Note: Descriptions are shown in the official language in which they were submitted.
K-8826(CA)
I~IRED UMBII,rC~L ~ABIE
Background of the lnverltion
In offshore seismic operations, arl l~nbi1ic.~1 cab:le is required
to pull a gun array, as well as to provLde air, power and electrical
conductors for shooting operations. Conventlonal practlce in this art
has been to use Jaclceted bundles which contain various c~ir hoses, tension
cables and e:Lectrical conductors or to use arlllored c,li)les con~:tining
hoses and conductors. Such bundLes do not :Lc~st lonlr becallse tow forces,
wave forces and cable hand].ing loads reduce the structurcll inteC~rity Or
the urr~ilical cable to a point where conductors break and leak. The
tension cables tend to abrade the electrica] conductor;, particularly
when the bundle is reeLed arolmd a sheave or a (lr~ under tensioll. More
specifically, the tenslon cables tend to put point ~recssllres, on the
electrical conductors, causing brea~age and insulation leaka~re. 'l'his
problern has been recogniæed hert?tofore, and one olution has been -to
L5 use a disc~ete wire rope tensiorl cable as a "clotheslirle" I`rolrl which to
inter~LLttentLy tie a round ~acketed buntlle oL electrica:l cables and ~ir
hoses. ~'hus, the wire rope cable provides the ~ensile slrength, and the
electrical/air bundle ad~acent to it is not slgrlific~ltly :loaded in tension.
l~lis method has worked reasonably well cbS l(~rl)r> a~. til(? bll~ldl.t,' C.ln be
drawn up "accordiarl style" without reelLnc, it ul). But;, a lor~er and
longer cables are needed ror towing gun subarrays rurther outboard of
the tow vessel, the cor~acted clothesline bundle is too ]ong and causes
too much drag to be effective and practicaL. Another problem with this
method is that very short-radills bends 1Orril in the b~mtile, antl as the
cycles of the be~nds increase, the bundle life is decreast-d.
An alternatlve to the cable~)undle system ls to use an umbi-
lical cable with tension wires, conductors and air hoses cabled into a
single "cable". ~le problem with this is that cyclically bending these
cables around sheaves causes the wires to crush the con~uctors and hoses
reducing cable life due to leakage.
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2. 1815~3215
Still another alternative is to build an armored
cable with an outer-shell tension member, and hoses and elec-
trical conductors within. This is feasible from a strength
standpoint and is reelable but has several problems: first, the
umbilical cable is excessively heavy; second, the terminations
are difEicult to seal; and third, the cables are expensive to
replace and have questionable reliability.
Still another serious problem with all of the above-
mentioned umbilical cable designs is that they tend to have a
large overall diameter as well as a poor cross-sectional shape,
thus causing high drag forces. The problem with high drag has
come about because of increasing requirements to tow guns in a
wide array and at higher speeds as shown in Figure 1, and more
particularly discussed hereinafter, as contrasted with narrower
widths used previously.
An umbilical referred to as Flexpak (TU) is manufactured
by Hydril Corporation (Bulletin 5086). The Flexpak (SU) umbilical
tends to "cup" into flow inasmuch as it utilizes tensioning
cables at both extremities and is not the equivalent of the
present invention.
An umbilical with a faired shape referred to as
Flexnose ~ is manufactured by Fathom Oceanology Ltd. (brochures
MSK 4, September 1976 and MSK 61, August 1976). The Flexnose
is a preformed clip-on or clip-together and is not equivalent
to the integrally molded faired umbilical of the present inven-
tion.
Summary of the Invention
The primary purpose of the present invention is to
provide an underwater cable which has a low drag coefficient
when deployed outboard of a tow vessel. Another purpose of the
present invention is to provide a reliable underwater cable
which is capable of being turned around a sheave while under
~;~
~ 3. 1815-3215
tension and of being wound upon a reel without damage to the
cable. Preferably, the cable is an umbilical seismic cable.
The present invention provides a seismic cable suit-
able for underwater towing comprising a torsionally resistant
torque-balanced tension member resistant to twisting, an elec-
trical bundle and an air hose arranged side-by-side and spaced
apart by a continuously elongate, flexible molded jacket which
is in situ molded between and around and thereby locked to the
tension member, electrical bundle and air hose, the jacket
having a faired shaped and the tension member being in the lead-
ing edge of the insitu molded jacket.
Preferably the electrical bundle has relatively
untensioned conductors twisted around a soft, flexible core
member.
_ief Description of the Drawinqs
Figure 1 is a plan view of a wide subarray configura-
tion.
Figures 2 and 3 are cross-sectional views of cable
configurations.
Figures ~ and 5 are cross-sectional views of various
cable reel-ups.
Description of Preferred Embodiments
As shown in Figure 1, a subarray 10 is towed with an
umbilical seismic cable 11 at a position which is well outboard
from vessel 12. While multiple floats are normally used, only
one is shown here for purposes of illustration. It i5 often
desired for seismic studies to tow floats far outboard on either
side of the tow vessel. The offset width 13 is directly
affected by the fluid dynamic drag forces experienced by
umbilical cable 11. Accordingly, the solution of the present
invention to the problems of getting greater offset width is to
provide a specially-built faired cable design with a tension
. ~ ~
3a. 1815-3215
member or tension members located at the forward or leading
edge of the cross-section thereof. Two examples of this con-
cept are shown in Figures 2 and 3. The faired cable construc-
tion is like an airplane wing shape with the purpose being to
reduce drag. A round cable has a drag coefficient of about
1.2 to 1.3, depending upon its linear diameter. A flat cable
with the same thickness has a drag coefficient of perhaps .13,
an order of magnitude reduction in drag.
Il .
The tension mernbers 20 and 30 in E~ligures 2 and 3 are at
the forwc~r~nost locati.ons followed by the electrical. cabl.es 21 and 31
and air hoses 22 and 32. Tension mernbers 20 arld 30 are prererably
anti-torsi.onal steel wire rope so that when the url~)il.ical cable is
under load it doesn'-t tend to tw:ist cmd i.s very tor.,iollrll.l.y stable.
Next to the tension me~ ers are the electrical b~dles 21 r;~ld 31. I~lese
bundles are purposely designed to be much more f'lexible in the axial
direct:Lon than the tension rnembers 20 c~nd 30. :It is pre['erab:Le to use
twisted paLrc; oI`:Insu:Lated conductors which rlre twi.-;t~ rollrld each
ottler and then layered a-round a circle. A soI't lnsert 23 and 33, such
as soft rubber, is inserted i.n ttle ~niddle of the circle so that i.t acts
rnuch like a ~hinese thuTnbpuller in that it has enou~l softnes<; that when
the cable is pulled, it will contract radia:lly, and therl when tension
ls slacked off, it expands. lhe electrical conducto.r.s 21 c~ld 31 r-~e
not tightly nestled. The twisted pairs of conductors in each layer are
not placecl too close toget}ler so that the e:IectrlcrlL con~ ctors can
flex, resulting :Ln an ax:Lally soft cable.
The next menlber ln the cable aft of the tensiorllllelnber and
the electrical bundles al~e alr hoses 22 arld 32. lile air hose also is
deslgned to be axially flexible. ~',ome of the air hos-:s n~y be used for-
hydraulic hose as needed.
Tenr;ion members 20 arld 30 carl be c()(lte(l w~.lh a -)~'t coatl.rlfr
to make them round and, where there is more thc~l one cable; they can be
circled together ~s S}10WIl i.n Figure 2 or placed s,ide by ~ide ,-~ shown
in 'Ficrure 3. ~n addition, elect.rical bwndles 21 and 31 carl be Jacketed
wlth a soft coat.i.ng material. 'lhe tl~ee el~merlts, tensiorllrlenlbers,
electrical bundles and air/hydraulic hoses, are pasic,ed tt~roudl an In-
Jector mold having a faired shape and the outer p].ast.ic jacket 24 and
34 are rnolded. Nitri.l.e rubber or polyuretharle are pr-eferred n~teria:Is,
3o both being durable and flexible.
The two wnbilical designs in Figure 2 and 3 behave somewhat
similarly due to water flow arownd them, but they are reeled up for stor-
age in different ways as showll in Figures 4 and 5. 'rhe faired flat design
i
5.
can be rolled up like a single rlbbon as shown in Fi~ure 5, whi:Le
the multiple tension cable umbillca~ shown in l~ re 4 will auto-
matically roll up with the nose toward the dr~n. It is wise to provide
adequate reel widtl~ to avoid mu~tiple layers Or the cabLe of Figure 4 on
the reel. It is important to desi~l the aLr atld electrical conrponents
of the cables to be extremely f]exLb]e Tn axlal extenslon ~md compression
so that reeling the cable on a drurn wlll not cause excessive stresses.
The faired ulnb-Llical design as shown in I~igure 5 ccm be ree:Led under a
much lower strain conditlon tt-an the multiple ten;l(,ll ca~'le ulllbilical
design Or Figure ll. TTIis is because the bending axis, or pitch axis,
of the electr:lcal components 21 and 31 and air hose comporlerlts 22 and
32 coincide with the bending axis Or the tension melrlber components 20
and 30.
I~le tension melrlbers 20 and 30 are torque balarlce(l so that the
cable does not twist under varying axial load conditTons. 'lhis is par-
t:Lcularly important L'or the flat, faLred desigrl o~ ~:lgure 5. In the de-
sign Or Fi~ure 4, tlle multiple palred cables can be canbined with opposite
lays to ensure structurlal symMetr7y anA thus avoid undesirable twisting.
A ~urther ac~varltage Or the cable Or F:igure 5 lfi that it can be
rolled up on a ribbon reel, meaT~r~ that it can be rollecl layer on top Or
layer, but it is not necessary to have it layer beside layer as in wind-
lng up the cab'le o['l~lgure 4. rL~Icrefore, it 1; l)o..Tbl.e I~J haVC~;1 very
thin roll of large dlameter as colllpared to a thicker reel of' srnaller
dlameter. There is another maJor advantage Or th:ls cable in the reaction
Or the tenslon melTIber into the reel wlthout havlng to load up any of the
conductors. By colnparisorl, with a round cable, the load in the tension
me}nber will squeeze the condllctors ln the process of' r'eeding into the
reel. 'l~lls is ef`fectlve]y taken out Or the design as ShOWIl In Figure 5
so lt can be used with outrigger reels. The advantage is that the load
is not red through the electrical conductors, but the electrical con-
ductors, air hose, and ar~thing t~lat is put in the cable, in effect, just
go along foI the ride and the tension rnember takes all the tension.
r~
6.
The present invention is useful not only as seismic cable
as above described but also can be utilized in cormection with other
towed bodies, e.g., a submarirle. In addition, by ch~r~?;ing the orienta-
tion of the faired cross-sectlon of the cable frorrl horizontal to vertical,
or some orientation therebetween, it can be used to connect towed bodies
which are directly or more directly behind and below the towing vessel.
The foregoir~, description of the invention is merely intended
to be explanatory thereof. Varlous ch~nges ln the d~?talls of the described
apparatus may be nlade w:Lthlrl the scope oI tlle app~-?r~ilc)(l cl~ rLs witllout de-
parting from the spirit Or the invention.
