Comparison of calculation approaches for monopiles for offshore wind turbines

Abstract

Briaud et al. (1984) postulate that the soil-pile behaviour is affected by the flexibility of the pile. Criteria for rigid or flexible behaviour have been suggested by various researchers, e.g. Poulus and Hull (1989). According to their recommendations, the piles used for the development of the p-y curves behave as flexible piles. In contrast, the monopile considered in this paper, and generally the monopiles for offshore wind turbines, behave more like rigid piles than flexible ones, i.e. they merely rotate when subjected to large horizontal loads and rocking moments implying a “toe kick”. Hence, the deformation behaviour of the piles and thereby the soil in the case of monopiles for nowadays offshore wind turbines is very different from the conditions from which the p-y curves are derived. These scale effects have not been taken into account in the p-y curve formulations currently recommended by API (1993). Commercial finite-element programs such as PLAXIS (2006) and ABAQUS (Simulia 2009) as well as the finite-difference program FLAC3D (Itasca 2007) do not suffer from these shortcomings. Further, much more complicated models for both soil and pile can be employed leading to a more accurate estimation of the load-deformation behaviour of the pile.