EP0840690A1

EP0840690A1 - Offshore oil rig

Info

Publication number: EP0840690A1
Application number: EP96926441A
Authority: EP
Inventors: Pierre-Armand Thomas
Original assignee: Technip Geoproduction SA
Current assignee: Technip Energies France SAS
Priority date: 1995-07-26
Filing date: 1996-07-22
Publication date: 1998-05-13
Anticipated expiration: 2016-07-22
Also published as: NO980339D0; FR2737179B1; NO980339L; EP0840690B1; JP3905557B2; BR9610018A; WO1997005011A1; OA10656A; JPH11509803A; NO314717B1; KR19990035950A; US6024040A; ES2140883T3; KR100442502B1; FR2737179A1

Abstract

An offshore oil rig comprising an upper body (1) located above the surface of the sea and connected to a completely submerged hollow base (3) via partially submerged substantially vertical connecting legs (2) forming a buoyancy tank. The submerged portion of each of the legs (2) comprises a series of at least two sections (10, 14), i.e. a first section (10) with solid walls defining an enclosed space and forming a buoyancy tank, and a second section (14) with a perforated side wall defining an inner space open to the surrounding marine environment.

Description

Offshore oil platform

The present invention relates to an offshore exploitation platform, in particular an offshore oil exploitation platform, of the type comprising an upper hull extending above sea level and connected to a base. hollow bottom totally immersed by partially immersed connecting legs forming buoyancy box and extending substantially vertically.

Platforms of this type are known as semi-submersible platforms. In order to ensure the stability of such platforms in the operating position, the lower base is ballasted, for example by filling it with sea water. In known platforms, the legs are formed by cylindrical columns with solid walls delimiting over their entire height an enclosed space forming a buoyancy box for the platform.

These platforms do not rest directly on the seabed and are simply anchored by catenary anchor lines. They are thus very sensitive to sea swell which causes upward and downward movements of the platform. The amplitude of these movements can reach important values. This phenomenon makes it difficult to exploit oil from the platform.

In an attempt to provide a solution to this problem, it has been proposed to extend the length of the legs so that the base is deeply immersed. The result obtained by implementing this solution remains imperfect and such platforms are complex to manufacture and install. In addition, they are temporarily unstable during their installation.

French patent application FR-A-2 713 588 describes a self-elevating platform comprising legs formed over the entire height of a metal trellis- lique. Integrated floats on the legs allow the platform to float. However, they are not intended to provide a reduction in the vertical movements of the platform. The object of the present invention is to propose an operating platform at sea which is not very sensitive to swell and whose length of the legs connecting the upper hull to the lower base is limited.

To this end, the subject of the invention is an offshore exploitation platform, in particular an offshore petroleum exploitation platform, of the aforementioned type, characterized in that the legs have on their immersed height at least two successive sections, a first section with solid walls delimiting an enclosed space and forming a buoyancy box and a second section with perforated side wall, the interior space of this second section being open to the surrounding marine environment.

According to particular embodiments, the invention may have one or more of the following characteristics:

- the second section with perforated side wall is a metallic lattice structure,

the second section with perforated side wall is disposed between the first section with solid walls and the base,

- The first section with full walls extends at least partially immediately below said shell, - The first and second sections are dimensioned so that the pressure force exerted on the first section with full walls compensates appreciably over a range of usual swell periods the acceleration force of the platform, the first and second sections are dimensioned so that the pressure force and the acceleration force are equal for two values of swell period included in the range of usual swell periods,

- the smallest swell period value for which the pressure force and the acceleration force are equal is greater than 4 seconds,

- the submerged height of the second section is between a quarter and three quarcs of the total submerged height of the leg,

the submerged height of the second section is between substantially 0.4 and substantially 0.65 times the total submerged height of the leg, the legs have a generally cylindrical outer shape,

the base comprises at least one passage crossing it substantially vertically right through,

- the base is filled with a ballast-forming fluid, in particular sea water,

- the hull is mounted movable along the legs and there are provided mechanisms for relative displacement and locking of the hull relative to the legs, - said second section with perforated side wall is disposed between two sections with solid walls according to the submerged height legs.

The invention will be better understood on reading the description which follows, given solely by way of example, and made with reference to the drawings in which:

- Figure 1 is a schematic elevational view of an oil platform according to the invention; - Figure 2 is a graph showing the transfer function of a platform of the state of the art as a function of the swell period; Figure 3 is a graph showing the evolution of the pressure and acceleration forces exerted on a platform of the prior art as a function of the swell period; and

- Figures 4 and 5 are graphs analogous to those of Figures 2 and 3 for a platform according to the invention.

In Figure 1, there is shown schematically a self-elevating oil platform of the semi-submersible type. It essentially comprises an upper hull 1 extending over the sea when the platform is in operation, and connected by legs 2 to a submerged lower base 3.

Conventionally, the upper shell comprises technical and residential buildings which are not shown, as well as a wellbore and well heads 4.

Furthermore, passages 5 are provided through the hull 1 to allow the passage of the legs 2. Lifting mechanisms 6 are arranged around the passages 5 and make it possible to lower the legs 2 and the base 3 and hoist the hull 1 above the surface of the water up to an altitude which puts it out of reach of the highest waves. The mechanisms 6 are for example pinion and rack mechanisms, the racks extending over the entire height of the legs 2. These mechanisms 6 also include means for locking the legs 2 relative to the shell 1 in order to ensure a rigid leg connection on the hull.

The legs 3 2 are for example four in number and have a generally cylindrical external shape. In the embodiment shown in Figure 1, they have a square section, but they can also have a circular or triangular section.

The legs 2 are all identical and have, according to their submerged height, two successive sections. A first upper section 10 is formed by a tube with a solid wall closed at its lower end by a bottom 12. This first section thus delimits an enclosed space isolated from the surrounding marine environment and forms a buoyancy box for the platform. The upper part of this first section extends above sea level on either side of the hull 1. The lower part thereof extends immediately below the hull 1 and is partially submerged.

The first section is extended by a second section 14 with perforated side wall, the interior of this second section being open to the surrounding marine environment. This second section is thus interposed between the first section 10 and the base 3 and is formed for example by a metallic lattice structure. This structure comprises four metal uprights 16 interconnected by a trellis 18 of metal tubes.

The second section is welded at its upper end to the lower end of the section 10 and at its lower end to the base 3. As shown in FIG. 1, in the operating position, the submerged height Zt of the first section 10 with a solid wall represents substantially a third of the total submerged height Zm of the legs 2. Thus, the second lattice section is completely submerged and extends in the embodiment shown over substantially two-thirds of the total submerged height of the legs 2. In general, the submerged height of the second section with perforated side wall is between a quarter and three quarters of the total submerged height of the legs 2. In practice, the calculations show that the submerged height of the second section is generally between substantially 0.4 and substantially 0.65 times the total submerged height of the legs. The base 3 is hollow and is generally square, rectangular or triangular. It is filled with seawater and thus forms a ballast for the entire platform. It can also include tanks incorporated inside it in which hydrocarbons are stored. Furthermore, a central passage 20 passes right through the base 3. This passage reduces the resistant surface offered to the water during the vertical movements of the platform. It can also allow the circulation of drilling tools. In the position shown in FIG. 1, the platform floats thanks to the submerged part of the first sections 10 with a solid wall. These sections are subjected to a pressure force denoted F _p exerted on their bottom 12. The pressure force F _p depends on the submerged height Zt of the first section 10.

It is expressed as a first approximation in the form:

F _p = A ^ "f (t) WHERE: A _{^ j} is the area of the flotation surface, i.e. the area of the bottoms 12, β is the wave number of the swell and f (t) is the rise in the level of the free sea surface as a function of time.

Moreover, the entire platform is subjected to a denoted F acceleration _force due principa¬ LEMENT the water movements and in particular their effect on the base 3. This acceleration force depends on the total immersed height Zm of the legs 2. It is expressed as a first approximation in the form: F _a = k _x Be ^pzm f (t) WHERE: k _λ is a constant for a given swell period and B is the sum of the mass of the base 3 filled with water and the added mass. The added mass is a fictitious mass taking into account the action of the sea water surrounding the base on the platform during the movements of the latter.

The two forces F _a and F _p applied to the platform are in phase opposition. Under these conditions, it is understandable that it is possible to size the first and second sections so that the submerged height Zt of the section 10 is such that the pressure force F _p exerted on this first section substantially compensates over a range of periods. usual swell the acceleration force F _a of the platform. In addition, the dimensioning can be such that these two forces are equal for two values of swell period included in the range of usual swell periods.

For this purpose, when sizing the platform, we first determine the flotation surface, that is to say the area of intersection of the legs with the surface of the water, as well as the base. By a conventional stability study, the total submerged height Zm of the legs to be used is then determined.

The submerged height Zt of the first solid wall section is determined by solving the equation reflecting the equality of the forces F _a and F _p applied on the platform. By a computer simulation of the behavior of the platform, it is then verified that the two values of swell period, for which the forces F _a and F _p are equal, are included in a usual swell period range. In particular, it is verified that the smallest value of swell period for the- which the two forces are equal is greater than 4 seconds.

If this is not the case, a new calculation of the heights Zm and Zt is carried out with a base of volume or of different shape. Indeed, the modification of the structure of the base, and in particular of its shape modifies the added mass. Thus, the heights Zm and Zt are modified as well as the swell period values for which the two forces F _a and F _p are equal.

In Figure 2 is shown the transfer function of a platform of the prior art, that is to say with legs formed of a single solid wall section extending from the base 3 to hull 1, depending on the swell period T expressed in seconds. The transfer function in heaving is the ratio between the amplitude of the heaving movement of the platform and the amplitude of a swell of one meter, the heaving being a quantity representative of the upward and downward vertical movements platform under the effect of the swell.

It can be seen on this curve that the heaving of the platform is significant over a range of periods from 18 to 28 seconds. This period range corresponds to the high values of the swell periods commonly encountered. Furthermore, heaving is extremely important for periods of swell close to 24 seconds.

FIG. 3 shows the evolution of the pressure force F _p and the evolution of the acceleration force F _a as a function of the swell period T expressed in seconds for a platform of the state of technology. On these curves, it can be seen that the amplitudes of the forces F _a and F _p for a period considered to be less than 28 seconds are very large. In addition, differences between the values of the forces F _a and F _p are large. Thus, the platform is subjected mainly to the acceleration force F _a , from which results the significant heaving translated on the curve of FIG. 2. For a period substantially equal to 31 seconds, the values of F _a and F _p are substantially equal, which corresponds to a substantially zero heaving in this figure.

For the platform according to the invention represented in FIG. 1, the transfer function is represented in FIG. 4 while the forces F _a and F _p are represented in FIG. 5.

It can be seen in FIG. 5 that thanks to the design of the legs in two successive sections, one of which is with solid walls and the other with perforated side wall, it is possible that the values of the forces F _a and F _p are rendered very close to each other over a wide range of swell periods between 0 and 24 seconds and corresponding to usual swells. In addition, the curves representing the forces F _a and F _p intersect at two points over this range of values, which corresponds physically, since these forces are in phase opposition, to a cancellation of the force of exci¬ resulting tation applying on the platform.

It can be seen in FIG. 4, that the forces of acceleration F _Λ and of pressure F _p compensate each other over the whole extent of the range of periods corresponding to the usual swells, the heaving of the platform is very weak . In particular, the maximum heaving obtained over this range corresponds to approximately 1/6 of the maximum heaving obtained with platforms of the state of the art.

In addition, in this figure, the curve is canceled for two different periods T (15.5 seconds and 23.5 seconds) and not only a value as in the case of known platforms. These two cancellation values result from the two points of intersection of the curves representative of the forces of acceleration F _a and pressure F _p .

The curves shown here were obtained with a platform whose submerged height Zt of the first section 10 with solid walls is equal to 50 m and whose total submerged length Zm of the legs is equal to 140 m. The volume of the base is equal to 33,000 m ³ , the area of the flotation surface (sum of the areas of the bottoms 12) is equal to 841 m ² . The added mass of the platform is equal to 194,750 tonnes.

As a variant, not shown, it is also possible to interpose between the lower end of the lattice sections 14 and the base 3 of the sections with solid walls forming additional buoyancy or storage boxes for the platform. Under these conditions, the lattice sections 14 are arranged between two sections with solid walls according to the immersed height of the legs. Furthermore, any other arrangement of successive sections, some of which have solid walls and others with perforated side walls, is also possible for producing the legs of the platform.

Note that with this type of platform, the length of the legs is independent of the depth of the place of operation.

In addition, the good stability of the platform allows the installation of well heads on the hull.

Claims

CLAIMS 1.- Offshore exploitation platform, especially offshore oil exploitation platform, of the type comprising an upper hull (1) extending above sea level and connected to a lower base (3) hollow totally submerged by connecting legs (2) partially submerged forming buoyancy box and extending substantially vertically, characterized in that the legs (2) have at least two successive sections on their submerged height (10 , 14), a first section (10) with solid walls defining an enclosed space and forming a buoyancy box and a second section (14) with perforated side wall, the interior space of this second section (14) being open on the surrounding marine environment.

2.- Platform according to claim 1, carac¬ terized in that the second section (14) with perforated side wall is a metallic lattice structure (18).

3.- Platform according to claim 1 or 2, characterized in that the second section (14) with perforated side wall is disposed between the first section (10) with solid walls and the base (3).

4.- Platform according to any one of the preceding claims, characterized in that the first section (10) with solid walls extends at least partially immediately below said hull

(1).

5.- Platform according to any one of the preceding reven¬ dications, characterized in that the first (10) and second (14) sections are dimensioned so that the pressure force (F _p ) exerted on the first section (10) with solid walls substantially compensates the acceleration force (F _a ) of the platform over a range of usual swell periods.

6.- Platform according to claim 5, carac¬ terized in that the first and second sections are dimensioned so that the pressure force (F _p ) and the acceleration force (F _a ) are equal for two Wave period values within the usual wave period range.

7.- Platform according to claim 6, charac¬ terized in that the value of the smallest swell period for which the pressure force (F _p ) and the acceleration force (F _a ) are equal is greater at 4 seconds.

8.- Platform according to any one of the preceding claims, characterized in that the submerged height of the second section (14) is between a quarter and three quarters of the total submerged height of the leg (2).

9.- Platform according to claim 8, carac¬ terized in that the submerged height of the second section (14) is between substantially 0.4 and substantially 0.65 times the total submerged height of the leg (2).

10.- Platform according to any one of the preceding claims, characterized in that the legs (2) have a generally cylindrical outer shape.

11.- Platform according to any one of the preceding claims, characterized in that the base (3) has at least one passage (20) passing through it substantially vertically right through.

12.- Platform according to any one of the preceding claims, characterized in that the base (3) is filled with a ballast-forming fluid, in particular sea water.

13.- Platform according to any one of the preceding claims, characterized in that the shell (1) is movably mounted along the legs (2) and in that there are provided mechanisms (6) for displacement and relative locking of the shell (1) relative to the legs (2).

14.- Platform according to any one of the preceding claims, characterized in that said second section (14) with perforated side wall is disposed between two sections with solid walls according to the immersed height of the legs (2).