A METHOD OF ESTABLISHING SUBTERRANEAN PIPELINES OF HEAT INSULATED, CONCENTRIC PIPES, SUCH A PIPELINE AND A PIPE ELEMENT THEREFOR

Abstract: In various processing or extractive systems it is actual to convey warm substances through subterranean, insulated pipes of such great length that it is required to add heat en route. Preferably this is arranged with the use of concentric pipes with an outer heat insulating coating. Because the temperature in the double pipe may change it has been found necessary to insert in long pipes expensive axial compensators such as U-bent pipe elements. With the invention this is avoided in a simple manner in that the pair of pipes is made of a central pipe (12), about which is mounted a row of pipe elements (2) of the type "district heating pipe", the inner pipe (4) of which will then form the outer pipe in the pair of concentric pipes, while a surrounding rigid insulation foam (6) and an outer mantle pipe (8) of plastic will serve to anchor the pipe in a self stabilizing manner in the earth, viz. by an earth friction locking effect. The pipe elements (2) may be prepared for mounting in that at one end of the inner pipe (4) there is welded in some inwardly protruding centering rib plates (10) for later welding to the central pipe (12).

Description: A method of establishing subterranean pipelines of heat insulated, concentric pipes, such a pipeline and a pipe element therefor. The present invention relates to a heat insulated pipe system with concentric pipes for a heating medium and a transportation medium, respectively, preferably mounted in the ground. In connection with many processing and extractive plants it is desirable to convey hot fluids through pipes over such long distances and perhaps at such low speed that the desired temperature cannot be maintained solely by the pipe being heat insulated. It could also come to a temporary■stop of the fluid flow, during which the temperature could not be allowed to decrease. It is known to arrange for electrical heating underneath the heat insulation, but often it is preferred to use con¬ centric pipes, in which the fluid pipe is surrounded by an insulated pipe for conveying hot water. The concentric pipes are relatively easy to arrange and to join by a subterranean mounting, but since it is anticipated that the pipes will expand due to heating after the mounting and possibly later on will contract and expand by varying operational conditions, then care is taken to enable such movements by mounting of U-bent pipe elements in all straight pipeline sections, e.g. as closely as every 100 m. This makes the installation quite expensive, and besides it has not been practice to widely safeguard the insulation against intrusion of water from outside. It is known that with the use of an outer steel pipe at the outside of the insulation layer it is pos¬ sible to construct the pipeline by joining pipe ele¬ ments, in which the inner conveying pipes are anchored in a pre-stretched condition, viz. by means of radial fin members welded in partly between the central pipe and the surrounding conveying pipe and partly between the latter and the exterior steel pipe. With this tech¬ nique, which is known e.g. from US-A-2,924,245, it is possible to lay out long, straight pipelines without the use of axial compensators, but the mounting work is very expensive, and the pipe elements are heavy and expen¬ sive; moreover, the outer steel pipes are vulnerable to corrosion attacks, whereby there is a risk of water intruding into the insulation layer. In connection with the present invention it has been recognized that it is possible to improve this technique widely by partial use of the special tech¬ nology associated with district heating pipe systems, which is now a highly developed area. Here the protec¬ tion of the insulation layer towards the surroundings has been brought to perfection, with the use of outer protective pipes of plastic and extremely tight joints therebetween, and moreover methods and sets of rules have been developed enabling the laying of long pipe¬ lines without the use of anchorings, U-bends or axial compensators. By the invention it has been found possible to make use of this technique and know-how for improving and simplifying the said concentric pipe systems. It should of course be considered that a further pipe component has been added, viz. a centrally disposed pipe which will not necessarily have the same temperature as the surrounding 'district heating pipe', but as already mentioned as known it is possible to fix the central pipe to the surrounding inner pipe, whereby these pipes will act as a unitary pipe, which, according to the invention, can then be regarded as the inner pipe of an ordinary district heating pipe. In the district heating pipe technology it is known that the pipes may be laid in the ground in long, straight and even slightly curved runs without any need for the provision of possibilities of expansion, not either by means of U-bent elements. The inner pipe is anchored to the outer protection pipe through a rigid insulation foam that sticks sufficiently to both pipes in order to anchor the pipes against mutual axial dis¬ placements, and the outer protection pipe is self an¬ choring by the frictional pressure exerted by the sur¬ rounding earth or sand material along a stretch of a sufficient length, this length being dependent of the force with which the pipe tends to expand axially. With the use of the interior double pipe considerably in¬ creased forces will occur by heating and cooling, but according to the invention it is recognized that the required anchoring or self stabilization will then only be a question of a corresponding increase of "the re¬ quired length" of the pipe stretch. Since conveying pipes of the type in question will normally come into use only when the stretches are rather long, e.g. more than a kilometer, it will almost always be possible to achieve a rigid frictional anchor¬ ing over a considerable part of the stretch. What will change in comparison to the laying of usual district heating pipes will only be the length of the so-called friction restrained stretches, i.e. those outermost lengths at both ends of a straight stretch, along which there will be no total frictional anchoring, and for the large picture it is not very significant whether just these end lengths will be somewhat longer than usually. By way of example it may be a question of an increase from 200 m to 400 , but in return it will be achieved that over a length of perhaps 10 or 50 kilometers be¬ tween these end portions it will not be required to mount as much as a single U-element or any other type of axial compensator. At the ends of the stretch, whether this is more or less long, an associated problem will arise, viz. that the expansion displacement will be noticeably larger than normally in connection with district heating pipe¬ lines, but luckily this problem is very insignificant, because the displacements are practically equally easy to take up whether they amount to for example 15 or 30 cm. Relative to the saved U-bent pipe elements with associated excavations it will be of no importance whether at the ends it is a little more or less costly to arrange for the required take-up of the expectable displacements, e.g. with the use of bent out connections forced to be laterally displaced over a certain length. Alternatively even an anchoring to the earth of the pipe ends could be provided, this just requiring that the anchoring be dimensioned somewhat stronger than usually. The invention can be realized by the use of ordi¬ nary district heating pipes that can be pushed onto an already assembled central pipe and be successively fixed thereto by means of the centering connector fins; also usable are district heating pipes with a laid-in central pipe of a similar length for successive assembling of both the central pipe and the surrounding pipe elements. The joints are relatively simple to work out, because the pipe elements are mutually displaceable until the final joining, whereby it is possible to work with a minimum of connector pipe elements. In the following the invention is described in more detail with reference to the drawing, in which:- Fig. 1 is a lateral view, partly in section, of a pipe element for use in a concentric dual pipe system, Fig. 2 is a length sectional view of a pipe joint in such a system, and Fig. 3 is a top view of an end portion of a straight stretch of the dual pipe system. In Fig. 1 is shown a pipe element 2 that is ba¬ sically a quite usual district heating pipe element consisting of an inner conductor pipe 4, an insulation layer 6 surrounding this pipe and an outer protection pipe 8 of plastic. The ends of the conductor pipe 4 project from the ends of the outer pipe parts 6 and 8. At one end of this pipe element, as shown to the right, there is secured to the inside of the conductor pipe 4, by weldings A, a number of radial rib plates 10, which project in front of the pipe end and extend in¬ wardly to an inner diameter D corresponding to the outer diameter of a centrally disposed conductor pipe 12. By the delivery of the pipe elements this pipe 12 may be stored inside the element 2, unless the elements 2 are to be inserted onto a long, already laid out pipe length 12. As shown at the middle the pipe element 12 may be slightly projecting towards the right from an already mounted, outer pipe element 2, and the pipe 12 in the element to be mounted thereafter may then be pushed towards the left and be joined with the first pipe 12 by a welding joint 14. In the same connection welding joints B are made between the outside of the already mounted central pipe 12 and the associated support edges of the rib plates 10-' or at least along the outermost, accessible length of these edges. Thereafter the pipe element 2 is pushed to the left along the pipe 12 for bringing together the ends of the pipes 4 and 4', and these ends are joined by a welding C, Fig. 2, just as by the mounting of district heat pipes. Then a sealed connection is established between the ends of the outer protective pipes 8 and 8' by means of a connection sleeve 16, likewise as known from the district heating area. In the element 2 the central pipe 12 has the same length as the conductor pipe 4, and when the butt join€ 14 is provided with the pipe 12 slightly projecting at the left end, then the right end of the pipe 12 will be located correspondingly inside the right end of the pipe 4, but still in centering engagement with the rip plates 10. When the element 2, as mentioned, is pushed towards the left into the position shown in Fig. 2 the right end of the central pipe 12 will be brought to project from the end of the pipe 4 as far as the corresponding pro¬ jection of the pipe 12' in the previously mounted ele¬ ment 2 ' , i.e. the next element in the row can now be mounted just as here described, and so forth. Through the weldings A and B a real anchoring is established between the central pipe 12 and the con¬ ductor pipe 4, whereby the pipes, also when they adopt mutually different temperatures, will act as a joined unit, occurring stresses just being distributed between the pipes. For rising temperatures in both of the pipes the expansion forces will be higher than for the 'dis¬ trict heating pipe' 4 alone, because the total cross sectional area of the iron has now been increased, but seen over a sufficiently large pipe length the pipe will nevertheless be stabilized by the surrounding earth pressure against carrying out expansion and contraction movements, an anchoring force transfer taking place through the rigid insulation foam 6. By the rigid mounting of the central pipe 12 along the straight pipelength it will be achieved that also this pipe remains straight and undamaged inside the assembled, through-going pipe 4, without any use of axial compensators, and at the ends of the straight length this anchoring will imply that the pipes without further stabilization will extend at least approximately concentric also in such bends which are provided at the ends of the stretch with the purpose of taking up the length variations that may occur along the outermost, so-called friction restrained partial lengths, this being illustrated in Fig. 3. At the right hand end there may occur a pushing out of the bend shown, and it may be a further pushing out than would have been the case with a single pipe 4, as shown by a dot and dash line and a dotted line, respectively, but there is no technical problem in absorbing this extended displacement of the bend. Since the two pipes are rigidly interconnected at a place 20 not very far from the bend it cannot come to any significant displacement between the pipes in or at the bend, i.e. the pipes cannot damage each other. The central pipe may be anchored otherwise to the the pipe 4, but it will be appreciated that the joining method illustrated in the drawing is easy to work with in practice.

Claims: C L A I M S 1. A method of establishing a heat insulated pipe¬ line with concentric inner pipes for conveying mutually different fluids, said inner pipes being used for mu¬ tual, rigid anchoring by locally welding in centering means between them, while the outermost of the inner pipes is anchored or preanchored to an outer pipe sur¬ rounding an insulation layer on the outside of said outermost inner pipe, the outer pipe being suited for contributing to a stabilization of long and. substantial¬ ly straight lined stretches of the pipeline against carrying out substantial axial expansions and con¬ tractions in response to temperature variations in the inner pipes, characterized in that in connection with a subterranean mounting of the pipeline there is placed on the central pipe outer pipe elements of the type "pre¬ fabricated district heating pipe elements", i.e. ele¬ ments having an interior conductor pipe with a surround¬ ing layer of a rigid insulation foam and an outermost protection pipe of plastic surrounding the foam layer, the insulation foam forming a rigid anchoring between the inner conductor pipe and the outer protection pipe, use being made of such pipe elements, the inner con¬ ductor pipes of which have a suitably larger diameter than the central pipe for the formation of a desired flow-through cross section between these pipes, the anchored connection between the central pipe and the surrounding inner pipe of these elements being estab¬ lished by the end to end joining of the respective dis¬ trict heat pipe elements by a welding in of the said centering means between these pipes. 2 A method according to claim 1, characterized in that the centering means in the form of radial rib plates (10) are mounted at at least one end of the pre¬ fabricated district heat pipe elements, inwardly pro¬ jecting from the inner conductor pipe thereof to a smaller diameter corresponding to the outer diameter of the central pipe (12) , and that these centering means are welded to the central pipe at the mounting place subsequently to the district heating pipe element being displaced along the central pipe to a position, in which the end of the inner conductor pipe (4) thereof abuts the correspondingly projecting end portion of the inner pipe (4 ) in the previously mounted district heating pipe ele ent. 3. A heat insulated pipe system laid in the ground by the method claimed in claim 1, with inner concentric pipes for conveying respective fluids, preferably a fluid to be transported and a heating fluid, respective¬ ly, said inner pipes being locally rigidly anchored to each other by welded in centering means, while they are surrounded by a heat insulating layer (6) which is again surrounded by a mantle pipe (8) , centering holding means (10) at least locally being mounted between these re¬ spective pairs of concentric pipes and providing a rigid axial anchoring between the pipe parts, characterized in that the pipe parts mounted about the central one of the pipes are pipe elements of the type "district heating pipes", i.e. consisting of an inner steel pipe and an outer mantle pipe of plastic and, provided therebetween, a rigid insulation foam forming the anchoring between the inner pipe and the mantle pipe. 4. A pipe system according to claim 3, charac¬ terized in that the said pipe elements are mounted and joined with the ends of their inner steel pipes (4) directly abutting each other. 5. A pipe element for use in the construction of a heat insulated pipe system according to claim 3, charac¬ terized in that it is constituted by an element of the type "district heating pipe", consisting of an inner steel pipe and an outer mantle pipe and, provided there¬ between, a layer of rigid insulation foam forming an anchoring between these two pipes, a number of inwardly protruding rip members (10) being welded in at an end area of the inner side of the inner pipe so as to extend inwardly to a diameter corresponding to the outer dia¬ meter of a central pipe, onto which the pipe element is to be mounted. 6. A pipe element according to claim 5, charac¬ terized in that as a ' loose part it comprises a length of the relevant central pipe, optionally housed inside the pipe element in sliding engagement with the inner ends of the rip members (10) .

Inventors: PEDERSEN HANS NOERGAARD, PEDERSEN, HANS, NOERGAARD
Applicants: ABB I C MOELLER A S, ABB I. C. MOELLER A/S
Application year: 1991
Main classification: F16L59
IPC codes: F16L59