The invention relates to a system for controlled lowering of an elongated
body such as a tube or cable into a volume of water such as a lake or sea, from
a relatively high level, especially the water level of said volume of water, to
a relatively low level, especially the bottom under said volume of water, comprising
an elongated tubular guide which is connected to one or more bouyancy bodies, and
braking means for braking the movement of said elongated body through said tubular
guide, said braking means incorporating at least one braking unit comprising:
a valve unit for inflating or deflating the inflatable body whereby the valve unit
is connected to a source of pressurized fluidum and is controlled by a control
- a flexible tubular inflatable body, the outer wall being supported by said tubular
guide and the inner wall of which acts as braking surface,
Such a system is known from the US specification US5575590. In this
document air is used to inflate or deflated the inflatable bodies of the braking
units. Each braking unit is connected to an individual source of pressured air
and the pressure inside the inflatable body of each of the braking units is preset
independent of the other braking units on a value which is dependent on the depth
at which the respective braking unit thereafter has to function. Because of the
increasing hydrostatic pressure the air or gas in the inflatable body is compressed
more and more with increasing depth. As a result the contact surface between the
inflatable body and the tube becomes smaller and smaller so that in fact in each
braking unit only a part of the initial braking surface will be active. Controlling
the actual braking force under these circumstances in the prior art system is rather
difficult or even impossible.
The object of the invention is now to eliminate the above indicated
In agreement with said object the invention now provides a system
of the above indicated type which according to the invention is characterised in
that the fluidum inside each inflatable body has a specific weight equal to the
specific weight of the surrounding water.
It is especially preferred that the fluidum inside each inflatable
body consists of the same water as the surrounding water.
In one further development each valve unit is connected to a separate
fluid reservoir attached to the corresponding braking unit and acting as source
of pressurized fluidum for said valve unit. Each braking unit is now able to operate
independent of the others. The large number of reservoirs could be a disadvantage
of this embodiment.
In another development a number of valve units (preferably all valve
units) are connected to a fluid conduit which extends along the tubular guide and
is connected to one fluid reservoir acting as source of pressurized fluidum for
said valve units. Only one reservoir is needed in this embodiment, however, a fluid
conduit is necessary between each valve unit and the common reservoir.
The control communication between the control centre and the valve
units can be performed in various ways using data transfer along electrical connections,
along acoustical communication paths, using radio waves, etc.
In all known prior art cases pressurised air is used as fluid for
inflating or deflating the inflatable bodies of the braking units and for determining
in correspondence therewith the applied braking force. Assume a water depth of
2000 m and assume a desired control range of 50 cm water column. In that case 4000
braking units would be necessary each capable of functioning at a different depth
and at a different hydrostatic bias pressure.
With increasing water depth the controllability of the valve units
becomes more and more a problem. The valve unit which has to operate for instance
at 1000 m depth should have a pressure control range between 999,75 m and 1000,25
m water column. Such a control range poses a serious problem. This problem will
be twice as serious for the most lower valve unit which has to operate in the pressure
range between 1999,75 m and 2000,25 m water column. An accurate control within
such a small control range under such relatively high bias pressure of the surrounding
sea water is hardly performable.
The whole problem is caused by the depth dependent hydrostatic pressure.
The question how to eliminate this problem appeared to be surprisingly simple.
The hydrostatic pressure acting on the outside of the inflatable bodies has to
be eliminated by the pressurizing control fluid inside the inflatable bodies. To
obtain such a situation a preferred embodiment of the system is characterised in
that the fluid has a specific weight equal to the specific weight of (sea)water.
It is especially preferred that the fluid is (sea)water.
By using (sea)water as fluid the hydrostatic pressure difference is
completely eliminated at all depths.
In a preferred system, in which the abovementioned fluid is (sea)water,
the number of braking units is equal to one, said one braking unit comprising:
- an elongated non stretchable outer tubular wall
- an elongated flexible tubular inflatable body, the outer wall of which is supported
by said outer tubular wall and the inner wall of which acts as braking surface,
- a valve unit connected to a pump unit through which (sea)water can be pumped
in or out the inflatable body to obtain the required braking stress.
The abovementioned elongated outer wall and the elongated flexible
tubular inflatable body preferably extends from just below sea level to just above
bottom level. In that case the whole tube is guided and controlled by only one
If for reasons which will not be discussed here it is preferred to
use two or more braking units then each of the braking units comprises
whereby all outer tubular walls are mechanically connected in series,
- a non stretchable outer tubular wall
- a flexible tubular inflatable body, the outer wall of which is supported by
said outer tubular wall and the inner wall of which acts as braking surface,
whereby all flexible tubular inflatable bodies and a valve unit are connected in
series by suitable conduits, and
whereby furthermore the valve unit is connected to a pump unit through which (sea)water
can be pumped in or out the series connected inflatable bodies to obtain the required
The pump can be of a rather simple design which only has to be able
to generate a pressure in the desired control pressure range, in the above example
a pressure between 0 and 50 cm water column.
The invention will be explained further with reference to the attached
Figure 1 illustrates schematically the general situation during lowering
of a tube or cable from a ship onto the sea bottom.
Figure 2 illustrates a cross-section through a system according to
Figure 3 illustrates a longitudinal section through a first embodiment
of a system according to the invention especially at the interconnection between
two sections of the elongated tubular guide.
Figure 4 illustrates a longitudinal section through a second embodiment
of a system according to the invention especially at the interconnection between
two sections of the elongated tubular guide and one end of the tubular guide.
Figure 5 shows an embodiment with only one elongated brake unit and
separate outside buoyancy bodies.
Figure 6 illustrates a cross section through the elongated brake unit
of figure 5.
Figure 1 illustrates a vessel 10, floating on the sea surface 12,
from which vessel 11 an elongated object such as a tube or cable 14 is lowered
onto the sea bottom 16. To limit the velocity with which the tube or cable 14 is
lowered and to maintain during the whole process an S-shape in the cable or tube
14 so that kinking or buckling of the cable or tube 14 is prevented, an elongated
tubular guide is used which in the illustrated embodiment comprises a number of
sections 18a, 18b, 18c ..... 18n. Each of those sections 18a etc. has a predetermined
buoyant capacity necessary to maintain the abovementioned S-shape. Furthermore,
each section comprises brake means which are clamped around the tubular cable 14
such that a braking force is applied to the tube or cable which is at least to a
large degree in balance with the buoyant force.
Figure 2 illustrates a cross-section through one of the sections 18x,
clamped around a tube 14. The section 18x comprises a non stretchable outer tubular
wall 20. Within said tubular wall a layer 21 is situated made from a material with
a lower specific gravity than water, which material generates the abovementioned
buoyancy force. As such suitable materials are known and furthermore the buoyancy
features are not the subject of the invention.Therefor a further detailed discussion
is considered superfluous.
Within the layer 21 the braking unit is positioned comprising a flexible
tubular inflatable body 22. At the inside surface this body may comprise a coating
of friction material 24. However, taking into account the relatively large contact
surface between the body 22 and the tube 14 in many cases this friction layer could
To improve handling of the whole system it is preferred that the tubular
configuration can be made from an initially flat or at least open configuration
by folding the flat configuration around the tube 14 and fastening the longitudinal
edges to each other. If the outer wall 20 is made of a rather stiff material such
as metal then it is preferred that the outer wall 20 is divided into a number of
segments interconnected by means of hinges. In the illustrated embodiment in Figure
2 there are two of such segments 20a and 20b interconnected by the hinge 23. On
the other hand one could make the outer wall from any strong flexible but non stretchable
material in which case the jacket could be attached as a blanket around the tube
14. The longitudinal edges of the outer wall are connected by means of a number
of cables or chains 26 which preferably are as short as possible. Because of the
presence of these cables or chains the flexible inflatable body 22 does not completely
surround the cable or tube 14 but fills only the volume between the outer wall
20 and the cable or tube 14, which volume has to be (nearly) constant.
To improve the strength of the configuration and to improve the transfer
of the braking forces from the braking layer 24 to the buoyancy layer 21 the braking
cushion 22 might be internally subdivided by means of dividing walls one of which
is indicated by 27 in figure 2. It will be clear that the resulting subvolumes
of the cushion 22 are all interconnected so that the pressure inside each subvolume
is always the same.
Figure 3 illustrates a longitudinal cross-section through a specific
embodiment of the elongated tubular guide comprising at least the segments 18M
and 18N. Each of the segments 18M, 18N,.. has an outer layer 20M, 20N,.., a buoyancy
layer 21M, 21N,..,and a braking cushion 22M, 22N,.. of which the inner side could
have a strengthening layer 24M, 24N,..of e.g. friction material. The various segments
are interconnected by suitable means such as the chains 30A, 30B. In this embodiment
the braking cushion 22M of segment 18M of the tubular guide is through a suitable
conduit 40M and a valve unit 42M connected to a reservoir 44M. The reservoir 44M
comprises for instance pressurized air which through the valve unit 42 can be filled
into the inflatable body 22M. On the other hand the valve means 42M are able to
let pressurized air escape from the body 22M through the conduit 40M into the surroundings.
All other segments of the tubular guide are in the same manner equipped with valve
means and reservoirs.
By controlling the inflating/deflating of the body 22M a predetermined
pressure can be set as soon as the segment 18x has reached its operating depth.
An electrical control line 46M extends from the valve means 42M to a central control
unit 48. All other valve means from all other segments are in the same manner connected
to this central control unit 48 as indicated by the dotted lines 46X and 46Y. Preferably,
the central control unit 48 is installed on board of the vessel 10 and can be operated
by the crew of said vessel to set and maintain the pressure in each of the inflatable
bodies in each of the braking means at the desired value.
Instead of a pressurized fluid reservoir 44 for each of the segments
of the elongated tubular guide, it will be clear even without detailed illustration
that one (larger) reservoir can be installed somewhere along the tubular guide
18. In that case each segment of the tubular guide comprises only a set of valve
means, which on the one hand are connected to the inflatable body of the respective
segment and are on the other hand through suitable conduit means connected to said
central reservoir. The valve means are in the same manner as in Figure 3 connected
to a central controller 48 for controlling the pressure inside the various inflatable
As already indicated above, it is preferred to fill the inflatable
bodies with a substance which has the same specific gravity as the surrounding
(sea) water and more especially to fill the bodies actually with (sea)water. By
filling these bodies with (sea)water, the problems with the hydrostatic pressure
are eliminated. The hydrostatic pressure outside and inside the inflatable bodies
22 is equal. A rather small overpressure is already sufficient to generate a rather
large braking force by each of the inflatable hollow bodies 22.
An example of an embodiment whereby sea water is used as the fluid
for inflating/deflating the hollow bodies is illustrated in Figure 4. This figure
shows parts of the segments 18R and 18S of the elongated body. Each of the segments
18S, 18R,.. has an outer layer 20S, 20R,.., a buoyancy layer 21S, 21R,..,and a
braking cushion 22R, 22SN,.. The various segments are mechanically interconnected
by suitable means such as the chains 30C, 30D. Although relatively long chains
are shown the relatively short chains as indicated in Figure 3 are preferred.
The actual difference between Figures 3 and 4 relies in the fact that
in Figure 4 all the inflatable bodies are connected in series by means of suitable
conduits. One of these conduits 36 is visible at the left hand side of Figure 4.
This conduit 36 is connected between the inflatable bodies 22R and 22S through
suitable connectors 38R and 38S. The whole system comprises only one valve means
50 which through a suitable conduit 52 is connected to one of the inflatable bodies
18... in the series, in the illustrated case the last inflatable body 18S of the
series. The valve means 50 cooperates with a pump 52 such that either (sea) water
from the surroundings is pumped from the port 56 through the valve means 50 and
the conduit 52 into the series circuit of inflatable bodies 22, or water is through
the conduit 52, the valve means 50 and the port 56 pumped out of the series circuit
of inflatable bodies. The inflating/deflating operation is under control of a central
controller 60 which preferably is installed on board of the vessel 10.
In the embodiment of figure 4 only one valve in combination with only
one pump is necessary to control the braking force developed by the combination
of all braking means. Controlling this configuration by means of the controller
is rather simple.
In stead of two or more braking units a preferred embodiment of the
system, comprises only one elongated braking unit 18 as schematically illustrated
in Figure 5. Figure 5 shows a view similar to the view in Figure 1. As illustrated
in cross section in figure 6 the elongated body 18 now comprises only one combination
of an outer layer 20, and a braking cushion 22. The pump/valve combination for
inflating/deflating the braking cushion is installed on board the ship 10 and not
shown separately in Figure 5.
In this embodiment the buoyancy capacity is supplied by a series of
annular buoyancy bodies 62a, 62b, ... which are attached with mutual intervals
around the elongated braking unit. The buoyancy bodies may have some flexibility
so that they can be clipped around the unit 18 and secured by means of a suitable
fastener 64 rather easily.