On behalf of Ailes Marine, a Joint Venture between Iberdrola and RES Offshore, a preliminary geotechnical site investigation project for the round 1 offshore wind farm Saint Brieuc was completed off the coast of Bretagne (France) in water depths of up to 52m HAT. Site conditions were extremely challenging: rough sea conditions, tides of up to 12.5m and tidal currents of up to 2.1 m/sec. The geotechnical conditions were challenging as well. A mixture of sands and hard bed rock including basalts and granites were to be expected. The project incorporated an UXO survey, a geophysical survey and a geotechnical survey.
The client required 100mm diameter high quality cores from the bed rock and CPTU test results from the overlaying sand layers allowing the engineer to complete the preliminary design of the turbine foundations. The works included the delivery of an interpretive geotechnical report.
The geophysical works and UXO survey works were undertaken from the survey vessel IXplorer. The results of the geophysical campaign were used to define the location of 11 boreholes and 11 deep CPTU tests. In order to reduce weather standby GeoSea proposed the use of its steady jack-up platform “Goliath” fitted with 85m long legs. Sampling works and CPTU works were undertaken simultaneously from 2 moonpools provided on the jack-up platform “Goliath”. The in-situ borehole testing consisted of HPD and PS Suspension logging tests. CPTU testing was executed using a hydraulic penetrometer pusher with a maximum nominal thrust capacity of 300kN and a cone tip capacity of 150kN. In stiff cohesive clays and rock GeoborS triple wireline core drilling was used as sampling method, providing 102mm diameter samples. The RKR Nordmeyer percussion sampling method was used as sampling technique in non-cohesive materials (e.g. gravel and loose sand) providing high sample recovery rates. The drill rods and CPT rods were protected from the high tidal currents by a heavy duty seabed casing.