TECHNICAL FIELD

The present invention concerns a slewing seal system, and is related to providing a slewing seal arrangement for a steerable thruster unit of a marine vessel. The invention concerns also a slewing seal assembly for a steerable thruster unit.

BACKGROUND OF THE INVENTION

Marine vessels employ steerable thrusters, (azimuthally rotatable thrusters) commonly for propulsion. However, all propulsion mechanisms require efficient sealing arrangements in order to prevent the sea water from leaking inside the hull of marine vessel. Also the sealing arrangement must prevent the lubricating oil from leaking to the sea.

The conventional propulsion systems of marine vessels already have sealing arrangements for propeller shaft. The propeller shaft is taken inside the hull of the marine vessel by means of a stern tube. The stern tube is provided with bearing and with a sealing assembly. The sealing assembly is intended to prevent the sea water from entering the hull and to prevent the lubricating oil from leaking to the sea. Such arrangement is known from WO2012042094.

The sealing of the steerable thruster units also requires efficient sealing ar- rangement. One problem is that sealing members wear or deteriorate over time and start to increasingly allow either sea water or oil or both pass the sealing. However requirements and conditions for sealing the steerable thruster unit differ from sealing the propeller shaft. Seal on the propeller shaft is for high rpm and "small" diameter. Seal on the slewing section is for large diame- ter and low RPM. Replacing of the sealing of the propeller shaft is also significantly easier operation compared to replacing the sealing of the steerable thruster.

The slewing seal systems are therefore usually replaced during a periodical inspection or maintenance, or in an emergency so as to cope with troubles such as accidental seawater and oil leakage. Usual period between replacements of steerable thruster unit seal system is on the average of seven years in the normal use. The replacement of seal assembly of the steerable thruster unit requires costly and time-consuming operation at the dry dock. Such replacement operation includes removing the steerable thruster unit from the hull and disassembly of the thruster unit.

An object of the present invention is to provide solutions to above mentioned problems. Another object of the invention is to provide means to lengthen the period between required seal assembly replacements and to reduce costs of thruster unit seal assembly replacements.

SUMMARY OF THE INVENTION

The present invention is characterized by the features defined characterizing part of the independent claims 1 , 4, 6 and 8.

The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

A steerable thruster unit comprises at least a casing, and a vertical drive shaft, which turns a propeller shaft and propeller. The steerable thruster comprises also a seal arrangement around a shank to prevent sea water from entering the hull and to prevent lubricating oil from leaking to the sea. The seal arrangement is of slewing seal kind to allow azimuthal rotation of the steerable thruster. The slewing seal arrangement is in most solutions arranged above the hull closing plate (thereby inside the hull of the vessel) and comprises at least a seal assembly and releasable locking mechanism. The hull closing plate is not water tight around the shank, but there is only a small gap between the plate and the thruster shank. The hull closing plate is used to give a kind of "stream line" to reduce turbulence. An additional advantage is that the slewing seals are protected against peak pressures due to a turbulent flow around the stemsection.

The slewing seal assembly comprises at least a set of at least two annular seals arranged around the vertical shank of the steerable thruster for sealing in order to prevent the sea water from entering the hull and to prevent the lubri- eating oil from leaking to the sea. Typical number for annular seals in the set is three, however the number of seals is not limited to three.

In typical case of three annular seals first two prevent the leakage of sea water inside, and the third seal, or the rest of annular seals, prevent the leakage of the lubricating oil to the sea. However the number of seals for sealing water and lubricating oil can vary. The annular seals are typically made of elastic materials such as rubber or elastomer.

The slewing seal assembly further comprises an upwardly elongated section above said set of annular seals and the section is housing a spare set of annular seals arranged around the vertical shank. The elongated section of the slewing seal assembly can further comprise at least two spare sets of annular seals arranged to be used for sealing. The spare set or sets of seals are of similar configuration as first or original set of seals.

The seal assembly further may comprise an arrangement for the annular seals to be fastened to form an integrated part. For example the seals may fixed to a casing or the set of seals is formed from a single block of material. Another option is that the casing can comprise plurality of casing members that are stacked together and fastened together to form an integral part. These arrangements enable that the seal assembly more simple to design and to as- semble. The most common way is to use (lip) seals according to fig. 2A, made from a combination of rubber, fabric and steel running against a liner. The seals are usually enclosed in a (metal) casing that can be build up from separate sections.

The releasable locking mechanism is comprised with the slewing seal ar- rangement for allowing the seal assembly to be moved and for allowing the spare set of annular seals to be taken into use. The spare set of annular seals is moved to the sealing position to replace the previously used set of seals by releasing the locking mechanism and thereby allowing the seal assembly to move downwards. The mechanism can be any fastening arrangement for fixing the seal assembly into its position known by a person skilled in the art, for example bolt fastening or mechanical fastening.

When seal assembly is moved downwards along the shank of the steerable thruster, the spare set of seal assembly slides into a sealing position around the shank and previous set of seals is moved below said sealing position. The seal assembly can be locked with the locking mechanism after the spare set of seals has been positioned into sealing position. However, another possible arrangement instead of the actual locking mechanism is that when seal assembly is moved to use spare set, the seal assembly can optionally be sup- ported on an annular collar or a shoulder provided around the shank, thereby eliminating the need for locking separate locking mechanism to fix the seal assembly into the position. However it must be ensured that seals stay in place and additional arrangement may be required in addition to gravity for fixing the seals in place. In the case of two or more spare sets of annular seals, however a locking mechanism is required so that the seal assembly can be moved several times and fixed in intended position.

The locking mechanism is arranged to be controlled so that locking mechanism is accessible from inside the vessel, for example through a maintenance space or a hatch that is above the slewing seal arrangement. Another option is that the locking mechanism is arranged to be controlled through an aperture of the hull closing plate. This arrangement enables changing the deteriorated seal set for a spare set of annular seals by a person, e.g. diver. Thereby the changing the set of annular seals does not require costly and time-consuming operation at the dry dock.

The spare set of seals or seals that are not in use should not touch the shank surface in order to avoid wear. The mating surface for the seals at the sealing position has slightly larger diameter to the surrounding surface to provide efficient sealing and also to protect the unused seals from wear. For the seals that are in use, the most common option is to have the seals running against a liner. A liner is a high quality bush with high wear resistance and no corrosion in sea water and or oil. All the changes in the azimuth angle will result in a movement between the lip seal and the bush.

When the spare seal set is pushed down for taking it in use, there is a transla- tion movement between the casing of the lip-seals and the non-rotating part of the stemsection. Standard O-rings can be used for sealing the gap between the seal casing and the stemsection. Other option is that the shank of the steerable thruster can be provided with a gap (a recess on the shank) between the set(s) of spare seals and the shank, in order to protect the seals from wear when the spare seals are not used. Another option for protecting the spare seals from wear is a spacer ring or anoth- er liner between the seals and the shank. The sealing position can also be provided with a formed surface or a recess to receive or accommodate the set of seals.

The present invention is used in steerable thrusters, or in azimuthally rotating thrusters, and in marine vessels. The present invention thus enables to pro- long the maintenance period by providing means to change the seals to spare seals in a simple and cheap way.

The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 illustrates schematically structure of the steerable thruster.

Fig. 2a illustrates schematically cross-section of a slewing seal arrangement at original position.

Fig. 2b illustrates the same arrangement with spare set of seal in sealing position. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In figure 1 is shown schematically a steerable thruster 10 of marine vessel. The steerable thruster 10 comprises an azimuthally rotatable pod with propeller, and also a vertical shank 4 that connects the pod to the vessel through a hull closing plate 5. Vertical axis of the shank is the rotation axis of the azi- muthal rotation. Arrows illustrate azimuthal rotation or changing of the azimuth angle (=steering angle) of the steerable thruster. A slewing seal arrangement 1 for a steerable thruster is used for sealing in order to prevent the sea water from entering the hull or steerable thruster and to prevent the lubricating oil from leaking to the sea. Stem section 13 is fixed to the hull of the ship. A closing plate 12 is fixed on top of the stem section 13 to provide a closed space.

In figs 2a and 2b is illustrated the slewing seal arrangement. The slewing seal arrangement 1 comprises a slewing seal assembly 2 and a mechanism 8 for locking and releasing the slewing seal assembly 2 for allowing the seal as- sembly to be moved and to be locked in intended position. The seal assembly comprises a set of at least two annular seals 3 arranged around the vertical shank 4 of the steerable thruster. Axis 14 of the shank is the rotation axis of the azimuthal rotation. Fig. 2a illustrates a set of three annular seals 3, which is located at sealing position against the mating surface of the shank 4 or the lin- er 9. The slewing seal assembly 2 is arranged above a hull closing plate 5. The hull closing plate 5 is provided with an aperture for the shank 4 of the steerable thruster to extend through the plate.

The mechanism 8 allows the seal assembly to be locked in its intended position, such that correct set of seals is in sealing position, in other words being in use. The mechanism 8 for locking is releasable so that the seal assembly 2 can be moved in a way that used set of seals is moved downwards and the spare set of seals 7 moves to the sealing position (Fig. 2b). When the spare seal set is moved down for taking it in use, there is a translation movement between the casing of the seals and the non-rotating part of the stem section. Standard O-rings 15 can be used for sealing the gap between the seal casing and the stem section. When the spare set of seals 7 is in sealing position, the mechanism 8 is locked again in order to fix the seal assembly in place. The mechanism 8 is arranged to be controlled from inside the vessel, for example through a maintenance space or a maintenance hatch arranged above or next to the slewing seal assembly. An alternative option is that the mechanism for locking and releasing the seal assembly is arranged to be controlled from outside of the vessel, through an aperture of a hull closing plate 5.

Further another possible arrangement is that the locking mechanism is provid- ed at either end of the seal assembly or seal casing instead of being on the side of seal assembly or seal casing.

Another option for the seal assembly 2 to be fixed to sealing position is that shank 4 of the steerable thruster is provided with an annular collar on which the seal assembly is supported. However this option is restricted to embodi- ment with one spare set of seals.

The slewing seal assembly further comprises above said set of annular seals 3 an upwardly elongated section 6 housing a spare set of annular seals 7 arranged around the vertical shank 4 of the steerable thruster.

It is also possible to that elongated section is provided with more than two spare sets of annular seals around the shank.

The seal assembly further comprises arrangement for the annular seals to be fastened to the casing to form an integrated part. This arrangement provides a single component for more simple maintenance and design. It is also possible that set of seals is formed from a single block of material which comprises the seals.

Another option is that the casing can comprise plurality of casing members that are stacked together and fastened together to form an integral part. This arrangement enables that the seal assembly more simple to design and to assemble. The spare set of seals or seals that are not in use are arranged so that they do not touch the shank surface in order to avoid wear. The mating surface (of the shank or the liner 9) for the seals at the sealing position 9 has slightly larger diameter to the surrounding surface, which means that the mating surface of the shank at the sealing position is elevated from the surrounding surface. This provides efficient sealing and also protects the spare seals from wear. Other option is that the shank of the steerable thruster can be arranged to have a gap between the set of spare seals and the shank in order to protect the seals from wear during the time when the spare seals are not used. Another option for protecting the spare seals from wear is a spacer ring between the seals and the shank.

When a spare set of seals 7 is taken into use by moving the seals of the spare set to the sealing position, as shown in fig 2b, the location where each new (spare) seal is set is not exactly the same location as for the corresponding seals of the previous set. The advantage of this is to get both new seal and new mating surface into use. This provides more efficient sealing. Fig 2b illustrates that seals of the spare set are slightly in different position compared to the original seals. This is illustrated in fig 2b so that at positions 1 1 of original seals the mating surface of liner 9 is slightly worn.

The present invention of the slewing seal arrangement with its embodiments can be utilized in a steerable thruster, especially in a marine vessel. The present invention provides the advantage to change the spare set of seals for a steerable thruster without the need of lifting the vessel from the water. This prolongs the maintenance period when the seal arrangement is required to be changed at the dry dock.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the invention is also applicable for sealing other locations in marine vessels having objects or components with rotational movement.

Reference numbers

1 . slewing seal arrangement 2. a slewing seal assembly

3. a set of annular seals

4. vertical shank

5. hull closing plate

6. elongated section 7. spare set of annular seals 8. releasable locking mechanism

9. mating surface / liner

10. Steerable thruster

1 1 . used mating surface

12. closing plate (between inside of the stemsection and the machine room)

13. stem section, fixed to the hull of the ship

14. rotation axis of the steerable thruster

15. O-ring seals