Field tests to investigate the cyclic response of monopiles in sand

Abstract

Design models for offshore wind turbine monopiles tend to be based on those developed for flexible piles in the offshore oil and gas sectors. However, wind monopiles tend to be shorter and wider, resulting in a stiffer structure that rotates rather than bends when laterally loaded. New methods have been proposed to calculate the load–displacement response of piles to monotonic lateral loading. Those based on correlations with in situ tests such as the cone penetration test seem particularly promising. There is a dearth of experimental test data where monopiles have been subjected to extensive cyclic loading with which to extend such models to consider these effects. To address this, field tests were performed on two 340 mm diameter driven piles with an embedded length of 2·2 m, which were cyclically loaded with up to 5000 loading cycles. The piles had a slenderness ratio (embedded length over diameter) of 6·5, which is a typical value used in the offshore wind sector. The test results show that the accumulated pile head response primarily depends on the soil strength and stiffness, the previous loading history and loading characteristics. A cyclic loading design procedure consisting of cone penetration test based static design and Miner's rule based superposition is presented.