WO1996016866A1

WO1996016866A1 - A ship propelled by a two-stroke slow-running internal combustion engine

Info

Publication number: WO1996016866A1
Application number: PCT/DK1995/000459
Authority: WO
Inventors: Henrik Willads Houmaand Christensen
Original assignee: Man B & W Diesel A/S
Priority date: 1994-12-02
Filing date: 1995-11-21
Publication date: 1996-06-06
Also published as: KR100351528B1; JP3382252B2; KR980700211A; JPH10509930A

Abstract

The engine room of a ship (1) is positioned so far forward in the ship that several cargo holds (3) are located aft of the engine room. The ship has a tall slender superstructure (6). The main engine (8) is connected with the propeller (7) through a transmission shaft (9) at least 40 m long. The combination of the length of the shaft, a slow-running main engine, and a tall, slender superstructure results in a imminent risk of inconvenient vibration in the superstructure generated by axial resonance vibration in the shaft (9). A hydraulically acting damper device (10) mounted near the aft end of the transmission shaft damps the axial vibration to an acceptably low level.

Description

A ship propelled by a two-stroke slow-running internal combustion engine

The invention relates to a ship propelled by a two- stroke slow-running internal combustion engine having a shafting system comprising the crankshaft of the engine, a transmission shaft interconnecting the crankshaft and the propeller of the ship, and a device acting on a shaft portion for damping axial vibration in the shafting system, the damper device having a piston which is arranged in a hydraulic cylinder with hydraulic chambers located axially on respective sides of the piston so that relative axial movements between the piston and the cylinder force the hydraulic liquid to flow between the two hydraulic chambers via a restricted flow passage.

Swiss patent No. 398179 describes such a ship with the damper device mounted at the front end of the crankshaft, the shaft portion with the piston being constituted by a shaft section bolted on to the front end of the crankshaft, and the hydraulic cylinder being bolted on to the engine frame at the front main bearing. This known damper device damps axial vibration in the crankshaft to a level where the vibrations are prevented from inflicting mechanical damage on the crankshaft. British patent No. 863752 describes an internal combustion engine with a damper device of a similar kind wherein, like in said Swiss patent, the piston is mounted on a shaft section at the front end of the engine, and the hydraulic cylinder is bolted on to the oilpan of the engine.

Japanese published patent application No. 6-147259 describes a damper device of the above type, wherein the piston is fixed to the crankshaft and the hydraulic cylinder is fixed to the engine frame. Japanese pub- lished patent application No. 6-147270 describes a damper device wherein the piston is constituted by a disc which is fixed to the engine frame and arranged between two collars fixed on the crankshaft and acting as end walls of the hydraulic cylinder.

German patent No. 1,675,554 describes a marine engine in which a hydraulic cylinder is fastened to the engine frame at the front end of the engine, and a piston journalled in the cylinder is fastened to the front end of the crankshaft of the engine. The piston is actuated by a hydraulic pressure generating a aftward axial force which raises the natural frequency for axial vibration of the crankshaft.

It is a common feature of the known hydraulic piston-cylinder devices for influencing the axial vibra¬ tion situation of the shafting system that they are mounted in extension of the crankshaft at the front end of the engine, and that they remedy vibration problems at the crankshaft itself. It is further known to remedy vibration problems in the hull of a vessel by mounting a vibration compensator at a suitable position on the hull itself. Such a vibration compensator comprises one or more rotating masses which are rotated synchronously with the number of revolutions of the engine to generate a periodically varying force which damps the hull vibra¬ tion generated by the propulsion system of the vessel. The Danish company, FLS Maskinfabrik, supplies, for example, a vibration compensator for mounting beside the thrust bearing in the shafting system for generation of an axial force which varies periodically in counterphase with the axial vibration in the shafting system. The disadvantages of this form of damping of hull vibration are partly that the vibration compensator is of a relatively complex construction and particularly, it is complicated to control it synchronously with the engine, partly that the hull and the associated structures, such as the bedplate of the thrust bearing, are influenced locally by the often quite large stresses from both the thrust bearing and the compensator.

In conventional ships, the hull has typically consisted of components being elongated in the longi¬ tudinal direction of the ship, such as decks, super¬ structures, etc, which were normally not exposed to inconvenient hull vibration. Through the past 25 years, the design of conventional commercial ships has under¬ gone major changes, and in particular superstructures have a markedly different appearance, which is due, among other things, to the steadily declining demands to the crew size with consequently reduced requirements for space in the superstructure. Today, the superstruc¬ ture is short so that the space gained can be used for payload. At the same time, the engine room has moved as far aft as possible, which has resulted in short transmission shafts with associated high natural fre¬ quencies for shaft vibration.

The object of the present invention is in a simple and economically advantageous manner to reduce or avoid hull vibration in and at the superstructure owing to axial vibration in the shafting system, even when the engine room is located further forward in the ship in consideration of hull design and cargo distribution and the handling of cargo at loading and unloading.

In view of this, the ship according to the inven- tion is characterized in that the ship, such as a container ship, has several cargo holds positioned aft of the engine room and has a tall superstructure projecting from the upper deck of the ship, that the transmission shaft has a length exceeding 40 m, that the shaft portion on which the damper device acts is positioned in the transmission shaft at a substantial distance from the engine, and that either the piston or the hydraulic cylinder is firmly connected with the hull of the ship. Thus, the damper device has a stationary part firmly connected with the hull of the ship, and a movable part following the axial movements of the transmission shaft portion.

Such a positioning of the damper device depart from the well established prior art teaching that the hydraulically acting damper device should be positioned on the crank shaft, particularly at its front end.

The simple hydraulically acting damper device damps the axial vibrations in the transmission shaft to a more harmless level, and also the superstructure vibrations may be reduced to a level where use of the known, complicated and rather cost demanding vibration com¬ pensator with rotating masses can be avoided.

When several cranes service the same ship in loading and unloading, it is advantageous that the superstructure and thus also the engine room are located so far forward that several holds are located aft of the engine room immediately accessible to a crane. If the stern is very slender, i.e., the hull has a small block coefficient to achieve a suitably low propulsion resistance at high speed, the advanced engine room may also be more easily fitted into the hull.

In consideration of look-out and navigation of the ship, the superstructure must be taller than the highest deck cargo, which results in a large height for the superstructure, particularly in container ships where up to seven or eight containers may be stacked on top of each other on the upper deck. The combination of the large height of the superstructure and its short length results in the possibility of inconvenient vibrations caused by longitudinal hull vibration.

The positioning of several holds aft of the superstructure, which is advantageous to loading the ship results in a transmission shaft of such a length with associated large mass and relatively low rigidity that its natural frequency for axial vibration is primarily determined by the shaft length, just as the vibration frequency of an organ pipe is only dependent on the pipe length. Disregarding the influence of the propeller, the natural frequency of the axial vibration will therefore be inversely proportional with the shaft length, and at a shaft length of 40 m the natural frequency may be less than 15 Hz. A slow-running two- stroke crosshead engine of a ship may typically have a nominal speed of about 2 rev/sec. , and the most substan¬ tial contribution to excitation of axial vibrations comes from the propeller, to be specific, the harmonic vibration, the order of vibration of which corresponds to the number of propeller blades. At a six-blade propeller rotated at 2 rev/sec. , the dominant excitation frequency will thus be 12 Hz, which is close to or coincides with (resonance vibration) the natural frequency of the shaft so that considerable vibration energy may be transferred to the hull. To prevent this, the vibrations are damped by fixing the movable part of the damper device on the transmission shaft at a substantial distance from the engine, while the station¬ ary part of the damper device is fixed to the hull of the ship so that an axial vibration results in relative movement between piston and cylinder with consequent pumping of hydraulic liquid between the chambers. The damper device requires no controlled synchronisation with the number of revolutions of the engine, as its movable part automatically participate in the axial vibration of the transmission shaft.

The propeller is positioned at the free end of the shaft, where the axial vibration is largest, while the fluctuations are smallest at the thrust bearing located at the aft end of the engine. To achieve the greatest possible relative movement between piston and cylinder, the damper device may be mounted at one of the aft-most journal bearings for the transmission shaft, suitably at the aft-most free-standing journal bearing.

Preferably the transmission shaft has at least one radially projecting collar constituting a part of the damper device. The collar may, for example, constitute the piston. This design is structurally simple and produces a damping force acting coaxially with the longitudinal axis of the shaft, which is not necessarily the case if one or more pistons positioned beside the transmission shaft and connected with it via a console are used instead. The transmission shaft may also be made with two radially projecting collars forming end walls of the cylinder, and the piston positioned between the two collars may then be firmly connected with the hull of the ship. The invention will now be explained in further detail below with reference to a an example of an embodiment of the invention shown very schematically in the drawing, in which

Fig. 1 shows a side view of a ship, and Fig. 2 shows a damper device mounted on a trans¬ mission member in the ship of Fig. 1.

A container vessel 1 has a hull 2 divided in a well-known manner into holds 3 by means of transverse bulkheads and an internal bottom or tank top. The holds are closed upwards by hatch covers 4, on which con- tainers may be stacked as deck cargo to a height indicated by the dashed lines 5. In the example shown, containers may be stacked in seven layers to a total height exceeding 20 m, which means that the superstruc- ture 6 of the ship must be 25-30 m high. At a typical width of the ship of about 32 m and a superstructure length of 8 m, there are nine floors providing about

2 2300 m which present-day crews of 12-14 men cannot utilize. Thus, the superstructure may be only 6 m long and still provide plenty of space. The tall slender superstructure has a low natural frequency for longi¬ tudinal vibration.

The superstructure 6 is positioned to provide place for several holds aft of it so that a container crane can service several holds without being impeded by the superstructure. In the example shown, there are five holds for 40 feet containers in the stern end.

To achieve a proper utilization of the space in the hull, the engine room is located below the superstruc- ture 6, i.e., at a large distance from the propeller 7 of the ship. An internal combustion engine 8 is con¬ nected via a transmission shaft 9 with the fixed pitch propeller. The transmission shaft consists of one or more intermediate shafts and a propeller shaft carrying the propeller. The front intermediate shaft is normally connected with the crankshaft via a thrust bearing at the aft end of the engine, so that the whole trans¬ mission shaft is substantially fastened in the axial direction at the thrust bearing. The engine is typically a large two-stroke crosshead engine with a nominal speed at full load operation in the range of 70 to 160 rpm, typically less than 110 rpm. In the example shown, the transmission shaft has a length of about 60 m, and the output of the engine is at about 50,000 k at a nominal speed of 94 rpm. The transmission shaft passes from the engine room through a shaft passage 9' further out through a stern tube to the propeller 7. The shaft 9 is supported in a conventional manner by a number of journal bearings distributed along the shaft length.

The number of blades of the propeller is adapted to its diameter and the output to be transmitted to the water. There may be , for example, between four and eight blades. With six blades and the above speed of 94 rpm, the dominant excitation frequency for axial vibration in the transmission shaft will be 9.4 Hz, which for a shaft length of 60 m largely corresponds to the natural frequency for axial vibration of the shaft.

The axial vibrations are transmitted through the thrust bearing to the hull of the ship where the vibrations may make transverse plate panels in the hull or superstructure or the superstructure as a whole vibrate in longitudinal direction in an undesired manner. To prevent this, the axial vibrations are damped by means of a damper device generally designated 10, which, as shown in Fig. 2, may be mounted on the side of one of the journal bearings of the transmission shaft, preferably the aft-most free-standing journal bearing 11. On a shaft portion 12 projecting from the journal bearing, the shaft is provided with a piston 13 in the form of a radially projecting annular collar which may be bolted or clamped to the shaft or may be fixed to it in any other manner, for example by welding, so that the piston is axially non-displaceable in relation to the shaft. It is possible to mount the piston so as to be rotatable in the circumferential direction in relation to the shaft.

The piston 13 is received in a hydraulic cylinder 14 bolted on to the side surface of the journal bearing, providing hydraulic chambers 15, 16 on respective sides of the piston. In a well-known manner, the piston may have an outer diameter of less than the internal diameter of the hydraulic cylinder, providing a throttl- ing passage between the two chambers 15, 16 between the piston and the inner side of the cylinder, but preferab¬ ly, the piston is approximately sealingly mounted in the cylinder, and at piston displacements the liquid in the chambers is forced to run through a circulatory passage comprising a connecting bore 17, 18 to each chamber and a throttling means 19. The throttling means may be designed with an outer housing 20 containing a stack of plate sections which individually delimit several narrow flow passages 21, each of a suitably small height so that the liquid flow through the passages is slowed down by the viscosity of the liquid.

The passage of the transmission shaft through the cylinder 14 is sealed in the usual manner, not shown, by means of, for example, co-rotating sealing rings made of PTFE, which are received in recesses in the inner side of the cylinder. The seals are made so that a small amount of liquid is continuously evacuated from the chambers 15, 16, whereby the heat transmitted by the damping is removed. The chambers are kept filled with liquid by means of a supply pipe, not shown, opening out in at least one of the chambers. At operation of the engine, there will be a periodically varying pressure difference between the chambers 15, 16, the maximum size of which is determined by the ratio between the maximum value of the varying axial force in the shaft and the area of the piston 13. The supply pressure to the chambers must be higher than half the pressure differ¬ ence between the chambers to prevent intake of air in the damper device, as a pressure drop in one chamber is largely offset by a corresponding pressure increase in the other chamber. At a shaft diameter of 780 mm and an external diameter of the piston of 1000 mm, the supply pressure may typically be of 5 bar.

If the need for damping is large, several pistons with associated cylinders may be used on the same transmission shaft.

As an alternative to mounting the cylinder on the side of a journal bearing, the cylinder may be connected directly with the hull 2, but this requires a separate bedplate for the cylinder.

The hydraulic liquid may, for example, be oil, water or an organic liquid. Tall, narrow superstructures and long transmission shafts may also become relevant in other types of vessels, such as vessels for trans- porting large, tall cargo as deck cargo.

In the above embodiment, the cylinder constitutes the stationary part of the damper device, and the piston the part movable with the shaft. In another embodiment, the piston is fixed on the cylindrical wall of the cylinder as a radially inwardly projecting collar, and the shaft portion has two collars separated in the axial direction and forming the end walls of the hydraulic cylinder. The piston is here incorporated in the stationary part connected with the hull, and the end walls are the movable part.

Claims

P A T E N T C L A I M S 1. A ship (1) propelled by a two-stroke slow- running internal combustion engine (8) having a shafting system comprising the crankshaft of the engine, a transmission shaft (9) interconnecting the crankshaft and the propeller (7) of the ship, and a device (10) acting on a shaft portion (12) for damping axial vibra¬ tion in the shafting system, the damper device having a piston (13) which is arranged in a hydraulic cylinder (14) with hydraulic chambers (15, 16) located axially on respective sides of the piston so that relative axial movements between the piston and the cylinder force the hydraulic liquid to flow between the two hydraulic chambers via a restricted flow passage (17, 18, 19), c h a r a c t e r i z e d in that the ship, such as a container ship, has several cargo holds (3) positioned aft of the engine room and has a tall superstructure (6) projecting from the upper deck of the ship, that the transmission shaft (9) has a length exceeding 40 m, that the shaft portion (12) on which the damper device acts is positioned in the transmission shaft at a substantial distance from the engine, and that either the piston or the hydraulic cylinder (14) is firmly connected with the hull (2) of the ship.

2. A ship according to claim 1, c h a r a c ¬ t e r i z e d in that the damper device is located at one of the aft-most journal bearings for the trans¬ mission shaft.

3. A ship according to claim 2, c h a r a c - t e r i z e d in that the damper device is mounted on the aft-most free-standing journal bearing (11) .

4. A ship according to any one of claims 1-3, c h a r a c t e r i z e d in that the transmission shaft (9) has at least one radially projecting collar constituting part of the damper device.

5. A ship according to claim 4, c h a r a c ¬ t e r i z e d in that the transmission shaft has two radially projecting collars forming end walls of the cylinder, and that the piston positioned between the two collars is firmly connected with the hull of the ship.