Patent Application
20100147047
The present invention relates to a method of making undersea pipes for conveying corrosive fluids, and in particular water, the method comprising assembling unit pipe elements together by welding.
The present invention relates more particularly to a subsurface connection installation between a floating support and an oil loading buoy.
The present invention relates more particularly to a bottom-surface connection installation comprising at least one undersea pipe providing a connection between a floating support and the bottom of the sea, in particular at great depth. Such undersea pipes are referred to as “risers” and they are made up of unit tubular elements made of steel that are welded together end-to-end.
More particularly, the present invention provides a riser type undersea pipe for making a connection between a floating support and the bottom of the sea, said riser being constituted by a rigid, catenary-type pipe that extends from said floating support to a point of contact with the sea bottom.
The technical field of the invention is thus the field of fabricating and installing undersea pipes and more particularly production bottom-surface connections for offshore extraction of oil, gas, or other soluble or phase-change material, or a suspension of mineral material, from an undersea well head in order to develop production fields located at sea or off-shore. The main and immediate application of the invention lies in the field of oil production, and also in reinjecting water and producing or reinjecting gas.
In general, a floating support includes anchor means enabling it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes means for drilling, storing, and processing oil, and means for off-loading to off-loading tankers that call at regular intervals to remove production. Such floating supports are referred to as floating production storage off-loading (FPSO) vessels or as “floating drilling and production units” (FPDU) when the floating support is also used for performing drilling operations with wells being deflected in the depth of the water.
An undersea pipe or “riser” of the invention may constitute either a “production pipe” for crude oil or gas, or a water injection pipe providing a connection with an undersea well head at the sea bottom, or indeed a “drilling riser” providing the connection between the floating support and a well head located on the sea bottom.
A multiplicity of lines are generally installed on FPSOs and it is necessary to implement either hybrid-tower type bottom-surface connections or else catenary type connections, i.e. connections that follow a catenary curve.
When the bottom-surface connection pipe is of the catenary type, it provides a direct connection between a floating support and a point of contact with the sea bottom that is offset from the axis of said support, said pipe taking up a so-called “catenary” configuration under the effect of its own weight, i.e. a curve having a radius of curvature that decreases from the surface down to the point of contact with the sea bottom, with the axis of said pipe forming an angle α relative to the vertical that varies in general from 10° to 20° at the level of the floating support up to, theoretically, 90° at the sea bottom corresponding to a theoretical position that is substantially tangential to the horizontal, as explained below.
Catenary type connections are generally made with the help of flexible pipes, however they are extremely expensive because of the complex structure of the pipe.
As a result, substantially vertical risers have been developed so as to bring the catenary-configuration flexible connection closer to the surface near the floating support, thus making it possible to minimize the length of said flexible pipe, and also to minimize the forces that are applied thereto, thereby considerably reducing its cost.
Once the depth of water reaches or exceeds 800 meters (m) to 1000 m, it becomes possible to make said bottom-surface connection with the help of a thick-walled rigid pipe since the considerable length of the pipe presents sufficient flexibility to obtain a satisfactory catenary configuration while remaining within acceptable stress limits.
Such rigid risers of thick strong materials in a catenary configuration are commonly referred to as steel catenary risers (SCRs) regardless of whether they are made of steel or of some other material such as a composite material.
Such “SCRs” are much simpler to make than flexible pipes and therefore much less expensive.
The geometrical curve formed by a pipe of uniform weight in suspension and subjected to gravity, known as a “catenary”, is a mathematical function of the hyperbolic cosine type (Cos h(x)=(ex+e−x)/2), relating the abscissa and the ordinate of an arbitrary point on the curve in application of the following formulae:
y=R
0(cos h(x/R0)−1)
R=R
0(Y/R0+1)2
in which:
Thus, curvature varies along the catenary from the surface where its radius of curvature has a maximum value Rmax down to the point of contact where its radius of curvature has a minimum value Rmin (or R0 in the above formula). Under the effect of waves, wind, and current, the surface support moves laterally and vertically, thereby having the effect of raising and lowering the catenary-shaped pipe in the vicinity of the sea bottom.
Thus, the pipe presents a radius of curvature that is greatest at the top of the catenary, and in generally at least 1500 m, and in particular lies in the range 1500 m to 5000 m, i.e. at the point where it suspended from the FPSO, with said radius of curvature decreasing down to the point of contact with the bottom. At that location, the radius of curvature is at a minimum in the portion that is suspended. However, in the adjacent portion that is resting on the sea bottom, said pipe is theoretically in a straight line so its radius of curvature is theoretically infinite. In fact, since some residual curvature remains, said radius is not infinite, but it is extremely large.
Thus, as the floating support moves on the surface, the point of contact moves forwards and backwards, and in the region that is lifted from or lowered onto the bottom, the radius of curvature passes in succession from a minimum value Rmin to a value that is extremely large, or even infinite in a theoretical configuration where the undersea pipe rests on the sea bottom substantially in a straight line.
This alternating flexing gives rise to fatigue phenomena that are concentrated throughout the foot region of the catenary, and as a result the lifetime of such a pipe is greatly reduced and in general not compatible with the lifetimes desired for bottom-surface connections, i.e. 20 to 25 years, or even more.
In addition, during these alternating movements of the point of contact, it is observed that the stiffness of the pipe associated with the above-mentioned residual curvature acts over time to dig a furrow over the entire length that is raised and then lowered back down again, so as to create a transition region in which there exists a point of inflection where the radius of curvature, which is at a minimum at the foot of the catenary, changes direction in said transition region and increases finally to reach an infinite value in a portion of undersea pipe that is resting in a straight line on the sea bottom.
These repeated movements over long periods dig a furrow of considerable depth in bottoms that are poorly consolidated, as are commonly to be found at great depths, thereby having the effects of modifying the curvature of the catenary and, if the phenomenon becomes amplified, of leading to risks of damage to the pipes, or to other undersea pipes lying on the sea bottom, or to the SCRs that provide connections between said undersea pipes resting on the sea bottom and the surface.
These pipes are made by welding unit pipe elements together end-to-end. The unit pipe elements are themselves assembled to form strings, in general strings of two to four unit elements welded end-to-end, which strings are then taken to sea. In known manner, these strings are assembled by being welded to one another at sea from a pipe-laying ship, in particular in a J-lay tower. The assembly welds are made preferably and for the most part from the outside of the pipe.
The most critical portion of a riser is situated at the assembly welds between unit pipe elements, in particular in the portion of the riser that is closest to the point of contact, and the major fraction of the forces in this low portion of the catenary are generated by the movements of the floating support and by the excitations that are applied to the top portion of the catenary, which is subjected to current and swell, with all of these excitations then propagating mechanically along the entire length of the pipe to the foot of the catenary.
The steels from which pipes are made are selected to withstand fatigue throughout the lifetime of installations, however, the welds between pipe elements, in this catenary foot region constitute weak points when said pipe conveys water or fluid that includes water, and more particularly salt water. In the presence of water, said welds are subjected to fatigue and corrosion phenomena that give rise over time, and under varying bending stresses, to cracks that lead to said pipes being destroyed.
To mitigate that problem, welds are made between pipe elements using a stainless steel or an alloy that withstands corrosion. Anti-corrosion alloys are well known to the person skilled in the art, and are constituted mainly by nickel-based alloys, in particular of the Inconel type, preferably of a specific grade, and in particular Inconel 625 or 825. Such Inconels also present excellent resistance to fatigue as a result of their high elastic limits, thereby making it possible to achieve lifetimes of 20 to 30 years.
In order to enable the welds to be strong and to be made under good conditions, proposals have been made to line the insides of two pipe elements for welding together with the same stainless steel or corrosion-resistant alloy over a length of a few centimeters in the vicinity of each end of the pipe elements for welding together, so that the penetration pass of the weld that constitutes the future wall in contact with the fluid is of the same metal as the welding filler metal, and in particular Inconel. Such a lining of stainless steel or anti-corrosion alloy, in particular of the Inconel type, is provided using an expensive arc method referred to as “cladding”, and generally performed using a tungsten inert gas (TIG) method or a plasma method, associated with a filler wire or with a powder of stainless steel or of corrosion-resistant alloy.
The object of the present invention is to provide a novel method of fabricating and installing undersea pipes for conveying corrosive fluids and in particular water, the method comprising welding together undersea pipe strings at sea on board a ship for laying undersea pipes, which method should:
In the present invention, the inventors have discovered that incipient cracks are located on the inside of the pipe in the vicinity of the small projection of the weld bead that extends towards the inside of the pipe, and not on the outside face comprising the main bulk of the weld bead on the outside of the pipe. More precisely, and as explained in the detailed description below given with reference to
During welding, uncontrollable localized quenching or shrinkage occurs, leading to contraction stress states of the metal that are localized in and close to the weld region, even though the remainder of the adjacent surface of the pipe is either at rest or in compression.
In general, these problems of localized contraction stress in welds are solved by annealing to relax the stress. Other means are known for treating such problems in welds in order to relax traction stress, but they are not compatible with the time constraints and the desired rates of laying at sea. However, in present circumstances, annealing treatments are not possible for the welds made between undersea pipe elements while laying the pipe at sea.
The present invention provides a method of making steel undersea pipes for conveying corrosive fluids and in particular water, the method comprising assembling unit pipe elements together end-to-end, the weld beads of steel or metal alloy of said welds being located on the outside of the pipe, the method being characterized in that localized peening is performed inside the pipe to increase the level of compression stress in the steel or the metal in the vicinity of said welds and in the adjacent peripheral inside surface of the pipe on either side of the welds so as to create a peened surface swath that is peened over a distance L that is limited in the axial longitudinal direction XX of said pipe, i.e. over a fraction only of the length of each of the two pipe elements that are assembled together by said welds, and as measured from their respective abutting ends.
More particularly, said peened swath extends over a distance L that is not less than half the thickness of the pipe wall, and more preferably over a distance L that is less than twice the thickness of the pipe.
More particularly, the weld comprises a main weld bead on the outside of the pipe and a projection or seam on the inside that is of smaller thickness and that projects into the inside of the pipe.
This internal projection or seam results from the partial melting of the ends of the unit elements that are assembled together by welding, said melting taking place during the welding heat treatment.
More particularly, said peened swath extends over a distance L corresponding to the width of the weld on the inside of the pipe, in particular the width of said internal seam, which seam presents a width lying in practice in the range 3 millimeters (mm) to 5 mm, plus a width on either side lying in the range 1 mm to 10 mm, so as to give a distance L lying in the range 5 mm to 25 mm.
The term “peening” is used herein to mean surface treatment by multiple impacts using one or more projectiles so as to increase the level of compression stress in a region of the surface under treatment.
According to the present invention, it is the entire surface of said swath, i.e. the cylindrical inside surface section on either side of the weld, overlapping the weld, that is subjected to these impacts, with no region of the surface outside said swath being subjected to such an impact.
The projectiles may be in the form of balls or the tips of pointed spindles, the projectiles striking the surface for treatment at their ends and, during impacts, the kinetic energy of the projectiles is transformed into plastic and elastic deformation energy in the surface being treated, thereby having the effect of increasing the compression stress in the material where it is treated, and as a result eliminating residual regions of traction stress.
Peening tools that can be used in the present invention are described in FR 2 791 293 in the name of one of the Applicants, however more rudimentary peening tools as described for example in U.S. Pat. No. 3,937,055 could also be used.
Peening in the present invention consists, so to speak, in cold forging to eliminate residual traction stresses by deforming the material in the peened surface. It should be observed that it is not desired to eliminate any extra thickness associated with an inside seam or projection of the weld bead, but only to apply compression in substantially uniform manner to the surface of the welding region and of the adjacent regions, using sufficient energy to plasticize and deform the metal so as to eliminate any residual traction stresses due to the welding operation.
Still more particularly, said ends of the unit pipe elements for welding together comprise, in longitudinal axial section, a straight end beside the inside of the pipe forming a root face that preferably occupies at least one-fourth of the thickness of the main portion of the pipe and that is extended towards the outside of the pipe by a sloping chamfer.
Under such circumstances, the inside projection or seam of the weld that stands proud is a made up molten metal from said root face and of the filler metal.
It will be understood that said chamfer faces towards the outside of the pipe so that it can receive a weld bead deposited between the two chamfers at the ends of two abutting pipe elements, thereby substantially forming a V-shape at the end of the two pipe elements for butt welding together.
In an advantageous implementation, material is removed by prior machining, preferably by grinding or by milling, from the inside surface of the pipe and from the weld bead over the surface that is to be peened, prior to said peening.
Also advantageously, said peening is performed at least in the transition region between the inside surface of the weld bead and the adjacent inside surface of the pipe.
More particularly, said peening is performed in such a manner as to establish compression or to increase compression over a thickness of 0.2 mm to 2 mm of said inside surface of the pipe and of said weld.
In one implementation, the limited distance L represents one to three times the thickness of the pipe.
Still more particularly, peening is performed in such a manner as to obtain compression stress that is greater than 5 megapascals (MPa), preferably greater than 50 MPa, in particular lying in the range 50 MPa to 1000 MPa, over the entire peened surface.
In a preferred implementation, said peening is performed with a peening device that is moved inside said pipe in translation and in rotation in the vicinity of said weld, the peening device comprising:
In a particular implementation, said peening tool comprises a vibratory surface that preferably extends over said limited distance L in the axial longitudinal direction XX, and a plurality of projectiles of rounded or pointed type suitable for being projected towards the inside surface for treatment by said vibrating surface in order to create a plurality of impacts.
In a preferred implementation, the method of the invention is characterized in that said first carriage includes means for moving said peening tool in translation relative to said first carriage in said longitudinal axial direction XX.
In particular, when the peening tool has a plurality of projectiles that are projected radially from a vibration surface, itself extending over at least some distance in the longitudinal direction, said means for imparting relative movement in longitudinal translation to the peening tool are suitable for moving said projectiles over at least a distance corresponding to the spacing between two successive projectiles so that the entire treated surface can be peened completely in substantially uniform manner.
Still more particularly, the method of the invention is characterized in that it includes the following steps:
It will be understood that the longitudinal movement of the peening tool in translation relative to said first carriage may be performed either continuously, or else essentially between two of said rotations of said peening tool. This makes it possible to avoid leaving any non-peened area between two impact regions of said successive projectiles, and thus to reach the most critical regions that are situated at the interface between the seam of the weld bead and the base metal of the pipe.
According to other characteristics that are advantageous:
In a particular embodiment, said peening tool has a plurality of projectiles, in particular of the rounded or pointed type that are projected from a vibrating surface of said peening tool against said surface for peening, in particular in a radial direction, where appropriate.
Nevertheless, in a particular embodiment, said peening tool is mounted to pivot relative to said second carriage, thus enabling the angle of inclination 3 of the projection direction Y1Y1 of said projectiles to be varied relative to said direction YY of movement in radial translation of said second carriage.
This embodiment makes it possible to optimize peening of the transition regions between the inside weld seam and the adjacent pipe wall, in particular when there is no prior machining of said inside weld seam.
In the method in which prior grinding is performed, the prior grinding of the surface for peening is performed with a grinder tool having a rotary grindwheel mounted in the place of or together with a said peening tool on a said first carriage.
More particularly, said welding is performed using carbon steel, stainless steel, or a corrosion-resisting alloy of the Inconel type having high elasticity, and good fatigue resistance, and preferably Inconel of grade 625 or 825.
Still more particularly, the method of the invention comprises the following successive steps:
1) in a workshop on land, assembling the respective ends of at least two unit pipe elements together end-to-end by said welding in order to form pipe strings; and
2) at sea, on board a laying ship fitted with a J-lay tower, assembling respective ends of said strings together by said welding to form a pipe.
The present invention also provides a bottom-surface connection undersea pipe having at least a portion including regions of said assembly welds between unit pipe elements that have been put into compression by a method of the invention.
More particularly, the present invention provides a bottom-surface connection undersea pipe of the invention that is characterized in that it is a catenary pipe of the SCR type with at least a portion thereof, including the region that comes into contact with the bottom and extending from the bottom over at least 100 m, and preferably 200 m, being assembled by a pipe-making method of the invention.
Finally, the present invention provides a peening device comprising at least a said peening tool mounted on a said first carriage suitable for moving in translation inside a pipe, the device comprising a said peening tool suitable for moving in longitudinal translation XX relative to said first carriage and in rotation about said axial longitudinal axis XX of the pipe in the vicinity of said welds, as defined above.
Other characteristics and advantages of the present invention appear in the detailed light of embodiments described below with reference to the accompanying figures, in which:
FIG. 3A′ shows a variant of
FIGS. 3B′ and 3C′ are fragmentary longitudinal sections corresponding to
In
Curvature varies along the catenary from the surface, where the radius of curvature has a maximum value, to the point of contact where the radius of curvature has a minimum value R0, R1, R2. Under the effect of waves, wind, and current, the floating support 10 moves, e.g. from left to right as shown in the figure, thereby having the effect of lifting or lowering the catenary-shaped pipe off or onto the sea bottom. In position 10c, the floating support is away from its normal position 10a, thereby having the effect of tensioning the catenary 1c and raising it, thereby moving the point of contact 14 towards the right from 14a to 14c; the radius of curvature at the foot of the catenary increases from R0 to R2, and the horizontal tension in the pipe generated at said point of contact also increases, and consequently the tension increases in the pipe and said floating support. In similar manner, when in position 10b, the movement to the right of the floating support has the effect of relaxing the catenary 1b and of resting a portion of pipe on the sea bottom. The radius R0 at the point of contact 14a decreases to a value R1, and similarly the horizontal tension in the pipe at the same time also decreases, as does the tension in the pipe at said floating support. This reduction in the radius of curvature at 14b gives rise to considerable internal stresses with the structure of the pipe, thereby generating fatigue phenomena that are cumulative and that can lead to the bottom-surface connection being destroyed.
Thus, the pipe presents a radius of curvature that is at its greatest at the top of the catenary, i.e. the point where it is suspended from the FPSO, and that decreases down to the point of contact 14 with the bottom 13. At this location, the radius of curvature at the suspended portion is at its smallest, however in the adjacent portion that is resting on the sea bottom, and assuming that said pipe is extending in a straight line, its radius of curvature becomes theoretically infinite. In fact said radius of curvature is not infinite but is very large, since, as a general rule, some residual curvature persists.
Thus, as explained above, as the floating support 10 moves on the surface, the point of contact 14 moves from right to left and in the region that is lifted off or rested on the bottom, the radius of curvature passes successively from a minimum value Rmin to a value that is extremely large, or even infinite, in a configuration that extends substantially in a straight line.
This alternating flexing gives rise to fatigue phenomena that are concentrated within the foot region of the catenary, and the lifetime of such pipes is greatly reduced, and in general is not compatible with the lifetimes that are desired for bottom-surface connections, i.e. 20 to 25 years, or even more.
Furthermore, as shown in
In
In general, the pipe elements do not have an internal cross-section that is perfectly circular, with the section being slightly ovalized. Furthermore, wall thickness may vary around the periphery. Thus, when the ends of the two pipe elements for assembling together are placed face to face, although the alignment of
It is then in this region presenting sharp angles α2 that there is a risk of generally-localized incipient cracks appearing under the effect of fatigue, which cracks initially propagate in the direction FF as shown in
The welding process involves the use of heating and melting powers, and thus of considerable amounts of energy, since it is desirable to minimize cycle time, particularly for the welding that is performed on board the laying ship 8, as explained above with reference to
During fatigue testing performed on lengths of pipe subjected to fatigue simulations corresponding to that which might be expected over a lifetime of 25 to 50 years, but actually carried out on a fatigue test bench, together with automated frequency spectrum and amplitude for the alternating stress cycles, the inventors have observed localized cracking phenomena at the interface between the base metal of a pipe element and the weld region, mainly where the root faces 16 melt and at the internal seam 62 of the weld bead 6. Because of localized quenching phenomena, combined with irregularities in local melting, weak points appear in which the material is in a residual traction stress state to significant level, generally in combination with the presence of a localized physical defect, such as angle. During movement of the pipe, it is specifically at such a location that incipient cracks will appear at 2k, as shown in
The device of the invention is constituted by a first carriage 3 having wheels 3e driven by a motor 3a and powered by an umbilical cord 3d. The wheels are connected to an axial main body 31 of the first carriage via a system of arms 3b mounted as a hinged parallelogram, preferably three parallelogram structures 3b, each carrying two wheels in alignment on the direction XX. The three parallelogram structures 3b are preferably uniformly distributed at 120° from one another, as shown in the cross-section of
A peening tool 5 is used that is of the kind described in FIGS. 6 and 7 of FR 2 791 293. More particularly, its vibratory surface is constituted by the end of a sonotrode. The metallic sonotrode is secured to a piezoelectric emitter via one or more acoustic amplifiers that, in known manner, present a profile that is adapted to amplify the amplitude of the oscillations of the sonotrode. The projectiles may be in the form of balls or in the form of spindles, or in the form of spikes. The ends of the projectiles strike the surface for treatment, and on impact their kinetic energy is transformed into plastic and elastic deformation energy, thereby creating or increasing the level of compression stress in the material at the point of impact.
Shifting the peening tool 5 in translation along the direction XX through a distance δx=e/5, for example, enables the surface for treatment to be peened between the point of impact of the various projectiles 51 when the peening tool is in a given longitudinal position, with this serving to peen the entire surface for treatment, and also to peen with insistence on some particular region, and/or to make peening uniform.
This peening serves to provide local deformation over a controlled thickness as a function of the energy transmitted by the sonotrode to said needles, the metal of the weld, and the base metal at the end of each of the pipe elements. This plastic deformation of the metal makes it possible to establish a generalized and substantially uniform compression stress state throughout the treated region 7, thereby having the effect of absorbing any residual localized traction stress state that might result from the welding process and the above-described undesirable localized quenching phenomena.
Achieving compression depends on the power and the accuracy of the peening process, and it is generally performed over a thickness lying in the range 0.2 mm to 2 mm, thereby advantageously preventing unwanted incipient cracks from appearing.
The quality of the pipe in the welding region is advantageously improved by internally grinding 63 the weld prior to peening so as to eliminate geometrical surface defects, thereby enabling peening to be performed over an inside surface of the pipe and the welding that is substantially cylindrical where it is peened. Grinding is advantageously performed using a grinder tool 19 as shown in
During prefabrication on land of the strings 2 as shown in
During on-site installation, the prefabricated strings generally have a length of about 50 m, as shown in
In the workshop, and on board the installation ship, at the end of the peening treatment of the welding region, it is advantageous to monitor the state of stresses in the treated region so as to ensure that traction stress states have been eliminated and replaced by compression stress states. The most appropriate inspection technique is the X-ray method that makes it possible to measure the inter-atomic distances within the surface of the material, and thus to characterize very accurately the stress state and level, regardless of whether stress is in traction, at rest, or in compression. Such means are available from the Applicant and are implemented using a robot similar to that described above, the peening tool 5 being replaced by the X-ray source and the associated sensors that are available from the supplier Stresstech (Finland). The signals recovered by the sensors are then sent to a signal processor unit, e.g. a computer, which deduces therefrom the real stress level that exists after and possibly also before the peening treatment of said welding region.
The present invention is described mainly for solving the problem associated with bottom-surface connections and more particularly in the region of the point of contact with the sea bottom in an SCR type connection. Nevertheless, the invention applies to any type of undersea pipe, whether it rests on the sea bottom, whether it is incorporated in a vertical tower, or indeed whether it constitutes a subsurface connection between two FPSOs, or between an FPSO and an unloading buoy.
The various types of subsurface connection are described in patent FR 05/04848 in the name of one of the Applicants, and more particularly in
| Number | Date | Country | Kind |
|---|---|---|---|
| 07 54425 | Apr 2007 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR08/50625 | 4/9/2008 | WO | 00 | 11/17/2009 |
