Abstract:
A system for controlled lowering of an elongated body such as a tube or cable, from a relatively high level, especially the water level to a relatively low level, especially the (sea) bottom. The system includes an elongated tubular guide that is connected to one or more buoyancy bodies. The system also includes a braking device for braking the movement of the elongated body through the tubular guide. The braking device includes a number of braking units incorporated in the tubular guide and distributed over the length thereof. The braking units are under control of a central control unit which controls the braking force applied by each braking unit.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a system for controlled lowering of an elongated body such as a tube or cable into a volume of water such as a lake or sea, from a relatively high level, especially the water level of said volume of water, to a relatively low level, especially the bottom under said volume of water, comprising an elongated tubular guide which is connected to one or more buoyancy bodies, and braking means for braking the movement of said elongated body through said tubular guide, said braking means incorporating at least one braking unit comprising: 
     a flexible tubular inflatable body, the outer wall being supported by said tubular guide and the inner wall of which acts as braking surface, 
     a valve unit for inflating or deflating the inflatable body whereby the valve unit is connected to a source of pressurized fluid and is controlled by a control unit. 
     Such a system is known from the US specification U.S. Pat. No. 5,575,590. In this document air is used to inflate or deflated the inflatable bodies of the braking units. Each braking unit is connected to an individual source of pressured air and the pressure inside the inflatable body of each of the braking units is preset independent of the other braking units on a value which is dependent on the depth at which the respective braking unit thereafter has to function. Because of the increasing hydrostatic pressure the air or gas in the inflatable body is compressed more and more with increasing depth. As a result the contact surface between the inflatable body and the tube becomes smaller and smaller so that in fact in each braking unit only a part of the initial braking surface will be active. Controlling the actual braking force under these circumstances in the prior art system is rather difficult or even impossible. 
     SUMMARY OF THE INVENTION 
     The object of the invention is now to eliminate the above indicated disadvantage. 
     In agreement with said object the invention now provides a system of the above indicated type which according to the invention is characterized in that the fluid inside each inflatable body has a specific weight equal to or at least nearly to the specific weight of the surrounding water. 
     It is especially preferred that the fluid inside each inflatable body consists of the same water as the surrounding water. 
     In one further development each valve unit is connected to a separate fluid reservoir attached to the corresponding braking unit and acting as source of pressurized fluid for said valve unit. Each braking unit is now able to operate independent of the others. The large number of reservoirs could be a disadvantage of this embodiment. 
     In another development a number of valve units (preferably all valve units) are connected to a fluid conduit which extends along the tubular guide and is connected to one fluid reservoir acting as source of pressurized fluid for said valve units. Only one reservoir is needed in this embodiment, however, a fluid conduit is necessary between each valve unit and the common reservoir. 
     The control communication between the control center and the valve units can be performed in various ways using data transfer along electrical connections, along acoustical communication paths, using radio waves, etc. 
     In all known prior ad cases pressurised air is used as fluid for inflating or deflating the inflatable bodies of the braking units and for determining in correspondence therewith the applied braking force. Assume a water depth of 2000 m and assume a desired control range of 50 cm water column. In that case 4000 braking units would be necessary each capable of functioning at a different depth and at a different hydrostatic bias pressure. 
     With increasing water depth the controllability of the valve units becomes more and more a problem. The valve unit which has to operate for instance at 1000 m depth should have a pressure control range between 999.75 m and 1000.25 m water column. Such a control range poses a serious problem. This problem will be twice as serious for the most lower valve unit which has to operate in the pressure range between 1999.75 m and 2000.25 m water column. An accurate control within such a small control range under such relatively high bias pressure of the surrounding sea water is hardly performable. 
     The whole problem is caused by the depth dependent hydrostatic pressure. The question how to eliminate this problem appeared to be surprisingly simple. The hydrostatic pressure acting on the outside of the inflatable bodies baa to be eliminated by the pressurizing control fluid inside the inflatable bodies. To obtain such a situation a preferred embodiment of the system is characterised in that the fluid has a specific weight equal to the specific weight of (sea)water. It is especially preferred that the fluid is (sea)water. 
     By using (sea)water as fluid the hydrostatic pressure difference is completely eliminated at all depths. 
     In a preferred system, in which the abovementioned fluid is (sea)water, the number of braking units is equal to one, said one braking unit comprising: 
     an elongated non stretchable outer tubular wall 
     an elongated flexible tubular inflatable body, the outer wall of which is supported by said outer tubular wall and the inner wall of which acts as braking surface, 
     a valve unit connected to a pump unit through which (sea)water can be pumped in or out the inflatable body to obtain the required braking stress. 
     The abovementioned elongated outer wall and the elongated flexible tubular inflatable body preferably extends from just below sea level to just above bottom level. In that case the whole tube is guided and controlled by only one combined component. 
     If for reasons which will not be discussed here it is preferred to use two or more braking units then each of the braking units comprises 
     a non stretchable outer tubular wall 
     a flexible tubular inflatable body, the outer wall of which is supported by said outer tubular wall and the inner wail of which acts as braking surface, 
     whereby all outer tubular walls are mechanically connected in series, 
     whereby all flexible tubular inflatable bodies and a valve unit are connected in series by suitable conduits, and 
     whereby furthermore the valve unit is connected to a pump unit through which (sea)water can be pumped in or out the series connected inflatable bodies to obtain the required braking stress. 
     The pump can be of a rather simple design which only has to be able to generate a pressure in the desired control pressure range, in the above example a pressure between 0 and 50 cm water column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained further with reference to the attached drawings. 
     FIG. 1 illustrates schematically the general situation during lowering of a tube or cable from a ship onto the sea bottom. 
     FIG. 2 illustrates a cross-section through a system according to the invention. 
     FIG. 3 illustrates a longitudinal section through a first embodiment of a system according to the invention especially at the interconnection between two sections of the elongated tubular guide. 
     FIG. 4 illustrates a longitudinal section through a second embodiment of a system according to the invention especially at the interconnection between two sections of the elongated tubular guide and one end of the tubular guide. 
     FIG. 5 shows an embodiment with only one elongated brake unit and separate outside buoyancy bodies. 
     FIG. 6 illustrates a cross section through the elongated brake unit of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a vessel  10 , floating on the sea surface  12 , from which vessel  11  an elongated object such as a tube or cable  14  is lowered onto the sea bottom  16 . To limit the velocity with which the tube or cable  14  is lowered and to maintain during the whole process an S-shape in the cable or tube  14  so that kinking or buckling of the cable or tube  14  is prevented, an elongated tubular guide is used which in the illustrated embodiment comprises a number of sections  18   a ,  18   b ,  18   c , . . . ,  18   n . Each of those sections  18   a  etc. has a predetermined buoyant capacity necessary to maintain the abovementioned S-shape. Furthermore, each section comprises brake means which are clamped around the tubular cable  14  such that a braking force is applied to the tube or cable which is at least to a large degree in balance with the buoyant force. 
     FIG. 2 illustrates a cross-section through one of the sections  18   x , clamped around a tube  14 . The section  18   x  comprises a non stretchable outer tubular wall  20 . Within said tubular wall a layer  21  is situated made from a material with a lower specific gravity than water, which material generates the abovementioned buoyancy force. As such suitable materials are known and furthermore the buoyancy features are not the subject of the invention, therefore a further detailed discussion is considered superfluous. 
     Within the layer  21 , the braking unit is positioned and which comprises a flexible tubular inflatable body  22 . At the inside surface this body may comprise a coating of friction material  24 . However, taking into account the relatively large contact surface between the body  22  and the tube  14  in many cases this friction layer could be omitted. 
     To improve handling of the whole system it is preferred that the tubular configuration can be made from an initially flat or at least open configuration by folding the flat configuration around the tube  14  and fastening the longitudinal edges to each other. If the outer wall  20  is made of a rather stiff material such as metal then it is preferred that the outer wall  20  is divided into a number of segments interconnected by means of hinges. In the illustrated embodiment in FIG. 2 there are two of such segments  20   a  and  20   b  interconnected by the hinge  23 . On the other hand one could make outer wall from any strong flexible but non stretchable material in which case the jacket could be attached as a blanket around the tube  14 . The longitudinal edges of the outer wall are connected by means or a number of cables or chains  26  which preferably are as short as possible. Because of the presence of these cables or chains the flexible inflatable body  22  does not completely surround the cable or tube  14  but fills only the volume between the outer wall  20  and the cable or tube  14 , which volume has to be (nearly) constant. 
     To improve the strength of the configuration and to improve the transfer of the braking forces from the braking layer  24  to the buoyancy layer  21  the braking cushion  22  might be internally subdivided by means of dividing walls one of which is indicated by  27  in FIG.  2 . It will be clear that the resulting subvolumes of the cushion  22  are all interconnected so that the pressure inside each subvolume is always the same. 
     FIG. 3 illustrates a longitudinal cross-section through a specific embodiment of the elongated tubular guide comprising at least the segments  18 M and  18 N. Each of the segments  18 M,  18 N, has an outer layer  20 M,  20 N, . . . , a buoyancy layer  21 M,  21 N,. . . . , and a braking cushion  22 M,  22 N, . . . , of which the inner side could have a strengthening layer  24 M,  24 N, . . . , of e.g. friction material. The various segments are interconnected by suitable means such as the chains  30 A,  308 . In this embodiment the braking cushion  22 M of segment  18 M of the tubular guide is through a suitable conduit  40 M and a valve unit  42 M connected to a reservoir  44 M. The reservoir  44 M comprises for instance pressurized air which through the valve unit  42  can be filled into the inflatable body  22 M. On the other hand the valve means  42 M are able to let pressurized air escape from the body  22 M through the conduit  40 M into the surroundings. All other segments of the tubular guide are in the same manner equipped with valve means and reservoirs. 
     By controlling the inflating/deflating of the body  22 M a predetermined pressure can be set as soon as the segment  18   x  has reached its operating depth. An electrical control line  46 M extends from the valve means  42 M to a central control unit  48 . All other valve means from all other segments are in the same manner connected to this central control unit  48  as indicated by the dotted lines  46 X and  46 Y. Preferably, the central control unit set and maintain the pressure in each of the inflatable bodies in each of the braking means at the desired value. 
     Instead of a pressurized fluid reservoir  44  for each of the segments of the elongated tubular guide, it will be clear even without detailed illustration that one (larger) reservoir can be installed somewhere along the tubular guide  18 . In that case each segment of the tubular guide comprises only a act of valve means, which on the one hand are connected to the inflatable body of the respective segment and are on the other hand through suitable conduit means connected to said central reservoir. The valve means are in the same manner as in FIG. 3 connected to a central controller  48  for controlling the pressure inside various inflatable bodies. 
     As already indicated above, it is preferred to fill the inflatable bodies with a substance which has the same specific gravity as the surrounding (sea) water and more especially to fill the bodies actually with (sea)water. By filling these bodies with (sea)water, the problems with the hydrostatic pressure are eliminated. The hydrostatic pressure outside and inside the inflatable bodies  22  is equal. A rather small overpressure is already sufficient to generate a rather large braking force by each of the inflatable hollow bodies  22 . 
     An example of an embodiment whereby sea water is used as the fluid for inflating/deflating the hollow bodies is illustrated in FIG.  4 . This figure shows parts of the segments  18 R and  18 S of the elongated body. Each of the segments  18 S,  18 R, . . . , has an outer layer  20 S,  20 R, . . . , a buoyancy layer  21 S,  21 R, . . . , and a braking cushion  22 R,  22 SN, . . . The various segments arc mechanically interconnected by suitable means such as the chains  30 C,  30 D. Although relatively long chains are shown the relatively short chains as indicated in FIG. 3 are preferred. 
     The actual difference between FIGS. 3 and 4 relies in the fact that in FIG. 4 all the inflatable bodies are connected in series by means of suitable conduits. One of these conduits  36  is visible at the left hand side of FIG.  4 . This conduit  36  is connected between the inflatable bodies  22 R and  22 S through suitable connectors  38 R and  38 S. The whole system comprises only one valve means  50  which through a suitable conduit  52  is connected to one of the inflatable bodies  18  . . . in the series, in the illustrated case the last inflatable body  18 S of the series. The valve means  50  cooperates with a pump  54  such that either (sea) water from the surroundings is pumped from the port  56  through the valve means  50  and the conduit  52  into the series circuit of inflatable bodies  22 , or water is through the conduit  52 , the valve means  50  and the port  56  pumped out of the series circuit of inflatable bodies. The inflating/deflating operation is under control of a central controller  60  which preferably is installed on board of the vessel  10 . 
     In the embodiment of FIG. 4 only one valve in combination with only one pump is necessary to control the braking force developed by the combination of all braking means. Controlling this configuration by means of the controller is rather simple. 
     Instead of two or more braking units a preferred embodiment of the system, comprises only one elongated braking unit  18  as schematically illustrated in FIG.  5 . FIG. 5 shows a view similar to the view in FIG.  1 . As illustrated in cross section in FIG. 6 the elongated body  18  now comprises only one combination of an outer layer  20 , and a braking cushion  22 . The pump/valve combination for inflating/deflating the braking cushion is installed on board the ship  10  and not shown separately in FIG.  5 . 
     In this embodiment the buoyancy capacity is supplied by a series of annular buoyancy bodies  62   a ,  62   b , . . . , which are attached with mutual intervals around the elongated braking unit. The buoyancy bodies may have some flexibility so that they can be clipped around the unit  18  and secured by means of a suitable fastener  64  rather easily.