CABLE INSTALLATION VESSELS FEATURE
WINDFARM DEVELOPMENTS OFFSHORE EAST ANGLIA
Since the second quarter 2011 and continuing to date, Modus Seabed Intervention Ltd have completed more than 700 days of subsea construction support activities off the East Anglian coast. Operations have included inter-array and export cable trenching, survey and construction support services utilising Modus’ fleet of tracked and free flying trenchers and high powered work class ROVs.
For most of this period, Modus were operating from three separate construction support vessels; trenching, cable lay, and ROV and diving. The extensive experience gained through these operations is providing invaluable feedback into technology and procedural development and optimisation; specified for the environmental extremes experienced in offshore windfarm construction.
TRENCHING CHALLENGES One of the field developments Modus has been involved with off the East Anglian coast required post-lay cable trenching services on infield and export power cables.
Highlighted challenges included; high currents, zero visibility, shallow water, and restrictions on handling cables and live (export) cables. These constraints meant that cable lowering by jetting was the only technically viable solution.
However, a significant presence of stiff and very stiff clays (in excess of 200kPa), on both infield and export routes was identified. Jetting technology is generally restricted to sand and clays less than 80kPa, therefore, Modus commenced the development of a bespoke jetting solution that would meet the burial
demands in these soils whilst not handling the cable.
The Modus CT1 trenching spread was mobilised in tracked
mode, with an upgraded and optimised high pressure jetting system.
TECHNOLOGY DEVELOPMENT Whilst stiff material was a considerable feature, it became clear following initiation, that the soil characteristics were extremely varied across short distances, and were often inconsistent with pre- identified conditions. In reaction to this, Modus developed site and cable specific trenching procedures and jetting swords to optimise trenching performance against issued data but also to handle unidentified variances.
As part of a continuous analysis programme, trenching performance and survey data from the first pass was further assessed against geotechnical information from pre construction site surveys and with additional data being gathered from the trenching support vessel using an ROV deployed CPT system.
Profiling and comparison of these results allowed further system optimisation to be undertaken where additional trenching passes were required, which included the further development of jetting sword design, jetting nozzle configurations (size, angle, position) and water pressure and flow settings.
The CT1 trenching spread was able to provide a multi-pass capability to lower the cables in seabed types ranging from sands and silts through to clays in excess of 200kPa. With the tracked configuration the system was also able to operate in close proximity to the cable pull in locations and to perform pre-trench GVI survey in strong currents when required.
ADDING VALUE
The reaction to the challenges on this project reflects the Modus goal to consistently offer differentiated and focussed engineered solutions to specific requirements; and to take a considered approach to the significant demands of offshore windfarm construction.
Having the ability to react quickly to the conditional extremes drives overall performance and ultimately reduces cost, which is particularly important given the vastness of future windfarm developments. Establishing a robust baseline process and approach is also essential due to the repeatability of operation in the inter array cable works.
SEABED INTERVENTION Seabed intervention is a universal and focused approach to the application of geological sciences, engineering and advanced remotely operated technology for subsea trenching and survey.
Modus Seabed Intervention is an international company that provides comprehensive support to the offshore construction industry for seabed intervention services.
Modus
www.modus-ltd.com
www.windenergynetwork.co.uk
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