Note: Descriptions are shown in the official language in which they were submitted.
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Towable Buoy
FIELD OF THE INVENTION
The present invention relates to a towable buoy, particularly, though not
exclusively,
to a communications buoy to be towed by a submarine.
BACKGROUND OF THE INVENTION
Towed communication buoys have been used for many years to provide
communications to submarines. Communications antennae and electronics mounted
within the buoy can provide a wide range of frequency coverage. Non-
communication
type sensors can also be mounted in the buoy. The buoy is physically,
electrically
and/or optically connected to a submersible vessel (platform) by a retractable
tow
cable. Modern towed communications buoys need to be operable at a large range
of
platform speeds and depths and to maintain stability whilst close to the sea
surface
over a range of sea state conditions.
SUMMARY OF THE INVENTION
According to a first aspect of this invention, there is provided a towable
buoy having a
tow point for connection to a tow cable of a vessel, wherein the position of
the tow
point is translationally moveable with respect to the remainder of the buoy.
According to another aspect of this invention, there is provided a method of
operating
a towable buoy having a tow point which is translationally moveable with
respect to
the remainder of the buoy, the method comprising towing the buoy behind a
moving
vessel by attaching a tow cable of the vessel to the tow point, and adjusting
the
position of the tow point with respect to the remainder of the buoy.
The invention is advantageous in that the tow point adjustment helps to
balance
hydrodynamic and hydrostatic forces on the buoy. At slow speeds the tow cable
orientation is dominated by the positive trim of the buoy resulting in an
almost vertical
link profile between the vessel and the buoy. At medium to high speed, buoy
and tow
cable drag takes dominance and so a more horizontal profile of the link
results. To
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improve buoy stability across the operating range, the tow point can be moved
such
that it can help balance hydrostatic forces on the buoy at low speeds, and
hydrodynamic forces at high speeds. As tow speed increases, the offset between
the
tow point and the buoy centre of gravity is preferably increased both
vertically and in
the fore/aft direction (horizontally) by translational movement of the tow
point with
respect to the remainder of the buoy.
Movement of the tow point can be achieved using either a passive or active
control
mechanism. The passive method could harness changes in tow cable tension
and/or
drag forces to translate the tow point. The simplest passive method would use
a spring
to oppose the tow cable tension and locate the tow point accordingly. An
active
mechanism could use a control loop system to translate the tow point position
according to the submarine's speed and or tow cable tension levels, with the
tow point
movement being under the action of a servomotor, for example.
It is intended that the buoy is stable in pitch and roll when towed a few
metres,
typically less than 10 m, below the sea surface by a submerged vessel at a
wide range
of speeds and depths, in sea states ranging from 0 to at least 6. More
particularly, the
buoy may pitch only slightly (typically less than +/- 10 degrees at sea state
6 at 6 m/s)
in response to wave perturbations.
To achieve the required stability, the buoy may have a significant offset
between its
centre of gravity (CoG) and centre of buoyancy (CoB), i.e. a significant
metacentric
height. The buoy may further comprise means to vary the centre of mass and/or
buoyancy of the buoy. The significant metacentric height may be achieved using
a
keel for the buoy. The keel may be moveable. In particular, the keel may be
movable
between a deployed position and a retracted position such that, when
retracted, the
buoy has a small space requirement for stowage. The keel may also be moved in
its
deployed position so as to alter the centre of mass as required. The buoy may
also
have an internal ballast tank adjustable to alter the hydrostatic
characteristics of the
buoy. The buoy may further include a lifting hydrofoil for increasing the
buoyancy of
the buoy. The hydrofoil may be fixed, or may be actively controlled so as to
alter its
shape, surface area and/or pitch angle for balancing the buoy in trim as the
towing
vessel speed and/or depth changes.
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BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described with reference to the
accompanying drawings, in which:
Figures 1 and 2 illustrate side and front elevations of a first example of a
buoy of this
invention;
Figure 3 illustrates the orbital particle motion that occurs within wave
structures;
Figure 4 illustrates a buoy performance simulation setup;
Figure 5a illustrates a buoy depth profile and Figure 5b a pitch profile for a
6 m/s tow
at SS6;
Figure 6a illustrates a buoy depth profile and Figure 6b a pitch profile for a
3 m/s tow
at SS6;
Figure 7a illustrates a buoy depth profile and Figure 7b a pitch profile for a
0 m/s tow
at SS6;
Figure 8a illustrates a buoy depth profile and Figure 8b a pitch profile for a
6 m/s tow
at SS3;
Figure 9a illustrates a buoy depth profile and Figure 9b a pitch profile for a
3 m/s tow
at SS3;
Figure 10a illustrates a buoy depth profile and Figure 10b a pitch profile for
a 0 m/s
tow at SS3;
Figure 11 illustrates the depth keeping performance of the buoy;
Figure 12 illustrates a side elevation of a second example of a buoy of this
invention;
and
Figures 13a to 13d illustrate various elevations of a third example of a buoy
of this
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
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Buoy Set-Up
A first example of a buoy is shown schematically in Figures 1 and 2. The buoy
I has a
hydrodynamic body 2, a hydrofoil 3, and a moveable tow point 4. The buoy I
houses
a payload, an adjustable ballast tank 5, a tow point actuator and a hydrofoil
actuator.
The tow point actuator is operable to move the tow point 4 between a high,
forward
position for high speed tow, and a low, aft position for low speed tow. The
hydrofoil
actuator is operable to adjust the shape, surface area and/or pitch angle of
the
hydrofoil 3. The adjustable ballast tank may include a chamber having an
opening to
the surrounding water, the volume of the tank being adjustable by moving the
position
of a piston within the chamber such that the tank contains a variable mass of
ballast
water. The remainder of the chamber may be filled with compressible gas. The
buoy is
connectable to a towing vessel by a tow cable (not shown) attached to the tow
point 4.
The buoy is configured as follows:
= Tow Point Translation - acting to balance hydrodynamic and hydrostatic
forces. At
slow speeds the tow cable orientation is dominated by the positive trim of the
buoy
resulting in an almost vertical link profile. At medium to high speed, buoy
and cable
drag takes dominance and so a more horizontal profile of the link results.
This can be
achieved using either a passive or active control mechanism. The passive
method
could harness changes in tow cable tension and or drag forces to translate the
tow
point. A simple passive method could use a spring to oppose the tow cable
tension and
locate the tow point accordingly. An active mechanism could use a control loop
system to translate the tow point position according to the submarine's speed
and or
tow cable tension levels.
= Buoy Buoyancy - Nett buoyancy (buoyancy subtract mass) is varied by changing
the
mass alone, through the ballast tank. This is important for low tow speeds
when the
buoy function has to support variable vessel depths and thus changes in tow
cable
mass (assuming tow cable is not neutrally buoyant).
= Metacentric Height - directly affecting roll and pitch stability. This is
important
when maintaining control in the dynamic wave zone. Stability improves with
increasing vertical separation of CoG and CoB.
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Orbital Particle Motion
Orbital particle motion is the movement of particles in the water column below
a
passing surface wave. Its effect on the buoy 1 will be apparent from the
following
sections, and a brief introduction to this motion will now be described. As
each wave
5 passes along the water's surface the particles in the water column below
complete an
orbital path; moving upwards on the leading edge of the wave, then
horizontally in the
direction of the wave at the top, then downwards at the back edge of the wave.
In deep
water this effect is experienced for many meters below the surface, eventually
decaying to less than 5% at a depth approximately half the wavelength. An
illustration
of the particle movement is shown in Figure 3 along with a graph of the
typical orbital
velocities that occur in sea state 6 at a depth 5m below the mean surface
elevation. As
can be seen, this orbital particle motion does include a significant vertical
and
horizontal velocity.
Buoy Performance
To evaluate the ability of the buoy I to maintain a shallow depth across the
operational envelope a series of simulations were performed, using the setup
shown in
Figure 4.
Performance of the buoy 1 has been evaluated for 0, 3 and 6 m/s submarine
speeds
(V) and in sea states (SS) 3 and 6. Submarine depth (D) was maintained at 102
in. The
link length (1) was varied according to the submarine speed and sea state
conditions.
The following two sections present the results of the two sea state conditions
evaluated.
Sea State 6
To maintain a depth of less than 3 m means that the buoy must attain a
position that
will be above the trough of many of the larger waves. Extracts of the
resulting buoy
depth and pitch profiles from the 6 m/s tow in sea state 6 are shown in
Figures 5a and
5b, respectively. The vertical translation of the buoy is generated purely by
the orbital
motion of the water particles.
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This helps to minimise depth variation of the buoy as it passes beneath the
waves. The
buoy however will react to the localised particle flows at a slower rate due
to its size
and the tow cable constraint, and so relative depth will change according to
the wave
height and period.
Buoy pitch was limited to 150 and was generated by the variable particle
vertical
velocity acting across the length of the buoy and variations in tow cable
tension. The
stability of the buoy is very good, with the relatively small variations in
both pitch and
depth being directly linked to wave motion.
At 3 m/s the buoy still attains vertical motion, caused by the wave particle
motion but
the increasing depth motion, occurring at the back of the waves, appears to
have a
larger influence than that driving it up. When each wave passes over, the buoy
gets
pushed below the trough of the wave and it takes several seconds for the
buoyancy to
bring the buoy back up to the optimum depth.
The effect of the down-flow being more influential than the up-flow is also
evident
during the 0 m/s case (see Figures 7a and 7b). Here the limited tow cable
prevents the
buoy from climbing above 2 m depth and so the advantage of the up-flow is
lost. The
effect of the down-flow however is not restricted and so the buoy experiences
this for
a longer time period.
Sea State 3
With its much smaller wave elevations, it is not necessary for the buoy to
enter the
wave structure so the buoy maintains a much safer distance from the surface
with little
chance of broaching.
Figures 8a and 8b (6 m/s case), Figures 9a and 9b (3 m/s case), and Figures 1
Oa and
10b (0 m/s case) show the reduced effect of the particle motion upon both the
depth
and pitch of the buoy at sea state 3. With the slight adjustments to trim and
tow point
position the buoy is able to maintain depth at all speeds evaluated.
Tow Cable Study
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The submarine pulls the tow cable and buoy through the water but it is up to
the buoy
to lift the tow cable towards the sea surface. The natural lie of a cable
being pulled
through the water is directly behind the submarine due to the drag forces
acting on it
(depending on the tow cable trim).
To achieve the same climb height (H) for a fixed length (350 m) of tow cable
at a tow
speed of 6 m/s requires as much as four times more lift than at 3 m/s. This is
tow cable
length dependent, reducing to a factor of three for longer tow cables.
To double the height (H) of the buoy from 100 m to 200 m without changing
submarine speed or cable length (350 m) requires an increase in lift demand by
a
factor of four. This reduces slightly to a factor of three for longer tow
cables.
Tow cable weight has a considerable impact on the amount of lift needed to
raise it.
The size of the impact is speed and tow cable length dependent with slow long
tows
being the most sensitive.
Impact of Tow Cable on Buoy
Assuming a typical tow cable having a diameter of 0.02m, mass in air of 0.962
kg/m,
CD Axial of 0.015 and CD Normal of 0.4, the drag forces generated in relation
to the
tow cable are far greater than the drag forces of the buoy alone. The
performance of
the buoy will be therefore dominated by the necessity to lift the tow cable
and so in
almost all cases except very low speeds (0 to 0.5 knots) the buoy stability
will be
dependent upon three forces, namely the buoy drag, lift and tow cable tension.
The ability of the towed buoy system to deliver the payload into its operating
zone is
dependent upon the buoy being able to generate the large lift forces needed to
control
the tow cable. The two most common methods of generating significant lift are
through water displacement (buoyancy) and dynamic lift using hydrofoils.
Buoy Lift
The lift coefficient of a hydrofoil is dependent upon its angle of attack with
respect to
the approaching water. Small changes in the angle of attack can result in
large changes
in the lift coefficient resulting in rapid changes to the buoy's depth. A buoy
that
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utilizes the lift generated by a hydrofoil needs to be stable in pitch to
prevent rapid
changes in depth.
The buoyancy approach removes the dependency upon through water speed and
angles of attack, ensuring a constant lift force is available to hold the tow
cable high in
the water.
The hydrofoil on the buoy ensures it is capable of generating the lift
required to
remain above the submarine. To maintain a relatively constant lift coefficient
the
stability of the buoy in pitch needs to be high. An increase in stability can
be achieved
by offsetting the hydrofoil generated forces and the tow cable connection
point on the
buoy. For small angles of attack the variation in drag produced by the
hydrofoil are
virtually negligible since drag is less than about 2% of the lift generated
force.
Instead of pitch generating lift fluctuations, these are caused by changes in
the buoy's
through water speed if the pitch is held approximately constant as the buoy
travels in a
passing wave. The orbital particle motion created by the waves interacts with
the
hydrofoil (hydroplane). The hydrofoil lift reduces as the buoy passes through
the
trailing section of the wave and so the tow cable tension pulls the buoy down.
As the
buoy enters the next wave the hydrofoil lift increases again pulling the buoy
and tow
cable up into the wave structure.
The buoy pitch stability is improved by providing a significant vertical
offset between
the buoy CoG and CoB. A mean nose up pitch orientation of around 8 degrees
generates the required lift. The buoy pitch varies by around +/_ 8 degrees
over a five
minute period, well within the optimum lift range of the NACA0012 hydrofoil
used in
the study. This ensures the changes in the drag coefficient of the hydrofoil
remain
negligibly small.
Optimized depth performance for a 3 m/s tow speed at 100 in depth is achieved
with
150 m of tow cable deployed and with 50 kg of displacement buoyancy assigned
to
the buoy by reducing its mass.
Figure 11 shows the depth performance in sea states 3, 5 and 6, with all
achieving at
least 85 % of the simulation period at less than 3 in depth without broaching
the
surface.
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Tow Speed
At 0 m/s the hydrofoil lift is limited to that generated by the particle
orbital motion
and the movement of the buoy as it is pushed around in the waves. This is
unreliable
and so additional buoyancy has to be introduced to support the tow cable. In
low sea
states (0 to 4), the buoy attains a compliant depth performance. In sea states
5 and 6
the performance is less reliable and there is an increased risk of surface
broaches.
At 6 m/s the buoy is less stable than at lower speeds. By increasing the
vertical and
horizontal offset between the hydrofoil and tow cable termination positions,
stability
improves sufficiently. For the 100 m depth case the tow cable length has to be
increased to 230 m. Depth performance with this revised buoy/tow cable
configuration
is consistent with that at 3 m/s.
Trim Control
The dynamic systems for inclusion in the buoy design to trim the buoy's
performance
are:
Moving Buoy Tow Point
At low speeds the metacentric forces act to stabilize the buoy in roll and
pitch but as
speed increases the hydrodynamic forces dominate and any stability provided by
the
buoyancy and mass becomes negligible. It is therefore important that the tow
point
can be moved to balance hydrodynamic forces at high speeds and hydrostatic
forces at
low speeds to improve stability across the operating range. As tow speed
increases,
the offset between the tow point and the CoG increases both vertically and
horizontally. This could be controlled passively by harnessing the changes in
the
tension or drag forces, or alternatively, actively by means of an electrically
powered
servo motor as part of a closed loop controlled system.
Variable Buoyancy
Buoyancy is crucial for very low submarine speeds when the hydrodynamic lift
of the
buoy is insignificant. The mass of the tow cable will have a large effect upon
the buoy
performance, and this will change according to the submarine depth. The depth
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keeping performance of the buoy is dependent on buoyancy across the operating
speed
range, therefore some variation in the buoyancy of the buoy will be required.
The
amount of buoyancy variation required is tow cable mass dependent with the
slow
deep submarine case being most reliant.
5 Active Hydrofoil
The depth keeping requirements for the buoy can be met with a fixed hydrofoil
which
reduces the complexity of the buoy. However, an active hydrofoil capable of
increasing its surface area and/or varying its pitch angle may be used.
Having the ability to make these adjustments will not only widen the
operational
10 range of the system but also provide additional flexibility for operations
such as
surfacing, deployment and recovery, ensuring minimal risk of damage to the
buoy and
towing platform.
Moving Keel
The stability of the buoy can be enhanced for high speed operation by
increasing the
separation between CoB and CoG by use of a moveable keel. The moveable keel
can
be retracted to minimize the height of the buoy for storage within the vessel
bay.
Control
The buoy contains actuators to tune tow-point location, ballast/buoyancy,
hydrofoil
configuration and keel position, but these are only used to tune the buoy
hydrodynamic characteristics for the specific speed and depth of operation. As
the
boat speed and depth do not vary continuously, the actuators on the buoy are
only
required to change position occasionally or slowly.
There are significant advantages to this approach to controlling the buoy.
Firstly, the
buoy has a minimum number of closed-loop control systems and no fast acting
actuators meaning it is much simpler to design, test and prove than a closed-
loop
control system. The slow response of the actuators also reduces their power
requirements and allows them to be made smaller. The position of each actuator
can
be predefined for the current boat speed and boat depth, with these values
being
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determined from modeling and tank trials. This significantly reduces the
modeling and
test time required to ensure that a complex closed-loop control system is
stable under
all conditions, and replaces it with the requirement to prove a number of
fixed
configurations of buoy for the differing operating conditions.
The fine control of the buoy depth is performed on a closed loop basis by
adjusting the
length of tow-cable deployed.
Buoy Design
Mechanical
As shown in Figures 1 and 2, the buoy 1 has a streamlined body 2 with a
hydrofoil 3.
Onboard actuators (passive and or active) control the buoyancy, the hydrofoil
length,
angle of attack and the tow-point location. The buoy may be manufactured from
composite materials with static buoyancy provided by sections of high density
foam.
The payload electronics are contained within pressure vessels, with the
actuators
mounted externally from the electronics.
Actuator Control
Control of active actuators in the buoy will be commanded from the towing
vessel,
with the actuators moving to predefined positions for the specific speed and
depth of
the vessel.
At low speeds, the buoyancy will be increased and the tow-point moved aft and
down.
This acts to stop the buoy pitching up at too great an angle as the natural
buoyancy
dominates the hydrodynamic lift of the buoy. At higher speeds, the buoyancy
can be
reduced as the hydrofoil provides all the lift required, and a forward located
tow-point
provides stability.
Additionally, as the depth of the boat increases, the lift required from the
buoy will
need to be increased to overcome the increased drag and weight of the longer
tow-
cable and so the hydrofoil and buoyancy need to be adjusted accordingly.
Alternative Buoy Designs
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Figure 12 illustrates a side elevation of a second example of a buoy of this
invention.
The buoy 101 has a hydrofoil body 102, a moveable keel 103, and a moveable tow
point 104. The body 102 houses a payload, an adjustable ballast tank 105, a
tow point
actuator, and an actuator for moving the keel 103.
The tow point actuator is operable to move the tow point 104 between a high,
forward
position for high speed tow, and a low, aft position for low speed tow. The
tow point
actuator includes a wheel 104a, which can rotate to move the tow point between
these
two positions. The wheel 104a is rotatably mounted on the forward end of the
body
102. This contrasts with the linear movement of the tow point 4 in the buoy I
of the
first example described previously. However, the tow points 4 and 104 share
the
common feature of enabling translational movement of the tow point with
respect to
the remainder of the buoy, under active or passive control. The two types of
tow point
are interchangeable on the buoys 1 and 101.
The keel actuator is operable to adjust the position of the keel 103. The keel
103 is a
heavy, streamlined body of generally circular cross section rotatably mounted
on one
end of keel arm 103a, whose other end is rotatably mounted on the underside of
the
body 102. Under the action of the keel actuator, the keel 103 can be moved
between a
high, retracted position and a low, deployed position. When in its retracted
position,
the buoy 101 occupies a smaller space for storage, e.g. onboard the towing
vessel.
When in its deployed position, the CoG of the buoy 101 is moved aft and
downwardly, increasing the metacentric height, and therefore the stability, of
the
buoy. The position of the deployed keel can be adjusted to vary the position
of the
buoy CoG. However, it is likely that the keel 103 will be typically fully
deployed to
maximise the stability of the buoy 101 in pitch and roll. The keel 103 may be
used to
house control electronics, if desired.
The adjustable ballast tank 105 is similar to the ballast tank 5 of the buoy 1
described
previously, and operates similarly.
The buoy 101 is connectable to a towing vessel by a tow cable (not shown)
attached to
the tow point 104. The buoy may be operated in a similar manner as for the
buoy 1
described previously.
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Figures 13a to 13d illustrate isometric, front, back and side elevations of a
third
example of a buoy of this invention. The buoy 201 has an upper hydrofoil body
202, a
pair of lower hydrofoils 203 each mounted from the hydrofoil body 202 by a
pair of
end plates 203a, and a moveable tow point 204. The body 202 houses a payload,
and a
tow point actuator. The tow point actuator may be a linear or rotary actuator
similar to
that used in the first or second examples of the buoy described previously for
moving
the position of the tow point 204. An adjustable ballast tank may be disposed
in one or
more of the upper hydrofoil 202 and the lower hydrofoils 203. The ballast
tank(s) may
be similar to those used in the first and second examples of the buoy
described
previously for adjusting the buoyancy of the buoy. One or more actuators may
be
provided for moving the hydrofoil 202 and/or the hydrofoils 203 for adjusting
the lift
generated by the buoy under tow. The buoy 201 is connectable to a towing
vessel by a
tow cable attached to the tow point 204. A tow point guide rail 206 extends in
an arc
from the upper hydrofoil 202 and between the two lower hydrofoils 203. The end
of
the tow cable, or tether, (not shown) incorporates a slider block at one end
which
slidably engages with the rail 206 to follow the contours of the rail
according to the
local catenary shape. The guide rail 206 acts as a manual means of adjusting
the tow
point position. A vertical profiled tether will self locate to the lower
central section of
the rail 206, and more horizontal tether will locate on the higher forward
rail section.
The buoy may be operated in a similar manner as for the buoys I and 101
described
previously.
Although the invention has been described above with reference to one or more
preferred embodiments, it will be appreciated that various changes or
modifications
may be made without departing from the scope of the invention as defined in
the
appended claims.
