D U CT HAV I N G A P R E- I N STALLE D ELON GATE D ELE M E NT

The present invention belongs to the general technical field of laying cables into pipes. A particular case concerns the further laying of cables into a pipe where a cable or a plurality of cables have already been installed.

In this case, the laying of an additional elongated element, such as an optical fiber cable for example, into a duct where another elongated element has already been installed may be particularly difficult. Indeed, the resident cable (the already installed elongated element) takes its own place within the internal space of the duct, and the laying of the additional elongated element may encounter severe issues such as sticking (or high friction) with the resident cable if its outer surface presents a high friction ratio, pinching or wedging between the resident cable and the duct... All these phenomena are known to drastically increase the friction and the required laying force to apply to the elongated element, thereby limiting the achievable length of the elongated element to be subsequently laid into a duct having a resident cable. In addition, if the optical fiber cable is used to measure a temperature of the resident cable, it is important to ensure that the optical fiber cable further installed into the duct is in close vicinity of the resident cable after the installation, so that an accurate temperature measurement will be possible.

The document US2004091015 A1 discloses a method where two guide ducts are first laid into the main duct on each side of the resident cable, to avoid pinching and/or wedging. However, this method increases the costs with two preliminary operations, in addition to the cost of the guide ducts. In addition, it is not possible to guaranty that for a long length (above 100 meters for example), the two guide tubes will be correctly positioned on each side of the resident cable all along the considered length, so that wedging or pinching may occur during the laying of the additional elongated element.

The present invention aims to solve the aforementioned drawbacks and is first directed to propose a method for laying over high lengths an elongated element into a duct having a pre-installed elongated element, with a reduced risk of pinching or wedging.

With this goal in mind, a first aspect of the invention is a method for laying an elongated element into a main duct having an internal space where a pre-installed elongated element is already present in the internal space, the method comprising the steps consisting in:

- filling the internal space with a liquid,

- laying by floating a sub-duct, e.g. a micro-duct, into the internal space, with a step of applying a pulling force to a front end of the sub-duct, with a pressurized fluid carried inside the sub-duct,

- laying the elongated element into the sub-duct,

- moving the elongated element into the vicinity of the pre-installed elongated element by eliminating the floating of the sub-duct.

The method according to the present invention comprises a step of laying a sub-duct into the main duct, before laying the elongated element (i.e. the so called additional elongated element in prior art description). The laying of the elongated element in this sub-duct presents no risk of wedging or sticking or pinching, as there is no other cable in this sub-duct. Regarding the laying of the sub-duct itself, as it is performed with floating, the sub-duct floats in the liquid present in the internal space, so that the sub-duct is above the pre-installed elongated element, away from the area of the internal space where there is a risk of wedging, pinching or sticking. The invention takes also advantage of the sub-duct, to carry via the sub-duct a fluid to create a pulling force at the front end of the sub-duct (i.e. the end entering first the main duct), thus facilitating the laying over great lengths. After the installation of the elongated element, eliminating the floating effect will force the sub-duct and/or the elongated element to be in close vicinity of the pre-installed elongated element. In the case where the pre-installed elongated element is a high voltage cable or an energy cable, and if the elongated element is a temperature sensor, the measurements will be more accurate with the elongated element in the vicinity of the pre-installed elongated element.

According to one embodiment, the step of laying by floating the subduct into the main duct comprises a step of pushing the sub-duct into the main duct with a pushing device arranged at an entrance of the main duct where the sub-duct is introduced into the main duct. This embodiment helps to increase the length of laying the sub-duct.

According to one embodiment, the step of laying by floating the subduct is achieved by selecting a density of the pressurized fluid and/or a density of the sub-duct material, so that a density of the sub-duct filled in with the pressurized fluid is lower, preferably just a bit lower, than a density of the liquid filled in the main duct. The floating effect is obtained via the adequate selection of values within three parameters: the density of the liquid filling the internal space of the main duct, the density of the sub-duct material, and the density of the pressurized fluid used to create the pulling force. This leads to several possibilities of choice, thus increasing the ability to the method to be adapted case by case.

According to one embodiment, the step of laying by floating the subduct is achieved by selecting a wall thickness of the sub-duct and an external diameter of the sub-duct, thereby defining a Standard Dimension Ratio (SDR: ratio between external diameter and wall thickness of the sub-duct), being a sufficiently defining parameter for the floating situation, in relation to a sub- duct's material having a predetermined density, and/or to a predetermined density of the pressurized fluid, such that a density of the sub-duct containing the pressurized fluid will be lower than a density of the liquid filling the internal space of the main duct. The present step of selecting the appropriate geometry of the sub-duct gives an additional parameter to adjust, to allow the laying by floating of the sub-duct. In other words, the present invention gives to the user a combination of four parameters in total to ensure that the laying of the elongated element will be done with the desired floating effect: the density of the liquid in the internal space of the main duct, the density of the pressurized fluid in the internal space of the sub-duct, the density of the material of the sub duct and the SDR of the sub-duct (ratio between external diameter and wall thickness of the sub-duct ). It should be kept in mind that the selection of the sub-duct geometry/material shall also take into account the pressure value which will be applied to the pressurized fluid, to avoid any bursting of the sub-duct, which is defined by the same SDR.

According to one embodiment, the density of the sub-duct containing the pressurized fluid and the elongated element is comprised in a range from 0.89 to 0.97 of the density of the liquid filled in the main duct. This range of densities reduces the upward and downward contact forces between the main duct, the sub-duct and the pre-installed elongated element, so that the movement of the sub-duct does not present high friction forces.

According to one embodiment, the step of eliminating the floating of the sub-duct is achieved by replacing the pressurized fluid used to apply the pulling force by a heavier fluid into the sub-duct. This operation is easy to carry out, as the sub-duct may easily be connected to a first pump arranged to feed a first fluid under pressure, and then the sub-duct is connected to the same first pump or a second pump which will fill the sub-duct with a second fluid, having a higher density than the first fluid, so that the sub-duct will sink towards the pre-installed elongated element. According to one embodiment, the step of applying the pulling force is achieved with compressed gas, and the step of laying the elongated element is achieved by jetting the elongated element into the sub-duct with a liquid. The liquid used to jet the elongated element having a higher density than the gas, the sub-duct will sink towards the pre-installed elongated element. According to one embodiment, the step of applying the pulling force is achieved with compressed air, and the step of laying the elongated element is achieved by jetting the elongated element into the sub-duct with water.

According to one embodiment, the step of applying the pulling force is achieved with a first liquid, and the step of laying the elongated element is achieved by jetting the elongated element into the sub-duct with a second liquid heavier than the first liquid. Since the second liquid is heavier than the first liquid, the sub-duct will sink towards the pre-installed elongated element.

According to one embodiment, the step of applying the pulling force is achieved with a liquid lighter than water, such as alcohol or a mixture of water and alcohol, and the step of laying the elongated element is achieved by jetting the elongated element into the sub-duct with water.

According to one embodiment, the step of applying the pulling force is achieved with water, and the step of laying the elongated element is achieved by jetting the elongated element into the sub-duct with a liquid heavier than water such as a mixture of water and salt and/or sugar.

According to one embodiment, the step of eliminating the floating of the sub-duct is achieved by removing the sub-duct out of the main duct. The elongated element will sink towards the pre-installed elongated element, and will be in direct contact with the latter. If the elongated element is a temperature sensor optical fiber or a temperature sensor optical cable, the temperature measurement will be more accurate.

According to one embodiment:

- the sub-duct is laid from a first end of the main duct to a second end of the main duct,

- the step of laying the sub-duct comprises a step of joining two or more sub sections of sub-duct via a permanent or disconnectable joint,

- and the removal of the sub-duct comprises a step of disconnecting at least one of the disconnectable joints or cutting the sub-duct next to the permanent joint, and a step of removing at least a sub section of sub-duct from the second end of the main duct. This embodiment allows to lay a long sub-duct being an assembly of several sub sections, allowing long lengths of cable without splice to be installed, and to take advantage of this assembly to disconnect it at the junctions, to remove the sub-duct sub section via the two ends of the main duct.

If the sub-ducts cannot be pulled out over the required length by above method, it is also possible to zip-cut one or more pulled out intermediate sections of sub-duct.

According to one embodiment, the method comprises a step of selecting the elongated element as being an elongated element having a density higher than a density of the liquid filled in the internal space. The laying of the elongated element into the sub-duct will increase its density and will force the sub-duct to sink towards the pre-installed elongated element.

According to one embodiment, the step of eliminating the floating of the sub-duct is simultaneously achieved with the step of laying the elongated element into the sub-duct, by selecting a density of the elongated element, so that a density of the sub-duct having received the elongated element is greater than a density of the liquid filled in the internal space of the main duct. It is taken advantage of the characteristics of the elongated element to increase the weight of the sub-duct assembly (i.e. sub-duct + pressurized fluid + elongated element), to make it sinking downwards to the pre-installed elongated element. There are two parameters, added to the four parameters that defined the floating phase, to make the sub-duct sinking when introducing the elongated element into it: the density of the elongated element, and the ratio between the internal diameter of the sub-duct and the external diameter of the elongated element.

According to one embodiment, the method comprises a final step of removing the liquid filled in the internal space of the main duct. The step will automatically force the sub-duct to be in contact with the pre-installed elongated element. According to one embodiment, the step of applying a pulling force to a front end of the sub-duct is achieved by installing a nozzle at the front end of the sub-duct, the nozzle being arranged to create a rearward jet with the pressurized fluid. Another aspect of the invention is a method of laying an elongated element in a series of main ducts, comprising the steps consisting in:

- laying the elongated element into a first main duct,

- buffering a length of the elongated element at an end of the first main duct,

- laying the elongated element at least into a second main duct,

wherein at least one of the steps of laying the elongated element into a main duct is performed according to the method according to one of the preceding claims. This aspect includes a step of constituting a buffer with a length of the elongated element by feeding an extra length of the elongated element through the sub-duct installed in the first main duct. Then, it is possible to re- conduct the process of laying the elongated element into another main duct and/or sub duct. The buffered length is determined in relation with the length of the second main duct, so that the elongated element may be laid into the total length of the second main duct.

Other characteristics and advantages of the present invention will appear more clearly from the following detailed description of particular non- limitative examples of the invention, illustrated by the appended drawings where:

- Figure 1 represents a cross section of a main duct having a pre- installed elongated element; - Figure 2 represents the cross section of the main duct of figure 1 , during a step of laying by floating a sub-duct into the main duct;

- Figure 3 represents the cross section of the main duct of figure 1 , after the completion of the method according to the present invention; - Figure 4 represents a general view of the laying by floating the subduct into the main duct.

In the present application, it is referred to elongated elements, which may be for example, cables, electric cables, high voltage cables, optical fibers or cables, temperature sensing optic fibers or cables. All these elongated elements may comprise for example a core, a coating, or a sheath. However, the wording elongated element is not limited to any of these specific examples.

It is also referred to ducts, which may be for example pipes, hollow cylinders, tubes, conduits: anything defining a channel in which an elongated element may be laid from a first location to a second location.

In the following, when a density is used, it is a density of a product/material with reference with water which is meant.

Figure 1 represents a main duct 20 having an internal space 25, where a pre-installed elongated element 30 is present: it may be a resident high voltage cable for example.

For economic reasons, or for monitoring a parameter of the internal space 25 (e.g. the temperature of the pre-installed elongated element 30), it may be necessary to lay a further elongated element into the main duct 20. It may be difficult to install such further elongated element into the main duct 20 having the pre-installed elongated element 30 in its internal space 25, as the elongated element to be installed may be wedged or pinched between the main duct 20 and the pre-installed elongated element 30, if during the laying operation, the elongated element occupies the location as shown by the dotted line A. In this case, the friction between the main duct 20 and pre- installed elongated element 30 will drastically shorten the maximum laying distance for the elongated element to be installed.

To overcome this drawback, the present invention includes a step of laying by floating a sub-duct 40, as shown on figure 2. Indeed, laying the sub- duct 40, if located as shown on figure 2, above the pre-installed elongated element 30, will not present any risk of wedging, as the floating effect will maintain the elongated element away form the area where there is a risk of wedging. To achieve a floating effect, a liquid with a defined density is first filled in the internal space 25 of the main duct 20. Consequently, a floating effect of the sub-duct 40 is achieved as soon as the density of the sub-duct 40 and its internal space 45 is lower than the density of the liquid filling the internal space 25 of the main duct 20. As a non limiting example, the internal space 25 is filled with water (density=1 ), a sub-duct 40 made of high density polyethylene (HDPE, density=0.95) is chosen, and the internal space of the sub-duct is filled in with water, resulting in a global density of the sub-duct 40 with its internal space 45 lower than 1 .

During the laying by floating of the sub-duct 40, the invention takes an advantage of the fact that the sub-duct 40 presents an internal space 45. Indeed, the internal space 45 is pressurized, to supply at a front end of the sub-duct 40 a pressurized fluid which is used to create a pulling force applied at the front end of the sub-duct. In this aim, a pulling pig having a nozzle is attached at the front end of the sub-duct 40, to create a jet in a rearward direction, in the internal space 25 of the main duct 20. This nozzle may be named as a "parachute pig", and redirects the pressurized fluid from the internal space 45 of the sub-duct 40 to a rearward direction, thus creating a pulling force onto the sub-duct 40.

Once the laying of the sub-duct 40 is done over the all length of the main duct 20, without wedging or pinching or sticking, due to the floating effect, the sub-duct 40 may easily receive an elongated element 10 in its internal space 45. The laying of the elongated element 10 into the sub-duct 40 may be performed with known techniques such as pushing, pulling, jetting or floating, for example. In the case where a proximity between the elongated element 10 and the pre-installed elongated element is required (if the elongated element is a temperature sensing optical cable for example), it is possible to bring the elongated element 10 in vicinity of the pre-installed elongated element 30 by removing or eliminating or abolishing the floating effect of the sub-duct 40, to achieve a sinking effect.

The eliminating of the floating of the sub-duct 40 may be achieved in several ways.

A first possibility may be to increase the density of the sub-duct 40 and its internal space 45. It may be achieved by introducing a heavy liquid in the internal space 45, and/or laying a heavy elongated element 10, so that the density of the sub-duct 40 and its internal space 45 will be greater than the density of the liquid filled in the internal space 25 of the main duct 20.

Coming back to the previous example, the laying by floating of the sub- duct 40 was achieved with the internal space 25 filled with water (density=1 ), the sub-duct 40 made of high density polyethylene (HDPE, density=0.95), and the internal space of the sub-duct filled with water, resulting in a global density of the sub-duct 40 with its internal space 45 just a bit lower than 1 (0.97). If the laying of the elongated element 10 into the sub-duct 40 is also done by jetting with water, and if the elongated element 10 has a sufficiently high density and diameter, such that the overall density of the sub-duct 40, having the water and the elongated element 10 in its internal space 45, will be greater than 1 , the sub-duct 40 will sink, as shown in figure 3. In case of a 12/8 mm micro-duct, a 4 mm cable of density 1 .5 will make a total density of 1 .03, sufficient to sink the whole.

Alternatives of this example may be to fill the internal space 25 of the main duct with a light liquid such as oil, and the sinking of the sub-duct 40 may be obtained by jetting an elongated element 10 of standard density (e.g. 1 .2 for a standard temperature sensing optical cable) with water. A second possibility to eliminate the floating effect may be to remove the liquid from the internal space 25 of the main duct 20, or to replace it by a lighter liquid having a density lower than the one of the sub-duct 40 and its internal space 45. A third possibility to eliminate the floating effect of the sub-duct 40 is to remove the sub-duct 40, provided that the density of the elongated element 10 is greater than the one of the liquid present in the internal space 25 of the main duct 20. Otherwise, the method will comprise a further step of removing the liquid from the internal space 25, to bring the elongated element 10 in contact or vicinity of the pre-installed elongated element 30.

The figure 4 represents a general view of the laying by floating of the sub-duct 40. The main duct 20, having the pre-installed elongated element 30 laid over its all length, is buried in a ground 60 and accessed via a man hole 61 . At this location, the main duct 20 is cut, to install at one end a coupling part 51 having a feeding inlet 51 a and a guiding duct 53 connected. A coiling device 54 and pushing means 52 are connected to the guiding duct 53 to push the sub-duct 40 into the main duct 20, which has been previously filled with a liquid through the feeding inlet 51 a.

A pulling pig 41 is attached to the front end of the sub-duct 40 which has been introduced first into the main duct 20. A pump 55 is connected to the other end of the sub-duct 40, to pressurize it with a pressurized fluid (water or air for example). The pulling pig is designed to exhaust the pressurized fluid in a rearward direction, so that a pulling force is created.

A working example may be defined as follows. A 12/8 millimeters sub- duct 40 made of high density polyethylene (HDPE density of 0.95, standard), filled with water (total density of 0.97), is installed into a water filled main duct 20 with inner diameter of 150 mm (with existing pre-installed elongated element 30, but not "seen" or touched by the floating sub-duct 40), over a length of 820 m. Inside the sub-duct 40, water is injected at a pressure of 40 bar (such pressure is below the bursting pressure for such a sub-duct 40). The water inside the sub-duct 40 is supplied to achieve a velocity of about 1 .6 m/s (which gives a Reynolds number of approximately 10000, in the range of a turbulent pipe flow), and a volume flow of 0.08 l/s. The impulse change of the outflowing water at the pulling pig 41 is a measure for the force that can be made with it.

When the total cross-sectional area of the nozzles in the pulling pig 41 is taken 20 times smaller than that of the inside of the sub-duct 40 (i.e. 2.5 mm2), the return velocity of the water will be 20 times larger than the forward velocity inside the sub-duct 40 (and reversed), so the change in velocity is 21 times that of the velocity in the sub-duct 40.

These conditions will result in a forwardly directed force of 21 x1 .6m/sx0.08kg/s = 2.7 N. This should be just enough (for a coefficient of friction of 0.1 ) to propel the floating sub-duct 40 over 820 m.

Pushing of the sub-duct 40 at the main duct 20 entry also helps. Pushing should not be done in this case with a pushing force higher than 50 N, because of the risk for buckling in the 150 mm inner diameter pipe, and the same buckling makes the pushing force only effective until about 5 N. However, still 500 m may be pushed in this way. Together with the pulling pig the installation over 820m may be achieved with the present invention. Last step of the example is to eliminate the floating effect of the subduct 40, and it is achieved by removing the sub-duct 40 out of the main duct 20, with a final removal of the water which was filling the main duct 20.

As a second working example the 12/8 mm sub-duct is made of composite plastic - glass with a density of 1 .6. The sub-duct is now jetted in with air. In this case the total density is 0.89, just floating. When the 4 mm cable of standard density of 1 .2 is installed with water, the total density becomes 1 .36, clearly sinking. Even with a density of the sub-duct of 0.98 a floating - sinking situation is created, but now amply floating and just sinking, while just floating (low friction) and amply sinking is to be preferred. The pulling effect of the pig is more difficult to calculate for air than for water. There is a higher velocity of the airflow, but the mass is less than that of the water flow. However, mixing of the airflow with the water in the internal space 25 will enhance the pulling effect. The pulling force can be found by trial and error.

It is understood that obvious improvements and/or modifications for one skilled in the art maybe implemented and being under the scope of the invention as it is defined by the appended claims. In particular, it may be considered advantageous to use a hollow rod made of glass fiber reinforced with plastic (GFRP) as sub duct, as its density during the jetting with air may be appropriate, and it is easy to remove it afterwards. It should be noted that this kind of rod inherently present a high stiffness, so that a pushing step will be efficient to introduce it into the main duct.