www.scourprevention.com

An innovative solution to seabed scour

Wave and tidal fl ows are disrupted as they pass structures placed in the sea, thereby causing seabed erosion which forms scour holes.

The process of scour hole formation

Wake vortices

Surface roller

Downfl ow

Horseshoe vortex

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Scour Prevention Systems Ltd offer a field proven, patented and innovative solution for the prevention of scour around offshore structures and cables.

Scour, the erosion of sediment around objects located in flowing water, can lower the seabed below the specified design level for a structure. This can affect structural stability, vibration frequency and hence accelerate fatigue damage to components.

Scour Prevention Mats consist of a matrix of end-of-life vehicle tyres laid side-by-side on the seabed. The tyres entrap sediment in their centres to form a stable protective layer, preventing lowering of the seabed wherever the mats are placed, protecting against scour. The ideal density of the tyres means that whilst mats are stable, they do not sink into the bed material as sediment shifts around them, as observed with higher density scour prevention techniques.

The mats are endorsed by E.ON Climate and Renewables after a full-scale offshore demonstration at Scroby Sands Offshore Wind Farm, off the coast of Great Yarmouth in the UK.

Advantages

Cost-effective.

Field-Proven: demonstrated at full scale offshore.

Lightweight: low risk to assets during installation.

Modular: flexibility in installation, transport and removal procedures.

Maintenance free: No edge scour or sinking.

Constructed from recycled materials.

Scour Prevention Mats

Scour Prevention Mats part-filled

Scour Prevention Mats empty

Scour Prevention Mats filled

Steve Johnson, Subsea Engineer,E.ON Climate and Renewables

“At the Scroby Sands Wind Farm, the mattress system has proven, beyond doubt, that it stabilises the immediate seabed area on which it has been installed, thereby eliminating the onset of scour.”

Scaled modelling of the scour prevention mats took place in the University of East Anglia’s wave fl ume hydraulics lab to study their performance under wave conditions.

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Before installation

One month after installation

Three-dimensional bathymetric surveys of the seabed around a monopile foundation before and after scour prevention mats were installed.

Scour holes develop around wind turbine foundations in areas with fl owing water and mobile seabeds. Lowering of the bed level infl uences the stability and vibration frequency of the structure, which can accelerate fatigue damage. Cables can also become exposed as they exit foundations, allowing them to hang freely across the scour hole between the J-tube and the seabed.

Scour PreventionScour Prevention Mats are laid onto the seabed around turbine foundations to provide a protective covering to the area. The mats prevent bed lowering, stopping scour pit formation.

The mats are modular and can provide protection for a range of foundation types, including monopile, gravity, suction caissons, tripods and jackets.

Scour RemediationTo repair existing scour, Scour Prevention Mats are placed on top of an infi ll material such as aggregate, geotextile sandbags, gravel fi lter units, or nets of tyres. The mats stabilise the infi ll material and prevent secondary scour, eliminating the need for costly maintenance.

Field trials at E.ON’s Scroby Sands Offshore Wind Farm proved the system’s effectiveness at remediating scour around a monopile. Mats were placed onto a bed of tyre nets substantially increasing the seabed level around the structure.

Protecting turbinesfrom scour

“The mattresses, used in conjunction with a ‘Tyre Filled Net’ system to infi ll existing scour holes around turbine monopiles at Scroby Sands Offshore Wind Farm, have successfully migrated seabourne aggregate back into the scour hole thereby eliminating scour around the monopile.”

Bob Durrant, Inventor of the Scour Prevention Mat and company founder.

Steve Johnson, Subsea Engineer,E.ON Climate and Renewables.

“We’ve been working with Scour Prevention for nearly three years on this project and never cease to be impressed by how well it works.”

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Subsea cables which become exposed on the seabed are susceptible to scouring, which further increases the extent and height of exposure. Cables may subsequently hang freely in the water column where they are subjected to hydrodynamic forces as well as hazards from dragged anchors, dropped objects and fi shing equipment. Scour Prevention Mats protect against such hazards by providing a stable protective layer over the cable in the following applications:

Protection for cable and pipeline crossings and junctions.

Protection for j-tube exits from turbines and platforms.

Maintaining the required cable burial depth in high energy environments with shifting seabeds.

Preventing development of freespanning cable sections - the “washing line effect.”

Freespan correction, using mats in conjunction with infi lling material such as tyre nets, aggregate, sandbags or gravel fi lter units, to repair existing scour holes.

Scour Prevention Mats do not suffer from secondary scour and are lightweight. Therefore there is no risk of mats becoming completely undermined and causing damage to cables by hanging off free-spanning sections, as has been reported for high-density alternatives. Instead, the mats fully stabilise the seabed at the level at which they are installed, regardless of bed level changes in the surrounding area. The application of mats for cable protection was demonstrated at Scroby Sands Offshore Wind Farm in September 2012. Five Scour Prevention Mats were installed onto the seabed, successfully stabilising the bed for over a year, despite continuous migration of sand waves across the site.

Protecting cablesfrom scour

Installation of Scour Prevention Mats at Scroby Sands Offshore Wind Farm using a lifting frame.

Scour Prevention Mats stabilise the seabed above subsea cables, preventing the development of freespanning sections.

Scour Prevention Systems Ltd.OrbisEnergy, Wilde Street, Lowestoft, Suffolk NR32 1XH, UK

T: +44 (0) 1502 509225E: info@scourprevention.com W: www.scourprevention.com

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Cost-effective.

Field-Proven to effectively stabilise the seabed.

Maintenance-free, one-off solution.

No secondary scour at mat edges.

Low density therefore lower risk to assets during installation.

No turbine downtime - possible to install using small vessels that do not obstruct turbine blades.

Modular design, eliminating the need for large installation vessels.

Ease of removal for maintenance or decommissioning.

May be manufactured worldwide without large manufacturing outlay costs.

Constructed from recycled materials.

It works.

Advantages