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CABLE PROTECTION/SCOUR MITIGATION


EFFECTIVE PROTECTION FOR FOUNDATIONS AND CABLES


The successful operation of marine energy projects depends on effective cable protection and foundation stability


Integral to the development of any commercial wave or tidal energy project, is the supporting electrical infrastructure of the intra-array and export cables. The array cabling connecting the individual devices is likely to vary depending on the specific device, but may involve an umbilical if the device is a floating design. The environments in which these devices operate may vary significantly depending on the technology deployed, but for some it will mean being located in regions of strong currents and/or waves. Assuring the stability of the device (whether floating or seabed mounted), and protecting the associated power cabling, is of vital importance for their successful implementation, continued operation and ongoing maintenance.


DESIGN PERSPECTIVE


From a design perspective with respect to the cabling, the use of DNV-RP-F109 for calculating on-bottom stability for small diameter cables and umbilicals can result in overly conservative designs on rocky and even normal, seabeds. The primary problem is inaccurate estimation of hydrodynamic loads. On rocky seabeds there is no allowance in present design approaches for the existence of intermittent gaps beneath the pipeline or cable. These gaps effectively relieve suction pressure along the cable and can dramatically reduce lift forces and increase stability. In addition, present design approaches ignore wave boundary layer effects, despite the knowledge that in most design conditions the wave boundary layer can be of a similar size to the cable diameter for cables with diameters of about 0.2m or less. Currently, the best way to overcome these challenges, argues HR Wallingford, a recognised world-leader in the analysis


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and modelling of scour processes, particularly with regard to marine scour and scour protection design, is to evaluate the performance of foundation and cable layout designs against the most realistic possible marine conditions in the laboratory. This allows designs to be checked, refined and further tested to assure their stability before they are deployed for real.


INVESTIGATING TIDAL IMPACT


By the early 2020s, MeyGen intends to deploy up to 398 MW of offshore tidal stream turbines which must remain securely anchored to the seabed. HR Wallingford has undertaken research for MeyGen and the Carbon Trust to investigate the impact of combined strong tidal currents and large waves on foundation and seabed cable stability. This included tests with waves


www.wavetidalenergynetwork.co.uk


and currents flowing together and in opposing directions, to simulate different tidal states. The research was undertaken in HR


Wallingford’s Fast Flow Facility, one of the world’s largest marine test facilities, which delivers a world-leading capability in wave, fast tidal current and sediment modelling.


The way in which waves, currents and sediments interact in the marine environment is extremely complex, with the Fast Flow Facility allowing the company’s scientists and engineers to examine these interactions at a larger scale and in more detail than has previously been possible, helping to more effectively optimise clients’ designs and minimising the water-based risks for projects. In this controlled environment, the performance of novel foundation designs can be evaluated and new and innovative scour protection systems tested.


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