GB2301128A

GB2301128A - Underwater excavation apparatus

Info

Publication number: GB2301128A
Application number: GB9610948A
Authority: GB
Inventors: Hector Filippus Alexand Susman; Kenneth Roderick Stewart; Michael Christopher Mcadie
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-05-24
Filing date: 1996-05-24
Publication date: 1996-11-27
Anticipated expiration: 2016-05-24
Also published as: GB9610948D0; GB2301128B

Abstract

Underwater excavation apparatus 100 for reducing the problems associated with water jetting at depths much in excess of 40 to 50 metres comprises a agitator device 265 having mechanical disturbance means and fluid flow disturbance means. The mechanical disturbance means and fluid flow disturbance means advantageously comprise a hollow drill bit having a plurality of holes 285 provided in a side wall thereof and also a plurality of paddles 275 disposed longitudinally and radially extending upon the drill bit and substantially equidistantly spaced one from another. The apparatus may be mounted on a sled. The apparatus 100 may also include a hollow body 170 having a inlet and an outlet, at least one pair of impellers 135, 140 coaxially displaced one from the other and rotatably mounted in the hollow body, and a drilling motor 10 for driving the impellers of the/each pair in contrary rotating directions.

Description

Improvements in or Relating to Underwater Excavation Apparatus This invention relates to an improved underwater excavation apparatus.

Problems exist in the excavation of seabeds, particularly seabeds composed of mud, sedimentary matter, or like viscid material. Water jet velocities in excess of 50 metres per second may be required for such excavation and the provision of such water jet velocities at depths much in excess of 40 to 50 metres causes much practical difficulty.

It is an object of one or more aspects of the present invention to seek to obviate or mitigate one or more of the aforementioned problems in the prior art.

According to a first aspect of the present invention there is provided an underwater excavation apparatus comprising an agitator device having mechanical disturbance means and fluid flow disturbance means.

Preferably the mechanical disturbance means and fluid flow disturbance means comprise a hollow drill bit having at least one hole provided in a side wall thereof.

Preferably the drill bit provides a plurality of holes in a side wall thereof and preferably also a plurality of paddles.

In one embodiment the paddles may be disposed longitudinally and radially extending upon the drill bit and substantially equidistantly spaced one from another.

In said embodiment a plurality of substantially longitudinally equally spaced holes may be provided between adjacent paddles.

Preferably the underwater excavation apparatus further comprises a hollow body having an inlet and an outlet, at least one impeller rotatably mounted in the hollow body and means for driving the impeller.

Advantageously there are provided at least one pair of impellers coaxially displaced from one another rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating directions.

Preferably, but not essentially, the inlet and outlet of the hollow body are provided at opposing ends thereof, the common axis of the impeller extending between the inlet and the outlet.

The means for driving the impellers may include a drilling motor.

The drilling motor may be a "Moineau", hydraulic or suitably adapted electric motor.

Alternatively and advantageously the drilling motor may comprise a stator and a rotor rotatably mounted in the stator, the stator being provided with a rod recess and an exhaust port, the rotor being provided with a rotor channel and at least one channel for conducting motive fluid from the rotor channel to a chamber between the rotor and the stator, the rod recess being provided with a rod which, in use, forms a seal between the stator and the rotor. Such a drilling motor is described in pending US 08/181,693 (SUSMAN et al).

In this embodiment the drill bit may be rotated around a longitudinal axis by means of the drill motor1 and fluid exhausted from the drill motor may be caused to exhaust through the drill bit.

Although not essential it is highly desirable that the rotor be provided with a seal for engagement with the stator.

Preferably, the seal is made from a material selected from the group consisting of plastics materials, polyethylethylketone, metal, copper alloys and stainless steel.

Advantageously, the rod is made from a material selected from the group consisting of plastics materials, polyethylethylketone, metal, copper alloys and stainless steel.

Preferably, the stator is provided with two rod recesses which are disposed opposite one another, and two exhaust ports which are disposed opposite one another, each of the rod recesses being provided with a respective rod, the rotor having two seals which are disposed opposite one another.

The drilling motor may advantageously comprise two drilling motors arranged with their respective rotors connected together each motor comprising a stator and a rotor rotatably mounted in the stator, the stator being provided with a rod recess and an exhaust port, the rotor being provided with a rotor channel and at least one channel for conducting motive fluid from the rotor channel to a chamber between the rotor and the stator, the rod recess being provided with a rod which, in use, forms a seal between the stator and the rotor.

Preferably, the drilling motors are connected in parallel, although they could be connected in series if desired.

Advantageously, the drilling motors are arranged so that, in use, one drilling motor operates out of phase with the other. Thus, in a preferred embodiment each drilling motor has two chambers and the chambers in the first drilling motor are 900 out of phase with the chambers in the second drilling motor. Similarly, in an embodiment in which each drilling motor has four chambers, the chambers in the first drilling motor would preferably be 450 out of phase with the chambers on the second drilling motor. This arrangement helps ensure a smooth power output and inhibits stalling.

The apparatus may provide means for steering the apparatus, in use.

Preferably the steering means comprises at least four apertures on the apparatus, the apertures being equally spaced around a plane through the apparatus, which plane is intended to be substantially horizontal in use, openable gates on each of the four apertures, and means for controlling the opening and closing of each gate, each gate preferably providing a portion which portion extends inwardly when the gate is open (so as to direct - or scoop - water through the respective aperture) the portion further closing the aperture when the gate is closed.

Preferably the control means comprises an electric or hydraulic actuator for each gate, each actuator being controlled by means of an umbilical extending above surface.

Alternatively, the steering means may comprise one or more openable flaps located on the outlet.

Each impeller may include a plurality of blades, the blades of one impeller being offset by 1800 with respect to the blades of the other impeller of the pair.

The impellers may be in the form of propellers. For example, the impellers may be in the form of propellers provided with water jets on the tips thereof as disclosed in GB 2 240 568.

The underwater excavation apparatus may be provided with ducting atop the hollow body which serves to direct excavated matter away from the apparatus.

The ducting may be arranged such that substantially equal flowrates of excavated matter are directed to either side of the underwater excavation apparatus, so as to seek to provide that the underwater excavation apparatus is not subjected to any reactive forces resulting from the expulsion of excavated matter and hence is not caused to deviate from its intended path.

The underwater excavation apparatus may be mounted upon a sled of the type used to execute seabed activities from a surface vessel.

The sled may comprise mounting means for the underwater excavation apparatus, runner means to allow the sled to move over the seabed, and means to attach the sled to a surface vessel.

The runner means may advantageously be provided with one or more ports along their lower edge through which pressurised fluid may be pumped.

According to a second aspect of the present invention there is provided an agitator device having mechanical disturbance means and fluid flow disturbance means for use in an underwater excavation apparatus.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, which are: Fig 1 a longitudinal cross-sectional view of a drilling motor for use in an underwater excavation apparatus according to the present invention; Figs 2A-2D a series of cross-sectional views along line A-A of the drilling motor of Fig 1 showing the motor in four different positions; Figs 3A-3D a series of cross-sectional views along line B-B of the drilling motor of Fig 1 showing the motor in four different positions; Fig 4 a partial longitudinal cross sectional view of a first embodiment of an underwater excavation apparatus according to the present invention; Figs 5A-5C a series of views of a first propeller for use in the apparatus of Fig 4; Figs 6A-6C a series of views of a second propeller for use in the apparatus of Fig 4;; Fig 7 a cross-sectional view along line A-A of Fig 4; Fig 8 a schematic side view of the apparatus of Fig 4; Fig 9 a schematic side view of the apparatus of Fig 4 connected to a hose reel provided, for example, at the stern of a ship; Fig 10 a partially sectioned end view of a sled mounted underwater excavation apparatus according to a second embodiment of the present invention; and Fig 11 a cross-sectional side view of the apparatus of Fig 10.

Two embodiments of an underwater excavation apparatus according to the present invention are disclosed herein, which apparatus each employ a drilling motor. In order to facilitate understanding of the embodiments of the underwater excavation apparatus disclosed, a detailed description will firstly be given of the drilling motor.

Referring to Fig 1 there is shown the drilling motor (drilling apparatus) generally designated 10. The drilling motor 10 comprises a first motor 20 and a second motor 50.

The first motor 20 comprises a stator 21 and a rotor 23. A top portion 22 of the rotor 23 extends through an upper bearing assembly 24 which comprises a thrust bearing 26 and seals 25.

Motive fluid, e.g. water, drilling mud or gas under pressure, flows down through a central sub channel 12 into a central rotor channel 27, and then out through rotor flow channels 28 into action chambers 31 and 32.

Following a motor power stroke, the motive fluid flows through exhaust ports 33 in stator 21, and then downwardly through an annular channel circumjacent the stator 21 and flow channels 35 in a lower bearing assembly 34. A portion 36 of the rotor 23 extends through the lower bearing assembly 34 which comprises a thrust bearing 37 and seals 38.

The ends of the stator 21 are castellated and the castellations engage in recesses in the respective upper bearing assembly 24 and lower bearing assembly 34 respectively to inhibit rotation of the stator 21. The upper bearing assembly 24 and lower bearing assembly 34 are a tight fit in an outer tubular member 14 and are held against rotation by compression between threaded sleeves 16 and 84.

A splined union 39 joins a splined end of the rotor 23 to a splined end of a rotor 53 of the second motor 50.

The second motor 50 has a stator 51.

A top portion 52 of the rotor 53 extends through an upper bearing assembly 54. Seals 55 are disposed between the upper bearing assembly 54 and the exterior of the top portion 52 of the rotor 53. The rotor 53 moves on thrust bearings 56 with respect to the upper bearing assembly 54.

Motive fluid flows into a central rotor channel 57 from the central rotor channel 27 and then out through rotor flow channels 58 into action chambers 61 and 62.

Following a motor power stroke, the motive fluid flows through exhaust ports 63 in stator 51, and then downwardly through an annular channel circumjacent the stator 51 and flow channels 65 in a lower bearing assembly 64. A portion 66 of the rotor 53 extends through a lower bearing assembly 64. The rotor 53 moves on thrust bearings 67 with respect to the lower bearing assembly 64 and seals 68 seal the rotor-bearing assembly interface. Also motive fluid which flowed through the flow channels 35 in the lower bearing assembly 34, flows downwardly through channels 79 in the upper bearing assembly 54, past stator 51 and through flow channels 65 in the lower bearing assembly 64.

The upper bearing assembly 54 and lower bearing assembly 64 are a tight fit in an outer tubular member 18 and are held against rotation by compression between threaded sleeve 84 and a lower threaded sleeve (not shown).

Figs 2A-2D and 3A-3D depict a typical cycle for the first and second motors 20 and 50 respectively, and show the status of the two motors with respect to each other at various times in the cycle. For example, Fig 2C shows an exhaust period for the first motor 20 while Fig 3C, at that same moment, shows a power period for the second motor 50.

As shown in Fig 2A, motive fluid flowing through the rotor flow channels 28 enters the action chambers 31 and 32. Due to the geometry of the chambers (as discussed below) and the resultant forces, the motive fluid moves the rotor in a clockwise direction as seen in Fig 2B.

The action chamber 31 is sealed at one end by a rolling vane rod 71 which abuts an exterior surface 72 of the rotor 23 and a portion 74 of a rod recess 75.

At the other end of the action chamber 31, a seal 76 on a lobe 77 of the rotor 23 sealingly abuts an interior surface of the stator 21.

As shown in Fig 2B, the rotor 23 has moved to a point near the end of a power period.

As shown in Fig 2C, motive fluid starts exhausting at this point in the motor cycle through the exhaust ports 33.

As shown in Fig 2D, the rolling vane rods 71 and seals 76 have sealed off the action chambers and motive fluids flowing thereinto will rotate the rotor 23 until the seals 76 again move past the exhaust ports 33.

The second motor 50 operates as does the first motor 20; but, as preferred, and as shown in Figs 3A-3D, the two motors are out of phase by 900 so that as one motor is exhausting motive fluid the other is providing power.

The seals 76 are, in one embodiment, made of polyethylethylketone (PEEK). The rolling vane rods 71 are also made from PEEK. The rotors (23, 25) and stators (21, 51) are preferably made from corrosion resistant materials such as stainless steel.

When a seal 76 in the first motor 20 rotates past an exhaust port 33, the motive fluid that caused the turning exits and flows downward, then through the channels 79, past the exhaust ports 63 and the flow channels 65.

Referring now to Fig 4 there is shown a partial view of a first embodiment of an underwater excavation apparatus according to the present invention, generally designated 100.

The apparatus 100 comprises a connector body 105 having a frustoconical internally threaded portion 110 for connection to drill pipe, coiled tubing or any pipe capable of transporting motive fluid for driving the drilling motor 10 provided within the apparatus 100. The connector body 105 has a through bore 115 which communicates with the central sub channel 12 of motor 20.

Rigidly connected to the connector body 105 is an outer tube 120, such that a portion of the connector body 105 is located with the outer tube 120. Around an outer surface of the portion of the connector body 105 there is rigidly connected a first part of a swivel 125. The swivel 125 comprises first and second parts rotatable with respect to one another. The second part of the swivel 125 is rigidly connected to an upper part 11 of the motor 10 which part is rigidly engaged with the stator 21. The swivel 125 is in this embodiment a known "stuffing box" including combined radial and thrust bearings.

It is, therefore, apparent that the rotors 23, 53 are rotatable with respect to the stators 21, 51 and with respect to the outer tube 120, while the stators 21, 51 are themselves rotatable with respect to the outer tube 120. The stators 21, 51 are further rigidly connected to motor housing 121.

The portion 66 of the rotor 53 is rigidly connected to one end of a drive shaft 130 by means of a female spine coupling provided in the drive shaft 130. At the other end of the drive shaft 130 there is provided a first impeller in the form of a first propeller 135.

The stator 51 is rigidly engaged with a second impeller in the form of a second propeller 140 by means of bolts 145 connecting the second propeller 140 to a flanged portion 150 on the end of the outer tubular member 18 of the motor 50.

The first and second propellers 135, 140 are connected between one another by a combined thrust and radial bearing 155.

It is, therefore, apparent that the first propeller 135 rotates with the rotors 23, 53, while the second propeller 140 rotates with the stators 21, 51.

At the end of the outer tube 120 there is provided a flanged portion 160. Below the flanged portion 160 there is provided a marine bearing 165. Connected to the flanged portion 160 by means of bolts 169 is a hollow body 170. The hollow body 170 carries at an inlet/outlet thereto four inlet/outlet guide vanes 175. At an outlet/inlet to the body 170 there are provided a plurality of outlet/inlet guide vanes 180. The guide vanes 175, 180 are provided so as to produce a predefined flow of water through the hollow body 170.

Within the inlet/outlet of the hollow body 170 there is provided a safety grid 185. Further equidistantly spaced circumferentially around the hollow body 170 are provided a plurality of (in this embodiment 8) longitudinal strengthening strips 186.

Circumferentially around the outlet/inlet of the hollow body 170 there is provided steering means in the form of four apertures 190 equally spaced on the hollow body 170 in a plane through the apparatus 100, which plane is intended to be substantially horizontal in use.

Each aperture 190 carries a gate 195. Each gate 195 provides a portion which portion extends inwardly when the gate 195 is open (so as to direct - or scoop - water through the respective aperture 190), the portion further closing the aperture 190 when the gate 195 is closed.

Each gate 195 is openable and closable by control means in the form of electric or hydraulic actuators 200 connected to the gate 195 by connecting members 205 and carried by a flange 210 provided around the hollow body 170. The actuators 200 are controlled by means of an umbilical (not shown) extending above surface.

Referring now to Figs 5A-5C and 6A-6C, there is shown detailed drawings of the first and second propellers 135, 140. As can be seen each propeller 135, 140 carries six blades. The propellers 135, 140 are substantially identical except that their blades are offset with respect to one another by 1800 so that the propellers 135, 140 rotate in contrary rotating directions.

Referring now to Fig 8, there is illustrated the first embodiment of the excavator 100 according to the present invention particularly suited for the excavation of a seabed composed of sedimentary mud, or like viscid matter, comprising motor 10, contra-rotating propellers 135, 140 and body 170. The drive shaft 130 of the motor 10 is extended by a hollow stub shaft 250, which is supported in a marine bearing 255. Said shaft 250 is further provided with a female thread 260 of a type commonly used for oil tools. Said thread 260 allows an agitator device 265 to be attached to the stub shaft 250.

The agitator device 265 in this embodiment is in the form of a rotatable drill bit and comprises a hollow cylindrical body 270 of a diameter substantially equal to that of the stub shaft 250 surmounted by a male thread 280, and a plurality of radially extending paddles 275 and a plurality of vent holes 285 provided between said radially extending paddles 275, which allow communication between the hollow cylindrical body 270 and the outside of the agitator device 265. The agitator device 265 may be connected directly to the stub shaft 250 or be attached by hollow extension shafts (not shown) which allow the distance between the excavator 100 and the agitator device 265 to be varied.

In use the excavator 100 is suspended from a surface vessel at a pre-determined distance from the seabed, while the agitator device 265 projects into the seabed.

The height of the excavator 100 above the seabed, the penetration of the agitator device 265 into the seabed and the distance between the agitator device 265 and the excavator 100 are dependent on such factors as the depth of excavation required in the seabed, the depth at which the excavation is taking place and the consistency of the seabed among others.

The supply of pressurised fluid, usually sea-water, to the motor 10 prompts contra-rotation of the impellers 135, 140. As before the upper second impeller 140 is rigidly attached to the motor housing 121 while the lower first impeller 135 is rigidly attached to the drive shaft 130. The stub shaft 250, being connected to the drive shaft 130, rotates at the same speed as the lower impeller 135, as does the agitator device 265. The rotation of the paddles 275 attached to the agitator device 265 disturbs the seabed and hence aids the excavating action of the water jet provided by the excavator 100 through body 170. The action of the paddles 275 is further enhanced by the use of the pressurised fluid utilised to power the motor 10. The fluid is vented from the base of the motor 10 into the hollow stub shaft 250 and is in turn passed to the agitator device 265.The pressurised fluid then passes through the plurality of vent holes 285 provided between the paddles 275 and into the seabed further enhancing the action of the agitator device 265.

The paddles 275 may be longitudinally displaced along the length of the agitator device 265 as shown in Figure 8 or arranged in a helical/spiral fashion depending on the nature of the material constituting the seabed and/or the depth of excavation required. The disturbance of the seabed, which is of a greater density than the surrounding sea-water and hence resists the action of the agitator device 265, requires power which is supplied by the drive shaft 130. The increased torque required to be supplied by the motor shaft 130 in order to overcome the resistance of the seabed results in a reduction in the shaft speed. As a result of this the rotation of the motor housing 121, and hence the upper impeller 140, increases in speed. Due to the relationship between rotational speed and torque dissipation the increase in rotational speed of the upper impeller 140 allows it to absorb significantly more torque than the slower rotating lower impeller 135. This has the advantageous effect of increasing the torque available to the agitator device 265. The excavator 100 therefore acts in a self compensating manner.

It should be noted that the mechanical torque utilised by the agitator device 265 is generated without resultant reactive torque on the excavator structure 100 as a whole. This allows the excavator 100 to be operated at great depths than heretofore possible and without the need to suspend the excavator from rigid support means such as drill pipe. Preferably the excavator structure 100 is suspended from a flexible hose which serves also to supply the motor 10 with pressurised fluid.

In use the excavator 100 may be operated such that the impellers 135, 140 provide a fast moving stream of water directed at the seabed. However it may be envisaged that the action of the propellers 135, 140 may be reversed such that water is drawn from below the excavator 100 and expelled above the excavator 100. Used in conjunction with the agitator device 265 this feature allows material removed from the seabed to be directed away from the excavation site. In order to accomplish this the excavator 100 is further provided with ducts (not shown) on top of the main body 170 to direct the expelled matter away from the excavator 100. The positioning of such ducts would ensure that the excavator 100 would not be subjected to any reactive forces as a result of the expelled matter, and hence would not be caused to deviate from its intended path.

Referring to Fig 9 there is shown the apparatus 100, to be lowered into the sea, connected to a hose reel 215 provided, for example, at the stern of a ship 220.

In use, the apparatus 100 is lowered to the desired position, for example, just above the seabed as is known in the art. The position of the apparatus 100 may be controlled by the positioning means by suitable controlled opening/closing of the gates 195 and operation of the propellers 135, 140.

Once in the desired position the apparatus 10 may be operated by pumping motive fluid into the drilling motor 10. The rotors 23, 53 consequently begin to rotate so driving the first propeller 135 in one direction.

Further, the second propeller 140 also begins to rotate by taking up reactive torque of the first propeller 135.

The propellers 135, 140, therefore, rotate at the same speed in opposite directions.

Figs 10 and 11 show a further embodiment of an underwater excavation apparatus according to the present invention 100' adapted for the excavation of a sea-bed composed of sedimentary mud, or like viscid matter, comprising a motor 10', contra-rotating impellers 135', 140' and a substantially tubular body 170'. The drive shaft 130' of the motor 10' is extended from the base of the tubular body 170' by a hollow stub shaft 250', which is supported in a marine bearing 255'. Said shaft 250 is further provided with a female thread (not shown) of a type commonly used for oil tools. Said thread allows an agitator device 265' to be attached to the stub shaft 250'.

The agitator device 265' comprises a hollow cylindrical body 270' of a diameter substantially equal to that of the stub shaft 250', a plurality of radially extending paddles 275' and a plurality of vent holes 285 provided between said radially extending paddles 275', which allow communication between the hollow cylindrical body 170' and the outside of the agitator device 265'.

The agitator device 265' may be connected directly to the stub shaft 250' or be attached by hollow extension shafts (not shown) which allows the distance between the excavator 100' and the agitator device 265' to be varied.

The excavator 100' is mounted on a sled 300 comprising a pair of runners 305', mounting plates 310', 315' to secure the excavator 100' to the sled 300', and an eye 320' to enable the sled 300' to be attached, via a cable, to a surface vessel. The mounting plates 310', 315' may be adjustable to allow the position of the excavator 100' relative to the sled 300' to be varied.

The runners 305' are further provided along their lower edge with a plurality of ports 325' through which fluid may be pumped.

The tubular body 170' of the excavator 100' is surmounted by a cover 330' incorporating a pair of ducts 335' which project either side of the excavator 100'.

In use the excavator 100' is lowered from a surface vessel such that the runners 305' of the sled 300' rest upon the seabed, while the agitator device 265' projects into the seabed. The height of the excavator 100' above the seabed, the penetration of the agitator device 265' into the seabed and the distance between the agitator device 265' and the excavator 100' are dependent on such factors as the depth of excavation required in the seabed, the depth at which the excavation is taking place and the consistency of the seabed among others.

The supply of pressurised fluid, usually sea-water, to the motor 10' prompts the contra-rotation of the drive shaft 130' and motor housing 121' with the resultant contra-rotation of the impellers 135', 140'. The upper impeller 140' is rigidly attached to the motor housing 121' while the lower impeller 135' is rigidly attached to the drive shaft 130'. The stub shaft 250', being connected to the motor shaft 130', rotates at the same speed as the lower impeller 135', as does the agitator device 265'. The rotation of the paddles 275' attached to the agitator device 265' disturbs the seabed and hence aids the excavating action of the water jet provided by the excavator 100'. The action of the paddles 275 is further enhanced by the use of the pressurised fluid utilised to power the motor 10'.The fluid is vented from the base of the motor 10' into the hollow stub shaft 250' and is in turn passed to the agitator device 265'.

The pressurised fluid then passes through the plurality of vent holes 285' provided between the paddles 275' and into the seabed further enhancing the action of the paddles 275'.

The paddles 275 may be longitudinally displaced along the length of the agitator device 265' as shown in the accompanying figures or arranged in a helical/spiral fashion depending on the nature of the material constituting the seabed and/or the depth of excavation required. The disturbance of the seabed, which is of a greater density than the surrounding sea-water and hence resists the action of the agitator device 265', requires power which is supplied by the drive shaft 130'. The increased torque required to be supplied by the motor shaft 130' in order to overcome the resistance of the seabed results in a reduction in the shaft speed. As a result of this the rotation of the motor housing 121', and hence the upper impeller 140', increases in speed.

Due to the relationship between rotational speed and torque dissipation, the increase in rotational speed of the upper impeller 140' allows it to absorb significantly more torque than the slower rotating lower impeller 135'.

This has the advantageous effect of increasing the torque available to the agitator device 265'.

It should be noted that the mechanical torque utilised by the agitator device 265' is generated without resultant reactive torque on the excavator structure 100' as a whole. This allows the excavator to be operated at great depths without the need to suspend it from rigid support means such as drill pipe.

In use the excavator 100' may be operated such that the impellers 135', 140' provide a fast moving stream of water vertically upwards from the seabed. Matter disturbed by the agitator device 265' is entrained in the water stream and passed through the tubular body 170' before being vented through the ducts 335'. The ducts 335' are configured such that the flowrate through each is substantially equal so that the excavator 100' tends not to be subjected to unbalanced reactive forces which could cause it to deviate from its intended path.

Pressurised fluid may be supplied to the sled runners 305' and vented through the plurality of ports 325' provided in the runner's 305' underside. This has the effect of reducing the coefficient of friction between the sled 300' and the seabed, thus aiding its progress over the seabed.

The excavator 100', either with or without the agitator device 265', may be mounted upon the sled 300' of the type currently known for use in subsea excavation operations. Mounting means provided upon such a sled 300 may allow the orientation, i.e inclination, of the excavator 100' relative to the seabed to be varied and hence optimise its operation.

The embodiments of the invention hereinbefore described are given by way of example only and are not meant to limit the scope thereof in any way.

Claims

1. An underwater excavation apparatus comprising an agitator device having mechanical disturbance means and fluid flow disturbance means.

2. An underwater excavation apparatus as claimed in claim 1, wherein the mechanical disturbance means and fluid flow disturbance means comprise a hollow drill bit having at least one hole provided in a side wall thereof.

3. An underwater excavation apparatus as claimed in claim 2, wherein the drill bit provides a plurality of holes in a side wall thereof.

4. An underwater excavation apparatus as claimed in either of claims 2 or 3, wherein the drill bit provides a plurality of paddles.

5. An underwater excavation apparatus as claimed in claim 4, wherein the paddles are disposed longitudinally and radially extending upon the drill bit and substantially equidistantly spaced one from another.

6. An underwater excavation apparatus as claimed in claim 5, wherein a plurality of substantially longitudinally equally spaced holes are provided between adjacent paddles.

7. An underwater excavation apparatus as claimed in any preceding claim, wherein the underwater excavation apparatus further comprises a hollow body having an inlet and an outlet, at least one impeller rotatably mounted in the hollow body and means for driving the impeller.

8. An underwater excavation apparatus as claimed in claim 7, wherein there are provided at least one pair of impellers coaxially displaced from one another rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating directions.

9. An underwater excavation apparatus as claimed in either of claims 7 or 8, wherein the inlet and outlet of the hollow body are provided at opposing ends thereof, the common axis of the impeller extending between the inlet and the outlet.

10. An underwater excavation apparatus as claimed in any of claims 7 to 9, wherein the means for driving the impellers includes a drilling motor.

11. An underwater excavation apparatus as claimed in claim 10, wherein the drilling motor is selected from one of a "Moineau", hydraulic or suitably adapted electric motor.

12. An underwater excavation apparatus as claimed in claim 10, wherein the drilling motor comprises a stator and a rotor rotatably mounted in the stator, the stator being provided with a rod recess and an exhaust port, the rotor being provided with a rotor channel and at least one channel for conducting motive fluid from the rotor channel to a chamber between the rotor and the stator, the rod recess being provided with a rod which, in use, forms a seal between the stator and the rotor.

13. An underwater excavation apparatus as claimed in claim 12, wherein the drill bit, in use, rotates around a longitudinal axis by means of the drill motor, and fluid exhausted from the drill motor is caused to exhaust through the drill bit.

14. An underwater excavation apparatus as claimed in either of claims 12 or 13, wherein the rotor is provided with a seal for engagement with the stator.

15. An underwater excavation apparatus as claimed in claim 14, wherein the seal is made from a material selected from the group consisting of plastics materials, polyethylethylketone, metal, copper alloys and stainless steel.

16. An underwater excavation apparatus as claimed in any of claims 12 to 15, wherein the rod is made from a material selected from the group consisting of plastics materials, polyethylethylketone, metal, copper alloys and stainless steel.

17. An underwater excavation apparatus as claimed in any of claims 12 to 16, wherein the stator is provided with two rod recesses which are disposed opposite one another, and two exhaust ports which are disposed opposite one another, each of the rod recesses being provided with a respective rod, the rotor having two seals which are disposed opposite one another.

18. An underwater excavation apparatus as claimed in any of claims 12 to 17, wherein the drilling motor comprises two drilling motors arranged with their respective rotors connected together each motor comprising a stator and a rotor rotatably mounted in the stator, the stator being provided with a rod recess and an exhaust port, the rotor being provided with a rotor channel and at least one channel for conducting motive fluid from the rotor channel to a chamber between the rotor and the stator, the rod recess being provided with a rod which, in use, forms a seal between the stator and the rotor.

19. An underwater excavation apparatus as claimed in claim 18, wherein the drilling motors are connected in parallel.

20. An underwater excavation apparatus as claimed in claim 18, wherein the drilling motors are connected in series.

21. An underwater excavation apparatus as claimed in any of claims 18 to 20, wherein the drilling motors are arranged so that, in use, one drilling motor operates out of phase with the other.

22. An underwater excavation apparatus as claimed in claim 21, wherein each drilling motor has two chambers and the chambers in the first drilling motor are 900 out of phase with the chambers in the second drilling motor.

23. An underwater excavation apparatus as claimed in claim 21, wherein each drilling motor has four chambers, the chambers in the first drilling motor would preferably be 450 out of phase with the chambers on the second drilling motor.

24. An underwater excavation apparatus as claimed in any preceding claim, wherein the apparatus provides means for steering the apparatus, in use.

25. An underwater excavation apparatus as claimed in claim 24, wherein the steering means comprises at least four apertures on the apparatus, the apertures being equally spaced around a plane through the apparatus, which plane is intended to be substantially horizontal in use, openable gates on each of the four apertures, and means for controlling the opening and closing of each gate, each gate preferably providing a portion which portion extends inwardly when the gate is open (so as to direct - or scoop - water through the respective aperture) the portion further closing the aperture when the gate is closed.

26. An underwater excavation apparatus as claimed in claim 25, wherein the control means comprises an electric or hydraulic actuator for each gate, each actuator being controlled by means of an umbilical extending above surface.

27. An underwater excavation apparatus as claimed in claim 24, wherein the steering means comprises one or more openable flaps located on the outlet.

28. An underwater excavation apparatus as claimed in claim 8, wherein each impeller may include a plurality of blades, the blades of one impeller are offset by 180 with respect to the blades of the other impeller of the pair.

29. An underwater excavation apparatus as claimed in either of claims 7 or 8, wherein the impeller(s) are in the form of a propeller(s).

30. An underwater excavation apparatus as claimed in claim 29, wherein the impeller(s) are in the form of a propeller(s) provided with water jets on the tip(s) thereof.

31. An underwater excavation apparatus as claimed in claim 7, wherein there is provided with ducting atop the hollow body which serves to direct excavated matter away from the apparatus.

32. An underwater excavation apparatus as claimed in claim 31, wherein the ducting is arranged such that substantially equal flowrates of excavated matter are directed to either side of the underwater excavation apparatus, so as to seek to provide that the underwater excavation apparatus is not subjected to any reactive forces resulting from the expulsion of excavated matter and hence is not caused to deviate from its intended path.

33. An underwater excavation apparatus as claimed in any preceding claim, wherein the apparatus is mounted upon a sled.

34. An underwater excavation apparatus as claimed in claim 33, wherein the sled is of a type used to execute seabed activities from a surface vessel.

35. An underwater excavation apparatus as claimed in either of claims 33 or 34, wherein the sled comprises mounting means for the underwater excavation apparatus, runner means to allow the sled to move over the seabed, and means to attach the sled to a surface vessel.

36. An underwater excavation apparatus as claimed in claim 35, wherein the runner means are provided with one or more ports along their lower edge through which pressurised fluid may be pumped, in use.

37. An agitator device having mechanical disturbance means and fluid flow disturbance means for use in an underwater excavation apparatus.

38. An underwater excavation apparatus as hereinbefore described with reference to Figs 4 to 9 or Figs 10 to 11.

39. An agitator device having mechanical disturbance means and fluid disturbance means for use in an underwater excavation apparatus as hereinbefore described with reference to Figs 4 to 9 or Figs 10 to 11.