Method and apparatus for laying an oil filled cable at depths below the laying equipment

Method and apparatus for laying an oil filled, electric cable at great depths and under conditions whereby the weight of the oil may cause internal cable pressures which will rupture the cable. In the method, the leading cable end is fed over a guide which is higher than the point at which the weight of the oil column between the guide and the cable source is equal to the normal oil pressure in the cable and is then fed downwardly but without exceeding an amount which would cause the oil column pressure after the guide to exceed safe limits for the cable. The pressure on the oil at the cable source is then reduced producing a torricellian vacuum in the cable at or near the guide which is maintained at a value such that the weight of the oil in the vertical section of the cable does not exceed safe limits. The apparatus includes the guide and pressure regulating and indicating devices at the cable oil source and the leading end of the cable.

The invention relates to methods and apparatus for laying an oil filled 
cable at depths with respect to the laying equipment which may cause the 
oil pressure to exceed the stress capabilities of the cable, and 
particularly to the laying of such a cable in a gallery which is at a 
great depth below sea level. 
It is known in the art that oil filled cables are cables having composite 
insulating, paper and/or similar materials, dielectric liquid (insulating 
or impregnating oil) and are of the self-contained type. The oil filled 
cables to which invention laying system applies is suited to the conveying 
of great amounts of energy at high voltages, e.g., by a 400 kilovolt D.C. 
system, 800 megawatts can be conveyed. 
It is known that the transfer of electric energy between two points remote 
from each other can be performed using either D.C. or A.C., the system 
chosen depending upon actually existing factors. The use of an oil filled 
cable rather than of other cable types, can be preferred because of its 
unique reliability properties. 
In the case where great amounts of electric energy have to be conveyed from 
one point to another one which is remote from the former with the sea 
intervening, it may be preferable, because of marine traffic or because of 
the presence of great ice masses (e.g., icebergs), to dig out a horizontal 
gallery beneath the sea and having a slight slope from its center towards 
its end, for allowing water drainage. Into such a gallery, said cable, 
wound in coils, can be lowered and then drawn along it by traditional 
methods. However, this system would require enormous excavations when the 
height of the wells, at the gallery ends, is hundreds of meters deep and 
the gallery involved is one of some tens of kilometers long, which latter 
means that coils of large dimensions have to be lowered into the wells, 
e.g., each carrying 500 meters of oil filled cable which causes numerous 
joints to be necessary in the finished oil filled cable. 
Accordingly, it would be better to have recourse to a system of 
transferring the cable vertically or longitudinally thereof into the well, 
and then pulling it along the gallery bound to a steel rope by the known 
and so-called "lateral bonding" system. However, this system, without the 
methods of the invention, would not permit the use of an oil filled cable 
of the same type, or with the same metallic band blindage and damage 
resistant structures which are generally helicoidally wound on cable 
metallic sheath and which are known to those skilled in the art by the 
name "frettage", as are used for conventionally laid cables, which are 
laid about one and a half meters below ground level, because special and 
expensive reinforcement structures would be necessary. 
It is believed to be clear that, for reaching the gallery, a cable should 
be vertically lowered into a well provided for this purpose. Since such a 
well is some hundreds of meters deep, then each cable section, when 
passing through the lowest point of its run, will be subjected to an oil 
column which will exert on the cable a pressure of some tens of relative 
atmospheres in addition to the pressure for which the cable has been 
dimensioned, and it would then be necessary to provide in advance for an 
overdimensioning of cable frettage layers, such overdimensioning being 
exclusively for the purpose of resisting the overpressures occurring 
during the laying of the cable. This fact would not only make even an oil 
filled cable very expensive, but also difficult to handle. The difficulty 
could be overcome by way of performing within the cable a great number of 
stop joints which could reduce the extent of the lengths lowered and, 
therefore, the highest laying pressure. This, however, would enormously 
increase the cost of laying, because of several connections, and would 
introduce many weak points, as generally, the joints are, from the point 
of view of electrical performance. Overpressures which would occur during 
laying would, however, have an influence not only on cable structure, but 
also on all accessories which are indispensable in a cable installation 
and for its maintenance. It should be considered, for example, what 
complicated equipment would be necessary for feeding a cable under some 
tens of relative atmospheres, instead of 0.5 to 3 relative atmospheres, as 
the case normally is for oil filled cables installed at a conventional 
level or for short lengths thereof. 
The purpose of the present invention is to teach a laying system which 
allows for the use of conventional type oil filled cables in long lengths 
thereof and even of some tens of kilometers, e.g., made by the method 
suggested in U.S. Pat. No. 3,918,281, without any structural modifications 
either of the cables or of connected installations. For example, with 
relation to both feeding and insulating oil compensation, simple pressure 
compensators, which are inexpensive and easily transported, e.g., of the 
type disclosed in U.S. Pat. No. 3,695,297 and in Argentine Pat. Nos. 
197,918 and 199,124. 
More particularly, one object of the present invention is a system for 
laying oil filled cables in a gallery which is at a great depth below sea 
level, in which the cables are lowered longitudinally thereof into a 
vertical well. The system is particularly adapted for laying cables for 
the transfer of great quantities of high voltage energy between two remote 
points and which have the structure used for cables laid at or close to 
sea level. In such system, torricellian vacuum is produced within the 
cable which acts at least on the vertical portion of the cable, while, at 
the same time, an insulating oil column with a pre-set pressure and 
consistent with operating limits of the cable, is maintained in the 
forward end, or end in the direction of advance, of the cable. A preferred 
embodiment of the invention comprises the following phases: 
1. the transferring of forward end of head of said oil filled cable from a 
skein thereof into a vertical well while passing it through a path 
comprising, at least for its initial portion, in the form of a suspended 
and tensioned cable and at the end of such portion at least a guide means, 
the tangent to the top of which is located at a height "h" which is 
greater than the height "h.sub.o " of insulating oil column which 
corresponds to zero relative pressure with respect to lower end of the 
skein; 
2. the reduction of pressure within oil filled cable, by acting on the 
internal pressure at the skein, so that oil level along said initial 
portion takes a height "h.sub.i " with respect to lower end of the skein, 
which is lower than or equal to said height "h" but, in any case, higher 
than said height "h.sub.o ", said pressure reduction step being performed 
when the pressure on said cable forward end reaches a pres-set, operating 
or limiting value during the transit of the forward end into the vertical 
well. 
A further object of the present invention is apparatus for carrying out 
said laying system which comprises at least the following means: 
i. an insulating oil feeding tank and a pressure measuring instrument for 
measuring the pressure of said oil connected to measure the internal 
pressure on said skein; 
ii. a cable suspension and transferring device arranged vertically above 
the well and at a higher position than any other suspension and guide 
means provided for the cable and in any case at a greater height, with 
respect to the lower end of the skein, than the portion of an insulating 
oil column at which the relative pressure is equal to zero.

The oil filled cable 10 shown in the drawings preferably is manufactured 
according to the processes described in said U.S. Pat. No. 3,918,281 in a 
long length, e.g., having a length of one or more tens of kilometers, and 
it is of the structural type dimensioned for being used in a conventional 
manner, which shall be defined, for ease of illustration, as dimensioned 
for use at sea level and with an insulating oil pressure, for example, 
between 0.5 and 3 relative atmospheres. Of course, it will be apparent to 
those skilled in the art that the cable 10 may be manufactured by any 
other appropriate process provided such process allows the cable lengths 
to be just as long, without any continuity problems and, in particular, a 
lead sheath free of defects such as inclusions, blow-holes or 
through-holes. 
In the embodiment illustrated, the fluid oil filled cable 10 has to be laid 
into a gallery 11 which is, for example, one or several tens of kilometers 
long and which is located at a great depth below sea level, such as, one 
or more hundreds of meters. The gallery 11 is generally horizontal or, 
preferably, as shown in the drawings, provided with a slight slope from 
central section c toward each of the ends and acts to drain water, etc. 
toward the ends. 
In the gallery 11, all facilities (not shown) which are necessary for 
drawing the cable 10 in the direction of the arrow P during its laying, 
for its maintenance and for communication of the operator with the 
environment outside of the gallery 11, etc. are disposed. Access to the 
gallery 11 is obtained through two vertical wells, one of which, well 12, 
is located on the shore in the area of the cable laying and the other 
well, 23, (FIG. 3) on the remote shore area to which the cable 10 must 
extend, said wells 12 and 23 having a depth of some hundreds of meters, 
e.g., between 300 and 400 meters. 
In both FIG. 1 and FIG. 2, the cable 10 is wound into a skein 13 and is 
carried by a water craft 14. It is to be understood, however, that, for 
various reasons, it may be less expensive and technically more appropriate 
to transfer the cable 10 from the water craft 14 onto solid ground to 
another skein, and then start the laying of the cable 10 from solid 
ground. 
Said water craft 14 also carries conventional cable laying devices, such 
as, for example, a first roller conveyor (not shown) which serves to feed 
the cable 10 while unwinding it from its skein towards the shore. A 
guiding means comprising at least a pulley or roller 15, which is 
supported by any conventional means (not shown), supports the cable 10 and 
guides its movement in the vertical well 12. Inflection of the cable 10 at 
the top of the roller 15 and the curved section of cable 10, which is 
indicated by the reference F, will hereinafter be called the curvature 
section. 
The roller 15 is so arranged that the tangent line "t" at the highest point 
or top of the roller 15, with respect to the lower end of the skein 13 
which, in FIGS. 1 and 2 is the lowest turn .pi.i, is at a height such that 
h&gt;h.sub.o, where h.sub.o is the height of the insulating oil column at 
which the relative pressure is equal to zero. In other words, the height 
h.sub.o is the point at which the upward force from the pressure applied 
to the oil at the craft 14 is balanced by the downward force due to the 
weight of the oil. Height h might also be measured with respect to a skein 
turn other than the lowest one, in the event that safety reasons should 
indicate it to be desirable. For example, for taking into account critical 
arrangement of horizontal windings, height h might also be measured with 
respect to upper skein turn .pi.s. However, height h is lower than the 
insulating oil column which would provide a pressure exceeding that for 
which the cable 10 is designed to withstand. 
The trailing or back head 16 of the cable 10 in the skein 13 is connected 
to a first cable pressure measuring device, e.g., a gauge 17, and to a 
tank 18. The latter is of the low pressure oil feeding type and, 
therefore, easy to handle, inexpensive and, preferably, is of the type as 
described in both U.S. Pat. No. 3,695,297 and in said Argentine patents. 
The other forward or leading head 19 of the cable 10 is connected (FIG. 2) 
at least to a second cable pressure measuring device, preferably, a vacuum 
gauge 20, and is also provided with an overpressure valve 22. The tank 18, 
gauge 17, roller, guiding means, suspension or transferring means 15, as 
well as the vacuum gauge 20 and overpressure valve 22, are comprised in 
the means forming the apparatus for carrying out the laying process of the 
present invention, which will be described hereinafter with reference to 
the drawings. 
The oil filled cable 10 is unwound from the skein 13 into which it has been 
previously wound, by pulling its forward head 19, the cable 10, at least 
in its initial portion of the path it follows, taking a suspended cable 
shape to a first (not shown) rolling means or roller from which it is then 
conveyed onto the other guiding or suspension means 15 supported on solid 
ground. Finally, by way of the curvature section F, the head 19 is 
transferred into the vertical well 12 where it is lowered, bound to a 
steel rope (not shown) by means of a system which is known as "lateral 
bonding". Preferably, the transferring of the cable 10 into said vertical 
well 12 takes place continuously and possibly at constant speed, even 
though stopping, during its laying, for technical or checking reasons, is 
permissible. 
As stated hereinbefore, the oil filled cable 10 is of the type that can be 
used at sea level and its normal insulating oil pressure can therefore be 
from about 0.5 to about 3 relative atmospheres. It follows, therefore, 
that it has a structure which is able to stand these pressures but not 
pressures significantly higher. Therefore, if one were to continue 
transferring the cable 10 so that it continues to be lowered into the 
vertical well 12, i.e., with the whole of its oil load, pressure on the 
advancing head 19 would progressively increase until it reached, at the 
well base, a value between 30 and 40 relative atmospheres. It is evident 
that such a cable 10 would get irremediably damaged long before it reaches 
said well base and would be damaged as soon as the loads involved, and 
therefore stresses on its structure, would exceed limits provided for by 
cable construction. 
In accordance with the present invention, the resulting oil pressure due to 
the weight thereof is reduced below a damaging value by subjecting the 
oil, at least downstream of the curvature section F and along the whole 
length of the cable 10 into the vertical well 12, to a torricellian 
vacuum, while maintaining on the advancing head 19 an insulating oil or 
impregnating oil column, corresponding to a pre-established positive 
pressure consistent with the limits provided for in the cable 
construction. Downstream of the curvature section F, the oil filled cable 
10 is relieved from any stress, and it does not undergo other concomitant 
stresses. Therefore, there is no risk of cable breakage which may permit 
air -- and with it electrically undesirable moisture -- to filter into the 
cable 10. 
The critical point is the head 19. It is known to those skilled in the art 
that joints and terminals always represent the weakest electrical zones of 
a cable, but the head 19 is under a positive pressure, and any 
micro-defect would be filled by insulating oil which would percolate and 
which, in turn, would oppose the ingress of air. 
A preferred embodiment of the system, according to the present invention, 
comprises acting on the trailing head 16 of the cable 10 by means of the 
tank 18 and reducing the insulating oil pressure to such a value that the 
oil level along said catenary takes a height h.sub.1 lower than or equal 
to height h, but higher than the height h.sub.o of an impregnating oil 
column corresponding to zero relative pressure, preferably, with respect 
to the lowest turn of the skein 13. Such pressure reduction is carried out 
when, by reason of the cable transit along the vertical well 12 as the 
head 19 advances, a pre-established positive pressure arises, consistent 
with the limits provided by the cable construction. 
Suppose, for example, this pre-established pressure occurs when the forward 
head 19 is in the position shown in FIG. 1 and for an insulating oil 
column of a height h.sub.2 corresponding, as hereinbefore-mentioned, to a 
pre-established pressure consistent with the limits imposed by cable 
construction. The windings of the skein 13 and internal head 16 remain 
under an insulating oil positive pressure, and therefore, they are immune 
to air infiltration. When pressure is reduced and the pressure, as read on 
the gauge 17, starts to decrease with respect to pressure at the height h, 
a liquid vein within the curvature section F breaks up and a free surface 
.sigma.1 of insulating oil within the portion between the skein 13 and the 
pulley 15 reaches the height h.sub.1 which may be equal to h but, 
preferably, lower, as it shown in FIGS. 1 and 2. Between the free surface 
.sigma.1 (FIG. 1) within said last-mentioned portion and the surface 
.sigma.2 of the insulating oil column pressing on the head 19 there is, 
therefore, a torricellian vacuum. 
As the cable 10 further descends along the vertical, the free surface 
.sigma.2 descends into the well 12 following the head 19 in its descent, 
whereas the free surface .sigma.1 remains in a constant or fixed position. 
This means that the cable 10 descends into the well 12 under torricellian 
vacuum condition. 
In case the height h.sub.1 of .sigma.1 coincides with the height h, then, 
torricellian vacuum would be obtained at least rearwardly of the curvature 
section F with respect to the advancing direction of the cable 10, i.e., 
within the vertical portion of the latter. 
It may happen that during descent of the cable 10 into the vertical well 12 
and when the cable 10 has already been introduced into the gallery 11 in 
which it advances in the direction of the arrow P, oil draining from 
papers has a tendency to cause a pressure increase on the head 19. The 
operator in charge can read this fact from the vacuum gauge 20 and he then 
opens the overpressure valve 22 and re-establishes the wanted equilibrium. 
If, on the other hand, due to micro-defects, the head pressure h.sub.2 
decreases down to values which tend to be too low, i.e., inconsistent with 
the purpose of avoiding air infiltration, the tank 18 is operated in order 
to give rise to the condition that h.sub.1 &gt;h. Due to this condition, 
insulating oil overflows from the portion of the cable 10 upstream with 
respect to the curvature section F to downstream of the cable 10 and it is 
permitted to overflow as much as necessary for re-establishing and 
maintaining on the head 19 the pre-established pressure, or a 
pre-established column h.sub.2 of insulating oil consistent, as already 
stated, with limits imposed by cable construction. 
When the cable 10, after descending along the whole length of the well 12, 
enters the gallery 11, the insulating oil tends to take a horizontal 
position, and the insulating oil column acting on the head 19, tends to 
decrease, until, after a certain amount of the cable 10 is in the gallery 
11, a condition arises for which such an insulating oil column would 
become equal to .phi., where .phi. is the internal diameter of the cable 
10. From this moment on, the situation becomes critical. If the cable 10 
continues to advance, a negative relative pressure, readable on the vacuum 
gauge 20 would arise at the head 19. As this must be prevented, the 
initial desired oil column pressure is re-established, for example, in the 
column h.sub.2 at the free surface .sigma.2, by way of again operating 
the tank 18 so that said level h.sub.2 is reached letting, as hereinbefore 
stated, insulating oil overflow beyond the curvature section F, for a 
condition h.sub.1 &gt;h. 
Advancing of the cable 10 along the gallery 11 in the direction of the 
arrow P is preferably obtained by way of a system known by those skilled 
in the art as "lateral bonding". The cable 10 is laterally fixed on a 
steel rope provided with especially made straps for cable fixing (not 
shown), which rope is pulled by an appropriate means, e.g., a small 
electric train, also not shown. 
After it is laid, the cable 10 may be, as shown in FIG. 3, a continuous 
element along the whole length of the gallery 11 (some tens of kilometers) 
and may be joined by means of stop joints at both heads 19 and 16, to 
cables 24 and 25, which cables 24 and 25, in turn, are respectively 
connected with feeding and utilization equipment, and constructed for 
resistance to high pressures. 
As will be observed, these cables 24 and 25 proportionally more expensive, 
per linear meter, than the cable 10, are each of only a few hundred meters 
in length, and therefore, they are of a negligible length with respect to 
the length of some tens of kilometers of the cable 10. Accordingly, the 
installation will nevertheless be very economical, due to the 
inexpensiveness of the cable 10 and the simple installations required for 
maintenance of such a cable. 
Of course, should the gallery 11 length be excessive or technical reasons 
favor it, elements of the cable 10 may be interrupted by further stop 
joints, which also allow for oil feeding. For example, one of these joints 
might be provided at the intermediate section c of the gallery 11. 
It will be apparent to those skilled in the art that means for carrying out 
the method of the invention and the facilities involved such as 
transportation devices, guides, cable suspension means, etc., may vary 
according to actual requirements. 
Although preferred embodiments of the present invention have been 
illustrated and described, it will be apparent to those skilled in the art 
that various modifications may be made without departing from the 
principles of the invention.