Review of design models for lateral cyclic loading of monopiles in sand

To date, monopiles are the most popular foundations used in the offshore wind sector, accounting for over 75% of existing turbine foundations. The cyclic lateral loading response of these foundations is one of the most important issues to be considered during the life-cycle design of monopiles. Current design guidelines rely on methods developed from field tests performed on long slender piles typically used in the oil and gas industry. Monopiles used for wind turbines tend to be relatively short and stiff. In order to investigate the response of a rigid monopile to long-term cyclic lateral loading, a series of field model experiments were conducted on instrumented steel pipe piles. The piles, which had a diameter of 340 mm and slenderness ratio of 6.5, were driven into dense sand at the University College Dublin geotechnical test site. Lateral loading tests were performed on two separate piles. In the cyclic lateral load tests, one pile was loaded with circa 5000 cycles at two load amplitudes (approximately 20% and 50% of the static resistance). In the second cyclic load test, more than 3000 load cycles were applied at three load amplitudes (30%, 40% and 70% of the ultimate resistance). The ability of current design models to predict the monopile response is considered.