Natural frequency degradation and permanent accumulated rotation for offshore wind turbine monopiles in clay

Abstract

9Offshore wind turbine (OWTs) monopile foundations are subjected to cyclic loading from wind, waves, 10 and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by 11 cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic 12 loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. 13 extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil 14 modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts 15 affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency 16 degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced 17 novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic 18 loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term 19 cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and 20 permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may 21 experience significant degradation. Further consideration is required for medium clays, as load magnitude 22 played a strong role in both natural frequency and permanent rotation estimation. 23 Keywords 24 offshore wind turbines; monopiles; p-y curves; cyclic loading 25 Nomenclature 26 DE Delaware 27

Post‐installation adaptation of offshore wind turbine controls

“…Furthermore, while the p‐y curves include corrections for cyclic loading, these are calibrated based on field data collected from piles subjected to a low number of loading cycles compared with the cycle count over the service life of an OWT . The implications of these and other shortcomings of the p‐y method are widely discussed in the literature on geotechnical modeling for OWTs . Although soil‐structure modeling errors are a significant source of uncertainty, this is not taken into account here.…”

“…The serviceability limit state ensures that the deflections and deformations of the support structure during normal operation do not exceed the tolerances for which the turbine can operate safely. For monopile OWTs, the serviceability criteria are often defined in terms of deflections and rotations (tilt) at the pile head and nacelle level, including both the initial tilt after installation and permanent rotation accumulated over the turbine's service life . The rotational requirements are usually the main concern as overturning moments are of greater importance than the horizontal forces .…”

“…11,13 The implications of these and other shortcomings of the p-y method are widely discussed in the literature on geotechnical modeling for OWTs. 8,9,11,13,14 Although soil-structure modeling errors are a significant source of uncertainty, this is not taken into account here. First of all, there are currently no statistical models available capable of reasonably capturing this uncertainty.…”

“…The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures . In particular, uncertainty in the support structures' fundamental natural frequency presents a challenge in estimating their fatigue life.…”

“…The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures. [8][9][10][11][12][13][14] In particular, uncertainty in the support structures' fundamental natural frequency presents a challenge in estimating their fatigue life. The fundamental natural frequency is usually placed in the relatively narrow ''soft-stiff'' frequency range in between the rotor frequency (1P) and blade passing frequency (3P).…”