EP0112877B1

EP0112877B1 - Underwater trenching machine

Info

Publication number: EP0112877B1
Application number: EP19830902110
Authority: EP
Inventors: Daniel Kai Carstens
Original assignee: Individual
Current assignee: CARSTENS, DANIEL KAI
Priority date: 1982-06-29
Filing date: 1983-06-28
Publication date: 1986-10-01
Also published as: DK152549C; DK152549B; WO1984000183A1; DK113084A; DE3366582D1; NO840667L; DK113084D0; EP0112877A1

Abstract

A machine for underwater trenching for pipelines in which a sled (1) comprising two pipe loops (2) arranged in vertical planes at opposite sides of the pipeline (4) and interconnected by transverse pipes (3) is supported on and guided for movement along the pipeline by rubber-faced rollers (6, 7, 8, 9). High pressure water is supplied to the loops through connection (20). Nozzles (18) in the front vertical runs of the loops (2) break up the soil in the face of the trench (5). Nozzles (15) of water jet pumps suck soil from the trench through the forwardly facing inlets (11), adjacent the bottom of the sled, of two eductor tubes (10) carried by the loops (2) at opposite sides of the pipeline, and discharge it through discharge outlets (14) directed respectively outwardly sideways and mainly rearwardly of the sled. The water and soil discharge produces a reaction which propels the sled forwards.

Description

This invention relates to an underwater trenching machine which will cut a trench under a pipeline and also remove the spoil from the trench to allow the pipeline to fall into the trench under its own weight.
Some underwater trenching machines comprise a sled which is tethered by a cable to a barge which, as it moves forwards on the surface of the sea, pulls the sled along the seabed. US-A-4114390 describes one such machine in which the sled is saddle-shaped frame and is supported on the pipeline by transverse rollers, the frame comprising tubes through which water is fed under pressure to fluidizing nozzles so that the non-cohesive bottom of the seabed is fluidized and the pipeline sinks into the fluidized bottom material as the sled is pulled along the pipeline by a cable connected to a winch on the barge. With 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 forward propulsion from the sled itself and independent of barge motion. To this end, tractor type machines are known, having hydraulic or electric drive, which are propelled by wheels or endless tracks in engagement with the pipeline itself (e.g. US-A-4087981) or the seabed. A problem is to get enough friction between the wheels and the pipe without damaging the latter or its anti-corrosive coating, or, where the tractor drives on the seabed, for the soil to be firm enough to prevent the track sinking into the mud. Another known machine, described in US-A-3576111, comprises a sled which uses high pressure water jets aimed in the direction opposite to that in which the sled is to move. These jets propel the sled forward. Some jets are aimed forward to cut the trench and others generally upwards to help push the spoil out of the trench, which 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 forward thrust on the sled more water must be discharged to the rear of the sled than to the front. Also a separate air lift system is required for spoil removal. The efficiency of this type of machine is very low, and decreases with depth of operation.
GB-A-1479459 describes an underwater trenching machine for dredging trenches on the seabed for receiving pipes or cables comprising a sled which is towed from a supply boat by means of a tow wire attached to the sled. Two pairs of downwardly directed water jet nozzles are mounted one behind the other at the forward end of the sled to loosen the material of the seabed which is then sucked up by two ejector pumps each having a suction inlet pipe of which the bottom ends are disposed aft of the water jet nozzles and below metal strips along the underside of the sled and forming a skid or runner. Each suction inlet pipe, which is inclined upwardly and rearwardly from its inlet end, is attached at its aft end to an angled ejector portion the aft end of which is secured to an upwardly, rearwardly and outwardly directed straight diffuser having an internal diameter increasing uniformly to the discharge aperture of the pump. Jet pipes aligned with the central longitudinal axis of the respective diffusers extend through the wall of the angled ejector portions and terminate in a nozzle which faces towards the discharge aperture of the respective diffuser. Water under pressure is supplied to the jet pipes to cause the ejector pumps to suck up the loosened seabed material and discharge it to the sides of the trench through the discharge apertures of the diffusers. Buoyancy tanks are provided on the sled for adjustment of the buoyancy. The pipe or cable to be laid in the trench is supported by pairs of front and rear guide rollers arranged in a V-shaped formation, and is laid into the trench on leaving the rear pair of rollers. The machine is such that it must be erected on the seabed. It is suggested on page 2 lines 98-103 of the description that the action of the nozzles and ejector pumps provide a reactive forward force component which can be used to propel the machine along the seabed providing the buoyancy of the machine is appropriately adjusted. In practice, however, it is not practicable to propel the sled by the said reactive' forward 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, acts to press the skids of the sled into the soil of the seabed. The reactive force provided by the ejector pumps has a downward component which also presses the skids into the seabed. The resultant high- frictional resistance prevents the sled from being propelled by the reactive forward force component of the ejector pumps. To reduce the frictional resistance would require very large buoyancy tanks capable of lifting the weight of the pipeline or cable (e.g. a pipeline of 60 cms diameter filled with water during laying can weigh several tons) in addition to the weight 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 alleviated by the lifting force imparted by the tow line to drag the skids out of and along the soil of the seabed.
US-A-4087981 (upon the disclosure of which the preamble of claim 1 is based) describes a buoyant self-propelled underwater trenching machine for a pipeline or cable to be laid in the seabed, 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 comprises an upright tube equipped with a plurality of vertically arranged nozzles disposed for directing water jets at the face of the trench for breaking up soil in the forward 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 sideways outwards and mainly rearward direction of the sled whereby to draw up portions of the spoil broken up by said nozzles and to discharge these portions symmetrically to opposite sides of the sled. To provide the necessary suction the upper end of the eductor tube is provided with a venturi composed of a venturi housing in which is positioned a high pressure water nozzle. The nozzle is directed towards an opening in the opposing wall of the venturi housing which is of smaller diameter than the eductor inlet to the housing and which connects with a divergent discharge tube which directs the spoil to the side of the trench. Propulsion of the machine is positively derived along the pipeline by drive rollers in frictional engagement with opposite sides of the pipeline, 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 side 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 least the upper part of the face of the trench prior to the advance of the forward guide rollers. The positive driving engagement of the drive rollers with the pipeline prevents any thrust from the eductor discharges 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 a height above the top of the trench (on 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 about 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 solely by the means used for soil removal from the trench. To this end, the invention consists in 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 educator 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, characterised 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 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 to provide the sole means of propulsion of 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 trench being cut and discharges it to the sides and rear of the trench, the water jet and spoil discharge producing a reaction on the sled in the forward direction which is used to propel the sled forwards.
The invention also consists in the 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 using said water jets to provide 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 providing the sole means for propelling the sled forwards on freely rotatable rollers mounted on the sled and resting on the pipeline.
In order that the invention may be more clearly understood, reference will now be made to the accompanying drawing, in which:-

Fig. 1 is a side view of one embodiment of a sled according to the invention, and
Fig. 2 is a plan of Fig. 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 kind 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 the two pipe loops respectively. The sled is further transversely guided on the pipeline 4 by pairs of rollers 8, 8', 9, 9', also resiliently faced, mounted for rotation about vertical axes at the front and rear of the sled respectively on the pipe loops at opposite sides of the pipeline 4.
The sled also comprises two eductor tubes 10, 10' carried from the respective pipe loops 2 and 2'. The inlet ends 11 of the eductor tubes, located adjacent the bottom of the sled, face forwardly and inwardly. Each inlet connects with a vertical section 12 which leads to an elbow 13 connecting with a horizontal discharge section 14, 14' which is directed outwardly sideways and mainly rearwardly of the sled, the discharge sections 14, 14' extending symmetrically to opposite sides of the sled. In each elbow 13 is a nozzle 15 aimed along the respective discharge section. Water is fed to the nozzles 15 by feed pipes 16, 16' which may be interconnected so that only a single hose connection 17 is required. Between each elbow 13 and its discharge section there is preferably a venturi pipe reducer 24, for example from 30 cms diameter to 25 cms diameter, corresponding to the diameters of the tube sections 12 and 14 respectively.
Each front vertical run of a pipe loop 2, 2' is provided with nozzles 18 to provide small jets of water aimed at the front of the trench 5. These break up the soil in front of the trenching machine. Further nozzles 19 may be provided along the bottom run of each pipe loop. High pressure water 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 removed from the trench through the eductor tubes. The high pressure water streams of the water jet pumps, aimed rearwardly along the discharge sections 14, 14', create suction in the eductors which lifts the spoil out of the trench 5 via their inlet ends 11 and discharges it to the side and rear of the sled through the discharge sections 14,14'. The reaction of the water jet pump and the spoil propels the sled forwards and keeps it in contact with the trench face, being guided along the pipeline 4.
Minimum negative buoyancy of the machine, for easy diver handling, is achieved by buoyancy tanks 21 at the top of the pipe loops. They also provide an uprighting force to keep the sled upright as it rides along the pipeline 4. Transverse rubber faced rollers 22, 23 may also be fitted to the machine, after it has been positioned on the pipeline, to prevent it from lifting. Each tank 21 is provided with a valve in its upper part to allow air to escape, and an open pipe 21' to allow a diver to fill the tank with air after the sled is positioned on the pipeline 4 and the rollers 22, 23 locked under the pipeline. The pipes 21' being open prevent the buoyancy tanks from becoming pressurised vessels.
The machine according to the invention has the advantage that the same power that is used for soil removal is also used for propulsion, only one power source being necessary. The spoil removal pressure source does not have to be as high as that needed to cut the trench. This lowers the cost of the pumps required on the barge. Further, the water jet pump efficiency does not decrease noticeably, if at all, with increasing depth of operation. The spoil is also discharged well to the side of the trench being cut.
While a particular embodiment has been described, it will be understood that various modifications may be made without departing from the scope of the invention. For example the machine may be equipped with more than one pair of eductors. If desired the spoil removal, propulsion and jetting water can be supplied through a common hose from a single pump source. The hose or hoses may 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 mechanical cutter, or, by a combination of mechanical cutting means and water jets, the loosened soil being removed by the eductors and discharged outwardly and rearwardly to propel the machine forwardly and against the face of the trench being cut. Further the rollers may be moved into or out of their operative positions by hydraulic or other power means.
The machine may also be used for laying cables.

Claims

1. A self-propelled underwater trenching machine for a pipeline (4) or the like to be laid, comprising a sled (1) having means for supporting it on and guiding it for movement along the pipeline or the like and comprising two side members (2, 2') spaced apart on opposite sides of the pipeline or the like by interconnecting transverse members (3) including freely rotatable transverse rollers (6,7,8,8', 9,9') 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 (21) on the sled, and front of each side member comprising a plurality of front water jets (18) 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 (10, 10') comprising an upwardly extending section (12) having an inlet (11) adjacent the bottom of the sled and connected at its upper end to a discharge section (14, 14') aimed'in a sideways, outwards and mainly rearward direction of the sled above the top of the trench (5) and along the axis of which a water jet from a nozzle (15) 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, characterised in that the upwardly extending section (12) of each eductor tube (10, 10') leads to an elbow (13) providing a smooth transfer to its respective discharge section (14, 14') which is just above the top of the trench and is substantially horizontal and of substantially uniform diameter throughout its length, the respective water jet nozzles (15) being located in the respective elbows and aimed along the respective discharge sections from which the water spoil mixtures are discharged to provide the sole means of propulsion of the sled along the pipeline or the like.

2. Machine according to claim 1, characterised by guide rollers (8, 8', 9, 9') 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 (18).

3. Machine according to claim 1 or 2, characterised by each side frame (2, 2') also comprising a further plurality of water jets (19) 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 (2, 2') comprise two loops of pipe spaced apart in parallel upright planes by interconnecting pipes (3) to straddle the pipeline or the like and provided with the freely rotatable transverse rollers (6, 7) to support the sled on and for movement along the pipeline or the like, the loops of the pipe having at least one inlet (20) for connection to a source of water under pressure, the front runs of the loops being provided with nozzles (18) for directing water jets at the face of the trench, and further water jet nozzles (19) being provided along the bottom runs of the pipe loops.

5. Machine according to claim 4, characterised in that the sled is provided with buoyancy tanks (21) at the top of the pipe loops (2, 2'), each tank being provided with a valve in its upper part to allow air to escape and an open pipe (21') in its lower part through which the tank may be charged with air, the sled also being provided with rollers (22, 23) which can be located below the pipeline after the sled has been positioned thereon.

6. Machine according to any preceding claim, characterised in that each side member (2, 2') carries one of each of a plurality of pairs of said eductor tubes (10, 10'), the eductor tubes of each pair having their discharge sections (14) directed sideways outwards and mainly rearwardly to opposite sides of the shed.

7. Method of underwater trenching which consists in propelling a sled (1) along a pipeline (4) or the like to be laid in the trench (5), 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 (10, 10') and associated water jets (15) which produce suction at the inlets (11) of the eductor tubes adjacent the bottom of the trench and discharge the spoil rearwardly at opposite sides of the trench respectively, characterised by using said water jets to provide 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 providing the sole means for propelling the sled forwards on freely rotatable rollers (6, 7, 8, 8', 9, 9') mounted on the sled and 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.