FIELD OF THE INVENTION

This invention relates to a method and apparatus for underwater trenching.- in particular but not exclusively for the burial of a submarine cable or pipeline.

BACKGROUND OF THE INVENTION

At present, such trenching and/or burial is effected by machines which employ water jets and/or mechanical cutters or diggers, which worl in conjunction with jet pumps, air lift pumps or dredge pumps to break up and remove seabed soil to form a trench, for example to receive a cable or pipeline. Trenching and burial of cables and pipelines is also achieved by fluidising the bed with water jets to form a fluidised region into which the cable or pipeline sinks by virtue of having a specific gravity greater than that of the fluidised bed. A further method is to plough a trench beneath the laid cable or pipeline, removing the bed material to one or both sides of the trench and allowing the cable or pipeline to fall into the trench. To pull the

plough requires a large force, especially where a trench of wide included angle has to be formed owing to lack of soil stability.

Another form of plough has a single hollow narrow blade with a foot piece which lifts a wedge of soil to form a space into which a cable or pipeline can be inserted.

All of these known, methods suffer from serious disadvantages.

SUMMARY OF THE INVENTION

According to the present invention, an underwater trench is formed, and/or a pipeline, cable or the like is buried, by means of a plough fitted with a vibrator for generating elastic-wave vibrations and operable at a frequency such as to loosen, break up or fluidise the bottom material.

The application of mechanical vibrations to the plough, so as to loosen and preferably fluidise the bottom material, enables greater trench depths to be produced for a given force, and enables harder bottom materials to be trenched, including friable rock as well as relatively unconsolidated materials such as clay or

trenching are substantially reduced compared with known trenching methods. The plough can be drawn from the surface, but the increased penetration and reduced forces resulting from application of the invention, make it possible to use a self-propelled trenching machine or plough in many situations, because the necessary force ^" is relatively small.

The plough may for example comprise ar srirςrϊe" vertical or sloping blade, or laterally spaced blades defining a channel between them. Multiple successive blades can be provided.

The vibrations or oscillations will normally but not necessarily be applied to the leading edge of the blade or the foremost blade, and/or the foot of the blade or foremost blade, in particular at or near its leading edge. Alternatively, a member excited by elastic wave vibrations can be provided separately from the plough blade or blades, to fluidise the bottom material in the vicinity of the blade or blades. This arrangement may be appropriate particularly in the case of soft bottom ground such as sand or mud.

The vibrations or oscillations can be excited in any convenient way, for example mechanically, but in general will be excited electrically, for example by power

submerged vessel. The excitation points, excitation frequencies and amplitudes are selected to achieve the most efficient loosening and fluidisation of the bottom material. Preferably, means are provided to enable the vibrations or oscillations to be tuned for maximum effect on any particular bottom material, alternatively or in addition the plough blade or blades may be replaceable to suit different bottom materials.

The excited plough blade or b-lades can for example be supported on a general-purpose or special-purpose underwater vehicle, in particular a tractor vehicle, which can be p'owered and/or steered. Alternatively, the blade or blades can be supported on a seabed or surface towed structure. The construction of suitable vehicles or structures is known.

For burying a pipeline or cable, a plough and oscillation or vibration-generating means in accordance with this invention can be used to fluidise the bottom material below a cable or pipeline so that the latter will sink into, or can easily be laid into, the fluidised region. The blade or blades (preferably positioned on both sides of the cable or pipeline) should be of sufficient length to enable the cable or pipeline to descend from the normal surface of the bottom to a desired depth. To assist the cable or

pipeline to descend to its desired depth beneath the seabed, the cable or pipeline may be forced downwards by a roller or guide.

Where an open trench is required, bottom material loosened or fluidised by the application of vibrations or oscillations is removed in any suitable way, for example by means of a generally conventional plough and/or by pumping- σr rσtatinςr device. Thus,- the- invention can be applied to a substantially conventional plough, by the application of elastic wave vibration generators to one or more blade surfaces of the plough, thereby loosening or fluidising the bottom material adjacent to the plough blade, reducing friction and adhesion between bottom material and blade and within the bottom material, and consequently reducing the necessary pull force, and traction in the case of a seabed vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, in the accompanying drawings

Figure 1 shows a plough for burying cable or pipeline, embodying the invention, and

Figure 2 shows schematically, in end view, a trenching device embodying the invention.

Figure 3 shows schematically a roller system to

Figure 4 shows a plough and rotating soild displacement system for burying a cable or pipeline in a wide open trench, the plough embodying the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS.

Figure 1 shows a seabed tract crawler vehicle 1 provided with an umbilical cable 2 connecting it to a surface or underwa-ter vessel or- structure". ^" The umbilical link: may provide traction but preferably provides only power enabling the vehicle 1 to drive itself.

The vehicle has a pair of spaced crawler tracks 3, and a super structure or body 4 from which depend a pair of laterally spaced flat, vertical or sloping, plough blades 5 which project below the crawler tracks to whatever depth is desired for trenching or burying a pipeline or cable 6.

The shaded area 7 of each plough blade, comprising its leading edge and at least the leading part of its foot, is provided with electrical transducers for exciting elastic wave vibrations in the blades.

In operation, the cable or pipeline 6 is laid on the seabed 8 along the required course, and the vehicle 1

blades excited at a frequency (which may be sonic, subsonic or ultrasonic) selected to loosen and fluidise . the material of the seabed. As a result, a fluidised region is formed between and around the plough blades, of a density which can be less than that of the cable or pipeline, so that the latter sinks into the fluidised material as shown in Figure 1. After the vehicle has passed, the seabed material settles again, burying the pipeline or cable.

Figure 2 illustrates apparatus for trenching. A suitable vehicle, powered or towed (not shown) carries a pair or pairs of plough blades or knives 11 which are hollow or are provided with ducts 12. These slope at an angle appropriate to the required slope of the trench walls, dependent on the stability of the bottom material. Each blade is provided with electrical elastic wave vibration exciters along its leading edge and optionally along at least the leading parts of its bottom edge. Optionally the plough may also have a horizontal leading blade also provided with elastic wave vibration exciters. The ducts 12 of the blades have mouths 13 at the bottom facing inwards, and outlets 14 at the top.

The passages 12 are provided with pumps, for example rotary pumps or ejector-type pumps. .

In operation, the plough is driven or towed along the seabed with its blades penetrating the seabed, the material of which is excited and thereby loosened or fluidised by the elastic wave vibrations applied to the blade edges. The loose material between the blades is withdrawn by the pumps through the ducts 12 and is expelled at the sides of the trench. The cable or pipeline 6 is laid into the trench thus formed, after or during th formation of the trench.

The application of elastic wave vibrations to the plough blades or knives greatly reduces the force necessary to penetrate and displace the seabed material. Figure 3. illustrates schematically a typical roller that can be used to force the cable or pipeline downwards to the bottom of the trench, when its flexural rigidity would otherwise prevent it from so doing under its own self weight.

Figure 4 illustrates schematically a rotating spoil displacement system incorporated into a plough embodying the invention. The rotating units will be positioned and operate in such a way as to disperse or displace the soil away from the sides of the trench such that it will not act as overburden forces to cause collapse of the trench before the cable or pipeline has descended to the ^' bottom on the trench.