Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- The present invention relates to the laying of a pipeline
su~h as an oil duct or a gas duct, in a body of water.
Whatever method is employed for such laying~ whether it be
by traction from a shore towards open water or by submersion
from the surface of the water from a floating device, buoyancy
elements/ such as floats , are normally associated with the
pipeline to give it a positive buoyancy, as well as cables or
ballasting chains resting on the bed of the body of water. The `
pipeline thus floats , at least temporarily in the midst of water.
When the whole of the pipeline is in position and is to go
into service (for example for the transport of petroleum or
natural gas), its buoyancy is destroyed by releasing its floats
or filling them with water~ The pipeline then comes to rest
on the bed. In certain cases, particularly in very deep water,
it is nevertheless preferred to maintain a permanent buoyancy
at a desired distance from the surface of the water.
In a general manner, the present invention aims to improve
the conditions of laying and of using the pipeline, particularly
so far as the creation and possible destruction of its buoyancy
are concerned. More particularly, it has the aim of improving
the protection of the pipeline against an environment that is
relatively corrosive with time, i.e. the body of water in which
it is submer~ed, and equally, at least in certain cases, to reduce
; the risk of lmplosion of the pipeline due to the depth of its
submersion.
To this effect, according to the invention, the pipeline is
enclosed within a continuous ballast tube and as laying of the
assembly of pipeline and ballast tube proceeds, the space be-
tween the pipeline and ballast tube is filled~ at least
30 temporarily, with a liquid less dense than water, consisting
preferably of a light hydrocarbon.
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The diameter and wall thickness of the ballast tube are
calculated so that, taking into account the low density of said
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liquid, the assembly of pipeline and ballast tube has a positive -
~buoyancy temporarily, that is to say while it is being laid, or
permanently, that is to say after it has been laid, in the case
where it is desired that, even in service, the assembly should
continue ~o float in the midst of water.
Conventional supplementary floats may be added to the ~`
ballast tube to increase its buoyancy effect without it being
necessary to reduce too much its wall thickness, especially in
the case of deep waters.
According to one embodiment applicable to the case where,
having been laid along the whole of its course, the assembly of
pipeline and ballast tube is to come to rest on the bed of the
15 body of water, the buoyancy of the assembly is destroyed by re-
placing , in the intermediate space referred to, the less dense
liquid by water, preferably soft water, or again by a grouting
of the type used in public works to carry out sealings and in-
jections into the soil, for example a grouting with a base of water
and a hydraulic or hydrocarbon binder such as cement, asphalt
or bitumen.
The grouting may be filled with substantially inert materials
such as sands, gravels or clays and contain various ingredients
for example suitable to facilitate its injection.
When the binder is to be set, the setting time is regulated
as a function of the length of the space to be filled in order
to avoid premature hardening. For example, one can use a
liquid having the following composition:
Portland Cement 50 kg.
Sand 0 to 20 kg. according to the length
of the pipeline
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Water 19 kg to 16-17 kg (if the sand is
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absent) and additives - plasticisers and
setting retarders of the order of 1 kg.
(regulated according to the length in-
jected)
It follows from what has been said that the pipeline is, in
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all cases, out of contact with the body of water (generally
sea water) which constitutes a corrosive medium, particularly
because of the presence of oxygen in said water, against which
`~ 10 the pipeline is usually protected by an appropriate coating.
; The pipeline now rests solely in contact with the liquid in the
` intermediate space defined by the ballast tube. Now this liquid
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- (light hydrocarbon or water) is, by nature, or because of its
confinement within a closed space, much less corrosive than the
` 15 body of water. In particular, the water (preferably soft)
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confined in this closed space, ceases to be corrosive when the
; small quantity of dissolved oxygen that it initially contains `~
is consumed without being able to be replaced by contact with the
body of water. This soft water may also advantageously contain
corrosion inhibitors of the water-soluble type.
Furthermore, the fact of maintaining in said intermediate
space a liquid less dense than water has the effect of reducing
by about 25% the external hydrostatic pressure exerted on the
pipeline, which gives advantages both technical (reduction of -the
risk of implosion) and economic (better production rom the
pipeline by optimisation of the ratio between its diameter and
its wall thickness).
All these advantages very largely compensate for the minor
disadvantage of a slight loss of buoyancy of the assembly of
pipeline and ballast tube, by comparison with an assembly in
> which the ballast tube is located by the side of the pipeline
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instead of around it. ~
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The ~ollowing description, with reference to the attached
drawing, given by way of non-limiting example, will enable the
; method of carrying the invention into effect to be better
understood, the particulars contained in both the text and the
drawing forming part of said invention.
Figure 1 is a schematic view, in longitudinal section, -
showing an assembly of a pipeline and a continuous ballast tube
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enclosing said pipeline, submerged in a body of water;
Figure 2 is a view, in transverse section, along the line
II - II of Figure l;
Figure 3 is a view, in transverse section, of a varia$ion -
of the pipeline/ballast tube assembly shown in Figure 2;
Figure 4 is a view on a larger scale of a detail of
Figure 3;
Figure 5 is a part view, in longitudinal section, showing
the connection of two successive sections of the pipeline/ballast
tube assembly.
In Figure 1, 1 is a pipeline, such as an oil duct or a gas
duct, laid in a body of water 2 (such as the sea) between two
opposed shores Rl and R2 thereof, by one or the other of the two
- known methods ttraction from the shore towards open water or
submersion beneath the surface of the water from a floating
device) previously mentioned and adapted to the particular
construction according to the invention, as will be described ;
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hereafter. The pipeline 1 preferably carries (see Figures 2 to
4) an external coating 3, for example of a bitumen base, as
protection against corrosion.
According to the invention, the pipeline 1 is enclosed for
substantially the whole of its length within a continuous ballast
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tube 4 which defines therewith an intermediate space 5. The
ballast tube is also preferahly provided with an external coating
6 as a protection against corrosion. As shown in Figure 2, the
pipeline 1 and the ballast tube 4 may be maintained co-axial with
each other by means of ribs 7 located in various positions.
Alternatively, as shown in Figure 3, the pipeline may be welded
on to the internal surface of the ballast tube. In this case in
order to avoid possible wear of the external coating 3 of the
pipeline, it may be advantageous to insert, in the region of the
interface between the two members, a protective pad or cushlon
9 of plastics material, shown in a larger scale in Figure 4.
In the course of laying the pipeline 1, the intermediate
space 5 between the pipeline and the ballast tube 4, is initially
filled with a liquid less dense than water, preferably a light
hydrocarbon. The pipeline thus assumes a positive buoyancy and
is maintained in position, in the midst of water, by means of
cables or ballasting chains 10 possibly carrying, at their
lower ends, weights 11 resting on the bed 2a of the body of
water. If necessary, the ballast tube 4 may be equipped with
conventional supplementary floats (not shown) in order to increase
its buoyancy effect, without it being necessary to reduce too
much its wall thickness, particularly in the case of deep waters.
~ hen the laying of the pipeline is completed from one shore
to the other of the body of water, one can maintain the con-
figuration of the assembly shown in Figure 1 by leaving the liquidless dense than water in position in the intermediate space 5.
This configuration is interesting particularly in waters of great
depth. In effect, the pipeline does not then descend to the level
of the water bed and furthermore it is only subjected to an
appreciably reduced hydrostatic pressure of about three-quarters
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of the pressure to which it would be subjected i~ it were in
- contact with the water. ThiY reduction in hydrostatic pre~sure
correspondingly reduces the risk of implosion of the pipeline
and furthermore allo~Ys a certain reduction in the -thickness of
the pipeline wall. These two ad-vantages result in a definite
and important economic gain. It goes without saying, of cour~e,
that it is necessary, in this case, to protect the outside of the '
~ ballast tube 4 against corrosion by contact with the body of
- water.
- 10 On the other hand, particularly in a body of water of slight
or average depth, it could be preferred to allow the assembly
of pipeline 1 and ballast tube 4 to rest on the bed 2a of the
body of water. In this case, the light liquid contained in the
- intermediate space 5 is replaced by a denser liquid such as
water, preferably soft, or a grouting. ~he circuit for emptying
; the intermediate space 5 is indicated by the arrows ~1 and ~2 in
igure 1. The densex liquid introduced at El disp~aces ~ ~ ~-
density the light liquid which is recovered at the end ~2
~hus~ the pipeline 1 is protected, during a~ important~
part of lts useful life, from corrosive contac~ with the body "'
o~ wate'rO If the intermediate space 5 is filled with water
' (pre~erably soft), the water is confined withi~ the closed ''
space 5 and ceases to be corrosive as soon as the small quantity
o~ dissolved oxygen that it contained initially has been con~umed'-
without being able to be replaced by'contaot with the body of
water. Due to this fact, the life of the pipeline is consi-
derablg increased even if~ after a certai~ time~ the ballast
tube has been gradually corroded by the body of water Said
soft water may furthermore be rendered even less corrosive by }
the addition of corrosion inhibitors of the water-solub'le type. '
~igure 5 relates to a feature of the invention applicable
to the case in which the pipeline is laid in successi~e sections~'
which are welded to each other. In -thi~ case, there in fact
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,' , ' xists a risk when welding t~lo successive sections th~t the light
hydrocarbon contained in the intermediate space 5 might,catch fire.
This risk is eliminated in the following manner: ';
- The intermediate space 5 between the pipeline and the ballast
" 5 tube 4 is only filled for a part of its length with the light
; hydrocarbon in question. There is placed between the free end
of a section T, already in position and the hydrocarbon mass '~
already,contained in the intermediate space 5 forming part of'this
section, a separator plug comprising bungs 12 (for example of
OlaSt,cs r~ate~ia~ "
lO,j~ foamed ~lastict and a column 13 of a non-inflammable light liquid `,
' `'~ consisting, for example, of a suspension in water of microspheres ,
,-- of a plastics material. ' ,
, It will thus be seen that when one comes to weld a new ~'
section T2 on to the previous section Tl, the mass of hydrocarbon
in the intermediate space 5 is located sufficiently far from the
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source of heat to eliminate any risk of catching fire. ~,~
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It wlll be understood that the embodiments described are ~'
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only examples and that it would b e possible to modify them,
notably by the substitution of technical equivalents, without
departing from the scope of the invention. - ' "~
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