Hose showing a limited axial deformation under internal pressure.

The present invention relates to a new flexible pipe structure having no appreciable variation in length under the effect of internal pressure.

The invention applies, in particular, to the production of flexible pipes for transporting pressurized fluids, such as water or hydrocarbons.

For many applications of such pipes, it is often desirable and sometimes necessary to have hoses which do not show any appreciable variation in length when they are subjected to an internal pressure with "bottom effect", that is to say application an axial force resulting from the action of internal pressure at the ends of the hose.

Indeed, an elongation of the hose in service, under the effect of internal pressure, may be a serious drawback in many applications.

This is particularly the case for underwater collection pipes which must be buried, that is to say arranged at the bottom of a trench, to avoid the risks of deterioration due to navigation.

If the pipe thus disposed tends to elongate when it is pressurized, it risks deforming vertically and re-emerging from the trench, which obviously cancels the effect of the burial.

The "multi-layer" reinforced flexible structures to which the improvement according to the invention applies, mainly comprise:<ul id="ul0001" list-style="none"><li>- a pressure resistance reinforcement comprising one or more plies of rings or wires, cables or profiles wound at a high angle α relative to the axis of the hose (this angle may exceed 85 °);</li><li>- a tensile strength armor, comprising one or more pairs of crossed plies of wires, cables or profiles wound at an angle at most equal to 55 °, this angle should not however be too small to preserve the flexibility of the pipe .</li></ul>

These different plies can be arranged in any order from the inside to the outside of the structure of the hose, and can be separated or not by thin plastic sheaths.

Such structures have for example been described in French patents 1,417,966 and 2,177,996.

US Patent 2,747,616 proposes adopting a winding angle of the fibers arranged in a helix greater than 55 ° to respond to the problem of increasing the bending stiffness, but this patent relates to the production of a rigid pipe and not a flexible tube.

We define the stiffness K of a weave, proportional to the modulus of elasticity E of the material constituting the plies thereof and to the quantity of this material by the product<maths id="math0001" num=""><img file="EP0147288A2_D0001.tif" /></maths>where e, the total useful thickness, is in fact the total thickness of the plies of the armor when these plies are made up of solid rectangular and contiguous wires. When it is a question of hollow, round or profiled wires, the useful thickness is deduced from the total thickness e by taking into account the voids or clearances made inside or between the wires, we have:<maths id="math0002" num=""><img file="EP0147288A2_D0002.tif" /></maths>where b is the ratio of the volume of resistant material constituting the armor to the total volume occupied by it.

The index p is assigned to the internal pressure resistance armor and t to the tensile resistance armor. So<maths id="math0003" num=""><img file="EP0147288A2_D0003.tif" /></maths>In the hoses according to the prior art, where one seeks above all to distribute the stresses well in the various layers of armor in order to reduce the total thickness and therefore the weight and the price of the hose, the ratio K_{t /}Kp is low (less than unity).

However, this type of structure cannot lead to flexible hoses which are stable in length under the effect of internal pressure. In the current state of the art, it is necessary, in order to reduce the elongation as much as possible, to wire the tensile strength armor with a very small angle α (20 °), which leads to great manufacturing difficulties and however, does not make it possible to sufficiently reduce this elongation for many applications, in particular in the case indicated above of the underwater collection pipes which must be buried.

To avoid variations in length under the effect of internal pressure, the invention provides:<ul id="ul0002" list-style="none"><li>- a flexible structure of the type described above, but whose stiffness ratio Kt / Kp is between 2.5 and 8 for a wiring angle of the tensile armor α (Fig. 4) between 20 ° and 50 °.</li><li>- a better result is obtained when the stiffness ratio and the angle α are linked by the relationships:<maths id="math0004" num=""><img file="EP0147288A2_D0004.tif" /></maths>where α, expressed in degrees is between 20 and 50.</li><li>- in order to reduce the influence of an error on the wiring angle, i.e. to increase the acceptable tolerance on this manufacturing parameter, a structure such as<maths id="math0005" num=""><img file="EP0147288A2_D0005.tif" /></maths>where α in degrees is limited between 25 and 45.</li></ul>

Thus, by producing a flexible such as the ratio of the stiffnesses of armor Kt / Kp = 3, where the tensile plies are wired at 35 °, one obtains a variation in length under pressure at least ten times lower than what l '' we know how to do today with conventional structures, while being able to accept a tolerance of 10% on the stiffness ratio and 2 to 3 ° on the wiring angle.

The attached figures show flexible armored pipes to which the invention applies:<ul id="ul0003" list-style="none"><li>FIG. 1 illustrates a first type of flexible flexible conduits,</li><li>FIGS. 2 and 3 represent other types of pipes, and</li><li>- Figure 4 is a diagram showing the winding angle of the tensile strength armor plies.</li></ul>

In FIG. 1, the reference 1 designates an internal conduit which can be made of plastic, for example nylon.

This duct is covered with a pressure resistance armor 2, which in the illustrated example consists of the winding of a strip or of a preformed profile, wound helically at an angle exceeding, for example, 85 ° with respect to the axis of the pipe.

The assembly is covered with armor 3 of tensile strength consisting of two crossed plies 3a and 3b of profiles wound helically with opposite steps in the two plies.

The absolute value of the angle α of winding of these sections has a value between 20 ° and 50 ° relative to the axis of the pipe.

An external sheath 4, for example made of nylon, covers the assembly thus formed.

Another type of flexible reinforced pipe is illustrated in Figure 2.

The same reference numerals have been used in this figure to designate elements similar to those illustrated in Figure 1.

In this second type of flexible pipe, the tensile strength armor comprises two pairs of crossed plies (3a, 3b) and (3c, 3d) of profiles wound in a helix at an angle which may be between 20 and 50 ° by relative to the axis of the pipe.

In the type of pipe illustrated in FIG. 3, a steel carcass 5 has been placed inside the plastic pipe 1 and the plies 2a and 2b of the armor for resistance to internal pressure have been placed on the side and the other of the tensile strength armor (3a, 3b).

In this embodiment illustrated in FIG. 3, the carcass 5 will not be taken into account for the calculation of the stiffness K of the armor of resistance to internal pressure if this carcass is not waterproof and, consequently, is not subjected to a differential pressure when a pressurized fluid circulates in the pipe (therefore does not contribute to the resistance of the pipe to internal pressure). In the structure according to the invention, a better distribution of stresses is obtained and therefore a reduction in the total thickness if one chooses to make the armor of resistance to pressure a material whose modulus of elasticity E_p is significantly lower than ^E_you

Thus, if the tensile armor plies are made of steel (E_t ≃ 200 GPa), it is possible for example to make the pressure resistance armor with a composite material (resin reinforced with glass fiber or other filaments), this material having a lower modulus of elasticity (E of 30 at 60 GPa). Such a material also has the advantage of withstanding high stresses while having a low density. A flexible hose is then obtained which is not only lightened, but stable under the effect of internal pressure. It is also possible to envisage making the armor for resistance to internal pressure from an alloy of aluminum, titanium, or any other material of low elastic modulus.