Field of the invention

The present invention relates to a method of operating a waterborne vessel propelled by at least one mechanical transmission azimuth thruster comprising a driveline connecting a motor located inside the hull of the vessel to a propeller mounted to an outboard azimuth unit of the azimuth thruster, which azimuth unit is arranged to be rotated to any horizontal angle and which driveline comprises a generally vertical shaft and a generally horizontal shaft which is connected to the vertical shaft via a bevel gearing comprising a first bevel gear mounted on the vertical shaft and a cooperating second bevel gear mounted on the horizontal shaft, which motor is drivingly connected to the vertical shaft and which horizontal shaft is drivingly connected to the propeller.

The present invention also relates to a mechanical transmission azimuth thruster for implementing the method.

Background

A mechanical transmission azimuth thruster comprises a driveline connecting a motor located inside the hull of the vessel to a propeller mounted to an outboard azimuth unit. The azimuth unit, or pod, is arranged such that it can be rotated to any horizontal angle, or azimuth. The driveline comprises a generally vertical shaft running in a rotatable column of the azimuth unit and a generally horizontal shaft which is connected to the vertical shaft via a bevel gearing, normally a right-angled bevel bearing, comprising a first bevel gear mounted on the vertical shaft and a cooperating second bevel gear mounted on the horizontal shaft. Consequently, the first and second bevel gears in mesh form the bevel gearing or gear set. The motor is drivingly connected to the vertical shaft and the horizontal shaft is drivingly connected to the propeller. An azimuth thruster may be configured as a pulling azimuth thruster, where the propeller is positioned at the leading end of the azimuth pod and is configured to provide a pulling thrust, or as a pushing azimuth thruster, where the propeller is positioned at the trailing end of the azimuth pod and is configured to provide a pushing thrust. Some azimuth thrusters are configured with both a pulling and a pushing propeller. Vessels propelled by mechanical transmission azimuth thrusters are normally equipped with at least two azimuth thrusters, typically arranged side-by-side as a pair, where the propeller of the starboard side thruster is arranged to rotate in the opposite direction as compared to the propeller of the port side thruster. Therefore, the bevel gears of the port and starboard side thrusters are usually configured as spiral bevel gears having opposite spiral angles, i.e. one left-handed set and one right-handed set.

In use, the rotating driveline components of a conventional mechanical transmission azimuth thruster, i.e. the vertical shaft, the gearing, the horizontal shaft and the propeller, are arranged to rotate only in one direction. A change of the propulsion direction is brought about by rotating the azimuth unit about the azimuth axis. Consequently, a reversal of the propulsion direction is brought about by rotating the azimuth unit 180 degrees.

As each gear in a gearing or gear set is only ever rotated in one direction, only the surface of the leading flank, or side, of the gear teeth will normally be worn or damaged. At the same time the surface of the trailing flank of the teeth is not loaded during meshing and is not subjected to the same wear or fatigue loading as the leading flank, which wear or fatigue loading may result in micro-pitting, surface pitting, subsurface fatigue / crack initiation and scuffing.

The roots of the teeth on both sides of the teeth are loaded even while the gear is rotated in a single direction. However, the load on the tooth root of the trailing flank of the gear has a minor influence on the overall fatigue life. The reason for this is that the loads generate compressive stresses on the root on the trailing flank and tensile stresses on the root on the leading flank.

If a gear tooth failure is detected during normal service intervals, the thruster and its underwater azimuth unit is normally dismantled from the vessel and serviced. This is done either at the site where the vessel is dry-docked or at a service centre. Spare part availability and lead times has the potential to prolong time spent in dry-dock, increasing vessel off-hire.

Bevel gears in azimuth thrusters are normally designed for 20 years of failure free service. However, the gears may experience early failure after as little as 5 years, which may result in the scrapping and replacement of the gearing.

To prevent failure, an azimuth thruster is normally serviced at 2, 5 and 10 years. At the 2 year service, the teeth of the bevel gearing are normally inspected. At the 5 year service, the water tight seals of the azimuth unit are normally replaced. At the 10 year service, the thruster is normally overhauled and the driveline bearings are normally replaced.

In conclusion, problems associated with gearing wear comprise short lifetime and low mean time before failure (MTBF) leading to vessel off-hire.

The present invention addresses these problems and seeks to provide a method and an associated mechanical transmission azimuth thruster which increase lifetime and reduces MTBF for bevel gears in azimuth thrusters, especially for thrusters arranged in pairs.

Summary of the invention

A first aspect of the invention relates to a method of operating a waterborne vessel propelled by a first mechanical transmission azimuth thruster comprising a propeller arranged to rotate in a first direction, and a second mechanical transmission azimuth thruster comprising a propeller arranged to rotate in a second direction which is opposite the rotational direction of the propeller of the first azimuth thruster, wherein each mechanical transmission azimuth thruster comprising a driveline connecting a motor located inside the hull of the vessel to the propeller mounted to an outboard azimuth unit of the azimuth thruster, which azimuth unit is arranged to be rotated to any horizontal angle and which driveline comprises a generally vertical shaft and a generally horizontal shaft which is connected to the vertical shaft via a bevel gearing comprising a first bevel gear mounted on the vertical shaft and a cooperating second bevel gear mounted on the horizontal shaft, which motor is drivingly connected to the vertical shaft and which horizontal shaft is drivingly connected to the propeller.

The method comprises the steps of:

- during a first period of time, operating the azimuth thruster utilising a first load direction of the bevel gearing; and

- during a second period of time after the first period of time, operating the azimuth thruster utilising a second load direction of the bevel gearing, which second load direction is opposite to the first load direction, which steps of changing the load direction of the bevel gearing (4) comprises one of the step of:

(i) at the termination of the first period of time, interchanging the bevel gearings (4) of the first and second azimuth thrusters,

(ii) at the termination of the first period of time, changing the rotational directions of the motor of the first and second azimuth thrusters, and interchanging the propellers (1) of the first and second azimuth thrusters, and

(iii) at the termination of the first period of time, interchanging the azimuth units, except the propellers, of the first and second azimuth thrusters.

Consequently, according to the invention the utilization of the bevel gearing is increased by reversing the load direction of the bevel gearing after the termination of the first period of time.

By reversing the load direction before the gearing has experienced excessive fatigue damage, breakdown and subsequent replacement of the gearing can be avoided, thereby limiting time spent servicing the thruster.

The first period of time may be a predetermined time of operation of the thruster. The predetermined time of operation may be within the range of 2 to 15 years and may be chosen so that it represents half of the bevel gearing's estimated lifetime. The first period of time may advantageously be chosen to coincide with the time up until a scheduled service, e.g. at 2, 5 and 10 years, allowing thruster to operate with a first load direction before the scheduled service, to be prepared for load reversal at the scheduled service, and to operate with a second load direction after the schedule service, which second load direction is opposite to the first load direction. Alternatively, the first period of time may run until damage or irregular operation of the bevel gearing is detected, thus allowing the load direction to be reversed when such damage or irregular operation is detected to avoid further deterioration of the gearing. For example, surface damages on the flanks of the bevel gear teeth usually create vibration and/or noise that increases as the deterioration of the teeth progresses. This vibration and/or noise can be detected audibly by the crew of the vessel or by installed measurement equipment. Also, damage to the gears can be detected during a scheduled service, e.g. by visual inspection or by using screening equipment, e.g. ultrasonic testing equipment or other non-invasive screening equipment that allow detection of subsurface damages. If the method comprises the step of interchanging the bevel gearings of the first and second azimuth thrusters at the termination of the first period of time, this will reverse the load direction of the bevel gears of the gearings but will not change the rotational direction of other rotating components of the driveline. At a scheduled service or if gear failure has been detected, the thruster is overhauled and the drive line is dismantled. The gearings can then be interchanged with little extra effort.

If the method comprises the steps of, at the termination of the first period of time, changing the rotational directions of the motor of the first and second azimuth thrusters, and interchanging the propellers of the first and second azimuth thrusters, this will change the rotational direction of the gearings, and thus the load directions, but does not require that the drivelines are dismantled.

If the method comprises the steps of, at the termination of the first period of time, interchanging the azimuth units, except the propellers, of the first and second azimuth thrusters, this will change the load direction of the gearings without changing the rotational direction of the propellers. Also according to this option, the thruster drivelines do not have to be dismantled.

A second aspect of the invention relates to a mechanical transmission azimuth thruster for implementing the above-discussed method, comprising a driveline for connecting a motor located inside the hull of a vessel to a propeller mounted to an outboard azimuth unit of the azimuth thruster, which azimuth unit is arranged to be rotated to any horizontal angle and which driveline comprises a generally vertical shaft and a generally horizontal shaft which is connected to the vertical shaft via a bevel gearing comprising a first bevel gear mounted on the vertical shaft and a cooperating second bevel gear mounted on the horizontal shaft, wherein the horizontal shaft is rotatably supported by two symmetrical tapered roller bearings and a radial bearing, the tapered roller bearings being arranged on the opposite side of the second bevel gear as compared to the propeller and the radial bearing being arranged between the second bevel gear and the propeller.

This will allow interchanging of drive line components between the thrusters. The vertical shaft of the driveline may be rotatably supported by first and second thrust bearings arranged above the first bevel gear, and by a radial bearing arranged between the first bevel gear and the first and second thrust bearings.

Alternatively, the vertical shaft of the driveline may be rotatably supported by a first thrust bearing arranged above the first bevel gear and a second thrust bearing arranged below the first bevel gear.

In addition, the vertical shaft may be rotatably supported by a radial bearing arranged above the first thrust bearing.

The first and second thrust bearings rotatably supporting the vertical shaft may be equally sized and arranged to handle thrust in both axial directions of the vertical shaft.

Description of the drawings

In the following, embodiments of the invention will be discussed with reference to the appended drawings.

Fig. 1 discloses the normal load direction for a bevel gear in a azimuthing thruster. Fig. 2 discloses a reversed load direction for a bevel gear in a azimuthing thruster.

Figs. 3 to 5 schematically disclose three driveline configurations allowing for interchanging port and starboard thrusters or thruster components.

In the drawings, like features are indicated by like reference numerals unless otherwise explicitly stated or implicitly understood by the circumstances. Detailed description of the invention

According to the invention, the utilization of a bevel gearing in a mechanical transmission azimuth thruster is increased by reversing the load direction of the gearing after a predetermined time of operation or service. This is illustrated in Figs. 1 and 2, where Fig. 1 discloses a bevel gear of an azimuth thruster which is subjected to a first load direction. After said predetermined time of operation, the load direction is reversed such that the bevel gear is subjected to a second load direction which is opposite to the first load direction, as is disclosed in Fig. 2.

The load direction can be reversed after a given amount of time in operation, e.g. during a scheduled service, e.g. when the vessel is dry-docked for service. In such a case, said predetermined time of operation may for example be within the interval of 2 to 15 years, e.g. at 5 or 10 years. Alternatively, the load direction can be reversed when a particular event occurs, i.e. when a gear tooth failure is detected, in which case said predetermined time of operation is up to that particular event occurring. In Figs. 1 and 2, the bevel gear is a spiral bevel gear, i.e. a bevel gear having the teeth formed along spiral lines. This is a common configuration in mechanical transmission azimuth thrusters. However, a mechanical transmission azimuth thruster may alternatively comprise straight bevel gears, i.e. having the teeth formed along straight lines. The load direction disclosed in Fig. 1 is the conventional load direction in an azimuth thruster having spiral bevel gears. Advantageously, said first load direction is this conventional load direction, and said second load direction is the opposite load direction, as is disclosed in Fig. 2.

A mechanical transmission azimuth thruster comprises a driveline connecting a motor located inside the hull of the vessel to a propeller mounted to an outboard azimuth unit. The azimuth unit, or pod, is arranged such that it can be rotated to any horizontal angle, or azimuth.

The driveline of a mechanical transmission azimuth thruster, which is housed inside the azimuth unit, typically comprises a generally vertical shaft 2 (see Figs. 3 to 5) running in a rotatable column of the azimuth unit, and a generally horizontal shaft 3 which is connected to the vertical shaft 2 via an angled bevel gearing 4 (normally a right-angle gearing) comprising a first bevel gear, or pinion, 5 mounted on the vertical shaft 2 and a cooperating second bevel gear 6 which is mounted on the horizontal shaft 3. The horizontal shaft, in turn, is connected to a propeller 1.

A bevel gearing can in theory always run in both directions, no matter whether the bevel gearing comprises straight or spiral bevel gears. Therefore, in order to change the load direction of the gearing of a mechanical transmission azimuth thruster, the rotational direction of the driveline may simply be reversed. However, the propeller of a mechanical transmission azimuth thruster is usually designed to be either left-handed or right-handed. Therefore, reversing the rotational direction of the driveline normally causes the propeller to rotate in an unfavourable direction. Consequently, reversing the rotational direction of the driveline in a thruster usually needs be accompanied by changing the propeller from a left- handed to a right-handed propeller or vice versa, as the case may be. This, however, may degrade the hydrodynamic properties of the vessel, especially in vessels having pair-wise mounted thrusters, potentially reducing the propulsion efficiency of the vessel.

Furthermore, when a bevel gearing changes the direction of rotation, not only will the direction of the gearing forces change, but the magnitude of the gearing forces will change as well. In a conventional mechanical transmission azimuth thruster, the driveline and its bearing arrangement are usually dimensioned for a single direction of rotation. Consequently, the limiting factor as far as load reversal is concerned, is usually the configuration of the driveline, i.e. the configuration of the rotating vertical and horizontal axes 2, 3 and their bearing arrangements, and not the configuration of the bearing teeth. In a vessel comprising pair-wise arranged port and starboard mechanical transmission azimuth thrusters - one having a left-hand rotating propeller and the other a right-hand rotating propeller - there are different methods of bringing about the load reversal.

Method 1 :

The load direction can be changed by interchanging the port and starboard bevel gears without changing the rotational direction of the starboard and portside drivelines. In other words, the first and second bevel gears of the portside thruster are switched to the position of the first and second bevel gears of starboard side and vice versa. This method does not change the rotational direction of the vertical and horizontal axes 2, 3 and the propellers 1, but reverses the load direction of the bevel gearings.

However, this method requires that the drivelines of the azimuth units, or pods, are dismantled so that the gearings can be interchanged.

This method can advantageously be used when the vessel is scheduled for service and the thrusters are routinely overhauled. During service the drivelines are usually dismantled anyway and the bevel gears of the gearings can be interchanged with little extra effort.

Method 2:

Another method is to change the rotational direction of starboard and portside motors and, in addition, interchange the starboard and portside propellers. This changes the rotational direction of the gearings and, consequently, also the load direction of the gearings. The rotational direction of the vertical and horizontal axes 2, 3 will also change, thus prompting interchanging the starboard side and port side propellers 1, i.e. switching each propeller into the place of the other.

As discussed above, this option has the drawback that it may affect the hydrodynamic properties of the vessel (as the rotational direction of the starboard and port side propellers will change), potentially reducing the propulsion efficiency somewhat. However, this method carries the advantage that the driveline of the thrusters do not have to be dismantled.

Method 3:

A third method is to interchange the azimuth units, except the propellers, of the thrusters, i.e. moving the starboard side azimuth unit into the position of the port side and vice versa, but maintaining the propellers at their original positions, i.e. but keeping the starboard side propeller at the starboard side and vice versa. This will change the rotational directions of the drivelines and, consequently, the load directions of the gearings without changing the rotational directions of the port and starboard propellers. According to this method the thruster drivelines do not have to be dismantled. Independent of the method used, the bevel gears in both thrusters should advantageously have the same configuration, e.g. the same spiral angle if the bevel gears are spiral bevel gears. In addition, the driveline, in particular the driveline bearings, should be arranged and dimensioned to withstand loads from both clockwise and counter-clockwise rotation.

As discussed above, the limiting factor as far as load reversal is concerned is usually the configuration of the driveline, i.e. the configuration of the rotating vertical and horizontal axes 2, 3 and their bearing arrangements.

In the following, three different driveline configurations capable of handling load reversal, i.e. be arranged and dimensioned such that they withstand loads from both clockwise and counter-clockwise rotation, will be discussed with reference to Figs. 3 to 5.

Figs. 3 to 5 schematically disclose drivelines of a mechanical transmission pulling azimuth thruster, i.e. an azimuth thruster where the propeller is arranged to provide a pulling thrust. The section of the driveline disclosed in the figures is the section located in the azimuth unit, i.e. the unit which is mounted outside the hull of the vessel. The driveline connects a motor (not disclosed) inside the hull (not disclosed) of the vessel (not disclosed) to a propeller of the thruster. As previously discussed, the drivelines disclosed in Figs. 3 to 5 comprise a generally vertical shaft 2 running in a rotatable column of the azimuth unit and a generally horizontal shaft 3 which is connected to the vertical shaft 2 via an angled bevel gearing 4, e.g. right-angle bevel gearing, comprising a first bevel gear, or pinion, 5 mounted on the vertical shaft 2 and a cooperating second bevel gear 6 which is mounted on the horizontal shaft 3. The horizontal shaft 3 is connected to the propeller 1.

In the driveline disclosed in Fig. 3, the vertical shaft 2 is rotatably supported by two equally sized thrust bearings 7a, 8a arranged above the pinion 5. As is known in the art, thrust bearings are bearings that permit rotation between parts but are also designed to support an axial load. The bearings 7a and 8a are symmetrically arranged to handle thrust in both axial directions. The vertical shaft 2 is also rotatably supported by a radial bearing 9a which is arranged between the thrust bearings 7a, 8a and the pinion gear 5. As is known in the art, radial bearings are arranged to accommodate loads that are predominantly perpendicular to the axis of the rotating shaft.

The horizontal shaft 3 is rotatably supported by two symmetrical tapered roller bearings 10, 1 1 and a radial bearing 12. The tapered roller bearings 10, 1 1 are arranged on the opposite side of bevel gear 6 as compared to the propeller 1 and the radial bearing 12 is arranged between the bevel gear 6 and the propeller 1. As is known in the art, tapered roller bearings are bearings that can take large axial forces, i.e. they are good thrust bearings, as well as being able to sustain large radial forces. The driveline disclosed in Fig, 4 is similar to the driveline disclosed in Fig. 3. However, in Fig. 4 the vertical shaft 2 is rotatably supported by equally sized thrust bearings 7b, 8b arranged on either side of the pinion 5, i.e. a first thrust bearing 7b arranged above the pinion 5 and a second thrust bearing 8b arranged below the pinion 5. The bearings 7b and 8b are arranged to handle thrust in both axial directions. Also, a radial bearing 9b is arranged above the first thrust bearing 7b.

The driveline disclosed in Fig, 5 is similar to the driveline disclosed in Fig. 4 except that the vertical shaft 2 is not supported by a radial bearing, i.e. the vertical shaft 2 is rotatably supported only by thrust bearings 7c, 8c arranged on either side of the pinion 5, i.e. a first thrust bearing 7c arranged above the pinion 5 and a second thrust bearing 8c arranged below the pinion 5. The bearings 7c and 8c are symmetrically arranged to handle thrust in both axial directions.

In the preceding description, various aspects of the method and apparatus according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the method and apparatus and their workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the apparatus, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, also lie within the scope of the present invention as defined by the claims.