WO2001014649A1

WO2001014649A1 - Apparatus and method for sea bed excavation

Info

Publication number: WO2001014649A1
Application number: PCT/GB2000/003208
Authority: WO
Inventors: Richard Lane; Phil Brown
Original assignee: PSL TECHNOLOGY Ltd
Current assignee: PSL TECHNOLOGY Ltd
Priority date: 1999-08-21
Filing date: 2000-08-21
Publication date: 2001-03-01
Anticipated expiration: 2002-02-21
Also published as: GB9919801D0; AU6852700A; ZA200201662B

Abstract

A sea-going vessel (1) provided with hoisting equipment (2) is employed for the controlled lowering and raising of excavation apparatus (3). The excavation apparatus (3) comprises a chamber (4) within which is contained agitation and suction means. The agitation means may comprise a propeller (9) employed to produce a high speed jet of sea water directed at the sea bed. Both agitation and suction means are controlled by communication means with the vessel (1). A method is described whereby an area of the sea bed is isolated by deployment of the chamber (4). Thereafter the sediment is disturbed by the agitation means before said disturbed sediment is conveyed to the vessel (1), for processing, by the suction means.

Description

Apparatus and Method for Sea Bed Excavation

The present invention relates to improved method and apparatus for enabling the extraction of sediment and other particulate materials from the sea bed.

In this specification, references to the sea bed should be construed broadly, and as such should be deemed to include river beds, lake beds and the like.

There are various applications, motivated by both environmental and commercial considerations where it may be beneficial to excavate an underwater sea bed. For example, diamonds occur in certain rock and sediment formations lying below the surface of the sea bed. This is the case on the southern west coast of Africa, where typically the highest concentrations of diamond deposits lie between approximately 0.3 metres and 0.8 metres below the gravel layer surface. Apparatus is required for extracting the top layer, preferably to a depth of approximately one metre, to allow the diamonds to be mined. Another application lies in the dredging of harbours or ports where the constant use of oil-based fuels to power marine craft leads to the polluting of the sea bed; creating the desire to provide a means for the periodic lifting and filtering or other cleaning of the sea bed material. A yet further application that is envisaged in consequence to growing environmental concerns relates to the extraction of drill cuttings in the vicinity of off- shore drilling sites.

The present invention is not limited to these applications but finds utility in any situation where it might be advantageous to excavate the sea bed. Usually, this will also involve the raising of the sea bed particles or other matter to the surface where would be positioned a ship or boat housing the means for performing some operation such as cleaning or separating the matter, for example.

In the art there are provided diverse means for excavating sea beds. One such means that is used in fairly loosely compacted sea beds involves the use of one or more suction units that are dragged by an overhead ship across the sea floor. Suction is provided by pumping apparatus located on the ship. However this technique is associated with a number of disadvantages, including the relative low efficiency of employing suction as a sole means to encourage the dislodging and uplifting of sea bed material. Movement of the suction means in loosely defined linear paths also leads to poor surface coverage of the entire sea bed; there inevitably being gaps between the linear paths and inconsistencies in the depth of penetration of the apparatus. Dragging heavy equipment across the sea bed may also lead to the inadvertent damage of pipelines or other sub sea structures.

It is an object of the present invention therefore to provide an improvement to this technique, and moreover to offer a systematic method for removing the sediment top layer of a sea bed with improved sea bed coverage at a consistent depth.

According to the present invention there is provided apparatus for use on a sea bed, the apparatus comprising a chamber defining a volume of sea water located above an area to be excavated, wherein the apparatus further comprises agitation means located within the chamber for agitating the sea bed in the said area and one or more suction means for receiving sediment or other sea bed material agitated by said agitation means.

Preferably the chamber is substantially open at a lower part thereof.

Alternatively the chamber is partially closed at a lower part thereof by a flexible membrane and securing means, wherein the flexible membrane defines an enclosed passageway between the chamber and the sea bed.

The suction means may communicate with one or more rigid or flexible delivery risers for conveying the sea bed material to a vessel or platform located on the sea surface. Typically the vessel or platform would be provided with pumping equipment for generating suction in the suction means. The chamber preferably acts as a barrier to contain the agitated material from the sea bed within a volume defined by the chamber. This serves to increase the efficiency of the apparatus m that the disturbed sea bed material is captured m the excavation area such that it is maintained as available for collection by the suction means .

The agitation means is preferably a device for etting sea water at the sea bed. It may for example comprise of a hydraulically driven propeller that is adapted to propel sea water within the chamber forcibly toward the sea bed. The propeller would encourage a circular flow of water in a horizontal plane, but other means may also be included, such as vanes, fins, to encourage such circular flow.

Alternatively, the agitation means comprises mechanical blades or other members adapted to agitate the sea bed by consequence of physically impacting the sea bed. A yet further alternative may be that the agitation means incorporates a sonic or ultrasonic device.

Preferably the suction means includes one or more inlets for the intake of sea bed material, wherein the or each inlet is disposed toward the perimeter of the chamber. Most preferably, an inlet is provided at each corner of the lower open side of the chamber.

The chamber may be further provided with a vent to balance the volume of water in the chamber. Tre vent may be associated with a valve or plug such that the vent is kept open m normal operation, but m the event that the apparatus becomes trapped in the sea bed can be closed enabling the suction process to be reversed thus freeing the apparatus. Reversal of the suction process may involve the use of the agitation means.

Preferably, the chamber has a skirt at the lower edge of its side walls, the skirt being robust and resilient.

According to a second aspect of the invention there is provided a method of excavating a sea bed comprising the steps of isolating an area of the sea bed; agitating the sea bed in the said area and sucking the agitated sea bed material to a surface vessel or the like.

Preferably the area is isolated by a physical barrier. The sea bed may be agitated by inducing high velocity fluid movement inside the isolated area.

Preferably the said method is conducted for a first period of time and then repeated for successive periods in respect of successive areas to be excavated.

Most preferably each successive excavation area is adjacent the preceding area such that optimum coverage of the sea bed is performed.

In order to better convey the invention embodiments will now be described by way of example only with reference to the accompanying Figures, in which:

Fig.l shows a schematic elevation of a subsea diamond mining system;

Fig. 2 shows in elevation excavation apparatus in accordance with the invention; Fig. 3 is a plan view of the excavation apparatus illustrated m Fig. 2;

Fig. 4 is a computer generated simulation of the movement of sea water and sea bed material inside the chamber of Figs. 2 and 3;

Fig. 5 is a plan view of an alternative embodiment of the excavation apparatus illustrated in Fig. 2; and

Fig. 6 shows m elevation an alternative embodiment of the excavation apparatus m accordance with the invention;

With reference firstly to Fig. 1, a sea going vessel 1 is provided with hoisting equipment 2 for the controlled lowering and raising of excavation apparatus, generally depicted at 3. The vessel may be provided with means for separating diamonds or material including diamonds from other waste material obtained from the sea bed. It should be appreciated that such means are not critical to the present invention; the invention hereto being concerned with the efficient and systematic collection of excavated material from the sea bed.

The excavation apparatus 3 is more clearly depicted in Figs. 2 and 3. The apparatus 3 includes a chamber 4 formed substantially as a cube, but with an open lower side. The lower side 5 is therefore approximately square, and m the example embodiment is dimensioned with sides of 5 metres. A twenty inch diameter lift line 6 feeds into the chamber and divides via a manifold into four arms 7, each arm 7 terminating with an inlet 8. The line 6 provides a suction medium for sucking and conveying sea bed material up to the vessel 1.

Descending through the centre of the chamber is a relatively powerful water driven propeller 9. The propeller 9 is adapted to thrust sea water downwardly ana forcibly onto the sea bed. The momentum of the propelleα sea water is sufficient to agitate the sea bed to such extent as to cause sediment and other particles to unsettle and travel around the chamber 4. It will be appreciated that the rotary action of the propeller 9 will cause the sea water and agitated sea bed material to move in a swirling direction, which will encourage the sea bed material to migrate to the corners of the chamber 4. Accordingly, the suction inlets 8 are ideally locateα at these corners for the efficient collection of the sea bed material.

This may be more clearly demonstrated by Fig. 4 nic illustrates a simulation of the movement of the seaoeα material in the chamber 4. The velocity of the material is dependent on its location in relation to both the propeller 9 and the suction inlets 8. Accordingly, material located directly under the propeller 9 ana towards the centre of the sides of the chamber 4 is affected least, while the material located towards the edges of the propeller 9 and adjacent to the suction inlets 8 is displaced most.

In an alternative embodiment, the chamber 4 may be provided with means to improve the swirling effect of the propeller 9 to ensure that the seabed within the area of the chamber 4 is efficiently agitated. Figure 5 illustrates such an alternative embodiment where baffles or directing blades 10 positioned below the propeller 9 are used to achieve this effect.

A further embodiment of the chamber 4 may be provided such that a flexible membrane 11 and chain 12 are incorporated on the lower side 5 of the chamber. Such an embodiment is illustrated in Figure 6. The flexible membrane 11 is employed to define a shallow passageway 13 across the sea bed. The propeller 9 is then used to generate a high velocity water flow across the sea bed. The water flow enters the passageway 13 at the inlet 14 before passing into the suction inlet 8 at the other end. As the water flow passes through the passageway 13 it carries with it sediment so permitting excavation of the sea bed. The chain 12 is employed to weigh down the flexible membrane 11 and so stop it billowing upwards when the high velocity water flow passes underneath.

As the sea going vessel 1 passes over the area to be excavated, the apparatus 3 is placed such that the chamber 4 is stationary on the seabed. The sediment is then agitated by the action of the propeller 9. The suction arms 7 draw the sediment through the inlets 8 to the vessel 1. Agitation of a single area is typically done for 30-45 seconds depending on the nature of the sea bed to allow an appropriate depth to be excavated.

Once in the vessel 1 the sediment can be filtered to remove the desired material, such as diamonds, and subsequently returned to the sea bed. The apparatus is then translocated to a neighbouring section of the sea bed and the process is repeated. The design of the apparatus is such that the entire sea bed can be covered improving the overall efficiency of the process.

Although unlikely, it might be possible as a result of the agitation process that the apparatus can sink into the sea bed and can become stuck. Accordingly, the chamber 4 is also provided with a valve (not shown) which is open during normal operation. However, in the event that the apparatus becomes stuck in the sea bed the valve can be closed and the suction process reversed to free the apparatus. Potentially, this may be achieved in two ways, namely: 1) the suction through the lift line 6 may be replaced by downward flow, or 2) the suction through the lift line may be deactivated and the upward reaction force of the propeller 9 be relied upon.

It will be noted from the example embodiments that the agitation of the sea bed is achieved without causing any direct impacting of the agitation device on the sea bed. This mitigates the possibility of damaging any existing structures that might pre-exist on the sea floor.

A further advantage of the present invention is that there is provided apparatus with improved capacity/efficiency for excavating a sea bed or the like as a result of improved coverage, higher concentration of material entering the suction arms and deeper penetration of the bed.

Further modifications and improvements may be added without departing from the scope of the invention herein intended.

Claims

1) Apparatus for use on the sea bed comprising a chamber defining a volume of sea water located above an area to be excavated, wherein the apparatus further comprises agitation means located within the chamber for agitating the sea bed in the said area and one or more suction means for receiving sediment or other sea bed material agitated by said agitation means.

2) Apparatus for use on the sea bed as claimed in Claim 1 wherein the chamber is substantially open at the lower end thereof.

3) Apparatus for use on the sea bed as claimed in Claim 1 wherein the chamber is partially closed at the lower end thereof by a flexible membrane and securing means, wherein the flexible membrane defines an enclosed passageway between the chamber and the sea bed.

4) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the suction means may communicate with one or more rigid or flexible delivery risers for conveying the sea bed material to a vessel or platform located on the sea surface.

5) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the vessel or platform would be provided with pumping equipment for generating suction in the suction means.

6) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the chamber acts as a barrier to contain the agitated material from the sea bed within a volume defined by the chamber so increasing the efficiency of the apparatus.

7) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the agitation means is a device for jetting sea water at the sea bed.

8) Apparatus for use on the sea bed as claimed in Claim 7 wherein the agitation means is a hydraulically driven propeller.

9) Apparatus for use on the sea bed as claimed in Claim 7 and 8 wherein the agitation means employs means for directing the jetting sea water.

10) Apparatus for use on the sea bed as claimed in Claim 9 wherein the means for directing the jetting sea water are one or more vanes.

11) Apparatus for use on the sea bed as claimed in Claim 9 wherein the means for directing the jetting sea water are one or more fins.

12) Apparatus for use on the sea bed as claimed in Claims 1 to 6 wherein the agitation means comprises mechanical blades or other members adapted to agitate the sea bed by consequence of physically impacting the sea bed.

13) Apparatus for use on the sea bed as claimed in Claims 1 to 6 wherein the agitation means is a sonic or ultrasonic device. 14) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the suction means includes one or more inlets for the intake of sea bed material.

15) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the or each inlet is disposed toward the perimeter of the chamber.

16) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the chamber comprises a vent to balance the volume of water in the chamber.

17) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the vent is associated with a valve or plug such that the vent is kept open in normal operation, but in the event that the apparatus becomes trapped in the sea bed can be closed enabling the suction process to be reversed thus freeing the apparatus.

18) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the reversal of the suction process involves the use of the agitation means .

19) Apparatus for use on the sea bed as claimed in any of the preceding claims wherein the chamber has a skirt at the lower edge of its side walls, the skirt being robust and resilient.

20) A method of excavating a sea bed comprising the steps of: 1. Isolating an area of the sea bed;

2. Agitating the sea bed in the said area; 3. Sucking the agitated sea bed material to a surface vessel or the like.

21) A method of excavating a sea bed as claimed in Claim 20 whereby the area is isolated by a physical barrier.

22) A method of excavating a sea bed as claimed in Claim 20 and 21 whereby the sea bed is agitated by inducing high-velocity fluid movement inside the isolated area.

23) A method of excavating a sea bed as claimed in Claims 20 to 22 whereby the said method is conducted for a first period of time and then repeated for successive periods in respect of successive areas to be excavated.

24) A method of excavating a sea bed as claimed in Claim 23 whereby each successive excavation area is adjacent to the preceding area such that optimum coverage of the sea bed is performed.