend, whilst too great a length ofthis lower part could result in complex and unfavourable dynamic behaviour.

The flexible structure constituting the said lower part of the platform preferably extends over at least one tenth ofthetotal heightofthe platform and advantageously over at least one tenth to at least five tenthsofthistotal height.

The flexible structure may comprise a plurality of vertical members which are connected to one another by horizontal members fastened by butt joints and arranged in horizontal planes distributed over the height of the structure.

Preferably at leastthe horizontal members ofthe flexible structure comprise tubular members.

The transition portions preferably each comprise an assembly of divergent members which diverge from the flexible structure.

The flexible structure advantageously ends in end horizontal members arranged in end horizontal planes and connected in these planes to the vertical members in end butt joints, and each transition portion advantageously comprises several assemblies of divergent members, the divergent members of one of these assemblies projecting from one ofthe said end butt joints.

The divergent members ofthe lower transition portion are preferably partially sunk into a concrete base which may be fixed rigidly to the sea bottom and forms the said base means.

An embodiment according to the invention will now be described, by way of example only, with reference to the accompanying drawings.

In the drawings: Figure lisa view in elevation and in perspective of an embodiment of an ocean platform with a flexible structure according to the present invention; Figure 2 is a view in elevation and in perspective, on a larger scale, of an upper portion ofthe platform; Figure 3 is a view in elevation and in perspective, on a largerscale, oftheflexiblestructure of another embodiment, the flexible structure being narrower than in Figure 1, and of lower and uppertransition elements, together with the base means and the bottom of the upper part of the platform; and Figure 4 is a view in elevation and in perspective of the flexible structure of Figure 1 and showing an alternative form ofthe base means.

Figure 1 illustrates a platform comprising a concrete bridge 1 supported bya rigid upperpart2connected to a flexible lower part 3.

The upper part 2, which can be seen better in Figure 2, is made, over the greater part of its length, of a metal lattice, in the general form of a polehedron with vertical generating lines and convex polygonal horizontal faces, as shown hexagonal, which informed by an assembly of vertical members 4, horizontal mem bers5,andinclinedstruts6which make the metal lattice rigid. At the top of the upper part 2, there is inserted within the metal lattice a buoyancy reserve 7 which preferably comprises, in known manner, a plurality offloats, such as tanks made of steel or concrete, so as to increase safety. Above this buoyancy reserve 7, there is a portion 8 for connection to the bridge 1 which also consists of a rigid metal lattice.Its general form is advantageouslythat of a truncated pyramid surmounted buy a polyhedron of narrower cross-section than that ofthe metal lattice located underneath the truncated pyramid. The members 4 and 5 and the struts 6 may be metal tubular elements or metal I-beams.

The lower part 3, which can be seen better in Figure 3, also has the general form of a polyhedron with vertical generating lines and convex polygonal horizontal faces, as shown hexagonal, butthis polyhedron informed by a plurality of vertical members or legs 9, of which there are six as shown and which are connected to one another by spaced apart groups of horizontal members or chords 10, of which there are six in each group as shown. The connections are made, for example, by welding at butt joints 11. The lower part 3 has no struts so that it has a certain flexibility. The members 9 and 10 comprise tubular elements made of steel. The cross-section ofthe lower part3 is clearly less than that ofthe upper part 2.

The lower part 3 is connected to the upper part 2 by a transition portion consisting of an assembly of divergent members 12 which, for example by means of welds, connect the butt joints 11 of the upper group 13 of horizontal members 10 ofthe lower part 3 to the lower buttjoints 14 oftheee members 4 and 5and struts 6 ofthe upper part 2.

The lower part 3 ends at its lower end in a lower group 15 of horizontal members 10. It is connected to a concrete base 16 of greater cross-section, which as shown has a hexagonal form, by a transition portion consisting of an assembly of divergent members 17 projecting from the butt joints 11 ofthe lower group 15,to which they are connected, for example by welding, and ending in the concrete mass ofthe base 16.

The base 16 itself can rest on the sea bed 18 simply by its own weight, this being the type of base shown in Figure 4, or it can be fastened to the sea bed 18 by means offoundation piles 19, as illustrated in Figures 1 and 3.

Shown on the right in Figures 1,2 and 3 is a bundle of conductors 20 which is attached from place to place along the platform by means of members 21 which grip the bundle and prevent itfrom being displaced horizontally.

In a preferred embodiment of platform for use with a water depth of 500 metres between the upper water levei 22 and the sea bed 18, the flexible lower part3, togetherwiththetransition portions 12,17 may extend, for example, over approximately 150 to 200 metres, the buoyancy reserve ofthe rigid upper part2 being provided in a zone located between 30 metres and 100 metres beneath the upper water level 22. The maximum inclination ofthe platform over a very long period of time would be 5". The vertical members 9 may have a diameter of the order of 5 metres, and the horizontal member 10 may have a diameterofthe order of 2.5 metres.Of course, since the invention is applicable to platforms used under highly diverse conditions, the relative lengths of the rigid upper part and oftheflexible lower part can vary in large proportions, and in particularthe rigid upper part may be reduced, in some cases, to a very short zone underneath the bridge. Likewise, the transition portion 12 may be lengthened and may comprise an entire network of members. The dimensions ofthe members of the lower part may, of course, be very different, depending on the circumstances of use. It would be possible, moreover, to provide an articulation in the rigid upper part or at the bottom of the latter, this articulation preferably being located at a ievel readily accessible to divers two make it easierto carry out monitoring and maintenance. Furthermore, although the invention generally makes it possible to dispense with the use of guys completely, nevertheless it may be advantageuos, in some special cases, to provide some guys to reducethe oscillations of the platform.

The buoyancy reserve may comprise steel floats installed outside the lattice ofthe rigid part 2, this lattice could be replaced by a single hollow column body,thetop of the rigid part2 could be made of concrete, and numerous other alternative forms and modifications cou Id be incorporated without departing from the scope ofthe invention.

Claims

1. An oscillating ocean platform comprising at its lowerend base means arranged onthesea bottom, a rigid upperpartwhich supports a bridge, a buoyancy reserve associated with said upper part, and a lower part which comprises a structure which has a smaller cross-section than said upper part, isflexible and comprises, at its upper and lower ends, upperand lowertransition portionsfor ensuring transmission of forces respectively between said upper part and said lower part and between said lower part and said base means.

2. A platform according to claim 1, wherein said lower part of said platform extends over at least one tenth ofthetotal heightofsaid platform.

3. A platform according to claim 2, wherein said lower partofsaid platform extends over at least half of the total height of said platform.

4. A platform according to any one ofthe preceding claims, wherein said lower part of said platform comprises a plurality of vertical members which are connected to one another by horizontal members fastened at buttjoints and arranged in horizontal planes spaced apart overthe height of said lower part.

5. A platform according to claim 4, wherein said horizontal members of said lower part comprise tubular members.

6. A platform according to any one of the preceding claims, wherein said transition portions each comprise an assembly of divergent members which diverge from said flexible structure of said lower part.

7. A platform according to any one of said preceding claims, wherein said flexible structure of said lower part ends in end horizontal members arranged in end horizontal planes and connected in said planes to vertical members at end butt joints, and each transition portion comprises a plurality of assemblies of divergent members, said divergent members of a respective one of said assemblies projecting from a respective one of said end butt joints.

8. A platform according to claim 7, wherein said divergent members of said lowertransition portion are partially sunk into a concrete base which is fixed rigidly to the sea bed and forms said base means.

9. A platform according to any one ofthe preceding claims, wherein said buoyancy reserve is arranged atthe upper portion of said upper part.

10. An oscillating ocean platform substantially as herein described with reference to the accompanying drawings.