Abstract:
A system for starting up a flowline suitable for conveying hydrocarbons is provided. The flowline is extended over the seabed from a wellhead and terminated at a joint end. The joint end is suitable for connection to a subsea riser extended in a catenary. The flowline is able to stretch and a lock fixes the joint end with respect to the seabed for maintaining the flowline in its stretched position, and preventing its return movement.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/FR2005/000558, filed 9 Mar. 2005, which claims priority of French Application No. 0402695, filed 16 Mar. 2004. The PCT International Application was published in the French language. 
     BACKGROUND OF THE INVENTION 
     The present invention refers to a method and a system for starting up a flowline suitable for conveying hydrocarbons for offshore production. 
     According to previously known methods, the flowline, a rigid-type pipeline for example, is first extended over the seabed and connected to an oil production wellhead or to a multiplicity of wellheads and its joint end is then connected to the free end of a subsea riser extended in a catenary, which can itself be rigid. This subsea riser is intended for linking to a surface installation, such as a platform for example. 
     To make this connection, a known method involves connecting horizontally the subsea riser to the joint end of the flowline, essentially parallel to the seabed surface. 
     After making the connections, flowline start-up involves conveying hydrocarbon, which arrives at the wellhead, through the flowline and the subsea riser up to the surface installation. 
     The subsea riser is therefore placed completely on the seabed before making the connection using remote-controlled robots (Remotely Operated Vehicle or ROV), which position themselves either on the flowline for pulling the subsea riser with a cable or on the free end of the subsea riser for drawing it as far as the joint end of the flowline. The other end of the subsea riser is then raised up to the surface installation. 
     Such a method is notably illustrated in document U.S. Pat. No. 5,975,803. However, other known methods allow the same result to be achieved. 
     The subsea riser therefore has a free end, which is connected to said joint end of said flowline, a first section, which extends over the seabed from this free end and a rising section, which extends asymptotically, right below the surface installation. A so-called touch-down zone, which corresponds to a dynamic area of the riser, between the first section and the rising section is able to feature a touch-down point which, in turn, is pulled against the seabed, especially by surface installation movements. 
     The flow line, for its part, is able to deform longitudinally. 
     In a rigid pipeline, the hydrocarbon passing through the wellhead is, moreover, hot, which causes expansion and elongation of the flowline. In flexible flowlines, injection of fluid under pressure inside also causes their elongation. 
     The result of this is that movement of the joint end of the flowline and the first section of the subsea riser is produced over a distance of the order of 1 meter, for example 5 meters. 
     However, the hydrocarbon pressure is not necessarily constant throughout the flowline and, furthermore, supply to the flowline can cease either because of an incident or to allow performance of a maintenance operation at the wellhead. 
     There is therefore a drawback in dimensional variations of the flowline associated with heating or cooling of the flowline or a pressure variation, which can then cause not only friction of said first section of subsea riser on the seabed, which can be abrasive, but also variation in the radius of curvature of the subsea riser between this section and the rising section or even displacement of the touch-down point position on the pipeline. These drawbacks contribute to its damage in particular. 
     According to the prior art illustrated in  FIG. 1A , the abovementioned drawbacks are overcome by connecting the riser to the flowline using a flexible pipe. This  FIG. 1A  shows a riser  1 , which extends between a free end anchored in the seabed and a surface installation  2 , and a flowline  3 , whose joint end  4 , also anchored, is connected to the riser  1  by a flexible pipe  5 . A first section  6  of the riser  1  is moored to the blocks  7  outside the axis of the flowline  3  such that dimensional variations in the flowline  3  are compensated by deformation of the flexible pipe  5 . The latter can, moreover, comprise a rigid U-shaped pipe member capable of deformation. 
     Implementation of such a system is uneconomical, given the different anchor points to be created. 
     A problem raised, which the present invention aims to solve, is to propose a method and system for starting up a flowline, which allows the service life of subsea risers to be prolonged by preventing their friction on the seabed and their deformation, at an advantageous cost. 
     SUMMARY OF THE INVENTION 
     With this purpose and according to a first object, the present invention proposes a method for starting up a flowline suitable for conveying hydrocarbons, said flowline being extended over the seabed from a wellhead and terminating at a joint end, said joint end being suitable for connection to a subsea riser extended in a catenary, said method comprising a first stage, in which elongation of said flowline is induced; said method comprising moreover a second stage, in which said joint end is fixed with respect to said seabed to maintain said flowline in its stretched position. 
     One feature of the invention therefore resides in fixing the joint end of the flowline after it has been expanded and stretched, for example when hot fluid is made to circulate through said flowline, this hot fluid being, for example, hot water injected under pressure to test the flowline. In this way, when the flowline cools because the hot water is no longer injected or the initially hot hydrocarbon stagnates in the flowline, the latter tends to retract, which induces internal stresses because it is restrained at its joint end, but its longitudinal dimensions do not vary. As a result, flowline deformations no longer cause movements of the subsea riser, which is connected to it by the joint end, such that the latter is no longer damaged. 
     In a particularly advantageous way, displacement of said joint end is authorized in the direction of elongation of said flowline and displacement of said joint end is prohibited in the opposite direction. During said first stage, as the flowline deforms under the effect of temperature and stretches, its joint end therefore moves in translation and as soon as it cools and tends to retract, it is restrained under stress by the joint end, which itself is kept fixed in translation in the direction of retraction of the flowline. In this way, the flowline can be fixed in its maximum state of deformation. 
     Preferably, said joint end is guided in translation during elongation of said flowline such that the shape of the locking means, which will be hereafter described, can be simplified. 
     Moreover, at an advantageous preliminary stage, said flowline is laid on the seabed and said subsea riser is installed, then said subsea riser is connected to the joint end during said preliminary stage. 
     According to another object, the present invention proposes a system for starting up a flowline suitable for conveying hydrocarbons, said flowline being extended over the seabed from a wellhead and terminating at a joint end, said joint end being suitable for connection to a subsea riser extended in a catenary, said flowline being able to stretch; said system comprising locking means for fixing said joint end with respect to said seabed for maintaining said flowline in its stretched position. 
     Locking means, which are described in the remainder of the description, are solidly fixed to the seabed and are perfectly anchored therein, allow the joint end to be kept in a fixed position with respect to said seabed. 
     According to a particular embodiment, said locking means include unidirectional arresting means suited to allowing displacement of said joint end in the direction of elongation of said flowline and to prohibiting displacement of said joint end in the opposite direction. In this way, the joint end can move in the direction opposite to the elongating flowline, which pulls it, and, on the other hand, it is immobilized in the opposite direction with respect to the seabed. 
     Preferably, the system comprises guidance means including a moving trolley, to which said joint end is suitable for connection, said moving trolley being able to slide on means forming rails. In this way, the joint end is perfectly guided in translation on the seabed in one direction, although the flowline does not necessarily deform uniformly and in one direction. 
     The system comprises a base anchored in the seabed, said rails being solidly fixed to said base, to anchor perfectly the trolley and the joint end connected thereto. Clearly, such a base is suitable for withstanding the traction exerted by the flowline while remaining immobile with respect to the seabed. 
     According to one particularly advantageous implementation feature of the invention, said unidirectional means comprise means forming a rack, mounted in the direction of said means forming rails, and means forming a ratchet mounted on said trolley, said means forming a ratchet being suitable for engagement in said means forming a rack. The means forming a ratchet are therefore suitable for inclined engagement between two teeth of the rack for inhibiting the trolley in translation in one direction and from swinging over the teeth, when the trolley is pulled in the opposite direction by flowline elongation. 
     According to a particularly advantageous embodiment of the invention, said trolley comprises first reception means suitable for receiving said joint end of the flowline. In this way, as will be explained in greater detail in the remainder of the description, the flowline terminated by its joint end is suitable for installation in the reception means, which themselves are pre-installed, by unwinding said flowline from a surface vessel. 
     Furthermore, according to a particular embodiment of the invention, said trolley comprises, moreover, second reception means suitable for receiving a free end of said subsea riser for interconnecting said joint end and said free end after installation of the flowline. 
     Other particular features and advantages of the invention will become apparent on reading the description given below concerning particular embodiments of the invention, provided for information only but unlimited, in reference to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagrammatic vertical cross-sectional view of a system according to the prior art; 
         FIG. 1B  is a diagrammatic vertical cross-sectional view of a system according to the invention; 
         FIG. 2  is a diagrammatic elevation view illustrating elements of the system represented in  FIG. 1B  during a first phase of a preliminary stage of the installation method; 
         FIG. 3  is a diagrammatic elevation view illustrating elements of the system represented in  FIG. 1B  during a second phase of the preliminary stage; 
         FIG. 4  is a diagrammatic elevation view illustrating elements of the system represented in  FIGS. 2 and 3  at the end of the second phase; 
         FIG. 5  is a diagrammatic view of a vertical section V-V through an element illustrated in  FIG. 4 ; 
         FIG. 6  is a diagrammatic plan view of elements illustrated in  FIG. 4 ; 
         FIG. 7  illustrates the system during an intermediate connection phase between the preliminary stage and a first stage, at which hot fluid is made to circulate through the flowline; 
         FIG. 8  is a diagrammatic view of details showing two possible states of an element represented in  FIG. 7 ; 
         FIG. 9  is a diagrammatic elevation view of the system illustrated in  FIG. 7  at a second stage; 
         FIG. 10  is a diagrammatic elevation view of the system illustrated in  FIG. 9  at an adjustment phase; and 
         FIG. 11  shows elements of a system according to an alternative embodiment during a preliminary phase of the implemented method. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Structural elements of the system according to the invention and, in parallel, its method of installation will be described with reference to  FIGS. 1 to 10 . 
       FIG. 1B  illustrates a system according to the invention between a wellhead  10  located on the seabed  12  and a surface installation  14  suitable for recovering a hydrocarbon and which is floating on the water surface  15 . This system comprises a flowline  16  terminating in a joint end  17 , which rests on the seabed  12 , and a subsea riser  18 , which prolongs it which joins the surface installation  14 . 
     Prior to connection, this riser  18  has a free end  20 , a first section  22 , which rests on the seabed, and an asymptotic section  24 , which joins the surface installation  14 . 
     An essential feature of the invention, which will be detailed hereafter, resides in the connection means  26  between the flowline  16  and the riser  18 , which allow prevention of relative movements of the joint end  17  and thus of the riser  18  resulting from deformation of the flowline under the effect of temperature or pressure variations. 
     Reference is now made to  FIGS. 2 and 3  illustrating in detail the method of installing the flowline  16  and the riser  18 . 
       FIG. 2  shows the seabed  12  and the joint end  17  of the flowline  16  during installation. 
     Prior to installation, a block or base  28  is anchored in the seabed  12  at a determined distance from the wellhead  10 . This base  28  has rails  30  or slideways and a moving trolley  32  running on these rails  30  in a direction oriented toward the wellhead  10  and which will be detailed in the remainder of the description. The trolley  32  is here in an inactive initial position offset toward the wellhead  10 . Furthermore, the trolley  32  has converging tubular reception means  34  comprising two reception parts  35  forming a funnel, suitable for receiving a first connection unit  36 , which is itself mounted swiveling at the joint end  17 . 
     The flowline  16  is therefore laid from an unrepresented surface vessel  18  located above the base and it then extends more or less vertically between the water surface  15  and the seabed  12 . The joint end  17  is then guided, whilst laying the flowline, such that the connection unit  36  is vertically adjusted within the reception part  35 , located toward the wellhead. It is understood that all the well-known methods of laying rigid pipes are suitable for implementation, especially the methods of laying known as “J” or “S” methods. 
     Subsequently, the entire flowline  16  is laid out toward the wellhead  10 , and it swivels at its joint end which is then held in a fixed position in a substantially horizontal plane, parallel to the seabed  12 , by virtue of the connection unit  36  which is itself held in the reception part  35 , itself solidly fixed to the trolley  32  and the base  28 . 
       FIG. 3  shows the flowline  16  installed in this way. After swiveling the flowline  16 , the joint end  17  is released and oriented in the direction of the other connection part  35 . 
     The riser  18 , which has a second connection unit  38  mounted swiveling on the free end  20 , will be similarly installed and connected to the other reception part  35 . After unwinding the riser  18  such that its first section  22  is laid in contact with the seabed  12  in line with the flowline  16  but in the opposite direction, its free end, which has swiveled, is then positioned facing the joint end  17  of the flowline  16 . 
       FIGS. 4 and 5  show the joint end  17  facing and at a distance of one meter, for example, from the free end  20 , the trolley  32  itself always remaining in its initial inactive position. 
     Moreover, the joint end  17  and/or the free end  20  are respectively mounted sliding in a sleeve, which itself is mounted swiveling on connection units  36  and  38  respectively. In this way, using a Remotely Operated Vehicle (ROV), the two facing ends  17  and  20  will be brought close to one another and imperviously interconnected to ensure passing of hydrocarbon from the flowline  16  to the riser  18  without leakage. 
       FIG. 7  also shows the two pipelines  16  and  18  connected by their ends  17  and  20 . The preceding installation phases constitute a preliminary stage of the implementation method and, from the position in which the pipelines are connected, as illustrated in  FIG. 7 , a second stage involves starting up the pipelines by injecting hot water, for example, into the flowline  16  from the wellhead  10  for checking and testing all the pipelines. The hot water is thereby recovered at a surface end of the riser  18 . 
     In the same way as the hot hydrocarbon, which will circulate through it, this hot fluid causes elongation of the flowline  16  such that the joint end  17  and the free end  20 , which is now connected to it, are caused to move in a direction opposite to the wellhead  10 , in the direction of arrow F. 
     The trolley  32 , which is solidly fixed in translation to the pipeline ends will therefore be similarly driven in translation along the rails  30  or slideways from its initial position, as also represented in  FIG. 7  and in the direction of arrow F. 
     However, displacement in translation of the trolley  32  in the direction of arrow F away from the wellhead, is authorized, while displacement toward the wellhead is prohibited due to unidirectional arresting means  39 , which will be described in detail with reference to  FIGS. 5 and 8 . 
     Dimensional variations in the flowline occur here after installation and connection of the flowline and the riser, which are then started up. Nevertheless, according to an alternative embodiment, installation and pressure testing of the flowline is envisaged prior to its connection. In this case, the joint end of the flowline is inhibited in translation in the same way as before, after elongating the flowline by pressurization. The riser is then able to be installed and connected to the flowline. 
       FIG. 5  illustrates a vertical cross section of the trolley  32  and the base  28 , on which the trolley rests through the rails  30 . Furthermore, a rack  42  is mounted solidly on the base  28  on each side of the trolley  32  near the rails  30  and extends parallel to the rails  30  with teeth oriented in the direction of arrow F. 
       FIG. 8  shows part of the rack  40  and part of the chassis of trolley  32 , which is suitable for sliding parallel to the rack  40 . Moreover,  FIG. 8  illustrates a moving part  42  forming a ratchet in two possible states. A first state shown in  FIG. 8A , in which the moving part  42  has swung over a tooth of the rack  40  and a second state shown in  FIG. 8B , in which the moving part  42  is engaged between two teeth of the rack  40 . Trolley  32  is therefore capable of being driven in translation in the direction of arrow F, the moving part  42  then being suited to swinging over the teeth and being inhibited in the direction opposite to arrow F, the moving part  42  then being engaged between the teeth. 
     Furthermore, the trolley  32  is sufficiently heavy and is maintained so as to slide on the rails  30  in such a way that it is inhibited vertically, in order, on the one hand, to prevent the trolley  32  from leaving the rails  30  and, on the other hand, to prevent the moving parts  42  escaping from the racks  40 . 
     Thus, the trolley  32  is caused to move in the direction of arrow F, away from the wellhead  10 , as the flowline deforms longitudinally. On the other hand, as soon as the temperature of the fluid passing through the flowline  16  reduces, it is inhibited by trolley  32 , which is itself inhibited in translation. 
     Therefore, in a situation of maximum deformation, as illustrated in  FIG. 9 , the trolley  32  is locked in a maximum active position and cannot return to its initial position unless the moving parts  42  are released. As a result, once the system has been started up under normal operating conditions, temperature or pressure variations in the flowline no longer cause elongation of the flowline, which is then fixed in its position of maximum elongation. 
     Nevertheless, displacement means illustrated in  FIG. 10  are provided and are intended for re-adjusting the position of the trolley on the base  28  to offset the position of the touch-down point on the riser to prolong its service life. The point of contact of the riser on the seabed is therefore displaced by varying the relative position of the free end of the riser  18  and of the surface installation. 
     These displacement means comprise, for example, a hydraulic pressure cylinder with one end solidly fixed to the rails  30  and the other end solidly fixed to the trolley  32 . 
     Moreover, according to another alternative embodiment of the invention, illustrated in  FIG. 11  showing the base  28 , the flowline  16  and the riser  18  during installation, the joint ends  20  and  17  are pre-connected on the laying vessel and the connection, which is fitted with a single connection unit  48  to be adjusted in a single reception part  50 , is guided toward the base  28  using a sling  52 . 
     Thereafter and in the same way as in the preceding variation, the reception part  50 , which is solidly fixed to a trolley, will be driven in translation in the direction of arrow F under the effect of elongation of the flowline  16  and will be inhibited in translation in the opposite direction.