NL1040430C2 - Method of laying a pipe under water. - Google Patents

Abstract

A method of laying a pipe under water includes the step of introducing matter to the interior of the pipe to permit the pipe to sink in the body of water. The matter comprising one of the group consisting of: solid matter; a liquid; a liquid and solid matter. The overall density of the pipe including the matter introduced to the interior of the pipe is greater than the density of the body of water in which the pipe is to be laid. The solid matter could be a cable. The liquid could be a salt solution which may comprise NaCl (sodium chloride) or ZnCl2 (zinc chloride), or any other suitable salt. Also claimed is a plastic pipe for laying at the bottom of a body of water wherein the interior of the pipe contains matter wherein the pipe is coiled onto a drum.

Description

Title: METHOD OF LAYING A PIPE UNDER WATER

The present invention relates to methods of laying plastic pipes under water and plastic pipes suitable for laying under water.

BACKGROUND

Various types of pipes are used for many different purposes. For example, pipes are used for transporting drinking water and sewage water, as well as other fluids such as oil. Also, pipes may be used as a protective shell around, for example, power cables or telecommunication cables. Often, pipes are laid underground, but in certain circumstances it may be necessary to lay a pipe under water. This may be the case, for example, where it is desired to provide an island with fresh drinking water from the mainland, and to transport the waste water from the island back to the mainland for treating. A further example would be to provide an island with power or telecommunications cables. Such islands may be located many miles offshore, or may alternatively be located in a freshwater lake. Alternatively, the geographical situation of a village or town may necessitate laying pipes across a fjord, a lake or a river. Another example would be to provide offshore wind turbines with power. A further example would be to connect wind turbines or islands, etc.

Depending upon the depth of the water, various techniques may be employed in order to lay a pipe underwater. The materials from which the pipes themselves are formed also influence the way in which the pipes are laid. For example, metal pipes such as steel pipes may be used, however these are disadvantageous where a long length of pipe needs to be provided, as metal pipes need to be formed in relatively short lengths and welded together to form a longer pipe. However, they have the advantage that, due to the weight of the metal, they are relatively easy to sink to the bottom of the sea, river or lake. Alternatively, polyethylene pipes or polypropylene pipes may be used. These have the advantages of being flexible, corrosion-resistant and able to be manufactured by continuous extrusion processes so that they can be, for example, several hundred meters long. However, such pipes exhibit a high degree of buoyancy and are difficult to sink. A known method of laying underwater polyethylene pipes is described with reference to Figure 1.

Figure 1 shows a pipe 1 being laid on the bottom of the sea 19 with the aid of a ship 13. The surface of the sea is indicated with number 17. A first end 9 of the pipe 1 is connected by a wire 11 to the ship 13, and the wire 11 exerts a pulling force F on the pipe 1 during laying of the pipe, to ensure that the pipe is laid in a controlled manner, thereby preventing damage to the pipe. The pipe is fitted with a plurality of concrete weights 3 which are spaced apart from each other and extend along the length of the pipe. To prevent damage to the pipe, the concrete weights are fixed in position on the pipe to prevent sliding during installation, and a rubber gasket may be fitted between the pipe and the concrete weights to protect the pipe and help to prevent the concrete weights from sliding. At the start of the sinking process, the pipe is full of air, and the sinking process starts by opening an air valve at the first end 9 of the pipe and controlling the inside pressure using a manometer if required to charge the pipe with compressed air. The air pressure inside the pipe needs to be regulated in order to achieve a controlled sinking at constant speed. During laying of the pipe, the pipe exhibits an S-bend configuration as shown in Figure 1. The S-bend configuration expresses a balance between the forces acting downwards (i.e. the concrete weights) and the forces upward (i.e. the buoyancy of the air-filled pipe section). During sinking, sea water enters the pipe via the second end 5, however, as can be seen in Figure 1, the air pressure and water level 15 in the pipe are controlled to ensure a controlled sinking process. The concrete weights 3 reduce the buoyancy of the pipe and enable the pipe to be laid on the sea bottom in a controlled manner.

Where lightweight pipes such as polyethylene pipes are submerged in this manner, the pipe generally will not lay flat on the bottom of the sea. This is because the pipe, even when filled with sea water, will remain too buoyant and will start to float up at locations where the concrete blocks are not present. This can cause undesirable curves and stresses in the pipe .

Furthermore, where it is desired to install a cable, such as a power and/or telecommunication cable, for example, through the pipe, the process of installing the cable through the pipe is complicated because the pipe is no longer straight, but instead is curved between each of the concrete blocks.

The present invention is concerned with providing an improved method of laying a plastic pipe under water, which avoids or alleviates the problems associated with current underwater pipe-laying methods, as described above. The present invention further provides a plastic pipe suitable for laying under water.

According to the present invention in one aspect thereof, there is provided a method of laying a plastic pipe at the bottom of a body of water comprising the step of introducing matter to the interior of the pipe to permit the pipe to sink in the body of water. The matter may be a liquid, solid matter or a combination of liquid and solid matter. The overall density of the pipe including the matter introduced to the interior of the pipe is greater than the density of the body of water in which the pipe is being laid.

This ensures that the pipe remains at the bottom of the body of water, and prevents it from floating upwards.

According to the present invention in another aspect thereof, there is provided a plastic pipe for laying at the bottom of a body of water by letting the pipe sink down to the bottom, wherein the pipe is coiled onto a drum for rolling out the pipe into the water, wherein the interior of the pipe contains matter, the matter comprising one of the group consisting of: solid matter; a liquid; a liquid and solid matter, for ensuring that the overall density of the pipe including the matter contained in the pipe is greater than the density of the body of water in which the pipe is to be laid.

The matter may be a liquid such as a salt solution. If the pipe is to be laid in a freshwater lake or river, the salt solution may be, for example, brackish water or sea water. Where the pipe is to be laid in the sea, a salt solution having a density greater than the surrounding sea water is advantageously used. Suitable salt solutions include, but are not limited to, sodium chloride (NaCl) solutions, or zinc chloride solutions (ZnC12), or calcium chloride (CaCl2), for example. The method may include introducing salinated water to the interior of the pipe. The salinated water may have a sodium chloride concentration of at least 5 parts per thousand. The salt solution may be a saturated salt solution, particularly where the pipe is to be laid in the sea or another body of water having a high density.

The exact salts present in the salt solution, and their concentrations can be selected taking into account the density of the water in which the pipe is to be laid, as well as the surrounding environment and the pipe used. For example, salts which may harm the surrounding area if they were to leak from the pipe should ideally be avoided.

Once the pipe or part of the pipe has been sunk, it may be fixed. The pipe is ideally fixed at both ends, for example between offshore wind turbines, or between the mainland and an island. The pipe is positioned in a desired position at the bottom of the body of water. For example, the pipe may be located in a trench at the bottom of the body of water. Positioning the pipe in a trench serves to further protect the pipe, and will also assist in preventing buoyancy of the pipe if the pipe is refilled with fresh water (or a lower concentration of salt solution) at a later stage.

If the pipe is to be used for transporting fresh water, for example, to prevent unwanted buoyancy the pipe may be entrenched or otherwise fixed to the sea bottom prior to removing salt solution from the pipe. The salt solution may be removed by pumping fresh water into the pipe, for example. Advantageously, the salt solution removed from the pipe is collected in a collection vessel such as a tank, for example. This allows the salt solution to be reused in subsequent pipe-laying activities. Additionally, it prevents potentially harmful concentrations of salts in the salt solution from contaminating the environment surrounding the pipe.

Where the pipe is intended to carry and protect a cable, for example a power cable or telecommunications cable, the cable may be installed in the pipe during the laying of the pipe. In this case, the matter may comprise solid matter in the form of the cable. A liquid may be introduced with the cable into the pipe, to aid the installation of the cable in the pipe. For example, the cable may be introduced into the pipe after or whilst the pipe is being filled with a salt solution. This permits the cable to be floated through the pipe using the flow or pressure of the salt solution. Additionally or alternatively, mechanical pushing of the cable may occur. The cable may be introduced into the pipe after the pipe is positioned at the bottom of the body of water. The presence of a salt solution or other liquid in the pipe reduces the friction between the cable and the interior surface of the pipe due to the upward pressure of the liquid during installation, which in turn permits relatively long lengths of cable to be installed in the pipe.

The present invention will now be described by way of example only and with reference to the following drawings:

Figure 2 shows a schematic diagram of a pipe being installed under water.

Figure 3 shows a pipe being installed under water in a similar manner to that depicted in Figure 2, but further shows an outlet pipe and salt solution collecting tank for retrieving and storing the salt solution.

Figure 4 shows a pipe being installed under water in a manner similar to that depicted in Figures 2 and 3, and further shows a cable being deployed in the pipe.

Figure 5 shows how a pipe is deployed from a vessel and is installed under water, three sequential views of the pipe laying process at different stages being depicted.

Figure 2 shows a pipe 21 being laid on the bottom of the sea 39. A salt solution is introduced into the pipe 21 at one end 25, causing the pipe to sink to the bottom of the sea 39. Air may be expelled from the pipe via valve 27 at pipe end 29. The density of the salt solution is greater than that of the sea water surrounding the pipe 21, thereby allowing the pipe to sink taking into account the density of the pipe. The concentration of the salt solution may be chosen with regard to the salinity of the sea locally, to further improve the sinking process, enabling a controlled and gradual sinking of the pipe. The other end 29 of the pipe is coiled around a drum secured on pipe-laying ship 33. The end of the pipe is connected to the drum 44 on the boat via a wire which to exerts a force F on pipe 21. A salt solution tank 43 is located on ship 34 (as shown in Figure 2) or alternatively may be located on the land 36 or a platform (not shown) . The tank 43 is in fluid contact with the interior of the pipe 21 via a first conduit 41. The flow of salt solution into the pipe is controlled to ensure a controlled sinking with a constant or nearly constant speed. Advantageously a plug 50 is used. The plug ensures that during introducing liquid into the pipe no air inclusion can take place. In a similar manner to that described in Figure 1, the pipe exhibits an S-bend configuration during laying, due to a balance between the forces acting downward (i.e. the salt solution in the pipe) and the forces acting upwards (i.e. buoyancy of the air-filled section of pipe above the salt solution level 35) . Controlling the flow rate of the salt solution into the pipe to achieve the desired rate of sinking is important together with the pulling force to ensure correct positioning of the pipe on the sea bottom, and also to ensure that the radius of curvature of the pipe at the surface of the sea 37 is not too small, which could cause the pipe to buckle. As in conventional pipe-laying methods, the S-conf iguration of the pipe is transformed to a J-configuration as thé pipe approaches the end of the sinking process. At this stage the wire 31 applies a predetermined pulling force to ensure a correct sinking speed and ensure a safe landing of the pipe at the bottom of the sea.

Once the pipe has been laid, the pipe may be positioned on the sea bed, or alternatively the pipe may be laid into a trench on the sea bed. The pipe is fixed at both ends. For example, the pipe may be fixed between two offshore wind turbines, or between an island and the mainland. The salt solution may then be flushed from the pipe. If the salt solution is, for example, concentrated NaCl solution, then the salt solution may simply be flushed out into the sea. Local sea water may be pumped into the pipe to flush out the salt solution, alternatively, fresh water or air may be used instead.

Where the salt solution is not compatible with the local environment, such as, for example, where the pipe is laid in a river and concentrated NaCl solution is used as the salt solution, the salt solution can be retrieved from the pipe and collected in an empty vessel e.g. on ship 33. Once the whole or the majority of the pipe has been laid, and the salt solution is no longer required to assist in sinking the pipe, the salt solution is removed from the pipe via pipe end 2 9 and is stored in a tank 10 on ship 33. The salt solution may be removed by pumping it out and/or pumping water from the surrounding body of water directly into the pipe, and/or by pumping in freshwater or compressed air. Advantageously a plug may be used. A plug 50 may be introduced into the pipe 21 via pipe end 25, and then water, for example, can be pumped into the pipe 21 via the pipe end 25. In this way the salt solution will be replaced by water. Thus, the salt solution is prevented from contaminating the local environment, and may also be reused. In this example, the salt solution tank is positioned on boat 33.

Figure 3 shows a pipe 21 being laid on the bottom of the sea 39. A salt solution is introduced into the pipe 21 at one end 25, causing the pipe to sink to the bottom of the sea 39. The other end 2 9 of the pipe is located above sea level. A tank 51 is used to introduce the salt solution in the pipe 21. The tank 51 may be provided on a platform or boat or the shore. No pump is needed and the liquid is introduced into the pipe through gravity and with the aid of hose 53. Air can leave the pipe 21 at both sides via pipe end 25 and pipe end 29, for example via a valve.

The density of the salt solution is greater than that of the sea water surrounding the pipe 21, thereby allowing the pipe to sink taking into account the density of the pipe. The concentration of the salt solution may be chosen with regard to the salinity of the sea locally, to further improve the sinking process, enabling a controlled and gradual sinking of the pipe. As can be seen from Figure 3 the level of the salt solution 3 5 in the pipe 21 will be the same on both sides, i.e. the rising parts of the pipe 21. When the pipe is almost totally filled with salt solution the S-shape will be turned into a J-shape and the pipe can be fully filled with the salt solution.

Figure 4 shows a pipe 21 being laid on the bottom of the sea 3 9 in a similar manner to that described with respect to Figures 2 and 3. However, the pipe depicted in Figure 4 is not intended for transferring water, but is instead intended to house a cable 55. The cable -55 may be a telecommunications cable or a power cable, for example. The pipe 21 is filled with salt solution and allowed to sink to a predetermined location on the bottom 3 9 of the sea, as described with respect to Figures 2 and 3 above. However, once only the end 29 of the pipe remains above the surface 37 of the sea, the cable 55 is introduced into the pipe 21. To achieve this, cable 55 is floated through the pipe 21. During installation, the cable is mechanically pushed into the pipe by the cable installation device 57. The cable installation device 57 may be located on the ship 33, on another nearby vessel, or alternatively on a floating platform. Once the cable 55 is installed in the pipe 21, the ends 29, 25 of the pipe can be fixed if they are not already fixed, and the cable may also be fixed.

Where a relatively heavy cable 55 is inserted into the pipe 21, the weight of the cable will be sufficient to ensure that the pipe remains on the seabed and does not float up to the surface. Accordingly, the salt solution can be flushed from the pipe, either by pumping the surrounding water into the pipe, or by pumping fluid stored in a tank 45 on the ship 33 or on a nearby floating platform into the pipe 21. The water stored in the tank 45 may be, for example, fresh water. The tank 45 may be connected to the pipe via an extension conduit 59 of the first conduit 41 or via a separate conduit (not shown) . As described with respect to Figures 2 and 3, the salt solution may exit the end of the pipe 21 via valve 52, and may either be released into the surrounding body of water, or may exit, as described in Figure 3, via an outlet pipe 53 to a salt solution collection tank 51 located at the surface .

The cable may be attached to a chain which may initially be inserted into the pipe, with or without an accompanying liquid, to ensure that the pipe sinks before or whilst the cable is introduced. Following installation of the cable, the chain may be recovered.

Although the present invention relates to laying of underwater pipes in general, the present invention is particularly advantageous when used for pipes which are intended to receive cables, where the pipe is for protecting the submerged cable. Presently, most cables intended for submersion in water are not provided in a pipe, but are instead encased in armour and are directly placed in the water without using a pipe. This method has numerous disadvantages, including the expense due to providing extra protection on the cable, and expense and complexity associated with submerging such cables. Previously, it was difficult to install cables in polyethylene pipes laid by using concrete weights as described with respect to the background prior art, because the pipes laid in this manner were not straight, instead exhibiting upward curves between each of the concrete weights, due to the buoyancy of the pipe. By contrast, the present invention teaches a method of laying a pipe which additionally enables cables to be readily installed within the pipe. Because pipes installed by the method of the present invention will not be prone to bending, insertion of the cable is made easier. Furthermore, by providing a relatively dense liquid in the pipe, there is reduced friction between the cable and the interior surface of the pipe during installation, as the relatively dense liquid gives the cable more buoyancy than it would have in the presence of air or fresh water.

In an alternative embodiment, the matter may be introduced to the interior of the pipe prior to the pipe contacting the body of water. For example, liquid and/or solid matter may be introduced to the interior of the pipe prior to coiling the pipe onto a drum for rolling out the pipe into the water. The matter may be introduced during extrusion of the pipe, or at any subsequent stage prior to laying the pipe in the water. In one embodiment, a cable and/or a chain is introduced to the interior of the pipe prior to coiling the pipe onto a drum.

Figure 5 shows the progression of the pipe laying process over time. A pipe 21 has a first end 29 coiled around drum 44 on pipe laying boat 33. The second end 25 of the pipe 21 is fixed to an offshore fixed platform 46. It will be appreciated that the platform 4 6 may be a wind turbine, for example. Initially, the boat 33 is positioned near to the platform 46. Gradually, the boat 33 moves away from platform 46 thereby deploying pipe 21 from the drum 44. In accordance with the present invention, the overall density of the pipe including the matter introduced to the interior of the pipe is greater than the density of the surrounding sea water in which the pipe is being laid, resulting in the sinking of the pipe.

The liquid may be but is not limited to a salt solution. A salt solution may be formed from any suitable soluble salt, such as NaCl or ZnC12 or CaC12, for example. One or more salts may be present in the solution. The concentration of the salt solution may be adapted taking into account the density of the surrounding body of water in which the pipe is to be laid, as well as the material from which the pipe is formed. The concentration of the salt solution is such that the salt solution has a density greater than that of the body of water in which the pipe is to be laid. For example, where the pipe is to be laid in a fresh water lake or river, a NaCl solution having a concentration of approximately 5 parts per thousand may be sufficient to allow sinking of the pipe. Alternatively, where the pipe is to be submerged in the sea, a NaCl concentration of approximately 40 parts per thousand may be required, depending upon the salinity of the local surrounding water. Where the surrounding water is particularly dense, a salt solution at a saturation concentration may be desirable.

The liquid and any salts present in the liquid can be chosen in accordance with the local environment. For example, where the pipe is to be laid in the sea, a concentrated NaCl solution may be used. Such a salt solution may, following installation, be simply forced out of the pipe following submersion of the pipe, without requiring an outlet pipe and collection tank for collecting the salt solution post installation of the pipe.

As an alternative to using a liquid, solid matter (such as a chain, a cable, gravel, stones, etc.) may be used. Alternatively a mixture of a liquid and solid matter may be used, e.g. a cable and water, mud and water, calcium carbonate and water, etc.

The drawings and their accompanying description are intended to explain the present invention by way of example only. Modifications to these embodiments may suggest themselves to the person skilled in the art, however, any such modifications that fall within the scope of the appended claims are within the scope of the present invention.

Claims (30)

A method of laying a plastic pipe on the bottom of a body of water, comprising the step of introducing matter into the interior of the pipe to allow the pipe to sink into the body of water, the matter being provided with one of the group consisting of: solid matter; a liquid; a liquid and solid matter, wherein the total density of the pipe comprising the matter introduced into the interior of the pipe is greater than the density of the body of water into which the pipe is to be placed. A method of laying a pipe according to claim 1, wherein the matter is a liquid. The method of laying a pipe according to claim 2, wherein the liquid is a saline solution. A method of laying a pipe according to any one of the preceding claims, wherein salt-containing water is introduced into the interior of the pipe. A pipe laying method according to claim 4, wherein the salt-containing water introduced into the interior of the pipe has a sodium chloride (NaCl) concentration of at least 5 parts per 1000. A method of laying a pipe as claimed in any one of claims 3-5, wherein the saline solution is saturated. A method of laying a pipe according to any of claims 2-6, wherein the liquid is removed the moment the pipe, or the major part of the pipe, is laid. A pipe laying method according to claim 7, wherein the liquid is removed by pumping fresh water into the pipe. The method of laying a pipe according to claim 7, wherein the liquid is removed by pumping sea water from the body of water into the pipe. A method of laying a pipe according to any of claims 7-9, wherein the liquid removed from the pipe is collected in a collection vessel. The method of laying a pipe as claimed in claim 1, wherein the matter comprises a cable. A method of laying a pipe as claimed in claim 1 or claim 11, wherein the matter comprises a chain. A pipe laying method according to claim 11, wherein the cable and a liquid are introduced into the pipe to allow the pipe to sink. A method of laying a pipe according to any one of the preceding claims, wherein the matter is introduced into the interior of the pipe before the pipe contacts the body of water. A method of laying a pipe according to any one of the preceding claims, wherein the material is introduced into the interior of the pipe prior to winding the pipe on a reel for rolling the pipe into the water. A method of laying a pipe according to any of claims 1-10, wherein a cable is introduced into the pipe when the pipe is positioned on the bottom of the body of water. A method of laying a pipe as claimed in any one of the preceding claims comprising the further step of fixing the pipe at both ends thereof. A method of laying a pipe as claimed in any one of the preceding claims comprising the further step of positioning the pipe in a desired position on the bottom of the body of water. A method of laying a pipe as claimed in any one of the preceding claims, wherein the pipe is located in a slot on the bottom of the body of water. A method of laying a pipe according to any one of the preceding claims, wherein the pipe is made of polyethylene. 21. Plastic pipe for laying a body of water on the bottom by sinking the pipe to the bottom, the pipe being wound on a reel for rolling the pipe into the water, the interior of the pipe contains matter, the matter comprising one of the group consisting of: solid matter; a liquid; a liquid and solid matter, for ensuring that the total density of the pipe comprising the matter in the pipe is greater than the density of the body of water into which the pipe is to be placed. The plastic pipe as claimed in claim 21, wherein the matter is a liquid. The plastic pipe as claimed in claim 22, wherein the liquid is a saline solution. The plastic pipe of any one of claims 21-23, wherein the liquid is salt-containing water. The plastic pipe of claim 24, wherein the salt-containing water has a sodium chloride (NaCl) concentration of at least 5 parts per 1000. The plastic pipe as claimed in any of claims 23-25, wherein the saline solution is saturated. The plastic pipe as claimed in claim 21, wherein the material comprises a cable. The plastic pipe of claim 21, wherein the material comprises a cable and a liquid. A plastic pipe according to any of claims 21, 27 or 28, wherein the material comprises a chain. The plastic pipe of any one of claims 21-29, wherein the pipe is made of polyethylene.