SCIENTIFIC DEVELOPMENT OF FRONDED SCOUR & EROSION CONTROL SYSTEMS

1. The Causes of Scour / Erosion.
Scour is a natural phenomenon, which occurs whenever a solid object is placed on loose sedimentary material, such as sand, in moving water. The flow of water close to any such obstruction causes local increases in water velocity together with eddies and vortices. These flow characteristics give rise to increased shear stress on the river bed or seabed so that sedimentary material is gradually lifted into suspension and then washed away, downstream, from the area around the object. The scour so created is cumulative but dependent upon the particular circumstances of the area; it may be a slow and gradual process or be very rapid. In shallow waters wave induced vortices can add significantly to the lifting of sediment into suspension.

Extensive research on local scour had identified the mechanisms which produce scour and the factors which determine the equilibrium depth of scour, when an obstacle such as a bridge pier is placed in flowing water, the resulting pressure field around the obstacle can give rise to vortex systems of three different types: Horseshoe-Vortex; Wake-Vortex; and Trailing-Vortex. The effects of these on scour creation then depends upon the composition of the surrounding river or seabed.

2. Artificial Seaweed.
It was recognised in the 1960's that natural seaweed plays an important role in retaining sand along coastlines due to the natural drag reducing shear stresses exerted by tides, currents and waves on the seabed. Thus the hydrodynamic possibilities of using buoyant fronds to arrest and control scour were appreciated some years ago. Trials of various forms of "Artificial Seaweed" were carried out by Dupont (USA), Royal Dutch Shell with Nicolon and Rijkswaterstaat (Netherlands) and later by ICI's subsidiary company, Linear Composites Limited (LCL) in both Norway and the UK, which were followed by full scale installation trials. All of ICI's trials achieved a degree of success and proved the viscous drag creation capability of the fronds but the main engineering problem encountered was the failure to provide proper safe anchorage to the river bed or seabed.

3. Continuous Lines of High Tensile Strength Fronds.
ICI's research showed that overlapping continuous lines of fronds were more effective in drag creation than Dupont's "tufts" of artificial seaweed, this finding was supported by parallel work by Delft Hydraulics in 1973. One example of early successful trials work was in 1965 for Phillips Petroleum's Arpet 'A' offshore platform on the Leman Bank - where a severe scour pit had developed around the base of all the platform's support legs ~ This was the first operational installation for ICI where 1m high buoyant fronds caused the scour pit to fill and then built up a 600mm high sediment bank which was found on annual surveys (recorded up to 1975 - 10 years later) to have remained stable. Successful installations continued but whilst the ICI fronds created the necessary drag ICI encountered problems with the techniques that they were using to anchor their fronds to the river bed or seabed.

4. Seabed Scour Control Systems Limited - Offshore Trials.
At the beginning of the 1980's Seabed Scour Control Systems Limited (SSCS) was established in the U.K. and using information and trials data published by Dupont, and in particular that provided by ICI's LCL and also by Royal Dutch Shell Chemicals began a period of extensive research and full scale development trials in conjunction with, and supported by, Shell (UK) in the North Sea, and later also with trials installations with Amoco, which culminated in multiple field trials successes. In 1984 SSCS commenced full production.
The SSCS scour control mat uses overlapping continuous lines of buoyant fronds as opposed to the deployment of close spaced clumps of fronds that failed so extensively elsewhere. On SSCS's trials it was found critically important that the fronds are in continuous discrete rows and that successive rows provide a substantial and unbroken overlap of fronds; in addition SSCS developed and patented a Safe-Anchor which when driven in the river bed or seabed provided massive hold down (independently witnessed and certificated by both Lloyds Register and by ABS).

The SSCS Frond Systems remain the only field proven scour control product that reduces current velocity by providing a strong and unbroken viscous drag barrier.

5. Conventional Scour / Erosion Control Methods.
Conventional and historic methods of dealing with scour have been a short-term approach. Most are both costly and time consuming, always requiring periodic inspection and maintenance. Typical traditional palliatives are the dumping of gravel, rocks, sandbags, gabions and graded ballast furnace slag. Such materials are dumped in an attempt to fill scour pits or holes. Such operations have only a limited chance of success and frequently require expensive support to assist with the routine positioning of the dumped material; on a scoured site most dumped materials create severe edge scour and then settle (or sink) faster; gabions corrode in a relatively short space of time and release all their content. The international oil and gas company, Amoco, established - on two of their Southern North Sea pipelines - that rock dumped into a scour pit took an average of 3 1/2 years to settle below the base level of the original scour pit into which the rock had been dumped.

6. The SSCS Systems for Seabed and River Bed Scour Control and Correction.
The SSCS Scour Control Mat and the Fronded Concrete Mattress use overlapping continuous lines of high tensile strength buoyant fronds to reduce current velocity and related turbulence in 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; and in a variable flow trial at the same 0.1m above the bed the flow was reduced to 8% of the free flow. A seabed or river bed is stabilised immediately on installation of an SSCS buoyant frond system. The Scour Control Mat is installed by divers and anchored to the sea or river bed, the Fronded Concrete Mattress may be installed by divers or, where visibility underwater is adequate, a work class ROV. Both are quick, easy and safe to install. 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 mat/mattress infill occurs quickly in normal tidal conditions given the sort of sediment sizes likely to be in suspension at scour susceptible sites. As the material bank forms, the exposed frond length is reduced, the mat creates less viscous drag and the rate of formation of the material bank slows until a steady state is reached. The sediment bank created is considerably more stable and more dense than the natural surrounding seabed or river bed: it 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 SSCS buoyant frond systems provides a permanent solution to the stabilisation and rebuilding of a natural seabed or river bed whilst eliminating the problems of secondary scour, damage to and fouling of structure, pipelines or the piles/piers which could occur with other alternatives. The SSCS record for fast creation of a sediment bank over the SSCS frond system after installation was on an Amoco installation to protect a severely scoured gas pipeline where a bank height of over 800mm was achieved in under 14 days, and a full height of approximately 1070mm was later achieved.

Today over 9,000 SSCS Scour Control Mats / Mattresses have been installed throughout the world, not only in UK waters and the North Sea but from the Gulf of Mexico to the Australian Bight, in China and Malaysia and are supported by a proven track record.

7. The SSCS Consultancy Process ~ Site Evaluation. SSCS carry out site assessment and environmental studies for scour or erosion potential on provision of the necessary data and design installations to correct, or to prevent, scour for a specific river or subsea site. SSCS also design and build specific systems to suit a site or a project.

Phased Flume Trials, commencing with ZERO (0) Frond Lines, in stages building up to FOUR (4) Frond Lines with a section of the Flume Tank obstructed by Fronds.