Abstract:
A method and a device for measuring the movement of a subsea pipeline. The measuring device has an accommodating mount anchored in the sea bed to accept the subsea pipeline. The subsea pipeline is liable to be made to move over a determined travel with respect to the accommodating support as the pipelines deforms. The movement has an amplitude that varies according to the deformation of the subsea pipeline. A plurality of frangible elements secured to one of either the deformable subsea pipeline and the accommodating mount. The frangible elements are intended to be broken in succession by the other of either the deformable subsea pipeline and the accommodating mount when the pipeline is caused to move.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/FR2008/001552, filed Nov. 4, 2008, which claims priority of French Application No. 0707960, filed Nov. 13, 2007, the disclosure of which is incorporated by reference herein. The PCT International Application was published in the French language. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for measuring the movement of a subsea deformable pipeline in relation to a sea bed. 
     One envisaged application field is that of on-bottom pipelines, or “flowline” pipes, that extend over the sea bed. They are intended to connect a wellhead, which projects from the sea bed, to a riser, which, from the sea bed, extends as a catenary to join a surface installation. The on-bottom pipeline, which is supported on and extends over the sea bed from the wellhead, has a connecting end for connecting the on-bottom pipeline to the riser, or to another on-bottom pipeline. 
     Therefore, a hydrocarbon which flows from the wellhead is brought up to the surface installation via the on-bottom pipeline and the riser. 
     Other technical fields are envisaged where a flexible pipeline is liable to deform under the effect of the thermal and/or mechanical variations of a liquid passing through it. 
     The hydrocarbons flow from the wellhead at a pressure and a temperature that vary over time. Moreover, when the flow has stopped, for any reason due to the operation, the pressure and temperature conditions of the on-bottom pipeline change dramatically. As a result, the on-bottom pipeline then expands or contracts when, for example, flow restarts. An on-bottom pipeline that is a thousand meters, for example, can be subject to meter-scale longitudinal dimension variations. 
     Thus, during the life of an oil field, which can be some years, the on-bottom pipeline is subjected to numerous expansion and retraction cycles with consequential amplitudes that bring about large stresses on the pipeline and the connecting parts. 
     It is known to minimise the stresses placed on the structure by designing structures that are capable of absorbing these stresses. To this end, the connecting ends are mounted on metal structures that can slide on a foundation anchored in the sea bed. In this manner, the connecting end can accommodate longitudinal movements. However, residual friction remains at the connecting ends and it is important then to assess these excursions to ensure that these stresses are compatible with the structure of the on-bottom pipeline. 
     It can also be envisaged to continuously monitor the behavior of the structure by recording data in real time. Thus, the lengthening of the structure can be monitored in real time and it can be determined if it is compatible with the maximum lengthening values that the connecting parts in particular can tolerate. However, this requires an expensive fragile device and a connection to the surface for processing the data. 
     Therefore, a problem that arises and which the present invention aims to solve is that of providing a device which enables the movements of a subsea deformable on-bottom pipeline to be measured and inspected, and at a favorable cost. 
     BRIEF DESCRIPTION OF THE INVENTION 
     With the aim of solving this problem, the present invention proposes a device for measuring the movement of a subsea deformable pipeline in relation to a sea bed, said subsea deformable pipeline being extended over said sea bed in order to transport liquids between two on-bottom installations, said subsea deformable pipeline being liable to deform according to the temperature of the liquids transported. The measuring device comprises an accommodating support anchored in said sea bed between said installations to accept said subsea deformable pipeline. When it deforms, subsea deformable pipeline is liable to be made to move over a determined travel with respect to the accommodating support. That movement has an amplitude that varies according to the deformation of said subsea pipeline. According to the invention, the device further comprises a collection or plurality of frangible elements secured to one of either of said subsea deformable pipeline and said accommodating support. The collection of frangible elements are arrayed along a mean direction that is substantially parallel with said determined travel. The frangible elements are intended to be broken in succession by the other of either of said subsea deformable pipeline and said accommodating support when said pipeline is caused to move over said determined travel, a measure or indication of said amplitude of said movement is a function of the number of broken frangible elements. 
     Therefore, one feature of the invention is the implementation of a collection of frangible elements, that can be located and observed, which, when the subsea deformable pipeline deforms both longitudinally and laterally, are broken in succession; wherein the number of broken frangible elements depends on the maximum deformation amplitude of the pipeline. Indeed, since the collection of frangible elements extends in and is arrayed along a direction that is parallel to the pipeline movement travel, the greater the deformation amplitude, the greater the number of broken frangible elements. The device in accordance with the invention therefore enables the measurement, by means of a viewing camera on board a robot for example, of the maximum amplitude, or maximum excursion, that occurs during the life of the oil field. This data is then compared with the values calculated during the design of the subsea on-bottom pipeline and the connecting ends thereof, in order to assess if they are compatible with the friction hypotheses put forward. Furthermore, such a measuring device is relatively inexpensive as it is extremely simple, and moreover it is reliable and robust. Furthermore, the measuring device in accordance with the invention cannot only be installed between a riser and an on-bottom pipeline, but also between two on-bottom pipelines. 
     According to a particularly advantageous embodiment of the invention, said collection of frangible elements includes rods, each rod having an end engaged in said subsea deformable pipeline or in said accommodating support and a free end projecting from said subsea deformable pipeline or from said accommodating support. In this manner, when the subsea pipeline deforms and is made to move, respectively, said accommodating support or said subsea deformable pipeline bears against the free ends of the rods and breaks them by shearing in synchronism with the relative movement of the subsea pipeline and said accommodating support. Advantageously, said rods have a groove or notch forming an incipient fracture, and this enables a brittle break of the rods when they are deformed by the relative movement of said accommodating support and the subsea pipeline. For better viewing, the free end of the rods is colored with a color that is distinct from the color of the sea bed, such that the images generated by the observation camera do not give rise to any doubt on the breakage or non-breakage of a rod. Indeed, when the rod is intact, the colored free end thereof appears clearly in the initial position thereof on the images of the observation camera. By contrast, when the rod has been broken, the colored free end thereof has generally been carried off by the on-bottom subsea currents, such that the remainder of the engaged broken rod simply shows a dot having a different color that contrasts with the other colored ends which remain intact. 
     Furthermore, each rod is kept oriented in its own direction that is substantially perpendicular to said determined travel, such that said subsea deformable pipeline or said accommodating support according to the embodiment, which bears on the free ends of rods, breaks the rods with maximum effectiveness. Furthermore, said rods are mounted and screwed into said one of either of said subsea deformable pipeline and said accommodating support, such as to make the mounting thereof simpler. Preferably, said rods are made of plastic, for example polyamide. In this manner, since this material is relatively rigid, and brittle, a minimum deformation of the rods causes the breakage thereof, and more specifically at the notch. 
     According to a particularly advantageous embodiment, said collection of frangible elements has at least one line of said rods, that are preferably evenly spaced, in a direction that is between the direction of said travel and a direction that is perpendicular to said travel, such as to be able to establish a relationship of proportionality between the number of broken rods and the amplitude of the movement of the subsea deformable pipeline. 
     According to a first alternative of the invention, that is particularly advantageous, said frangible elements are secured to said accommodating support, while said subsea deformable pipeline is suitable for breaking said frangible elements. In this manner, the collection of the frangible elements is kept in a fixed position in relation to the sea bed, and it is the movements of the deformable pipeline that break the frangible elements. To this end, and according to an advantageous feature, the measuring device in accordance with the invention further comprises a carriage slidingly mounted on said accommodating support, said pipeline being mounted securely to said carriage, and said carriage is suitable for breaking said frangible elements when said subsea pipeline is made to move and it drives, thereby, the carriage. 
     According to a second alternative, said frangible elements are secured to said subsea deformable pipeline, while said accommodating support is suitable for breaking said frangible elements. In this manner, it is the subsea pipeline which, in deforming, drives the frangible elements, which are then broken against said accommodating support which itself is kept in a fixed position on the sea bed. 
     Advantageously, in accordance with this second alternative, the measuring device further comprises a sleeve that fits tightly around said subsea deformable pipeline and that supports said frangible elements. The sleeve is then totally secured to the subsea pipeline and it is slidingly mounted inside a ring anchored on the sea bed. Said ring is then suitable for breaking said frangible elements when said subsea pipeline is made to move and it drives, thereby, the sleeve through the ring. 
     According to another subject matter, the present invention proposes a method for measuring the movement of a subsea deformable pipeline in relation to a sea bed, said subsea deformable pipeline being extended over said sea bed in order to transport liquids between two on-bottom installations, said subsea deformable pipeline being liable to deform according to the temperature of the liquids transported, said method being of the type according to which an accommodating support is provided that is anchored in said sea bed between said installations to accept said subsea deformable pipeline, said subsea deformable pipeline being liable to be made to move over a determined travel with respect to said support when it deforms, said movement having an amplitude that varies according to the deformation of said subsea pipeline; according to the invention, the measuring method further comprises the following steps: there is provided a collection of frangible elements secured to one of either of said subsea deformable pipeline and said accommodating support, said collection of frangible elements extending in a mean direction that is substantially parallel with said determined travel, said frangible elements being intended to be broken in succession by the other of either of said subsea deformable pipeline and said accommodating support, when said pipeline is caused to move over said determined travel, said amplitude of said movement is measured as a function of the number of broken frangible elements. This measurement is carried out visually, for example by means of an observation camera which generates images that can be then observed at the surface. The measurement involves counting the number of broken frangible elements in relation to the number of frangible elements initially installed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will emerge upon reading the following description of a particular embodiment of the invention, given indicatively but in a nonlimiting manner, with reference to the appended drawings in which: 
         FIG. 1A  is a schematic longitudinal and vertical section view of the device in accordance with the invention, according to a first alternative; 
         FIG. 1B  is a top schematic view of the device illustrated in  FIG. 1   
         FIG. 2  is a top schematic view of a first detailed element of the device illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic view of a second detailed element of the first detailed element shown in  FIG. 2 , and according to a perpendicular; and 
         FIG. 4  is a perspective schematic view of the device in accordance with the invention, according to a second alternative. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIGS. 1A and 1B  show a sea bed  10  on which rests an on-bottom pipeline  12  extended longitudinally in a given direction, a measuring device  14  in accordance with the invention and a riser  16  intended to join a surface installation. The measuring device  14  includes an accommodating support  18  anchored in the sea bed  10 . Installed on this accommodating support  18  is a carriage  20  that is longitudinally translationally moveable in a direction D that is substantially parallel with said given direction of the on-bottom pipeline  12 . The carriage  20  is translationally moveable with respect to the accommodating support  18 , which is provided with guiding means, that are not shown, in order to, precisely, translationally guide the carriage  20 . 
     The on-bottom pipeline  12  has a connecting end  22  kept in a fixed position on the moveable carriage  20 , via a clamp  24 . Therefore, the deformations of the on-bottom pipeline  12 , that are mainly linked to the thermal variations to which it is subjected, cause lengthening or retraction of this on-bottom pipeline  12  which then in turn makes the connecting end  22  move longitudinally in the direction D, and consequently, the carriage  20  to which it is secured. The carriage  20  is therefore made to move alternately over a determined travel in the course of the thermal variations of the on-bottom pipeline  12 . Of course, this alternate movement of the carriage  20  can be for relatively long periods which can amount to several months or even several years. The measuring device  14  in accordance with the invention then enables the amplitude of these alternate movements to be measured by means of a collection  26  of frangible elements comprising plastic rods  28 . It will be seen that the collection  26  of frangible elements extends in a mean direction that is substantially parallel with the direction of the alternate movements. 
     These rods  28  are made of plastic, polyamide for example, and are screwed on a front face  29  of a support plate  30 , that is installed substantially horizontally on the accommodating support  18  and is fixed there. The advantage of polyamide is the rigidity thereof and, as a result, the ability thereof to fracture with a brittle break. The carriage  20  covers the support plate  30  and has a window  32  through which the rods  28  extend and project. Furthermore, the two transverse opposite edges  34 ,  36  of the window  32  form two opposite cutter bars that are substantially perpendicular to the direction D of movement of the carriage  20 . These two transverse opposite edges  34 ,  36  are then liable to be translated flush with the front face  29  of the support plate  30 . Therefore, it is understood that the lengthening of the on-bottom pipeline  12 , due to an increase in the temperature of the liquid or of the hydrocarbon passing through the pipeline, will then push back the carriage  20  in a direction V opposite the on-bottom pipeline  12  and, consequently, one edge  34  of the two transverse opposite edges will break the rods  28 . If the temperature of the hydrocarbon drops back to a normal operating temperature, then the on-bottom pipeline  12  retracts, and drives the carriage  20  in an opposite direction P. 
     Reference will now be made to  FIG. 3  in order to describe in greater detail the method of fixing the rods  28  on the support plate  30 .  FIG. 3  shows a partial view of the support plate  30  with the front face  29  thereof in which a tapped opening  38  is provided in a substantially perpendicular manner. This opening has a depth e, that is less than half the thickness of the plate  30 , and it is extended by a channel  40  that opens onto a back face  42  of the support plate  30 . Furthermore, the rod  28  is made up of a threaded rod  43  with a screwing head  44  on top. The rod  28  has an end  46  screwed into the opening  38  and a free end supporting the screwing head  44 . It will be seen that the screwing head  44  precisely enables the end  46  to be screwed into the opening  38 . Furthermore, the threaded rod  43  has a groove  50  with a depth of approximately 2 mm, forming a notch between the end  46  engaged in the support plate  30  and the free end  48 . This groove  50 , which forms an incipient fracture, enables an easier break, with less force, of the threaded rod  43  when one of the transverse opposite edges  34 ,  36  strikes the free end  48  of the rod  28 . Furthermore, the channel  40  enables the opening  38  to be brought to hydrostatic pressure when the support plate  30  that is provided with the rods  28  thereof is installed in the sea bed. In this manner, the breaking of the threaded rod  43  is even more brittle. 
     Reference will now be made to  FIG. 2 , which illustrates, from above, the support plate  30 , though which a plurality of openings  38  is made, with a specific geometry as will be described hereafter. As an example, this support plate  30  has a width W of 340 mm for a length L of 500 mm and a thickness of 50 mm. The tapped openings  38 , made in the support plate  30 , have a diameter of 15 mm. Above all, they are made in a series of lines  52 ,  54 ,  56 ,  58  that are parallel with each other and slanted at 90° in relation to the length L of the support plate  30 . Along the lines thereof, the openings  38  are spaced apart from each other by a distance of approximately 35 mm, whereas along the length L, the openings  38  are spaced, from one series to another, by a distance of 100 mm. Furthermore, along the width W, there is always two openings  38  of two adjacent series corresponding, which define a row d that is parallel with the width W. The collection of the openings  38  extends in a mean direction that is substantially parallel with the length L. 
     Therefore, each of the openings  38  of the 32 openings in total shown in  FIG. 2 , of the support plate  30 , have a rod  28  of the type illustrated in  FIG. 3  screwed therein. Furthermore, the screwing heads  44  are colored with a color that is distinct from the color of the sea bed. 
     Thus, returning to the embodiment illustrated in  FIGS. 1A and 1B , where the carriage  20  is translated over the accommodating support  18  in the direction V opposite the on-bottom pipeline  12 , the transverse edge  34  of the window  32  would then simultaneously press against the two rods of the first row r 1  of the support plate  30  illustrated in  FIG. 2 , and would also, as the carriage  20  moves, break them simultaneously by shearing at the groove  50 . While continuing the travel thereof, the transverse edge  34  of the window  32  would then press simultaneously against the two rods of a second row r 2  adjacent to the first row r 1  in order to break them in turn. 
     The same applies to the following rows, r 3  up to r 16 , assuming that the amplitude of movement of the carriage  20  is substantially equal to the length L of the support plate  30 . The rows of rods  28  are evenly spaced from each other by a value of 20 mm. The presence of two rods per row limits the risk of a rod being wrongly broken by the relative movement of the carriage  20  and the accommodating support  18 . If this risk does not exist, it is pointless keeping two rods  28  per row; if however there is a considerable risk, it is appropriate to provide more than two rods  28  per row. 
     In reality, the support plate  30  is oversized so that a certain number of rods  28  can be kept intact on the support plate  30 , and they can be viewed thanks to the colored screwing head  44  thereof, compared to the already broken rods. Therefore, when the on-bottom pipeline  12  is put into operation, for a determined period, for example 12 months, the carriage  20  will have been able to move back and forward on the accommodating support  18  depending on the temperature of the hydrocarbon which flowed inside over time, and reach a maximum amplitude corresponding to a maximum of rows of broken rods  28 . 
     In this manner, when, after 12 months, the support plate  30  is inspected by means of a viewing camera, the number of rows of intact rods remaining in relation to the initial number of rows is then observed, and the maximum amplitude of the movement of the carriage  20 , and consequently of the connecting end  22 , is deduced therefrom. In the example shown in  FIG. 2 , in the case where, for example, seven rows, r 1  to r 7 , of rods  28  have disappeared, while the other rows are intact, it is deduced therefrom that the maximum excursion, or maximum amplitude, of the carriage  20  on the accommodating support  18 , is 140 mm. 
     Reference will now be made to  FIG. 4  which illustrates a second alternative of the invention, according to which the frangible elements, which are also formed by rods, are no longer secured to the accommodating support  18  but to the on-bottom pipeline. 
     With the aim of facilitating the description of this alternative, the similar elements of the measuring device which were illustrated in the previous figures, and which have the same functions, have the same reference assigned with a prime mark: “′”. 
     Therefore,  FIG. 4  shows an on-bottom pipeline  12 ′ portion engaged in a longitudinal sleeve  30 ′. This longitudinal sleeve is kept in a fixed position on the on-bottom pipeline  12 ′ via clamps  60 . Furthermore, four lines  52 ′,  54 ′,  56 ′,  58 ′ of rods that are respectively diametrically opposite in twos are screwed into the thickness of the longitudinal sleeve  30 ′. 
     The accommodating support is made up of a ring  18 ′ provided with a border which surrounds it in a secured manner. This ring  18 ′ is anchored on the sea bed by partially burying said border. It is then mounted in a fixed position in relation to the sea bed. Alternatively, it can be installed on a base that is not shown. The ring  18 ′ allows the longitudinal sleeve  30 ′ to slide when the latter is driven by the on-bottom pipeline  12 ′. Furthermore, the ring  18 ′ has two circular opposite shearing edges  34 ′,  36 ′ that are intended to break the rods  28 ′ when the sleeve  30 ′ is translated through the ring  20 ′. 
     Moreover, according to yet another alternative of the invention that is not shown, where the aim is to measure not exclusively the longitudinal deformations of an on-bottom pipeline but rather the lateral deformations thereof, the carriage that is translationally moveable on an accommodating support can be installed in a direction that is transverse to the on-bottom pipeline. In this manner, the lateral movements of the pipeline cause movement of the moveable carriage, which, itself, causes the frangible elements to break. 
     Of course, the embodiments described above are in no way limiting, and any other embodiment can be envisaged. In particular, such a measuring device can be installed between two interconnected on-bottom pipelines.