Note: Descriptions are shown in the official language in which they were submitted.
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Arrangement and method in a ship
TECHNICAL FIELD
The invention relates to an arrangement in a ship according to the preamble of
claim 1.
The invention also relates to a method in a ship according to the preamble of
claim 12.
The arrangement and the method can be used in a ship comprising a hull, at
least
one propulsion engine, transmission means, at least one propeller connected
via
the transmission means to the at least one propulsion engine and a support
structure comprising an upper portion being supported at the hull, a lower
portion,
and a front edge, a control unit for controlling the at least one propulsion
engine.
The ship can have only one propulsion engine or two or more propulsion engines
situated at the stern of the ship. The propeller can comprise a single
propeller or
two contra-rotating propellers.
The arrangement is suitable to be used especially in large ships e.g.
cruisers,
tankers transporting oil or liquefied natural gas, vehicle carriers, container
ships
and ferries.
BACKGROUND ART
JP patent publication No. 2004182096 discloses a pod-type propulsion apparatus
comprising a support structure being pivotably attached to the hull of a ship
and a
chamber attached to the support structure. The chamber comprises a motor being
connected to a first end of a shaft, the second opposite end of the shaft
protruding
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from the front end of the chamber and being connected to a propeller. The
rotation
angle of the support structure is limited when the speed of the ship increases
in
order to prevent cavitation.
RU patent publication No. 2009957 discloses a device to reduce cavitation in a
ship. The propeller at the stern of the ship is connected via a shaft to a
motor
within the hull of the ship. There are flexible casings with oscillation
drives on the
blades of the propeller. A cavitational noise sensor is located on the hull of
the
ship. An oscillation frequency control block for the flexible casings is
connected
in series with the noise sensor. The propeller shaft is fitted with a
collector with
brushes. The sensor generates a signal which is proportional to the acoustic
radiations and feeds it as an input signal to the oscillation frequency
control block.
The oscillation frequency control block in turn generates a return signal to
the
drives for the flexible oscillation casings in order to reduce cavitational
noise to
the minimum.
JP patent publication No. 09136694 discloses automatic speed control of a
water
jet pump used in a ship. A pressure sensor detects the delivery pressure of
the
water jet pump when the ship is moving. A calculator calculates the number of
revolutions that can be applied to the water jet pump in order to avoid
cavitation
generation in water current based on the output signal of the pressure
detector. A
signal selector compares the calculated number of revolutions and the number
of
revolutions indicated by a steering control unit. The signal selector outputs
a
control signal to a drive motor of the water jet pump by selecting the signal
that
indicates the smaller number of revolutions.
JP patent publication 09109991 discloses a cavitation prevention type ship fin
stabilizer. The stabilizer includes a fin being pivotably supported to the
hull
through a shaft. A fin driving mechanism adjusts wing angle of the fin by
turning
the shaft. A water exhaust nozzle at the lower rear edge of the fin controls
cavitation generation of the fin while cruising. A feed water pump supplies
water
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and discharges water from the exhaust nozzle. An under water microphone
situated after the fin, detects noise caused by the cavitation of the fin. A
water
injection controller controls the water injection from the exhaust nozzle by
controlling the feed pump based on the noise detection signal of the
microphone.
Cavitation occurs when liquid changes its phase into vapour at a certain flow
region where local pressure is very low due to high local velocities. At least
four
different cavitation types relating to a rotating propeller in water can be
distinguished: a) tip vortex from the suction side, which is regarded as
normal
operation until a certain level is exceeded, b) sheet cavitation at suction
side, c) tip
vortex from the pressure side, d) bubble cavitation.
The control bridge is at the stem of the ship i.e. 200 to 400 meters ahead of
the aft
in a big ship. The control bridge is also 15-40 meters above sea level. This
means
that the captain or navigating officer sitting on the navigation bridge do not
normally feel or hear cavitation caused by the propellers at the aft of the
ship.
There is thus a need to make the captain and/or the navigating officer aware
of
situations where worse degree cavitation is emerging. Such situations might
typically occur when the ship is suddenly accelerated with full power or when
the
propulsion unit and/or the ship is turned at big turning angels.
SUMMARY OF THE INVENTION
The object of the invention is an arrangement and method to manage situations
where cavitation occurs in a ship.
The arrangement in a ship according to the invention is characterized by the
features in the characterizing portion of claim 1.
The method in a ship according to the invention is characterized by the
features in
the characterizing portion of claim 12.
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The arrangement in a ship according to the invention comprises a hull, at
least one
propulsion unit comprising a propulsion engine, transmission means, at least
one
propeller connected via the transmission means to the propulsion engine, and
an
oscillation sensor situated in the vicinity of the at least one propeller in
order to
sense oscillations caused by cavitation of the at least one propeller. The
arrangement comprises further a control unit for controlling the at least one
propulsion engine, said oscillation sensor being connected to the control
unit,
whereby the output signal of the oscillation sensor is analyzed in the control
unit
in order to determine whether a worse degree cavitation is emerging, and
indicate
that a worse degree cavitation is emerging on a display unit at the navigation
bridge and/or regulate the rotation speed and/or the power of the at least one
propulsion engine when worse degree cavitation is emerging.
It is easier to recognize cavitation of the propeller when the oscillation
sensor is
situated near the origin of the phenomena i.e. near the propeller. When the
propulsion unit is about to enter into an unwanted operation phase with
harmful
worse degree cavitation the control unit sends a warning to a display unit at
the
navigation bridge and/or controls the speed and/or the power of the propulsion
engine.
When the measurement is done directly from the oscillations caused by the
cavitation of the propeller, the amount of tuning parameters in the system is
limited to a minimum. The only tuning parameters are sensitivity in the
propeller
blade frequency, the type of burst and the amplitude. The signal processing of
the
raw measurements is also rather simple, which means that the indication of
worse
case cavitation will be fast. The speed ramps of the propulsion unit are
normally
relative low, which means that there is plenty of time to react before more
severe
cavitation emerges.
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First stage tip vortex cavitation is normal in operation e.g. in the case the
ship
drives at full speed. First stage tip vortex is also normally taken into
account in
hydrodynamic design and the operation efficiency of the propeller is not
harmfully affected by first stage tip vortex. In case the cavitation gets
worse, it
5 will be harmful for the whole propulsion mechanics and may cause instant
or long
run damage. A worse class cavitation will result in a collapse of the
efficiency of
the propeller, which decreases the maneuverability of the ship dramatically.
The best position of the oscillation sensor for sensing the tip vortex of the
propeller is on a support structure situated behind the propeller in the
driving
direction of the ship at a place where the tip vortex 'rope' hits the support
structure. The support structure is behind the propeller in a normal pulling
type
propulsion unit i.e. in a propulsion unit where the propeller is at the front
end of
the chamber.
The support structure creates a different density in the hydrodynamic
environment
as the propeller blade passes in front of the support structure. The tip
vortex and
the support structure affect together. The interaction between the tip vortex
and
the support structure will create propeller blade frequency bursts when the
tip
vortex gets worse. This point is the borderline to start actions in propulsion
control for avoiding more severe cavitation.
There could be a possibility to disabled the control unit in certain
conditions by a
separate buttom on the steering bridge.
The invention can advantageously be used in large ships e.g. cruisers, tankers
transporting oil or liquefied natural gas, vehicle carriers, container ships
and
ferries. The power of the propulsion unit in such large ships is in the order
of at
least 1 MW.
BRIEF DESCRIPTION OF THE DRAWINGS
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Some specific embodiments of the invention are described in the following in
detail with reference to the accompanying figures, in which:
Figure 1 shows an arrangement according to the invention in a ship comprising
a
pod propulsion unit.
Figure 2 shows an arrangement according to the invention in a ship comprising
a
rudderpod unit.
Figure 3 shows an arrangement according to the invention in a ship comprising
a
conventional axial propulsion unit.
DETAILED DESCRIPTION OF SOME SPECIFIC EMBODIMENTS
Figure 1 shows an arrangement according to the invention in a ship with a pod
propulsion unit.
The arrangement comprises a propulsion unit 100 situated at the stern of the
ship.
The propulsion unit 100 comprises a support structure 10, a chamber 20, a
propulsion engine comprising a first electric motor 30, a shaft 40, and a
propeller
50. The support structure 10 has an upper portion 11, a lower portion 12, a
front
edge 13 and a rear edge 14. The upper portion 11 of the hollow support
structure
10 is pivotably attached to the hull 200 of the ship. The chamber 20 is
stationary
attached to the lower portion 12 of the hollow support structure 10. The shaft
40
has a first end which is connected to the first electric motor 30 and a second
end
protruding from the front end 21 of the chamber 20 and being connected to the
propeller 50. The propeller 50 is thus situated at the front end 21 of the
chamber
20. The first electric motor 30 can be an induction motor or a synchronous
motor.
The propeller 50 can comprise a single propeller or two contra-rotating
propellers.
The driving direction of the sip is shown by the arrow 51 in the figure.
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The propulsion unit 100 comprises further a turning mechanism 60 for turning
the
propulsion unit 100 in relation to the hull 200 of the ship around a turning
axis Y-
Y. The turning mechanism 60 is situated within the hull 200 of the ship and
comprises a gear rim 61 and a second electric motor 62. The shaft 63 of the
second electric motor 62 is connected to a pinion gear 64 and the pinion gear
64 is
connected to the circumference of the gear rim 61. The upper portion 11 of the
support structure 10 is connected to the gear rim 61. The second electric
motor 62
will thus rotate the pinion gear 63, which rotates the gear rim 61, which
rotates the
support structure 10 and thereby the propulsion unit 100. The second electric
motor 62 can be an induction motor or a synchronous motor. There can naturally
be two or more second electric motors 62 situated around the circumference of
the
gear rim 61.
The arrangement comprises also a control unit 400 for controlling at least the
first
electric motor 30 and alternatively also the second 62 electric motor. The
control
unit 400 will control the first 30 and the second 62 electric motors based on
the
commands from the navigation bridge. The rotation speed of the electric motors
30, 62 can be controlled e.g. by frequency converters.
The arrangement for cavitation indication comprises further an oscillation
sensor
300 being situated on the front edge 13 of the support structure 10. The
oscillation
sensor 300 is situated on the front edge 13 of the support structure 10 at a
height
corresponding to the position of the tip of the propeller 50 blade in the
situation
when the tip of the propeller 50 blade during the rotational movement is
passing
in front of the front edge 13 of the support structure 10. The oscillation
sensor 300
is connected to the control unit 400. This position of the oscillation sensor
300 is
optimal in view of sensing especially tip vortex created by the rotating
propeller
50.
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Figure 2 shows an arrangement according to the invention in a ship comprising
a
rudderpod unit.
The arrangement comprises a propulsion unit 100 situated at the stern of the
ship.
The propulsion unit 100 comprises a support structure 10, a chamber 20, a
propulsion engine comprising a first electric motor 30, a shaft 40, a
propeller 50,
and a rudder 70. This arrangement corresponds to the pod arrangement of figure
1
except for the rudder 70 and the fact that the support structure 10 is in the
rudderpod stationary supported at the hull 200 of the ship. The rudder 70 is
pivotably supported around a turning axis Y-Y.
Figure 3 shows an arrangement according to the invention in a ship comprising
a
conventional axial propulsion unit. This arrangement has no pod arrangement
outside the hull 200 of the ship. The propulsion unit 100 comprises a
propulsion
engine 30 within the hull 200 of the ship, a shaft 40 having a first end
connected
to the propulsion engine 30 and a second end extending through an opening at
the
rear end of the hull 200 and being connected to a propeller 50 situated
outside the
hull 200. The propulsion engine 30 could be e.g. a diesel engine or an
electric
motor. The rudder 70 is situated after the propeller 50 in the driving
direction Si
of the ship. The rudder 70 is further pivotably supported around a turning
axis Y-
Y.
An oscillation sensor 300 is situated on the front edge 71 of the support
structure
70 i.e. the rudder 70 at a height corresponding to the position of the tip of
the
propeller 50 blade in the situation when the tip of the propeller 50 blade
during the
rotational movement is passing in front of the front edge 71 of the rudder 70.
The
oscillation sensor 300A could alternatively be attached to the bottom of the
hull
200 of the ship as also shown in the figure. The position of the oscillation
sensor
300A on the hull 200 would in such case be in a radial direction above the tip
of
the blade of the propeller 50.
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It is advantageous to have the oscillation sensor 300 attached to a support
structure, which is situated behind the propeller 50 in the driving direction
of the
ship. The oscillation sensor 300 will in such a position sense effectively
especially
tip vortex of the propeller 50. A suitable support structure to attach the
oscillation
sensor 300 is e.g. the support structure for the pod or a rudder, but it could
be any
support structure situated behind the propeller 50 in the driving direction Si
of the
ship. In the case of a pod unit having the propeller 50 at the rear end of the
chamber 20, a separate support structure is needed behind the pod for the
oscillation sensor 300.
The propeller 50 could be formed of a single propeller or of two contra-
rotating
propellers.
The oscillation sensor 300, 300A could be e.g. a pressure sensor, an acoustic
sensor, or an acceleration sensor.
When the oscillation sensor 300 is situated on the support structure 10, 70
after
the propeller 50 in the driving direction Si of the ship, the optimal position
is at a
base height position corresponding to the height of the tip of the blade of
the
propeller 50 when the tip of the blade is in an uppermost position. The
allowed
vertical deviation V1 from the base height position is equal to or less than
25%
of the diameter D1 of the propeller 50. An oscillation sensor 300 situated on
a
support structure 10, 70 after the propeller 50 measures cavitation
propagating
backwards from the propeller 50.
When the oscillation sensor 300A is situated on the hull 200 above the
propeller
50 in the redial direction, the base longitudinal position is exactly above
the tip of
the propeller 50 blade when the tip of the blade is in an uppermost position.
The
allowed horizontal deviation H1 of the base longitudinal position is equal to
or
less than 50% of the diameter D1 of the propeller 50. An oscillation sensor
300A
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situated in a radial position above the propeller 50 measures cavitation
propagating in the radial direction from the propeller 50.
The oscillation sensor 300, 300A must be positioned so that it is sensitive to
5 cavitation emerging from the propeller 50.
The control unit 400 could be a separate unit or it could be integrated into
some
other control unit in the ship.
10 The invention could also be implemented in a ship having two or more
propulsion
units. E.g. a ship provided with two propulsion units at the aft of the ship
would
need an oscillation sensor for each propeller and a control circuit for each
propulsion motor.
The examples of the embodiments of the present invention presented above are
not intended to limit the scope of the invention only to these embodiments.
Several modifications can be made to the invention within the scope of the
claims.
