Patent Publication Number: US-9849938-B2

Title: Thruster assembly in a marine vessel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage filing under section 371 of International Application No. PCT/FI2012/050993, filed on Oct. 16, 2012, and published in English on Apr. 24, 2014, as WO 2014/060636 A1, the entire disclosure of this application being hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to an assembly in a marine vessel comprising a unit arranged in a water tight chamber opening in a hull of a vessel, wherein the unit is adapted to extend into the water below the vessel when at its mounted position, which assembly comprises a connection path between in inside and outside of the vessel. 
     BACKGROUND ART 
     Document WO 97/27102 describes a method and apparatus for removing a propeller assembly from an opening of a floating vessel, wherein the propeller assembly is designed to close an opening in the vessel hull when the propeller assembly is in its mounted position. According to this prior art, a watertight hoisting chamber is provided around the propeller assembly and inside that hoisting chamber a drive shaft is provided, which leads to a drive motor positioned in the vessel and outside of the hoisting chamber. When the known propeller assembly is to be removed, the drive shaft is removed first. Then its passage through the hoisting chamber wall is closed in a watertight manner. After that the propeller assembly is connected to hoisting means and then a flange of the propeller assembly is loosened. Then, the propeller assembly can be lifted from the hoisting chamber. The propeller assembly is also known as thruster. The method according to the prior art requires assembly/disassembly work which has to be carried out under water. 
     Further, large vessels can have a draught which is up to 20 meters under water surface level. In this case, considerable forces act to push the thruster upwards and inside the vessel due to the difference in pressures between the water pressure outside and the air pressure inside the vessel. With large thrusters, the forces pushing up the thrusters which are not compensated by thruster weight may reach up to 2000 kN. When the fixing screws of the flange are removed, the propeller assembly or thruster is lifted by these forces in uncontrolled manner. Having such a heavy mass which moves in uncontrolled manner is dangerous. Further, loosening of the fixing of the flange while under these forces is difficult. 
     Document WO 20111279878 A1 describes a method of maintenance of a unit arranged in the watertight hoisting chamber and closing an opening in a vessel hull, wherein the unit is adapted to extend into the water below the floating vessel. The method comprises the steps of clamping the unit in its mounted position by clamping means, releasing fixing means which fix the unit into its mounted position while holding the unit clamped in its position and at least partly flooding the hoisting chamber, then releasing the clamping of the unit and hoisting the unit away from its mounted position. A clamping means is used to clamp the unit in its mounted position before the fixing means, which are normally used for mounting the unit into its position, are removed. In this way, the opening can be held closed and the unit is held in position, so that the work for releasing the fixing means can be carried out while it is still dry in the hoisting chamber. After the fixings are removed, the hoisting chamber is at least partly flooded either by provision of an extra valve for flooding the chamber or by a controlled release of the clamping of the unit. When the clamping is released, the water in the at least partly flooded hoisting chamber puts some pressure on the unit from the vessel inside, so that the pressure differences at the unit between inside and outside the vessel are reduced. Therefore, the forces pushing up the unit can be reduced. Even if the method is beneficial as such there has emerge some need to further develop the method. 
     In view of the above prior art, it is the object of the invention to provide an assembly in a marine vessel which provided improved closing operation of a unit arranged in a water tight chamber opening in a hull of a vessel. 
     DISCLOSURE OF THE INVENTION 
     Object of the invention are met with an assembly an assembly in a marine vessel comprising a unit arranged in a water tight chamber opening in a hull of a vessel, wherein the unit is adapted to extend into the water below the vessel when at its mounted position, which assembly comprises a connection path between in inside and outside of the vessel which connection path is provided with a sealing system and which sealing system divides the connection path to outside connection path portion and inside connection path portion, In is characteristic to the invention that the assembly comprises a gas inlet system opening to the outside connection path portion. 
     According to an embodiment of the invention the gas inlet system comprises a conduit extending through the unit. 
     According to an embodiment of the invention the gas inlet comprises a conduit extending through the vessel hull. 
     According to an embodiment of the invention the conduit is in connection with a source of inert gas. 
     According to an embodiment of the invention the source of inert gas comprises an air-nitrogen converter. 
     According to an embodiment of the invention the conduit is in connection with a source of pressurized air. 
     According to an embodiment of the invention the sealing system is arranged to a periphery surface of the flange. 
     According to an embodiment of the invention the sealing system is arranged to a lower surface of the flange plate portion. 
     According to an embodiment of the invention the assembly comprises a control system for maintaining a predetermined supply of gas to the inlet gas system. 
     According to an embodiment of the invention the control system comprises a valve. 
     According to an embodiment of the invention the control system comprises an orifice plate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which 
         FIG. 1  shows a section of a vessel with a hoisting chamber and a mounted unit; 
         FIG. 2  shows an enlarged schematic view of a portion of  FIG. 1  where a part of the hoisting chamber and a part of the unit is shown; 
         FIG. 3  shows a detail of a clamping means cooperating with the unit; 
         FIG. 4  shows the sequence of dismounting a thruster unit from the vessel hull; 
         FIG. 5  shows a detail of hoisting chamber and a flange portion connection; 
         FIG. 6  shows an embodiment of the flange; 
         FIG. 7  shows a further embodiment of the flange; and 
         FIG. 8  shows a still further embodiment of the flange. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a vessel hull  100  which floats in water as is indicated with the broken line in  FIG. 1 . Inside the vessel, there is provided a hoisting chamber  1  which is a fixed construction mounted to the vessel  100 . Inside the hoisting chamber  1  there is a unit  2  which extends through the vessel hull  100  to the outside. This is the unit which requires maintenance. Here, the unit is a so-called thruster  2 . The thruster  2  has a propeller  21 , a gear housing  22 , a flange  24  and an electric motor  23  for driving the propeller  21 . Another term often used in practice for the flange  24  is mounting can; hereinafter the term flange is used for that part. Of the thruster  2 , the propeller  21  and the gear housing  22  are the elements immersed in water, while the flange  24  closes the opening in the vessel hull  100  when the unit is mounted to the vessel. The propeller  21  and the gear housing  22  may be rotated around an axis substantially perpendicular to the rotational axis of the propeller  21 . This kind of thruster is often used in connection with large vessels for position control and for maneuver assistance. The flange  24  may also be developed and contain a gear box and drive means for rotating the thruster around its substantially vertical axis. 
       FIG. 1  further shows that hoisting chamber  1  has guide rails  12  at its wall extending along the chamber in its height direction; here two guide rails  12  are shown. Also, clamping means  11  are shown which will be discussed in more detail under reference being made to other drawing figures. It is noted that only a pair of clamping means  11  is shown in  FIG. 1  although typically up to eight clamping means are provided which are arranged on a circle at equal angular intervals around the flange  24 . 
     Further,  FIG. 1  shows a cover  4  of the hoisting chamber, which preferably water tightly closes the hoisting chamber at its upper end. One of the functions of this cover is of course to avoid that someone may fall into this chamber (in the shown example vessel&#39;s draught i.e. the depth of the chamber is about  18  meters) and, on the other hand, the cover is additional protection against immersion of water into the vessel if the opening in the vessel bottom is not tightly closed for whatever reason. 
       FIG. 2  shows an enlarged view of a section II of the arrangement of the thruster  2  at a portion close to the flange  24  of the thruster. The flange  24  of the thruster further comprises hoisting eyes  25  and a flange plate portion  26 , which cooperates with seals  13  for water tightly closing the opening in the vessel hull  100 . Screws  14  form the fixing means and a number of screws are provided along the flange plate portion  26  in a rim around the unit  2 .  FIG. 2  shows a part of the hoisting chamber wall, which hoisting chamber wall carries two guide rails. It should be understood that the flange and the bottom opening in the hoisting chamber may be formed suitably according to the need. They may have for example circular, rectangular or polygon form, or a combination thereof. 
     Although not shown, unit  2 , in particular the flange  24  thereof, has guide means which cooperate with the guide rails, when the unit is moved inside the hoisting chamber. Although two guide rails  12  are shown in  FIG. 1 , any suitable number of guide rails can be provided. Also, clamping means  11  are shown fixed to the hoisting chamber wall. Functional cooperation of the clamping means  11  with the flange  24  of the unit  2  will be described by reference being made to  FIG. 3 . 
     Further,  FIG. 2  shows an electric motor  23  which is fixed to a gear box having a coupling so as to be in drive connection with a propeller  21  of the unit  2 . Of course, other types of motors may be used as well. 
     Turning now to  FIG. 3 , a detail of a clamping means  11  as shown of the right-hand side of  FIG. 2  is shown in more detail in  FIG. 3 . The clamping means  11  has a hydraulic cylinder  111  which has a cylinder rod  112 . By controlling flow of hydraulic fluid to and from the cylinder  111 , movement of the rod  112  can be controlled. At the end of the rod  112  there is shown a clamp  114 , which is adapted to cooperate with a clamping pad  115  provided on the flange  24  of the unit  2 . Also,  FIG. 3  shows the fixing screws  14  serving as the fixing means and a seal  13  for water tightly sealing the connection between the unit  2  and the vessel hull  100 . It is noted that the clamping pad  115  here has the form of a shallow recess cut into the plate-shaped member  241  which is fixed to the flange  24  and the flange plate portion  26 . 
     Furthermore,  FIG. 3  shows a guide surface  113  which is inclined outwardly with increasing height of the hoisting chamber. The guide surface  113  cooperates with the clamp  114  of the clamping means  11 . With the cylinder bolt  117  the hydraulic cylinder  111  is fixed to the hoisting chamber wall, so that the cylinder  111  can pivot around this cylinder bolt  117 . Further, an urging means  116  is provided, which urges the cylinder  111  of the clamping means  11  towards the outside of the hoisting chamber so as to ensure that the clamp  114  is always guided by the guide surface  113 . 
     Now starting out from the position of the cylinder  111  shown in dotted lines in  FIG. 3 , in which the cylinder  111  extends almost parallel to the hoisting chamber wall and the rod  112  is retracted into the cylinder, functions of the clamping means are described. When the flange  24  is to be clamped for holding it (and the unit  2 ) in its mounted position, the hydraulic cylinder  112  is controlled so as to extend the rod  112 . The clamp  114  moves guided by the guide surface  113  towards the clamping pad  115 , which guided movement is supported by a guide shoe  118  provided close to the clamp  114  and cooperating with the guide surface  113 . Once the cylinder  111  has fully extended its rod  112 , the clamp  114  sits into the shallow recess  115  as the clamping pad provided on the flange  24 , i.e. on the plate-shaped member  241  thereof. The hydraulic cylinders  111  are strong enough to securely clamp the unit into its mounted position, so that the position of the unit in regard to the opening in the vessel hull can be securely held or maintained while the fixing means  14  are screwed off. In order to securely avoid that any unintentional release of the clamping of the flange  24  may happen, it is suggested that the cylinders  111  are provided with locking valves which cut off the fluid connection of the hydraulic cylinder to the hydraulic system in order to maintain the cylinder in its actual position. 
     Once the fixing means  14  have been removed and the hoisting means (such as hoisting lines, not shown) is fixed to the lifting eyes  25 , the clamping may be gradually released. In particular, looking at  FIG. 3 , the cylinder  111  is controlled so as to slowly retract the rod  112 . Because the fixing means  14  have been removed, and due to the differential pressure between inside the hoisting chamber  1  and the water pressure at the bottom of the vessel  100 , the flange  24  will move upward following the cylinder rods  112  movement while keeping the engagement between the shallow recess  115  and the clamp  114 . Once the seal  13  lifts up from its counter surface, water may rapidly flow into the hoisting chamber  1 . After the hoisting chamber is flooded to a required level, the cylinder  111  may be controlled to further retract the rod  112 . 
     When the pressure differences and forces have been leveled out, the unit  2  following its own weight will again sit on the seal  13  with its flange plate portion  26 . 
     According to an embodiment of the invention the unit further comprises a number of lifting devices  500  arranged in co-operation with the flange  24  and the hoisting chamber  1  in order to apply controllably force to the flange in the upward direction of the hoisting chamber. Such a lifting device is shown the  FIGS. 2, 3, 5 and 6 . The lifting devices  500  comprise a number of lifting jacks, particularly hydraulic jacks, arranged under the flange plate portion  26  around the unit  2 . The lifting devices are preferably arranged substantially symmetrically in respect of center of load to be lifted. The lifting devices  500  are arranged to fit between an extension of the flange plate portion  26  and a lifting base arranged to the hoisting chamber in a retracted position while the flange  24  sits on the seal  13  with its flange plate portion  26 . The flange plate portion  26  is provided with a local extension only at the location of the jack  500  against which the jack is supported during the lifting so that the flange portion may pass the guide surface  113 . 
     Each lifting device is provided with or is in connection with a force control system  510 , which facilitates controlling of alignment of the flange while installed or removed. The lifting devices may be used during assembly/disassembly of the unit, whether it is a thruster or a closing cover or other kind of unit. 
     Next in the procedure the hydraulic jacks  500  are activated and the flange  24  is lifted from the seat controllably by the hydraulic jacks. The flange is lifted so that the cylinders  111  and their rods  112  adjustably control the lifting of the flange and the flange  24  will move upward following the cylinder rods  112  and hydraulic jacks  500  movement while keeping the engagement between the shallow recess  115  and the clamp  114 . This way the removal of the flange is performed controllably and movement may be kept translational i.e. moving the flange aligned with the opening in the hosting chamber. A low tension on the hoisting lines may also be is applied. 
     According to an embodiment of the invention the unit is lifted at a first level, the hydraulic cylinders  111  are fully retracted in to their end position. Advantageously the lift is about 150 mm. Once the cylinders have been retracted, following the urging force of the urging means  116 , each cylinder  111  will again lie flat or substantially flat against the hoisting chamber wall. The unit  2  may then be lifted to be taken out of the hoisting chamber. A bottom closing cover  6  (shown  FIG. 4   c )) may be installed to the hoisting chamber to seal the opening. Also a safety hatch may be installed on top of chamber and the chamber may be emptied i.e. water pumped away. 
     As depicted in  FIG. 6  according to an embodiment of the invention, the closing cover  6  is arranged of at least two parts, a first part  601  and a second part  602 . The parts are here called as top part and lower part, because the first part forms a top (inner) portion of the closing cover and the lower part form the lower (outer) portion of the cover. The lower part has a form such that it can be fitted into the opening in the bottom of the hoisting chamber through the hull. The form may be circular or a polygonal. It is, however, important that particularly the sealing system may properly function at possible corner areas, which are therefore advantageously suitably rounded. The upper part has a flange plate portion  26  by means of which it may be attached to the chamber. In the  FIG. 6  the cover  6  is in a position supported by the jacks  500  just before mounting. There is also shown how the upper part  601  may at a removed position by dotted line. The second part  602  comprises a periphery wall  622  and a water tight inner section  620  bordered by the periphery wall. The first part comprising a flange  24  is extending over the periphery wall of the second part, to be outside the area of the lower part  602 . The first part and the second part are removably attached to each other. Advantageously the periphery wall outer surface of the second part  602  and the flange plate portion lower surface of the first part are perpendicular to each other. The upper part extends wider than the lower part. The first and the second part are provided with counter surfaces which are joined together when the first and the second part are attached with each other. The surfaces are preferably provided with planar counter surfaces. 
     In  FIG. 7  there is shown a closing cover according to an embodiment of the invention. The lower part is provided with a sealing system  604  at its outer periphery wall surface  622 . The sealing system is controllable so that it may be in active state or in passive state. When it is in active state it seals the gap between lower part and the wall of the hoisting chamber. Preferably there are arranged at least two successive sealing units  606 ,  608  in the longitudinal direction of the hoisting chamber. The seal units may be inflatable seals so that they may be pressurized by a working fluid when activated and depressurized when deactivated. The seals are provided with or are in connectable with a controllable working fluid supply system  618 , such as pressurized air/pneumatic system. The sealing system  604  is arranged in a connection path  630  from outside of the vessel to inside of the vessel along interconnecting surfaces of the closing cover and the hull of the vessel. 
     The lower part  602  is also provided with attaching means to attach the second part  602  to the vessel hull at least when the upper part  601  is removed. The attaching means comprises a number of holding means  610 . The holding means are in this embodiment pins, which may move partially from outer periphery wall surface  622  of the lower part and co-operate with the hoisting chamber, in which a mating recess  614  has been arranged thus, when activated or pushed out, locking the lower part  602  to its locking position in the hoisting chamber. The hoisting chamber is provided with a counterpart  614  for each of the holding pin, such as a recess, or a slot, so that the holding pins, when protruding from the wall of the lower part into the pin recess, locks the movement of the lower part. The locking pins are arranged to the lower part longitudinally at one side of the sealing system, the part being at inner side i.e. dry side when installed. 
     When servicing the closing cover while the vessel is floating, the closing cover is attached through its periphery wall  622  to the surrounding opening and the sealing system  604  is activated by inflating the sealing units at the outer cylindrical surface. Now the first part may be removed from the second part by e.g. removing respective screws  624 , and any serviceable object revealed by the removing of the first part may be serviced. Thus, the top part  601  may be removed from the lower part  602  after the seal units  606 ,  608  are activated and the holding pins  610  are inserted in to recesses  614  in the counter surface of the hoisting chamber. 
     In this way there will be free access to the main seal mating surface under the top part for inspection and/or repair while sealing and locking of the lower part is activated. 
     The first part is extending radially from inner side of the periphery wall of the second part to outer side of the periphery wall. 
     In  FIG. 8  there is shown a closing cover according to another embodiment of the invention. The lower part is also provided with a sealing system  604 ′ at its upper part, on lower surface of a plate  626  on which the upper part  601  is attached. The sealing system comprises two successive O-rings  606 ′,  608 ′ in the radial direction of the hoisting chamber. The sealing system is arranged in a connection path  630  from outside the vessel to inside of the vessel along interconnecting surfaces of the closing cover and the hull of the vessel. 
     The lower part is also in this embodiment provided with a number of holding means  610 . The holding means are pins in this embodiment, which pins may move partially from outer periphery wall surface  622  of the lower part and co-operate with the hoisting chamber. The hoisting chamber is provided with a counterpart  614  for each of the holding pin, such as a recess, or a slot, so that the holding pins, when protruding from the wall of the lower part into the recess, locks the movement of and tightens the sealing system  604 ′ the lower part. The locking pins are arranged to the lower part longitudinally at one side of the sealing system, the part being at inner side i.e. dry side when installed. 
     When servicing the closing cover while the vessel is floating, the lower part of closing cover is attached through its periphery wall  622  to the surrounding opening tightening the O-rings  606 ′,  608 ′. Now, the first part may be removed from the second part by e.g. removing respective screws  624 , and any serviceable object revealed by the removing of the first part may be serviced. Thus, the top part  601  may be removed from the lower part  602  after the holding pins  610  are securely inserted in to recesses  614  in the counter surface of the hoisting chamber. 
     In this way there will be free access to the main seal mating surface under the top part for inspection and/or repair while sealing and locking of the lower part is activated. 
     The first part may be is a flange plate extending radially from inner side of the periphery wall of the second part to over the periphery wall. 
     When installing the unit again in its mounted position, in case the lifting devices  500  or the jacks have been removed, the jacks are remounted at their positions explained above. Next the chamber is flooded e.g. with a water pump up to a proper level corresponding the ship&#39;s draft. Next the bottom closing cover is demounted in a similar manner explained above in connection with the flange  24 . 
     The unit will be lowered into the flooded hoisting channel until it lands and rests on the actuated i.e. extended jacks, which are positioned at the bottom of the channel. Then, the cylinders  111  are controlled to extend their rods  112  and, guided by the guide surface  113 , each clamp  114  will move into engagement with the shallow recesses  115  on the flange  24 . Now the working pressure on each hydraulic jack can be released in a controlled manner and the jacks will be retracted down with appropriately low actuating pressure on the cylinders  111 . This takes place while the hoisting chamber is filled with water. When the clamping means have clamped the unit  2  into the mounted position, water can be pumped out of the hoisting chamber so as to dry the working space there. It is noted that the cylinders of clamping means are also maintained in a locked state, for safety reasons. 
     Once the water has been pumped off, persons for re-applying the fixing means  14  may climb down the hoisting chamber to set the screws. After the screws have been set and fixed, the clamping means can be retracted by retracting the rod  112  into the cylinder  111 . This is recommended, because the cylinder rod  112  is then protected against corrosion when it is located in the cylinder  111 . 
     Returning back to  FIG. 5 , in which a still another embodiment of the invention is described. As is depicted also in  FIG. 5  by the dotted line the assembly comprises a connection path  630  which runs through a potential route to the water underneath the vessel from outside the vessel to inside of the vessel along interconnecting surfaces of the unit  2  or the flange thereof and the hull of the vessel  100 . There is a sealing system  604  which divides the connection path  630  to outside connection path portion  630 ′ and inside connection path portion  630 ″. The outer side connection path portion  630 ′ opens into the surroundings i.e. water when the vessel is floating. According to an embodiment of the invention the unit  2  is provided with a gas inlet system  640 ,  640 ′ arranged to open into the outside connection path portion  630 ′. Naturally only one inlet system is needed and the separated conduits in  FIG. 5  are for purpose of example. 
     The gas inlet system  640 ,  640 ′ comprises a conduit  642 ,  642 ′ extending through the unit or though the hull and is connectable to a source of gas  644  arranged in the vessel. The source of gas is advantageously a pressurized air system of the vessel. The source of gas may also comprise an air-nitrogen converter  645 , in which case the gas which is fed to the space is advantageously nitrogen. Thus, the gas inlet system is arranged to inject dry nitrogen into the space, so that a pressure above the water hydrostatic pressure at prevailing draft is controllably maintained in the outside connection path portion. This prevents any sea growth in the space and also prevents sea water to enter into the space. Advantageously only service air is needed to feed this system. There is also a control system  646 ,  646 ′ for maintaining a predetermined supply of gas to the inlet gas system. The control system may comprise e.g. a pressure sensor. The control system may comprise e.g. a valve  646 ,  646 ′ and or an orifice plate  647 . 
     Finally,  FIG. 4  shows a sequence of steps a), b), c) in which a unit in the form of a thruster  2  is dismounted from a vessel hull  100 . On the left in  FIG. 4 , a step a) is shown in which the electric drive motor  23  is removed from the thruster  2 . After removal of the electric motor  23 , cover  61  or covers are mounted to the thruster  2  to close all openings against the immersion of water when the hoisting chamber  1  is to be flooded. The drive connection of the thruster  2  usually consists of a flexible coupling; it may however also or additionally include a gear box. Also, a hoist means  7  is already fixed to the flange  24  of the thruster  2  as is indicated at the right lifting eye in step a) of  FIG. 4 . Further, the clamping means are activated so as to clamp the flange  24  against its seat in the vessel hull  100 . Once the clamping is set, the fixing means (screws) can be removed and the persons leave the hoisting chamber. 
     Then, the clamping means is gradually released so as to let water flow into the hoisting chamber. Once the hoisting chamber is filled to the required water level, the clamping means can be released and the lifting devices activated, that is, as described under reference to  FIG. 3 , the rods of the cylinders are retracted. Then, continuing to step b) in the middle of  FIG. 4 , it is shown that the hoist means  7  of a crane  5  are used to pull up the thruster  2  by lifting it at the lifting eyes. During lifting, the thruster  2  is guided in the hoisting chamber by means of the guide rails which are indicated with several parallel lines on the chamber walls in step b) of  FIG. 4 . When the thruster  2  is completely withdrawn from the hoisting chamber  1 , the status of step c) in  FIG. 4  will be established in that a provisional cover  6  is inserted into the hoist chamber so as to close the opening in the vessel hull using the flange seat at the bottom of the chamber. It is noted that clamping with the clamping means is possible because the cover  6  has the same clamping pads as are provided with the flange  24  of the thruster  2 . Depending on how long the hoisting chamber has to stay closed, the cover may be additionally fixed with fixing means after the water has been removed from the chamber. Basically, however, it should be sufficient to press the cover down by a clamping means. Also, an additional cover  4  is put on the top of the hoisting chamber for safety reasons as discussed above. 
     In  FIG. 4 , dismounting the thruster and putting it on deck of the vessel has been described. Mounting of the thruster after maintenance or for replacement is done in the opposite order of steps. That is: removing cover  4 , clamping cover  6 , fixing hoist means  7  to cover  6 , removing any fixing means (if any), and gradually releasing the clamping force so as to flood the hoisting chamber  1 . Once the required water level is reached in hoisting chamber  1 , cover is lifted after clamping has been released. The cover  6  is removed from hoisting chamber  6 . Then, proceeding back to step b), the thruster  2  fixed to the hoist means  7  will be led down the hoisting chamber  1  while being guided by the guide rails  12 . Once the thruster  2  is set in its mounting position, clamping means  111 ,  112 ,  114 ,  115  are activated to press the flange  24  of the thruster  2  against its seat to seal the bottom of the hoisting chamber  1 . The hoisting chamber  1  is then pumped empty and in the dry space fixing means like screws are set and fixed. After that, clamping may be released. Hoist means  7  are separated from the lifting eyes of the thruster and covers  61  are removed. Thereafter, the electric motor  23  can be lowered into position and fixed for operation. After connection work has been done, the thruster  2  is ready for use again. 
     While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. In the above example, the invention has been described using a thruster as the unit which passes through the vessel hull. However, it may be any, in particular large, unit of any vessel or ship for the maintenance of which the invention may be applied. In particular, these units can be oil or gas well connectors, drilling equipment, pumping equipment, etc. Due to the clamping provided to hold the unit in sealing contact with its sealing seat, and having an automatic engagement of the clamping means provided, it can be achieved that no person has to enter a water-filled hoisting chamber, that is, all steps can be carried out in dry environment without diving work. Of course, modifications can be seen in using divers for setting the clamping means or other work. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such a combination is technically feasible.