EP0221986A1

EP0221986A1 - Offshore structures

Info

Publication number: EP0221986A1
Application number: EP86903465A
Authority: EP
Inventors: Geoffrey Milson John William
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-05-16
Filing date: 1986-05-16
Publication date: 1987-05-20
Also published as: WO1986006771A1; GB8512436D0

Abstract

La construction de diverses structures est assistée en congelant artificiellement l'eau ou un matériau contenant de l'eau dans ou à proximité d'une partie inférieure de la structure ou entre des éléments de la structure. Dans le cas d'une plate-forme de gravité (1), par exemple, l'eau dans le vide (8) sous la base (7) de la plate-forme est congelée par un réseau de tuyaux (9) refroidis par un appareil de congélation (13). Ceci crée une zone congelée sous la structure qui s'étend avantageusement dans le sol naturel et qui augmente la résistance de la structure contre les forces latérales exercées par exemple par de la glace flottante (6). D'autres applications de congélation sous-marine sont également décrites.The construction of various structures is assisted by artificially freezing water or a material containing water in or near a lower part of the structure or between elements of the structure. In the case of a gravity platform (1), for example, the water in the vacuum (8) under the base (7) of the platform is frozen by a network of pipes (9) cooled by a freezer (13). This creates a frozen area under the structure which advantageously extends into the natural soil and which increases the resistance of the structure against the lateral forces exerted for example by floating ice (6). Other underwater freezing applications are also described.

Description

Offshore structures

This specification relates to structures adapted to be sited offshore such as immersed tube tunnels or gravity platforms, artificial islands etc. for oil or gas exploration or other purposes where a static offshore structure is required.

It is known to construct an underwater tunnel by prefabricating it in tube sections, each of which are closed at the end by temporary bulkheads or diaphragms so that they can be floated into a position over their final location where they can be lowered onto the seabed. Here each section in turn is joined onto the previously placed section, and when the joint is completed and waterproofed, the adjacent bulkheads are removed to open the tunnel between the sections.

Various methods have been devised for making watertight joints between tube sections. It is usual to make a temporary seal immediately after the sections are first brought together. This enables the space between the bulkheads on each side of the joint to be drained, and personnel can then enter the space to construct the permanent joint between the sections from the inside in dry conditions. These methods have proved satisfactory in the depths of water in which immersed tube tunnels have been built hitherto but tunnels at considerably greater depths ae now under consideration and there are doubts as to whether the established methods will be as effective at these depths. Some methods of achieving the temporary seal involve the placing of concrete underwater around the outside of the joint, which becomes an increasingly difficult process with increasing depths of water. Other methods involve the use of a rubber sealing ring between the faces of the elements that meet at the joint. However, the rubber sealing ring admits of little or no flexibility for adjusting the relative positions of the tube sections to correct for construction tolerances, so that there is a demand for great accuracy in making and placing the sections. As the depth of water increases this accuracy becomes more difficult to achieve. Thus both of the currently accepted methods for making a temporary seal between two immersed tube sections involve increasing difficulties as the depth of water increases. Furthermore the deeper tunnels that are now under consideration are generally located in less sheltered waters so that there is an increased likelihood of tube section sinking and joining operations being inter¬ rupted by bad weather.

It is also known to provide offshore structures such as gravity platforms and artificial islands which rest on the seabed. Such structures have to resist the overturning and translatory forces from water and wind (and sometimes earthquakes) that are common to all localities, but in the Arctic and Antarctic they also have to resist the forces exerted by floating ice and these can be very large.

A gravity platform resists all these lateral forces by its own weight and by the shearing resistance between the base of the platform and the ground beneath. Thus the creation of contact -between the base of the platform and the ground effective to resist such forces is crucial to the performance of the structure.

With such gravity platforms most of the under- water part of the structure is usually.constructed away from its final site and then floated to the site and there sunk to rest in the required position. The ground surface beneath the platform is generally prepared as much as possible but it cannot be made sufficiently smooth to achieve contact with the entire area of the platform base and means must be provided for filling the voids under the base with material strong enough to resist the overturning and translatory forces. Sometimes the platform may be lowered onto relatively small temporary foundations, but the problem of filling the space between them remains. Generally this problem has been met by injecting material to fill the space, but then there is always doubt as to whether the space has been filled completely upto the platform base, and the injected material has to be transported to the site. In the Arctic and Antarctic there is the added problem that the sea is icefree for only a short period each summer so that the time available for preparing the seabed, floating-in and sinking the structure and filling beneath it is particularly short.

In the case of artificial islands, these are constructed in the sea by placing suitable fill material on the seabed. This can be either soil that has been excavated by a dredger and conveyed to the site by barges, pipelines or the dredger itself or soil that has been excavated on land and conveyed to the site by barges, pipelines or by vehicles operating on the frozen sea in "winter. The sides of the island may be contained and protected by suitable slopes with armouring or by caissons or cofferdams.

In polar regions these islands may be subjected to considerable forces from floating ice which is mostly resisted by the strength of the soil in the islands and by the ground beneath. It is known that the soil in the islands freezes downwards from the top, creating a permafrost layer and also that this freezing substantially increases the strength of the soil. However the permafrost layer is of limited thickness so that the most likely zone of failure under the ice forces is in the unfrozen ground below the frozen layer.

Viewed from one broad aspect there is disclosed herein a method of construction of an offshore structure in which a first immersed portion is to be connected to or supported by a second immersed portion of the structure or by a foundation, wherein such supporting or connection is e^'ffeσted or assisted by freezing water or water containing material adjacent said first immersed portion.

The method may be of use in the construction of an immersed tube tunnel, for example. In this application_* the connection between tunnel forming tube sections, or between such a section and another portion of the structure such as a ventilation building or a pier, can be assisted by freezing the water between the portions of the structure to be connected. Thus in a preferred method at least one of the immersed portions is a tunnel forming tube section and the connection of the portions is assisted by forming a frozen watertight seal therebetween.

A seal of this kind can provide flexibility for adjusting the relative positions of the immersed portions of the structure, while generally minimising the use of floating plant susceptible to weather conditions on the surface of the water. In general, the frozen seal will be temporary and will be main¬ tained until a permanent seal between the immersed portions has been made.

Generally the region where the frozen seal is to be formed should be enclosed to reduce the escape of cold into the surrounding water. This can be done by providing a collar e.g. of steel or reinforced concrete which, in the case of a connection between two tunnel forming tube sections of generally rectangular cross-section, might enclose the sides and the roofs of the sections on each side of the connection and the freezing can then be restricted to the water between the collar and the outside of the sections. The loss of cold can be further reduced by incorporating an insulating layer in the collar. The connection can be further enclosed by dumping filling material over it, and indeed filling material could be used to cover the joint without the presence of a collar, the method then involving freezing the water in the filling material. If necessary a membrane could be placed over any gap between the sections to prevent the material from entering.

The freezing can be effected by circulating a conventional heat transfer fluid through- a freezing pipe system built into the surface of the tube section or other portion of the structure adjacent to the water to be frozen. The heat transfer fluid can be chilled by conventional refrigeration equipment or by the use of liquid nitrogen located in a convenient position. In a preferred method the ends of two tunnel forming tube sections are positioned adjacent each other for connection, each section being provided with a temporary sealing bulkhead longitudinally spaced from its end to be connected such that a water filled chamber is defined between the two bulkheads when the section ends are positioned adjacent each other. Usually one of the sections will be positioned- before the other, so that it will be convenient for the refrigeration equipment or liquid nitrogen to be located within the previously positioned section, in which case it will be necessary to connect it to the freezing pipe system built into the section on the far side of the connection before the water filled chamber can be dewatered. This connection can be made by a diver entering the chamber through a pressure lock provided in the bulkhead of the previously positioned section, the diver then making the necessary connection while inside the chamber. Alternatively a temporary connection can be made between the two sections by jacking out a tube from the bulkhead in the previously positioned section across the water filled chamber to seal against an opening provided in the bulkhead of the newly positioned section, the tube enabling the new section to be supplied from a cooling system in the previous section. Since this tube has to be thrust out against the water pressure it will preferably be of the minimum diameter to provide access. Since the diameter will be small,' there should be little difficulty in accommodating small errors in alignment between the forward end of the jacked out tube and the opening in the bulkhead. It would then be a simple matter to connect the freezing pipe system in the new section to the system in the previous section through this tube. As is the case with known methods of forming a connection between tube sections, when the chamber between bulkheads is dewatered to enable the permanent seal to be made, the water pressure on the bulkhead of the new section will no longer be there to balance the corresponding pressure on the bulkhead at the other, forward end of the new section. In a preferred method this force on the new se-ction is transferred to the previous section through the ice or frozen filling material formed at the connection, for example by forming shear keys in the outside of the sections on each side of the connection, and if a collar is used, then also in the inner face thereof. In this way the force can be transferred from the new section to the previous section by shearing forces in the frozen material. Alternatively, the adjacent end faces of the sections being joined may be provided with sufficient area for the force to be transferred by direct σompressive thrust through the ice. As discussed hereinafter, it is possible to strengthen the ice by injecting e.g. fibre into it. Alternatively jacks can be inserted between the sections at the connection to transfer the force. A further method of resisting the force is to place backfill adjacent to the sides of the new section before the chamber between the bulkheads is dewatered.

A special case often arises when the last tunnel forming tube section in a succession thereof is placed- in position because it frequently has to be placed between two previously placed sections or between a previously placed section and a fixed structure, such as a ventilation building or a built insitu section of tunnel. In any case an appreciable gap cannot be avoided at the last connec¬ tion to be made. If that connection is between two immersed tunnel tube sections the method previously described can be used to seal it, although no measures will be needed to transfer the unbalanced horizontal force on' the last section because that will have been removed already by completion of the connection at its other end. If a connection has to be made up to a 'fixed structure, such as a ventilation building, a freezing pipe system c ιan be built into the faces of that structure around the connection and the shape of the collar may have to be adapted to fit against the fixed structure.

The tunnel tube sections may be of rectangular cross-section, but the principles can be applied to tunnels of any shaped cross section. The methods described herein could of course be used for making some connections in a tunnel, while conventional methods could be used for making the other connections. In a preferred method, the offshore structure is a gravity platform which rests under its own weight on its foundation, the method including freezing the water or water containing material disclosed beneath the base of the platform so as to increase the resistance of the structure to lateral forces imposed thereon.

The foundation of the structure might be the natural ground forming the bed of a sea, or a lake, or a river etc., or the ground might be specially prepared.

If the platform rests on its foundation with little or no penetration into the ground beneath and, as is generally the case, the ground is not smooth throughout, then there will be voids beneath the platform base and the water in these voids will be frozen.' It is advantageous if the frozen zone extends to the ground beneath the platform base in addition to.such voids. This would increase the strength of the interface of the frozen material 'and the ground, and it would also make the structure less vulnerable to any soft ground that had been left on the site before the platform had been placed in position.

The material to be frozen in the voids beneath the platform base can be simply the ambient water, or this can be added to, or modified, to improve the effectiveness of the frozen material and the performance of the structure. For example, in a preferred method, before it is frozen one or more additives can be injected into the voids beneath the structure for such purposes as increasing the viscosity of the water before it is frozen to facili- tate the control of local freezing, or to improve the properties of the ice, including its strength and ductility. This includes natural and artificial fibres, chemicals, polymers and other additives. In some circumstances material such as gravel might be introduced into the water to be frozen to displace some of the water and thereby reduce the latent heat that has to be removed in order to create the frozen zone. Freezing could be effected over substantially the entire area of the platform base, or alternatively frozen zones could be selected that were most critical to the stability of the structure. In one possible method, freezing could be effected initially in one or more critical location(s) to support the platform and the installations to be placed upon it, together with any wind and water forces imposed upon them, and freezing could be subsequently^' extended to other locations, for example, to meet floating ice forces.

It is frequently desirable that the gravity platform be movable from one site to another, and to achieve this in one preferred method the frozen material is heated and thereby thawed. By such heating the freezing effect is readily reversed and the platform or elements thereof can be floated away. The source of the heat for thawing might be warm atmospheric air if available, but preferably the heat would come from external thermal or electrical sources.

In another preferred method the offshore structure is an artificial island having a lower region comprised of fill material, the method including freezing the fill material in said lower region so as to increase its shear strength and thereby increase the resistance of the structure to lateral forces imposed thereon. With such a method, the support provided by the lower region of the island to the rest of the island is assisted by the freezing process, so stabilising the island. In practice, it is the water in the pores of the fill material which is frozen to increase its shear strength, examples of such fill material being sand or gravel.

The frozen zone will generally be disposed at a reasonable distance beneath the upper surface of the island, and if the island has an artificially or naturally frozen surface layer then the frozen zone will generally be disposed below this frozen layer, preferably extending downwards from it. The island rests on the natural ground of the seabed or the like and it may be desirable for the frozen zone to be disposed at least partly adjacent to the natural ground, for example, by extending down¬ wardly to it from the upper part of the island. Indeed it may be advantageous to freeze the natural ground itself, thereby improving the stability of the island on the seabed or the like.

While this preferred method is applicable to such islands in temperate and warm areas of the world, it is particularly useful in polar regions where the strength of the island below the frozen permafrost layer can be substantially increased by forming at least one wall _of frozen material extending downwards from the permafrost layer. Such a wall of frozen material forms a connection between the permafrost layer and the material beneath to reduce the likelihood of failure in the region below the permafrost layer. The wall would preferably be orientated to resist forces, particularly ice forces, from the direction in which they were most likely to develop. The frozen wall might extend down into the natural ground beneath the fill material of the island. Subsequent frozen walls can be orientated in other directions and located appro¬ priately until the island was capable of resisting forces likely to arise from any direction. whether the method of the first aspect is applied to gravity platforms or to artificial islands, freezing can be effected by use of conventional refrigeration apparatus and/or use of e.g. imported liquid nitrogen as a source of cold. However, in cold, e.g. polar regions much .of the cold required for the freezing can be obtained from the air, whose temperature may be well below freezing point for most of the year and particularly so when any ice forces on the structure are likely to approach their maximum. In the summer, when the structure would be placed in position in such regions, for much of the time the air temperature would fall below freezing at night which would enable the freezing process to start to a sufficient extent to stabilise the structure until the ice forces developed, but in some cases it might be necessary to supplement the air cooling with conventional ref igeration plant at that stage.

Preferably the method of stabilising an offshore structure such as a gravity platform or an artificial island comprises passing a working fluid and low temperature atmospheric air through a heat exchanger to reduce the temperature of the working fluid, and conveying the cooled fluid to a station where it effects freezing of the material in and/or adjacent the lower region of the structure. Conventional refrigeration liquids are suitable for use as the working fluid.

Viewed from another broad aspect there is disclosed herein a structure adapted to rest offshore on the seabed or the like, including means for freezing the material in and/or adjacent a lower region of the structure. Such a structure, whether a gravity platform or an artificial island, is advantageously stabilised by the frozen material as discussed hereinbefore.

Preferably, the freezing means is adapted to cool a suitable working fluid and to convey it to the lower region of the structure. This could be achieved, for example, by an arrangement of pipes extending to the lower region of the structure. The freezing means may comprise conventional refrigeration plant. However, if the structure is to be sited in a cold, e.g. polar, region then the freezing means may comprise a system for exchanging heat with low temperature atmospheric air which may if necessary be assisted by conventional refrigera- tion plant. In a preferred embodiment, the heat exchanging system includes an arrangement of fans and pumps near the top of the structure to cool the working liquid, and an^'arrangement of pipes and valves to enable the distribution and degree of the freezing process in the lower region of the structure to be controlled. This would preferably be facilited by a comprehensive system of flowmeters and temperature measuring devices, and it would also be possible to install one or more pressure measuring devices in the region where material is to be frozen. In certain circumstances the expansion of water as it froze could create unaccep¬ table local loading on parts of the structure and these could be relieved by appropriate pressure relieving means.

In order to reduce the loss of cold from the region in which material is to be frozen, it is .possible to provide suitable insulation. Generally such insulation would be provided to prevent cold moving upwards into the structure, as it would usually be advantageous for the freezing to spread downwards. When the structure comprises a movable gravity platform then heating means are preferably provided to thaw the frozen material. Although separate heating means may be provided, such as heating pipes or heating cables, in a particularly compact and simple arrangement the heat is supplied using the same means as that used for freezing purposes.

The freezing means may include a network of pipes located in the base of the platform. However in some cases the freezing could be accelerated by providing a freezing element suspended below the structure. This may be fixed in position so that water could circulate around it or it might be suspended so that it could be raised upto the platform base during installation thereof and subse¬ quently lowered either into any voids beneath or onto the seabed before freezing commenced. In the latter case the freezing element would be supplied through flexible pipes. If a recess is provided to accommodate the element when it is raised it will be less liable to damage when the platform is being moved. One.or more such freezing elements may be provided.

The base of the platform might be shaped in various ways to suit the circumstances of particular sites. For example a skirt might be provided to penetrate the ground slightly around the perimeter of the base to contain the water or material beneath it, or the entire area of the base might be divided into cells by similar walls penetrating the ground. The freezing means, such as a network of pipes, may be incorporated in such projecting skirts or walls. Again there might be no skirts or walls beneath the structure and the development of the frozen layer could be entirely controlled by the application of the cold over the area of the base that could be divided into sections for the purpose. Alternatively the skirt could be arranged to distort to bear on the ground or it might take the form of an inflatable tube or material might be placed around the element to create a seal. The surface of the base might also be shaped to provide a key with the frozen material and this could vary, according to the circumstances, from a simple doming to an intricate pattern of depressions and excrescences. In this way the frozen material would be integrated with the platform base to provide a strong foundation structure.

From the foregoing description it will be seen how the principles of this disclosure in its first broad aspect are^' applicable to a method of forming an offshore structure by joining two or more prefabricated elements by freezing the water between them. These elements, formed for example of precast concrete, may be initially separate and then joined by the frozen water or they may be initially connected to each other and then their connection reinforced by the frozen water. The method according to this aspect can be applied in conjunction with the methods described above in relation to gravity platforms and artificial islands, whereby the frozen zone is disposed in and/or adjacent a lower region of the structure, or it can be applied separately from the above methods, e.g. by joining elements disposed anywhere in the submerged part of the structure. The various preferred features described above, such as the type of freezing means to be used, the provision of insulation adjacent the frozen zone, displacing some of the water to be frozen, increasing its viscosity, or strengthening the ice, may also be used when joining such prefabricated elements.

In one broad aspect there is disclosed herein a method of freezing underwater material in and/or adjacent a lower region of an offshore structure, wherein atmospheric air is used as a source of cold. Such a method is suitable for use whenever the atmospheric air is sufficiently cold, and would preferably be operated using a heat exchanging system.

Finally, in another broad aspect there is disclosed herein a method of modifying the properties of water to be frozen in and/or adjacent a lower region of an offshore structure by injecting one or more additives into the water. Such additives could increase the viscosity of the water or increase the strength and ductility of the frozen material to be formed, and could for example comprise artificial fibres or polymers.

Certain specific embodiments of the various broad aspects of the disclosure will now be described,

< by way of example only, with reference to the accom¬ panying drawings, in which:- Figure 1 is a diagrammatic sectional view of a gravity platform;

Figure 2 is a partially cut-away perspective view of another gravity platform;

Figures 3 to 7 are sectional views of various gravity platforms, showing modifications to the platform base;

Figure 8 is a sectional view of a part of a modified platform base;

Figure 9 is a partially cut away perspective view of an artificial island supporting an offshore installation;

Figure 10 shows sectional views of three typical soil fill artificial islands; and

Figure 11 is a sectional view of an artificial island in which the fill material is frozen to strengthen the island.

Figure 12 is a view showing the sequence of construction of an immersed tube tunnel; Figure 13 is a sectional view of a connection between two tunnel forming tube sections;

Figure 14 is a view similar to Figure 13, but showing another method of forming the connection; Figure 15 is a sectional view of a part of a connection between tube sections, showing a modifi¬ cation using a shear key;

Figure 16 is a view similar to Figure 15, but showing another modification; and Figure 17 is a sectional view of a connection between a tube section and another portion of the tunnel.

Referring to Figures 1 and 2, these show gravity platforms 1 generally known as "arctic cones" and each comprising a topside structure 2 and a main platform structure 3 which is generally symmetrical about the centre line 4. The topside structure has the usual operations and accommodation modules. The gravity platform is shown resting on the seabed 5 and projecting through ice 6 floating on the sea. The platform has a flat base 7 resting on the uneven seabed and forming voids 8 beneath the base. Within the platform base 7 there is arranged a grid of pipes 9 and a thermal insulation layer 10 is disposed above the grid, which can either extend throughout the area of the base or it can be provided in selected areas. The pipe grid is supplied with working liquid by distribution pipes 11 which are fed by one or more vertical feed pipes 12. The working liquid is conveyed from freezing apparatus 13 contained in the topside structure 2 via the feed pipe(s) 12 and the distribu¬ tion pipes 11 to the pipe grid 9 to effect freezing of the water in the voids 8. The working liquid then returns via recovery pipes 14 and one or more vertical return pipes 15 to the freezing apparatus. Coσling of the working liquid may be effected by conventional regrigeration plant or by heat exchangers making use of low temperature atmospheric air, or by a combination of both systems. The platform may also include means for injecting additives into the voids 8 to modify the properties of the frozen material or to reduce the amount of water and hence the amount of latent heat of fusion which has to be overcome. The platform shown in Figure 1 simply rests on the seabed with no penetration into the ground beneath. It is conventional before sinking a gravity platform to remove ground from the site of the structure so that it rests in a- depression or ground may be removed and replaced by more suitable material for supporting the structure. Indeed a mound of suitable material might be raised above the seabed to reduce the depth of the structure. In all cases the material beneath the platform, whether this be water, or water plus additives, or the original ground, or fill material, can be frozen to strengthen the foundations. Figure 1 shows a zone 16 of the seabed itself which has been frozen by the freezing means.

Although the gravity platform shown is an arctic cone, the freezing principles could equally be applied to a gravity structure of any shape. These can be made of reinforced concrete, steel or other suitable materials and may be constructed and floated to the site as a unitary structure or as a number of separate prefabricated elements which are assembled prior to sinking. Figures 3 to 7 show various modifications of the platform base. Although these all show the space below the platform structure as being sealed, this does not necessarily have to be the case. Figure 3 shows a peripheral rigid skirt 17 which penetrates the seabed, and dotted lines indicate walls 18 which might be included to create cells 19 of frozen material. Such walls 18 might also be included in the other illustrated arrangements. Figure 4 shows a deformable skirt 20, which can provide for the expansion of the material to be frozen. The platform base may be domed as in Figure 5, or it may be supported on an inflated tube 21 as in Figure 6. Figure 7 shows a flat platform base having fill material 22 deposited around its periphery to seal the space beneath the base. It will be appreciated that various combinations of the platform base designs illustrated are envisaged. Figure 8 shows further possible features of the platform base. In this example a recess 23 is provided in the base to receive a freezing element 24 suspended by suspension cables 25 which pass through watertight seals 26 to winches (not shown) in an access gallery 27. The freezing element is. connected to the pipe grid 9 by flexible hoses 28. With this arrangement the freezing element can be lowered right onto the seabed and it can be raised into the recess, for example when the platform is to be moved.

The structure illustrated in Figure 9 is a conventional artificial island 29 in which a caisson 30 surrounds a mass of fill material 31 such as sand. The island supports an offshore installation 32. Other conventional artificial islands are shown in Figures 10 (a) , (b) and (c) . Figure 10(a) shows an island comprising sand and gravel fill 33 which has been dumped to a 1:3 slope design and covered by armour 34. The top surface 35 is protected by a peripheral wall 36 of armour material. Figure 10(b) shows an island having pumped fill 37, e.g. sand, up to the level of a flat berm 38, and dumped sand and gravel fill 33 above the berm. A sheet pile 39 provides additional protection. Figure 10(c) illustrates a similar design to Figure 10(b), except that no flat berm is provided. Like gravity platforms, artificial islands resist lateral forces by their own weight, and the strength of the fill material from which they are formed can be increased by freezing the material in one or more locations. Figure 11 shows the application of a freezing technique to an artificial island which has an edge 40 which can either be a caisson (Figure 9) or an armoured slope (Figure 10) . The island has a permafrost layer 40 which is penetrated by conventional ground freezing pipes 41 connected to freezing apparatus 42 and extending into the normally unf ozen -material 43 in a lower region of the island structure. In this way the weak region below the permafrost layer 40 can be reinforced by a wall of frozen material 44 extending downwards into the island structure. Possibly this wall can extend below the level of the original seabed. The freezing apparatus can be either conven¬ tional freezing plant or it can make use of cold atmospheric air if available.

In the described embodiments, the frozen material engages the surrounding unfrozen load bearing material to provide a bond or connection between the upper part of the structure and the surrounding material, whether this is the original ground or fill material which has been moved to the site.

Figure 12 shows three stages A,B and C in the sequence of construction of an immersed tube tunnel from tunnel forming tube sections 50, 51, 52 and 53, to be connected together respectively at connections 54, 55 and 56. Each » section comprises a tubular body having longitudinally spaced from each end 57 a temporary diaphragm or bulkhead 58 such that when the ends of two sections are positioned adjacent each other to be connected a chamber 59 is defined between two bulkheads. The chamber is initially filled with water until a temporary seal has been made, when it is dewatered to enable personnel to enter the chamber to make a permanent sealing joint from the inside of the tunnel. The direction of construction is from left to right as shown in Figure 12. At stage A the first section 50 is joined to the second section 51 by connection 54 where there is a temporary frozen seal, the chamber between bulkheads has been dewatered, and the formation of a permanent seal is in progress. At this stage floating plant 60 is used to lower the third section 52 into position on the previously prepared subgrade 61. The end of section 52 is positioned adjacent that of section 51 so that the frozen seal can be formed at their connection 55, a certain amount of flexibility being available in the relative positioning of the ends to be connected.. This is illustrated as stage B, before the chamber 59 at connection 55 has been dewatered. This connection is then dewatered and while a permanent seal is being made there the fourth section 53 can be lowered into position and a frozen seal formed at the next connec¬ tion 56. This is shown as stage C, by which time the permanent seal at the earlier connection 54 will have been completed and the bulkheads removed.

The manner in which a f ozen seal "65 can be formed between the ends 57 of two adjacent sections will be described with reference to Figures 13 and 14. Figure 13 shows the end of each section closed off by a bulkhead 58 so that the major length of each section is dry. The bulkhead of the previously positioned section is provided with a pressure lock 62 for the entry and exit of divers from the water filled chamber 59. Each section has a freezing pipe system 63 built into the outer walls at the section end, the pipes being arranged both near the longitudinal end faces 64 of the walls and along the lateral outer surfaces of the walls. This ensures that any water between the end faces is frozen, and also that the frozen seal 65 extends longitudinally on both sides of the adjacent section end faces 64. The freezing pipe system in the walls of the previously positioned section is connected directly to a cooling system located in that section, while the freezing pipe system in the newly positioned section is connected to the same cooling system via lines 66 which pass across the water filled chamber 59. A collar 67 is placed over the walls of the sections where the frozen seal 65 is to be formed for the purpose of reducing the escape of cold into the surrounding water. The collar 67 is advantageously provided with an insulating layer 68.

The procedure for connecting the two tunnel' sections shown in Figure 13 is a follows. After the sections are in position with their ends 57 adjacent each other a diver enters the chamber 59 via the pressure lock 62 and uses the lines 66 to connect the freezing pipe system in the newly sunk section to the cooling system in the previous section. The cooling system can then be operated to freeze the water adjacent the walls of the sections so as to form a temporary frozen seal. This is assisted by the insulating effect of the collar 67. Once the frozen seal is watertight the chamber 59 can be dewatered by means of a valve (not shown) which releases water into the previously sunk section, thus enabling a permanent sealing joint to be made in the annular space 69. After the permanent joint has been completed the cooling system is disconnected from the freezing pipe systems built into each section and the pipe systems are then carefully sealed off. A modified arrangement for forming a frozen seal is shown in Figure 14, in which there is no necessity to use divers. Instead of a pressure lock, the bulkhead of the previously sunk section is provided with a waterproof annular seal 70 through which an access tube 71 can be jacked out into the chamber 59 to seal against a compressible seal 72 provided round an opening 73 in the bulkhead of the newly sunk section. Each end of the access tube 71 is closed by a watertight door 74, and the opening 73 is likewise closed by a watertight door 75. Jacking equipment is not shown but will generally be required to push the access tube out against the water pressure. Once the tube has sealed against the bulkhead of the newly sunk section the doors 74 and 75 can be opened to allow the freezing pipe system in the new section to be connected to the cooling system in the previous section.

Figure 14 also shows an alternative method of insulating the temporary frozen seal 65. In this method a membrane 90 is placed over the region where the section ends meet and this is then covered over by backfill 91. The backfill is then frozen by operation of the freezing pipe systems, and, in combination with the membrane, provides a frozen seal.

It should be noted that for convenience the use of a collar has been shown combined with the use of divers, while the use of backfill has been shown combined with the use of an access tube, but that the methods do not necessarily have to be combined in this way. For example, the use of divers can be combined with use of backfill, and the use of an access tube can be combined with use of a collar. Indeed, the frozen seal. can be formed without any enclosure outside the sections if necessary. Referring again to Figure 12, it will be apparent that in stage B of construction, for example, the tunnel tube section 51 has been dewatered at its left hand end but that the bulkhead at its right hand end is still subject to water pressure P exerted by the water in chamber 59. As a result there is a horizontal force acting leftwardly on section 51 at a time when the permanent joint has not yet been completed at its left hand connection 54. Figure 15 illustrates one method by which frozen material can be used to resist such a horizontal force, namely by shaping the outer surface of the wall 76 to form a shear key 77. In this way the horizontal force on the section can be transmitted from the outer surface of the wall by shear forces in the frozen water 92 and into the surrounding collar which may also include a shear key 78. The exact shape of the shear key of both the tube section and the collar will be adapted to the magnitude of the horizonal force for the project concerned. In some cases the collar might be omitted and the force transmitted instead through frozen backfill.

An alternative arrangement for resisting horizontal thrust is shown in Figure 16. In this case the end faces 64 of the sections being joined are designed with sufficient area for the force to be transmitted by compressive forces in the frozen water (plus strengthening additives if necessary) between the faces. It might still be necessary to extend the freezing pipe systems 63 for some longitudinal distance away from the connection in each section to ensure a good frozen seal, as shown in Figure 16.

Finally, Figure 17 shows an arrangement in which a tunnel forming tube section 79 is to be connected to another part of the structure, such as a ventilation building 80. This is equipped with an opening shut off by a bulkhead 81, the face around the opening including its own freezing pipe system 82. A specially shaped collar 83 is provided to enclose what might be an appreciable gap between the tube section and the building, and frozen seals 84 and 85 are respectively formed between the section 79 and the collar 83, and between the collar and the building 80.

Modifications to the broad aspects of this disclosure and to the specific embodiments of such aspects referred to or suggested herein may be apparent to those skilled in the art and this dis¬ closure is intended to encompass any such modifica¬ tions. Viewed from one broad aspect, there is disclosed herein the concept of artifically freezing water or other material to assist in civil engineering or construction work, and there will be many cases, where this will be of use.

Claims

1. A method of construction of an offshore structure in which a first immersed portion is to be connected to or supported by a second immersed portion of the structure or by a foundation, wherein such supporting or connection is effected or assisted by freezing water or water containing material adjacent said first immersed portion.

2. A method as claimed in claim 1, wherein at least one of the immersed portions is a tunnel forming tube section and the connection of the portions is assisted by forming a frozen watertight seal therebetween.

3. A method as claimed in claim 2, wherein the frozen seal^' is temporary and is maintained'until a permanent seal beween the immersed portions has been made.

4. A method as claimed in claim 2 or 3, wherein a collar is provided to enclose the region where the frozen seal is to be formed to reduce the loss of cold therefrom.

5. A method as claimed ^"in claim 2, 3 or 4, wherein the ends of two tunnel forming tube sections are positioned adjacent each other for connection, each section being provided with a temporary sealing bulkhead longitudinally spaced from its end to be connected such that a water filled chamber is defined between the two bulkheads when the section ends are positioned adjacent each other.

6. A method as claimed in claim 5, wherein one of the sections is positioned before the other, and a tube is jacked out from the bulkhead in the previously positioned section across the water filled chamber to seal against an opening provided in the bulkhead of the newly positioned section, the tube enabling the new section to be supplied from a cooling system in the previous section.

7. A tunnel forming tube section for use in a method as claimed in any of claims 2 to 6, including a freezing pipe system built into the wall or walls of the section where the frozen seal is to be formed.

8. A method as claimed in claim 1, wherein the offshore structure is a gravity platform which rests under its own weight on its foundation, the method including freezing the water or water containing material disclosed beneath the base of the platform so as to increase the. resistance of the structure to lateral forces imposed thereon.

9. A method as claimed in any of claims 1 to

6 or 8, wherein an additive is added to the water to be frozen so as to modify its properties.

10. A method as claimed in cla^'im 1, wherein the offshore structure is an artificial island having a lower region comprised of fill material, the method including freezing. the fill material in said lower region so as to increase its shear strength and thereby increase the resistance of the structure to lateral forces imposed thereon.