Previous guidelines based on oil and gas platform methods underestimated the support the seabed provides turbine foundations, resulting in over-conservative design calculations for monopiles.
Following field tests in clay and sand conditions, researchers believe more accurate calculation methods based on offshore wind’s loading conditions and larger diameter monopile dimensions could help manufacturers better optimise turbine foundations and structures.
The research, spread across eight papers recently made available on an open-access basis by the geotechnical engineering journal Géotechnique, was carried out as part of the Ørsted-led Pile Soil Analysis (PISA) joint-industry research project.
The new guidance means in some conditions, monopile designers could use 30% less steel than they would have used under calculations based on oil and gas platforms, enabling cost savings for foundations, according to the Carbon Trust, which ran the PISA project.
This cost saving could be achieved because designers can more realistically assess how the soil will react as load is applied to the monopile, the Carbon Trust's director for offshore wind Jan Matthiesen explained. The new design methods more accurately capture the physics of the system relating to the the different geometry of monopiles over oil and gas jacket piles, and can be customised for different sites, he claimed.
Meanwhile, developers could deploy smaller transportation vessels to deliver the smaller monopiles, spend less time installing foundations due to their shorter length, and better understand the risk and reliability of monopiles over their service life because their design is better quantified, Matthiesen suggested.
"Not only has (this research) led to significant steel savings for monopile foundations, but the use of PISA allows greater confidence in our design calculations," said geotechnical design and installation manager at Ørsted, Miguel Pacheco Andrade.
A turbine foundation typically costs about 8.2% of the levelised cost of energy (LCOE) of a 1GW UK offshore wind farm starting operations in 2022, according to research body ORE Catapult, which was not involved in the study.
The field testing for the eight papers was carried out in the North Sea: in clay conditions in Cowden, UK, and in sand conditions in Dunkirk, France.