Abstract:
A pipeline for the transportation of a cryogenic liquid, thermally insulated and comprising three coaxial pipes formed of sections, one inner pipe made of a material having a low thermal dilation coefficient, a middle pipe and an outer pipe made of steel, the annulus space between the inner pipe and the middle pipe being provided with thermal insulation, wherein: the pipes are linked two by two using linking parts so as firstly to join the sections of each respective pipe and secondly to take up the loads produced by thermal contraction when the pipes change temperature. the section of the middle pipe is of a sufficient length such that when assembled, in cooperation with the linking parts, it prevents the formation of a thermal bridge between the inner and outer pipes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The technical scope of the present invention is that of pipelines for the transportation of very low temperature fluids, and in particular for the transportation of liquefied natural gas under ground, in area or under sea.  
         [0003]     2. Description of the Related Art  
         [0004]     Patent FR-2748545 describes a pipeline for the transportation of liquefied natural gas comprising two coaxial pipes. This type of pipeline has thermal insulation properties but also suffers from drawbacks. Indeed, the contraction of the inner pipe (by around 1 m for a pipe of a length of 4,000 m) requires the use of a steel linking part between the two pipes placed before the end bend of the straight part, such a linking part creating a thermal bridge and resulting in: 
        a deterioration in the thermal efficiency of the assembly,     a lowering in the temperature of the external pipe, thereby embrittling it and making it sensitive to impacts and concentrations of stress.        
 
         [0007]     Moreover, the use of a simple external envelope cannot be considered sufficient in terms of reliability.  
       SUMMARY OF THE INVENTION  
       [0008]     The aim of the present invention is thus to supply a pipeline for the transportation of cryogenic liquids, such as liquefied natural gas, that has efficient thermal insulation, with no thermal bridges at the ends of the pipe sections and which is extremely reliable.  
         [0009]     The invention thus relates to a pipeline for the transportation of a cryogenic liquid, thermally insulated and comprising three coaxial pipes formed of sections, one inner pipe made of a material having a low thermal dilation coefficient, a middle pipe and an outer pipe made of steel, the annulus space between the inner pipe and the middle pipe being provided with thermal insulation, wherein: 
        the pipes are linked two by two using linking parts so as firstly to join the sections of each respective pipe and secondly to take up the loads produced by thermal contraction when the pipes change temperature,     the section of the middle pipe is of a sufficient length such that when assembled, in cooperation with the linking parts, it prevents the formation of a thermal bridge between the inner and outer pipes.        
 
         [0012]     According to one characteristic, the annulus space between the middle pipe and the outer pipe is provided with thermal insulation.  
         [0013]     According to another embodiment of the invention, the annulus space between the middle pipe and the outer pipe is filled with a gas that is pressurized or depressurized with respect to the pressure of the surrounding sub-marine environment.  
         [0014]     According to yet another characteristic, the linking parts are in the shape of two Us joined at their base and incorporating a drill hole parallel to the axis of the pipes so as to allow the gas to circulate between the different sections of a same ring and thereby allow said annulus to be pressurized or depressurized.  
         [0015]     According to yet another characteristic, the linking parts connecting the inner pipe -and the middle pipe are fastened respectively by tubular spacers having a thermal dilation coefficient situated between the thermal dilation coefficient of the linking part and that of the inner and middle pipes connected to it.  
         [0016]     According to another characteristic: 
        the inner pipe has a wall thickness of less than approximately 8 mm,     the middle pipe has a wall thickness of less than 15 mm and more than 6 mm approximately, and     the outer pipe has a wall thickness of over 12 mm approximately.        
 
         [0020]     According to another characteristic, the inner pipe is made of a material whose dilation coefficient is less than 2.10 −6  m/m/K for temperatures of between −162° C. and 20° C.  
         [0021]     According to another characteristic, the inner pipe is made of an iron-nickel alloy incorporating a proportion of nickel of around 36%.  
         [0022]     According to another characteristic, the middle pipe is made of a material adapted to low temperatures, for example stainless steel or 9% nickel alloyed steel.  
         [0023]     According to another characteristic, the thermal insulation is a silica-based insulating material under controlled industrial vacuum.  
         [0024]     A first advantage of the pipeline according to the invention lies in the considerable reduction in the risk of the pipe failure.  
         [0025]     Another advantage lies in the fact that the linking part between two adjacent pipes is able to withstand differential thermal contraction.  
         [0026]     Yet another advantage lies in the fact that the inspection of the second annulus allows the condition of the pipeline to be verified both reliably and at little cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     Other characteristics, particulars and advantages of the invention will become more apparent from the description given hereafter by way of illustration and in reference to the drawings, in which:  
         [0028]      FIG. 1  is a longitudinal section of one embodiment of the invention,  
         [0029]      FIG. 2  is a longitudinal section of another embodiment, and  
         [0030]      FIG. 3  shows an embodiment of the link between two pipes according to the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     Generally speaking, the pipelines in question are used to carry a cryogenic liquid from one point to another, for example to fill or empty a ship of liquefied gas when it is far from the liquefaction plant. The pipelines are thus immersed in the sea and must rise vertically by means of a bend. The variations in length of the part of the pipeline immersed in the sea or on supports leads to variations in the position of the vertical upright part of the pipeline. These variations cause stresses in the vertical part which must be controlled. The same stresses must be controlled when the pipeline is under ground or in area.  
         [0032]      FIG. 1  partially shows a pipeline  1 , sectioned along the longitudinal axis XY, intended for the transportation of a cryogenic liquid formed of three pipes: an inner pipe  2  inside which the cryogenic fluid circulates, a middle pipe  3  and an outer pipe  4 . Each of these pipes is formed of sections  2   a  and  2   b;    3   a ,  3   b  and  3   c;    4   a  and  4   b  which are welded end to end to make the full pipeline. According to the invention, the pipe sections are joined using parts  5  and  6  that take up the load. The purpose of these parts  5  and  6  is to limit, according to one of the essential aspects of the invention, the loads applied to the end of the pipe. This proves particularly important when the pipe has a bend. The bent part of the pipeline is not shown in the drawing, but the expert will easily understand the advantage of reducing the contraction of this pipeline.  
         [0033]     The inner pipe  2  is delimited in the Figure by two sections  2   a  and  2   b , the middle pipe  3  by three sections  3   a ,  3   b  and  3   c  and the outer pipe  4  by two sections  4   a  and  4   b.    
         [0034]     Sections  2   a ,  3   a  and  4   a  are, for example, the ends of a bend, and sections  2   b ,  3   c  and  4   b  the ends of the undersea pipeline that may be several kilometers long. Other arrangements may naturally also be envisaged, for example by mounting one after another several similar devices such as will be described hereafter.  
         [0035]     The sections of the inner pipe  2  and middle pipe  3  are joined by part  5  and the sections of middle pipe  3  and outer pipe  4  by part  6 . The linking parts  5  and  6  have a section in the shape of two U joined at their base. Naturally, each pipe section is welded to the corresponding branch of the U, the inner pipes  2  and middle pipes  3  being respectively attached to the inner  5   a  and outer  5   b  branches of part  5  and the middle pipe  3  and outer pipe are respectively attached to the inner  6   a  and outer  6   b  branches of part  6 .  
         [0036]     The inner diameter of part  5  is equal to the inner diameter of the inner pipe  2  such that there is a perfect join between these two elements. The outer diameter of part  5  is equal to the outer diameter of the middle pipe  3  such that there is also a perfect join here. The distance between the two branches  5   a  and  5   b  of the U is determined according to that desired between the inner pipe  2  and outer pipe  3 . This distance will define the thickness of insulation  7  to be incorporated between the two pipes  2  and  3 .  
         [0037]     The inner diameter of part  6  is equal to the inner diameter of middle pipe  3  such that there is a perfect join between them. The outer diameter of part  6  is equal to the outer diameter of outer pipe  4  such that there is once again a perfect join. The distance between the two branches of the U of part  6  is also determined according to the distance desired between the middle pipe  3  and outer pipe  4 .  
         [0038]     Part  5  incorporates a drill hole  51  with an axis parallel to axis XY of the pipes. This drill hole  51  allows the circulation of air between sections  11   a  and  11   b  of the first ring  11 , located between the inner pipe  2  and the middle pipe  3 , thereby allowing said ring  11  to be pressurized or depressurized.  
         [0039]     Part  6  also incorporates a drill hole  61 , parallel to axis XY of the pipes. This drill hole  61  allows the circulation of air between sections  10   a  and  10   b  of the second ring  12 , located between the outer pipe  4  and the middle pipe  3 , thereby allowing said ring  12  to be pressurized or depressurized.  
         [0040]      FIG. 1  shows that parts  5  and  6  are not placed one after the other around the diameter. Indeed, according to the invention, the creation of a thermal bridge between inner pipe  2  and outer pipe  4  is prevented. Indeed, a single linking part between the three pipes would cause substantial local loss in frigories and a substantial lowering of the temperature of the outer pipe  4  (which may drop to temperatures of under −100° C.). Since the outer pipe is made of carbon steel for financial reasons, its temperature cannot drop to under around −50° C. if good ductility is to be preserved. The invention proposes a solution that is particularly advantageous in that it is unnecessary for a single linking part to be insulated which, on the inner side, would be subjected to cryogenic temperatures and, on the outer side, to the marine environment. Additionally, this part must absorb the shearing stresses linked to the displacements of the pipeline under the effect of the thermal contractions, thereby further complicating the solution.  
         [0041]     Thus, the production of a three-pipe pipeline incorporating a separate linking part to join the pipes two by two enables localized thermal problems to be overcome. Lastly, the introduction of a third pipe helps to rigidify the pipeline, thereby significantly improving its reliability. It is easy to accept that the probability of the simultaneous failure of the middle and outer pipes is almost inexistent. The second annulus  10  allows the condition of the pipeline to be controlled by depressurizing or pressurizing it.  
         [0042]     Insulation  7  is placed between the inner pipes  2  and middle pipes  3 . Annulus  10 , located between the middle  3  and outer  4  pipes, is filled, in this embodiment, with a gas that can be pressurized with respect to the undersea pressure. This gas is, for example, nitrogen. The use of a pressurized gas namely allows any water leakage to be detected. The continuous monitoring of this pressure enables the simple and inexpensive monitoring of the condition of the pipeline.  
         [0043]     So as to limit the thermal losses from the linking part  5  it is also possible for insulation  9  to be placed around the part  5  and the middle pipe  3 . For example, a sleeve of a length  1  of 0.5 to 1.5 m approximately is largely sufficient to ensure this insulation.  
         [0044]     A variant embodiment of the invention, shown in  FIG. 2 , consists in placing insulation  8  in the annulus  10  located between the middle pipe  3  and outer pipe  4 . This arrangement does not prevent the introduction of a pressurized or depressurized gas as explained above.  
         [0045]     Insulations  7 ,  8  and  9  are, for example, silica-based or microporous under controlled industrial vacuum either individually packed or along the length of the annulus.  
         [0046]     So as to reduce the circumferential stresses in the welds, due to the thermal contractions and dilations, it may be advantageous for the linking parts  5  to have a thermal dilation coefficient of between the thermal dilation coefficient of the linking part  5  and that of the inner  2  and middle  3  pipes connected to it. It may be difficult to obtain such parts.  FIG. 3  thus shows an embodiment enabling the performances of the pipeline to be improved. In this embodiment, the linking parts  5  connecting the inner pipe  2  to the middle pipe  3  are fastened to pipes  2  and  3  respectively by tubular spacers  52  and  53  having a thermal dilation coefficient of between the thermal dilation coefficient of the linking parts  5  and that of the inner  2  and middle  3  pipes connected to it. Spacer  53 , of outer and inner diameters respectively identical to the outer and inner diameters of pipe  3 , is placed between part  5  and sections  3   a  and  3   b  of pipe  3 . Spacer  52  of outer and inner diameters respectively identical to the outer and inner diameters of pipe  2 , is placed between part  5  and sections  2   a  and  2   b  of pipe  2 .  
         [0047]     Such an arrangement may also be applied to linking part  6 . But since the difference in temperature between the middle  3  and outer  4  pipes is less than the temperature difference between the middle  3  and inner  2  pipes, such an arrangement is not strictly speaking required.  
         [0048]     By way of a non-exhaustive example, the inner pipe  2  has a wall thickness of less than approximately 8 mm, the middle pipe  3  has a wall thickness of between approximately 6 and 15 mm, and the outer pipe  4  has a wall thickness of over approximately 12 mm.  
         [0049]     The inner pipe is advantageously made of an iron/Nickel alloy, with a Nickel component of around 36%, of which the dilation coefficient is less than 2.10 −6  m/m.K for temperatures of between −162° C. and 20° C. The middle pipe  3  is made of a material adapted to low temperatures, for example a stainless steel or 9% nickel alloyed steel and the outer pipe  4  of ordinary steel.