Chapter 2: Field tests to investigate the cyclic response of monopiles in sand

Monopiles have been used to support the majority of offshore wind turbines installed to date (Doherty and Gavin, 2011; EWEA, 2014). These open-ended steel tubes typically have a large diameter, D in the range 4–10 m, and are driven or drilled into the sea bed. While the embedded length of a monopile will depend on a number of factors, serviceability requirements (limitation of rotation) typically govern the design. For the range of monopiles diameters currently deployed, pile embedment lengths of <50 m are usually sufficient to meet design requirements. The piles used in the offshore wind sector are unique in that they have a low slenderness, with ratios of the embedded length to the diameter L/D in the range 3–8 (Byrne et al., 2015), and are as a result relatively rigid. When lateral loads are applied to stiff piles, rotation occurs. The accumulated pile head response as a result of environmental cyclic loading from wind and waves is critical to the lifetime performance of these dynamically sensitive structures.