Scour naturally occurs where sediment is eroded from a seabed or river bed as a result of the force of waves and currents, often in shallow water locations, but also in deep waters.

 

Marine structures are immediately vulnerable to the erosion of sediment at their base. This is ‘scour’ and its effect is due to the action of waves and tidal currents, which as a result of the presence of these structures are redirected and accelerated, increasing the velocity of water flow across the seabed or river bed, and creating eddies and vortices that increase the speed and extent to which sediment is removed.

 

At any structure the result is that seabed or river bed is increasingly, and can be rapidly, lowered from a previously obtained and natural equilibrium level.

 

The scour pits that result and form at these structures can jeopardise the integrity and stability of the structures and must therefore preferably be anticipated and accounted for during the design of the structures and the necessary protection methods planned for.

 

 

In the early 1960s it was recognised that natural seaweed plays an important role in retaining sand along coastlines due to its natural drag reducing the forces exerted by tides, currents and waves.

 

This led to investigations then into the hydrodynamic effectiveness of the use of buoyant fronds (artificial seaweed) to reproduce the same effect, the results of which proved that the buoyant fronds provided the same natural protection as natural seaweed.   

 

In the early 1980’s Seabed Scour Control Systems Ltd (SSCS) carried out extensive research and full scale development trials into the design and deployment of the buoyant fronds. These trials, which involved the deployment of the SSCS Frond Mats to correct pipeline freespans were carried out with Shell and Amoco on their existing pipeline installations in the Southern Sector North Sea.

 

These development trials were quickly followed by full production of the Mats to rectify existing Shell and Amoco pipeline freespans.

 

The success of the SSCS Mats is in their basic design and their ability to immediately reduce current velocity by providing a strong and unbroken viscous barrier.

 

 

 

Each SSCS Frond Mat uses overlapping continuous lines of buoyant fronds to reduce current velocity and turbulence in the immediate vicinity of the location being protected thereby eliminating the conditions which create scour.

In constant flow trials the velocity at a height of 0.1m above the bed was reduced in the trial of 4 (four) rows of fronds to 26% of the constant free flow.

 

In variable flow trials at a height of 0.1m above the bed the flow was reduced to 8% of the free flow.

 

A seabed or river bed is therefore stabilised immediately on installation of an SSCS buoyant frond.

 

The build-up of the material bank within the fronded area is non-linear. 1.25m high fronds create the greatest viscous drag when first deployed and the initial 200mm to 300mm of frond infill occurs quickly in normal tidal conditions given the sediment type likely to be in suspension at scour susceptible sites. As the sediment bank forms, the exposed frond length is reduced, creating less viscous drag and the rate of formation of the sediment bank slows until an equilibrium is reached. The sediment bank created is considerably more stable and dense than the natural surrounding seabed or river bed and will have been consolidated by the vibratory movement of fronds during the sedimentary period and once in place is permanently reinforced by the frond material.

The use of the SSCS buoyant fronds provides a permanent solution to the stabilisation and rebuilding of a natural seabed or river bed while eliminating the problems of edge scour and secondary scour and damage to and fouling of structures and pipelines which can occur with other methods.

 

It has been proven that a sediment bank with a height of 800mm can be achieved within the SSCS frond system in under 14 days, and that a full height of approximately 1070mm was later achieved.

 

 

 

 

 

 

Historic methods of dealing with scour have proven to be a quick fix, short term approach. They have been costly and time consuming and have required, frequent and periodic inspection and maintenance. Typically these have been the dumping of gravel, rock and sandbags and the placement of gabions, and grout bags and piers.

 

None of these methods have proven to provide an equally permanent solution such as that provided with the Frond Mat.

 

Rather they have required frequent, costly and time consuming, replenishment and replacement as a result of settlement, sinkage, dissipation, instability and edge scour.

 

The replenishment and replacement of these other methods is known to be required after only two years following their installation, and is known to have to be repeated as repeatedly and frequently thereafter with significant cost and time impacts. 

 

 

SSCS recognised that this was a major problem with other methods that are used for scour protection and to be a problem that repeatedly contributes to their failure to provide a permanent solution.

 

Each SSCS Frond Mat presents, as part of its design, a ‘soft natural edge’ which eliminates edge scour, and the propagation of secondary scour.

 

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