CA1234994A - Underwater trenching machine - Google Patents

Abstract

UNDERWATER TRENCHING MACHINE

ABSTRACT OF THE DISCLOSURE

There is described a machine for underwater trenching for pipelines in which a sled comprising two pipe loops arranged in vertical planes at opposite sides of the pipeline and interconnected by transverse pipes is supported on and guided for movement along the pipeline by rubber-faced rollers. High pressure water is supplied to the loops through connection nozzles in the front vertical runs of the loops break up the soil in the face of the trench.
Nozzles of water jet pumps suck soil from the trench through the forwardly facing inlets, adjacent the bottom of the sled, of two eductor tubes carried by the loops at opposite sides of the pipeline, and discharge it through discharge outlets, directed respectively outwardly sideways and mainly rearwardly of the sled, The water and soil discharge produces a reaction which propels the sled forwards.

Description

is invell~ic)n rclcltcs to arl undclh~er ~renct~
n,~chin~ hhiCh ~ill cut a trellcll under a pipeline and al50 r~n,ove the spoil from tl,e tr~ncll to allo~ tt-c pi?eline to fall into the trench undeJ- its o~n ~eight.
Some under~ater trenching machines co~,prise a sled ~hich is tethered by a cab]e to a bar~e ~hich, as it moves for~ards on the surface of the sea, pulls the sled along the seabed. US-A-4114390 describes one such machine in ~hich the sled is saddle-sllaped frame an~ is supported on the pipeline by transverse rollers, tl,e fi~ame comprising tubes through which ~ater is fed under pressure to flllidi~ing nozzles so that the non-cohesive bottom of the seabed is fluidized and the pipeline sinks into the fluidized bottom material as ~he sled is pull-d alono the pipeline by a cable connected to a hinch on the barge.
~'ith such machines problems arise due to an increase or decrease in the cable tension as the barge rides on the sea. It is therefore desirable to produce the for~ard propulsion from the sled itself and independent of barge motion. l'o this end, tractor type machines are knohn, havillF
hydraulic or electric orive, ~hich are propelled by h~heels endless tracks in engagement h~ith the pipeline itself (e.g.
-A-4087981) or the seabeo. A prcblem is to ~et enoneh friction between the wheels and the pipe hithout damaging the latter or its anti-corrosive coating9 or, hhere the tractor drives on the seabed, for the soil to be firm enough to prevent the track sinking into the mud. Another knohn machine, described in US-A-3576lll, comprises a sled ~hich uses high pressure water jets aimed in the direction opposite 3 to that in which the sled is to mo~e. These jets propel the sled for~ard. Some jets are aimed forward to cut the trench and others generally uphards to help push the sl~oll out of the trench, ~hich is assisted by discharging air at the bottom of the sled. The main disadvantage of this system is that in order to get a net forhard thrus~ on ~i~

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I t~ lc(l ~r~ t l)l (lihc~ l lc~ c~,]
c,f lhc s~ed than ~, t~e front. Al~o ~ ~paratL ~
lift s-s~em is requirecl fol spoil rcmo~al. ~he ef1i-ciency of this t~e o~ mclchine is ~ely ~ow, and decr~ases with depth of operation.
GB-A-147~459 describes an ul-clel-water trenchin6 n:achine for dredging trenches on the seabed for recei~ing pipes or cables comprising a sled which is towed from a supply boat by means of a tow wire ~ttached to the sled. Two pairs of downwardly directed wat~
jet nozzles are mounted onè behind the other at the forwdrd end of the sled to loosen the material oî
the seabed which is then sucked up by two ejector pumps each having a suction in~et pipe of which the bottom ends are disposed aft of the water jet noz~]es and below meta~ strips along the underside of the sled and forming a skid or rullner. Each suction inlet pipe, which is inclined upwardly and rearwarcily from its inlet end, is attached at its aft end to an angled ejector portion the aft end of whieh is seeured to an upwardly, rearwardly and outhardly directed straight diffuser having an internal niameter inereasing uniformly to the discharge aperture of the pump~ Jet pipes aligned with the eentral longi-tudinal axis of the respective diffusers extend throughthe wall of the angled ejector portions and terminate in a nozzle whieh faces towards the discharge aperture of the respeeti~e diffuser. ~'ater under pressure is supplied to the jet pipes to cause the ejector pumps to suek up the loosened seabed material and diseharge it to the sides of the trench through the discharge ~pertures of the diffusers. Buoyancy tan~s are provided on the sled for adjustment of the buoyancy.

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i ~ ' ( ' 1 ' C ~ ' t C) i) ~ t 11 ( ' I T ' ~ C I ~ o ) t ~ ~1 ir~ ~f '.I~c)nt ~ cl ]~c~ c ~C)~
in a V-shaped fol~maLioll, an(l is 1ai~l into ~h( tr~llch or~ ~eavin6 th~ rear pair of 1o~1er~ maehine is such that it mllst be elccted on ti,~ seabed. It is suggested on page 2 1ines C)~-103 o~ thc description that the action of the nozzles and ejector pumps pro~ide a reactiYe forward force component hhich can be used to propel the machine along the seabed providing the buoyancy of the machine is appropriately adjusted. In practice, hohever, it is not practicaDle to propel the sled by the said reactiYe for~ard force component alone since the weight of the pipeline or cable to be laid, supported on the front and rear pairs of rollers on the sled, aets to press the skids of the sled into the soil of the seabed. ~Ihe reactive force provided by the ejector pumps has a do~.nwa1d çomponent which also presses the skids into the seabed.
The resultant high-frictional resistance prevents tlle sled from being propelled by the reactive forward force component of the ejector pumps. To reduc~
thè frictional resistance would require very large buoyancy tanks capable of lifting the weight of the pipeline or cable (e.g. a pipeline of 60cms diameter filled with water during laying can weigh several tons) in addition to the ~eight of the machine, and such a lifting force would increase the friction between the pipeline or cable and the pairs of supporting rollers, with an abortive result and substantial risk of damage to the pipeline or cable. The frictional resistance between the skids and the seabed can only be alleYiated by the lifting force imparted by the tow line to drag the skids out of and along the soil of the seabed.

~ S-A-4~&~9~ cril)~ ant ~ f - ~
under~ater trenching machine for a pipe~in~ or ca~e t, be laid in the seabed, co~,prisin~- ~ sled havin6 m~ans îor support,ing it on and ~uidin6 it for mo~emcnt along the 5 pipeline or the like and comprising two side members spaced apart on opposite sides of the pipeline or the like by interconnecting transverse members including freel~ rotatabl~
transverse rollers adjacent the top of the sled hhere~,~
the sled forms a kind of saddle ~hich straddles the pipelin~
10 or the like, supported thereon by said transverse rollers and maintained in a generally upright position ~n the pipe-line or the like by buo~anc~ tanks on the sled. The front of each side member comprises an upright, t,ube equipped hith a pluralit~ of verticall~ arranged nozzles disposed 15 for directing water jets at the face of the trench for breaking up soi~ in the forh~ard path of the sled. Each side member carries an eductor tube having its inlet adjacent the bottom of the sled and its discharge section above the top of the trench and aimed in a sideh~ays outhards 20 and mainly rear~ard direction of the sled whereby to drah~
up portions of the spoil bro~en up by said nozzles and to discharge these portions symmetricall~ to opposite sid~s of the sled. To provide the necessar~ suction the upper end of the eductor tube is provided with a venturi co~posed 25 Of a venturi housing in ~hich is positioned a high pressure hater nozzle. The nozzle is directed to~ards an openi-ng in the opposing wall of the venturi housing h~hich is of smaller diameter than the eductor inlet to the housillg and ~hich connects ~ith a divergent discharge tube which 3 directs the spoil to the side of the trench. Propulsion of the machine is positiYely derived ~long the pipeline by 3L~dt3~

1 drive rollers in fric~ional engagement h~ith oppocite sides of the pipelir.e,at least one drive roller being driven by means of an electric or hydraulic motor and suitable reduction gearing. Forward guide rollers which engage the sides of the pipeline in advance of the water jet nozzles at the front of each siàe member sense the pipeline direction and produce a torque to guide the machine on to the centerline of the pipeline. The jets from the forward nozzles must be sufficiently powerful to cut away at le~st the upper part of ~he face of the trench prior to the advance of the forward guide rollers. The positive driving engagement of the drive rollers ~ith the pipeline prevents any thrust from the eductor discha-ges from propelling the sled forwards. The sole propulsion of the machine is derived from the drive rollers.
No propulsion is provided by the eductors. The reduced size of the outlet from the venturi housing to the discharge section is liable to become clogged by larger pieces of spoil entering the housing through the eductor inlet, and since the discharge from the eductors takes place at â height above the top of the trench lon which the pipeline initially rests) which is approximately equal to the depth of the trench being dug, any imbalance between the sideways thrusts produced by the eductors tends to tilt the machine a~out the axis of the pipeline.
An object of this invention is to provide an underwater trenching machine and method which overcomes the above disadvantages, in which the machine is propelled ~ y by the means used for soil removal from the trench. To this end, the invention consists in a sel-propelled underwater trenching machine for a pipeline or the like to be laid, comprising a sled having means for supporting it on and g~iding 1 it for mo~ement ~lollg the pipeline or ~he li~e and comprising two side mem~ers spaced apart on opposlt~
sides of the pipeline or the like by in~erconnectlr,s transverse members including freely rctatable tran~.~e~-rollers adjacent the top of the sled whereby thesled forms a kind of saddle which straddles the plpe-line or the like, supported thereon by said transvers~
rollers and maintained in a generally upright p~s~
on the pipeline or the like by buoyancy tanks on the sled, the front of each side mem~er comprising a plurality of front water jets directed at the face of the trench for breaking up soil in the forward path of the sled, and each said side member carryin~
an eductor tube comprising an upwardly e~tending section having an inlet adjacent the bottom of the sled and connected at its upper end to a discharge section aimed in a sideways outwards and mainly rearward direction of the sled above the top of the trench and along the axis of which a water jet from a nozzle is directed to draw up portions of spoil broken up by saidfront water jets and to discharge these portions to opposite sides of the sled, charactel--ised in that the upwardly extending section of each eductor tube leads to an elbow providing a smooth transfer to its respective discharge section which is just above the top of the trench and is substantiaily horizontal and of substantially uniform diameter throughout its length, the respective water jet nozzles being located in the respective elbows and aimed along the respective discharge sections from which the water-spoil mixtures are discharged ~ P~o~ g '~ to provide ~he-~o~e means ~e- the sled along the pipeline or the like.
The high pressure pump jets produce suction in the eductors that lifts the mud out of the ~rench ~ ~3~

being cu-t and discharges it to -the sides and rear oE the -trench, the water je-t and spoil clischarge producing a reaction on the sl.ed in the forward direction which is used to propel the sled forwards.
The invention also consists in -the method of underwa-ter trenching which consists in propelling a sled along a pipeline or the li]ce to be laid in the trench, breaking up soil in the forward path of the sled, and removing spoil from the trench as the sled advances by a pair of eductor tubes and associated water jets which produce suction a-t -the inlets of the eductor tubes adjacent the bottom of the trench and discharge the spoil rearwardly at opposite sides of -the trench respectively, characterised by said water jets constituting a common source of power bo-th :cor spoil removal and propulsion of the sled along the pipeline or the like, -the forward thrus-t produced by the discharge o:E -the wa-ter-spoil mixture propelling the sled forwards, on freely ro-tatable rollers res-ting on the pipeline.
In order -tha-t the invention may be more clearly understood, reference will now be made to the accompanying drawing t in which:-Figure 1 is a side view of one embodiment of a sled according to the invention~ and Figure 2 is a plan of Figure 1.
Referring to the drawing, -the sled 1 comprises a frame formed of two loops of pipe 2, 2' spaced apart in parallel upright planes by interconnecting pipes 3 to form a ~ind of saddle which straddles a pipeline 4 to be laid in a trench 5 cut by the sled. The sled rests on the pipeline 4 by transverse rollers 6, 7, faced with rubber or other resilient material, mounted between the pairs of front and rear vertical runs of ~3~
- . -1 t~c t~ r-ipc ~o~>s r~r~cti~c~. T~lc ~d i~ 7`t~
tr~nsversely uided on t~l~ pir)e]irle ll b~ irs of rollers B,8' 9 9,91 ~ EIlSo resiliently ~ced, mounted ~or rot~tion about vertica~ axes at t~le front and re.;r 5 of the sled respecti~el~ on Lhe pipe loops at opposite sides of the pipeline 4.
Thc sled also comprises two eductor tubes 10,10' carried fro~ the resp~ctive pip~ loop~ 2 an~ 2'. The inlet ends 11 of the eductor tubes, locatcd adJacent the l)l)t-tc,m of ~;~e s]ed,may face fol~ard1y and in~ardl~ ch i~1ctconnects .ith a vertical section 12 ~hich leads to an elbo~ 13 con~ectin~ ~ith a horizontal ~ischarge section 14,14' ~.hich is directed out~ardly sideways and mainly r~ar~ardly of the sled, the discharge sections 14,14' extendin~ syr~etrically to vpposite sides of the sled.
In each elbot~ 13 is a no~zle 15 aimed along the respecti~e discharge section. I~ater is fed to the no~zles 15 by feed pipes 16,16' ~hich may be interconnected so that only a single hose connection 17 is required. Bct~.een each elbo~ 13 and its disc~lrge section there is pre~er-ably a ~enturi pipe reducer 24, for e~ample ~ro~ 30 cms dia~
m~ter to 25 cms diameter, eorresponding to the diameters ol the tube secticns 12 and 14 respectively.
E~ch front ~ertical run o~ a pipe locp 2,2' is provided with nozzles 1~ to provide small jets of ~ater aimed at the front of the trench 5. These breal~
up the soil in front of the trenching machine. Further nozzles 19 may be pro~ided along the bottom run o~ each pipe loopO High pressure ~ater from a pump unit on a barge is supplied to the pipe loops through a hose connection 20 at the top rear end or front end of one of the pipe loops/
The soil loosened by the nozzles 18,19 is remo~ed from the trench through the eductor tube~. The hih pressure water streams of the ~ater jet pumps, aimed c3l~ ,1lorl~ t~ ` } I . I T ~ ~` f . ~ ~ C l ~-ICtiOsl in ~hc ln3uctor~ ~liC~ 5 th~' f`r~Oi~ O~lt oi tl~
trench 5 via their inlet er~d~ nc3 disch~r~ it to l~le side ~nd rear of the sled throucrh the discharE~ sections 14,14'~ The reaction of the ~ater ~et pump and the ~poil propels the s~ed for~ards and keeps it in cont~ct ith the trench face, b~inc ~uided ~lon~ the pipcline 4.
~linimum nc~ati~e buo~ancy of t~e m~chine, for easy di~er handlin~,is achieved by buoyancy tan~s 21 ~t the top ~f the pipe loops. They also provide an up-righting force to Xeep the sled upright as it rides ~10ng the ]pipeline 4. Transverse rubber faced rollers ~2,23 may also be fitted to the machine, after it ~s been ~osit;oned on the pipeline, to prevent it frsm li~tin~. Each tar~ 21 is pro~ided ~i*h a val~e in its upper part to a1lo~ air to escape, a~ an open pipe 21' t~ allo~ a diver to fill tne tan~ ~ith air af~er the sled is positioned on the pipeline 4 and the rollers 22,~3 locked under the pipeline, The pipes 21' being open prevent the buoyancy ta~cs from becoming pressurise~ vesselsD
The discharge sections 14,14' are of uniform dia-meter to maint.ain maximum ~elocity of dischal~e.
The ~oz~les 18 aimed at the front of the trench are preferably aimed up~ardly to bloh some soil out of the trench and reduce the amount of soil to be remo~ed by the eductors. In-~ardly directed nozzles on the front pipe runs l.oosen the soil betheen the pipe runs.

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( I

I,cl~ t.l~ l2~it t~ r~ r t~ u~ l f(~
remova~ is nlso use~ for ~ror~ ;ion, onl~ c~re po~r ~ourc bein~ neces~e~ar~. T~e sroi~ rc~val prese.ur~
.i source does not have to be as hi~h a~ that needed to e~t the trench. This ~o~ers the cost of the pumps required o~ the bar~e, ~urther, the ~ater jet pump efficicncy does not decrease noticeably, if at all, ~ith increasin~ de~th of operation. ~he spoil is also 1~ discharged well to th~ side Or the trench being cut.
liThile a particular embodiment has been described ~
it ~ill be understood that various modificaticns m~y be .ade without departing from the scope of the i~cntion.
~or e~ample the machine may be equipped ~ith m~re than one pair of eductors. If desired the spoil re~oval, propulsion ~nd ~jetting ~ater can be supplied through a common h~se from a single pump source. The ~se or hoses ma.y be connected to the front end of one or both of the pipe loops. In another embodiment the soil at the face of the trench may be broken up by a mech-anical cutt.er~ or, by a combination Or mechanical cutting means and ~ater jetsl the loosened soil bein~
removed by the eductors and discha~ged out~ardly and rear~ardly to propel the ~achine for~ardly and against the face oI` the trench bein~ out~

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A self-propelled underwater trenching machine for a pipeline or the like to be laid, comprising a sled having means for supporting it on and guiding it for movement along the pipeline or the like and comprising two side members spaced apart on opposite sides of the pipeline or the like by interconnecting transverse members including freely rotatable transverse rollers adjacent the top of the sled whereby the sled forms a kind of saddle which straddles the pipeline or the like, supported thereon by said transverse rollers and maintained in a generally upright position on the pipeline or the like by buoyancy tanks on the sled, the front of each side member comprising a plurality of front water jets directed at the face of the trench for breaking up soil in the forward path of the sled, and each said side member carrying an eductor tube comprising an upwardly extending section having an inlet adjacent the bottom of the sled and connected at its upper end to a discharge section aimed in a sideways outwards and mainly rearward direction of the sled above the top of the trench and along the axis of which a water jet from a nozzle is directed to draw up portions of spoil broken up by said front water jets and to discharge these portions to opposite sides of the sled, character-ised in that the upwardly extending section of each eductor tube leads to an elbow providing a smooth transfer to its respective discharge section which is just above the top of the trench and is substantially horizontal and of substantially uniform diameter through its length, the respective water jet nozzles being located in the respective elbows and aimed along the respective discharge sections from which the water spoil mixtures are discharged to provide means for propelling the sled along the pipeline or the like.

2. Machine according to claim 1, characterised by guide rollers mounted for free rotation about vertical axes at the front and rear of the sled respectively at opposite sides of the pipeline or the like, the front guide rollers being mounted rearwardly of the front water jets.

3. Machine according to claim 1 or 2, characterised by each side member also comprising a further plurality of water jets along the bottom of each side member and directed at the base of the trench.

4. Machine according to claim 1 or 2, characterised in that the side members comprise two loops of pipe spaced apart in parallel upright planes by interconnecting pipes to straddle the pipeline or the like and provided with the freely rotatable transverse rollers to support the sled on and for movement along the pipeline or the like, the loops of the pipe having at least one inlet for connection to a source of water under pressure, the front runs of the loops being provided with nozzles for directing water jets at the face of the trench, and further water jet nozzles being provided along the bottom runs of the pipe loops.

5. Machine according to claim 1 or 2, characterised in that the sled is provided with buoyancy tanks at the tops of the side members, each tank being provided with a valve in its upper part to allow air to escape and an open pipe in its lower part through which the tank may be charged with air, the sled also being provided with rollers which can be located below the pipeline after the sled has been positioned thereon.

6. Machine according to claim 1 or 2, characterised in that each side member carries one of each of a plurality of pairs of eductor tubes, the eductor tubes of each pair having their discharge sections directed sideways outwards and mainly rearwardly to opposite sides of the sled.

7. Method of underwater trenching which consists in propelling a sled along a pipeline or the like to be laid in the trench, breaking up soil in the forward path of the sled, and removing spoil from the trench as the sled advances by a pair of eductor tubes and associated water jets which produce suction at the inlets of the eductor tubes adjacent the bottom of the trench and discharge the spoil rearwardly at opposite sides of the trench respectively, characterised by said water jets constituting a common source of power both for spoil removal and propulsion of the sled along the pipeline or the like, the forward thrust produced by the discharge of the water-spoil mixture propelling the sled forwards, on freely rotatable rollers resting on the pipeline.

8. Method according to claim 7, characterised by breaking up soil in the forward path of the sled by directing water jets at the face of the trench.

9. Method according to claim 7 or 8, characterised by breaking up the soil beneath the sled by water jets along the bottom of the sled.