SEABED TRENCHING APPARATUS

Abstract: An apparatus for forming a trench in a seabed, for burial of pipelines and cables, comprises a front portion (10) to which a towing means (22) is attached, and a rear portion (14) which is pivotally attached to the front portion. The rear portion is provided with a first trench-­ forming plough (32) which forms a trench of a constant profile. A second trench-forming plough (34) is pivotally mounted to the first trench-forming plough (22) and is adjustable by means of hydraulic rams (40), so that in relatively firm seabeds a steep-sided trench can be formed, whereas in softer seabeds the walls can be made shallower to prevent their collapsing.

Description: The present invention relates to apparatus for use in the formation of subsea trenches for burial of pipelines and cables. When laying pipelines and cables on the seabed, it is sometimes necessary to form a trench in which the pipeline or cable can be seated. In a seabed formed of relatively soft and granular materials, a wide, shallow-sided trench is required to achieve the stability necessary for the trench walls to be self- supporting during pipeline or cable installation. In harder materials, such as clay or soft rock, a steeper sided trench is acceptable. The formation of such a trench can be achieved using an underwater plough, a component or components of which are known as shares. This term is used to describe both the seabed engaging "share", and the spoil moving "mouldboard". Plough shares can be detachable items, or an integral part of the plough structure. EP-A-0221581 discloses a plough having fixed plough shares which are preceded by two disc cutters mutually inclined to cut slits in the sea bed ahead of the plough in a V relationship. However, if the composition of the seabed changes, it may be necessary either to replace the plough or the plough shares to produce a trench of the correct profile. Not only is this time consuming, but it is also expensive, since it is necessary to have replacement ploughs or plough shares of the correct profile as well as the personnel to carry out such replacements. It is an object of the present invention to provide apparatus capable of forming seabed trenches of different profiles, depending on the composition of the seabed. In accordance with the present invention, there is provided apparatus for forming a trench in a seabed, comprising a first plough having two fixed plough shares and thereby adapted to form a trench of a fixed profile, and characterised by a second plough having two plough shares, one situated on either side of the first plough, movable relative to the first plough and thereby adapted to produce a trench having a selectively variable profile. This reduces the force needed to tow the apparatus. In harder seabed soils it allows the formation of a stable, steep-sided trench with a minimum cross-sectional area, and at the same time provides a facility whereby shares on the second plough means may be adjusted to produce a stable shallow sided trench with increased cross-sectional area in soft cohesive and granular seabed soils. In these areas the plough pulling forces will generally be less than for the equivalent trench profile in harder seabed soils. Preferably, the second, movable plough means is situated behind the first plough means. The second plough means may then be adjusted so that the finished trench profile is determined solely by the first plough means, or alternatively so that the second plough means alters the profile of the trench produced by the first plough means. In this way, the profile of the trench can be altered according to the type of seabed encountered. Preferably, the second plough is longitudinally displaced from the first plough. The second plough means may be adapted to be able to alter the profile of the trench produced by the first plough means over the full depth of the trench, or in the upper portion of the trench only. Preferably, the second plough means comprises two plough shares, one situated on either side of the first plough means. By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawings, in which: Fig. 1 is a aide elevation of an embodiment of seabed trenching apparatus in accordance with the present invention, positioned on the seabed; Fig. 2 is a rear elevation of the apparatus shown in Fig. 1, in a first configuration; Fig. 3 is a rear elevation of the apparatus shown in Fig. 1, in a second configuration; and Fig. 4 is a profile of two trenches which may be formed using a modification of this invention. Referring firstly to Figs. 1 to 3, the trenching apparatus comprises a forward towing and support portion 10 which is attached to a trenching portion 14. The towing and support portion 10 is fitted with a support skid 16, which is pivotally mounted at pivot 18 to enable the skid to pass over uneven surfaces. The portion 10 is also provided with a towing cable attachment 20, by means of which a towing cable 22, extending from a surface ship, is attached to the trenching apparatus, whereby the apparatus may be towed along the seabed. The trenching portion 14 comprises an upper elongate frame member 24 and two lower elongate frame members 26. The upper frame member 24 is rigidly attached to the forward portion 10, and each lower frame member 26 is hingedly attached to the forward portion 10 at a vertical hinge 12. A lifting cable attachment 28 is provided on the upper frame member 24, by means of which the trenching apparatus may be lifted from, or lowered onto, the seabed. The upper frame member 24 is also provided with other articles such as a hydraulic fluid pressure generator 30, but these other articles are not involved in the understanding of the present invention, and will not be described further. The undersurface of the lower frame members 26 is provided with a first plough means in the form of two fixed plough shares 32, one attached to respective frame members 26 and a second plough means in the form of two secondary plough shares 34a, 34b, positioned behind, and situated one either side of, the fixed plough shares 32a, 32b. The fixed plough shares 32a, 32b are symmetrical about a central ploughing edge 36, which is inclined rearwardly, such that the material removed by the fixed plough shares 32a, 32b is displaced approximately equally on each side, and so that the plough shares will form a trench whose profile is symmetrical about a vertical plane. The secondary plough shares 34a, 34b extend outwardly from respective slots 37a, 37b in the fixed plough shares 32a, 32b and are hingedly attached to the fixed plough shares 32a, 32b by means of hinges 38, situated at the base of the slots 37a, 37b whereby the secondary plough shares 34a, 34b are pivotable about an axis normal to the surface 43 of the fixed shares 32a, 32b. The secondary plough shares 34a, 34b are each pivotable by means of a double-acting hydraulic ram 40 extending between, and pivotally connected by means of mountings 42 to a respective secondary plough share and the fixed plough shares 32a, 32b. By retracting the rams 40 fully, the two secondary plough shares are pivoted upwardly into the position shown in Fig. 2, wherein the edges of the secondary plough shares are aligned with the edge of the fixed plough shares 32a, 32b. By extending the rams 40, the secondary plough shares 34a, 34b are pivoted and thereby protrude beyond the edge of the fixed plough shares 32a, 32b. The position of the secondary plough shares 34a, 34b at their maximum protrusion is illustrated in Fig. 3. In use, the trenching apparatus is lowered onto the seabed with a towing cable 22 attached to the cable attachment 20. The trenching apparatus is then towed along the seabed by a towing boat, thereby causing the trenching apparatus to form a trench in the seabed, into which a pipeline or cable may be laid. If the seabed is formed of relatively hard materials, such as clay or soft rock, a relatively steep-sided trench will not collapse. In such a case, the rams 40 are fully retracted as in Fig. 2, thereby making the profile of the trench solely dependent upon the fixed plough shares 32a, 32b. If, however, the seabed is formed of softer or more granular materials, it is necessary to produce a more shallow-sided trench, otherwise the walls of the trench may collapse and fill in the trench. In this case, the rams 40 are extended, thereby causing the secondary plough shares to change the profile of the trench formed by the fixed plough share, by making the slope of the upper portions of the trench cut by the fixed share more shallow-sloping. The degree to which the rams are extended, and the trench profile modified, depends upon the properties of the seabed, but the two extreme positions are shown in Figs. 2 and 3 respectively. As seen in Fig. 3, the apparatus of this particular embodiment produces a base having the profile of its lowermost portions with relatively steep-sided walls, as determined by the fixed plough shares 32a, 32b, and having the profile of the uppermost portions with relatively shallow-sloped walls, as determined by the protrusion of the secondary plough shares 34a, 34b beyond the fixed plough shares 32a, 32b. An alternative embodiment is illustrated schematically in Fig. 4, and is similar to the first embodiment except that the secondary plough shares 34a, 34b are hinged at a hinge 38′ situated at or near the level of the lowermost portion of the fixed plough shares 32a, 32b. The result of this is that a trench having walls of constant slope is produced. In the left-hand side of Fig. 4, which illustrates one secondary plough share in one extreme position, the rams 40 are retracted, causing the profile of the trench to be determined solely by the fixed plough share 32a, whereas in the right-hand side of Fig. 4, which illustrates one secondary plough share in the other extreme position, the profile of the trench originally formed by the fixed plough share 32b has been modified by the secondary plough share 34b which now protrudes beyond the fixed plough share 32b by virtue of the extension of the rams 40. In both embodiments, the one trenching apparatus can be used to form trenches having different profiles, without the need to replace the plough shares or the apparatus itself if the composition of the seabed changes. Also, by virtue of the fact that the fixed ploughshares always forms a trench, when a shallower trench is then formed by projecting the secondary plough shares, much less seabed material has to be displaced by the secondary plough shares, and the towing force required is generally much less than if all the seabed material for a shallow-sided trench is removed by a single plough share. The invention is not restricted to the details of the foregoing embodiments. For example, the movable plough shares may be positioned in front of the fixed plough share if desired.

Claims: 1. Apparatus for forming a trench in a seabed, comprising a first plough having two fixed plough shares (32a, 32b) and thereby adapted to form a trench of a fixed profile, and characterised by a second plough having two plough shares (34a, 34b), one situated on either side of the first plough, movable relative to the first plough and thereby adapted to produce a trench having a selectively variable profile. 2. Apparatus as claimed in claim 1, wherein the second plough is situated behind the first plough. 3. Apparatus as claimed in claim 2, wherein the plough shares (34a, 34b) of the second plough may be withdrawn to lie laterally no further outward than, or laterally inward of, the first plough. 4. Apparatus as claimed in any of claims 1 to 3, wherein the second plough is longitudinally displaced from the first plough. 5. Apparatus as claimed in any claims 1 to 4, wherein the plough shares (34a, 34b) of the second plough are pivotally mounted on the apparatus. 6. Apparatus as claimed in any of claims 1 to 5, wherein the plough shares (34a, 34b) of the second plough pivotally mounted on the first plough. 7. Apparatus as claimed in any of claims 1 to 6, wherein the plough shares (34a, 34b) of the second plough are movable by means of a fluid-operated ram (40). 8. Apparatus as claimed in any of claims 1 to 7, wherein the second plough is adapted to alter the profile of the trench produced by the first plough over the full depth of the trench. 9. Apparatus as claimed in any of claims 1 to 7, wherein the second plough is adapted to alter the profile of the trench produced by the first plough in an upper portion of the trench only. 10. Apparatus as claimed in any of claims 1 to 9, comprising a front portion comprising means (20) for attaching a towing means (22), and a rear portion (14), upon which the first and second ploughs are mounted, the front and rear portions being pivotally connected.

Inventors: LEY, TIMOTHY GEOFFREY, SEAMANS, JOHN WILLIAM
Applicants: LAND & MARINE ENGINEERING LIMITED
Application year: 1988
Main classification: E02F5
IPC codes: E02F5, B67C3