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CABLE PROTECTION/SCOUR MITIGATION


CABLE PROTECTION/SCOUR MITIGATION – SOME ALTERNATIVE CONSIDERATIONS?


The installation of inter-array and export power cables remains one of the higher risk and therefore cost elements of any marine renewable project


Development of scour at a structure in a current flow field


The reasoning and necessity for cable burial should therefore be well understood. For example, should a risk-based approach be adopted versus current practices? The former balances risk/cost against insurance and serviceability while the latter constitutes cable install-protection and best mitigation via robust survey, route planning, burial and/or protection (armouring, backfill, rock placement, mattressing, fronds and increasingly innovative and proprietary solutions).


COSTS


With funding remaining scarce, it is difficult to see a departure from the more traditional methods of cable installation, protection and where required, scour mitigation. As the unit cost of subsea cable is a notable CAPEX item and its protection a significant OPEX item, there are still technical and cost benefits in conventional cable installation and protection methods. Where burial is deemed prudent and beneficial (as in high energy environments associated with tidal turbines and wave energy converters), industry has developed a panoply of burial tools and proprietary methodologies underpinned by guidance notes and recommended practices to deal with burial in both ‘rock’ and ‘soil’ conditions via Cable Burial Risk Assessment methodologies.


SCOUR


The question of scour remains a variably defined issue; to what level does scour potential need to be understood to trigger relatively simple and technically and cost-effective mitigation measures?


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Seabed scour resulting in cable free-spans


Controversially, understanding the basics of scour may arguably sometimes be sufficient but the question, when are those ‘sometimes’? As suggested above with cable protection, are there benefits and savings to be made from more ‘frugal’ scour analyses?


A competent geoscientist has a fair grasp on the input parameters (i.e. sediment grain size, current velocity and direction) and the ability to determine the likelihood for problematic scour using established nomographs (Hjülstrom/Soulsby curves). Or, should we model scour via complex physical, numerical and computational fluid dynamic models and possibly reach the same conclusions and mitigation methods?


www.wavetidalenergynetwork.co.uk


Hjülstrom nomograph INNOVATIVE METHODS AND TECHNIQUES


Industry has responded recently with innovative methods and techniques to mitigate against scour and this is most certainly not to undermine or diminish this excellent and cutting edge work. However, a debate is required on the level of detail necessary to understand when scour becomes problematic, how scour may affect a development and what level of mitigation is sufficient not only to satisfy scopes of work, codes of practice and specifications, but to ensure fit-for-purpose design criteria and serviceability. As the warning on fireworks used to say: “light blue touch paper and retire to a safe distance!”


Cetus Innovate Limited


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