GB1565352A

GB1565352A - Stabilization of erodible marine or fluviatile sediments

Info

Publication number: GB1565352A
Application number: GB2704775A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-09-06
Filing date: 1976-09-06
Publication date: 1980-04-16

Description

(54) IMPROVEMENTS IN OR RELATING TO THE STABILISATION OF ERODIBLE MARINE OR FLUVIATILE SEDIMENTS (71) I, MICHAEL JOHN STEPHENS, a British subject of Royal Building, 11 St.

Andrew's Cross, Plymouth PL1 2DS, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the stabilisation of erodible marine or fluviatile sediments, and particularly to a method of stabilising a marine or fluviatile sediment which can be performed from a vessel on the surface without using divers to supervise the operation from the sea bed, although divers may be used to advantage in shallow water.

Although the present invention will be particularly described herein with reference to its use in stabilising sediments on the sea bed it will be appreciated that the invention is applicable to any circumstances where sedimentary layers are being eroded by water movement, including river currents or waves on the sea shore.

Erosion of sedimentary layers on the sea bed may be particularly dangerous, and is certainly undesirable, in the vicinity of civil engineering works or structures, both during construction and after completion.

this is true both of permanent structures and of temporary structures and is equally applicable to offshore work such as oil drilling rigs or storage platforms as it is to works near the sea shore, such as sea walls, piers, walls, embankments and the like.

Sedimentary erosion takes place when the local ambient fluid energy is sufficient to overcome both the gravitational force and any adhesive resistance of the sediment substrate, thereby acting to raise this from the sea bed and to carry it into contact with any structure in the vicinity. The hydraulic effects of local currents may be increased by the introduction of a man-made structure, and this disturbance may be sufficient to cause sedimentary erosion to take place where it had not previously existed. This can lead to a weakening of the foundation for such structures due to erosion of the surface immediately around the structure; and also to scouring of the structure itself due to the fact that the eroded sediment is carried by the current and impinges on the sides of the structure.With steel or other structures this may encourage sufficient deterioration rapidly to weaken the structure dangerously: the time taken for this situation to be reached may often be far less than the anticipated working life span of the structure.

Particularly for deep water application it is usually not possible to stabilise an erodible sedimentary layer simply by overlaying it with a hardenable material such as concrete. Although this sometimes overcomes the immediate problem of scouring around the foundations, it is not always possible to apply the concrete to a sufficiently large area to prevent waterborne sediment from impinging on a structure and, moreover, it frequently causes merely the geographical relocation of the primary problem, and scouring around the edges of the concrete layer takes place.

One previous proposal for stabilising a substrate involves overlaying it with what is called a "reverse filter": this comprises layers of particulate material graded inversely, that is with the smallest material, preferably of a particle size approximately the same as that of the sediment to be stabilised, lowermost, and with particle sizes increasing to a much coarser grade at the top. The reverse filter is applied by depositing successively coarser layers over the erodible sediment until a particle size is reached which is able to resist the ambient fluid energy and to stabilise the layers below it (this being determined, by calculations on the local submarine energy conditions).

Such reverse filters, however, suffer from the hydraulic effect of their increasing porosity, they are not easy to lay and are nevertheless often eroded.

The present invention seeks to provide a method of stabilising a marine of fluviatile sediment, which overcomes the disadvantages inherent in the use of the known reverse filter, of subsequent erosion due to the hydraulic effects of the current. The present invention also comprehends apparatus for performing the method.

According to the present invention there is provided a method of stabilising an erodible marine or fluviatile sediment by depositing directly over it a slurry of a non-setting paste-like or colloidal matrix material incorporating particles of a solid material. Advantageously the non-setting paste-like or colloidal matrix material is mud.

In the performance of the method the slurry is transported to the required position over the marine sediment to be stabilised by pumping it through an underwater pipeline having a fixed or a movable laying head by means of which the slurry is spread over the sediment. The method is performed in such a manner that the slurry is deposited in the position it will occupy over the sediment and will not normally be moved from this position subsequent to deposition, apart from a minor redistribution of slurry using a special distribution head or seabed plough, which may be required on some occasions to level the slurry surface.

Preferably the matrix material is flocculated prior to admixture of the particles. The maxtrix and the particles may be admixed to form the slurry immediately prior to pumping the slurry over the sediment to be stabilised, or may be mixed previously and subsequently transported to the area to be stabilised.

The particles should be larger than the grain size of the erodible sediment and may be of any relatively dense material, although conveniently stones or gravel are used since these are readily available in a marine environment.

Having been laid the slurry might be eroded slightly as the mud gradually de-waters, but during the de-watering stage the mud acts as a baffle to the pressure differences normally operating across the surface of porous sediments and thus, although the upper surface layers may be stripped, the matrix should not be entirely removed below one or two grain thicknesses of the particles and can therefore act physically to bind the particles together in a framework the interstices of which are filled by the matrix.

The consistency of the matrix must be such that it is compatible with the rapid attainment of high cohesion and for this purpose high specific gravity drilling muds are eminently suitable. The use of chemically inert matrix materials is to be preferred to avoid the desorption of toxic chemicals from the mud in areas of ecological risk, such as fish nursery areas. In certain circumstances it has been found that a suitable density for mud to be used as the matrix material immediately prior to admixture with the particulate material is 1.3 grams/cc.

The present invention also comprehends apparatus for performing the method outlined above, comprising a container for the particulate material, a container for the matrix material, means for mixing the matrix and the particulate materials together, a pump capable of operating to pass the particles and the matrix, and a pipeline for directing the slurry from the pump to the required position over the sediment to be stabilised. The apparatus may also include a laying head designed to carry the pipe outlet and operative to flatten and spread the slurry delivered therefrom over a selected width of the sea bed. The laying head may also be provided with means for flattening, at least partly, the sea bed immediately prior to laying the slurry.

Although the layer of slurry does not immediately attain its greatest resistance to erosion, it is immediately more resistant to erosion than the erodible sediment it covers and, accordingly, it will be found that it is usually only necessary to lay a relatively thin layer of the slurry in order to provide the necessary stabilising effect. In order to cover the area to be stabilised the pipeline may be trailed from a vessel carrying the containers of matrix and particulate material, and the vessel may move in a series of parallel lines, taking account of any surface currents to ensure that the laying head at the end of the pipeline is drawn along successive parallel lines over the area to be stabilised. The laying head serves to spread the slurry to form a layer with a substantially planar surface; it may also be provided with control and/or monitoring systems, such as T.V. cameras.

A suitable construction for the pipeline comprises a flexible outer casing with an internal effectively flexible power-driven screw. Rotation of the screw effects transfer of the slurry longitudinally of the pipeline and assists in maintaining this in a thoroughly intermixed condition prior to deposition onto the sediment to be stabilised.

The screw may be made from a resilient material so that it is truly flexible, or may alternatively be formed as a plurality of sections joined together end to end by flexible couplings or universal joints. This allows the outer casing to flex although the material of the screw is not itself flexible.

It is preferred that there are provided means for monitoring and adjusting the density of the matrix material prior to its admixture with the particulate material.

Adjustments to the density may be made by flocculating the material, particularly if mud is used.

The laying head at the lower end of the pipeline may comprise a pair of substantially parallel skids interconnected by cross members which support the end of the pipeline. The transverse supports may be formed as or may carry inclined deflectors for deflecting the slurry emerging from the deposition head laterally outwards as the distribution head is drawn over the surface to be stabilised.

The present invention also comprehends a sediment stabilising layer when made by the method of this invention and/or by the apparatus of this invention.

Reference will hereinafter be made to the accompanying drawings which schematically illustrate the performance of the invention. In the drawings: Figure 1 is a side view of apparatus for laying a stabilising layer onto a seabed floor; and Figure 2 is a plan view of a laying head which forms part of the apparatus illustrated in Figure 1.

Referring to the drawings there is shown a vessel, generally indicated 11, carrying two containers 12, 13 in the former of which is carried a particulate material such as gravel, and in the latter of which is carried a matrix material such as mud.

Adjacent the latter container 13 is a tank 14 in which the density of the mud can be monitored and adjusted, and this has an outlet to a mixing chamber 15 to which also the particulate material in the gravel container 12 can be fed. From the mixing chamber 15 the slurry formed therein is driven by a pump 16 to a pipeline 17 having an internally located chain of screw links 18 forming a power-driven Archimedean screw and an outer flexible cover. At the lower end of the pipeline 17 is a deposition head 19 carried on a laying head or seabed plough 21 which will be described in greater detail below.

In the illustration it will be seen that the vessel is drawing the laying head 21 by means of a towing cable 22 over a sedimentary layer 20 which is shown as being disturbed and eroded by a current generally indicated by the arrow A. As the slurry of mud and gravel is deposited over the sedimentary layer 20 it acts to stabilise this against further erosion even though a small amount of the mud fraction may be stripped away by the current.

The seabed plough 21 comprises two parallel skids 23, 24 joined by transverse struts 26, which in turn are linked by two longitudinal ribs 27 to form a support structure on which the deposition head 19 is carried. The forward ends of the skids 23, 24 are joined by respective coupling points 25 to the towing cable 22 which also supports an electrical cable for a monitoring and control system indicated schematically as a television camera 30 mounted on a tripod 29 on the support structure of the sealed plough where it can scan the slurry being deposited by the deposition head 19.The longitudinal ribs 27 carry a number of ground plane deflectors 28a and slurry deflectors 28b; the former act to spread and level the sediment to be stabilised and the latter act to spread the slurry laterally as it is deposited, to ensure a uniform distribution of the slurry layer in a wide strip with a substantially planar surface.

WHAT I CLAIM IS: 1. A method of stabilising an erodible marine or fluviatile sediment by depositing directly over it a slurry of a non-setting paste-like or colloidal matrix material incorporating particles of a solid material.

2. A method of stabilising a marine sediment as claimed in Claim 1 in which the slurry is transported to the required position over the sediment to be stabilised, by pumping it through an underwater pipe having a movable laying head by means of which the slurry is spread over the sediment.

3. A method as claimed in Claim 1 or Claim 2 in which the matrix material is flocculated prior to the admixture therewith of the particles.

4. A method as claimed in claim 1 or claims 2 and 3, in which the matrix and the particles are admixed to form the slurry immediately prior to pumping the slurrv over the sediment to be stabilised.

5. A method as claimed in any preceding claim in which the matrix material is a colloid.

6. A method as claimed in any preceding claim in which the matrix material is mud.

7. A method as claimed in any preceding claim in which the particles are stones or gravel.

8. Apparatus for performing the method of Claim 1 comprising a container for the particulate material, a container for the maxtrix material, means for mixing the matrix and the particles, a pump capable of operating to pass the particles and the maxtrix, and a pipeline for directing the slurry from the pump to the required position over the sediment to be stabilised.

9. Apparatus as claimed in Claim 8 in which the pipeline comprises a flexible outer casing and an internal effectively flexible power-driven screw.

10. Apparatus as claimed in Claim 9 in which the screw is formed as a plurality of sections joined together by flexible

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. density of the matrix material prior to its admixture with the particulate material. Adjustments to the density may be made by flocculating the material, particularly if mud is used. The laying head at the lower end of the pipeline may comprise a pair of substantially parallel skids interconnected by cross members which support the end of the pipeline. The transverse supports may be formed as or may carry inclined deflectors for deflecting the slurry emerging from the deposition head laterally outwards as the distribution head is drawn over the surface to be stabilised. The present invention also comprehends a sediment stabilising layer when made by the method of this invention and/or by the apparatus of this invention. Reference will hereinafter be made to the accompanying drawings which schematically illustrate the performance of the invention. In the drawings: Figure 1 is a side view of apparatus for laying a stabilising layer onto a seabed floor; and Figure 2 is a plan view of a laying head which forms part of the apparatus illustrated in Figure 1. Referring to the drawings there is shown a vessel, generally indicated 11, carrying two containers 12, 13 in the former of which is carried a particulate material such as gravel, and in the latter of which is carried a matrix material such as mud. Adjacent the latter container 13 is a tank 14 in which the density of the mud can be monitored and adjusted, and this has an outlet to a mixing chamber 15 to which also the particulate material in the gravel container 12 can be fed. From the mixing chamber 15 the slurry formed therein is driven by a pump 16 to a pipeline 17 having an internally located chain of screw links 18 forming a power-driven Archimedean screw and an outer flexible cover. At the lower end of the pipeline 17 is a deposition head 19 carried on a laying head or seabed plough 21 which will be described in greater detail below. In the illustration it will be seen that the vessel is drawing the laying head 21 by means of a towing cable 22 over a sedimentary layer 20 which is shown as being disturbed and eroded by a current generally indicated by the arrow A. As the slurry of mud and gravel is deposited over the sedimentary layer 20 it acts to stabilise this against further erosion even though a small amount of the mud fraction may be stripped away by the current. The seabed plough 21 comprises two parallel skids 23, 24 joined by transverse struts 26, which in turn are linked by two longitudinal ribs 27 to form a support structure on which the deposition head 19 is carried. The forward ends of the skids 23, 24 are joined by respective coupling points 25 to the towing cable 22 which also supports an electrical cable for a monitoring and control system indicated schematically as a television camera 30 mounted on a tripod 29 on the support structure of the sealed plough where it can scan the slurry being deposited by the deposition head 19.The longitudinal ribs 27 carry a number of ground plane deflectors 28a and slurry deflectors 28b; the former act to spread and level the sediment to be stabilised and the latter act to spread the slurry laterally as it is deposited, to ensure a uniform distribution of the slurry layer in a wide strip with a substantially planar surface. WHAT I CLAIM IS:

1. A method of stabilising an erodible marine or fluviatile sediment by depositing directly over it a slurry of a non-setting paste-like or colloidal matrix material incorporating particles of a solid material.

couplings or universal joints.

11. Apparatus as claimed in any of Claims 8, 9 or 10, in which there are further provided means for monitoring and adjusting the density of the matrix material prior to its admixture with the particulate material.

12. Apparatus as claimed in any of Claims 8 to 11, further comprising a laying head adapted to support the outlet end of the pipeline and having means for the attachment thereto of a towing line or cable by means of which the distributor may be towed over the seabed.

13. Apparatus as claimed in claim 12, in which the said laying head includes guides vanes for deflecting the slurry laterally after it has been delivered from the outlet end of the pipeline.

14. Apparatus as claimed in Claim 12 or Claim 13, in which there is further provided a monitoring device, such as a television camera, mounted on the said laying head.

15. Apparatus as claimed in any of Claims 12 to 14, in which there are further provided control means for controlling the deflection of and/or the thickness of the slurry layer as it is deposited.

16. Apparatus as claimed in any of claims 12 to 15, in which the laying head comprises a pair of substantially parallel skids interconnected by cross members which support the end of the pipeline, the cross members being formed as, or carrying, inclined deflectors for deflecting laterally the slurry emerging from the pipeline.

17. A method of depositing an erosion inhibiting layer over an erodible marine or fluviatile sediment in which the material from which the stabilising layer is to be formed is pumped down a pipeline the outlet end of which is located directly over the sediment to be stabilised.

18. A method of stabilising a marine sediment substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

19. A sediment stabilising layer, comprising a particulate material embedded in a matrix, when formed by a method as claimed in any of Claims 1 to 7 or 18.

20. Apparatus for forming a sediment stabilising layer of particles embedded in a matrix, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.