Design Recommendations for Monopiles Installed in Rock using an Insert Pile Concept

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Monopiles have been the preferred foundation solution for offshore wind turbines for many years. The monopile foundation concept is developed for installation to target penetration depth into soils, such as clay, silt and sand, and sometimes gravel. However, with the recent expansion of the offshore wind market to areas where rock is found within the range of expected pile penetration depths, novel solutions are needed.

This paper presents recommendations for the design of monopiles installed in rock. The presented work takes its basis from an offshore wind project with monopile foundations in Southeast Asia, where the site conditions required that the monopiles were installed into rock. To select a viable foundation solution a decision-based matrix was established, to be able to take an informed decision on most suitable solution. This methodology can be extended and adjusted for future projects.

An insert pile foundation concept was selected, and the concept will be discussed in depth, including discussions of design considerations to be taken. This will cover both the geotechnical in-place verifications, the driveability assessment, and the geotechnical interface to design of the grouted connection between, rock and pile. The insert pile concept requires, beside the verification of the permanent phase, a verification of a temporary phase where the monopile is still not fixed to the rock. Recommendations will also cover items such as: how to interpret the transition from soil to rock, what are the design driver for the design of the insert pile, what are the expected limitations of the insert-pile concept, and which additional design criteria such a design require compared to a traditional monopile design for impact driven piles.

The paper concludes that the insert pile concept is a viable solution when rock is met at depths which require the monopile to go into the rock, but at the same time have sufficient soils above rock to ensure the temporary stability.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

7 - Pile design and installation in challenging soil conditions: glauconite, carbonated soils, cemented soils, gravels and rocks