The invention relates to a sealing structure for sealing a shaft which is situated underwater and rotatable about an imaginary longitudinal axis, the structure comprising a structural part which is arranged transversely to the shaft through which the shaft can be passed or is passed, at least two coaxial ring seals which mutually enclose a pressure chamber for sealing the shaft and the structural part with respect to one another, as well as pressure means for supplying a fluid to the pressure chamber at a positive pressure, as well as a discharge for discharging the fluid.

US 3934952 A discloses a leak-free seal for a propeller shaft of a ship. The object of US 3934952 A is to prevent the leaking of oil. It is known from this publication, to pump water into an annular space at a pressure which is higher than the pressure of the seawater for cooling and lubricating a sealing ring and for lowing away dirt outboard.

GB2096554 discloses an assembly comprising a bearing and a seal for a propeller shaft of a ship. The object of GB2096554 is to prevent migration of oil or seawater via an intermediate chamber. This known seal has an annular pre-chamber filled with water and a continuous chamber filled with oil. The fact that the use of oil entails a risk of environmental load is a drawback, for example the leaking away of oil. Furthermore, due to the use of both oil and water, a separating tank is required in order to separate oil and water.

Another example of a sealing structure for a ship is known from JP-A-11304005. This known sealing structure forms part of the rear seal of a propeller shaft sleeve containing a propeller shaft. In the sealing structure, a continuous flow of fluid from the source has to be generated. The excess fluid escapes under the outer ring seal(s). The supply of a fluid to the pressure chamber provides various advantages which relate to an improved separation between the water outside the sealing structure and the space within the latter, in particular the lubricant-filled space between the propeller shaft and the propeller shaft sleeve. As a result of the counterpressure generated by the fluid in the pressure chamber, the load on the ring seals is first of all significantly lowered, thus reducing wear and increasing the service life thereof. In addition, this makes it possible to prevent the lubricant from leaking from said space to the environment, which would thus result in environmental pollution. A further sealing structure for driving a ship is known from the document EP 1 586798 BI. The document describes a radial shaft seal which makes use of a number of adjacent pressure chambers which are separated by sealing elements in order to seal said shaft with respect to its environment. More specifically, the sealing elements are designed as radial elastomenc lip seals by means of which the shaft is sealed with respect to the environment. A radial shaft seal with only elastomeric sealing elements has the drawback that the pressure difference per sealing element is limited. This is due to the fact that the maximum acceptable pressure difference per element is low and a number of sealing elements has to be placed in series. In addition, the pressure in the pressure chambers between the sealing elements has to be actively controlled. As a result thereof, it is possible for the sealing structure to seal against higher pressures than would be possible for each individual sealing element. The liquid to be supplied to the pressure chambers is passed through a fixed throttle, which results in a reduction in pressure. The required liquid is taken from the environment and freely flows back into the environment.

As the required liquid freely flows back into the environment, the known sealing structure can only be used if no or only a limited counterpressure is present. This is the case, for example, if the underwater sealing structure is situated relatively closely to the water surface and if the structural part is in communication with the ambient pressure above the water surface. If this is not the case, for example with structures which are completely submerged or structures which have to operate at relatively large depths, at which depths the pressures are much greater, the known sealing structure cannot be used, as a free flowing out of the liquid is practically impossible.

If structural parts have to be used at greater depths than those that are customary for propeller shafts of a ship, another kind of seal has therefore until now usually been used. It is for example possible to use special mechanical shaft seals to provide a seal. However, such mechanical seals are less attractive, inter alia due to their technical complexity and costs. In case the structural part forms part of a tidal turbine for example, it is known to pressurize the housing in which the turbine is located, for example by filling the housing with a gas, such as nitrogen or another suitable gas. Due to the increased pressure in the housing itself, the pressure difference across the shaft running through the housing is reduced and the quality of the sealing of the shaft seal increases. However, a drawback of pressurizing the (housing of the) structural part is that this is a technically complicated operation, in particular if the pressure has to be maintained at a sufficiently high level for a relatively long period of time. In practice, this means that, for example, a supply conduit has to be provided to the shaft structure. This is all the more difficult as such structures are often difficult to access.

Furthermore, the pressure inside the structural part has to be increased to an extreme level in order to be able to compensate for the pressure difference across the seal at great water depths, (for example of more than 50 m, up to even 100 m or more), which may result in technical complications.

It is an object of the invention to provide a seal of the type mentioned in the preamble in which one or more of the abovementioned drawbacks of the known shaft seals are overcome. It is a further object of the invention to provide a sealing structure by means of which a rotatable shaft can be sealed with respect to a structural part at relatively great depths.

According to an aspect of the invention, at least one of the objects is achieved by a sealing structure for sealing an underwater shaft which is rotatable about an imaginary longitudinal axis, the structure comprising a structural part which is arranged transversely to the shaft through which the shaft can be passed or is passed, at least two coaxial ring seals which mutually enclose a pressure chamber for sealing the shaft and the structural part with respect to one another, as well as pressure means for supplying a fluid to the pressure chamber at a positive pressure, as well as a discharge for discharging the fluid, wherein the discharge is connected to a discharge part which comprises a pump which is configured to pump discharged fluid further downstream.

According to an embodiment of the invention, the fluid flowing out of the pressure chamber is discharged to the environment by force or recirculated in the direction of the pressure chamber. Forced discharge is effected by means of one or more pumps. These can increase the pressure in the fluid flowing out sufficiently, from the relatively low pressure prevailing in the structural part up to the relatively high ambient pressure (caused by the water column and thus dependent on the depth at which the structural part is submerged).

As has already been mentioned above, in a particular embodiment the fluid is fed back to the pressure chamber(s). To this end, the sealing structure may have a discharge part which is connected to the pressure chamber in order to return discharged fluid to the pressure chamber. More particularly, the discharge part may be configured in such a manner that fluid which has already been discharged is circulated to the pressure chamber via the pressure means. This embodiment has the advantage that the quality of the circulating fluid can readily be controlled. If the fluid is recirculated in a closed system, the risk of contamination of the seal, for example as a result of bio-organisms or substances entering the fluid, is relatively small. In addition, no fluid ends up in the environment, thus resulting in an environmentally friendly seal.

In other embodiments, the discharge part is configured to discharge into the underwater environment, at least if the seal is in use, in order to supply discharged fluid thereto. This embodiment can be configured as a simple structure.

According to an embodiment, the discharge part has a reservoir for temporarily storing discharged fluid. The reservoir may be connected to the pump for the further discharge of fluid.

In a further embodiment, the discharge part comprises a heat exchanger for cooling the discharged fluid. The fluid can provide local cooling at the location where the structural part is connected to the shaft. Due to the rotation of the shaft, the connection may become hot and this may, in time, result in damage to the structure and or shaft. Certainly (but not only) when the fluid is recirculated, the fluid can be heated up, for example in the pressure chamber, so that its cooling effect is reduced. By passing the fluid through a heat exchanger, the fluid may be brought to the desired (low) temperature.

In a further embodiment, the sealing structure comprises three, four or more successive coaxial ring seals of which in each case two adjacent ring seals delimit a pressure chamber. By creating a number of adjacent pressure chambers in this manner, it is possible to withstand a higher pressure difference than is the case with a single pressure chamber. In this case, the pressure means are preferably configured such that they, viewed from the side of the highest pressure (e.g. the side adjacent to the underwater environment) to the side of the lowest pressure (e.g. the interior space of the structural part), generate a gradually decreasing pressure in the successive pressure chambers. In this embodiment, the pressure chambers cooperate in order to gradually absorb the positive pressure in the longitudinal direction of the shaft.

In a further embodiment, the steps during the stepwise reduction of the pressures in the successive pressure chambers are substantially equal, so that the various seals are • -" ^■ · » « _* »» bv iw / w \ ( j 9

5

loaded virtually in the same manner, which significantly improves the service life of the seal. Nevertheless, if desired, the pressure can also be adjusted (usually reduced) in different pressure steps.

In a further embodiment, the pressure chambers are connected in series, so that the seal is still operational when one or more of them are damaged.

The sealing elements, more particularly the ring seals, of the sealing structure can take numerous forms, such as for example slip ring seals, depending on the

circumstances and the desired sealing properties. In an advantageous embodiment, the ring seals comprise lip seals, the lip of which is turned to the side with relatively high pressure.

As has already been explained before, in contrast with the field of ship

propulsion, no sealing structures provided with ring seals and pressure chambers are being used at present in the field of tidal turbines. Tidal turbines currently mainly use mechanical or similar seals, in which the nacelle of the turbine is optionally placed under excess pressure (with respect to an atmospheric pressure) in order to reduce the pressure difference across the seal. However, various drawbacks are associated with the use of such seals and with placing the nacelle of the tidal turbine under pressure using gas, such as nitrogen, in particular when these are situated at considerable depth, for example - but not limited to - depths of 20 m or more. One of the drawbacks is the fact that placing the nacelle under pressure is a technically complicated operation, especially when the pressure has to be maintained at a sufficiently high level for a relatively long period of time. The nacelle is then connected to a gas supply above the water surface via a gas supply conduit.

WO2007/125349 discloses a tidal turbine. The turbine has a sealing system between the casing and the shaft in order to prevent the ingress of water into the housing. The sealing system discharges to a collecting tank.

It is therefore a further object of the invention to provide a nacelle with a simple and durable sealing structure which makes it possible to use the nacelle at great water depths.

According to a further aspect of the invention, this object is achieved with a nacelle of an underwater turbine comprising a shaft which is rotatable about an imaginary longitudinal axis, a structural part of the housing which is arranged transversely to the shaft and through which the shaft is passed, at least two coaxial ring seals which mutually enclose a pressure chamber for sealing the shaft and the structural part with respect to one another, as well as pressure means for supplying a fluid to the pressure chamber at positive pressure, as well as a discharge for discharging the fluid, wherein the discharge is connected to a discharge part which comprises a pump which is configured to pump discharged fluid further downstream.

The inventors have come to the surprising view that a nacelle for a tidal turbine operating at a relatively great depth can indeed be sealed properly using a sealing structure of the kind mentioned in the preamble which is used in the shipping industry for sealing the propeller shaft of a ship. If the discharge of such a sealing structure is connected to a discharge part with a pump for pumping the fluid discharged from the pressure chamber, the pressure in the fluid can be increased to such an extent that it can be recirculated or can be released into the environment of the nacelle (where a high water pressure prevails). The sealing structure can furthermore be configured such that the local changes in the water pressure can be tracked automatically, without requiring active pressure regulation of the pressure(s) in the pressure chamber(s). Local changes in water pressure may, for example, result from changes in tide or the weather conditions on the water surface.

If the sealing structure has a sufficient number of successive ring seals and associated pressure chambers, the pressure difference absorbed by the seal may be so great that even when the nacelle is placed at a great depth (of, for example, more than 50 m or even more than 80 m), a low pressure, for example a pressure of around 1 atmosphere, may prevail in the interior of the nacelle.

In principle, the fluid may consist of any suitable liquid and/or gas composition. In a further embodiment, however, the fluid is water (for example seawater) which has been taken from the underwater environment. In particular, the fluid does not comprise oil, more particularly, however, the fluid is only water, even more particularly only seawater which has been taken from the underwater environment. By correctly choosing the fluid (for example water), it is possible to obtain an environmentally friendly seal. In order to ensure the supply of fluid, the nacelle may, in a further embodiment, be provided with a fluid-introducing element for introducing fluid, in this case water from the underwater environment.

In many cases, the shaft will have to be lubricated in order for the shaft to be able to rotate with respect to the nacelle for relatively long periods without resulting in damage. It is known to lubricate the shaft by providing lubricant between the shaft and the nacelle or at least a structural part therein by means of a lubricating element.

Lubricant may, for example, be provided in a lubricant chamber adjoining the shaft. Under certain circumstances, this lubricant may flow away and pass into the environment. This not only has a polluting effect on the environment, but often also means that the lubricant supply has to be replenished regularly in order to be able to lubricate the shaft to a sufficient degree. According to a further preferred embodiment of the invention, however, the lubricating element and the lubricants are formed, respectively, by the one or more pressure chambers and the fluid flowing therein, in particular the water taken from the environment. Consequently, this embodiment does not require separate lubrication and the risk of polluting the underwater environment with lubricants is eliminated. Furthermore, compared to the usual lubricants, the fluid remains relatively cool so that heating up of the shaft with all its negative consequences is less likely to occur.

Further advantages, features and details of the present invention will be explained in the following description of some embodiments thereof. In the description, reference is made to the attached figures, in which:

Fig. 1 shows an axial cross section of a propeller shaft structure with a first embodiment of the sealing structure according to the invention;

Fig. 2 shows an alternative embodiment of the sealing structure for the propeller shaft structure from Fig. 1 ;

Fig. 3 diagrammatically shows a further embodiment of the sealing structure in which the fluid is collected and recirculated;

Fig.4 diagrammatically shows yet another embodiment of the sealing structure in which the fluid is collected and released into the environment; and

Fig. 5 shows the desired Q-H curve for a pump in an embodiment of the sealing structure according to the invention.

Fig. 1 shows an example of the application of an embodiment of a sealing structure according to the invention on a propeller shaft structure. First of all, Fig. 1 shows a propeller shaft 1 and a propeller shaft sleeve 2 denoted overall by reference numeral 2 surrounding the propeller shaft 1. This propeller shaft sleeve 2 comprises a sleeve portion 3, a rear structural part 4 which is passed to the outside through the ship's hull 5 and a front structural part 6. The rear structural part 4 supports the rear bearing 7, the front structural part 6 and the front bearing 8. In these bearings 7, 8, the propeller shaft 1 is accommodated so as to be rotatable about its axis 9.

In order to seal the propeller shaft 1 with respect to the rear structural part 4, the rear seal which is denoted overall by reference numeral 10 is provided. Furthermore, for the front structural part 6, the front seal which is denoted overall by reference numeral 11 is provided. In the space 12, delimited by the propeller shaft 1, the propeller shaft sleeve 2 and the seals 10, 11, an amount of liquid lubricating oil is provided, the hydrostatic pressure of which is inter alia determined by the height of the lubricating oil column which is determined by the lubricating oil level 14 of the amount of lubricating oil 15 in the lubricating oil container 16, as well as the difference in height across the conduit 13 by means of which this lubricating oil container 16 is connected to the space 12.

On board the vessel of which the respective propeller shaft structure forms part, there is a fluid supply 17 for supplying a fluid (that is to say a gas, a liquid or a mixture of the two). The fluid may be compressed air, which compressed air may be supplied by the vessel's network. This compressed air has a pressure which is typically between approximately 7.5 and 8.5 bar. However, the fluid may also be water, for example seawater which has been taken from around the vessel.

Via a fixed throttle device 18 having a single fixed passage as will be explained later, the fluid is connected to the space 19 above the lubricating oil level 14 in the lubricating oil container 16. The throttle device 18 has a passage which is such that a fluid flow of, for example, approximately 25 standard litres per minute is delivered. From the lubricating oil container 16, a conduit 20 runs to the rear seal 10. There, the conduit 20 is connected to a pressure chamber 21 , which is determined between the lip seals 22, 23. The lip of the front lip seal 23 is turned towards the front and the space 12, while the lip of the other lip seal 22 is turned towards the rear. The further lip seal 24 is also turned towards the rear.

As is illustrated in Fig. 1, the conduit 20 is furthermore connected to the discharge valve. The throttle device 18 is chosen such that some amount of fluid is continuously escaping via the discharge valve 25. This means that the pressure in the pressure chamber 21 is equal to the liquid column which is situated between the discharge valve 25 and the water level 26 at which the respective vessel is. Since the conduit 20 is both connected to the pressure chamber 21 and to the discharge valve 25, no second conduit has to be installed which runs separately to the discharge valve 25.

The fluid together with any leaking liquid can be discharged from the pressure chamber 21 by means of a first discharge conduit 50 which runs to the collection reservoir 51. The collection reservoir is provided with a deaerator 52. Via a second discharge conduit 53, the reservoir 52 is connected to the aforementioned conduit 20 which runs to the pressure chamber 21. In the second discharge conduit 53, a fluid pump 54, for example a centrifugal pump or similar pump, is provided in order to pump the fluid discharged via the first conduit 50 in the direction of the conduit 20.

In a similar manner, a further pressure chamber 31 which is situated between the lip seals 32 and 33 is pressurized. These lip seals have a lip which is turned to the rear and the space 12 containing the lubricating oil. To this pressure chamber 31, a fluid container 34, for example a container for oil, is connected to which a pressure can be applied via conduit 35 and throttle device 36. This pressure depends on the pressure as adjusted and measured by the valve 25. This means that the pressure in the pressure chamber 31 fluctuates together with the water column up to the water level 26.

Therefore, if the pressure of the lubricating oil in the space 12 increases as a result of an increase in the draught or a large wave, the pressure in the pressure chamber 31 is also increased. The lip seal 32 is thus not subjected to additional load when the pressure of the lubricating oil in the space 12 increases as the higher lubricating oil pressure is counteracted by the pressure of the fluid (oil) in the pressure chamber 31. This increases the service life of the lip seal 32.

A second throttle device 37 is connected in series with the fixed throttle device 36. This second throttle device opens into the environment. The branch conduit 38 which runs to the lubricating oil container 34 is therefore loaded with a pressure which is approximately half the pressure in the conduit 35, if the throttle devices 36 and 37 are identical.

The fluid together with any leaking liquid can be discharged from the pressure chamber 31 by means of a first discharge conduit 55 which runs to the collection reservoir 57. The reservoir 57 is connected to the aforementioned fluid container 34 (lubricating oil container) via a second discharge conduit 58. In the second discharge conduit 58, a second fluid pump 56, for example a centrifugal pump or similar pump, is provided in order to pump the fluid discharged via the first conduit 55 in the direction of the fluid container 34 in order to make recirculation thereof in the direction of the pressure chamber 32 possible.

The abovementioned seal is not limited to an embodiment with a single pressure chamber. In other embodiments, more pressure chambers are present, for example three pressure chambers 31 , 39, 40, which are formed between four lip seals 32, 33, 41 and 42. The conduit 35 is now connected to three throttle devices 36, 37, 43 connected in series, the last of which opens into the ambient atmosphere. This may in each case be a piece of pipe having a small internal diameter. Via correspondingly arranged branch conduits 38, 44, the lubricating oil containers 34, 35 are placed under gradually decreasing pressures, so that the pressure chambers 31 , 39 are also at gradually decreasing pressures. The pressure is lowest in the front pressure chamber 40, since this is only subjected to the lubricating oil column of the lubricating oil in the associated lubricating oil container 47, which is in communication with the atmosphere via the conduit 48. By means of a number of pumps 70a, 70b and 70c, the fluid introduced into the pressure chambers 31, 39 and 40, respectively, can be discharged via discharge conduits 49a, 49b and 49c to the lubricating oil containers 34, 45 and 47, respectively.

Another embodiment of the invention is illustrated in Fig. 3. In the embodiment from Fig. 3, a rotating pipe seal is provided in a cannula 106 (diagrammatically indicated by dashed lines) of a sea turbine for generating electrical power, for example of a type similar to that described in EP 1 586798 BI. The pressure (Po) in the cannula is relatively low, for example equal to 1 bar. The rotating pipe seal comprises a fixed housing 60 and a bushing 59. The housing is attached to a fixed pipe and the bushing to a pipe which is rotatable about the axis. Several lip seals form part of the housing (in the illustrated embodiment the lip seals 61 to 66, but a larger or smaller number of seals is obviously also possible). Between every two of these lip seals, a pressure chamber 71 to 75 is provided, as well as annular throttle devices 81 to 84.

With reference to Fig. 3, a pressurized (PI) fluid, for example (sea)water which comes from a fluid supply element 102, is supplied (at a volumetric flow Qv4) and passed to the first pressure chamber 71 via a conduit 76. The fluid flows into the first pressure chamber 71. The fluid can leave the first pressure chamber via a conduit 77 and can be passed through to the second pressure chamber 72. In this case, the fluid first passes into the first throttle device 81. Downstream of the first throttle device 81 , the fluid flows into the second pressure chamber 72 via conduit 78 and into the second throttle device 82 via conduit 79. The pressure in the second pressure chamber 72 is lower than the pressure in the first pressure chamber 71. In this way, the fluid is passed through to each of the successive pressure chambers 73-75. Due to the successive and alternating sequence of throttle devices 81-84 and pressure chambers 71-75, the pressure (PI,P2,P3,P4,PO) is gradually reduced in regular steps, as a result of which the load on all the lip seals is very limited.

Thus, the fluid flows from the supply conduit 76 to the discharge conduit 103 via the various pressure chambers. Via discharge conduit 103, the fluid (volumetric flow Qvl) is passed to a reservoir 104 present in the cannula. The reservoir 104 is connected to an automatic deaerator 105 and to a pump module 107 placed in the cannula 106. The pump module 107 inter alia comprises a pump 108 with which the fluid can be pumped (at a volumetric flow Qv2). The pump 108 sends the fluid (volumetric flow Qv3, in which case Qvl = Qv3 + Qv4 applies in principle) back in the direction of the supply conduit 76 of the first pressure chamber 71.

Fig. 4 shows an alternative embodiment of the invention. Instead of the fluid being recirculated, it is discharged to the environment after it has passed through the pressure chambers. The fluid supplied via supply part 102, that is to say the seawater which comes from outside the nacelle 106, has a high pressure (PI) and is passed to the pressure chambers at this pressure. However, the pressure (Po) of the fluid leaving the last pressure chamber is much lower. In order to still be able to discharge the fluid, for example via the discharge element 109, the fluid pressure has to be increased to PI or more. This pressure increase is effected by a pump 110 which is connected to the conduit 111.

Referring to Fig. 3, the pressures and volumetric flows in the sealing structure at different operating conditions are mapped in order to assess the system performance of the circulation system. For example, if it is assumed that the water pressure PI can vary within a bandwidth of ± 1 bar due to for example the tidal movements, the nominal water pressure is 5 bar (corresponding to a depth of approximately 50 m) and the pressure Po in the (turbine) housing or cannula is assumed to be atmospheric pressure (Po = 0 bar) and if it is assumed for the volumetric flows that Qvl = Qv3 + Qv4, the pressures in the various pressure chambers and the associated volumetric flows can be calculated. The minimum volumetric flow required in the pressure chambers will depend, inter alia, on the development of heat in the seal, which in turn depends on speed and pressure. Based on a volumetric flow Qvl through the throttle, the values shown below in Table 1 are calculated.

Ideally, the following should apply: Qvi = Qv2 = Qv3 and Qv4 = 0, but in practice, Qv2, Qv3 and Qv4 still depend on the pump solution. Fig. 5 shows the associated desired Q-H curve for the pump.

The present invention is not limited to the embodiments thereof which have been described here. The rights sought are also defined by the attached claims, which allow for numerous modifications and changes.