Fugro has been breaking new ground with onshore pile tests for two new offshore windfarm projects.
Eolien Maritime France (EMF), an offshore wind development company, is developing two offshore windfarms founded on Monopiles driven into calcarenite weak rock.
Turbines around 150 metre high with more than 50 metre diameter blades will be placed on seven7 metre diameter piles in the calcarenite weak rock, and in an industry first, Fugro conducted the onshore tests to understand the behaviour of the monopiles in this type of rock.
Pile tests of such significant diameter are very rare and these types of loads applied to the piles as part of the tests have never been done before in this type of rock.
Monopile foundation testing
To learn how monopile foundations will penetrate weak calcarenite rock and their resistance to environmental forces, large diameter lateral pile tests were performed in an old quarry in Gouvieux, in the North of France using technology specially designed and built for the project by Fugro.
Fugro Pile Monitoring Specialist, Michel Maron, explained: “Pile tests of such significant diameter – 0.76 and 1.2 metres – are very rare and these types of loads applied to the piles as part of the tests have never been done before in this type of rock.
“It is not clear how the rock will react to the monopiles. Our onshore pile load tests were completed to address these concerns, as well as optimise the monopile design and installation.”
For the tests, conducted over a three-month period at the end of last year, Fugro developed a purpose-built load frame and monitoring system using the latest sensors and data acquisition technologies.
The loading system, which can pull a pile up to 250 tons horizontally until failure at five meters above ground level is the only one of its kind.it uses a hydraulic power pack and electronic controls which allow maximum lateral load, speed of loading, number of cycles and loading steps to be modified on site as needed. The high flow hydraulic assembly is adapted to mimic the cyclic loading of the environmental conditions, such as the waves and wind, at the offshore windfarms.
The whole testing rig is easy and safe to operate and can be set up rapidly between test piles to perform the test.
Michel Maron said: “The cyclic amplitude of the force of 250 tons and the fact that the force is applied at five metres above ground, is completely new to this type of pile testing. Installing new technology sensors, such as optic fibre strain gauges, that resist the pile installation is also an innovation.”
Preliminary interpretation of the results were supervised by internationally recognised experts in geotechnics and pile driving – Dr Alain Puech and Dr Mark Randolph of the University of Western Australia, both employed by Fugro.
The test results are currently being used as high quality data for benchmarking the adopted design methodology for static and cyclic, laterally loaded testing of monopiles.