EP1509671A1

EP1509671A1 - Multi-catenary type rising column

Info

Publication number: EP1509671A1
Application number: EP03756012A
Authority: EP
Inventors: François-Régis PIONETTI
Original assignee: Saipem SA
Current assignee: Saipem SA
Priority date: 2002-05-31
Filing date: 2003-05-27
Publication date: 2005-03-02
Anticipated expiration: 2023-05-27
Also published as: WO2003102358A1; FR2840350A1; ATE336637T1; DE60307629D1; AU2003258777A1; EP1509671B1; US7189029B2; FR2840350B1; US20050254900A1

Abstract

An undersea pipe of the riser type providing a connection between a floating support and the sea bottom. The riser has a rigid pipe of the catenary type extending from the floating support to a point of contact with the sea bottom. The catenary riser comprises a lower pipe portion terminating at the point of contact. The apparent weight per unit length in water of the lower pipe portion being at least 25% less than that of the remainder of the pipe constituting the catenary riser.

Description

MULTI-CATENARY TYPE RISING COLUMN

The present invention relates to a bottom-surface connection installation comprising at least one submarine pipe ensuring the connection between a floating support and the seabed, in particular at great depth. These underwater pipes are called "risers" or "risers" as explained below. These risers consist of unitary tubular elements welded or screwed together end to end made of rigid materials such as steel or composite material.

More particularly, the present invention relates to an underwater pipe of the riser type ensuring the connection between a floating support and the seabed, said riser being constituted by a rigid pipe of catenary type extending from said floating support up to 'at a point of contact at the bottom of the sea. The technical sector of the invention is the field of manufacturing and installing bottom-surface production links for the underwater extraction of oil, gas or other soluble or fusible material, or a suspension of mineral matter, from submerged wellheads for the development of production fields installed in the open sea off the coast. The main and immediate application of the invention being in the field of petroleum production, as well as in the re-injection of water and the production or re-injection of gas.

A floating support generally comprises anchoring means to remain in position despite the effects of currents, winds and swell. It also generally comprises means for drilling, storing and processing petroleum as well as means for unloading towards tanker removers, the latter appearing at regular intervals to carry out the removal of production. The name of these floating supports is the English term "Floating Production Storage Offloading" (meaning "floating means of storage, production and unloading") whose abbreviated term "FPSO" will be used throughout the description next, or even "FDPU" or "Floating Drilling & Production Unit" (meaning "floating drilling and production means"), when the floating support is also used to carry out drilling operations with well deviated in the height of the wafer of water. A catenary riser according to the invention can be either a "production riser" for crude oil or gas, or a water injection riser, ensuring the connection with a submarine pipe resting at the bottom of the sea, or again a "drilling riser" ensuring the connection between the floating support and a wellhead located at the bottom of the sea.

In FPSOs where a multiplicity of lines is generally installed, it is necessary to implement either bottom-surface links of the tower-hybrid type, or "chain" type links.

When the bottom-surface connection pipe directly connects a floating support to a point of contact at the bottom of the sea which is offset from the axis of said support, said pipe takes on its own configuration called of "chain", forming a curve whose radius of curvature decreases from the surface to the point of contact at the bottom of the sea, and the axis of the said pipe forms an angle α with the vertical whose value generally varies from 10 to 20 degrees at the level of the floating support up to, theoretically, 90 degrees than the level of the sea bottom corresponding to a theoretical position substantially tangential to the horizontal as will be explained below.

Chain-type connections are generally made using flexible pipes, but their cost is extremely high due to the complex structure of the pipe.

Thus, it has been necessary to develop substantially vertical risers, so as to bring the flexible connection in the chain configuration towards the floating support closer to the surface, which makes it possible to minimize the length of said flexible pipe, as well as the forces which are applied, thus considerably minimizing its cost.

In addition, in the case of oil production, said crude oil traveling over very long distances, several kilometers, we seek to provide them with an extreme level of insulation to, on the one hand minimize the increase in viscosity which would lead to a reduction in the hourly production of the wells, and on the other hand to avoid blocking the flow by depositing paraffin, or formation of hydrates when the temperature drops to around 30-40 ° C. These latter phenomena are all the more critical, particularly in West Africa, since the seabed temperature is around 4 ° C and the crude oils are of the paraffinic type.

As soon as the water depth reaches and exceeds 800-1000m, it becomes possible to make said bottom-surface connection using a rigid pipe with thick wall, because the length of the pipe being considerable, its flexibility allows obtain a satisfactory chain configuration while remaining within limits acceptable constraints.

These rigid risers made of very thick resistant materials, in chain configuration, are commonly called by the term Anglo-Saxon.

"Steel Catenary Riser" meaning "steel riser in the form of a chain" of which the abbreviated term "SCR" or "catenary riser" will be used in the present description whether it is made of steel or of another material such as a composite material.

These "SCR" or "catenary risers" are much simpler to produce than flexible pipes and therefore less expensive.

The geometric curve formed by a uniform weight pipe suspended under gravity, called "chain" is a mathematical function of hyperbolic cosine type (Coshx = (e ^x + e ^"x ) / 2, connecting the abscissa and the ordinate of any point on the curve according to the following formulas: y = Ro (cosh (x / R ₀ ) - 1) R = Ro. (Y / Ro + 1) ² in which:

- x represents the distance in the horizontal direction between said contact point and a point M of the curve,

- y represents the altitude of point M (x and y are therefore the abscissa and ordinate of a point M on the curve with respect to an orthonormal reference frame whose origin is at said contact point)

- R ₀ represents the radius of curvature at said point of contact, ie at the point of horizontal tangency.

- R represents the radius of curvature at point M (x, y) Thus, the curvature varies along the chain from the surface, or its radius has a maximum value R _max , up to the point of contact, or its radius a a minimum value R _m i _n (or R ₀ in the above formula). Under the effect of waves, wind and current, the surface support moves laterally and vertically, which has the effect of lifting or resting the pipe in the form of a chain, at sea level.

Thus, the pipe has a radius of curvature which is maximum at the top of the chain, in general, at least 1500, in particular from 1500 to 5000m, that is to say at the point of suspension on the FPSO, and which decreases up to '' at the point of contact with the ground. At this point, the radius of curvature is minimal in the suspended portion. But, in the adjacent part resting on the bottom of the sea, said pipe being theoretically in a straight line, its radius of curvature is theoretically infinite. In fact said radius is not infinite but extremely high, because there remains a residual curvature.

Thus, according to the movements of the floating support on the surface, the contact point moves back and forth and, in the raised or rested area on the bottom, the radius of curvature passes successively from a minimum value R _min to a extremely high value, even infinite in the case of a theoretical configuration where the underwater pipe rests on the seabed substantially in a straight line.

These alternative flexions create fatigue phenomena concentrated throughout the chain end zone and the lifetime of such pipes is greatly reduced and generally incompatible with the lifetimes sought for bottom-surface connections, that is to say 20-25 years or more.

In addition, we observe that during these alternative movements of the point of contact, the stiffness of the pipe, associated with the residual curvature mentioned above, will in time dig a furrow over the entire length raised then rested and create a transition zone in which will exist an inflection point where the radius of curvature, minimum at the foot of the chain, will then change direction in said transition zone, and will increase to finally reach an infinite value in the portion of underwater pipe resting in a straight line on the bottom of the sea.

These repeated movements over long periods will create a groove all the more important in poorly consolidated soils that are commonly encountered at great depths, which will have the effect of modifying the curvature of the chain and lead, if the phenomenon s amplifies, at the risk of damage to the pipes, either at the level of the underwater pipes resting on the seabed, or at the level of the SCR ensuring the connection between these underwater pipes resting on the seabed and the surface.

The most critical portion of the chain is therefore located in the portion close to the point of contact and, most of the forces in this lower part of the chain are in fact generated by the proper movements of the floating support and by the excitations which occur in the upper part of the chain subjected to current and swell, all of these excitations then propagating mechanically throughout the pipe to the chain end. The currents located at the seabed, as well as the influence of the swell at this depth, are known to be weak and do not create efforts significant hydrodynamics on the lower part of the chain.

The floating support on the surface has considerable buoyancy and remains insensitive to the vertical loads generated by the chains suspended on its plating, on the other hand, the horizontal tension H created by each of the chains must be counterbalanced, either by a balanced distribution on port side and starboard of all the chains, or by strengthening the anchoring of the floating support, on the side opposite the chains.

Patent EP 0 952 301 is known which describes an FDPU associated with a bottom-surface link in a chain, through which the drill string travels, said bottom-surface link serving not only as a guide, but also as a return path. for drilling mud loaded with debris from said drilling. In the lower part of said bottom-surface connection, the curvature is the most accentuated, the rotating rod train rubs against the wall of the bottom-surface connection risking damaging or even destroying the latter. Thus, the problem posed is to provide an underwater bottom-surface connection pipe capable of withstanding the cumulative fatigue at the point of contact with the sea bottom, created by the movements of the surface support, as well as by the effects of swell and current, mainly in the area near the surface where the effects of said swell and said current are generally the most important.

This problem is further accentuated when the pipe structure incorporates a high performance insulation system, the latter then making the pipe even more sensitive to fatigue problems due to the complexity of its internal structure. Thus, another problem posed is to provide a bottom-surface connection pipe whose horizontal tension at the level of said point of contact with the sea and at the level of said floating support is as low as possible, thus minimizing the imbalance created at the level of the anchoring of the floating support and of phenomena of creation of grooves at the level of said point of contact. Another problem posed is to provide a bottom-surface connection pipe of the drilling riser type having improved mechanical characteristics, in particular to reduce the risk of damage to the riser caused by the rotating rod trains introduced into the riser and rubbing against the inner wall of said riser. One solution to the problems posed is an underwater pipe of the riser type ensuring the connection between a floating support and the bottom of the sea, said riser being constituted by a rigid catenary type pipe extending from said floating support to a point of contact at the bottom of the sea, characterized in that said catenary riser comprises a lower portion of pipe ending at said contact point, portion lower pipe whose apparent linear weight in water is lower than that of the rest of said pipe constituting said catenary riser.

The catenary pipe according to the invention therefore comprises at least two pipe portions corresponding to two different chain curves, it is therefore called multi-catenary pipe. More particularly, said lower portion of lightened pipe extends over a length of at least 100m from said contact point.

The lightening of said lower portion of pipe compared to the rest of the pipe has the effect of significantly increasing the radius of curvature Ro at the bottom at the point of contact with the seabed, compared to this that it would be with a pipe having the characteristics of said current part in the lower pipe portion. This increase in the minimum radius of curvature at the point of contact has the effect of considerably reducing the phenomena of fatigue as well as the phenomena of grooving of grooves. Indeed, during movements back and forth of the chain, the pipe portion is alternately bent to reach the minimum radius of curvature, then returned substantially in a straight line, which, because the minimum radius is larger, generates much lower alternating stresses in the device according to the invention compared to the prior art, thus reducing fatigue throughout the service life which generally exceeds 25 years. In addition, the groove created at the bottom of the chain by the residual curvature will be less accentuated, which improves the resistance over time of the bottom-surface connection.

Preferably, for depths greater than 1000 m, said lower portion of lightened pipe extends over a length of 200 to 600 m. More particularly, said lower pipe portion is lightened with an apparent linear weight lower in water by at least 25%, preferably lower by 25 to 80% compared to the part of the rest of the pipe which is adjacent to it.

It is also possible to create a said lower portion of pipe according to the invention having an apparent linear weight lower than the rest of the pipe by weighing down an upper portion of the pipe extending from said floating support.

Advantageously, a multi-catenary pipe according to the invention therefore comprises an upper portion of pipe which extends from said floating support, said upper portion of pipe being weighed down so as to have an apparent linear weight greater than that of said part of the remainder of the pipe adjacent to it, the mass

The weighting of said upper portion of pipe has the effect of increasing the apparent weight in the water of the pipe at this level and therefore, of increasing the tension in said pipe and thus of decreasing the inclination of the axis. pipe with respect to the vertical at the junction with the floating support; moreover, the increase in the mass of said pipe increases the stability of said upper portion of pipe, which then becomes less sensitive to the effects of current and swell.

The excitation at the head of the pipe being reduced, its repercussion on the foot, at the level of the point of contact is less important, which reduces the movements and therefore the fatigue of the lower portion of the pipe which is the critical portion of the bottom-surface connection as mentioned previously.

Preferably, said upper weighted pipe portion extends over a slice of water corresponding to at least the zone of influence of the swell, that is to say preferably 150 to 200 m. In this zone, large currents are generally observed, the latter generally being substantially uniform in slices of water corresponding to thermoclines. This increases the mass and weight of the pipe in the upper slice of water which constitutes the most disturbed area of the bottom-surface connection. In one embodiment, said upper portion of weighted pipe has an apparent linear weight greater than at least 50% than that of the current portion of pipe adjacent to said upper portion, preferably said apparent linear weight is greater from 100 to 300 % to that of said part of the rest of the pipe adjacent to it. When said multi-catenary pipe according to the invention provides the connection with a bottom located at a depth of at least 1000 m and said upper portion of weighted pipe preferably extends over a length of 150 to 250 m from the surface .

In a preferred embodiment, an upper portion of the weighted pipe and a lower portion of the lightened pipe are combined: a) a said upper portion of the weighted pipe which extends from said support floating up to the upper end of a middle portion of the pipe and whose apparent linear weight is greater than that of said portion of middle pipe, and b) a said lower portion of lightened pipe which extends from the 'lower end of said middle portion of pipe to said point of contact at the bottom of the sea and whose apparent linear weight is less than that of said middle portion of pipe.

More particularly, when said multi-catenary pipe according to the invention provides the connection between a floating support on the surface and the point of contact at the bottom of the sea situated at a depth of at least 1000 m, it has: a) a so-called upper portion of weighted pipe extending over a length of 150 to 250 m from said floating support, corresponding to a weighting of 100 to 300% relative to said adjacent median portion of pipe, and b) said median portion of pipe whose length is between

75 and 120% of the water height between the surface and said point of contact, and c) a said lower portion of lightened pipe with an apparent linear weight 25 to 80% lower than that of said corresponding middle portion of pipe to the current part of said pipe, and extending over a length of 200 to 600 m from said point of contact.

According to an advantageous alternative embodiment, said lower pipe portion is made lighter by increasing its buoyancy using buoyancy elements, preferably floats surrounding said pipe.

More particularly, the multi-catenary pipe according to the invention is constituted by a pipe of the "pipe in pipe" type comprising two internal and external coaxial pipes and having buoyancy elements associated with said pipe, preferably insulating, preferably still made by syntactic foam around said external pipe.

According to another advantageous alternative embodiment, said multi-catenary pipe comprises a said upper portion of weighted pipe extending from said floating support, said upper portion of pipe being weighed down because the thickness of the tubular wall of steel pipe is greater than that of the rest of the pipe, with in particular complementary hoops or localized masses which can be made integral, at regular intervals or not, of said weighted pipe.

The present invention makes it possible to produce more resistant pipes which has a tubular wall thickness greater than the wall thickness of said part of the rest of the pipe which is adjacent to it, but of which said lower pipe portion is lightened by floating elements.

In a preferred embodiment, a multi-catenary pipe according to the invention comprises at least one transition pipe portion which provides the connection between the upper end of said lower pipe portion and the rest of the pipe, said portion of transition pipe having an intermediate apparent linear weight and, preferably the value of which varies progressively in stages or regularly between the value of the apparent linear weight of said lower portion of pipe and the value of the apparent linear weight of the current part of pipe which is adjacent to its other end.

A pipe according to the invention can therefore comprise the following successive pipe portions: a) an upper weighted pipe portion, b) a first transition pipe portion, c) a middle pipe portion, d) a second pipe portion transition, and e) a lower portion of lightened pipe. The gradual variation of the apparent linear weight of said portions of transition pipe can be obtained by a gradual variation in the thickness of their tubular steel wall or by a gradual variation in their buoyancy by varying the amount of syntactic foam.

Thus, to produce these so-called transition pipe portions with progressive variation in stages of the apparent linear weight, one or more unitary pipe elements can be used, the apparent weight of which, in particular the thickness, is uniform for each unitary element but with intermediate values between those of adjacent unit elements.

These portions of the transition pipe can therefore extend over lengths of 12, 24 or 48 m, they make it possible to avoid sudden breaks in inertia detrimental to good behavior over time with regard to the fatigue of the pipe. bottom-surface connection.

In an advantageous embodiment, said multi-catenary pipe according to the invention constitutes a drilling riser ensuring the connection between a derrick placed on said floating support and a well head at the bottom of the sea or, preferably, the end an underwater pipe resting at the bottom of the sea, itself connected at its other end to a so-called wellhead.

Other characteristics and advantages of the present invention will appear in the light of the following embodiments, with reference to FIGS. 1 to 3.

FIG. 1 is a side view of a pipe in a simple chain configuration 1 _a , suspended from a floating support 2 of FPSO type, and the lower end of which rests on the sea floor, represented in three different positions 1 _a , 1b, 1c- Figure 2 is a sectional side view detailing the trench dug by the chain foot during the lifting and resting movements of the pipe on the seabed.

FIG. 3 is a side view of a pipe in a single chain configuration 1 _a , suspended from a floating drilling and operating support of the FDPU type, and the lower end of which rests on the bottom of the sea before penetrate the ground to reach the oil tables.

Figures 4 and 5 are side views of a floating FPSO support to which is suspended a multi-catenary pipe according to the invention 1 having three chain curves (8, 7, 6). FIG. 6 shows a side view, in exploded section, of the transition zone between the upper pipe portion 8 and the intermediate portion 7.

Figure 7 shows, in side view, the relief of the lower part by means of continuous floats or distributed around the pipe.

In Figure 1 there is shown a side view of a bottom-surface _binding 1 according to the prior art was suspended in 3 to a floating support 2 of the FPSO type, and resting on the seabed 4 at point contact 5.

The curvature varies along the chain from the surface, or its radius has a maximum value R _ma χ, up to the point of contact, or its radius has a minimum value R _m i _n . Under the effect of waves, wind and current, the surface support 2 moves, for example from left to right as shown in the figure, which has the effect of lifting or resting the pipe in the form of a chain, at the bottom of the sea. In position 2 _C the floating support deviates from normal position 2 _a , which has the effect of tightening the chain 1 _c by lifting it, and of moving the contact point 5 towards the right ; the radius of curvature Rmi _n at the bottom of the chain increases, as does the horizontal tension H _a at said contact point, as well as the tension in the conducted at said floating support. Similarly, in position 2 _b , moving the floating support to the right has the effect of relaxing the chain 1 and resting part of the pipe on the sea floor. The radius Rmi _n at the point contact 5 decreases, as does the horizontal tension H _b in the pipe at the same point, as well as the tension in the pipe at said floating support. This reduction in the radius of curvature at 5b creates considerable internal stresses within the structure of the pipe, which generates cumulative fatigue phenomena which can lead to the ruin of the bottom-surface connection. Thus, the pipe has a radius of curvature which is maximum at the top of the chain, that is to say at the point of suspension 3 on the FPSO, and which decreases up to the point of contact 5 with the ground 4. At this point , the radius of curvature is minimal in the suspended portion, but in the adjacent part resting on the seabed, said pipe being theoretically in a straight line, its radius of curvature is theoretically infinite. In fact said radius is not infinite but extremely high, because there remains a residual curvature.

Thus, as explained above, according to the movements of the floating support on the surface 2, the contact point 5 moves from right to left and, in the raised or rested area on the bottom, the radius of curvature passes successively by a value minimum R _m j _n to an extremely high value, even infinite in the case of a configuration substantially in a straight line.

These alternative flexions create fatigue phenomena concentrated throughout the chain end region and the service life of such pipes is greatly reduced and generally incompatible with the service lives sought for bottom-surface connections. say 20-25 years or more.

In addition, as illustrated in FIG. 2, it is observed that during these alternative movements of the point of contact, the stiffness of the pipe, associated with the residual curvature mentioned above, will in time dig a groove 12 over the entire length lifted then rested, and thus create a transition zone in which there will be an inflection point 11, where the curvature changes direction in the transition zones 11a-11b, to finally reach an infinite value in the portion of underwater pipe resting in a straight line on the bottom of the sea, said portion being raised only in an exceptional manner, for example during the maximum accumulation in the same direction, to the left, of all the disturbing elements (swell-wind-current) acting on the floating support and on the chain, or even when resonance phenomena appear at level of the chain itself.

FIG. 4 represents in side view a chain with multiple curvature according to the invention made up, in its upper part, of a portion of heavier pipe 8 compared to the current part 7 located just below and in continuity, and in its lower part, of a portion of lightened pipe 6 relative to said main part 7. From this arrangement according to the invention, it follows that the radius of curvature Ro in the lower part increases significantly, which considerably reduces the phenomena of fatigue explained above. In addition, the upper portion 8 being weighed down will have a higher tension, which will further increase the stability of the upper part of said chain, which will then become less sensitive to the effects of current and swell, the latter having significant effects. up to depths of 100 to

150m from the surface. The excitement at the head being reduced, their repercussion at the bottom of the chain 5 is less significant, which reduces the movements, therefore the fatigue in this critical lower portion 6 of the bottom-surface connection.

By way of illustration, a simple chain 1 _a (FIGS. 1 and 5) installed by a depth of 1200 m of water, consists of a pipe of the "pipe in pipe" type with an outside diameter of 323.85 mm, of apparent linear weight in water ω = 177.42 kg / m and having a radius of curvature R ₀ (= R _m in) at the point of contact 5 of R ₀ = 500m, corresponding to acceptable behavior over time of the pipe opposite fatigue. The horizontal tension H at the point of contact is H = 89 tonnes and the head angle is oc = 17.1 degrees with respect to the vertical. The floating support 2 on the surface is therefore also subjected to the same horizontal tension force at the other end of the pipe, ie H = 89 tonnes and its anchoring on the side opposite to the bottom-surface connection must be reinforced as much .

A multi-catenary pipe 1 according to the invention, that is to say made up here of three different chain curves (FIGS. 4 and 5) is made up: • of a middle portion 7 of the pipe described above, over a length of 1000m, • of an upper portion 8 weighed down 200m in length suspended from the floating support 2, weighed down by increasing the thickness of steel to reach an apparent linear weight in water ω = 440.45 kg / m, and, • a lower portion 6 of lightened pipe by adding floating elements of the syntactic foam type to reach the apparent linear weight in water ω = 68.13 kg / m. The lower portion 6 has a radius of curvature at the contact point Ro = 733.9m, which leads to a horizontal tension of 50 tonnes.

The table below compares the main results relating to driving in a single chain and to multi-catenary driving according to the invention.

In general, and by way of illustration but not limitation, an upper portion 8 with a length of 150 to 250 m is used, weighed down by 100 to 300% relative to the current middle portion 7 located just below, the length of the portion . median current 7 being for example between 75 and 120% of the water ^• height, the lower portion of pipe 6 being lightened by 25 to 80% relative to the median current part 7, and representing a length of 200 to 600 m , extending to the bottom of the sea 4 beyond the point of contact 5 for several tens, even several hundreds of meters. Thus, during movements from front to back of the floating support 2, the lower portion 6 of the raised chain is always of the lightened type and not of the type corresponding to the underwater pipe resting on the bottom of the sea and connecting the well heads. A multi-catenary pipe 1 according to the invention therefore has the following advantages:

The radius of curvature R ₀ at the bottom of the chain is increased by approximately 50%, which provides the driving with a behavior in fatigue over time considerably improved,

The horizontal tension force at the point of contact, and therefore the horizontal tension force at the head on the floating support are divided substantially by two, thus drastically reducing the imbalance in the anchoring of said floating support 2,

• the misalignment of the axis of the pipe with respect to the vertical is considerably reduced, the angle of the axis of the pipe with respect to the vertical at the floating support being α = 9.6 ° instead of α = 17.1 ° with respect to a pipe 1 _a of the prior art.

In Figure 3 there is shown in side view a floating support 3 of the type

FDPU comprising a bottom-surface link 1 _a in chain configuration called drilling riser connecting the base of the 2ι drilling derrick to a device

5ι located on the bottom of the sea, said device guiding the pipe in the area of penetration of the seabed. Said drilling riser 1 _{has the} function of guiding the rod train actuated and manipulated from said drilling derrick 2ι, the return of drilling mud taking place in the space existing between said rod train and said drilling riser . In the prior art, the rod train being strongly curved in the lower part of the chain tends to rub on the walls of the drilling riser, thereby significantly increasing the wear and the risks of damaging said riser. The multi-catenary pipe according to the invention thus makes it possible, not only to reduce the problems of fatigue of the foot area of riser by increasing the radius of curvature at the foot, but also to reduce the internal wear of said drilling riser under the effect of the friction of the rod train in rotation with the wall of said riser. The fatigue of the drill string itself, by reducing the curvature in the chain end region of the drilling riser will thus be considerably reduced; in the same way, the power necessary for the actual drilling will be radically reduced.

The device according to the invention thus makes it possible to improve the fatigue strength of the drilling riser in the region of the chain end, to reduce the phenomena of wear by the drill string rotating in the interior of said riser and significantly improve the alignment, at the FDPU, between said drilling riser and said derrick.

In the case where the multi-catenary pipe according to the invention is a drilling riser which extends from a floating support with derrick 2-ι of the FDPU type, the weighting and the length of the upper portion 8 are advantageously increased, so as to minimize the angle α made by the vertical and the axis of the pipe at the floating support 2, and therefore so as to reduce the offset of the axis of the pipe relative to the axis of the derrick 2 which facilitates the introduction of drill pipes and drilling operations in general.

In the same way, the thickness of steel of the lower portion 6 of the riser is advantageously increased to reduce the risks of wear by friction of the drill string, insofar as this additional weight is compensated for by an increase in the floating elements, for example shells syntactic foam, to obtain an average weight in water as described above.

At the connection between two types of chain, it avoids creating a sudden change of inertia detrimental to good behavior over time with regard to fatigue, the bottom-surface connection and advantageously incorporates portions of transition pipe in 9ι, 9 ₂ , for example over a length of 12, 24 or 48m in length, consisting of a gradual increase or by increments in the thickness of the pipe from the bottom part to the top part, and / or a progressive integration of buoyancy elements so that the lower part called "transition pipe" is lighter than the upper part of said transition pipe.

In FIG. 6 is shown the transition zone 9ι between the upper portion consisting of a pipe 8 made heavier by increasing the thickness of its wall 11 ι, as well as by the addition of circles 12 regularly or not spaced apart, and the pipe portion 7 of lower thickness 11 ₃ . Said transition zone 9-ι is provided by a pipe length of 24 m, thickness 11 ₂ between the values 1 ^ and 11 ₃ , preferably equal to the average of the two said values. ^. . -.

It remains in the spirit of the invention when the linear mass of the pipe is increased by means of localized loads attached to the pipe, distributed, uniformly or not, along the upper pipe portion 8, in replacement or in combination with the increase in thickness 1 ^ or the hoops 12 described above.

In Figure 7, there is shown the lightened pipe portion 6 equipped, on the left side with a continuous buoyancy element of great length 10ι, consisting of syntactic foam shells, and on the right side, with buoyancy elements individual 10 ₂ regularly spaced and giving the same average buoyancy per meter of pipe as said continuous element 10ι.

Claims

1. Underwater riser type pipe (1) providing the connection between a floating support (2) and the seabed (4), said riser (1) being constituted by a rigid catenary type pipe extending from said floating support (2) to a contact point (5) at the bottom of the sea (4), characterized in that said catenary riser comprises a lower portion of pipe (6) ending at said contact point (5) , lower portion of pipe (6) whose apparent linear weight in water is less than that of the rest (7, 8) of said pipe constituting said catenary riser (1).

2. Pipe according to claim 1, characterized in that said lower portion of lightened pipe (6) extends over a length of at least 100m from said contact point (5).

3. Pipe according to claim 1 or 2, characterized in that it ensures the connection with a bottom to a depth of at least 1000m and said lower portion of lightened pipe (6) extends over a length of 200 to 600m .

4. Pipe according to one of claims 1 to 3, characterized in that said lower pipe portion (6) is lightened with an apparent linear weight in water lower by at least 25%, preferably from 25 to 80 % relative to the part of the rest of the pipe adjacent to it (7).

5. Pipe according to one of claims 1 to 4, characterized in that it comprises an upper portion of pipe (8) which extends from said floating support (2), said upper portion of pipe (8) being weighed down so as to have an apparent linear weight greater than that of said part of the rest of the pipe which is adjacent to it (7).

6. Pipe according to claim 5, characterized in that said upper portion of weighted pipe (8) extends over a slice of water corresponding to at least the area of influence of the swell, preferably from 150 to 200m.

7. Pipe according to one of claims 5 or 6, characterized in that said upper weighted pipe portion (8) has an apparent linear weight greater than at least 50% than that of the part of the rest of the adjacent pipe ( 7) to said upper portion, preferably the apparent linear weight is 100 to 300% greater than that of said part of the rest of the pipe which is adjacent to it (7).

8. Pipe according to one of claims 5 to 7, characterized in that it ensures the connection with a bottom (4) located at a depth of at least 1000m and said upper portion of weighted pipe (8) extends over a length of 150 to 250 m from the surface.

9. Pipe according to one of claims 1 to 8, characterized in that it has: a) said upper portion of weighted pipe (8) which extends from said floating support to the upper end of a middle part of the pipe (7) and whose apparent linear weight is greater than that of said middle pipe portion (7), and b) a said lower portion of lightened pipe (6) which extends from the end bottom of said middle portion of pipe (7) to said point of contact (5) at the bottom of the sea (4) and whose apparent linear weight is less than that of said middle portion of pipe (7).

10. Pipe according to claim 9, characterized in that it provides the connection between a floating support (2) on the surface and the contact point (5) at the bottom of the sea (4) located at a depth of at least 1000 m and it has: a) said upper portion of weighted pipe (8) extending over a length of 150 to 250 m from said floating support, corresponding to a weighting of 100 to 300% relative to said median portion of pipe (7) adjacent, and b) a said middle portion of pipe (7) whose length is between 75 and 120% of the height of water between said surface and said point of contact (5), and c) a said lower portion of lightened pipe (6) with an apparent linear weight 25 to 80% lower than that of said middle portion of pipe (7) corresponding to the current part of said pipe, and extending over a length of 200 to 600m from said contact point.

11. Pipe according to one of claims 1 to 10, characterized in that said lower pipe portion (6) is lightened by an increase in its buoyancy using buoyancy elements associated with said pipe, preferably floats (10ι, 10 ₂ ) surrounding said pipe (1), preferably still made of syntactic foam.

12. Pipe according to claim 11, characterized in that it is constituted by a pipe "pipe in pipe" comprising two internal and external coaxial pipes and having buoyancy elements preferably insulating, preferably still constituted by syntactic foam around said external pipe.

13. Pipe according to one of claims 1 to 12, characterized in that it comprises a said upper weighted pipe portion (8) extending from said floating support (2), said upper pipe portion (8) being increased by the fact that the thickness of the tubular wall (11 ι, 12) of the steel pipe is greater than that (11 ₂ ) of the rest of the pipe (7, 8).

14. Pipe according to one of claims 1 to 13, characterized in that said lower portion of lightened pipe (6) is lightened by buoyancy elements (10) and has a thickness of tubular wall greater than the wall thickness of said pipe part which is adjacent to it (7).

15. Pipe according to one of claims 1 to 14, characterized in that it comprises at least one portion of transition pipe (9-ι, 9 ₂ ) which provides the connection between the upper end of said lower portion of pipe (6) and the rest of the pipe (7, 8), said portion of the transition pipe (9ι, 9 ₂ ) having an intermediate apparent linear weight and, preferably varying gradually in steps or regularly between the value of the linear weight apparent of said lower portion of pipe (6) and the value of the apparent linear weight of the part of the rest of the pipe which is adjacent to it at its other end.

16. Pipe according to one of claims 1 to 15, characterized in that it constitutes a drilling riser ensuring the connection between a derrick (2ι) placed on said floating support (2) and a well head at the bottom of the sea (4) or, preferably, the end (5) of an underwater pipe resting at the bottom of the sea (4), itself connected at its other end to a so-called wellhead (5ι).