Optimizing pile driving fatigue for offshore foundations in very dense sand : a case study

• Published in: Volume 6: Polar and Arctic Sciences and Technology; Offshore Geotechnics; Petroleum Technology Symposium
• Main Authors: Ozsu, Erdem, Ta, An-Ninh, Stuyts, Bruno, Jaeck, Christophe
• Format: Conference Object
• Language: English
• Published: American Society of Mechanical Engineers (ASME) 2013
• Subjects: Technology and Engineering; Arctic
• Online Access: https://biblio.ugent.be/publication/8696185; http://hdl.handle.net/1854/LU-8696185; https://doi.org/10.1115/OMAE2013-10664; https://biblio.ugent.be/publication/8696185/file/8696190

With the rapid development of offshore wind energy in Europe, a large number of piled structures are being installed. Driven pipe piles are adopted as a foundation solution for the majority of offshore wind turbine support structures. In soils consisting of very dense sand, pile driving induces large-amplitude stress cycles in pile material, which have to be accounted for in fatigue calculations. These stress cycles can be calculated using one-dimensional wave equation analysis. Different ways of reducing pile driving damage are presented. Depending on the soil surrounding the pile and the target penetration depth, an optimum driving sequence can be established which minimises pile damage. As damage depends more on induced stresses than on the number of hammer blows, reducing the hammer energy at some point during driving can be beneficial for reducing the accumulated damage. In this paper, an optimum driving sequence is developed for a generic soil profile consisting of very dense sand. The pile driving damage calculated with the optimum sequence is compared to the damage calculated when driving close to maximum hammer efficiency. Additionally, using a larger hammer can also be beneficial for reducing induced stresses when keeping the transmitted energy at a similar level. The paper also highlights the advantages of using pile driving monitoring or pile driving back-analysis for verifying the stress levels in the piles during driving. Offshore design standards allow a reduction of the damage fatigue factor for inspected members. This principle may be extended to monitored piles. The differences between data from pile driving monitoring and data from pile driving back-analysis are discussed and the potential impact on the damage fatigue factor is highlighted Finally, the potential conflict of pile driving fatigue requirements and pile capacity requirements is discussed. Both considerations should eventually lead to an optimized design which satisfies the required design equations.