EP3353049A1

EP3353049A1 - A method and an arrangement for maneuvering a marine vessel

Info

Publication number: EP3353049A1
Application number: EP15770868.6A
Authority: EP
Inventors: Robert De Lange; Albert DROST; Norbert BULTEN
Original assignee: WAERTSILAE NETHERLANDS BV; Wartsila Netherlands BV
Current assignee: WAERTSILAE NETHERLANDS BV; Wartsila Netherlands BV
Priority date: 2015-09-25
Filing date: 2015-09-25
Publication date: 2018-08-01
Anticipated expiration: 2035-09-25
Also published as: CN108137146A; CN108137146B; EP3353049B1; KR20180049113A; WO2017050386A1; KR101881725B1

Abstract

The present invention relates to a method of maneuvering a marine vessel, the marine vessel having a hull with a longitudinal centreline and at least two tunnel thrusters arranged in the hull at right angles to the longitudinal centreline, each tunnel thruster having a fixed pitch propeller with a rotational speed in a preferred rotational direction for delivering a positive thrust, the positive thrust of a first tunnel thruster of the at least two tunnel thrusters having a direction opposite to that of a second tunnel thruster of the at least two tunnel thrusters, the method comprising the steps of, delivering, when no maneuvering action is desired, a constant equal positive thrust by both tunnel thrusters, and increasing, when a maneuvering action is desired, the rotational speed of the first tunnel thruster to increase the positive thrust of the first tunnel thruster.

Description

A METHOD AND AN ARRANGEMENT FOR MANEUVERING A MARINE VESSEL

Technical field

[0001 ] The present invention relates to a method and an arrangement for maneuvering a marine vessel in accordance with the preamble of claim 1 .

Background art

[0002] It has been common practice for decades to provide marine vessels at their bow and/or aft with tunnel thrusters arranged in the hull of the marine vessel transverse to the longitudinal direction of the marine vessel. The purpose of such tunnel thrusters is to move the marine vessel or an end thereof sideways for, for instance, docking or dynamic positioning purposes. The propeller used in such tunnel thrusters may be either a fixed pitch propeller (FPP) or a controllable pitch propeller (CPP). A typical feature of a fixed pitch propeller (FPP) is that its direction of rotation has to be changed for changing the direction of flow in the thruster tunnel. Thus the steering force or thrust created by the FPP is dependent only on the rotational speed and direction of the propeller. The changing of the flow direction in the tunnel of a CPP is done by changing the pitch of the propeller whereby the direction of rotation of the propeller may be maintained. Thereby the steering force or thrust is dependent on both the pitch angle and the rotational speed of the CPP.

[0003] Nowadays marine vessels are oftentimes provided with two or more tunnel thrusters arranged side by side in the hull of the marine vessel, as schematically shown in Figure 1 , to ensure that the steering force is sufficient for the most demanding applications either in docking or when keeping the position of the marine vessel constant at sea. There are several reasons for the use of two tunnel thrusters.

[0004] Firstly, irrespective of the type of the tunnel thruster, i.e. FPP or CPP, the tunnel thruster has always a preferred direction of operation where the efficiency of the propeller is at its best. Thereby, by arranging the tunnel thrusters in the hull of the marine vessel such that their preferred directions of operation are opposite, the applicable steering forces in both directions are equal.

[0005] Secondly, only one tunnel thruster needs to be used in light maneuvering tasks, i.e. the active one that may be used in its preferred direction of operation. Thereby the other tunnel thruster may be considered a passive spare tunnel thruster or a tunnel thruster that is used in hard maneuvering tasks, though opposite to its preferred direction of operation.

[0006] Thirdly, the use of two smaller tunnel thrusters in place of one large one capable of handling the most severe maneuvering tasks in a direction opposite to its preferred direction of operation saves both space in the vertical direction in the hull of a marine vessel and energy at least in light maneuvering tasks.

[0007] However, lately it has been learned that the tunnel thrusters arranged in tandem and run in a traditional manner have a few weaknesses that could be corrected.

[0008] Firstly, it is a known fact that a fixed pitch propeller has a higher efficiency ratio than a controllable pitch propeller, mostly due to the fact that the propeller blades of the CPP have to be designed to work in all applicable blade angles, whereby their design is not optimal at any blade angle.

[0009] Secondly, even if a fixed pitch propeller would seem to be an optimal choice for a tunnel thruster, the fact that it has to be run in a direction opposite to its preferred direction of operation requires that such situations have to be taken into account in its design. It means that some of the best features of an FPP blade have to be sacrificed to make it operable in the opposite direction.

[0010] Thirdly, it has been learned in practice that the accuracy of maneuvers is not as good as it could be, especially when it is a question of small maneuvers requiring low power and, consequently, small flow through the tunnel thruster.

[001 1 ] Fourthly, it has been learned in practice that traditional way of operating a pair of tunnel thrusters in a case where the direction of sideways movement of the hull of the marine vessel is about to be changed from port to starboard or vice versa is slow, i.e. leads to long response times, the tunnel thrusters have to be stopped for reversing their rotational direction.

[0012] In view of the above an object of the present invention is to aim at optimization in maneuvering a marine vessel taking into account a number of different aspects relating to the use of a tunnel thruster in steering and maneuvering a marine vessel. Here, in this specification, the word 'maneuvering' is to be understood broadly to cover any intended relative movement of the marine vessel, its aft or its bow in sideways direction. In other words, each time at least one tunnel thruster arranged in the hull of the marine vessel transverse to the centreline of the hull is producing thrust the operation is called 'maneuvering'. Thus, both keeping the marine vessel in place irrespective of wind, waves and/or water current (so called dynamic positioning) and actually moving the vessel in sideways direction is included in the term 'maneuvering'.

[0013] Another object of the present invention is to find an overall economical method and arrangement for maneuvering a marine vessel.

[0014] Yet another object of the present invention is to aim at minimizing the use of tunnel thrusters in a direction opposite to their preferred direction.

[0015] A further object of the present invention is to improve the accuracy of the maneuvers, especially in view of small-scale movements.

[0016] A yet further object of the present invention is to minimize the creation of additional noise and vibration when maneuvering a marine vessel.

Disclosure of the Invention

[0017] The above and other objects of the invention are met by a method of maneuvering a marine vessel, the marine vessel having a hull with a longitudinal centreline and at least two tunnel thrusters arranged in the hull at right angles to the longitudinal centreline, each tunnel thruster having a fixed pitch propeller (FPP) with a rotational speed in a preferred rotational direction for delivering a positive thrust, the positive thrust of a first tunnel thruster of the at least two tunnel thrusters having a direction opposite to that of a second tunnel thruster of the at least two tunnel thrusters, the method comprising the steps of, delivering, when no maneuvering action is desired, a constant equal positive thrust by both the first and the second tunnel thrusters of the at least two tunnel thrusters, and, increasing, when a maneuvering action is desired, the rotational speed of the first tunnel thruster to increase the positive thrust of the first tunnel thruster of the at least two tunnel thrusters

[0018] Other characteristic features of the method of maneuvering a marine vessel of the present invention will become apparent from the appended dependent claims.

[0019] The present invention, when solving at least one of the above-mentioned problems, also brings about a number of advantages, of which a few has been listed in the following:

• the investment in tunnel thrusters with a fixed pitch propeller is lower than those with a controllable pitch propeller, • the energy consumption of a tunnel thruster using the fixed pitch propeller is lower than that of a tunnel thruster using the controllable pitch propeller,

• the accuracy of the maneuvers when using the maneuvering method and arrangement of the invention is far better than when using traditional maneuvering methods and arrangements,

• the tunnel thrusters may be run, for the most part of their lifetime, in their preferred direction of rotation, and

• changes in the direction of the sideways movement of the hull of the marine vessel may be done very smoothly and quickly.

[0020] However, it should be understood that the listed advantages are only optional, whereby it depends on the way the invention is put into practice if one or more of the advantages were obtained.

Brief Description of Drawing

[0021 ] In the following, the method and an arrangement for maneuvering a marine vessel of the present invention is explained in more detail in reference to the accompanying Figures, of which

Figure 1 illustrates schematically two tunnel thrusters arranged at the bow of the marine vessel transverse to the longitudinal centreline of the marine vessel,

Figure 2 illustrates a comparison between a fixed pitch propeller and a controllable pitch propeller in a power consumption - required thrust chart,

Figure 3 illustrates the thrust delivery as a function of thrust demand for two prior art of tunnel thrusters,

Figure 4 illustrates the difference in power reduction between a FPP- type and CPP- type tunnel thrusters,

Figure 5 illustrates the rotational speed of the tunnel thruster as a function of thrust demand for two prior art tunnel thrusters,

Figure 6 illustrates the thrust delivery as a function of thrust demand for two tunnel thrusters run in accordance with a novel running scheme in accordance with a first preferred embodiment of the present invention, Figure 7 illustrates the difference in power reduction between a CPP- type tunnel thruster and the FPP- type tunnel thruster run in accordance with a novel running scheme in accordance with a first preferred embodiment of the present invention,

Figure 8 illustrates the rotational speed of the tunnel thruster as a function of thrust demand for two tunnel thrusters run in accordance with a novel running scheme in accordance with a first preferred embodiment of the present invention,

Figure 9 illustrates the thrust delivery as a function of thrust demand for two tunnel thrusters run in accordance with a novel running scheme in accordance with a second preferred embodiment of the present invention,

Figure 10 illustrates the difference in power reduction between a CPP- type tunnel thruster and the FPP- type tunnel thruster run in accordance with a novel running scheme in accordance with a second preferred embodiment of the present invention,

Figure 1 1 illustrates the rotational speed of the tunnel thruster as a function of thrust demand for two tunnel thrusters run in accordance with a novel running scheme in accordance with a second preferred embodiment of the present invention,

Figure 12 illustrates the arrangement for maneuvering a marine vessel in accordance with a preferred embodiment of the present invention, and

Figure 13 illustrates an exemplary embodiment for a manual control of the rotational speed of the tunnel thrusters.

Detailed Description of Drawing

[0022] Figure 1 illustrates schematically a marine vessel 10 provided at its bow with two tunnel thrusters 12 and 14 having drive units 16 and 18 and propellers 20 and 22, the propellers being arranged such that their preferred directions of operation are opposite. The tunnel thrusters have been positioned such that their axis is at right angle to the centerline of the hull of the marine vessel. If it is assumed that the preferred direction of operation of a propeller is to draw water past the drive unit, the tunnel thruster 12 is, when operating in its preferred direction, pushing the bow of the marine vessel to the port direction, i.e. to the left, and the tunnel thruster 14 to starboard direction or right, i.e. in the directions opposite to the arrows showing the direction of water flow through the tunnel thrusters. The following description discusses the present invention by using the tunnel thruster arrangement of Figure 1 as a preferred tunnel thruster arrangement of the present invention. However, it has to be understood that the number of tunnel thrusters in a tunnel thruster arrangement may be more than two, too.

[0023] The starting point of the present invention is the fact that a vast majority of the maneuvering tasks of marine vessels are small-scale tasks where the demand for thrust is relatively low, less than 30 % of the maximal available thrust. Such tasks include, among others, dynamic positioning applications, which mean keeping the marine vessel in place irrespective of wind, wave and/or water current conditions. From efficiency point of view fixed pitch propellers are preferred choices for the tunnel thrusters as their construction is simpler than that of controllable pitch propellers, whereby the investment in such tunnel thrusters is lower.

[0024] Also, as illustrated in Figure 2, the fixed pitch propeller is more energy efficient than the controllable pitch propeller. The chart of Figure 2 compares the power (y- axis) required from the drive unit (for example an electric motor) to produce the thrust (x- axis) required by a tunnel thruster. In full 100% thrust the power required by the tunnel thruster having a controllable pitch propeller (CPP) represented by curve A is some 6% higher than that required by the tunnel thruster having a fixed pitch propeller (FPP) represented by curve B. However, when getting to the area of lower thrust, i.e. the required thrust being 50% or less of the maximal available thrust, it may be seen that the curves A and B start departing such that the curve B representing the power consumption of a tunnel thruster having a FPP propeller narrows to 0 % when the required load narrows to 0 %. The curve A representing the tunnel thruster having a CPP propeller stays above 25% power consumption, whereby the difference in required power at low required thrust is enormous, i.e. the power requirement of the CPP- type tunnel thruster is of the order of 10- fold compared to that of a FPP- type tunnel thruster. The main cause for the higher power consumption of the CPP- type tunnel thruster is that its operating principle is to change the thrust by adjusting the propeller blade angle, whereby even if the blade angle is almost 0 degrees the propeller is still rotating and thus consuming energy. The higher power consumption at higher thrusts is based on the fact the hydrodynamic design of a CPP is not as optimal as that of an FPP, partly because of the CPP- blade design that has to take into account changing blade angles and cannot therefore be designed to be optimal for a single blade angle, and partly because of a larger hub of a CPP- type tunnel thruster housing the pitch deflection mechanism of the blades. The operating principle of the FPP- type tunnel thruster is, on its part, to adjust the thrust by changing the rotational speed of the propeller, whereby, naturally, the power consumption decreases with the decreasing rotational speed as the thrust demand gets lower. Also, the propeller blades may be designed to be hydro-dynamically more optimal as the blade angle is fixed.

[0025] Figure 3 illustrates the basic case, i.e. a traditional, prior art way of running two FPP- type tunnel thrusters, Figure 4 compares the power consumption between the FPP- type tunnel thrusters run as shown in Figure 3 and CPP- type tunnel thrusters and Figure 5 shows the effect of thrust demand to the rotational speed of a FPP- type tunnel thruster. Figure 3 is a chart where the X- axis represents the thrust demand for both tunnel thrusters in percentage units of maximum available combined thrust of the two tunnel thrusters and Y- axis the thrust delivery of a single tunnel thruster in percentage units of maximum available thrust delivery of the single tunnel thruster. In other words, like the chart shows the graphs of the two tunnel thrusters are positioned one on top of another such that, for instance, when total thrust demand is 50 %, both tunnel thrusters are run such that they deliver 50 % of their maximum thrust. The positive or negative values in the thrust demand (X- axis) mean thrust maneuvering the marine vessel or the aft or the bow thereof in port or starboard direction. The positive and negative values in thrust delivery (Y- axis) indicate the thrust value of a single tunnel thruster. Positive value means thrust in the preferred direction of operation of the FPP- propeller and negative value means thrust in a direction opposite to the preferred direction of operation of the FPP- propeller. The preferred direction may be taken as the direction where the propeller draws water past the drive unit of the tunnel thruster. The scale on the Y- axis, i.e. positive values above the X- axis and negative therebelow, represents the thrust of the first tunnel thruster shown by graph C, whereas the scale for the second tunnel thruster represented by graph D is the opposite, i.e. positive values below X- axis and negative thereabove (this is better visible in connection with Figure 6). As may be seen in Figure 3, the combined thrust delivery of the two tunnel thrusters is a linear function of the thrust demand of a single tunnel thruster. This means, in practice, that, in order to deliver the desired thrust in one direction, i.e. either in port or in starboard direction, one tunnel thruster is run at a desired relative thrust in its preferred direction of operation producing a positive thrust and the other at the same desired relative thrust in a direction opposite to its preferred direction of operation producing a negative thrust. To clarify, when combining the thrusts the calculation is performed as follows: Combined thrust = (Thrust of the first tunnel thruster - Thrust of the second tunnel thruster)/2 (the minus- sign taking into account the opposite preferred directions of operation of the tunnel thrusters). The equation, by dividing the "Thrust of the first tunnel thruster - Thrust of the second tunnel thruster"- factor by two, takes into account the fact that the combined thrust of the two tunnel thrusters is in fact at its highest 200%, whereby to be able to use 100% relative thrust as the highest relative combined thrust value, the factor has to be divided by two. Here, for example, the first tunnel thruster is the one creating positive thrust to a direction moving the marine vessel or the aft or the bow thereof to starboard direction and the second tunnel thruster the one creating positive thrust to a direction moving the marine vessel or the aft or the bow thereof to port direction. I.e. thereby the positive combined thrust moves the marine vessel or the aft or the bow thereof to starboard direction and negative combined thrust to port direction

[0026] When considering the same in view of power consumption, Figure 4 shows, in line with Figure 2, how, throughout the full range of thrust demand (X- axis), the power consumption (Y- axis) of a CPP- type propeller (graph CPP) is higher than that of an FPP- type propeller (graph FPP). The third graph (PR) illustrates the power reduction in percentage units when using FPP- type propellers in place of CPP- type propellers. It may be seen that the power reduction is, naturally, at its highest at the area where the thrust demand is between -50% and +50%, i.e. at the area typical for dynamic positioning applications. Anyway, the overall power reduction over the entire range of thrust demand (-100% ... +100%) is 34%.

[0027] Figure 5 illustrates the rotational speed (Y- axis) of the FPP- type tunnel thruster as a function of combined thrust demand (X- axis). It is easily seen that the rotational speed does not correlate linearly to the thrust demand, but especially near the origin quite a high change in rotational speed is required to effect a small change in the thrust.

[0028] In view of the above prior art study it appears that the use of a fixed pitch propeller in tunnel thrusters is well based, and should be taken as the starting point for further optimization. As was already discussed in the introductory part of the specification the prior art way of operating the FPP- type tunnel thrusters, i.e. either running the propellers one at a time in its preferred direction, or, in heavier circumstances, both propellers such that they deliver thrust in either port or starboard direction, whereby, in dynamic positioning applications where the direction of the movement of the hull of the marine vessel may be almost constantly changing the propellers have to be started and stopped continuously, results in very slow response in the demand for changing the direction of movement. This may be explained in more detail by means of an example where the direction of sideways movement of the marine vessel, or its bow or its aft should be changed. Thus the situation is, at its worst, such that water is flowing through both tunnel thrusters in the same direction, and in order to change the direction of movement of the marine vessel, or the bow or aft thereof, the rotational direction of both propellers have to be changed. Now, within the thruster tunnel, the propellers have to first fight the inertia of the water flow, meaning waste of energy, until the water flow may be turned in both tunnels to the desired opposite direction. This kind of change in the direction of water flows takes time and during that time there is, in practice, no maneuvering action in either direction. Additionally, the sudden and sometimes almost continuous change in the flow direction in the thruster tunnels causes noise and vibrations that may be considered as less desired features.

[0029] To improve the maneuverability of a marine vessel, the present invention suggests running the tunnel thrusters such that, at low or zero maneuvering action, two tunnel thrusters of the at least two tunnel thrusters deliver positive thrust, i.e. thrust in their preferred direction of operation, and, for a significant part of the operation, i.e. in a maneuvering action, of the at least two tunnel thrusters one tunnel thruster is responsible for the thrust delivery for the desired maneuvering. Basically, a first novel and inventive way to correct the problem discussed above is to be prepared to change the direction of movement of the marine vessel, its bow or its aft at the low thrust area, i.e. the area in use at dynamic positioning, for instance, by running both propellers in their preferred direction for producing positive thrust, whereby there would be no need to change the direction of water flow in the thruster tunnels at all, but just to adjust the velocity of the flow, i.e. the volume flow. A second novel and inventive way is to run the tunnel thrusters at different rotational speeds and, by doing that, to adjust the direction in which the actual or relative movement of the marine vessel, its bow or aft takes place.

[0030] By the above discussed ways the arrangement of the present invention is always prepared to changes in the direction of movement of the marine vessel, its bow or its aft without, in most of the applicable occasions, any need to change the flow direction in the thruster tunnels. Even if the flow direction in a thruster tunnel may need to be changed in some exceptional situations, it takes place only in one thruster tunnel not in both like in prior art methods and arrangements. Also, by continuously delivering, at low thrust demand, oppositely directed thrust forces there is not a single period of time that there is no efficient maneuvering action at all going on, whereby any temporary instability in the dynamic positioning, for instance, is prevented as well as vibration and noise is significantly reduced. Now that at the origin of the charts 6 and 9 both tunnel thrusters are producing thrust in opposite directions, and when moving in either direction from the origin, there is always thrust present in either direction increasing the stability of the marine vessel. [0031 ] Figure 6 illustrates the running scheme of the at least two of tunnel thrusters in accordance with the first preferred embodiment of the present invention in a thrust demand - thrust delivery chart in the manner the running scheme of prior art was illustrated in Figure 3. In other words, Figure 6 is a chart where the X- axis represents the thrust demand for both tunnel thrusters in percentage units of maximum available combined thrust of the two tunnel thrusters and Y- axis the thrust delivery of a single tunnel thruster in percentage units of maximum available thrust delivery of the single tunnel thruster. In other words, like the chart shows the graphs of the two tunnel thrusters are positioned one on top of another outside the thrust demand of +45 % and -45 % and separate therebetween. The positive or negative values in the thrust demand (X- axis) mean thrust maneuvering the marine vessel or the aft or the bow thereof in port or starboard direction. The positive and negative values in thrust delivery (Y- axis) indicate the thrust value of a single tunnel thruster. Positive value means thrust in the preferred direction of operation of the first FPP- propeller and negative value means thrust in a direction opposite to the preferred direction of operation of the first FPP- propeller. The preferred direction may be taken as the direction where the propeller draws water past the drive unit of the tunnel thruster. The scale on the Y- axis, i.e. positive values above the X- axis and negative therebelow, represents the thrust of the first tunnel thruster shown by graph C, whereas the scale for the second tunnel thruster represented by graph D is the opposite, i.e. positive values below X- axis and negative thereabove. The chart of Figure 6 has a vertical line L by means of which the relation between the thrust demand and the thrust delivery may be better explained. The point where line L intersects X- axis is the thrust demand or need which is required for a certain maneuvering action, i.e. here in this example +15 % of the maximum combined thrust of the two tunnel thrusters. And the points where the line L intersects the graphs T1 and T2 indicate the thrust delivery of the tunnel thrusters T1 and T2 needed to result in the combined thrust of 15% of the two tunnel thrusters. In other words, using the equation discussed in connection with Figure 3, now that tunnel thruster T1 delivers thrust of about +35% and tunnel thruster T2 thrust of about +5%, the combined thrust is (Thrust of T1 - Thrust of T2) divided by two, i.e. (35% - 5%)/2 = 15%.

[0032] The chart may also be understood as follows. Above the X- axis (thrust demand) the first tunnel thruster represented by graph T1 rotates in its preferred direction of operation delivering positive thrust and the second tunnel thruster represented by graph T2 in a direction opposite to its preferred direction of operation delivering negative thrust. Below the X- axis the second tunnel thruster rotates in its preferred direction of operation delivering positive thrust and the first tunnel thruster in a direction opposite to its preferred direction of operation delivering negative thrust, naturally. At the right hand side of the Y- axis the "dominating" or active tunnel thruster is the first tunnel thruster, i.e. the first tunnel thruster is responsible for that the marine vessel, its bow or its aft moves in the direction that the positive thrust produced by the preferred direction of rotation of the first tunnel thruster moves it. However, somewhere above a certain predetermined thrust demand (here about 40%) the second tunnel thruster unit by being turned to rotate in a direction opposite to its preferred direction starts assuming responsibility by aiding in moving the marine vessel, its bow or its aft by the negative thrust it produces in addition to the positive thrust of the first tunnel thruster. At the left hand side of the Y- axis the "dominating" tunnel thruster is the second tunnel thruster, i.e. the marine vessel, its bow or its aft moves in the direction that the positive thrust produced by the preferred direction of rotation of the second tunnel thruster moves it. However, somewhere above a certain predetermined thrust demand (here about -40%) the first tunnel thruster by being turned to rotate in a direction opposite to its preferred direction starts assuming responsibility by aiding in moving the marine vessel, its bow or its aft by the negative thrust it produces.

[0033] Thus, the at least two tunnel thrusters are run such that when, for instance in dynamic positioning, no movement in either direction is desired, i.e. the true thrust demand is 0, both tunnel thrusters are set to deliver a predetermined positive thrust, for instance 5% to 10% of its maximum capability in their preferred direction (here 5% set point shown), whereby the equal positive thrust forces acting in opposite directions overrule one another. Now, in accordance with a preferred embodiment of the present invention, when movement in either direction is desired, one of the tunnel thrusters (from hereon a first tunnel thruster) is turned to be an active tunnel thruster (by increasing its rotational speed) the other one (from hereon a second tunnel thruster) remaining a passive or idling one (the rotational speed staying constant). The rotational speed as well as the positive thrust delivery (graph T1 ) of the first tunnel thruster pushing the marine vessel, its bow or its aft in the desired sideways direction by using its preferred rotational direction is, thus, increased, whereas the positive thrust delivery (graph T2) of the second tunnel thruster is maintained (here at 5%). However, if the combined thrust demand exceeds a predetermined value, in this example 40% of the total available thrust, which means that the thrust delivery of the first tunnel thruster is raised to a borderline value, in this exemplary embodiment to 85 % (in other words, total combined thrust demand of 40 % for each one of the first and the second tunnel thruster means 85 % thrust delivery for the first tunnel thruster as it also has to counter the 5 % thrust delivery of the second tunnel thruster running in opposite direction), the second tunnel thruster is taken into action by first decelerating its rotational speed to zero, by turning it to rotate in the direction opposite its preferred direction and thus turning the water flow in the thruster tunnel in opposite direction and finally raising the speed of the second tunnel thruster so that it delivers a share of its maximum thrust equal to that of the first tunnel thruster. Naturally, the latter action means that the rotational speed or the positive thrust delivery of the first tunnel thruster has to be reduced in a corresponding manner so that the total or combined thrust delivered by the two tunnel thrusters remains on the diagonal of the chart.

[0034] Figure 7 illustrates the effect of the change in the tunnel thruster running scheme on the power consumption of the tunnel thrusters. Again, the use of the FPP- type tunnel thrusters are compared to CPP- type tunnel thrusters. The power consumption of the FPP- type tunnel thruster remains below that of the CPP- type tunnel thruster except for the area of about 40 % or -40% thrust demand where the power reduction graph (PR) turns to negative. However, in view of the total operating range the power reduction included in the use of FPP- type tunnel thrusters is very significant.

[0035] Figure 8 illustrates the rotational speed (y- axis) of the FPP- type tunnel thruster run in accordance with the scheme of Figure 6 as a function of total or combined thrust demand (x- axis). Here, again, line L shows that in accordance with the running scheme of Figure 6 to reach the desired total thrust of +15%, the first tunnel thruster T1 needs to be run at 60% of its full speed and tunnel thruster T2 at about 22% of its full speed. In other words, the delivered thrust of +35% (see Figure 6) of the first tunnel thruster T1 requires the + 60% rotational speed and the delivered thrust of +5% (see Figure 6) of the second tunnel thruster T2 requires the + 22% rotational speed. At origin, i.e. a position where the thrust demand is 0 the rotational speeds of both tunnel thrusters are maintained at an equal positive value of about 20 % to 25 % of the full speed value of the tunnel thrusters. The positive value meaning that the tunnel thrusters are rotated in their preferred direction and they produce a positive thrust. Such a rotational speed of the tunnel thruster produces some 5% - 10% positive thrust (see Figure 6) of the maximum available thrust from the tunnel thruster. Likewise, the term positive thrust means thrust created by a tunnel thruster rotating in its preferred direction.

[0036] When the marine vessel, its bow or its aft needs to be moved sideways in water the rotational speed of one of the tunnel thrusters, for instance the first tunnel thruster T1 , is increased while keeping the rotational speed of the second tunnel thruster T2 constant. If the first tunnel thruster T1 reaches its predetermined borderline value for the thrust demand (shown in Figure 6) the second tunnel thruster T2 is taken to participate the thrust delivery. First the rotational speed of the second tunnel thruster T2 is decelerated to 0 whereby the positive thrust of the second tunnel thruster T2 reduces to 0 and no more acts against the positive thrust of the first tunnel thruster T1 . Next the rotational direction of the second tunnel thruster T2 is changed and it starts to create a negative thrust, which has the same direction as the positive thrust of the first tunnel thruster T2. Now that the second tunnel thruster T2 participates the thrust creation in the same direction as the first tunnel thruster T1 , the rotational speed of the first tunnel thruster T1 may be reduced and its share of the thrust creation reduced to meet that of the second tunnel thruster T2 at about 45 % thrust demand. Thereafter, i.e. above the 45 % thrust demand value the rotational speeds of both tunnel thrusters are changed equally.

[0037] Figure 9 illustrates a running scheme of the tunnel thrusters in accordance with a second preferred embodiment of the present invention. The functional differences may be seen in the graphs T3 and T4, where all "corners" of the graphs T1 and T2 of Figure 5 are rounded. In practice, this means smoother operation of the tunnel thrusters and less noticeable accelerations or decelerations in the sideways movement of the marine vessel. Additionally, it reduces the stresses subjected to the various components in the tunnel thruster arrangements, especially the blades, gears wheels and the drive units.

[0038] Figure 10 illustrates the effect of the change in the tunnel thruster running scheme on the power consumption of the tunnel thrusters. When comparing to Figure 7 it is easy to see that the "rounding of the corners" in the thrust delivery results in "rounding of the corners" in power consumption, too. Now, the power reduction graph (PR) remains positive over the full operating range of the tunnel thrusters.

[0039] Figure 1 1 illustrates the rotational speed (Y- axis) of the FPP- type tunnel thruster run in accordance with the scheme of Figure 9 as a function of thrust demand (X- axis). If compared to the representation of Figure 9 at origin, i.e. a position where the thrust demand is 0 the rotational speeds of both tunnel thrusters are maintained at an equal positive value of about 30 % to 35 % of the full speed value of the tunnel thrusters. The positive value meaning that the tunnel thruster is rotated in its preferred direction. Such a rotational speed of the tunnel thruster produces about 10% positive thrust of the maximum available thrust from the tunnel thruster (see Figure 9). Likewise, the term positive thrust means thrust created by a tunnel thruster rotating in its preferred direction. The rounding of the corners in the running scheme of Figure 9 is accomplished by, when moving away from the origin, simultaneously increasing the rotational speed of the first tunnel thruster T3 and decreasing the rotational speed of the second tunnel thruster T4. However, as the rotational speed of the second tunnel thruster T4 is only decelerated to a value of about 20 - 25 % with a certain thrust demand/delivery and thereafter kept constant up to a situation where the combined thrust demand has raised to its borderline value, the running scheme of the tunnel thrusters continues substantially as presented and explained in connection with the first preferred embodiment. Another small change in the running scheme may be seen near the borderline value where the increasing of the rotational speed of the first tunnel thruster is not stopped immediately as in the embodiment of Figure 8, but steadily reducing the speed increase and then turning it steadily to deceleration. By performing the speed changes smoothly the entire operation is smoother and its effects on the marine vessel as vibrations and noise are less noticeable.

[0040] As may be seen in the embodiments discussed in Figures 6 - 1 1 the present invention brings about a number of advantages when compared to prior art. Firstly, for a majority of the true operating or maneuvering range of the tunnel thrusters (here between -40% and +40% combined thrust demand) the direction of rotation of the propellers of the tunnel thruster is not changed. Only in very exceptional cases combined thrust of more than +/- 40% is needed. In practice, this means that the volume flow in the thruster tunnels is not normally changed, which equals to reduced energy consumption, noise and vibration. Also, drive or gearbox, which runs almost always in the same direction may be considered as an advantage as the currently applied load reduction to 70% for the gears, based on the idling gear approach, might be increased to 90%. This would allow more power on the same thruster unit, resulting in reduced cost. The load reduction is applied since the teeth of the gearwheels are loaded on both sides when the gearwheels are operated in both directions. This double sided loading has an impact on the fatigue loading and therefore a load reduction to 70% is applied.

[0041 ] As to the above discussed two preferred embodiments of the present invention it should be understood that the running schemes may vary a great deal within the coverage of the present invention. For instance, it is possible to increase or reduce the maximum thrust delivery a single tunnel thruster is allowed produce before a second tunnel thruster is turned to assist a first one with a thrust in the same direction. It is also possible to reduce or increase the "idle" positive thrust where the tunnel thrusters function with the same thrust but in opposite directions. Also, the constant low positive thrust of the "passive" tunnel thruster may be increased or reduced from the values shown in the two presented preferred embodiments. Furthermore, it is obvious that the number of tunnel thrusters used in the arrangement of the present invention may be more than two. One option is that the tunnel thrusters appear in pairs whereby there are two equal pairs of tunnel thrusters, one pair having its preferred direction of operation in port direction and the other pair in starboard direction. In such a case each pair may be run like a single tunnel thruster of the above embodiments. In another case, one tunnel thruster of one pair may be run individually with another tunnel thruster of another pair in the manner of the above embodiments. A further option is a case where there are three tunnel thrusters so that there are two smaller tunnel thrusters and a larger one. In such a case the two smaller ones may have their preferred direction of operation in one direction and the larger one in the opposite direction. Now the two smaller ones may be run either one at a time or together. However, in either case the smaller tunnel thrusters may be considered as one tunnel thruster and the larger one as the other tunnel thruster and driven in the manned of the above embodiments. Furthermore, it is an option that there are three tunnel thrusters of which two are alike having their directions of operation in opposite directions, and third one being used only, when either one of the two tunnel thrusters needs assistance. In such a case the two first mentioned tunnel thrusters may be run as discussed in the earlier embodiments.

[0042] Figure 12 illustrates the arrangement for maneuvering a marine vessel in accordance with a preferred embodiment of the present invention. The arrangement comprises the pair of tunnel thrusters 30 and 32 each tunnel thruster having the drive unit and the fixed pitch propeller. The drives of the tunnel thrusters 30 and 32 are coupled to a control unit 34 for adjusting the rotational speed of the tunnel thrusters 30 and 32. The control unit 34 is part of a propulsion control system 36 of the marine vessel, which may be controlled either automatically, for instance by the dynamic positioning system 38, or manually 40 by means of a lever.

[0043] Figure 13 illustrates an exemplary embodiment for a manual control of the rotational speed of the tunnel thrusters. The x- axis of the chart represents the control lever position from port (left) to starboard (right) and the y- axis the relative rotational speed of the propellers of the tunnel thrusters. At the area above the x- axis the propellers of the tunnel thrusters are rotating in their preferred rotational direction and below the x- axis in the direction opposite to their preferred direction. The solid line represents the first tunnel thruster and the dashed line the second tunnel thruster. The manual control takes place, for instance such that when a maneuvering action is desired, the control lever is moved to the right, whereby the rotational speed of the first thruster increases linearly in relation to the control lever movement. Simultaneously the rotational speed of the second thruster is kept constant at a small value in its preferred rotational direction. When the rotational speed of the first thruster has reached its maximum value at lever position 5, the rotational speed of the second thruster is first reduced to 0, and then the rotational direction is reversed. Thereafter, when the control lever is still moved to the right the rotational speed of the first thruster is maintained in its maximum value and the rotational speed of the second thruster is increased such that it reaches its maximum speed value while the control lever reaches its maximum value (10 in the Figure). Figure 13 discloses clearly how both tunnel thrusters may be run in their preferred direction for a considerable part of their total control range, i.e. both tunnel thrusters may be run three quarters of their control range in their preferred direction. And when the fact that the central area, i.e. lever positions 1 - 4 in either direction, form the majority (at least 80%) of the maneuvering tasks, including the dynamic positioning, the tunnel thrusters very seldom are run in a direction opposite to their preferred direction.

[0044] The above discussed embodiments of the present invention reveal that in practice the rotational speed (rpm) control of the tunnel thrusters takes place by means of more or less automated means. In case it is a question of dynamic positioning where the marine vessel is kept in place irrespective of wind, waves or current the positioning and maneuvering may be performed fully automatically. The operator needs only to instruct the control system to maintain the position of the marine vessel whereafter the system by using, for instance, GPS or some other appropriate position detecting system and a software stored in the control computer performs the required maneuvering tasks, the main features of which have been discussed in connection with the embodiments of Figures 6 - 1 1 . Although ordinary docking of a marine vessel may be performed automatically, too, it is normal practice that the operator of the marine vessel performs the docking manually by using one or more levers like the one discussed in connection with Figure 13. However, as may be seen in Figure 13, each movement of the lever from one position to another means different things to different tunnel thrusters. Thereby it is clear that both the lever position and the direction of its movement instructs the software to perform in proper order and timing the required maneuvering tasks, the main features of which have been discussed in connection with the embodiments of Figures 6 - 1 1 . For instance, bringing the lever from position 6 to the port (from the left) to position 5 instructs, in accordance with Figure 13, the following tasks to be performed: 1 ) turning the rotational speed of the first tunnel thruster to its preferred direction of rotation, and 2) increasing the rotational speed of the first tunnel thruster to 10 % of its full rpm. However, if the same lever position 5 is approached from the right i.e. from lever position 4, the tasks are different, as follows: 1 ) increasing the speed of the second tunnel thruster to full speed, 2) turning the rotational speed of the first tunnel thruster to a direction opposite to its preferred direction of rotation, and 3) increasing thereafter the rotational speed of the first tunnel thruster to 10 % of its full rpm in the direction opposite to its preferred direction of rotation.

[0045] It should be understood that the above is only an exemplary description of a novel and inventive method and an arrangement for maneuvering a marine vessel. It should be understood that though the specification above discusses certain types of running schemes of a pair of tunnel thrusters, the discussed running schemes do not limit the invention to the schemes discussed. Thus the above explanation should not be understood as limiting the invention by any means but the entire scope of the invention is defined by the appended claims only. Also it is clear that the drive of the propeller of the tunnel thruster may be arranged not only by mechanical gear and shaft drive but also by means of an electric or a hydraulic motor arranged down in the thruster hub. From the above description it should be understood that separate features of the invention may be used in connection with other separate features even if such a combination has not been specifically shown or discussed in the description or in the drawings.

Claims

1 . A method of maneuvering a marine vessel, the marine vessel having a hull with a longitudinal centreline and at least two tunnel thrusters arranged in the hull at right angles to the longitudinal centreline, each tunnel thruster having a fixed pitch propeller (FPP) with a rotational speed in a preferred rotational direction for delivering a positive thrust, the positive thrust of a first tunnel thruster of the at least two tunnel thrusters having a direction opposite to that of a second tunnel thruster of the at least two tunnel thrusters, the method comprising the steps of,

a) delivering, when no maneuvering action is desired, a constant equal positive thrust by both the first and the second tunnel thrusters of the at least two tunnel thrusters, and

b) increasing, when a maneuvering action is desired, the rotational speed of the first tunnel thruster to increase the positive thrust of the first tunnel thruster of the at least two tunnel thrusters.

2. The method of maneuvering a marine vessel as recited in claim 1 , characterized in, while practicing step b), maintaining the positive thrust of the second tunnel thruster of the at least two tunnel thrusters by maintaining its rotational speed constant.

3. The method of maneuvering a marine vessel as recited in claim 1 , characterized in, while practicing step b), reducing the rotational speed of the second thruster for reducing the positive thrust of the second tunnel thruster of the at least two tunnel thrusters.

4. The method of maneuvering a marine vessel as recited in claim 1 , characterized in, while practicing step b), reducing the rotational speed of the second tunnel thruster to a value between its original value and zero for reducing the positive thrust of the second tunnel thruster of the at least two tunnel thrusters but still maintaining the thrust positive.

5. The method of maneuvering a marine vessel as recited in any one of the preceding claims, characterized in, while practicing step b), either increasing the rotational speed of the first tunnel thruster to a predetermined level to deliver a positive thrust or increasing the rotational speed of the first tunnel thruster to deliver a predetermined positive thrust.

6. The method of maneuvering a marine vessel as recited in claim 5, characterized in, when the first tunnel thruster has either reached its predetermined rotational speed or its predetermined positive thrust, operating the second tunnel thruster to reduce its rotational speed.

7. The method of maneuvering a marine vessel as recited in claim 6, characterized in, operating the second tunnel thruster to reduce its rotational speed by i. first reducing its rotational speed to zero and ii. secondly increasing its rotational speed in an opposite direction to deliver a negative thrust.

8. The method of maneuvering a marine vessel as recited in claim 7, characterized in, simultaneously with steps i) and ii) reducing the rotational speed of the first tunnel thruster to reduce the positive thrust it delivers.

9. The method of maneuvering a marine vessel as recited in claim 7, characterized in, simultaneously with steps i) and ii) maintaining the rotational speed of the first tunnel thruster to keep the thrust it delivers constant.

10. The method of maneuvering a marine vessel as recited in claim 8, characterized in settling the positive thrust of the first tunnel thruster and the negative thrust of the second tunnel thruster to equal values.

1 1 . The method of maneuvering a marine vessel as recited in claim 10, characterized in, if more thrust is desired, increasing the rotational speed of the first and the second tunnel thrusters equally to increase the thrust they deliver equally.

12. An arrangement for maneuvering a marine vessel for practicing the method of any one of claims 1 - 1 1 , the marine vessel (10) having a hull with a longitudinal centreline and at least two tunnel thrusters (12, 14) arranged in the hull at right angles to the longitudinal centreline, each tunnel thruster (12, 14) having a drive unit (16, 18) and a fixed pitch propeller (20, 22), the arrangement further comprising a control unit (34) having a memory, into which at least one running scheme of the at least two tunnel thrusters (12, 14) is stored.