METHOD AND APPARATUS FOR CONTROLLING PROPULSION DRIVE OF SHIP

The object of the invention is a method for controlling the propulsion drive of a ship or another water-craft according to the preamble part of Claim 1, and an apparatus for controlling the propulsion drive of a ship according to the preamble part of Claim 6.

The general power transmission of ships is based on a propeller drive, with the propeller shaft rotated by a power engine such as a diesel engine or an electric machine. The propeller generates the thrust that is adjusted by controlling the rotation speed of the engine driving the propeller, or by changing the blade angle of the propeller blades.

It is well known that the propeller thrust is based on the differential pressure between the front and back surfaces of the propeller blades. However, a harmful effect called cavitation may occur with propeller drives when the rotating speed or load of the propeller is too high for the prevailing operational conditions. Bubbles are created in the water, weakening the propeller thrust and causing erosion and wear that damage the propeller and the surface structures near the propeller. When cavitation increases and exceeds the critical level, the entire propeller thrust collapses. Cavitation also causes vibration that stresses the ship body and the apparatus mounted to it and increases the noise level. As the cavitation stress is particularly directed at the propellers, the phenomenon also has a significant harmful impact on the propeller shaft bearings.

As the propeller begins to cavitate, its efficiency deteriorates. If speed is increased, cavitation will also increase, and thrust will eventually collapse. In such a situation, the drive rotating the propeller must be controlled to return to a range in which the propeller works according to its design. Many factors influence the generation of cavitation, and these will be observed when designing propeller drives. The number of factors behind cavitation is large, and co-occurrence of several factors increases the potential of the propeller to cavitate. It is thus not always possible to prevent cavitation with structural design and operating instructions. As a reparative adjusting measure, the propeller speed will be decreased to a range in which cavitation will not occur. This can be carried out for example by measuring the rate of rotation and limiting the speed when the rate of rotation increases so that it is no longer within the characteristic curve. However, in such a case cavitation has already occurred, and its harmful effects have caused strain to the drive performance and ship structures.

Cavitation could also be avoided if, when controlling the ship, the propulsion drive was always kept at an operating point where no cavitation occurs. This would, however, require very lengthy experience and deep knowledge of the conditions as well as knowledge of the multiplicative effects of the various factors causing cavitation, hi practice, this is not possible. When steering a ship, the large mass and slow reactions of the ship often result in poor steering response. For example changing direction in high velocity or drive in strong wind may result in situations in which the propeller cavitates but the operator is only able to notice it after a delay. The quality of the water, for example dirtiness, also influences the likelihood of cavitation.

The purpose of the present invention is to create a new and efficient ship propulsion drive control system that removes deficiencies listed above, improves the adjustability of a ship propulsion drive and improves the performance of the propulsion system and the whole ship, hi order to achieve this, the method according to the invention is characterized by the features specified in the characteristics section of Claim 1. Correspondingly, the apparatus according to the invention is characterized by the features specified in the characteristics section of Claim 6. Certain preferred embodiments of the invention are characterized by the features listed in the dependent claims.

In the solution according to the invention, the propulsion drive is so adjusted that the preventive adjustment prevents propeller cavitation, hi the method a group of propulsion drive's characteristic curves is determined as a function of a minimum of one controlled variable. These curves determine an operating point in which the propeller causes cavitation. This set of characteristic curves defines the cavitation-free range as well as the cavitation range for the propulsion drive as a function of controlled variables. The said characteristic curves are used when adjusting the propulsion drive to avoid propeller cavitation. Current values corresponding to the characteristic curves are continuously measured and compared with saved characteristic curves. Propulsion drive is adjusted to keep the drive within the cavitation-free range. At the same time, the propulsion drive is naturally controlled to achieve the best possible efficiency and thrust.

According to one embodiment of the invention, part of the characteristic curves are calculated. Such data include propeller rotating speed, propeller shaft's turning angle and the blade angle; the effect of these can be determined already in the design stage.

According to another embodiment of the invention, at least part of the characteristic curves are defined with scale model experimentation or with an at-sea experiment at the commissioning stage, or based on experience. These characteristic curves, such as the effect of the direction and speed of a sea current or the effect of draft, can be modeled using a scale model in a cavitation tunnel, or with other methods, hi addition, the joint effect of several factors can be determined based on experiments.

According to yet another preferred embodiment of the invention, the adjustable variable to be used to control the propulsion drive is selected according to the ship type or the characteristics and use of an individual ship. Accordingly, one or more of the following adjustable variables will be used: rate of rotation, blade angle, torque, power, propeller shaft's turning angle, propeller's angle of tilt. If the ship has several adjustable variables, several characteristic curves are created and used either separately or together.

According to a preferred embodiment of the invention, characteristic curves are defined for at least one parameter. One or more of the following parameters will be used: propeller geometry, water stream direction, the speed of the ship, the position of the propeller in relation to the ship's speed vector, the ship's drift angle, the direction of the wind, wind velocity, the direction of the sea current, the speed of the sea current, water density, water depth.

According to yet another application according to the invention, the apparatus is used to control a fixed propeller, with a bearing arrangement mounted to the ship body and not able to turn. The propeller may have fixed or adjustable blades.

According to another embodiment of the invention, the thrusting equipment includes a propeller that is able to turn in relation to the ship body. Yet in another embodiment the thrusting equipment includes both a fixed and a turning propeller.

The variables to be measured, monitored and adjusted are usually accessible. This facilitates the implementation of the invention. Installation of new measuring instruments or other such equipment is not required to be able to apply the invention.

In the following, some of the preferred embodiments of the invention will be described in detail by referring to the drawings, where:

- Figure Ia illustrates an operating situation of a ship,

— Figure Ib illustrates another operating situation of a ship,

- Figure 2a presents some characteristic curves of a ship's propulsion curve, suitable for use in a solution according to the invention,

— Figure 2b shows other characteristic curves of a ship's propulsion curve, suitable for use in a solution according to the invention, and

- Figure 3 illustrates an apparatus in accordance with the invention.

The movement of a ship or a corresponding watercraft is based on the thrust created with propellers. In the example illustrated in Figures Ia and Ib, the ship's propulsion drive utilizes turning thrusters 4, the propellers 3 being mounted to a turning thrust apparatus 5, which is able to turn 360 degrees around a vertical rotation axis. The ship is steered by turning the thrusters 4, and the speed and thrust is adjusted by changing the rotating speed of propellers 3. In Figure Ib, the ship runs straight at an even speed, where the thrust of thrusters 4 and the direction of the ship 6 are in line with the ship's centerline 8. In the case illustrated in Figure Ib, no asymmetric force making the ship deviate from its course is targeted at the ship. In this so-called normal situation, the control of the ship and its propulsion drive is easy and straightforward. As long as the propeller's rotating speed remains under nominal speed at each load and speed or acceleration of the ship, the propeller will not cavitate, as the propeller is designed for such conditions.

In the situation illustrated by Figure Ia, a lateral wind 10 is targeted at the ship 2, as a result of which the course 12 will deviate by angle β from the ship's centerline 8. The ship's propellers 4 are turned from the ship's centerline by angle α. Since the thrust of the propellers no longer corresponds to the ship's course, the cavitation-free range of the propeller changes significantly. The thrust direction of the propellers does not correspond to the ship's course, that is, the ship's speed vector; at the same time, the ship's resistance to motion increases due to the increasing drift angle. When the angle of rotation of the thrusters and the rate of rotation of the propellers exceeds the thresholds defined based on the system design, the propeller begins to cavitate and the thrusting power drops. Increase of power and rate of rotation will not increase the thrust; on the contrary, the thrust decreases. The operating point of the propulsion drive moves outside the cavitation-free operating range. At the same time, additional stress and erosion is targeted at structures

such as the propellers and their support and turning devices. Although cavitation as such is a known phenomenon, sensory observation of the crossing of the cavitation threshold in an operating situation is not possible. Cavitation may continue for a long period unobserved.

In a solution according to the invention, one or more characteristic curves are saved into the control system of the propulsion drive. These curves define the cavitation threshold for the prevailing conditions. Figures 2a and 2b illustrate some characteristic curves that are suitable for use when implementing the invention. Figure 2a shows a thrust characteristic curve, with the torque coefficient K _Q and the thrust coefficient KT shown as a function of the cavitation number σ. More precisely, Figures 2a and 2b present the torque coefficient decupled to allow for a clearer presentation of the variables within the same figure. Therefore, the figures adopt the expression 10K _Q , referring to the tenfold value of the torque coefficient. The torque coefficient K _Q is a variable without dimension, defined in a known way based on the torque, water density, propeller rate of rotation and propeller diameter. Correspondingly, the thrust coefficient K _T is a variable without dimension, defined in a known way based on the thrust, water density, propeller rate of rotation and propeller diameter. Cavitation number σ is a variable with no dimension, defined based on the speed of the body and the pressure and influenced by the size of the body, flow speed, temperature, water characteristics etc. As the cavitation number decreases, a point is achieved where the thrust and torque drop. In the situation described in Figure 2a, this happens when the cavitation number is approximately 1.5.

Figure 2b describes an open water curve measured in a cavitation tunnel. The curve illustrates the tenfold torque coefficient 10K _Q and the thrust coefficient K _T , as well as the propeller efficiency ηo as a function of advance coefficient J, resulting in J=V _a /(n*D), with V _a = propeller advance speed, n = propeller rate of rotation and D = propeller diameter. The curves in Figure 2b describe the variables listed above with various propeller drafts.

Curves presented in Figure 2a define threshold values for the cavitation number, line 11. The cavitation numbers below these values are not allowable when the drive is kept in a cavitation-free range. Thus the area to the right from line 11 is the operating range. Similarly, control operations can utilize the open water curves presented in Figure 2b, which can be used to determine the smallest advance coefficient, lines 13, located in the cavitation-free range, to the right from lines 13 in Figure 2b. In Figure 2b, separate values are shown for various drafts with a broken line, dotted broken line and dotted line. The

same line types show the thresholds of cavitation-free ranges defined for various drafts. When measurement and control instruments show that the values are approaching a cavitation limit, or that there is a danger of exceeding the limit, the propulsion drive control limits the variable so that the drive operating point remains in cavitation-free range. In the case illustrated in Figure Ia, the system adjusts either the propeller rate of rotation, propeller blade angles (if adjustable), thruster angle of tilt (if adjustable) and the thruster turning angles α. Turning angles may differ between the various thrusters, or the thrusters may be turned to different directions to create the required steering effect.

Figure 3 is a diagram presentation of an apparatus providing a function according to the invention. The example shows the propulsion drive components required to rotate and steer one propeller. Propeller 14 is mounted onto the shaft 18 of the motor 16 rotating it. Motor 16, which in this configuration is an alternating-current motor such as a permanent magnet type synchronous motor or a squirrel cage motor, receives its power input from frequency converter 20, which receives power from the ship's electrical power system 22. Frequency converter 20, which may consist, as is well known, of a rectifier, a DC circuit and an inverter, is controlled by the control unit 24, which produces the control signals for the frequency converter via cables 26 and utilizes the electric measurement data from the frequency converter, brought to the control unit via cables 28.

The propulsion drive is controlled from the ship's bridge with a control stick 30, which is used to issue the necessary steering commands. The commands are transferred to the control unit 24 via the drive control cables 32. Alternatively, steering commands may be issued with the autopilot 33 or the dynamic positioning (DP) device 35, utilizing a GPS positioning device. The rotating speed of the motor or the propeller is measured using a tachometer, from which the output 34 will be taken as current value data to control unit 24. The current rate of rotation can also be deduced from the frequency converter control data.

The control unit issues the required control signals to achieve the desired thrust and speed from the propulsion drive.

The propulsion drive's characteristic curves as functions of various variables and parameters are saved to the memory 36. Variables include propeller rate of rotation, blade angle, torque, power, turning angle of the propeller shaft, and propeller's angle of tilt, depending on the structure and properties of the ship and the propulsion drive. According to the invention, these variables are also used to control the drive so that the propeller

remains in a cavitation-free range in a manner selected on an application-specific basis. Characteristic curves have also been defined for various parameters that influence the generation of cavitation. Such parameters include propeller geometry, water stream direction, the speed of the ship, the position of the propeller in relation to the ship's speed vector, the ship's drift angle, the direction of the wind, wind velocity, the direction of the sea current, the speed of the sea current, water density, water depth. Some of these parameters, such as the propeller geometry, are fixed for the ship, unless alterations are made. Variables and parameters with no fixed value are measured during operation. The measurement values, that is, the current values are transferred via signal wires 42 to the operating point definition unit 38. During propulsion drive operation, the actual operating point of the drive is determined, and the resulting current operating point is transferred to the cavitation limit monitoring unit 40. The monitoring unit 40 is a comparing and difference detecting element with the minimum function of determining whether the current operating point is in the cavitation-free-range or outside it. When the cavitation limit monitoring unit 40 detects that the propulsion drive is closer to cavitation than the predetermined value determines, the monitoring unit issues a control command to the control unit 24, which adjusts the propulsion drive towards the cavitation-free operating range. In other words, when the propulsion drive is moving to the left side of line 11 and lines 13, respectively, in Figures 2a and 2b, the monitoring unit 40 brings the propulsion drive back to the right side of lines 11 and 13, respectively. The control unit 24 controls the propulsion drive in the most optimal way possible, according to the control commands issued by the control stick 30 at the bridge or other control devices 33 or 35, as well as the combined control of the cavitation limit monitoring unit 40.

hi the above the invention has been described with the help of a certain embodiment. However, the description should not be considered as limiting the scope of patent protection; the embodiments of the invention may vary within the scope of the following claims.