1 FIELD OF THE INVENTION The present invention relates to the sphere of flexible pipes. Flexible pipes are notably used in the petroleum industry to convey oil from a wellhead located on the sea bottom to a treatment facility on the surface. The pipes 5 placed on the sea bottom, commonly referred to as flow lines, mainly undergo static mechanical stresses. On the other hand, the pipes connecting the sea bottom to the surface, commonly referred to as risers, undergo static and dynamic mechanical stresses. In the present description, the term "wire" designates elongate elements one 10 dimension of which is very large compared to the others. BACKGROUND OF THE INVENTION The structure of a flexible pipe, notably described in the API 17 B and API 17 J standards established by the American Petroleum Institute as "Recommended Practice for Flexible Pipe", generally comes in form of a tube made up of plies of armour wires 15 providing mechanical strength and polymer sheaths providing sealing. An example of a flexible pipe structure shown in Figure 1 consists of several layers or plies described hereafter from the inside to the outside of the pipe. Carcass 1 consists of a metal strip helically wound with a short pitch and intended to increase the resistance to collapse due to the external pressure applied to the pipe. 20 The metal strip can be made from a sheet or a wire, each spire being stapled to adjacent spires. Sealing sheath 2 is formed by extrusion around the carcass of a polymer material, generally selected from among polyolefins, polyamides and fluoropolymers. Vault 3 made of stapled or interlocking metal wires provides resistance to the 25 internal pressure in the pipe. 5069614_1 (GHMatters) P96110.AU 2 Optionally, a second polymer sealing sheath 4 is extruded around vault 3. Tensile armour plies 5 consist of metal wires helically wound at angles ranging between 200 and 55'. The plies can be maintained by tape 6. Polymer sheath 7 provides an external protection for the flexible pipe. 5 Petroleum fluid production in flexible or rigid pipes may require injecting chemicals into the production fluid, either with a view to prevention or remediation of fluid stream maintenance or pipe resistance problems. Thus, it may be necessary to inject anticorrosion additives into the production fluid in order to limit the effects of the transported fluid corrosion on the resistance of the 10 constituent metals of the pipe. Various additives are used, with variable compositions depending on the fluids transported and the metals used. Besides, additives can be used to limit fluid stream maintenance problems in the pipe, and notably gas hydrate formation problems. Gas hydrates form in the presence of certain gases such as methane and water, 15 under certain thermodynamic pressure and temperature conditions. A thermodynamic stability domain is thus defined in the plane considered, which indicates the area where hydrates may form. These conditions can be encountered when the pressure is high and the temperature low, which may notably occur during production stop phases. Furthermore, kinetic conditions define the rate of appearance of the hydrates; they 20 depend in a very complex manner on the physicochemistry of the fluids in place, and on the flow regimes. As a preventive measure, various additives can be used to limit or prevent hydrate formation, such as, for example, additives referred to as thermodynamic, often based on alcohol, such as ethanol, or Mono Ethanol Glycol whose presence shifts the 25 conditions of appearance of gas hydrates in the pressure and temperature domain. Additives referred to as kinetic also exist and act upon the agglomeration capacities of 5069614_1 (GHMatters) P96110.AU 3 the hydrate particles that can, in the presence of these additives, be transported without coagulating and creating plugs, thanks to surfactant properties. Critical situations are obtained during production stops. In this case, a conventional procedure consists in replacing the production oil by dead oil, which is commonly 5 referred to as dead oil displacement, involving a pipe loop architecture and possibly relatively long-lasting operations. In cases where pipes or control devices such as distributors, commonly referred to as manifolds, are caught in hydrate plugs, a ship may be required to carry out work over operations, notably using coiled tubings. Alternatively, means for heating the 10 production fluid can be available, through heating distributed along the pipes, so as to allow preventive or curative action. Additives injection can also be considered as a curative measure, optionally associated with pipe depressurization or heating. Therefore, in several situations, it is of interest to be able to inject various fluids into 15 the production stream in the flexible pipes. However, this requires in current practice injecting at specific points, which are typically the collector and distributor means commonly referred to as manifolds that are present on the pipes or the wellheads. Alternatively, products can be injected through a coiled tubing into a flexible pipe, but at a limited distance from the surface, and therefore over a limited portion of the 20 flexible pipe. The principle of the invention is to provide a flexible pipe architecture allowing direct injection of the fluid into the pipe, at one or more points of the pipe. One or more metal tubes are therefore provided, which are arranged between the carcass and the sealing sheath, and open onto one or more points of the pipe. 25 5069614_1 (GHMatters) P96110.AU 4 SUMMARY OF THE INVENTION In general terms, the object of the present invention is a flexible pipe comprising, from inside to outside, a carcass and a polymer sealing sheath, the carcass consisting of at least one helically wound metallic element. The invention is characterized in that 5 at least one hose is arranged in an annular space that separates the carcass from the sealing sheath, the hose comprising an injection orifice at one of the two ends of the pipe and a discharge orifice opening between the two ends of the pipe. According to the invention, the flexible pipe can comprise at least two hoses arranged in the annular space, the discharge orifices of the hoses opening onto 10 different points between the two ends of the pipe. Said at least one hose can be helically wound. Said at least one hose can be wound in a succession of at least a first helical segment and a second helical segment, the winding angle of the helix of the first segment being opposite to the winding angle of the helix of the second segment. 15 The winding angle of the helix can range, in absolute value, between 5' and 250 with respect to the axis of the pipe. An elongate element can be arranged between the carcass and the sealing sheath, the elongate element being juxtaposed with said at least one hose. Said at least one hose can be made of a metallic material selected from among a 20 stainless steel or a duplex steel. The flexible pipe can also comprise a pressure vault arranged above the sealing sheath, at least one tensile armour ply and a polymer protective sheath. The invention also relates to a method of transporting a petroleum effluent by circulation in a flexible pipe according to the invention, characterized in that a fluid is 25 injected into the injection orifice of said at least one hose. 5069614_1 (GHMatters) P96110.AU 5 The fluid can comprise anticorrosion compounds, antihydrate compounds and anti-asphaltene compounds. BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will be clear from reading the 5 description hereafter, with reference to the accompanying figures wherein: - Figure 2 shows in detail an embodiment of a flexible pipe with an injection hose according to the invention, - Figures 3 to 6 show various injection hose winding modes, - Figure 7 shows a pipe according to the invention provided with three injection hoses, 10 - Figures 8 and 9 show two embodiments of a flexible pipe according to the invention. Figure 1 shows a flexible pipe according to the prior art. DETAILED DESCRIPTION Figure 2 diagrammatically shows a cross-sectional view along axis AA' of a flexible pipe. The pipe comprises, from inside to outside, a carcass 1, a sealing sheath 2, a vault 15 3, armour plies 5 and a protective sheath 7 similar to those described with reference to Figure 1. According to the invention, the pipe furthermore comprises a ply 9 including one or more hoses 8 arranged, by being helically wound for example, in the annular space contained between carcass 1 and sealing sheath 2. Hose 8 extends along the pipe and opens, for example through a valve, onto a point located between the ends 20 of the pipe. For example, the hose opens onto a point located in an axial position at a counted distance with respect to the end of the pipe, said distance ranging between 1 % and 99 %, preferably between 10 % and 90 % of the length of the pipe. Hose 8 allows to inject a fluid, for example an antihydrate fluid, an alcohol or a glycol for example, a gas, anticorrosion additives, anti-asphaltene additives, from one end of the 25 pipe to the halfway point on the pipe onto which hose 8 opens. Thus, the present 5069614_1 (GHMatters) P96110.AU 6 invention allows a fluid to be directly injected into the pipe, at different points of the pipe. Hose 8 is preferably made from materials withstanding the corrosion due to the fluids circulating in the pipe and to the fluids transported in hose 8. These materials can 5 be 304 or 316 grade stainless steel for example, or EN 1.4016 (AISI 430) grade duplex steel. Figures 3 to 6 show four embodiment examples of ply 9 of the flexible pipe described with reference to Figure 2. Figure 3 shows a hose T helically wound in a flexible pipe according to the 10 invention. The hose is wound in a helix of radius R and angle a that can range between 50 and 250, preferably between 7' and 200, an excellent value for angle a of the helix ranging between 7' and 150. The other plies of the flexible pipe are not shown for easier readability of Figure 3. The ply formed by winding hose T also comprises three juxtaposed elongate elements El, E2 and E3, for example wires of rectangular section, 15 made of polymer or metal. Elements E1, E2 and E3 are wound in a helix of same radius R and same angle with respect to axis AA' as the winding of hose T. For easy readability of Figure 3, only the winding of element E2 is shown in dotted lines. These elements El, E2 and E3 allow to fill the annular space between two adjacent spires of wound hose T and to prevent the sealing sheath from collapsing between two spires of wound hose T. 20 Figure 4 shows a portion of a pipe according to the invention comprising three injection hoses TI, T2 and T3 arranged in three adjacent helices inscribed in the same radius R and having the same winding angle with respect to axis AA' of the flexible pipe. The annular space between the hoses is filled by elongate elements E4 and E5. For easier readability of Figure 4, the windings of elements E4 and E5 are not shown. 25 With reference to Figure 4, hoses TI, T2 and T3 are adjacent, then elements E4 and E5 5069614_1 (GHMatters) P96110.AU 7 are arranged behind the three hoses. Alternatively, each one of elements E4 and E5 can be inserted between two hoses TI, T2 and T3. Figures 5 and 6 show a hose T wound in helical segments with inversion of the helix angle of "S-Z" type. "S-Z" type windings are notably described in document API 17E. An 5 "S-Z" type winding comprises a succession of identical winding patterns recurring periodically and comprising each helically wound segments with a constant helix angle in a positive direction and helically wound segments with a constant helix angle in the negative direction. Segments with inverted winding directions provide progressive variation of the winding angle between the segments of constant helix angle. The "S-Z" 10 type winding is advantageous because the machines required for making it are simpler than those used for the constant-pitch helical winding. With reference to Figure 5, between the winding direction inversion points, of axial positions X1 and X2, hose T is wound in a direction over an angular travel of 3200. Then, between axial positions X2 and X3, hose T is wound in the opposite direction over the same angular travel. The 15 absolute value of the winding angle of the helical segment between X1 and X2 and of the helical segment between X2 and X3 can range between 5' and 250, preferably between 7' and 200, an excellent absolute value for this angle ranging between 70 and 150. With reference to Figure 6, hose T is wound in a helical segment making one turn and a half between the direction inversion points of axial position X1' and X2'. Hose T is 20 then wound in a helical segment making one turn and a half between the direction inversion points of axial position X2' and X3. Figure 7 shows a flexible pipe of axis AA' provided with three injection hoses 8a, 8b and 8c to illustrate the possibility of injecting a fluid in different axial positions in the pipe according to the invention. For easier readability, the other constituent elements of the 25 flexible pipe are not shown in Figure 7. Each one of hoses 8a, 8b and 8c respectively comprises a connection 10a, 10b and 10c at end B of the flexible pipe. 5069614_1 (GHMatters) P96110.AU 8 Hose 8a extends from connection 10a to valve l la opening approximately onto the middle of the pipe length. Hose 8a allows a fluid to be injected from end B into the pipe at valve l l a. Hose 8b extends from connection 1 Ob to valve 11 b opening approximately onto a 5 point three quarters down the length of the pipe. After valve 11 b, an elongate element 8b', a second hose or an armour wire for example, extends as the continuation of hose 8b up to the other end. Element 8b' is helically wound similarly to hose 8b between the carcass and the sealing sheath. Hose 8b allows a fluid to be injected from end B into the pipe at valve 11 b. 10 Hose 8c extends from end B to end C of the flexible pipe. Hose 8c is provided at end B with a connection 1Oc, a valve 1 Ic arranged approximately in the middle of the pipe length and a valve 11 c' arranged at approximately four fifths of the pipe length. Hose 8c allows a fluid to be injected from end B into the pipe at valves 1 Ic and 1 Ic'. Preferably, the distance between the axial position of valves l la, 11 b, 11 c, 11 c' 15 and end B ranges between 1 % and 99 %, preferably between 10 % and 90 % of the pipe length measured between ends B and C. Figure 8 shows a flexible pipe 10 connected to wellhead 11 arranged on the sea bottom. Pipe 10 connects wellhead 11 to floating vessel 12 on the sea surface. Flexible pipe 10, commonly referred to as a riser, allows the petroleum effluent coming from 20 wellhead 11 to be carried to floating vessel 12. Pipe 10 is made up of an architecture according to the invention, i.e. it comprises at least one hose 15 extending from the end located on floating vessel 12 to a position between wellhead 11 and floating vessel 12. For example, hose 15 opens onto low point 13 where hydrates are likely to form first. A pumping system allows hose 15 to be supplied with fluid to be injected at floating 25 vessel 12. 5069614_1 (GHMatters) P96110.AU 9 Figure 9 shows a flexible pipe 20 of flow line type connecting subsea wellhead 22 to a pumping station 23. Pumping station 23 is connected to floating vessel 24 by flexible pipe 21 of riser type. Pipes 20 and 21 are assembled end to end at the level of pumping station 23 so as to carry the petroleum effluent coming from wellhead 22 to floating 5 vessel 24. Pipe 20 is made according to the invention, i.e. it comprises one or more hoses 25 extending from pumping station 23 and opening onto one or more points located between pumping station 23 and wellhead 22. For example, hose 25 opens onto positions 25a, 25b and 25c. Hose 25 is connected to pumping device 27 arranged on platform 24 by means of line 26 extending between platform 24 and station 23. Thus, 10 the fluid to be injected is pumped by device 27, carried through line 26 and hose 25 in order to be delivered inside flexible pipe 20 at injection points 25a, 25b and 25c. 5069614_1 (GHMatters) P96110.AU