Note: Descriptions are shown in the official language in which they were submitted.
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TOW1NG ASSEMBLY
The present invention relates to faired towing cables used on ships for
towing a submersible body launched at sea and to the handling of these
cables. It relates more particularly to towing cables which are faired using
scales or portions articulated to one another. R also applies to any type of
faired
elongate element intended to be at least partially submerged.
The context of the invention is that of a naval vessel or ship intended to
tow a submersible object such as a variable-immersion sonar incorporated into
a towed body. In such a context, in the non-operational phase, the submersible
body is stored on board the ship and the cable is wound around the drum of a
winch used for winding in and paying out the cable, namely for deploying and
recovering the cable. Conversely, in the operational phase, the submersible
body is submerged behind the ship and towed by the latter using the cable, of
which the end connected to the submersible body is immersed. The cable is
wound in/paid out by the winch through a cable guiding device that allows the
cable to be guided.
In order to obtain a high degree of immersion at high towing speeds,
the towing cable is faired to reduce its hydrodynamic drag and to reduce the
vibrations caused by the hydrodynamic flow around the cable. The cable is
covered with a segmented fairing made up of rigid fairing elements having
shapes intended to reduce the hydrodynamic drag of the cable. The purpose
of the sheath made up of the fairing elements is to reduce the wake turbulence
produced by the movement of the cable through the water, when this cable is
immersed in the water and towed by the ship. For great immersion depths that
go hand-in-hand with high towing speeds of at least 20 knots, the fairing
elements need to be rigid. Flexible fairings are of benefit only for
economically
profiling chains or cables for buoys subjected to marine currents or, at
worst,
towed at speeds of 6 to 8 knots. In the case of the use of rigid fairing
elements,
segmenting the fairing into fairing elements is necessary sa that the cable
can
pass through guide elements of the pulley type, and sa that lateral cable
deflection can be tolerated in case the ship changes heading and also sa as
to be able to be wound onto the drum of a winch.
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2 , .
In the normal operating state, the fairing elements are mounted with the
ability to rotate about the longitudinal axis of the cable. This is because it
is
necessary for the fairing elements to be able to rotate freely about the cable
so as to be correctly oriented with respect to the stream of the water.
However,
each fairing element is connected to its two neighbors axially and in terms of
rotation about the cable in such a way as to be able to pivot with respect to
these about an axis parallel to the axis x by a maximum angle that is small,
of
the order of a few degrees. This link between the fairing elements in
particular
allows the fairing as a whole to pass fluidly through ail the guide elements.
As
a result, the rotation of one of one fairing element leads to a rotation of
its
neighbors and so on and so forth through the entire set of fairing elements.
Thus, both when the cable is deployed in the water and when it is wound
around the drum, any change in orientation of one of the fairing elements has
a knock-on effect on ail of the fairing element fairing the cable. Thus, when
the
cable is deployed at sea, the fairing elements naturally orient themselves in
the direction of the current generated by the movennent of the vesse!.
Likewise,
the guide device is conventionally configured to orientate and guide the
fairing
elements that pass through it in such a way that these exhibit a predefined
orientation with respect to the drum of the winch, ail the fairing elements,
as
the cable is raised, adopting one and the same orientation relative to the
drum,
which orientation allows the cable to be wound in keeping the scales parallel
to one another from turn to turn.
Now, the applicant company has found that, when the faired cable is
wound around the drum of a winch so as to recover the towed body, the fairing
sometimes becomes severely damaged or even crushed as it passes through
the guide devices, this being something which may render the entire sonar
system unavailable. It may even happen that this damages the guide device.
By way of example, certain variable-immersion sonar systems installed on
certain ships and operated in the normal way by military crews encounter
fairing-element-crushing problems approximately once a year and sometimes
far more frequently. This crushing may have limited consequences but may
also degenerate or jam the winch or damage it, and thus cause the entire
towing system and therefore the sonar to become unavailable.
It is one object of the present invention to limit the risks of damage to
the fairing of a towed cable.
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3
To this end, the applicant company has first of ail, in the context of the
present invention, identified and studied the cause of this problem of the
fairing
elements becoming crushed by observing the faired cable in an operational
situation and by modeling the faired cable in an operational situation and by
modeling the various forces acting on it, notably the hydrodynamic and
aerodynamic flows, and the force of gravity.
Du ring the operational phase, the faired cable is towed by the ship and
has one end immersed. Very often, the tow point of a cable or of a fairing is
a
point on a pulley which is situated a certain height above the water. What is
meant by the tow point is the position of the point at which the cable bears
against a device on board the ship, which is closest to the immersed end of
the cable or respectively of the fairing. As the ship moves forward, under the
action of drag, the cable moves away from the transom to disappear beneath
the water a little further afield than a point vertically below the tow point.
The
length of faired cable that is airborne is increased in comparison with the
simple height of towing above the water because the cable is inclined with
respect to the vertical. It is found that the last fairing element still
engaged with
the ship, namely the fairing element which is at the tow point, often resting
on
the pulley or resting on a guiding device on board the ship, is oriented
correctly
in the direction of the flow even though it is considerably higher up in the
air
(leading edge facing into the flow and trailing edge trailing). The first
fairing
element in the water (namely the fairing element that has just been immersed)
is assumed to adopt a correct orientation in the flow stemming from the speed
of the ship (leading edge facing into the flow and trailing edge trailing).
However, between these two remarkable fairing elements, the string of fairing
may twist because, in the air, it is subjected only to vibration, to an
insignificant
flow of air and to the effect of gravity. Under the effect of the influences
of the
sea, of the towing conditions and of the waves, situations whereby this
airborne
string twists are regularly observed. The first cause of twisting is the
effect of
gravity as soon as the cable moves away from the vertical, something which it
has to do as soon as the towing speed becomes sufficient. Under the effect of
gravity, the string of fairing between the tow point and the sea will twist to
one
side (in the air) and then straighten up (in the water). This is the nominal
situation of the string of fairing. This twist is dependent on the intrinsic
stiffness
of the string of fairing and ars on the length airborne. A situation in which
the
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4
airborne part of the fairing 2 is a little twisted, namely experiences torsion
about
the axis of the cable, is depicted in figure 1A. In figure 1A, the vertical
direction
in the Earth's frame of reference is represented by the axis z and the
orientation of the section of certain fairing elements in zones A, B and C
delimited by dotted une has been depicted. In the situation depicted in figure
1A, the last fairing element 3, which is engaged with the ship, is oriented
vertically (trailing edge uppermost) as depicted in zone A. The fairing
elements
that are in the air between the pulley P and the water surface S are lying
down
under the effect of gravity. In other words, as visible in zone B, the
trailing edge
of the fairing elements is oriented downward (between the pulley P and the
water surface S, the fairing elements have rotated about the cable). By
contrast, the fairing elements that are in the water have straightened up
under
the action of the flow of water acting in the direction of the arrow FO as
depicted
in zone C (trailing and leading edge both situated at approximately the same
depth).
Occasionally, depending on the sea conditions, with green seas or
waves breaking more or less over the transom of the ship, the airborne part of
the cable then temporarily experiences flow in the opposite direction to that
prevailing lower down and which corresponds to the speed of forvvard travel of
the ship. These packets of water are perfectly capable of twisting the string
of
fairing still further and of placing it in opposition to the position expected
in the
normal towing stream. When that happens, the fairing is twisted and makes a
half-turn about the cable in its airborne part. That means that two fairing
elements of the airborne part of the string of fairing have trailing edges
that
between them form an angle of 180 degrees around the cable. The part of the
fairing situated between these two fairing elements is twisted or in torsion.
Starting out from this situation, it may happen that these parts of fairings
which
are therefore the wrong way round with respect to the mean stream imposed
by the speed of the ship then suddenly find themselves immersed in this mean
stream again (because of the movements of the ship, that of the waves, etc.)
so the part of the fairing that is the wrong way around is therefore urged to
return to the right direction (the direction associated with the normal mean
stream). It may then:
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¨ cancel its half-turn and return to its initial position by making the
opposite rotation to the rotation that led it to become the wrong way round.
It
then finds itself correctly oriented.
¨ or add to the existing half-turn a further half-turn which returns it to
the
5 correct orientation in the stream but has the effect of twisting the
airborne part
of the fairing above it by 1 turn (or 3600) and of similarly twisting a
portion below
it by one turn (or 360 , but this time in the other direction). The part which
was
initially the wrong way round has returned to the correct orientation in the
main
stream associated with the speed of the ship, but this has resulted in two
twistings by one turn, one of the above it in the air and the other below it
in the
water. The name given to this is a full twist of the fairing. This full twist
is a
stable situation of the string of fairing or of the fairing 2. It is depicted
in figure
1B. This situation may be described as follows: between the tow point R and
the water surface S, the string of fairing makes a full turn in the direction
of
the arrow F1 about the cable. The string of fairing 2 passes through the
surface
S and remains correctly oriented over a certain length L of a few meters or
sometimes less. The string of fairing 2 then makes a complete revolution in
the
water, in the opposite direction, depicted by the arrow F2, to return to the
correct orientation in the stream. In other words, the fairing undergoes a
double
full twist about the cable. The double twist comprises an airborne full twist,
situated above the water surface and an immersed full twist situated below the
water surface. Ail of the part of the fairing that is situated below this
double full
twist is now completely unaffected by what happens above it (its fairing
elements are correctly oriented in the stream).
The configuration in which the fairing undergoes a double twist is stable
but highly degraded and carnes a high risk of subsequently introducing a great
deal of disturbance into the entire system.
The applicant company has discovered that when a fairing experiences
a complete double twist, under certain conditions, the fairing will become
very
much deteriorated in the water and this deteriorated part will cause a great
deal of damage to the faired cable or even to the entirety of the faired
system
as the cable is being wound in and, more specifically, as it passes through
the
cable guiding device.
By analyzing the complete double twist, the applicant company has
found that the submerged twist can be considered to be "caught" on the cable.
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In other words, the position of the submerged twist is fixed with respect to
the
cable along the axis of the cable. By contrast, its airborne counterpart, the
airborne twist, remains situated at the same point between the tow point R and
the water surface S. It is not fixed with respect to the cable along the axis
of
the cable but fixed with respect to the water surface S or to the tow point.
When
the cable is hauled in or lowered, the fairing elements experiencing the
submerged twist follow the movement of the cable which is being hauled in or
lowered, while the airborne twist remains fixed with respect to the water
surface. From this it follows that a paying-out of the cable causes the
submerged twist to sink to a greater depth while the airborne twist remains in
the same place with respect to the water surface (so the 2 twists move further
apart). Figure 1C depicts a situation in which the cable has been paid out
with
respect to the situation of figure 1B (see arrow). The distance L2 represents
the distance between the part of the fairing affected by the submerged twist
and the point at which the fairing enters the water is greater than the
distance
L1 which represents this same distance in the situation of figure 1B.
Conversely, a hauling-in of the cable, with respect to the situation of figure
1B,
in the direction of the arrow represented in figure 1D, causes the submerged
twist to rise while the airborne twist still remains in the same place with
respect
to the water surface (so the two twists move doser together).
It is then necessary to examine what happens for a twist of one turn that
is immersed and towed in that state. This twist which deploys over a small
height forces the fairings to travel backwards or across the stream. The
action
of the stream on these fairing elements is therefore very great (proportional
to
the surface area, angle, density of the water and the square of the speed) and
this action manifests itself in the form of powerful torsional moments which
tend to force the fairing elements to align in the stream but they corne up
against the stiffness of the turn of twist which therefore increases. What
happens then is that a balance is struck and that the one-turn twist finds
itself
very much restricted in height and the fairing experiences violent loadings
which will tighten the submerged twist under the effect of the towing speed.
In
other words, the full turn of the fairing about the cable will take place over
an
ever-shortening distance. Observations at sea have shown that the string of
fairing could effect one full turn around the cable over a length of under 50
cm.
During towing, the hydrodynamic stream applies a very high torque to the
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7
incorrectly oriented fairing elements which may go so far as to damage the
fairing or even as to completely break the fairing elements.
When a submerged twist is hauled in, the fairing has been very highly
stressed for a long time and retains the memory of its deformation (namely of
its twisting), and the submerged twist cornes out of the water still very
tightly
twisted during hauling and does flot disappear during the hauling. This is
referred to as remanent twist. Depending on the length of time for which the
fairing has been exposed to this submerged twist and towed, the submerged
twist may be able to become permanent or very slow to be reabsorbed, making
it completely unable, for a fairly long period of time, to engage in the cable
guide device even though the continuity of the fairing is unbroken. On the
airborne twist side there is no damage; although there is a twist applied it
is
not at any time capable of damaging the cable.
When the still very tightly twisted submerged twist then arrives at the
guide device, for example the pulley, the fairing elements affected by this
submerged twist are unable to position themselves correctly in the guide
device, notably in the pulley, and they jam in the guide device. If that
happens,
then the entire string of fairing that enters the guide device afterwards will
be
methodically destroyed if hauling is continued because each fairing element
will, in sequence, follow the orientation of the one before it. This situation
may
even cause the guide device to break.
The invention proposes a guide device configured in such a way as to
limit the risk of damaging the cable fairing.
Ta this end, one subject of the invention is a towing assembly
comprising an elongate element faired by means of a fairing comprising a
plurality of fairing elements, the fairing elements which comprising a canal
intended to accept the elongate abject and being profiled in such a way as to
reduce the hydrodynamic drag of the elongate abject when the elongate abject
is at least partially immersed, said fairing elements being pivot-mounted on
the
elongate element around the longitudinal axis of the canal, the towing
assembly further comprising a towing and handling device intended to tow the
faired elongate element while the latter is partially immersed, the towing
device
comprising a winch allowing the faired elongate element to be wound in and
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8
. .
paid out through a guide device that allows the elongate element to be guided,
the guide device comprising a first groove the bottom of which is formed by
the
bottom of the groove of a pulley, the first groove being delimited by a first
surface having a profile that is concave in a radial plane of the pulley, the
width
of the first groove and the curvature of the profile of the first curved
surface in
the radial plane being determined in such a way as to allow the fairing
element,
under the effect of the rotation of the fairing element about the axis of the
elongate element under the effect of the traction of the elongate element with
respect to the guide device along the longitudinal axis thereof, to flip from
a
turned-over position in which the fairing element is oriented with its
trailing
edge toward the bottom of the first groove into an acceptable position in
which
it is oriented with the leading edge toward the bottom of the first groove.
Advantageously, the width of the first groove and the curvature of the profile
of
the first curved surface in the radial plane are determined as a function of
the
radius R of the pulley, of the maximum length CAR, measured parallel to the
chord separating the trailing edge of the fairing elements of the fairing from
the
axis of the elongate element, of the maximum chord length LC of the fairing
elements and of the maximum thickness E of the fairing elements.
Advantageously,
the guide device comprises a first groove the bottom of
which is formed by the bottom of the groove of a pulley, the first groove
being
delimited by a first surface that is concave, of which the cross section a
radial
plane of the pulley is a first concave curve comprising the bottom coinciding
with the bottom of a second, reference, groove delimited by a second curved
surface of which the cross section in the radial plane BB is a V-shaped
reference curve, the aperture of the V being at least equal to twice a
threshold
angle as, and the width of the V lv, measured along a straight line d parallel
to
the axis of the pulley, is at least equal to a threshold width Is, given by:
/.9 = 0.7 * lid
lid = 2 (LC + E) * sin(as)
R
as = ai * R ¨ CAR
where ai is a limit angle greater than 450 and less than 90 , where R is the
radius of the pulley and where CAR is the maximum distance separating the
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9
trailing edge BF of the fairing elements of the fairing from the axis of the
elongate element, measured parallel to the chord CO of the fairing
elements, where LC is the chord length of the fairing elements and E is the
maximum thickness of the fairing elements,
in which the first curie is coincident with the second curie at two endpoints
of the reference curie, the first curie is at every point comprised between
each of the endpoints and the bottom coincident with the second curie or
doser to the axis of the pulley than the second curie along the radius of the
pulley in the radial plane.
Advantageously, the limit angle ai is given by the following formula:
Tr 1
cri = ¨4 + -2Arctan (C n
where Cf is the coefficient of friction between the material that forms the
exterior part of the tau l of the fairing element and the material that forms
the
surface delimiting the groove of the pulley.
Advantageously, the first groove is the groove of the pulley.
Advantageously, the concave first curie has a U-shaped profile
between the endpoints.
Advantageously, the fairing elements comprise a fairing element
comprising a nose accepting the elongate element and comprising a leading
edge and a tail of streamlined shape extending from the nose and comprising
a trailing edge, the concave first curie is defined in a radial plane of the
pulley
such that, when the fairing element extends with the leading edge
perpendicular to the radial plane, whatever the position of a fairing element
in
the first groove, when the nose of the fairing element is bearing on the
concave
first curie and the elongate element is exerting on the fairing element, in
the
radial plane, a force to press the nose of the fairing element against the
pulley,
said pressing force Fp comprising a component CP perpendicular to the axis
of the pulley and a lateral component CL, the trailing edge of the fairing
element is flot in contact with the concave first curie or is in contact with
a part
of the concave first curie that forms, with a straight line dp of the radial
plane
perpendicular to the axis xa extending from the axis of the elongate element x
as far as the trailing edge of the fairing element, an angle y that is at
least equal
to an angle of slip at. The angle of slip is given by the following formula:
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at --= Arctan (Cf)
where Cf is the coefficient of friction between the material that forms the
5 exterior
part of the tail of the fairing element and the material that forms the
surface delimiting the groove of the pulley.
Advantageously, the first curve has a U-shaped profile and has a central
zone of width equal to g*lid, where lid is the ideal width and g is comprised
between 0.7 and 1, between the endpoints coinciding with the endpoints of the
10 reference
curve having a width equal to g*lid, the central zone being delimited
by the following two curves:
- an upper curve having a first radius of curvature R1 radius equal
to 1/2* g*lid passing through the bottom and the center of which
is situated on a straight line perpendicular to the axis of the
pulley passing through the bottom,
- a lower curve INF comprising a central portion CENT extending
substantially parallel to the axis of the pulley symmetric with
respect to a plane perpendicular to the radial plane passing
through the bottom and extending, along the axis of the pulley,
over a first width equal to g*lid and comprising, on each side of
the central portion CENT, lateral portions LAT1 and LAT2
connecting the central portion to the endpoints 133, 134 and
having a second radius of curvature R2 equal to 1/4*g*lid.
Advantageously, the fairing elements are rigid.
Advantageously, the fairing comprises a plurality of fairing portions,
each fairing portion comprising a plurality of fairing elements joined
together
along the axis of the elongate element and articulated to one another, the
fairing portions being free to rotate about the axis of the elongate element
relative to one another.
Advantageously, the fairing portions have respective heights along the
axis of the canal, these heights being defined as a function of the angular
stiffnesses k of the respective fairing portions, and as a function of the
chord
length LC of said fairing elements of said respective portions so as to
prevent
a full twist from forming on said respective portions.
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Advantageously, the fairing portions have respective heights that are
less than a maximum height hmax such that:
n- * k
hmax < __________________________________________
F LC2
where F is a constant comprised between 250 and 500.
Advantageously, at least one fairing element comprises a leading edge
and a trailing edge, comprising a bearing edge comprising a first bearing edge
that is mitered with respect to the leading edge, the first bearing edge being
arranged in such a way that the distance between the leading edge and the
bearing edge, measured perpendicular to the leading edge, decreases
continuously, along an axis parallel to the leading edge, from a first end of
the
first bearing edge to a second end of the bearing edge, said fairing element
being referred to as a mitered fairing element.
Advantageously, the bearing edge is arranged in such a way that the
distance between the bearing edge and the leading edge decreases
continuously, along an axis parallel to the leading edge, from the first end
of
the first bearing edge to a first lateral face of the fairing element closer
to the
second to the first bearing edge than to the first end of the bearing edge.
Advantageously, the bearing edge is the trailing edge.
Advantageously, the mitered fairing element is sized in such a
way as to be more resistant to a pressure loading, applied in a direction
perpendicular to the leading edge and connecting the leading edge to the
trailing edge, than the other fairing elements.
Advantageously, the mitered fairing element comprises two parts back
to back along the first bearing edge, the fairing element being configured to
be
kept in a deployed configuration when subjected to the hydrodynamic flow of
the water, the two parts being arranged, relative to one another about the
first
bearing edge, in such a way that the fairing element has a trailing edge
parallel
to the leading edge and a cross section that is constant along the leading
edge
and configured in such a way as to allow relative pivoting between the two
parts about the first bearing edge when a torque inducing relative pivoting
between the two parts, applied about an axis formed by the first bearing edge,
exceeds a predetermined threshold so that the fairing element passes from
the deployed configuration into a configuration fol ded about the bearing
edge.
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Advantageously, of said portions, at least one comprises at least one
end fairing element, adjacent to one single other fairing element belonging to
said portion, having a bearing edge comprising a first bearing edge which is
mitered with respect to the leading edge, the first bearing edge being
arranged
in such a way that the distance between the leading edge and the first bearing
edge, considered perpendicular to the leading edge, decreases continuously,
along an axis parallel to the leading edge, from a first end of the first
bearing
edge to a second end of the first bearing edge, further away from the other
fairing element than the first end, along the axis parallel to the leading
edge.
Advantageously, the fairing elements are rigid.
Other features and advantages of the invention will become apparent
on reading the detailed description which follows, given by way of non-
limiting
example and with reference to the appended drawings in which:
- figure 1A, already described, depicts a faired cable, faired by
means of rigid fairing elements joined axially together, towed partially
immersed from its immersed part as far as a guide pulley in a situation in
which
the cable does flot experience a double twist, figure 1B depicts the cable of
figure 1A in the same state of immersion (namely of winding-in and of paying-
out) as in figure 1A, but experiencing a double twist; figure 1C depicts the
cable
of figure 1A with the double twist of figure 1B in a configuration in which
the
cable has been paid out in relation to figure 1B; figure 1D depicts the cable
of
figure 1A exhibiting the double twist of figure 1B in a configuration in which
the
cable has been hauled in in relation to figure 1B,
¨ figure 2 schematically depicts a ship towing a towed object by
means of a faired cable,
¨ figure 3 schematically depicts a portion of faired cable according
to the invention faired using a fairing according to the invention,
- figure 4a depicts a cross section of a fairing element of the fairing
according to the invention on the plane of section AA depicted in figure 2,
figure
4b schematically depicts a side view of the fairing element of figure 4a in
the
direction of the arrow b,
- figure 5 schematically depicts a portion of faired cable according
to the invention entering a cable guide pulley,
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- figures 6a and 6b depict cross sections of a pulley according to
the prior art, on the lateral face of the fairing element entering with the
trailing
edge toward the bottom of the groove, at the moment at which it cornes to bear
against the pulley (figure 6a) and then afterwards when the cable has been
pulled to the right in figure 5 (figure 6b), namely when the cable has been
hauled in and its tension has crushed the fairing element,
- figure 7 depicts a partial cross section on a radial plane BB (see
figure 5) of one example of a pulley according to a first embodiment
embodiment of the invention, and a reference curve,
- figure 8a schematically depicts a section of a pulley, according to
a second embodiment of the invention, in a plane formed by a lateral face of
the first fairing element coming into contact with the pulley (equivalent to
the
plane M in figure 5), comprising the point of contact with the pulley, figures
8b
and 8c depict sections of the pulley on planes successively occupied by the
same lateral face of the fairing element as the cable is wound in,
- figures 9a and 9b depict sections, on radial planes, of two
exarnples of pulleys according to a third embodiment,
¨ figure 10 schematically depicts, in a plane BB, lower and upper
curves of a first bathtub curve,
- figures 11a to 11c depict, in successive planes parallel to the plane
M, cross sections of the pulley and the orientations successively adopted by
the lateral face of the reference fairing element as the cable is wound in,
the
fairing element arriving at the pulley of figure 7 upside down,
- figures 12a to 12c schematically depict in side view, a fairing
element according to a first embodiment of the invention and a portion of
fairing
comprising a fairing element according to the invention entering a pulley, in
perspective (12a), in side view as it enters the pulley (figure 12b), and
viewed
in section on the plane M visible in figure 12a, and viewed in section on the
plane Q visible in figure 12d,
- figure 13 schematically depicts an example of a fairing element
according to a second embodiment of the invention,
¨ figure 14 depicts, in a radial plane of the pulley, a portion of a
concave first curve complying with an advantageous feature of the invention,
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14
¨ figure 15 depicts a circle, constructed with respect to a fairing
element and satisfying the advantageous feature of the invention.
From one figure to another, the same elements bear the same
references.
The invention relates to a fairing intended to cover an elongate
object, for example a flexible object such as a cable, or a rigid object such
as
an offshore drill string, intended to be at least partially immersed. The
elongate
element is conventionally intended to be towed by a floating vessel. The
fairing
is intended to reduce the forces generated by the current on this elongate
element when it is immersed in the water and towed through the water by a
naval vessel.
Another subject of the invention is a towing assembly as depicted in
figure 2, comprising an elongate element 1 faired by means of a fairing
according to the invention. In the continuation of the text, the invention
will be
described in the case where the elongate element is a cable, but it does apply
to other types of flexible elongate element.
The cable 1 tows a towed body 101, for example comprising one or
more sonar antennas. The towed body 101 is mechanically anchored to the
cable 1 in an appropriate manner. The towed body 101 is put into and removed
from the water by means of a winch 5 arranged on a deck 103 of the ship 100.
The towing assembly according to the invention also comprises a
device for towing and handling the faired cable, comprising:
¨ a winch 5 for winding in and paying out the faired cable 1,
¨ a guide device 4 for guiding the cable 1, the guide device being
positioned downstream of the winch when viewed from the end 6 of the cable
1 which is intended to be immersed. In other words, the cable 1 is wound
around the winch 5 (or paid out by means of the winch) through the guide
device 4.
The guide device 4 is advantageously mounted on a bearing
structure 7 intended to be fixed to the ship and which may or may not be
capable of pivoting.
The guide device provides guidance for the cable 1, which means to
say makes it possible to limit the lateral deviation of the cable with respect
to
CA 02977734 2017-08-24
15
. .
the winch in a direction parallel to the axis of rotation of the drum of the
winch.
It is also advantageously configured to modify the direction of the cable
between its end 6 intended to be immersed and the winch 5 in a plane
substantially perpendicular to the axis of the winch while at the same time
making it possible to safeguard the radius of curvature of the cable so that
it
does not drop below a certain threshold in this plane.
In the nonlimiting example depicted in figure 3, the guide device is a
pulley 4. The guide device may further comprise, amongst other things, a
fairlead to safeguard the radius of the cable and/or a reeling device so that
the
cable can be stowed correctly on the drum and/or at least one deflector
forming
a surface that makes it possible to alter the orientation of a fairing element
with
respect to the deflector by rotation of the fairing element about the axis of
the
cable under the effect of the traction of the cable as it is being wound
in/paid
out. The latter function may be performed by a pulley.
Figure 3 schematically depicts a portion of cable 1 covered with a fairing
11 according to the invention. This fairing 11 comprises a plurality of
fairing
portions 12a, 12b. Each fairing portion 12a, 12b, comprises a plurality of
fairing
elements 13, 13a. Figure 3 depicts two fairing portions 12a, 12b, each
comprising 5 fairing elements, although in practice, the fairing may comprise
far more fairing portions comprising far more fairing elements.
The fairing elements are advantageously rigid. What is meant in the
present patent application by fairing elements that are rigid is that the
fairing
elements are configured in such a way that they do not deform substantially
under the effect of the hydrodynamic stream when immersed and possibly
towed in the direction of the leading edge. In other words, the fairing
elements
maintain substantially the same shape when subjected to the hydrodynamic
stream. The fairing elements may potentially deform under the effect of forces
stronger than those developed by the hydrodynamic stream. They are, for
example, made of hard plastics materials such as, for example, polyethylene
terephthalate (PET) or polyoxymethylene (POM).
Each fairing element 13, 13a has a hydrodynamic profile, of the kind
depicted in figure 4a, in a plane AA perpendicular to the axis x of the cable
(or
axis of the canal 16). In other words, each fairing element 13, 13a is
profiled
in such a way as to reduce the hydrodynamic drag of the cable 1 when the
cable 1 is being towed. The fairing elements 13a are fairing elements
exhibiting
CA 02977734 2017-08-24
16
the same features as the fairing elements 13 but able to differ from the
fairing
elements 13 in terms of the features explained hereinafter because of their
position in the portions 12a, 12b. Each fairing element 13 comprises a wide
nose 14 intended to accept the cable 1 and a tait 15 of streamlined shape
extending from the nose 14. The nose 14 houses a canal 16 of axis
perpendicular to the plane of the sheet, intended to accept the cable 1. The
nose 14 comprises the leading edge BA and the tait 15 comprises the trailing
edge BF which are the endmost points of the fairing element 13 in the plane of
section. The fairing element 13 more particularly in this plane has a wing-
shaped profile. The profile of the fairing element allows a less turbulent
flow of
water around the cable. The hydrodynamic profile exhibits, for example, a
teardrop shape or an NACA profile which is a profile defined by the National
Advisory Committee for Aeronautics, NACA.
Figure 4b depicts a view of the fairing element in the direction of arrow
B, which is the same view as in figure 3. The fairing element has a shape that
is elongate from the leading edge BA to the trailing edge BF. In side view,
the
fairing element 13 has a substantially rectangular shape delimited by the
trailing edge BF and the leading edge BA which are parallel to the axis xc of
the canal 16 and connected by two lateral faces 17, 18. The lateral faces 17,
18 extend substantially perpendicular to the trailing edge BA. The lateral
faces
are arranged at the respective ends of the canal 16.
In figure 4a, the chord length of the fairing element 13, which has been
referenced LC, is the maximum length of the straight-line segment referred to
as chord CO connecting the trailing edge BF and the leading edge BA of the
fairing element 13 in a direction perpendicular to the axis of the canal xc.
In
other words, the chord is the straight-line segment connecting the endmost
points of a section of the fairing element. The maximum thickness E of the
fairing element is the maximum distance separating the first longitudinal face
22 from the second longitudinal face 23 in a direction perpendicular to the
chord CO in the plane of section of the fairing element. In the embodiment of
figure 4b, the distance separating the trailing edge and the leading edge is
constant along the axis of the canal xc parallel to the leading edge BA. This
distance is the chord length. The longitudinal faces 22 and 23 run parallel to
the leading edge BA.
CA 02977734 2017-08-24
17
The fairing elements 13 are intended to be mounted on the cable 1 in
such a way as to be able to pivot about the longitudinal axis of the cable 1,
namely about the longitudinal axis of the canal 16.
The fairing elements 13 belonging to one and the same portion of fairing
12a or 12b are joined together by means of a coupling device 20 that allows
relative rotation of said fairing elements 13 with respect to one another
about
the cable 1. The coupling device 20 joins the fairing elements together both
axially, namely along the towing cable, and also in terms of rotation about
the
cable 1. The coupling device 20 allows relative rotation of the fairing
elements
with respect to one another about the axis of the cable, namely of the canal
16. This excursion is permitted either freely with a stop. The rotation of one
fairing element about the cable therefore does flot cause the adjacent fairing
element to turn. The excursion may be achieved in a constrained manner, with
more or less strong return toward the aligned position (position of no
relative
rotation of the fairing elements relative to one another about the cable). In
the
latter instance, rotation of one fairing element about the cable causes the
adjacent fairing elements of the same portion to rotate about the cable.
Advantageously, the clearance between adjacent fairing elements is near
zero, which means that any relative rotation of the fairing elements leads to
elastic deformation of the coupling device. That allows the fairing elements
of
one and the same portion to adopt an orientation with respect to the cable
that
allows it to offer the least resistance to the current brought about by the
movement of the cable through the water. The coupling device allows this
relative rotation with a maximum amplitude, namely a maximum angular
excursion. Thus, the rotation of one fairing element causes the neighboring
fairing elements and, through a knock-on effect, ail of the fairing elements
of
the same portion 12a or 12b to rotate. As the cable is raised, all the fairing
elements of one and the same portion adopt one and the same orientation
relative to the drum thereby allowing the cable to be wound in keeping the
scales parallel to one another from turn to turn.
Advantageously, the coupling device 20 allows the relative rotation of
the fairing elements with respect to one another in such a way as to allow the
cable to be wound around a winch, the lateral excursion of the cable being
caused, for example, by changes in heading of the ship. The coupling device
allows these movements of relative rotation of these fairing elements with
CA 02977734 2017-08-24
18
respect to one another with maximum respective angular excursions. The
coupling device 20 depicted in figure 3 comprises a plurality of individual
coupling devices 19 comprising, for example, a fishplate, each allowing a
fairing element to be connected to a fairing element adjacent to said fairing
element, which means to say allowing the fairing elements of one and the
same portion to be coupled one to the next. In other words, each individual
coupling device allows a fairing element to be connected to another fairing
element adjacent to said fairing element only. The adjacent fairing elements
form pairs of fairing elements. The fairing elements of the respective pairs
of
fairing elements of one and the same portion of fairing are connected by means
of distinct individual coupling devices. The coupling device thus allows each
fairing element of a portion of fairing to be connected individually to each
of its
adjacent fairing elements. Advantageously, the individual coupling devices are
configured in such a way as to deform elastically upon relative rotation of
the
fairing elements around the cable. This refers to a twisting of the individual
coupling devices.
Advantageously, the fairing elements 13 are immobilized translationally
with respect to the cable 1 along the axis of the cable x. That makes it
possible
to prevent the fairing elements 13 from becoming squashed together or spread
out along the cable 1, either of which could have the effect of causing the
fairing 11 to jam during the winding-up of the faired cable around the drum of
the winch 5 or even when passing through the guide device 4. For this purpose,
each portion of fairing 12a, 12b comprises an immobilizing device 21
collaborating with a fairing element 13a of said portion 12a, 12b and intended
to collaborate with the cable 1 so as to immobilize the fairing element 13a
translationally along the axis of the cable. According to the embodiment of
figure 3, the fairing element 13a is the fairing element furthest from the end
6
intended to be submerged situated in the direction of the arrow f (referred to
as the head-end fairing element) Because the fairing elements are joined
together, the immobilization achieved by the immobilizing device on one
fairing
element 13a has a knock-on effect on the other fairing elements of the same
portion. There is no need to install one immobilizing device per fairing
element,
and this makes it possible to limit costs and fitting time as well as limiting
the
weight of the faired cable. As an alternative, the portion comprises several
immobilizing devices each one collaborating with one fairing element of the
CA 02977734 2017-08-24
19
portion. The immobilizing device for example comprises a ring 21 fixed to the
cable by crinnping and collaborating with the fairing element 13a so as to
immobilize it translationally with respect to the cable along the axis x of
the
cable 1.
According to the invention, the fairing portions 12a, 12b are free to
rotate relative to one another about the axis of the canal 16, namely about
the
axis of the cable 1 when they are mounted on the cable 1. In other words, the
fairing elements 13 belonging to two distinct portions of fairing 12a, 12b are
free to rotate relative to one another about the axis of the canal, namely
about
the cable 1. Each portion 12a, 12b is relatively flexible in terms of rotation
about
the cable even if a certain torsional stiffness is observed. This flexibility
only
amplifies with deployed length. For this reason, breaking the fairing down
into
fairing portions which are free to rotate relative to one another makes it
possible to limit the risks of the formation of double twists and therefore to
limit
the risk of damage to the fairing, because the twists in the portions of
fairing
are flot transmitted from one portion to another. The fairing may be installed
ail along the cable. In other words, the fairing extends over the entire
length of
the cable. As an alternative, the fairing extends along the cable over a
length
less than the length of the cable.
The fairing is intended to act as a fairing for an elongate element. It is
also intended to be towed by means of a towing device as described in the
present patent application.
The heights h of the respective fairing portions, namely their lengths
along the axis x of the cable, are less than a maximum height hmax. As an
alternative, at least one of the portions has a height less than this maximum
height hmax. In figure 3, the two portions have the same length, but this is
not
compulsory. The maximum height hmax is chosen to be small enough to
prevent the formation of a complete airborne twist in the portion, for example
of a complete twist in the portion. The affected portion may make a complete
turn on itself and realign in the stream, and because it is uncoupled from its
neighbors, this portion no longer disturbs them and there is no longer any
airborne torsion or immersed torsion. This configuration makes it possible to
prevent old immersed full twists from entering the guide device and therefore
limits the risks of damage to the fairing. Moreover, this configuration makes
it
possible to avoid having to set in place a monitoring procedure performed by
CA 02977734 2017-08-24
20 =
the crew, or a monitoring device aimed at detecting immersed twists, and a
mechanical or manual procedure aimed at reabsorbing a detected double twist
or aimed at helping an immersed remanent twist coming out of the water to
enter the guide device without causing damage.
A portion of fairing T experiencing a twist by an angle 0 about the axis
x of a cable (or of the canal 16) is subjected to a torque C applied about the
axis x of the cable 1. The torque C that makes it possible to obtain this
torsion
angle is given by the following formula:
ka
C = ¨
h
where k is the angular torsional stiffness of the portion of fairing angularly
in
torsion about the axis of the cable (or of the canal) expressed in Nm2/radian,
h is the height of the portion of fairing, namely the length of the portion of
fairing
along the axis of the cable or the longitudinal axis of the leading edge.
The maximum height hmax is dependent on the torsional stiffness of
the portions of fairing. The higher the stiffness of the portions of fairing
about
the axis of the cable, the greater the height they may have. The longer the
chord of the fairing, the more affected the portion of fairing will be by the
influences of the sea and the towing conditions, and the lower the maximum
height of the portions of fairing will be. The torsional disturbances
generated
by the influences of the sea and the towing conditions are proportional to the
surface area of the fairing elements of the portion (and therefore to the
chord
length) and to the lever arm (and therefore to the chord length of the
fairing).
The maximum height hmax is therefore given by the following formula:
g * k
hmax < F LC2
where F is a constant calculated according to a configuration identified as
being the most restrictive and which takes account of the flow and reflow of
the wake and LC is the chord length of the fairing elements of the portion of
fairing.
The constant F is comprised between 250 and 500. F is dependent on
the maximum speed at which the cable is to be towed. If the cable is to be
towed at a speed of 20 knots, F is fixed at 400. F is lower if the maximum
speed decreases.
CA 02977734 2017-08-24
21
Typically, for fairings with an angular torsional stiffness k of the order of
4 to 5 Nm2/rad, and a chord length LC of 0.125 m, the maximum height is of
the order of 2 m if the constant is fixed at 400.
The fairing according to the invention offers advantages even when
there is no desire to wind the cable around a winch. Specifically, the fact
that
the fairing according to the invention minimizes the risks of the formation of
double twists means that the risks of fairing damage associated with the aging
of the immersed twists can be limited without these entering a guide device.
The fairing according to the invention therefore limits the requirements in
terms
of cable maintenance.
Advantageously, the guide device of the towing assembly according to
the invention is configured in such a way as to make it possible to modify the
orientation of a fairing element of the fairing with respect to the guide
device
by rotation of the fairing element about the axis of the cable under the
effect of
the traction of the cable with respect to the guide device (along the axis of
the
cable), when the fairing element exhibits an orientation in which it is
bearing
on the guide device and in which the Une of action of the force applied by the
cable to the guide device extends substantially in the direction extending
from
the axis of the cable as far as the trailing edge of the fairing element.
Advantageously, the guide device is configured to turn a fairing element
round from a turned-round position in which it is oriented tait down into an
acceptable position in which it is oriented tait up. The orientations up and
down
are defined with respect to a vertical axis associated with the winch.
These configurations facilitate the winding of the faired cable onto the
winch. Specifically, when it is desired to wind the cable around the drum of
the
winch, the first fairing element of each portion to leave the water rises up
toward the guide device and, flot being connected to the fairing elements of
the preceding portion, turns over trailing edge downmost under the effect of
gravity, taking with it the next fairing elements of that same portion of
fairing. If
the guide device does not allow such a turning-over, the fairing elements will
arrive on the drum of the winch incorrectly oriented (it is preferable for the
fairing elements to be wound in with their trailing edges uppermost in order
ta
avoid damage to the fairing because the leading edge is stronger).
Ta this end, the guide device comprises a guide or a set of guides that
allows the fairing element to be flipped or its orientation changed. This
guide
CA 02977734 2017-08-24
22
,
,
or set of guides may for example comprise a pulley and/or deflector or any
other device allowing the orientation of the fairing elements about the axis
of
the cable to be altered. One nonlimiting example of this type is described in
the French patent application published under the number FR2923452. These
devices are conventionally arranged upstream or downstream of the pulley as
seen from the winch. They are conventionally concave, which means to say of
the type having a groove, so as to define a housing intended to accept the
fairing element in order to flip it. These guides may be able to follow the
cable
if the cable deviates laterally parallel to the axis of the pulley (or of the
winch),
for example by being mounted with the ability to pivot about a substantially
vertical axis.
Hitherto, ail towing pulleys have been configured in such a way as to
cause the fairing elements to pass with the nose toward the bottom of the
groove and the tau l facing out of the groove. This arrangement is logical
because the towing cable, through which the forces pass, has to be located in
the nose of the fairing elements, namely near the leading edge. Ail towing
pulleys therefore have a narrow V-shaped groove. This arrangement is
rendered necessary because of the connections between ail the fairing
elements. On leaving the sea and arriving at the towing pulley, the fairing
elements which, during their airborne path, have a tendency to orientate
themselves with the trailing edge downmost (so upside down) thus find
themselves straightened up by degrees thanks to the connections between the
fairing elements. When a fairing element is correctly positioned in the groove
of the pulley, during hauling-in (and also during paying-out) ail the
successive
ones will become straightened by degrees and pass in the best way through
the pulley.
Moreover, the devices that allow the fairing to be turned over (or
straighteners) do flot perform well when they are installed downstream of the
pulley, when viewed from the free end of the cable, because the position of
the
cable at this point has at least two degrees of freedom: longitudinal and
lateral,
and present-day straightening devices are incapable of correctly following the
cable in these two directions or else are devices that are complicated.
ln the case of a narrow V-groove pulley, if the guide device has no
turning-over device downstream of the pulley as seen from the free end of the
cable or if this device does not perform well, fairing elements entering the
CA 02977734 2017-08-24
23
. .
pulley tail-down may be able to jam in the groove and, if they are flot
engineered to withstand the force applied by the cable in this orientation,
they
will deform and cause the subsequent fairing elements to deform. This
situation is depicted in figures 5 and 6a to 6b. Figure 5 depicts a portion of
a
faired cable 1 entering a pulley P of groove 50. In this figure, the cable 1,
which
is therefore entering the pulley in the direction of the arrow, is being wound
in.
In this figure, the axis xp of the pulley is perpendicular to the plane of the
page.
The fairing elements 13 of a first group of fairing elements 12a are oriented
with their trailing edge BF facing toward the outside of the groove and
leading
edge toward the groove. The notable fairing element 13a is the head-end
fairing element of the portion 12b, namely the fairing element 13a of the
portion
12b which is furthest away from the end 6 of the cable that is intended to be
immersed. The fairing element 13a arrives at the pulley P with its trailing
edge
BF toward the groove of the pulley and its leading edge BA toward the outside
of the groove. This notable fairing element 13a belongs to a second group of
fairing elements 12b.
If the pulley P is a pulley of the prier art, the cross section of the pulley
of the prier art in the plane M passing through the lateral edge 18 connecting
the trailing edge BF and the leading edge BA of the head-end fairing element
is as visible in figure 6a. Figure 6b is a cross section of the pulley P of
the prier
art in another plane comprising the lateral edge 18 of the head-end fairing
element 13a situated to the right of the plane M in figure 5 because, between
figure 5 and figure 6b, the cable 1 has been hauled in, nannely pulled in the
direction of the arrow depicted in figure 5, causing the notable fairing
element
13a to advance in the groove. The groove of the pulley has a V-shaped cross
section with an aperture angle of between 20 and 50 The bottom of the V
has a shape that substantially complements the leading edge so that when a
fairing element enters the pulley with the leading edge uppermost, the
subsequent fairing elements connected to this fairing element will aise adopt
this orientation as the cable is wound in. By contrast, if a head-end fairing
element 13a arrives with the trailing edge facing toward the groove 105 as is
the case in figure 6a, the groove is too narrow for the fairing element to
turn
over with its trailing edge uppermost under the effect of the traction of the
cable
with respect to the groove of the pulley along its axis. The tension in the
cable
forces the head-end fairing element 13a to drop down toward the bottom of the
CA 02977734 2017-08-24
= W 24
groove. Specifically, as the cable is pulled through the pulley along its
axis, it
develops a force, on the fairing element, that is directed along the line of
action
of the force indicated by the arrow in figure 6a. Now, if the fairing element
is
flot engineered to withstand this stress, it deforms and breaks (or becomes
damaged) as depicted in figure 6b.
In order to alleviate these disadvantages, the invention seeks to give
the pulley itself a function of turning the fairing elements over about the
axis of
the cable.
To this end, the invention consists in providing a towing assembly
comprising a guide device for guiding the cable, which is positioned
downstream of the winch when viewed from the end of the cable intended to
be immersed, the guide device comprising a first groove the bottom of which
is formed by the bottom of the groove of a pulley, the first groove being
configured in such a way as to allow a fairing element of the fairing to be
flipped, by rotation of the fairing element about the axis of the cable x
under
the effect of the tension in the cable, from a turned-over position in which
the
fairing element is oriented with its trailing edge (or tau) toward the bottom
of
the first groove, into an acceptable position in which it is oriented with its
leading edge (or nose) toward the bottom of the first groove, which means to
say with its trailing edge toward the outside of the groove. The dimensions
and
the shape of the profile of the first groove, notably the width of the first
groove
and the curvature of the profile of the first curved surface (which will be
defined
later on) in the radial plane are determined as a function of the radius R of
the
pulley, of the maximum length CAR, measured parallel to the chord separating
the trailing edge BF of the fairing elements of the fairing from the axis x of
the
elongate element 1, of the chord length LC of the fairing elements and of the
maximum thickness E of the fairing elements so as to allow the fairing element
to be flipped from the turned-over position into the acceptable position.
When the trailing edge (or tau) is oriented toward the bottom of the first
groove, that means that the trailing edge (or the thin end of the tau) is
situated
a shorter distance than the leading edge (or than the nose) away from the axis
of the pulley xp. The axis of the pulley is the axis about which the pulley
pivots
with respect to the winch, namely with respect to the fixed part of the winch.
Advantageously, the axis of the pulley is substantially horizontal, namely
intended to run parallel to the water surface when the sea state is cairn when
CA 02977734 2017-08-24
' 11' 25
the towing device is fixed to a naval vessel or ship. The bottom 26 of the
groove
of the pulley forms a circle of radius R the center of which lies on the axis
of
the pulley.
Figure 7 depicts a cross section of the pulley P of figure 5 in the radial
plane BB of the pulley P, in the case where the pulley P is a pulley according
to one preferred embodiment of the invention. A radial plane of a pulley is a
plane which is formed by a radius r of the pulley and the axis xp of the
pulley
about which the pulley pivots. The radius r has a length R.
The first groove 24 is delimited by a first surface of which the cross
section in the radial plane 66 is the first concave curve 25 (U-shaped curve
depicted in bold in figure 7). The first concave curve 25 comprises a bottom
26
of the first groove 24. The bottom is the point of the first groove 24 which
is
closest to the axis xp of the pulley.
Figure 7 also depicts a V-shaped reference curve 28. The V-shaped
reference curve 28 is the cross section, in the radial plane BB, of a second
curved surface delimiting a second, reference, groove 29 or virtual second
groove. The bottom of the second groove, namely the bottom of the reference
groove 28, is the bottom 26. The bottom V is the point of intersection of the
two branches 31, 32 of the V.
According to the invention, the aperture of the V, av, is at least equal to
twice a threshold angle as, and the width of the V lv, measured along a
straight
line d parallel to the axis of the pulley, is at least equal to a threshold
width Is,
given by:
is = 0.7 * lid
where lid = 2 (LC + E) * sin(as)
R
as = ai * R ¨ CAR
lid is an ideal width of the V,
where ai is a limit angle greater than 45 and less than 90 , where R is the
radius of the pulley and where CAR (indicated in figure 4a) is the maximum
CA 02977734 2017-08-24
= '.26
length, separating the trailing edge BF of the fairing elements of the fairing
from the axis of the cable, measured parallel to the chord CO of the fairing
elements, where LC is the chord length of the fairing elements and E is the
maximum thickness of the fairing elements.
In one preferred embodiment of the invention, the width of the V is at
least equal to lid. The turnover is therefore accomplished more easily.
Advantageously, the limit angle ai is given by the following formula:
ai = 7r/4 + Arctan (Cf)
where Cf is the coefficient of friction between the material that forms the
exterior part of the tau l of the fairing element and the material that forms
the
surface delimiting the groove of the pulley. The material that forms the
exterior
part of the tail of the pulley is the material that forms the fairing element
when
the fairing element is made from a single material.
In the embodiment of figure 7, the first curve 25 coincides with the
second curve 28 at the endpoints 33, 34 of the second curve 28. The endpoints
33, 34 of the second curve are the points on the second curve which are
spaced apart by the width lv along a straight line parallel to the axis of the
pulley xp. They delimit the first groove and the second groove along an axis
parallel to the axis of the pulley and along an axis parallel to the radius of
the
pulley passing through the bottom 26. The first curve 25 is, at every point
comprised between each of the endpoints 33, 34 and the bottom 26, coincident
with the second curve or doser to the axis of the pulley xp than the second
curve along the radius of the pulley in the plane of section BB.
As a result, in order to ensure the desired turnover, the first concave
curve 25 delimiting the first groove 24 may have the profile visible in figure
7,
or alternatively nnay, between the endpoints, at any point other than the
bottom
and the endpoints 33, 34, lie below the curve 28 and at least at a distance
from
the axis that is equal to the distance separating the bottom of the pulley
from
the axis of the pulley (radius R of the pulley). In other words, the first
concave
curve, at all points, lies in the space delimited by the curve 28, the
straight line
dl parallel to the axis passing through the bottom 26 and the straight lines
d3
and d4 which are parallel to the radius r passing through the points 33 and
34.
CA 02977734 2017-08-24
27
The first concave curve 25 is the curve delimiting the first groove 24
intended to receive the faired cable in a radial plane (see figure 7).
Figure 14 depicts, in dotted line, in a radial plane, a portion 250 of a
concave first curve complying with an advantageous feature of the invention.
The fairing element 13 extends with its leading edge perpendicular to the
radial
plane. This feature is as follows: the concave first curve is defined in a
radial
plane BB of the pulley such that, when the fairing element extends with the
leading edge BA perpendicular to the radial plane BB, whatever the position of
a fairing element in the first groove 24, when the nase 14 of the fairing
element
13 is bearing on the concave first curve and the cable 1 is exerting on the
fairing element 13, in the radial plane, a force to press the nose of the
fairing
element against the pulley, said pressing force Fp comprising a component
CP perpendicular to the axis of the pulley and a lateral component CL (which
means to say a component parallel to the axis of the pulley), the trailing
edge
BF of the fairing element 13 is flot in contact with the concave first curve
or is
in contact with a part 251 of the concave first curve that forms, with a
straight
line dp of the radial plane perpendicular to the axis xa extending from the
axis
of the cable x as far as the trailing edge of the fairing element, an angle y
that
is at least equal to an angle of slip at. The angle of slip is given by the
following
formula:
at = Arctan (Cf)
This feature makes it possible to prevent the fairing element from
blocking the cable in the groove when the cable moves laterally in the groove,
namely when it moves parallel to the axis of the pulley. What happens is that
if this angular condition is respected, the fairing element can be sure of
slipping
in the event of lateral thrust from the cable. In other words, a pulley having
a
profile as defined with reference to figure 14 makes it possible to ensure
that
the fairing element will overturn from a turned-over position into an
acceptable
position.
The concave first curve 25 and, therefore, the profile of the first groove,
is obtained by those skilled in the art by simulations starting from this
definition.
In practice, for an angle at of the order of 100, a first curve forming a
curved line having at every point a radius of curvature at least equal to half
the
chord length LC of the fairing element makes it possible to ensure that the
CA 02977734 2017-08-24
r die 28
fairing element will slip in the event of lateral thrust from the cable. A
curved
line is a line that has no sharp or salient angle (in the mathematical sense
of
the term). Specifically, if, as can be seen in figure 15, a circle Cr is
plotted that
passes through the nose of the fairing element 14 and the trailing edge BF of
the fairing element 13, with its tangent T at the trailing edge forming an
angle
at with the straight line dp, the radius RA of this circle is approximately
equal
to 55% of the chord length LC of the fairing element, which is greater than
the
value of 50% adopted hereinabove.
Advantageously, the dimensions and
shape of the first groove profile are determined in such a way as to allow the
flipping of a reference fairing element of maximum length CAR, measured
parallel to the chord separating the trailing edge BF of the fairing elements
of
the fairing, a fairing element chord length LC and a maximum thickness E, and
possibly also as a function of the coefficient of friction Cf between the
reference
fairing element and the pulley. These dimensions and profile are
advantageously defined in such a way as to ensure that the fairing element
flips from a turned-over position into an acceptable position between without
deforming this reference fairing element.
In the embodiment of figure 7, the width of the first groove Igb is equal
to the width of the V, Iv. As an alternative, the first groove extends beyond
the
endpoints. It may comprise the groove of the pulley only or comprise the
groove of the pulley and be delimited, on each side of the pulley, by
deflectors
or cheeks that are vertical (which means to say perpendicular to the axis of
the
pulley) or substantially vertical. The first groove may also be the groove of
the
pulley which, beyond the V or above the V comprises walls that are vertical
(which means to say perpendicular to the axis of the pulley) or substantially
vertical. The walls and cheeks as defined make it possible to prevent the
cable
from leaving the first groove in the event of lateral movement.
In the ernbodiment of figure 7, the first groove is the groove 24 of the
pulley. As an alternative, the first groove comprises the groove of the
pulley.
The bottom of the first groove is the bottom of the groove of the pulley. By
contrast, the first groove extends beyond the groove of the pulley. ut is, for
example, delimited at least on one side of the pulley with respect to a plane
perpendicular to the axis of the pulley, by a deflector or a cheek. The
deflector
or cheek may be fixed with respect to the pulley or able to rotate with
respect
to the pulley about the axis of the pulley. Advantageously, the first groove
CA 02977734 2017-08-24
29
comprises lateral edges making it possible to limit the lateral movement of
the
cable. The lateral edges may extend completely within the part situated
between the two endpoints or alternatively partially and extend also partially
beyond these points.
The pulley, and more specifically the groove of the pulley, has a profile
that is constant. In other words, it is the same in ail the radial planes of
the
pulley.
The first curve 25 and the second curve 28 are symmetric with respect
to a plane perpendicular to the axis xp of the pulley and comprising a radius
of
the pulley passing through the bottom 26. This plane is then the median plane
of the groove.
The way in which the pulley profile according to the invention as
depicted in figure 7 was obtained will now be explained in greater detail. The
applicant started from the observation that the V of figure 6a needs to be
opened out so that the tau l can move clear to the side as the cable is being
wound in. Figure 8a depicts a partial cross section of a pulley 40 according
to
a second embodiment, in the plane M which is a plane formed by a lateral face
18 of the head-end fairing element 13a of the segment 12b coming into contact
with the pulley. The lateral face comprises the point of the fairing element
that
is first to corne into contact with the pulley. The pulley has an open V-
shaped
profile making it possible to achieve turnover. In this figure, the pulley 40
has
a V-shaped groove 44. The notable fairing element 13a is resting against a
first branch 45 of the V, with ifs leading edge facing toward the bottom 46 of
the groove 44. The groove aperture ag is such that the angle formed between
the line of action of the force (depicted by the arrow indicated in the
fairing
element) and the second branch 47, af, is greater than 90 . In this case, the
tail has been given a clearance path which allows it to turn over in the
direction
of the arrows indicated in figure 8a to adopt the position depicted in figure
8c
while passing via the position depicted in figure 8b following the movement
indicated by the arrows by pivoting about the axis of the cable under the
action
of the tension of the cable (which is exerted along the line of action of the
force)
when the cable is hauled along the groove. As visible in figure 8a, the
direction
of the line of action of the force is substantially parallel to the first
branch 45.
This is why the aperture of the V ag in the plane M, which is at least equal
to
twice the limit angle ai is substantially equal to af. As a result, the
aperture of
CA 02977734 2017-08-24
r - . , 30
the V ag is greater than 90 . To take account of friction between the tau l of
the
fairing element and the surface of the groove, the limit aperture ag= 2*ai is
at
least equal to 950 and preferably at least equal to 1000
.
The angular feature is flot enough to obtain correct overturning of the
fairing elements. It is necessary for the width of the groove lgm, in the
plane
M, to be at least equal to a limit width li which is given by the following
formula:
/i --= 2 (LC + E) * sin ai
Now, as can be seen in figure 5, the profile of the groove of the pulley in
the
plane BB is the projection, onto a plane that makes an angle 13 with the plane
M, of the profile of the groove in the plane M. The angle f3 is dependent on
the
length CAR which is the maximum length separating the trailing edge BF of
the fairing elements of the fairing from the axis of the cable measured
parallel
to the chord CO of the fairing element 13a. It is defined as follows:
CAR = R ¨ R cosf3
CAR = R(1¨ cos f3)
CAR)
13 = arccos(1
R
The V previously defined is therefore to be corrected by the bias introduced
by
the angle (3. The aperture av of the V formed by the second curve 28 in the
plane BB is at least equal to a threshold angle as. The threshold angle as is
given by the following formula:
ai
as = ________________________________________
cosie
Hence as ,---- ai * ____ R
R-CAR
Therefore, the width of the V lv in the plane BB is at least equal to the
ideal
width lid given by the following formula:
lid = 2 (LC + E) * sin as
CA 02977734 2017-08-24
41è 31
The first curve 25 delimiting the first groove 24 has, at least from the
first endpoint 33 to the second end point 34, a concave shape.
It may, at least from the first end point 33 as far as the second end point
34 have a V shape or alternatively exhibit several sharp salient angles AS as
depicted in figures 9a and 9b. In other words, the curve substantially forms a
broken line. In these figures, the curves exhibit a sharp or salient angle in
the
region of the bottom 26 and are symmetric with respect to the plane
perpendicular to the axis of the pulley and comprising a radius of the pulley.
These profiles perform better at turning over the fairing elements than does
the V-shaped profile. These profiles are advantageously, although flot
necessarily, symmetric with respect to a plane perpendicular to the axis of
the
pulley passing through the bottom 26. As an alternative, the first curve has
sharp or salient angles and has a tangent substantially parallel to the axis
of
the pulley xp at the bottom. The bottom is then the point on the curve
situated
on the median plane of the groove.
Advantageously, as depicted in figure 7, the first curve 25 is, between
the endpoints 33, 34, a curved line. In other words, this is a concave curve
with
no sharp or salient angle (within the mathematical meaning of the term).
Mention is made of a U-shaped profile. What this means is that the curve
substantially never has more than one tangent at any one point. lts derivative
is substantially continuous.
When the first groove (or first curie) has a V-shaped cross section (V-
shaped first curve) it needs to have a width at least equal to lid for
turnover to
be guaranteed. When the first groove (or first curve) has a cross section such
that the first curve is U-shaped, then it can have a smaller width potentially
as
low as 0.7*Iid, because it has no sharp angles in which the tau l of the
fairing
element may jam. In that case, the aperture of the V may also be below the
threshold angle. In other words, the V needs to have a width at least equal to
0.7*lid. By contrast, overturning may prove more difficult than when the V has
a width at least equal to !id. Below this threshold, there is no certainty
that
overturning will occur.
Advantageously, in the case of a first groove having a U-shaped profile,
the first groove has a bathtub-shaped bottom. The groove with a bathtub-
shaped bottom offers the advantage of ensuring certain and fluid reorientation
CA 02977734 2017-08-24
= V = Ir 32
of the fairing element and allows the fairing element to be oriented in a
substantially lying-down position in the bottom of the groove.
That means that the first curve has a central zone, this central zone has
a width equal to g*lid, where lid is the ideal width and g is comprised
between
0.7 and 1, between the endpoints coinciding with the endpoints of a V-shaped
reference curve 128 having a width equal to g*lid. The central zone is
delimited
by the two curves (see hatched zone) 10:
- an upper curve SUP having a first radius of curvature R1 radius
equal to 1/2* g*lid passing through the bottom and the center of
which is situated on a straight line perpendicular to the axis of
the pulley passing through the bottom,
- a lower curve INF comprising a central portion CENT extending
substantially parallel to the axis of the pulley symmetric with
respect to a plane perpendicular to the radial plane passing
through the bottom and extending, along the axis of the pulley,
over a first width equal to1/2*glid and comprising, on each side
of the central portion CENT, lateral portions LAT1 and LAT2
connecting the central portion to the endpoints 133, 134 and
having a second radius of curvature R2 equal to 1/4*glid. Each
lateral portion extends over a width equal to Yeglid along the
axis of the pulley. The centers of the lateral portions are
symmetric with respect to one another about the vertical plane
PV passing through the bottom and perpendicular to the axis of
the pulley xp.
The central zone may be one of the two curves. The lower curve is the
preferred embodiment of the invention.
Advantageously, the central zone of the first curve is formed by a pulley
having a groove the width of which is the width of the central zone.
Advantageously, the first curve comprises upper parts extending
substantially perpendicularly above the endpoints of the V so as to prevent
the
cable from leaving the first groove in the event of a vertical movement of the
cable. These cheeks are secured to the pulley or belong to the pulley or are
fixed with respect to the axis of the pulley.
CA 02977734 2017-08-24
= 33
The first curves comprised between the upper curve and the lower curve
offer the advantage of satisfying the angle condition making it possible to
prevent the fairing element from inhibiting the lateral movement of the cable.
Figures 11a 11c depict, in successive planes parallel to the plane M,
orientations successively adopted by the lateral face of the reference fairing
element comprising the first point to corne into contact with the pulley, as
the
cable is being wound in. The fairing element 13a arrives with its trailing
edge
downward (figure lia in the plane M) and when the cable is pulled, the element
pivots about the axis of the cable (see figure 11 b) under the effect of the
tension of the cable, until it reaches the substantially lying-flat position
in which
the leading edge faces toward the bottom of the groove and the leading edge
faces toward the outside of the groove (figure 11c). This profile makes it
possible to facilitate and simplify the flipping of a fairing element because
the
flattened central portion of the groove of the pulley means that there is a
significant distance between the axis of the reaction of the groove of the
pulley
on the fairing element (the axis leading from the trailing edge toward the
center
of the portion of circle formed by the central portion) and the axis of
rotation of
the fairing element (extending along the trailing edge axis ¨ toward the axis
of
the canal xc or axis of the cable x) because of the significant distance
between
the axis of the cable and the center of the portion of circle formed by the
central
portion. This profile aise allows the cable and its fairing, which are
positioned
substantially lying flat, to corne and rest without danger against the flanks
of
the pulley when the cable is urged laterally (namely parallel to the axis of
the
pulley) if for example the ship changes heading. If the cable and the leading
edge of the fairing are positioned on the correct side, they remain there. If
they
are on the incorrect side, the profile of the pulley allows a gentle near-
overturning which allows the cable (which is where the forces are applied) to
corne and press against the flank of the pulley. This slippage is present but
less fluid in the other configurations of pulley.
To sum up, the pulley according to the invention and, more generally,
the guide device according to the invention, makes it possible to ensure the
straightening of a fairing element coming to bear against the pulley with an
orientation in which the trailing edge faces toward the bottom of the groove
of
the pulley and the leading edge is vertically aligned with the trailing edge.
The
fairing element cardes along with it the fairing elements to which it is
connected
CA 02977734 2017-08-24
y 4 4 1 34
in rotation about the cable, namely the fairing elements of the same portion.
The pulley according to the invention also allows the straightening of the
fairing
elements of a cable organized into a single portion in which the fairing
elements are ail joined together in rotation about the cable if an inter-
fairing-
element connection should break for example under the effect of a double
twist, thereby allowing the faired cable to pass through the pulley without
deformation of the fairing elements. It also allows the straightening of the
head-
end fairing element of a fairing comprising a single portion extending over a
length shorter than the length of the cable starting from the end intended to
be
immersed. It also allows the straightening of the fairing elements of a faired
cable comprising fairing elements which are ail free to rotate about the cable
independently of one another. It furthermore, because of its width, allows
guidance of a cable organized into a single portion exhibiting remanent twist
(very tightly twisted immersed torsion flot reabsorbed on passing through the
pulley) without deformation of the fairing elements, something that is flot
possible using a narrow V-shaped pulley.
The guide device of the invention is simple and effective because it does
flot require the fitting of a cable-follower device (namely a device able to
follow
the cable as it moves laterally and vertically with respect to the pulley).
The pulley according to the invention, and, more generally, the guide
device according to the invention, because of its profile, does flot turn the
fairing element over as far as a situation in which the trailing edge is
situated
in vertical alignment with the leading edge. For example, in the case of the
pulley with a bathtub-shaped bottom, the fairing element is turned over into a
position in which it is substantially flat (with the trailing edge raised
slightly
uppermost). It therefore needs to pivot by approximately 1/2 of a turn as
opposed to 1/2 of a turn (if it were to have to adopt the position in which
the
trailing edge was above and in vertical alignment with the leading edge)
thereby facilitating the operation whereby the pulley straightens the fairing
element.
Advantageously, the guide device comprises, between the winch and
the pulley, a straightening device allowing the fairing elements leaving the
pulley and heading for the winch to be oriented about the axis of the cable in
such a way that they exhibit a predetermined orientation with respect to the
drum of the winch, for example with the leading edge downmost and the trailing
CA 02977734 2017-08-24
e 35
edge vertically in line with the leading edge. These devices are truly
effective
only when the position of the cable is perfectly known (which it is as it
leaves
the pulley).
In the embodiment of figures 4a and 4b, the fairing elements of the
portions have a cross section which is constant, which means te say fixed,
along the leading edge. What is meant by cross section is the profile of the
fairing element in a transverse plane, namely a plane running perpendicular te
the leading edge BA, namely te the axis of the canal xc. What is meant by a
cross section that is constant is a cross section that exhibits substantially
the
same shape and the same dimensions in ail transverse planes regardless of
their positions along the leading edge between the lateral faces 17, 18. In
other
words, the trailing edge BF is substantially parallel te the leading edge BA
across the entire width I of the fairing element. The width I of the fairing
element
is the distance between the two lateral faces 17, 18 along an axis parallel te
the leading edge BA.
The trailing edge BF constitutes a bearing edge parallel te the leading
edge BA.
As an alternative, as visible in figures 12a te 12c, at least one fairing
element 130 of the fairing is a mitered fairing element. A mitered fairing
element is a fairing element which comprises a bearing edge BAPa comprising
a first bearing edge Bza which is mitered with respect te the leading edge
BAa,
the miter being produced in such a way that the distance between the leading
edge BAa and the mitered first bearing edge Bza, considered along an axis
perpendicular to the leading edge BAa, and te the axis xc of the canal 16,
varies linearly along the axis xc. What is meant by a first bearing edge Bza
that
is mitered is a first bearing edge Bza which extends longitudinally
substantially
along a straight line which is angled or inclined with respect te the leading
edge
BAa. The first bearing edge Bza extends longitudinally in a first plane
containing a plane or parallel te a plane defined by the leading edge BAa and
the chord CO of the fairing element. In other words, the first bearing edge
Bza
is at an angle with respect te the leading edge BAa in this first plane.
The bearing edge BAPa extends longitudinally between two ends El
and E2. The bearing edge BAPa is arranged in such a way that the distance
between the bearing edge BAPa and the leading edge BAa decreases
continuously, from a first end El of the first bearing edge Bza te a first
lateral
CA 02977734 2017-08-24
1 e. I 36
face 180 of the fairing element closer to the second to the first bearing edge
Bza than to the first end of the bearing edge, along an axis parallel to the
leading edge BA.
In the embodiment of figure 12b, this lateral face 180 is the lateral face
of the fairing element 130a furthest away from the free end 6 of the cable
(visible in figure 2) in the opposite direction to the arrow. The other
lateral
face 170 is the lateral face of the fairing element 130a closest to the free
end
6 of the cable. This feature makes it easier to turn the fairing element 130
over
when it cornes to bear against the pulley via its trailing edge, as the cable
is
being wound in, namely as the cable is being pulled with respect to the axis
of
the pulley xp, in the direction of the arrow f. Specifically, figure 12b
depicts the
position P', on the pulley 4 of figure 7, of the point at which the fairing
element
130a cornes into contact with the pulley 4 as a result of the traction of the
cable
with respect to the axis of the pulley xp in the direction of the arrow. This
point
is situated at a distance B' (indicated in figure 12b) from the cable 1
perpendicular to the axis of the cable x. Also depicted is the position P, on
the
pulley 4, of the point at which a fairing element 13 that would have had the
shape depicted in figures 4a and 4b would have corne into contact with the
pulley P. This point is situated at a distance dB from the cable 1
perpendicular
to the axis of the cable x. The distance dB' is less than the distance dB,
which
means that the overturning of the fairing element is easier and therefore that
the overturning of the fairing elements of the portion is aise easier. This is
valid
for the pulley of the invention but is aise valid for any guide device,
particularly
of the type that allows the orientation of the fairing element with respect to
the
guide device to be modified by rotating action of the fairing element about
the
axis of the cable. In particular, the mitered bearing edge makes it easier to
reorient a fairing element in any guide device that allows the orientation of
the
fairing element with respect to the guide device to be modified by rotating
action of the fairing element about the axis of the cable (or of the canal)
when
the fairing element cornes to bear against a bearing surface of the guide
device
via the bearing edge. In other words, the mitered bearing edge in particular
facilitates the reorientation of the fairing element by any guide device
comprising a surface that opposes the traction of the faired cable as the
cable
is being wound in or paid out. The invention works for example with guide
devices that make it possible to follow the cable in the event of lateral
and/or
CA 02977734 2017-08-24
= 4A = 37
vertical movement of the cable. In general, the presence of a mitered fairing
element makes it possible to limit the risks of damage to the fairing, notably
in
the presence of a double twist, by facilitating the flipping of a fairing
element
as it enters a guide device, thereby limiting the risks of the fairing
becoming
jammed in the guide device.
This embodiment also offers an advantage in the case of a pulley of
constant profile, and more particularly a pulley according to the invention.
Specifically, the point of contact P' is situated in a plane M' situated at a
shorter
distance D' than the distance D at which the plane M (comprising the point P)
is situated, with respect to the axis of the pulley, parallel to the axis of
the cable
x. As a result, the groove of the pulley is not as deep in the plane M' as in
the
plane M. Specifically, the profile of the groove in the plane M (or M') is the
projection of the profile of the groove in a radial plane passing through the
plane P (or respectively P') onto the plane M (or, respectively, M') forming
an
angle E3 (or respectively 13' less than 13) with the radial plane at the point
considered. Now, the fact that the groove is not as deep in the plane M' as it
is in the plane M means that the pulley is flatter in the plane M than in the
plane
M', at least at the bottom (namely at the level of the central part of the
curve
delimiting the groove). If the fairing element cornes into contact with the
central
portion of the pulley in the bottom of the bathtub, the central portion is
flatter in
the plane M' than in the plane M, or in other words, the radius of the contact
surface at the point P is greater in the plane M' than in the plane M, making
it
easier for the fairing element to flip under the effect of the traction of the
cable
with respect to the axis of the pulley.
In the embodiment of figure 12b, the mitered fairing element comprising
the miter is the fairing elennent 130a at the head-end of the portion, namely
the
fairing element furthest from the end of the cable that is intended to be
immersed. That makes it possible to facilitate the flipping of the fairing
element
130a during the winding-in of the cable and to facilitate the flipping of the
entire
portion 120 because, since the fairing element is connected in terms of
rotation about the cable to the other fairing elements of the portion, as it
moves
about the cable it carnes ail the fairing elements of the portion 120 along
with
it. The head-end fairing element 130a is a fairing element which is adjacent
to
just one other fairing element 130b belonging to the same portion 120. The
first bearing edge Bza of the head-end fairing element 130a is arranged in
such
CA 02977734 2017-08-24
= P 38
a way that the distance between the leading edge BAa and the first mitered
bearing edge Bza decreases continuously, along an axis parallel to the leading
edge BAa, from a first end El of the first bearing edge Bza to a second end
E2 of the first bearing edge Bza, further away from the other fairing element
130b than the first end El, along the axis parallel to the leading edge BAa.
As an alternative, the mitered fairing element is the fairing element at
the tail-end of the portion, namely the fairing element closest to the end of
the
cable that is intended to be immersed. That makes it possible to facilitate
the
flipping of the fairing element during the paying-out of the cable (when the
fairing element cornes to bear on the pulley on the other side of the pulley
with
respect to the axis of the pulley) and to facilitate the flipping of the
entire portion
because the fairing element (by a propagation of the rotational movement over
the entire portion). The tail-end fairing element is a fairing element which
is
adjacent to just one other fairing element belonging to the same portion. The
first bearing edge is arranged in such a way that the distance between the
leading edge BAa and the first mitered bearing edge decreases, along the
leading edge BAa, from a first end of the first bearing edge facing the other
fairing element to a second of the first bearing edge further away from the
other fairing element than the first end, along the axis parallel to BAa. The
other end of the first bearing edge is closer to a lateral face than the first
end
of the bearing edge. This embodiment, like the preceding one, makes it
possible to ensure the flipping of ail the fairing elements of the portions of
fairing without having to provide only mitered fairing elements over the
entire
fairing, as so doing would have the effect of limiting the performance of the
fairing in terms of drag reduction.
The other fairing elements are net mitered fairing elements. They do flot
have a mitered first bearing edge. The bearing edge is the trailing edge and
is
substantially parallel to the leading edge over its entire length.
Advantageously, each portion comprises at least one (head- or tail-)
end fairing element comprising a mitered edge.
In an alternative form, a fairing comprising a single portion as defined
above may comprise a fairing element with a mitered bearing edge. This
portion extends for example over a length less than the length of the cable
starting from the end intended to be immersed. In this case, the head-end
fairing element of the portion is advantageously a fairing element comprising
CA 02977734 2017-08-24
. é= f 39
a mitered bearing edge designed as for the head-end fairing element
described hereinabove.
In another alternative form, the portion extends over the entire length of
the cable.
In ail the configurations of fairing (of the type comprising one portion,
several portions or comprising fairing elements which are all free to rotate
independently of one another about the elongate element), ail the fairing
elements could be mitered fairing elements. That would make it easier to flip
each fairing element in the event of a breakage of an inter-fairing-element
connection downstream of the fairing element as seen from the pulley, when
the fairing elements are initially connected. In cases where the fairing
elements
are free to rotate independently of one another, that makes it easier to flip
each
fairing element as it arrives on a guide device. More generally, the mitered
fairing element makes it possible to avoid the need to join the fairing
elements
together and therefore makes it possible to limit the cost of the fairing and
the
time taken to assemble the fairing.
If there is a wish to reorient the fairing elements when the cable is being
wound in, the miter is produced in such a way that the distance between the
leading edge BA and the first mitered bearing edge decreases, along the axis
xc, from the end of the first bearing edge closest to the end of the cable
intended to be immersed as far as the end of the bearing edge opposite to the
end of the cable that is intended to be immersed and vice versa if the wish is
to facilitate the flipping during the paying-out of the cable.
In the embodiment of figures 12a and 12b, the bearing edge BAPa is
the trailing edge BF. It comprises the first mitered bearing edge Bza and a
second bearing edge Bla which runs parallel to the axis x and is situated a
fixed distance away from the leading edge along the axis x. The first mitered
bearing edge is connected to the lateral face 180 and to the second bearing
edge Bla in the direction of the leading edge, by fillet radii or chamfers.
The
maximum chord length LC is the distance between this second bearing edge
Bla and the leading edge. As an alternative, the bearing edge has no second
bearing edge Bla extending parallel to the axis x. The miter extends
substantially over the entire width of the fairing element and is
advantageously,
although not necessarily, connected to the lateral faces by fillet radii or
chamfers.
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As visible in figures 12c and 12d which depict cross sections of the
fairing element on respective planes N and Q, depicted in figure 12a, parallel
to the leading edge and perpendicular to the lateral faces 170, 180, the
fairing
element comprises a thick first portion 130a1 visible in figure 12c and a thin
second portion 130a2 having a second thickness smaller than the first
thickness el of the thick part. The second thickness e2 is substantially equal
to the thickness of the tail end 15 opposite to the end of the tail that is
connected to the nose 14 of the fairing element. The first edge comprises a
first portion Bzal extending into the thick first portion 130a1 of the fairing
element and a second portion Bza2 extending into the thin part. The first
portion of the first bearing edge Bzal is connected to the longitudinal faces
122, 123 by respective chamfers 132, 133 respectively. In other words, the
fairing element comprises chamfers connecting the first portion of the first
bearing edge Bzal to the respective longitudinal faces 122, 123. That means
that the trailing edge can be made thinner in the thick part of the fairing
element, thereby limiting the risks of the fairing element becoming jammed on
the guide device. As an alternative, the chamfers extend over the entire
length
of the first bearing edge.
As an alternative, the first portion of the leading edge Bzal is connected
to the lateral faces by respective bulging surfaces. What is meant by bulging
surfaces is surfaces with convex curvature. This embodinnent also makes it
possible to limit the thickness of the bearing edge. As an alternative, the
curved
surfaces extend over the entire length of the first bearing edge. The chamfers
and curved surfaces are two nonlimiting technical solutions that make it
possible to obtain the feature whereby at least a first portion of the first
bearing
edge Bzal has a thickness e3 less than the thickness of the fairing element
in any longitudinal plane parallel to the leading edge and perpendicular to
the
lateral faces of the fairing element intersecting the first portion of the
first
bearing edge Bzal. The thickness of the fairing element in a plane of section
is the distance separating the first longitudinal face 122 from the second
longitudinal face 123 in a direction perpendicular to the chord CO in the
plane
of section of the fairing element. Advantageously, the first portion Bzal has
the same thickness as the second bearing edge Bla which runs parallel to the
axis x and is situated a fixed distance away from the leading edge along the
axis x.
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A bearing edge of a fairing element according to a second embodiment
of the invention will now be described with reference to figure 13. Everything
already stated regarding the installation of the fairing element on a fairing,
the
configuration of the fairing, the thickness of the bearing edge and the
arrangement between the first bearing edge and the second bearing edge
remains valid.
In figure 13, the bearing edge BAPb connects the two lateral faces 270,
280. The fairing element 230 is formed of two parts 231, 232 back to back
along the first mitered bearing edge Bzb. The fairing element is configured to
io be kept in a deployed configuration (visible in figure 13) when
subjected to the
hydrodynamic flow of the water, in which configuration the two parts 231, 232
are arranged, relative to one another about the first bearing edge, in such a
way that the fairing element has a trailing edge parallel to the leading edge
and
a cross section that is constant along the leading edge. In other words, the
chord length is constant. The fairing element is kept in the deployed position
as long as the torque inducing relative pivoting between the two parts about
an axis formed by the first bearing edge Bzb is less than or equal to a
predetermined threshold. The longitudinal direction of the first bearing edge
is
the direction of the axis formed by the bearing edge. The threshold is higher
than the torque that may be applied by the hydrodynamic flow of the water on
the fairing element when the fairing element is immersed and possibly being
towed along the trailing edge ¨ leading edge axis. The fairing element is also
configured in such a way as to allow relative pivoting between the two parts
231, 232 about the first bearing edge Bzb (see arrow) when a torque inducing
relative pivoting between the two parts 231, 232, applied about the axis
formed
by the first bearing edge Bzb, exceeds the threshold so that the end fairing
element passes from the deployed configuration into a configuration folded
about the bearing edge. The axis formed by the first bearing edge is an axis
contained in the first bearing edge and parallel to the longitudinal axis of
the
first bearing edge. In the folded configuration, the fairing element does not
have a constant cross section and the trailing edge is not parallel to the
leading
edge over its entire length. In the folded position, the fairing element is
folded
along the first bearing edge Bzb. In the deployed position, the fairing
element
is unfolded. This embodiment makes it possible to linnit or avoid reductions
in
performance in terms of the reduction of the hydrodynamic drag of the fairing
CA 02977734 2017-08-24
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element while at the same time facilitating the progress of the fairing
element
through the pulley and the overturning of this element.
The first part 231 extends on one side of the first bearing edge and is
delimited by the first bearing edge Bzb, the second bearing edge (if there is
one) Blb, the leading edge BA, one lateral face 280 and the portion of the
other
lateral face 270 extending between the leading edge BA and the first bearing
edge Bzb.
The second part 232 is delimited by the first bearing edge Bzb, the part
of the first lateral face 270 extending from Bzb as far as the trailing edge
BF
and the part of the trailing edge BF situated between Bzb and the first
lateral
face 270.
The first part 231 is, for example, made from a material that is rigid and
the second part 232 is made from a material that is flexible or soft and does
flot deform appreciably when the torque inducing relative pivoting of the two
parts about the first bearing edge is less then or equal to the threshold and
which does bend when the torque exceeds the threshold, notably when the
point of intersection between the trailing edge and the first lateral face 270
cornes into abutment against a guide device. The second part may, for
example, be made of polyurethane. The first part may be made of a
polyurethane with a rigidity higher of the first part or alternatively may be
made
of POM or of PET. As an alternative, both parts have a rigidity such that they
do flot deform under the effect of a torque higher than the threshold but are
connected by a pivot connection about the first bearing edge and the fairing
element comprises a stabilizing device configured to keep the two parts in the
deployed relative position when the relative pivoting torque is less than or
equal to the threshold and so as to allow the two parts to rotate relative to
one
another so that they pass into the relative position of folding around the
first
bearing edge when the torque exceeds the threshold. The coupling device is,
for example, a device comprising a deliberate weak link or a compression
spring.
Advantageously, at least one mitered fairing element or each mitered
fairing element is dimensioned so as to be better able to withstand a pressure
load applied, in a direction perpendicular to the leading edge connecting the
leading edge and parallel to an axis to the trailing edge, than the other
fairing
elements of the portion considered (which are not mitered) or alternatively,
and
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43
more generally, than the non-mitered other fairing elements. This feature
makes it possible to limit the risks of deformation and breakage of the
fairing
elements as they enter the guide device, turn over, and pass through this
guide
device. To this end, this fairing element is, for example, made from a harder
material than the other fairing elements and/or comprises ribs providing this
additional reinforcement. Advantageously, the fairing comprises at least one
reinforced mitered end fairing element collaborating with the immobilizing
device. That makes it possible to reduce the cost and possibly the weight of
the fairing because only one the mitered fairing element or elements differs
or
differ from the others, ail the others being identical.
The invention also relates to an assembly comprising a ship, the towing
assembly being carried on board the ship. The ship is intended to move at a
nominal speed in a nominal sea state. The towing assembly is installed on the
ship in such a way that the tow point is situated at a nominal height.
