Insulated pipes for buried or trenchless installation

Abstract

The invention relates to a temperature insulated pipe section (10) with a central metal tube (12), a thermal insulation layer (16) of a polymer foam, and (optionally) a jacket layer (18) protecting the thermal insulation layer from mechanical damage. The insulated pipe section has an outer casing layer (20) and a rough, friction increasing outer surface layer (14) that comprises protruding particles (14.1) that are embedded in a matrix (14.2) holding the particles (14.1). The friction increasing outer layer encloses the outer casing layer (20).

Description

[0001] Insulated pipes for buried or trenchless installation

[0002] The invention relates to pipes for trenchless and / or buried installations, in particular insulated steel pipes with composite cladding for temperature-insulated pipes, in particular district heating pipes.

[0003] District heating pipes are used to transport a liquid heat transfer medium - usually water - to provide heat to consumers. For example, it is known to transport heat from block-type thermal power stations or other industrial heat sources via district heating pipes to industrial or residential consumers. In order to keep the loss of heat energy as low as possible during transport, i.e. while the heat transfer medium is flowing through the district heating pipe, district heating pipes are insulated. For this purpose, thermal insulation made of a polymer foam or other insulation materials is typically applied to the outside of a central tube. A district heating pipe is therefore typically composed of a central tube carrying the heat transfer medium and a jacket layer applied to the outside of the central tube. The central tube typically is a steel pipe.

[0004] Between the central tube and an outer jacket layer a thermal insulation layer preferably made of polymer foam or other insulation material is provided. The polymer foam or other insulation material typically comprise polyurethane or is a polyurethane foam. On the outside, the thermal insulation layer is covered with a protective jacket layer, which can be a polyethylene (PE) jacket pipe, for example. The central tube for the heat transfer medium has an internal diameter in the order of 100 to 2000 mm, i.e. between 250 mm and 700 mm, for example.

[0005] The invention addresses the problem of improving the installation and the durability of pipes in general and for district heating pipes in particular.

[0006] The invention concerns a pipe section, in particular district heating pipe or other temperature-insulation pipe section that is embedded in soil when installed. The pipe section comprises a central metal tube for carrying a fluid or gaseous medium at a temperature different from the surrounding soil. The pipe section comprises a thermal insulation layer in order to reduce a heat transfer between the fluid conveyed by the central metal tube and the soil surrounding the pipe with the pipe section. The pipe section comprises an outer casing layer made of plastic material, in particular made of fibre-reinforced plastic. The pipe section has a rough, friction increasing outer surface layer that comprises protruding particles that are at least partly embedded in a matrix holding the particles. The particles are as hard or harder than the matrix that holds the particles. The friction increasing outer layer encloses the outer casing layer.

[0007] The invention includes the recognition that metal pipes in general and district heating pipes in particular are prone to elongate when the metal pipe becomes warmer and to shorten when the metal pipe becomes colder. For pipes installed above ground as well as for buried pipes extension loops, potentially with expansion cushions, are provided that allow for a compensation of length variations without causing too high structural forces within the pipes' walls. However, in particular for buried pipelines, the expansion loops require particular design and engineering consideration in terms of soil compression potential and restriction of movement at the joints of the expansion loops. Trenchlessly installed pipes cannot have arches or extension loops for compensating length variations over the length of the trenchless installation. Instead, sometimes costly shafts are sunk from the surface to the trenchlessly installed pipes to anchor the pipes in certain segments, e.g. with concrete weight blocks or steel anchors. Otherwise, the length of trenchless installation is limited by the required maximum distance between the expansion loops.

[0008] The inventors found that the forces for elongation of the steel due to temperature changes can nevertheless be controlled if the outer surface of the pipes is rough so as to increase the friction between soil and pipes. For increasing surface roughness of the outer surface of the pipes, an outer friction-increasing layer that comprises protruding particles that are as hard or harder than a plastic matrix in which they are embedded is provided.

[0009] Traditionally, low-friction outer surfaces are preferred for pipes that are trenchlessly installed. The inventors found that trenchless installation is possible with pipes having a friction increasing outer surface and that the friction increasing outer surface improves the pipe's resistance with respect to heat-induced (and other) elongations because the friction increasing outer surface increases the friction between the pipe and the soil, the pipe is embedded in. Thus, forces and tensile stress induced by heat are locally absorbed and do not cause axial movements of parts of the pipe that otherwise would lead to even higher stress in some parts of the pipe.

[0010] The matrix that is holding the particles preferably is made from a thermoset resin such as polyester or vinyl ester resin. Alternatively, epoxy resin may be used to provide an even stronger outer surface. Using a thermoset resin for the matrix holding the particles results in a strong bonding of the particles to the outer surface of the pipe and prevents particles from getting lost during the expansion cycling or when a pipe is trenchlessly installed.

[0011] An increase of outer surface friction compared to the smooth surfaces of a jacket layer or a thermoset resin can already be achieved by integrating particles into the matrix with grain sizes of 0.2 mm. The particles preferably have grain sizes up to 6.3 mm. Preferably particles with grain size ranges of 0.4 to 0.8 mm or 0.7 to 1 .2 mm and preferably between 3.2 and 5.0 mm are being used, depending on the desired surface roughness and friction to the surrounding soil required. Grain size ranges can be established based on ISO 3310-1 in combination with ISO 2591-1 and a grain size range of 0.4 to 0.8 mm implies that only approx. 10 % of all grains are smaller than the lower end of the range, in this case 0.4 mm and approx. 10 % of all grains are larger than the upper end of the range, in this case 0.8 mm.

[0012] Besides the grain size, the protrusion of the particles from the matrix is a relevant determining factorto the effect of the surface roughness vis-a-vis the soil. Protrusion of the grain material can be expressed as a protrusion ratio, for instance a fraction of measurable depth from highest outer surface points to lowest outer surface points over a certain length relative to the average grain size. The protrusion ratio should be at least 30 %, preferably at least 50 % or in some instances at least 70 %, i.e. it is preferred if the particles protrude more than half of their radial extension from the surface surrounding protruding particles. The protrusion can be measured by means of a depth caliper. The radial extension of the particles may be estimated according to the grit size. Therefore, the protrusion ratio can be estimated as measured protrusion divided by the grit size of the particles.

[0013] The matrix holding the protruding particles may partially or even completely cover the particles. In such embodiment, it is both, the particles and the matrix that covers the particles, which are protruding from the surface between the particles.

[0014] Further the effect of the surface roughness on the surrounding soil depends on the particle coverage of the surface. Depending on the particle sizes, particle coverage can be from 30 % to 100 %. Larger particles have a better effect with lower coverage ratios from at least 30 %, preferably at least 50 %. Smaller particles require a higher coverage ratio of at least 60 %, preferably at least 75 %. For a particle-covered surface portion of the pipe section, the particle coverage C is the ratio between the area covered by particle APand the total area of the surface portion As:

[0015] C=AP / As

[0016] Further, the particles preferably have a hardness according to the Mohs scale of at least 6, preferably at least 7.

[0017] The friction increasing outer layer preferably provides a friction factor according to NEN 9997-1 :2011 of at least 0.5, preferably more than 0.6 to the surrounding soil. Likewise, a coefficient of static friction p between the friction increasing outer layer and the surrounding soil preferably is at least 0.6 and preferably at least 0.7 with the surrounding soil, with such soils being for example sand or clay or drill-grout. The coefficient of static friction is the axial force Fathat is needed to overcome the static friction (causing a friction force Ft; i.e. Fa = max(Ff)), i.e. before sliding begins when a radial force Frexerted by the soil acts on the outer surface on the pipe:

[0018] Ps= F a / F r

[0019] Accordingly, a pipe section, in particular a section of a district heating pipe, is proposed, which has a central metal tube, in particular a steel tube for carrying a heat transfer medium, a heat insulation layer made of a polymer foam or other insulation materials, in particular polyurethane foam, and a jacket layer, in particular a jacket pipe made of polyethylene (PE), which protects the heat insulation layer from mechanical and electrolyte damage. The district heating pipe has an outer composite casing layer (composite cladding) made of fibre-reinforced plastic. The invention is not limited to district heating pipes but is suitable for all sorts of pipes that may undergo temperature changes during use.

[0020] According to the invention, the pipe is provided with an outer friction-increasing layer as part of the composite cladding that comprises protruding particles that are as hard as or harder than a plastic matrix in which they are embedded. While the invention is particularly suited for district heating pipes, the invention is not limited to such pipes and may be used in combination with other pipes that comprise metal tubes that can elongate when heated or otherwise.

[0021] In alternative embodiments, the matrix holding the protruding particles may be the jacket layer. In such case, no further composite cladding, in particular no further outer casing layer made from fibre reinforced plastic is needed. However, an outer casing layer made from fibre reinforced plastic is preferred and may complement or even replace a traditional jacket layer.

[0022] According to a preferred embodiment that is suited for trenchless installation, the outer casing layer has a layer thickness of at least 4 mm, preferably at least 10 mm, in order to provide sufficient mechanical protection and, above all, to be able to absorb and transmit the tensile and compressive forces that occur, so that the thermal insulation layer is exposed to as few shear forces as possible during installation when tensile and compressive forces are introduced into the outer casing layer. The outer casing layer is dimensioned to absorb radial forces during thrusting the district heating pipe during trenchless installation. Embodiments for buried installations may have an outer casing layer (composite cladding) that has a layer thickness of less than 4 mm, for instance between 2 mm and 4 mm.

[0023] The central metal tube preferably has an internal diameter between 100 mm and 1600 mm.

[0024] Another aspect of the invention relates to a pipeline composed of a plurality of pipes, each of said pipes having at least one or more central metal tube(s) for carrying a heat transfer medium, a thermal insulation layer made of a polymer foam or other insulation materials,

[0025] (optionally) a jacket layer protecting the thermal insulation layer from mechanical and electrolyte damages, and an outer friction-increasing layer embedded on top of a composite casing layer.

[0026] The central metal tubes (steel pipes) are welded together at their end faces in such a way that the welded junction between two central metal tubes can transmit tensile forces and is tight with respect to the heat transfer medium.

[0027] According to preferred embodiments, district heating pipes have an outer composite casing layer made of fibre-reinforced plastic, whereby the outer casing layers of interconnected district heating pipes are joined together by means of fibre-reinforced plastic in such a way that tensile forces of preferably at least 50 kN (kilo-Newtons) can be transmitted from one district heating pipe to the next district heating pipe via the junction of the outer casing layers. A composite, fibre reinforced casing layer (composite cladding) has the advantage that the protruding particles of the outer friction-increasing layer can be strongly bonded to the composite, fibre reinforced casing layer by means of a suitable resin, preferably the same resin that is used for the composite, fibre reinforced casing layer.

[0028] Optionally, one end of the district heating pipe has a pulling head made of metal, in particular steel, which is sheathed in glass-fibre reinforced plastic. The pulling head itself is directly connected to both the central metal tube and the outer casing layer in such a way that pulling forces are introduced into both the central metal tube and the outer casing layer.

[0029] Preferably, the outer casing layers of interconnected pipes are scarf joined - i.e. connected to each other by a scarf joint - in the area of the junction over a length of at least 80 mm each, for instance between 50 mm and 1000 mm. Along the scarf joints, no outer frictionincreasing layer may be provided since the scarf joints are much shorter than the joined pipes. However, it is also possible to provide an outer friction-increasing layer also on the scarf joints.

[0030] According to a further aspect of the invention, a method of installing a district heating pipe is provided. The method comprises the steps of

[0031] Providing a first district heating pipe that comprises a force transmitting structure and an outer friction-increasing layer

[0032] Providing further district heating pipe sections without force transmitting structure and an outer friction-increasing layer, wherein each district heating pipe section comprises a central metal tube, an optional corrosion protection layer, a thermal insulation layer, an optional jacket layer, a casing layer and an outer friction-increasing layer.

[0033] The method further comprises

[0034] Joining the faces of the central metal tubes by welding,

[0035] (optional application of a corrosion protection layer),

[0036] Restoring the heating insulation layer,

[0037] (optionally restoring the jacket layer,) and

[0038] Joining the casing layers, for instance by way of sleeving or scarf joining.

[0039] Optionally, the scarf joints can be provided with an outer friction-increasing layer. However, in most applications, this additional step is not needed.

[0040] The invention will now be explained in more detail by means of examples with reference to the figures. From the figures shows:

[0041] Fig. 1 : A detail of a prior art scarf joint between outer casing layers of two interconnected district heating pipes;

[0042] Fig. 2: a detail of an outer casing of two interconnected district heating pipes with a rough outer surface layer according to the invention;

[0043] Fig. 3: a detail of an alternative embodiment of an outer casing of two interconnected district heating pipes with a rough outer surface layer according to the invention;

[0044] Fig. 4a: a detail of a rough, friction increasing, outer surface layer according to the invention in an idealistic representation; Fig. 4b: a detail of a rough, friction increasing, outer surface layer according to the invention in a more realistic representation;

[0045] Fig. 5a, b: are pictures of a friction increasing surface according to the invention;

[0046] Fig. 6: illustrates a particle coverage C of a surface portion of a particle covered pipe section;

[0047] Fig. 7: a section of a district heating pipe for illustrating thrusting of the district heating pipe;

[0048] Fig. 8: an embodiment of a district heating pipe that comprises a metallic protection shield; and

[0049] Fig. 9: a section of a district heating pipe embedded in soil.

[0050] Figure 1 shows a detail of a prior art district heating pipe illustrating a scarf joint between pipe segments. Like conventional district heating pipes, the district heating pipe 10 has a central metal tube 12.1 and 12.2, which may be provided with an optional corrosion protection layer (not shown) on the outside. The central metal tube 12.1 and 12.2 - and, if provided, the corrosion protection layer 13 - is surrounded by a thermal insulation layer 16, which in the example case is formed by polymer foam, namely polyurethane foam. The thermal insulation layer 16 is in turn surrounded by a jacket pipe 18, which in the example case is made of polyethylene. In this respect, the structure of the district heating pipe 10.1 and 10.2 corresponds to what is usual for district heating pipes.

[0051] In addition, the district heating pipe 10.1 and 10.2 has a casing layer 20 of fibre-reinforced plastic on the outside of the jacket pipe 18. The casing layer 20 has a fibre matrix formed for example by glass fibres, which is embedded in a synthetic resin matrix, in particular an epoxy resin, a polyester resin or a vinyl ester resin matrix.

[0052] The casing layer 20 made of fibre-reinforced plastic serves for two purposes:

[0053] On the one hand, it is intended to protect the district heating pipe 10 from external damage and, on the other hand, it is intended to transmit tensile forces occurring due to temperature-induced elongation to the soil that surrounds the pipe 10. The casing layer 20 may comprise protruding particles. The casing layer 20 may thus have an outer friction-increasing layer as illustrated in the following figures.

[0054] As shown in figures 2 to 4, an outer friction-increasing layer 14 that comprises protruding particles 14.1 is enclosing the casing layer 20. The particles 14.1 are as hard as or harder than a plastic matrix 14.2 in which the particles 14.1 are embedded. The outer frictionincreasing layer 14 can cover the entire pipe 10 including the joints between the pipe sections, see figure 2. In this case, the outer friction-increasing layer 14 is applied after joining the pipe sections. Alternatively, the outer friction-increasing layer 14 is applied prior to joining the pipe sections. Then, a laminate 26 of fibre-reinforced plastic for joining the fibres of the casing layers 20.1 and 20.2 will initially interrupt the outer friction-increasing layer 14 in the region of a joint. However, the laminate 26 may also be provided with an outer frictionincreasing layer 14 (not shown in figure 3).

[0055] The matrix 14.2 that is holding the particles 14.1 preferably is made from a thermoset resin such as polyester or vinyl ester resin. Alternatively, epoxy resin may be used to provide an even stronger outer surface. The resin preferably is the same resin that is used for the composite outer casing layer (composite cladding) 20. Figure 4a is an idealistic representation of the outer friction-increasing layer 14. Typically, the particles 14.1 are more or less covered and thus embedded in the matrix 24.2 as shown in figure 4b. However, the particles 14.1 nevertheless provide a height H of protrusion that causes an increased tube-soil friction and helps to transmit longitudinal forces induced by changing temperatures to the surrounding soil, thus preventing displacements due to elongation of the pipe 30. In the case wherein the matrix 14.2 holding the protruding particles 14.1 completely covers the particles 14.1 , it is both, the particles and the matrix that covers the particles, which are protruding from the surface between the particles by a height H, see figure 4b.

[0056] Preferably, the particles 14.1 protrude by at least 0.5 mm, preferably more than 1 ,5 mm from a circumferential surface that is formed by the matrix that holds the particles. In other words, the distance H (protrusion height) between surface valleys between the particles and the peaks of the protruding particles 14.1 is at least 0.5 mm, preferably more than 1 .5 mm.

[0057] Preferably, at least half of the particles 14.1 preferably have a grain size (grit size; S) of at least 0.5 mm, preferably more than 2 mm, see figure 4. Further, the particles 14.1 preferably have a hardness according to the Mohs scale of at least 6, preferably at least 7. The friction increasing outer layer 14 preferably comprises at least 5 particles 14.1 per square centimeter in average. Since the particles are not evenly distributed, not every square centimeter of the outer surface is comprising at least 5 particles. That is why the average number of particles is given. Figures 5a and 5b are pictures of a friction increasing outer layer 14 according to the invention. The pictures illustrate the distribution of particles 14.1 .

[0058] The effect of the surface roughness on the surrounding soil depends on the particle coverage of the surface. Depending on the particle sizes, a particle coverage can be from 30 % to 100 %. Larger particles have a better effect with lower coverage ratios from at least 30 %, preferably at least 50 %. Smaller particles require a higher coverage ratio of at least 60 %, preferably at least 75 %. For a particle-covered surface portion of the pipe section, the particle coverage C is the ratio between the area covered by particle Ap and the total area As of the surface portion:

[0059] C = Ap / As

[0060] With respect to the illustration in figure 6, the total area As of the surface portion is d * I and the area covered by particle Ap ist the area of all particle - represented in a darker color in figure 6 - together.

[0061] The friction increasing outer layer 14 preferably provides a friction factor (pipe-soil friction coefficient) according to NEN 9997-1 :2011 of at least 0.5, preferably more than 0.6 to the surrounding soil.

[0062] Likewise, a coefficient of static friction psbetween the outer surface of the pipe 10 and the surrounding soil 42 preferably is at least 0.6 and preferably 0.7, if the soil 42 is comprised of sand or clay or drill-grout. The coefficient of static friction is the axial force Fathat is needed to overcome the static friction (causing a fiction force Ft; i.e. Fa = max(Ff)), i.e before sliding begins when a radial force Frexerted by the soil acts on the outer surface on the pipe: s—F a / F r

[0063] This is illustrated in figure 9. The district heating pipe can be pushed orth rusted during trenchless installation. Forthrust- ing a district heating pipe, the first, most distal front section 10 of the district heating pipe is provided with a force transmitting structure 22 that comprises a metallic protection shield 28; see figure 8.

[0064] The metallic protection shield 28 radially extends between the central metallic tube 12 and the outer casing layer 20. The metallic protection shield 28 is fixed to the central metal tube 12 and attached to the outer casing layer 20. Preferably gussets 38 are provided that extend between a distal surface of the metallic protection shield 28 and an outer surface of the central metal tube 12. The gussets 38 are welded to both, the metallic protection shield 28 and the central metal tube 12. Where the outer casing layer 20 is attached to the metallic protection shield, the outer casing layer may have an increased thickness as shown in figure 2.

[0065] For thrusting district heating pipe, a thruster (thrusting machine) is used. Thrust pads 40 grab the district heating pipe and exert a radial clamping force Fc on the outer casing layer 20; see figure 7. For instance, the radial clamping force Fc can amount up to 17 MN and typically exceeds 3 MN. Thus an axial push force (thrusting force FT) between 600 kN and 1 ,5 MN can be achieved. It is a further effect of the friction increasing outer surface layer 14 that even higher thrusting force FT can be achieved without increasing the radial clamping force Fc. This compensates for a higher friction between the soil 42 and the pipe 10 during trenchless installation.

[0066] During thrusting, the outer casing layer 20 must withstand the radial clamping force Fc. Further, the outer casing layer 20 transmits the axial force FT towards the distal end of the district heating pipe and the metallic protection shield 28. The metallic protection shield 28 thus pushes the central metal tube 12 and the outer casing layer 20 and the central metal tube 12 move forward together. Thus, no or little shearing and radial forces are acting on the insulation layer 16 while the district heating pipe is thrusted during trenchless installation.

[0067] Since a complete district heating pipe 30 is typically composed of several district heating pipes sections 10, which are each connected to each other at their longitudinal ends, the tensile and / or compressive forces must also be transferred across the corresponding connection points from one district heating pipe section 10.1 to the next district heating pipe section 10.2. The connection of the central metal tubes 12.1 and 12.2 is made in the usual manner, for instance by welding the central metal tubes 12.1 and 12.2 together at their end faces. The weld 32 also ensures that the district heating pipe is tight, i.e. that no heat transfer medium can escape from the district heating pipe at the junctions between the district heating pipe sections 10.1 and 10.2. At the junction 24, an optional corrosion protection layer and the thermal insulation layer 16 are applied in the usual manner, so that no gap occurs in the corrosion protection layer and in the thermal insulation layer 16 where the sections 10.1 and 10.2 of the district heating pipe 30 are connected; see figures 1 to 3.

[0068] The gap in the jacket layer 18 is also closed in a manner known per se.

[0069] One way to restore the thermal insulation layer 16 and the jacket layer 18 in the area of the junction 24, or to close the gap that initially exists, is to first place a sleeve 34 with the diameter of the jacket pipe 18 around the junction, so that a cavity remains between the sleeve 34 and the restored corrosion protection layer, which can then be filled with polyurethane foam 36.

[0070] Finally, the casing layers 20.1 and 20.2 of the interconnected district heating pipe sections 10.1 and 10.2 are joined together by means of the laminate 26 of fibre-reinforced plastic in such a way that the newly applied fibre-reinforced plastic bonds directly to the fibres of the casing layers 20.1 and 20.2. For this purpose, these fibres are first prepared and exposed in the area of the junction, as is known, for example, by tapering and scarf joining the ends of casing layers 20.1 and 20.2. This ensures that the casing layers 20.1 and 20.2 can also transmit the tensile forces occurring during trenchless installation across the junction 24. The same applies to the central metal tubes 12.1 and 12.2, which are connected to each other with corresponding tensile strength by their welding.

[0071] The tapered or sleeved length L over which the respective casing layer 20 of fibre-rein- forced plastic is tapered or sleeved in the region of the junction 24 depends on the tensile forces to be transmitted and is, for example, between 4 and 100 cm on both sides of the junction, but preferably at least 10 to 30 cm in each case.

[0072] In order for the casing layer 20 to be able to transmit the tensile forces occurring during trenchless installation in the case of a district heating pipe with a central metal tube with an internal diameter of 700 mm, the casing layer 20 is preferably constructed as follows:

[0073] The thickness of the casing layer 20 can be varied depending on the tensile forces to be transmitted, but is usually between 4 mm and 24 mm. It has a fibre content of at least 50 % by weight, preferably at least 60 %. The casing layer 20 is composed of several layers of fibre reinforced plastic (FRP), whereby the individual layers can be formed in the form of rovings, UD fabrics, glass fibre mats, fibre fabrics or a combination of these and are applied to the thermoplastic cladding surface in a wet-on-wet process using a vinyl ester, polyester or epoxy resin, preferably in a machine winding process. At least the outer two individual laminate layers of the outer casing layer 20 are preferably cross-wound, i.e. the fibres and the synthetic resin matrix are applied with a first winding direction and a second fibre layer is applied with a winding direction opposite to this winding direction. The advantage of cross-winding is that the media tube 12 can be installed in any axial direction.

[0074] Preferably, the casing layer 20 has an approximately continuously decreasing layer thickness at the longitudinal ends of each of the district heating pipe sections 10 prior to being joined to form a district heating pipe 30. Preferably, the casing layer 20 tapers over a length of approximately between 100 mm and half a meter, in order to avoid or limit thickening as far as possible during the sheathing of the casing layer 20 in the respective junction area. This length of decreasing thickness is also provided with a peel ply on its outer side prior to scarf joining. This contributes to the fact that the longitudinal ends of the pipes do not have to be ground before scarf joining after welding the end faces of the district heating pipes 10.

[0075] The district heating pipe sections 10 are typically only joined to each other at the point of installation. For a longer district heating pipe 30, a large number of district heating pipe sections 10 are therefore joined to each other.

[0076] Ways to install a pipeline comprises of pipe sections 10 are as follows: buried but open trench installation with soil backfill to ensure locking of surface with geology trenchless installation by pulling or thrusting. In both cases, axial forces need to be transmitted over the entire length of the pipeline section. Horizontal directional drilling (HDD) is a steerable trenchless method of installing underground pipelines in a shallow arc along a prescribed bore path by using a surface-launched drilling rig. For HDD installation, preferably a pipe segment with a pull-head covered with cladding is used, Forthrusting the front of the pipeline may be configured differently to ensure that the thrusting forces are equally introduced into the metal pipe and the cladding. Thrusting requires pipe segments with sufficient radial strength, HDD doesn’t. • offshore installation. For offshore installation, a pipeline segment with a fastener and with a rough coat is grabbed. The surface with increased friction improves the performance of the gripping and holding during installation. Pipe segments for a pipeline for offshore installation need radial strength and possibly axial strength to hold the entire pipeline without rupturing the insulation layers.

[0077] When installing the district heating pipe 30, one district heating pipe section 10 after the other can be connected to the rest of the district heating pipe 30 and the district heating pipe 30 can be pulled forward piece by piece by the length of one district heating pipe section 10. Subsequently, another district heating pipe sections 10 can be joined to the district heating pipe 30. The joining includes the steps of welding the central metal tubes 12, optionally applying the corrosion protection layer, restoring the thermal insulation layer 16, adding the jacket layer 18 (jacket pipe 18) and scarf joining the casing layer 20, as described above.

[0078] The result is an installation method in which first a first district heating pipe section 10.1 is provided with a force transmitting structure 22, in particular a protective shield 22.2 of the type described above. In addition, a second district heating pipe section 10.2 and further district heating pipe sections are provided, the total length of the joined district heating pipe sections corresponds to the desired district heating pipe 30.

[0079] The second district heating pipe section 10.2 is connected to the first district heating pipe section 10.1 that is provided with the force transmitting structure 22, in the manner described. Subsequently, the composite of the interconnected district heating pipes 10.1 and 10.2 is pushed forward by approximately the length of the district heating pipe 10.2 by means of thrust pads 40 of a pipe thrusting machine (not shown). A third district heating pipe section can then be connected to the end of the second district heating pipe section 10.2 in a manner as described and the composed first part of the district heating pipe 30 can again be pulled or thrusted forward by the length of the third district heating pipe section. These steps are repeated until all district heating pipe sections 10.n are connected to form a complete district heating pipe 30 composed of joined heating pipe sections 10. The steps of joining two district heating pipes at their open ends and the subsequent axial advancement of the composite of district heating pipe sections thus alternate.

[0080] As already mentioned, each joining of the district heating pipe sections comprises the steps of: Welding the faces of the central metal tubes 12,

[0081] (optional Application of a corrosion protection layer),

[0082] Restoring the heating insulation layer 16,

[0083] Restoring the jacket layer 18 and

[0084] Scarf joining the casing layers 20.

[0085] The advantage of this method is that the district heating pipe can be installed without digging an open trench, so that above-ground infrastructure, for example, is not affected. Thus, a district heating pipe can be pushed or pulled under rivers, roadways, buildings or the like.

[0086] The forces FT required to push (thrust) and / or pull the district heating pipe forward are advantageously introduced into the sections 10 of the district heating pipe 30 created by connecting individual district heating pipe sections 10 by means of the force transmitting structure in such a way that no shear forces act on the insulation layer 16. The casing layer 20 thus has a dual function, namely on the one hand the function of protecting the jacket layer 18 and the thermal insulation layer 16 from mechanical and / or electrolytic damage and on the other hand of transmitting tensile forces between the interconnected district heating pipe sections 10 - and preferably to the same extent as the tensile forces are also transmitted by the interwelded central metal tubes 12. Further, the casing layer 20 provides the friction increasing outer surface layer 14 that comprises the protruding particles 14.1 that are at least partly embedded in the matrix 14.2 holding the particles 14.1. The matrix 14.2 that holds the particles 14.1 preferably comprises a thermoset resin that is the same resin that is comprised in the composite casing layer (composite cladding) 20. Thus, the bonding of the protruding particles 14.1 is strong enough for preventing particles 14.1 from getting lost when the pipe 20 is thrusted for trenchless installation.

[0087] As total area of a particle covered surface portion

[0088] Ap total area covered by particles in the particle covered surface portion a d, I Dimensions of a particle-covered surface portion L Scarf joint length

[0089] S Grain Size

[0090] H protrusion height, distance between surface valleys between the particles and the peaks of the protruding particles FT thrusting force

[0091] Fc clamping force

[0092] Faaxial force along the pipe

[0093] Frradial force exerted by the soil on the outer pipe surface

Claims

Claims1. Pipe section, in particular district heating pipe or other temperature-insulation pipe (10) that is embedded in soil when installed, with a central metal tube (12) for carrying a fluid or gaseous medium at a temperature different from the surrounding soil, and a thermal insulation layer (16), characterized in that the pipe section has an outer casing layer (20) made of plastic material, preferably fibre-reinforced plastic and a rough, friction increasing outer surface layer (14) that comprises protruding particles (14.1) that are at least partly embedded in a matrix (14.2) holding the particles (14.1), wherein the particles (14.1) are as hard or harder than the matrix (14.2) that holds the particles (14.1), said friction increasing outer layer (14) enclosing the outer casing layer (20).

2. Pipe section according to claim 1 , wherein the particles protrude by at least 30 % measured as a fraction of measurable depth from highest outer surface point to lowest outer surface point relative to average grain size, preferably more than 50 % from a circumferential surface (14.3) that is formed by the matrix (14.2) that holds the particles (14.1).

3. Pipe section according to claim 1 or 2, wherein the particles have a grain size (S) of at least 0.2 mm, preferably in grain size ranges of 0.4 to 0.8 mm or 0.7 to 1 .2 mm or 3.2 to 5.0mm, but preferably not more than 6.3 mm.

4. Pipe section according to at least one of claims 1 to 3, wherein the particles (14.1) have a hardness according to the Mohs scale of at least 6, preferably at least 7.

5. Pipe section according to at least one of claims 1 to 4, wherein the friction increasing outer layer comprises a particle coverage of at least 30 % on average, preferably at least 50 %.

6. Pipe section according to at least one of claims 1 to 5, wherein the friction increasing outer layer provides a friction factor according to NEN 9997-1 :2011 of at least 0.5, preferably more than 0.6 relative to soil comprised of sand, clay or drill-grout that surrounds the pipe after installation.

7. Pipe section according to at least one of claims 1 to 6, characterized in that matrix (14.2) that holds the particles (14.1) of the friction increasing outer surface layer (14) comprises a thermoset resin.

8. Pipe section according to at least one of claims 1 to 7, characterized in that the casing layer (20) has a layer thickness of at least 2 mm, preferably at least 4 mm for pipes that shall be trenchlessly installed.

9. Pipe section according to at least one of claims 1 to 8, characterized in that the central medium pipe has an inner diameter between 100 mm and 2000 mm.

10. Pipe section according to at least one of claims 1 to 8, characterized in that a jacket layer (18) protecting the thermal insulation layer from mechanical damage is arranged between the thermal insulation layer (16) and the- outer casing layer (20).11 . Pipe (30) composed of a plurality of pipe sections, each of the pipe sections (10.1 , 10.2) comprising a central metal tube (12) for conducting a fluid or gaseous medium at a temperature other than the surrounding soil, a thermal insulation layer (16) e.g. of a polymer foam, and(optionally) a jacket layer (18) protecting the thermal insulation layer from mechanical damage wherein the central metal tubes (12) are welded together at their end faces in such a way that the welded junction between two central metal tubes (12) can transmit tensile and compressive forces and is tight with respect to the transferred medium, wherein the pipe sections (10.1 , 10.2) have an outer casing layer (20) of fibre-rein- forced plastic, the outer casing layers (20) of interconnected pipe sections (10) beingconnected to one another by means of glass fibre-reinforced plastic in such a way that tensile forces can be transmitted from one pipe section (10.1) to the next pipe section (10.2) via the junction of the casing layers (20) and wherein the pipe sections (10.1 , 10.2) have a friction increasing outer surface layer (14) that comprises protruding particles (14.1) that are at least partly embedded in a matrix (14.2) holding the particles, wherein the particles are as hard or harder than the matrix that holds the particles, said friction increasing outer layer enclosing the outer casing layer (20).

12. Pipe according to claim 11 , characterized in that the outer casing layers (20) of interconnected pipe sections (10) are connected to one another by means of glass fibre-reinforced plastic in such a way that tensile forces of at least 50 kN (Kilo-Newton) can be transmitted from one pipe section (10.1) to the next pipe section (10.2) via the junction of the casing layers (20).

13. Pipe according to claim 11 or 12, characterized in that it comprises at one end a pipe section according to any one of claims 1 to 7.

14. Pipe according to at least one of claims 11 to 13, characterized in that the outer casing layers (20) of interconnected pipe sections (10) are each scarf joined over a length of between 50 mm and 1000 mm in the region of the connection.

15. Method of installation of a pipe, the method comprising the steps of:Providing a first pipe section (10) according to at least one of claims 1 to 9,Providing further pipe sections (10.2, 10.n) according to at least one of claims 1 to 9, each pipe section (10) comprising a central metal tube (12), a thermal insulation layer (16), (optionally) a jacket layer (18), a casing layer (20) and a friction increasing layer (14) with protruding particles (14.1),Joining the faces of the central metal tubes (12) by welding,Restoring the heating insulation layer (16),(Optionally) Restoring the jacket layer (18) andJoining the casing layers (20).

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