Abstract:
High-pressure pipe element made of a hooped tube for making lines referred to as kill lines and choke lines that equip oil production installations, notably offshore. The high-pressure pipe element is an assembly consisting of two connections ( 3 ) and ( 4 ) respectively welded to the two ends of a metallic tube ( 1 ). The metallic assembly is hooped. Hooping ( 2 ) covering tube ( 1 ) comprises a sufficient number of hooping layers to withstand a determined internal pressure. Transition zones ( 5 ) and ( 7 ) in the vicinity of the welds comprise additional hooping layers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the sphere of high-pressure pipes intended to equip an oil drilling and/or production installation. More precisely, it relates to an application of the hooping technique for reinforcing metal tubes by means of composite reinforcing elements. 
     BACKGROUND OF THE INVENTION 
     The hooping technique consists in winding a reinforcing element, generally in form of a fiber band coated with a polymer, around a metallic core in order to increase the resistance of the core to the internal pressure without increasing its weight significantly, considering the low weight of the bands. 
     The core can be a metal tube, for example made of steel. The reinforcing element is an elongate element. It can have the shape of a strip, of a wire, or preferably of a series of wires or wick coated with a polymer matrix. 
     The reinforcing element can be wound around the core while introducing a tension therein. Thus, the reinforcing element wound around the core is subjected to a tensile stress, which causes the metallic core to be stressed. The prestress undergone by the core is similar to the prestress that would be produced by an outside collapse pressure. 
     Oil is produced from an offshore reservoir using a pipe generally referred to as riser, which allows the wellhead installed at the sea bottom to be connected to the surface. The riser is an extension of the tubing carrying the oil from the well bottom to the wellhead. The riser is provided with at least two auxiliary lines called kill line and choke line, whose main function is to establish a hydraulic connection between the sea surface and the wellhead at the sea bottom. More particularly, the auxiliary lines allow to supply the well with fluid by circulating below the closed blowout preventer, and/or to discharge a fluid from the well without passing through the inside of the riser which is not high pressure resistant. The fluid conveyed, resulting from an influx in an underground reservoir, can circulate at a pressure of 700 bars. 
     The present invention proposes using hooped tubes so as to reduce the weight of the auxiliary lines. Its aim is to provide a simple and economical embodiment of a high-pressure pipe element made of a hooped tube. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a high-pressure pipe element comprising at least one hooping layer on a metallic core. The core consists of a main part to which two connection means are welded at each end. The main part comprises a number of hooping layers inducing a determined prestress in the core, and the pipe comprises additional hooping layers in a zone stretching on either side of the two welds. 
     According to the present invention, the additional hooping layers can stretch over at least 50 mm of the main part. 
     The ends of the connection means may not be hooped and can have a sufficient thickness to withstand at least the same internal pressure as the hooped main part. 
     The outer surface of the connection means can form a cone between the main part and each end of the connection means. The cones can be covered at least partly by the additional hooping means. 
     According to the present invention, the pipe element is coated over the total outer surface thereof with a protective layer made of glass fiber embedded in a coloured polyamide matrix. 
     The pipe element according to the present invention can be used to make an auxiliary line of a drilling riser, and the auxiliary line can be a kill line, a choke line, a booster line or a mud return line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other details, features and advantages of the invention will be clear from reading the description hereafter of a non limitative embodiment example, with reference to the accompanying drawings wherein: 
     FIG. 1 shows a pipe element made of a hooped tube according to the invention, 
     FIG. 2 shows the transition zone of the first end of a hooped tube, on the female connection side, 
     FIG. 3 shows the transition zone of the second end of a hooped tube, on the male connection side. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a high-pressure pipe element made of a hooped tube according to the invention. This element comprises four different parts: a metal tube  1  or core, a first connection  3 , a second connection  4  and hooping layers  2 . Tube  1  has an inside diameter D and a thickness e which are substantially constant over the total length of tube  1 . Connections  3  and  4  are tubular parts obtained for example by machining, forging or molding. According to the present invention, metal tube  1  and connections  3  and  4  are manufactured independently of one another. Tube  1  is generally manufactured from a rolled blank. Then, connections  3  and  4  are welded onto tube  1 . Connection  3  is welded to one end of tube  1 , the weld being shown by reference number  8  in FIG.  1 . Then, connection  4  is welded to the other end of tube  1 , the weld being shown by reference number  9  in FIG. 1. A metallic assembly is thus obtained. Winding a reinforcing element around this metallic assembly allows hooping  2  of the metallic assembly to be obtained. The reinforcing element can be made from a polyamide matrix reinforced by carbon fibers. The hoop comprises a sufficient number of layers for the tube to withstand a determined internal pressure. The metallic assembly consisting of connections  3 ,  4  and tube  1  is hooped over the total length thereof, except for its ends part  33  of connection  3  and part  63  of connection  4 , which may cooperate with other elements, are not hooped. 
     The hooping principle according to the invention consists in inducing a compressive prestress in the metal core or tube  1  by means of the composite reinforcing bands. Thus prestressed, the internal pressure resistance capacity of tube  1  is increased since the effective pressure stress is reduced by the prestress value induced upon manufacture. In other words, the allowable pressure in this pipe element is increased by the internal pressure value that would equalize the hooping pressure. 
     In the description hereafter, what is referred to as&lt;&lt; first transition zone&gt;&gt; is the zone surrounded by circle  5  which stretches on either side of weld  8 . What is referred to as&lt;&lt; second transition zone&gt;&gt; is the zone surrounded by circle  7  which stretches on either side of weld  9 . On pipe element  1 , the zone located between the first and the second transition zone is referred to as&lt;&lt; current zone&gt;&gt;; it is shown by reference number  6  in FIG.  1 . 
     Current zone  6  comprises a determined number of hooping layers allowing to. withstand a determined internal pressure. The number of hooping layers is generally constant over the total length of current zone  6 . 
     Welds  8  and  9  are spots where a heterogeneity is observed, and potentially a fatigue brittleness. The purpose of the present invention is to overcome these mechanical risks in transition zones and to increase the internal pressure resistance of the hooped tube in the vicinity of welds  8  and  9 . 
     Thus, a pipe element according to the present invention has the advantage of being more resistant to the internal pressure in the vicinity of transition zones  5  and  7  than in the vicinity of current zone  6 . Transition zones  5  and  7  are therefore covered with a hooping with a higher number of layers than the number of layers covering current zone  6 . 
     FIG. 2 shows more in detail first transition zone  5 . Metal tube  1  is connected to connection  3  by weld  8 . In the vicinity of weld  8 , the metal thickness e and the diameter D of tube  1  are similar to the metal thickness e 1  and to the diameter D 1  of connection  3 . Eighteen hooping layers (from layer  10  to layer  27 ) can cover current zone  6  and first transition zone  5 . Layer  10  is deposited directly on metal tube  1 , layer  27  is the outer surface of the hooped tube. Prior to winding layer  10 , the metal tube can be subjected to a surface preparation operation, of&lt;&lt; Rilsan&gt;&gt; type for example, to provide good adhesion of layer  10  to metal tube  1 . The tension applied in the layers ranges evenly from 2400 N for layer  10  to 2320 N for layer  27 . In order to increase the internal pressure resistance in the vicinity of weld  8 , additional hooping layers are deposited on first transition zone  5 . Twenty-two hooping layers (from layer  28  to layer  49 ) can be deposited in the vicinity of weld  8  and above hooping layers  10  to  27 . Additional hooping layers  28  to  49  cover first transition zone  5 . Layer  28  is deposited on layer  27  and layer  49  is the outer surface of the hooped tube at the level of first transition zone  5 . The tension applied in layers  28  to  49  can be 1000 N. For example, hooping layers  28  to  49  can stretch from weld  8  over a length L 1  at least equal to 50 mm on the side of metal tube  1 , and over a length L 2  at least equal to 25 mm on the side of connection  3 . Beyond length L 1  on tube  1 , the number of additional layers  28  to  49  progressively decreases from twenty-two to zero. On tube  1 , layer  49  located outside stretches the least far, layer  28  stretches the furthest. 
     Connection  3  can be divided in three parts having each at least one distinct function. 
     Part  31  is the end of connection  3  that is welded to tube  1 . In FIG. 2, part  31  stretches over length L 2  from weld  8 . The geometry of part  31  is similar to the geometry of tube  1 : the inside diameter D 1  and the metal thickness e 1  of part  31  are substantially identical to inside diameter D and metal thickness e of tube  1 . Part  31  is hooped by layers  10  to  49  described above. Metal thickness e 1  and the number of hooping layers are so selected that the internal pressure resistance of part  31  is higher than the internal pressure resistance of current zone  6 . 
     The function of part  33  of connection  3  is notably to cooperate with another connection. Part  33  is not hooped. The metal thickness e 3  of part  33  is so selected that the internal pressure resistance of part  33  is at least equal to the internal pressure resistance of current zone  6 . In FIG. 2, the inside diameter D 3  of part  33  is similar to inside diameter D 1  of part  31  and metal thickness e 3  of part  33  is greater than metal thickness e 1  of part  31 . 
     Part  32  is the part of connection  3  located between part  31  and part  33 . The function of part  32  is to provide the transition between part  31 , which has a metal thickness e 1  and a hooping consisting of forty layers, and non-hooped part  33  which has a metal thickness e 3  greater than e 1 . On part  32 , the invention provides a progressive increase of metal thickness e 2  and, simultaneously, a progressive decrease of the number of hooping layers. Thus, the increase of metal thickness e 2  and the decrease of the number of hooping layers are so selected that the internal pressure resistance of part  32  is constantly at least equal to the internal pressure resistance of current zone  6 . Progressive transition from the geometry of part  31  to the geometry of part  33  prevents stress concentrations. In FIG. 2, the diameter D 2  of part  32  is substantially identical to diameters D 1  and D 3  of parts  31  and  33 . At the interface between part  31  and  32 , the metal thickness e 2  of part  32  is substantially equal to the metal thickness e 1  of part  31 . The further from the interface between part  31  and  32 , the more metal thickness e 2  increases. The outer surface of part  32  can be a cone of angle α. The inside of part  32  is a cylinder of diameter D 2 . This increase of metal thickness e 2  is accompanied by a progressive decrease in the number of hooping layers. The number of hooping layers decreases progressively from forty layers at the level of the interface between part  31  and part  32  to zero layer when metal thickness e 2  is sufficient to withstand the internal pressure. 
     FIG. 3 shows in detail second transition zone  7 . Metal tube  1  is connected to connection  4  by weld  9 . In the vicinity of weld  9 , the metal thickness e and the diameter D of tube  1  are identical to metal thickness e 4  and to diameter D 4  of connection  4 . Eighteen hooping layers  10  to  27 , described above in connection with FIG. 2, cover current zone  6  and second transition zone  7 . In order to increase the internal pressure resistance in the vicinity of weld  9 , additional hooping layers are deposited on second transition zone  7 . For example, in the vicinity of weld  9  and above hooping layers  10  to  27 , seven hooping layers (from layer  50  to layer  56 ) are deposited. The seven additional hooping layers  50  to  56  cover second transition zone  7 . Layer  50  is deposited on layer  27  and layer  56  is the outer surface of the hooped tube at the level of second transition zone  7 . The tension applied in layers  50  to  56  can be 1000 N. For example, hooping layers  50  to  56  can stretch from weld  9  over a length L 3  at least equal to 50 mm on the side of metal tube  1  and over a length L 4  at least equal to 25 mm on the side of connection  3 . Beyond length L 3 , the number of additional layers  50  to  56  decreases progressively from seven to zero. On tube  1 , layer  56  located outside stretches the least far, layer  50  stretches the furthest on tube  1 . 
     Connection  4  can be divided in three parts having each at least one distinct function. Parts  61 ,  62  and  63  of connection  4  are respectively similar to parts  31 ,  32  and  33  of connection  3 . 
     Part  61  is the end of connection  4  that is welded to tube  1 . In FIG. 3, part  61  stretches over length L 4  from weld  9 . The geometry of part  61  is substantially identical to the geometry of tube  1 : the inside diameter D 4  and the metal thickness e 4  of part  61  are substantially identical to inside diameter D and metal thickness e of tube  1 . Part  61  is hooped by layers  10  to  27  and  50  to  56  described above. Metal thickness e 4  and the number of hooping layers are so selected that the internal pressure resistance of part  61  is higher than the internal pressure resistance of current zone  6 . 
     The function of part  63  of connection  4  is notably to cooperate with another element. Part  63  is not hooped. The metal thickness e 6  of part  63  is so selected that the internal pressure resistance of part  63  is at least equal to the internal pressure resistance of current zone  6 . In FIG. 3, inside diameter D 6  of part  63  is smaller than inside diameter D 4  of part  61 , and metal thickness e 6  of part  63  is greater than metal thickness e 4  of part  61 . 
     Part  62  is the part of connector  4  located between part  61  and part  63 . The function of part  62  is to provide the transition between part  61 , which has a metal thickness e 4  and a hooping consisting of twenty-five layers, and non-hooped part  63  whose metal thickness e 6  is greater than e 4 . In part  62 , the invention provides a progressive increase of metal thickness e 5  and, simultaneously, a progressive decrease of the number of hooping layers. Thus, the increase of metal thickness e 5  and the decrease in the number of hooping layers are so selected that the internal pressure resistance of part  62  is constantly at least equal to the internal pressure resistance of current zone  6 . The progressive transition from the geometry of part  61  to the geometry of part  63  prevents stress concentrations. In FIG. 3, the diameter D 5  and the metal thickness e 5  of part  62  vary. At the interface between parts  61  and  62 , metal thickness e 5  of part  62  is substantially equal to metal thickness e 4  of zone  61 , and diameter D 5  is substantially equal to diameter D 4 . The further from part  61 , the more metal thickness e 5  increases and the more diameter D 5  decreases. The inner surface of part  62  can consist of a cone of angle β. The outer surface of part  62  can consist of a cone of angle δ. This increase of metal thickness e 5  is accompanied by a progressive decrease in the number of hooping layers. On part  62 , the number of hooping layers decreases progressively from twenty-five layers in the vicinity of the interface between part  61  and part  62  to zero layer when thickness e 5  is sufficient to withstand the internal pressure alone. 
     The outer surface of the pipe element according to the invention can be covered with a protective shell. This protective shell can be obtained by winding, i.e. an elongate element is wound around the tube with contiguous spires that stick to one another. The elongate element can be wound without tension. The elongate element can be made from a polyamide matrix containing glass fibers or Kevlar. The protective shell can also be used to give the external colouring of the hooped tube, white for example. 
     In FIG. 1, part  70  is a wearing part. Wearing part  70  is intended to cooperate with a female connection  3  of another pipe element and it is fastened to connection  4 . 
     Wearing part  70  can be the female part of the link with connection  4 . The wearing part is screwed onto connection  4 . Seal elements  71 , lip seals for example, are arranged between the inner surface of wearing part  70  and the outer surface of connection  4  to provide a sealed link. 
     The wearing part  70  of a first tube can be the male part of the link with the connection  3  of a second tube. Wearing part  70  is in contact with connection  3  through cylindrical surface  72 . Sealing can also be provided by seals. 
     After a certain number of connections between pipe elements, the male connection may be damaged. In this case, wearing part  70  has to be changed, with the corresponding joints of female part  3 .