How The Frond System Works
The Frond System
What is Scour
The Cause of Scour
Seabed soil types and a combination of current, tidal and wave forces combine to create an environment which leads to seabed sediment being lifted and transported.
The result is the natural depletion of soil in one area and its aggradation in another.
This natural occurrence known as scour, or erosion, is exacerbated when objects are placed on the seabed, such as pipelines, platforms, protection structures, umbilicals and cables.
Local Scour
Any fixed subsea installation will disrupt tidal flow, causing the current to first decelerate in advance of it then accelerate beyond it, to form vortices and eddies which cause sediment to be scoured, i.e. lifted from the seabed and transported.
Reversed tidal flows will cause scour to develop significantly on all sides of the installation to form steep sided scour pits, and in the case of pipelines, cables and umbilicals, beneath them, to cause free spanning.
Global Scour
Shallower ripples and wider depressions can also form beneath and extend to some distance from the installation. This is referred to as global scour and can impact on an individual installation or form through interaction between adjacent installations.
How the SSCS Frond System Works
The SSCS Frond System comprises continuous lines of overlapping buoyant polypropylene fronds that when activated create a viscous drag barrier that significantly reduces current velocity. The frond lines are secured to a polyester webbing mesh base that is itself secured to the seabed by anchors pre-attached to the mesh base by polyester webbing lines.
The action of reducing current velocity immediately prevents seabed sediment in the immediate area of the fronds from being transported i.e. ‘scoured out’, and causes sediment transported across the fronded area to fall into, and collect, within the fronds.
By reducing current flow the SSCS Frond Systems prevent scour from occurring and rectify scour by reversing the effects that cause it.
A planned and selective deployment of the Frond Systems will immediately prevent and remedy scour, and provide permanent scour protection.
The following photographs, courtesy of Shell UK Exploration & Production Ltd prove and demonstrate the effectiveness of the SSCS Frond Systems. The originally installed Frond Mats have now been in place at this Shell site for some 23 years and continue to provide protection.
INITIAL SEDIMENT BUILD-UP COVERING MAT BASE AND THE FOOT OF THE FRONDS
CONTINUED SEDIMENT BUILD-UP IN CENTRE OF THE MAT AND SLOPING TO AND BEYOND THE MAT EDGES.
RE-INFORCED SEDIMENT BANK NEAR FULL DEVELOPMENT WITH A FEW SHORT LENGTHS OF FRONDS EXPOSED
DIVER INSPECTING FULLY DEVELOPED SEDIMENT BANK WITH SHORT RANDOM FRONDS SHOWING AND A TYPICAL "RIPPLE" PATTERN APPEARING ON SEDIMENT SURFACE
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ONE YEAR AFTER INSTALLATION AT THE PREVIOUSLY SCOURED PIPLINE WITH A FULLY DEVELOPED SEDIMENT BANK FORMED OVER THE FRONDS WITH MARINE LIFE COLONIES ON THE FINAL BANK
The following underwater video excerpts show the SSCS Frond Mats being installed and the subsequent post installation surveys, immediately following installation and one year after installation.