EP1985766A2

EP1985766A2 - Vehicle for laying an elongate product in a trench in the sea floor

Info

Publication number: EP1985766A2
Application number: EP08251502A
Authority: EP
Inventors: John Reece
Original assignee: Soil Machine Dynamics Ltd
Current assignee: Soil Machine Dynamics Ltd
Priority date: 2007-04-26
Filing date: 2008-04-23
Publication date: 2008-10-29
Also published as: GB0708082D0; EP1985766A3

Abstract

A vehicle (40) for laying a pipe (42) in a trench (44) in the sea floor (46) is disclosed. The vehicle has two operational modes, a first mode for cutting a trench in a sea floor with a soft upper layer and a hard lower layer and a second mode for a sea floor of hard material throughout the depth of the trench. The vehicle has three cutting chains (56,58,60), the forward most chain, that is only used when the second mode cuts a shallow central section of the trench from a first side. A centrally positioned cutting chain (56), that is used in both modes, cuts the next portion of the trench to its full depth from the opposite side that the forward most cutting chain (58) operates. A rear most cutting chain (60) cuts the final portion of the trench to its full depth, therefore partially overlapping with the previous portion, and operates from the same side of the trench (44) as the forward most cutting chain (58).

Description

The present invention relates to a vehicle for laying an elongate product in a trench in the sea floor, and relates particularly, but not exclusively, to a vehicle for cutting a trench into rock or other hard material and laying a pipe therein.
It is well known for tractor-type unmanned sub-sea vehicles to be used to cut trenches into the sea floor in order that a pipe or other elongate product can be located therein. In soft sea beds this trenching operation can be carried out using a ship-hauled plough or a water jetting device. However, in hard sea beds, such as stiff clays or rock, a mechanical cutter must be used. Such cutters typically use tungsten carbide picks mounted on a chain, rotating wheel or drum that progressively breaks up the hard material into small pieces and it is then removed from the trench that has been formed.
Figure 1 shows a schematic representation of a trench cutter of the prior art, viewed from the front. A chain cutter 10 cuts a trench 12 in sea-floor 14. The chain cutter 10 is formed as a belt consisting of a series of hinged plates with each plate carrying tungsten carbide teeth (not shown) arranged in rows across the width of the plate. With the pipe 16 already laying in position on the sea-floor 14, the vehicle operating the chain cutter 10 is lowered onto the sea floor 14 and the trench 12 is cut under the pipe 16. In order to cut the trench 12, the pipe 16 must be lifted from the sea floor 14 to accommodate cutter chain 10. Trench cutting vehicles of this type have significant problems. The pipe must be lifted a considerable height from the sea-bed, typically at least two metres, to allow the entire chain cutter, mechanical supports and drive systems to be mounted underneath. This applies stresses to the pipe that can result in damage and can mean that this type of trench cutting vehicle cannot always be used. Because a single cutter is used, where a very wide pipe is being placed in the trench, a very wide cutter must be used. This significantly reduces the efficiency of the chain cutter. Furthermore, the vertical sides of the trench are not stable in some sea-beds and additional cutters or jetters are needed to ensure that the trench remains open and stable whilst the pipe is laid therein.
In an attempt to overcome some of the problems of the prior art of figure 1 an alternative trench cutter, shown in figure 2, uses multiple angled cutters 20, 22, 24 and 26 to cut the sea floor 28 underneath pipe 30. In the example shown, four cutters are used arranged in symmetrical pairs to cut the trench two different depths, whilst the pipe 30 remains on sea floor 28. Because the pipe is quite rigid, it sags very little over the length of the trenching machine and therefore stays at sea floor level despite the ground underneath having been removed until well after the vehicle has passed.
A trench forming vehicle of this type has the significant problems, particularly in hard ground, that the cutters must be kept well away from the pipe as any contact between pipe and chain cutters will result in damage to the pipe. The cutters must also be kept well apart from each other in order to prevent them from touching and damaging each other. As a result, a central region underneath the pipe is not cut. In softer sea floors this is only a limited problem as the central portion collapses and material is removed by the cutting blades. However, in sea floors of harder material a central portion can remain in tact thereby preventing the pipe from entering the trench. The problems of chain cutters contacting each other or the pipe are further exacerbated in hard ground as the hard ground causes vibration, and slight movement, of the chain cutters.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to an aspect of the present invention there is provided a vehicle for laying an elongate product in a trench in a sea floor, the vehicle comprising:-

a vehicle body adapted to travel along the sea floor;
first cutting means for cutting a first portion of a trench in the sea floor from a first side of said trench;
second cutting means for cutting a second portion of said trench from a second side opposite said first side, wherein said first and second trench portions overlap with each other and said first cutting means is located in front of said second cutting means in the direction of travel of the vehicle.

By providing two cutting means, typically chain cutters, one in front of the other, that cut portions of the trench from overlapping sides and that those portions overlap with each other, the advantage is provided that where a trench is being cut in sea floor that has a softer upper layer but harder lower layer, within the height of the trench that is being cut, is always cut to the correct depth, irrespective of the hardness of the lower layer of the sea floor. As a result, the pipe will always end up sufficiently buried in the trench and there is no danger that the harder material in the lower portion of the sea floor will prevent the pipe from reaching its required depth. Furthermore, it is not necessary to lift the pipe at all from the sea-bed thereby decreasing the likelihood of damage to the pipe.
The vehicle may further comprises third cutting means for cutting a third portion of said trench.
By introducing a third cutting means the advantage is provided that in the sea floor that is formed from similarly hard material throughout the whole depth of the trench the vehicle is able to cut a trench, ensuring that all of the hard material is cut away within the trench whilst at the same time not requiring the pipe to be lifted very far above the sea floor. In particular, the pipe does not need to be lifted as high above the sea floor as seen in the prior art.
In a preferred embodiment the third cutting means is located at a forwards most position in the direction of travel of the vehicle and cuts said third portion of said trench from said second side, said first cutting means is located behind said third cutting means in the direction of travel and said second cutting means is located behind said first cutting means in the direction of travel.
By cutting a central portion of the trench first from one side using the third cutting means, then cutting a second portion of the trench to its full depth from the other side and then cutting the final portion of the trench from the same side as the central portion was cut, the advantage is provided that this is the most efficient method for cutting the trench using three cutters.
The first, second and third cutting means may comprise a chain cutter.
By using a chain cutter the advantage is provided that a compact cutter can be provided with a very low height that when in use, efficiently transports spoil from the cutting face backwards before it is removed from the trench.
In a preferred embodiment at least one of said chain cutters is adapted to engage the sea floor at an angle of 45°.
The vehicle may further comprise product handling means for lifting the product from the sea floor.
In a preferred embodiment the product handling means comprises fore and aft pipe lifters.
By using two pipe lifters towards the front and towards the rear of the vehicle, the advantage is provided that a pipe is held at the desired height whilst ensuring that the stresses applied to the pipe are distributed over the maximum length of the pipe possible.
In another preferred embodiment said vehicle has two working modes:-

in a first mode, for use in forming a trench in a sea floor that has a softer upper layer and harder lower layer within the depth of the trench, the third cutting means and product handling means are inoperative and the first and second cutting means operate to undercut the product that lies on the sea floor; and
in a second mode, used for forming a trench in a sea floor formed of harder material though substantially the whole depth of the trench, the product handling means operates to lift product from the sea floor, the third cutting means cuts a third trench portion below said product from said second side, said first cutting means cutting a first trench portion from said first side thereby forming a first face of said trench and said second cutting means cutting a second trench portion from said second side thereby forming a second face of said trench.

By having two working modes, one in which the third cutting means operates and one in which it is inoperative, has the advantage that the vehicle can be used most effectively and efficiently in all sea floor types. For example where a soft layer covers a harder layer on the sea floor the pipe lifting apparatus need not be used, but if the vehicle encounters sea floor that is formed from hard material throughout its depth, it is equally able to produce a trench whilst at the same time applying minimal strain to the pipe.
The vehicle may also further comprise pumping means for pumping sea and spoil cut from said sea floor out of said trench.
In a preferred embodiment the pumping means comprises a respective dredge pump for each cutting means, each said pump located behind its respective cutting means.
According to another aspect of the present invention there is provided a method of laying an elongate product in a trench in a sea floor, comprising:-

cutting a first portion of a trench in the sea floor from a first side of said trench;
cutting a second portion of said trench from a second side of said trench opposite said first side, wherein said first and second trench portions overlap with each other and said first portion of said trench is cut in front of said second portion of said trench.

The method may further comprise cutting a third portion of said trench.
In a preferred embodiment the third portion of said trench is formed in front of said first portion and is formed from said second side.
In another preferred embodiment the third portion of said trench is formed centrally, said first portion of said trench is cut forming a first face of said trench on said first side and said second portion of said trench is cut forming a second face of said trench on said second side.
The method may further comprise lifting the product from the sea floor.
In a further preferred embodiment the product is lifted at a location in front of the position where the portions of the trench are formed and at a location to the rear of the position where the portions of the trench are formed.
The method may also further comprise pumping material from said trench portions using at least one dredge pump.
Preferred embodiments of the present invention will now be described, by way of example only, and not in any limitative sense, with reference to the accompanying drawings in which:-

Figure 1 is a schematic front view of a pipe laying vehicle of the prior art;
Figure 2 is a schematic front view of another pipe laying vehicle of the prior art;
Figure 3 is a perspective view of a pipe laying vehicle of the present invention viewed from the front;
Figure 4 is a perspective view of the pipe laying vehicle of figure 3 from the rear;
Figure 5a is a front view of the pipe laying vehicle of figure 3 in an inoperative position;
Figure 5b is the same view as that of 5A with the vehicle in an operative position;
Figure 6a is a side view of the pipe laying vehicle of figure 3 in an inoperative position;
Figure 6b is the same view as that of 6a with the vehicle in an operative position;
Figure 7a is a rear view of the pipe laying vehicle of figure 3 in an inoperative position;
Figure 7b is the same view as that of 7a with the vehicle in an operative position;
Figure 8 is a schematic representation showing stages of operation of the vehicle of figure 3 in a second mode of operation;
Figure 9 is a schematic representation showing stages of operation of the vehicle of figure 3 in a first mode of operation; and
Figure 10 is a front view of the vehicle of figure 3 showing three stages of lifting a pipe.

Referring to figures 3 to 8, a vehicle 40 for laying an elongate product, typically a pipe 42, in a trench 44 that the vehicle 40 cuts in sea-floor 46 has a body, in the form of frame 52, that is adapted to travel along the sea floor using caterpillar tracks 54. The vehicle 40 is provided with three cutting means in the form of first, second and third cutting chains 56, 58 and 60. The cutting chains are of a type known to persons skilled in the art. Each chain cutter is formed as a belt approximately 800mm wide formed from interlinked plates of high strength steel with high hardness steel and tungsten carbide used to form teeth 62 and in other high wearing areas. The chain cutters are of a type known as Dint header type chain cutter assemblies.
The cutting chains 56, 58 and 60 are able to move from inoperative positions, shown in figures 5a, 6a and 7a, to operative positions, shown in figures 5b, 6b and 7b.
The vehicle 40 also has pipe handling apparatus in the form of fore and aft pipe lifters 64 and 66, that are each formed from a pair of crescent-shaped grab arms 68.
The vehicle 40 has two modes of operation. In a first mode, depicted schematically in figure 8, a pipe 42 is being laid in a trench 44 in sea floor 48. The first operational mode is for use where the sea floor 48 is formed from a shallow layer of soft material 100 which sits on top of a deeper layer of hard material 102. The pipe 42 sits on top of soft material layer 100 and the vehicle 40 is lowered so as to straddle pipe 42 with a caterpillar track 54 on either side, as seen in figures 5a, 6a and 7a. A first cutting chain 56 cuts a first trench portion from a first side 106 of the trench. The first cutting chain 56 is set at an angle of 45° to the sea floor 48, as seen in figure 8a. As the first trench portion 104 is cut the soft material immediately below pipe 42 falls into first trench portion 104 and is excavated by a combination of the first cutting chain 56 and the dredge pumps 78. As can also be seen in figure 10B this leaves the pipe suspended above the partially formed trench. However, because the pipe is being supported on the sea-floor it is able to sag into the bottom of trench 44 over a long distance and in the area immediately adjacent the vehicle remains at approximately the same height as the sea floor 48. A second trench portion 108 is cut using second cutting chain 58 which approaches the trench from a second side 110 that is opposite the first side 106. The first trench portion 104 and second trench portion 108 overlap in the area adjacent the apex at the bottom of the trench 44.
When the second cutting chain 58 has passed further along the trench 44 is formed as shown in figure 8d. Trench 44 has a first face 114 and a second face 116 that join at a right-angled apex 118. As a long trench is formed the pipe 42 sags into the base of the trench adjacent apex 118. The rigidity of the pipe means that it does not reach the bottom of the trench until the vehicle has moved some considerable distance forward and as a result there is no danger of the pipe sagging into the first and second trench portions 104 and 108 whilst the trench is being cut.
In a second operational mode, depicted in figures 9 and 10, a pipe 42 is again laid in a trench 44 cut in sea floor 48. This second mode of operation is for use where the material from which the sea floor 48 is formed is hard, typically formed from clay or rock, throughout all or most of the depth that the trench is cut. This second mode of operation works in substantially the same way as the first except that a third cutting chain 60 is used to initially cut a third trench portion 120 in sea floor 48. This third trench portion 120 is cut immediately below pipe 42. This cutting step is undertaken because the hard material that extends throughout the sea floor 48 will not simply fall away as the soft layer 100 does in the first operational mode.
In order to gain access to the sea floor 48 immediately below pipe 42 it is necessary to lift pipe 42 above sea floor 48. However, because third cutting chain 60 is angled at 45° it is not necessary to lift the pipe as high as it is in the example of the prior art shown in figure 1. In fact it is typically the case that the pipe can be lifted about half the distance from the sea floor that is necessary in devices that work on the principle shown in figure 1. Referring to figure 10, in 10a, the vehicle 40 has been lowered to the sea floor 48 and straddles pipe 42 with a caterpillar track 54 on either side of the pipe 42. The fore and aft pipe lifters 64 and 66 are lowered to adjacent the pipe 42 and the grab arms 68 are closed using hydraulic ram 70 around pipe 42, as shown in figure 10b. The pipe is then lifted and once lifted two roller beds 72 are lifted into position to support the pipe from underneath. As a result, when vehicle 40 moves forward on caterpillar tracks 54 the pipe remains stationary and rolls on the series of rollers that form the pair of roller beds 72.
Once lifted into this position, as shown in figure 10c, the first, second and third cutting chains 56, 58 and 60 can be moved from their inoperative positions, shown in figures 5a, 6a and 7a and brought into their operative positions in figures 5b, 6b and 7b. As third cutting chain 60 is the forwards most cutting chain in the direction of travel D it is the first cutting chain to cut a portion of the trench, this being third trench portion 120. As seen in figure 9c, the first trench portion 104 is cut by first cutting chain 56 from first trench side 106. Behind first cutting chain 56 the second cutting chain 58 cuts second trench portion 110 from second trench side 110. The overlap portion 112 is again present adjacent apex 118 of first and second faces 114 and 116.
It will be appreciated by persons skilled in the art that the above embodiment has been described by way of an example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

Claims

A vehicle for laying an elongate product in a trench in a sea floor, the vehicle comprising:-
a vehicle body adapted to travel along the sea floor;

first cutting means for cutting a first portion of a trench in the sea floor from a first side of said trench;

second cutting means for cutting a second portion of said trench from a second side opposite said first side, wherein said first and second trench portions overlap with each other and said first cutting means is located in front of said second cutting means in the direction of travel of the vehicle.
A vehicle according to claim 1, further comprising third cutting means for cutting a third portion of said trench.
A vehicle according to claim 2, wherein said third cutting means is located at a forwards most position in the direction of travel of the vehicle and cuts said third portion of said trench from said second side, said first cutting means is located behind said third cutting means in the direction of travel and said second cutting means is located behind said first cutting means in the direction of travel.
A vehicle according to any one of the preceding claims, further comprising one or both of the following features:-
a) wherein at least one of said first, second and third cutting means comprises a chain cutter; and

b) wherein at least one said cutting means is adapted to engage the sea floor at an angle of 45°.
A vehicle according to any one of the preceding claims, further comprising product handling means for lifting the product from the sea floor.
A vehicle according to claim 5, wherein said product handling means comprises fore and aft pipe lifters.
A vehicle according to claim 5 or 6, wherein said vehicle has two working modes:-
in a first mode, for use in forming a trench in a sea floor that has a softer upper layer and harder lower layer within the depth of the trench, the third cutting means and product handling means are inoperative and the first and second cutting means operate to undercut the product that lies on the sea floor; and

in a second mode, used for forming a trench in a sea floor formed of harder material though substantially the whole depth of the trench, the product handling means operates to lift product from the sea floor, the third cutting means cuts a third trench portion below said product from said second side, said first cutting means cutting a first trench portion from said first side thereby forming a first face of said trench and said second cutting means cutting a second trench portion from said second side thereby forming a second face of said trench.
A vehicle according to any one of the preceding claims, further comprising pumping means for pumping sea and spoil cut from said sea floor out of said trench.
A vehicle according to claim 8, wherein said pumping means comprises a respective dredge pump for each cutting means, each said pump located behind its respective cutting means.
A method of laying an elongate product in a trench in a sea floor, comprising:-
cutting a first portion of a trench in the sea floor from a first side of said trench;

cutting a second portion of said trench from a second side of said trench opposite said first side, wherein said first and second trench portions overlap with each other and said first portion of said trench is cut in front of said second portion of said trench.
A method according to claim 10, further comprising cutting a third portion of said trench.
A method according to claim 11, wherein said third portion of said trench is formed in front of said first portion and is formed from said second side.
A method according to any one of claims 10 to 12, wherein said third portion of said trench is formed centrally, said first portion of said trench is cut forming a first face of said trench on said first side and said second portion of said trench is cut forming a second face of said trench on said second side.
A method according to any one of claims 10 to 13, further comprising one or both of the following steps:-
a) lifting the product from the sea floor; and

b) pumping material from said trench portions using at least one dredge pump.
A method according to claim 14, wherein said product is lifted at a location in front of the position where the portions of the trench are formed and at a location to the rear of the position where the portions of the trench are formed.