Trenching Machines and Methods

The present invention relates to machines and apparatus for making trenches and to methods of making trenches using the machines or apparatus of the invention. The invention is particularly, but not exclusively, concerned with making sub-sea trenches, that is, trenches in the seabed, in particular for locating pipelines, cables, umbilicals or the like therein. Pipelines, cables, umbilicals and the like will hereinafter be referred to as "elongate workpieces".

It is well known to cut trenches in the seabed and to locate elongate workpieces in the trench. After laying an elongate workpiece, the trench can be back-filled to provide mechanical protection for the elongate workpiece against damage from objects crossing the seabed, such as fishing gear, anchors and so on. Burying a pipeline can also provide insulation to improve the passage of pipeline contents.

Methods of trenching use trenching machines which fall into one of three broad categories: towed machines such as ploughs or jetting machines, free swimming ROVs (remotely operated vehicles) and tractors. The tractors are usually mounted on continuous (endless) articulated tracks and are moveable under their own power, in a similar manner to earth moving vehicles used on land. The present invention is particularly, but not exclusively, directed towards trench cutting machines in the form of tractors, and in particular tractors constructed for operation on the seabed.

In a typical trenching operation, an elongate workpiece is laid on the seabed and a trenching apparatus is placed over the elongate workpiece. The trenching apparatus is used to cut a trench, typically alongside the elongate workpiece, and the elongate workpiece is then manoeuvred into the trench. Various cutting arrangements are known, including ploughs, jetting arrangements and chain cutters. The present invention is particularly concerned with trenching machines employing chain cutters.

Chain cutters as such are known. A typical prior art trenching machine employs at least a pair of chain cutters arranged at an angle (for example 60° to the vertical) to cut a V-shaped trench. Cutting a V-shaped trench is inefficient in that, for a given depth of cut, the amount of spoil which must be removed is high. This generates large piles of spoil on either side of the trench which may interfere with subsequent

operations. Also when using angled cutters, there is a strong tendency for soil to remain in the trench which reduces the effective burial of the elongate workpiece which can be achieved. In conventional trenching machines there is a significant danger of interference between the cutters and the elongate workpiece which can cause substantial damage to the elongate workpiece. This is particularly so at the time when the trenching machine is being located in its position of use, and as the elongate workpiece is being mounted to pass through the trenching machine.

WO 99/54556 describes a trenching machine in the form of a tractor, which includes a single chain cutter unit. The chain cutter is mounted on an arm which is pivoted at an upper part of the tractor. By pivoting the arm, the chain cutter can be moved from a use position to a stowed position in which the cutter is raised. In the stowed position, the elongate workpiece can be mounted on the trenching machine without interference from the cutter.

According to a first aspect of the present invention there is provided a trenching machine comprising: a body portion; first and second arms mounted on the body portion; and first and second trench cutter units mounted on the respective arms, wherein the arms are moveable between a first position in which the cutter units adopt a stowed position and at least one use position in which the cutter units can adopt a trenching condition.

In a first preferred embodiment of the invention, when the arms are in the first position, the cutter units are spaced apart and when the arms are in the use position the cutter units are relatively closer together.

Preferably in this embodiment when the arms are in the first position the cutter units are spaced apart to an extent sufficient to allow passage therebetween of the an elongate workpiece to be trenched.

In a second preferred embodiment preferably the trenching machine further comprises lifting means for raising an elongate workpiece between the cutter units to a position above the cutter units, when the cutter units are in the stowed position.

In another preferred embodiment when the arms are in the first position, the cutter units are in a relatively higher position and when the arms are in the use position the cutter units are in a relatively lower position.

Most preferably, the first and second arms are pivotally mounted on the body portion.

In preferred embodiments the pivot axes of the respective arms are arranged to be generally parallel to the longitudinal axis of a work piece to be disposed in the trench.

In further preferred embodiments, when the arms are in a use position, the cutter units are selectively moveable relative to the arms to adopt any of a desired range of trenching conditions. Preferably, the cutter units are moveable relative to the arms by rotation of the cutter units about an axis.

Preferably said axis of rotation of the cutter units is orthogonal to the pivot axes of the respective arms.

In preferred embodiments the cutter units are chain cutter units.

In still further preferred embodiments in one or more use positions of the arms, the cutter units are juxtaposed.

Preferably the trenching machine further comprises securing means operable to join the first and second cutter units together.

Preferably in one or more use positions of the arms the cutter units are in spaced apart relation.

In still further preferred embodiments, in their trenching condition, the cutter units are arranged substantially vertically. That is, the cutter units are arranged substantially parallel to a vertical plane. Preferably the cutter units are spaced equidistantly from a plane passing through the elongate workpiece in use. More

specifically, in preferred arrangements the line of action of the cutter units lies in a substantially vertical plane.

In preferred embodiments the trenching machine includes supporting means configured to support the workpiece on passing through the body of the machine substantially without deformation of the workpiece from a generally linear condition, with respect to a nominally horizontal plane.

According to a second aspect of the invention there is provided a method of laying an elongate workpiece in a trench, comprising the steps of: providing a trenching machine as defined in the first aspect of the invention; arranging the trenching machine over the elongate workpiece; moving the first and second arms to their first position, if not already in said first position; moving the elongate workpiece to a supported position within the machine; and moving the arms to a use position in which the cutter units adopt a trenching condition.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made, by way of example only, to the following drawings, in which:

Figure 1 shows a plan view of a trenching machine according to the invention;

Figure 2 shows a side elevation of a trenching machine according to the invention;

Figure 3 shows an end view of a trenching machine according to the invention;

Figures 4A to 4D show various configurations of the cutter units in the trenching machine according to the invention;

Figures 5A to 5E show stages in the mounting of an elongate workpiece in the trenching machine of the invention; and

Figures 6A and 6B show forward and aft dredging arrangements of the trenching machine according to the invention.

Referring now to the drawings, trenching machine 10 comprises a body portion 12 which, in the illustrated machine, is defined by a plurality of frame members such as 14a, 14b, 14c, 14d. The body portion 12 is carried on ground engaging means 16 which allow for forward and reverse motion of the machine over the seabed and, where required, for steering of the machine 10. In the illustrated example, and in preferred embodiments, the ground engaging means 16 comprise endless articulated tracks 16a, 16b of known type. The trenching machine 10 is most preferably moveable under its own power by movement of tracks 16a, 16b, in the manner of a land-based tracked vehicle.

The trenching machine 10 includes a path along which the elongate workpiece 18 is supported within or on the machine during trenching operations. The path is such that (as can be seen from Figure 1) the workpiece remains essentially straight and is not deformed in a generally horizontal plane. This contrasts with the conventional trenching methods in which the trench is formed to one side of the elongate workpiece and the elongate workpiece is then moved sideways into the trench as it passes through the trenching machine. Such sideways movement necessarily causes deformation of the elongate workpiece which is a potential cause of damage to the elongate workpiece and which may further limit the types of elongate workpiece with which a conventional trenching method may be employed. The straight path of the elongate workpiece 18 through the trenching machine 10 allows, for example, larger steel pipes with large bend radii to be trenched. As can be seen from Figure 2, some curvature of the elongate workpiece 18 in a generally vertical plane is permitted as the elongate workpiece passes through the trenching machine 10 of the invention. However, the radius of curvature of the workpiece 18 is sufficiently large so that the ability of the machine to work with different types of elongate workpiece is not compromised.

The trenching machine 10 further comprises first and second arms 20, 22 which are pivotally attached at pivot points 24a, 24b respectively to a portion of the body such as 14a. The arms are arranged to pivot about an axis nominally parallel to frame members 14c, 14d, that is, nominally parallel to the longitudinal axis of the elongate

workpiece 18 and also nominally parallel to the normal straight line movement of the trenching machine 10. A chain cutter unit 26a, 26b is attached to each respective arm 20, 22 distal from the pivots 24a, 24b. Means 28 are provided for moving the arms 20, 22 between raised and use positions. Preferred means 28 are hydraulic actuators such as piston and cylinder arrangements 30. Each chain cutter unit 26a, 26b is pivotally mounted on its respective arm 20, 22 for movement about an axis 26c which is orthogonal to the axis of the pivots at 24a, 24b and to the direction of forward movement of the trenching machine 10. Thus the chain cutter units 26a, 26b can be raised and lowered between lower trenching positions in which the chain cutter units 26a, 26b can engage the seabed and raised positions in which the chain cutter units 26a, 26b are spaced from the seabed. Means such as a hydraulic actuator (e.g. piston and cylinder arrangement) 32 are provided for moving the chain cutter units 26a, 26b about axis 26c.

Various configurations of the arms 20, 22 and chain cutter units 26a, 26b are illustrated in particular in Figures 4A to 4D. Figure 4A illustrates an initial configuration used, for example, during deployment of the trenching machine 10, that is, during movement of the trenching machine 10 to, or on, the seabed prior to trenching operations. This configuration may also be adopted for recovery of the trenching machine 10. To achieve the configuration shown in Figure 4A (if the trenching machine is not already in that configuration), piston and cylinder arrangements 30 are used to raise arms 20, 22, by pivoting about pivot points 24a, 24b, so that the chain cutter units 26a, 26b are spaced apart, preferably to their maximum extent. The chain cutter units 26a, 26b are maintained in their raised configuration. In this configuration, there is free space between the chain cutter units 26a, 26b and between lower portions of the chain cutter units 26a, 26b and the seabed. The dimensions of the free space are selected to be greater than the typical maximum diameter of an elongate workpiece 18. Thus, potentially damaging contact between the trenching machine 10 and the elongate workpiece 18, as the trenching machine 10 is brought to its position of use, is avoided.

Figure 4B illustrates the loading of the elongate workpiece onto the trenching machine 10, along the supporting path noted above. Further details of the loading operation are explained below. The configuration of the arms 20, 22 and of the chain cutter units 26a, 26b illustrated in Figure 4B is the same as that shown in

Figure 4A so that the chain cutter units 26a, 26b are spaced sufficiently far apart to allow unimpeded passage of the elongate workpiece from its initial position on the seabed (Fig 4A) to its loaded position on the trenching machine 10 (Fig 4B). Thus, in the loading operation, damage to the elongate workpiece 18 by unintended contact with trenching machine 10 or, more specifically, with chain cutter units 26a, 26b is avoided.

Figure 4C illustrates a condition of the trenching machine 10 after loading of the elongate workpiece 18 in preparation for trenching operations. It can be seen that arms 20, 22 have been moved by piston and cylinder arrangements 30 to their lowermost position, so that the chain cutter units 26a, 26b are juxtaposed. It is noted, however, that juxtaposition of the chain cutter units 26a, 26b is not a prerequisite for trenching operations. In certain trenching operations, trenching may advantageously be carried out with the chain cutter units 26a, 26b spaced somewhat apart. For example, a spaced apart configuration of the chain cutter units 26a, 26b may be advantageous where a relatively wide trench is required. Wider trenches may be required where the trench 52 must be stable for a longer period of time (i.e. before it is filled).

Figure 4D illustrates one condition of the arms 20, 22 and chain cutter units 26a, 26b during trenching operations. The arms 20, 22 are in a lowered state, under the control of piston and cylinder arrangements 30. The chain cutter units 26a, 26b have been lowered by rotation about pivot axis 26c under the control of piston and cylinder arrangement 32. The required depth of trench determines the extent to which the chain cutter units 26a, 26b are lowered. During trenching operations elongate workpiece 18 passes over the chain cutter units 26a, 26b and through the trenching machine 10 along the support path.

It is also possible, in particular when cutting a narrow trench, to employ only a single chain cutter unit 26a or 26b. In such a case, the respective arm 20 or 22 carrying the chain cutter unit 26a or 26b to be used is lowered, and the chain cutter unit 26a or 26b is moved about axis 26c to its operational position. The other chain cutter unit 26b or 26a is maintained in a position in which the seabed is not engaged. For example, the chain cutter unit 26b or 26a may be in its upper position with respect to rotation about axis 26c and the arm 22 or 20 may be raised. It is therefore

apparent that the arms 20, 22 and chain cutter units 26a, 26b may be operated independently when required.

The chain cutter units 26a, 26b preferably each comprise a single chain cutter, but each unit 26a, 26b may possibly include a plurality of chain cutters. Each chain cutter unit 26a, 26b is provided with its own drive motor. A typical chain cutter unit 26a, 26b is about 0.5m wide, giving a total width of trench of about Im. Wider or narrow cutter units 26a, 26b may be used as appropriate, for example depending on the desired trench width and the soil conditions. The provision of a motor on each cutter unit 26a, 26b is advantageous over using a single cutter, in that the available torque is increased which increases cutting efficiency, in particular in difficult soil conditions. In some preferred embodiments, engagement means are provided on the respective chain cutter units 26a, 26b, by which the units 26a, 26b may be joined, so that they support each other. In other preferred embodiments, the drives of the respective chain cutter units 26a, 26b may be linked to increase torque availability at any given point.

The provision of two symmetrical chain cutter units 26a, 26b also results in increased stiffness, which is enhanced by joining the cutters together. Increased stiffness of mechanical cutters is an important consideration, for example in ensuring more effective torque availability.

At the end of trenching operations, the chain cutter units 26a, 26b are returned by piston and cylinder arrangement 32 to their raised state as seen in Figures 4A, 4B and 4C and the arms 20, 22 will normally also be raised to the configuration shown in Figures 4A and 4B to allow unloading of the elongate workpiece 18.

Aspects of the loading of the elongate workpiece 18 onto the trenching machine 10 are illustrated in Figures 5A to 5E. In Figures 5A to 5E, the chain cutter units 26a, 26b and arms 20, 22 are not shown, for clarity of illustration.

Figure 5A shows the elongate workpiece 18 resting on the seabed, with the trenching machine 10 located in position above it. Trenching machine 10 is provided with a grab or lifter 34 mounted on an arm 36. Arm 36 is pivotally mounted at pivot point 38 on the trenching machine 10 and is moved under the control of a hydraulic

actuator 40 such as a piston and cylinder arrangement. Grab 34 embraces the elongate workpiece 18 and lifts the elongate workpiece 18 to a position (Figures 4B, 5B) above the height of the chain cutter units 26a, 26b which, during this lifting operation, are in the raised and spaced apart position shown in Figure 4B. Thus, the elongate workpiece 18 is lifted without any risk of interference from the chain cutter units 26a, 26b. The elongate workpiece 18 is mounted on the supporting path in the trenching machine 10 and the grab 34 is released (Figure 5C). With the elongate workpiece 18 in position in the supporting path, the chain cutter units 26a, 26b can be moved to the operational positions shown in Figures 4C and 4D so that trenching can commence.

Figures 5D and 5E illustrate particular examples of means of supporting the elongate workpiece along the supporting path. In this example, the supporting path includes at least one set of rollers 37 which can be moved between positions in which they engage the elongate workpiece (Figure 5E) and positions in which they are spaced apart from the elongate workpiece (Figure 5D). Cylindrical rollers may be used but in particularly preferred constructions, the rollers 37 are diabolo (or diablo) rollers (i.e. rollers formed from conjoined substantially frusto-conical elements). The rollers 37 are mounted on supporting members 39 which are mounted on the body of the trenching machine 10 and are moveable about pivot points 41 between the engaged and spaced apart positions. A hydraulic actuator 42 is provided to effect movement of the members 39 and the associated rollers 37. Thus, the rollers 37 are closed on the elongate workpiece 18 when the chain cutter units 26a, 26b are in their closed condition (Figures 4C and 4D) and may improve traction of the trenching machine 10 by permitting the trenching machine 10 to "pull" on the elongate workpiece 18, thereby improving the work rate of the trenching machine 10, especially in marginal seabed conditions. In alternative constructions the rollers 37 may be replaced by endless track-type gripping devices. Track-type devices as such are known in the art.

Figures 6A and 6B show dredging units at the forward and aft ends of the trenching machine 10 (forward and aft being relative to the normal direction of travel of the trenching machine 10). The forward dredging units 44 include suction heads 46 provided in front of the chain cutter units 26a, 26b and act to carry soil away from the area between the tracks 16 of the trenching machine 10. The aft dredging units

48 include suction heads 50 and act to remove spoil from the trench 52 cut by the chain cutter units 26a, 26b prior to the landing of the elongate workpiece 18 in the trench 52.

By using a pair of chain cutter units 26a, 26b which are oriented substantially vertically the trenching machine 10 of the invention maximises the efficiency of the trenching operation by disturbing the minimum amount of seabed and ensuring that the trench which is cut has the smallest cross-sectional area for a given depth. In contrast, trenching machines which rely on angled cutters define trenches with inclined side walls and necessarily must remove more soil and rock to achieve a given depth of trench. This reduces work rates.

A particular advantage of the trenching machine 10 according to the invention is that the chain cutter units 26a, 26b can be mounted in the middle of the track centre of steering. This is advantageous in enabling the tractor (trenching machine 10) to steer about the chain cutter units 26a, 26b, allowing the cutter units 26a, 26b to be more rigidly mounted within the body of the trenching machine 10 and so improving their structural connection and rigidity. This enhances the ability of the trenching machine 10 to cut through rock and poor soil conditions, improving both the work rate and the quality and uniformity of the resulting trench.

Although the present invention has principally been described in relation to its application to a sub-sea tractor, the skilled person will appreciated that the invention is susceptible to being adapted for use with a plough, ROV or other sub-sea trenching apparatus.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.