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Offshore wind turbines (OWTs) are situated in intricate marine environments and consistently subjected to cyclic loadings, leading to an accumulation of foundation displacement along with stiffness variation. Employing the multi-spring system, improvements are made to the p-y and t-z model of multi-springs, according to the bounding surface plasticity-based theory, supporting the ongoing loading and unloading behaviors at the caisson-soil interface, which is also verified by the monopod caisson's tests under both uni-directional and multi-directional cyclic lateral loadings. The comparative analysis further indicates that ignoring the combined effect of p-y and t-z springs may result in overstating the deformation of the caisson. Then, incorporating the enhanced multi-spring model into the soil-structure module, allows for the implementation of the dynamic analysis procedure designed for caisson-based OWTs under multi-directional cyclic lateral loading. An increase in the cycle count leads to a larger load deflection angle for the case studied, initiating a continuous accumulation of plastic displacement and the disturbance at caisson-soil interface. This process gives rise to a certain degree of shrinkage within the soil stress boundary, ultimately affecting the overall stability of the structure. • Employing the multi-springs system, enhancements to the p-y and t-z models are presented. • Integrating the multi-springs model into SSI module, the dynamic analysis for caisson-based OWTs is implemented. • The shrinkage of soil stress boundary ultimately affects the overall stability of the structure. [ABSTRACT FROM AUTHOR]

Suction caisson has been widely used as the foundation for the offshore wind turbines. The behaviors of suction caisson under lateral loading caused by the combined effects of wind, wave and current, are quite significant for engineering design. In this paper, a novel multi-springs model is proposed to estimate the foundation displacement behavior and soil resistance distribution under lateral loading. In the proposed model, the p-y and t-z springs are divided into multiple groups and their combined effect is considered to reflect the uneven distribution of interface normal stress outside the caisson. In this way, the enhancement and direction reversal of vertical friction resistance can be reflected, which is also validated by the centrifuge tests of suction caisson. Afterwards, the multi-springs model is adopted to study the static performance of suction caisson under monotonic lateral loading. It is found that the soil displacement field is gradually dominated by the caisson rotation, whose rotation centre moves upwards and stabilizes at about 0.7-0.8 L. Compared to p-y and Q-z springs, t-z spring plays the key role in resisting the foundation overturning at the slightly rotation angle of caisson. [ABSTRACT FROM AUTHOR]

Pile foundations supporting long-span bridges, offshore platforms and offshore wind turbines often suffer from repeated lateral loads due to wind, waves and currents. To predict the degradation of the pile's lateral resistance and cumulative lateral deformation through cyclic loading, a theoretical cyclic p-y model is derived for a laterally loaded pile in undrained clay, formulated within the framework of the bounding-surface elastoplastic theory. Unlike the conventional p-y curves which models the degradation of the pile's lateral resistance based on the cycle number and amplitude, bounding surface contraction is employed in this study to model the softening of the soil resistance during laterally cyclic loading. To ensure practicability, the softening model parameter can be easily calibrated by the in-situ T-bar testing technique. A series of verifications with centrifuge tests having various pile-head conditions, pile-soil relative stiffness and load amplitudes are performed. The good agreement demonstrates its ability to predict the degradation of pile lateral resistance. • A cyclic p-y model based on the bounding-surface elastoplastic theory is formulated for a laterally loaded pile in clay. • The lateral stiffness degradation is modeled by the contraction of bounding surface due to the accumulated plastic strain. • The soil softening model based on in-situ T-bar testing technique is introduced to describe the contraction process. • Verifications with centrifuge tests are performed. [ABSTRACT FROM AUTHOR]

Rigid foundations supporting offshore wind turbines (OWT) like short piles and caissons can sustain multi-directional lateral loading. Moreover, the monotonic and cyclic response of these foundations can be affected by recent loading history aligned along directions different from the present one. To represent such recent loading history effects, a multi-directional p-y model is proposed for rigid foundations in undrained clays. The model is constructed by extending the uni-directional p - y model previously proposed by the authors based on Total-displacement-loading Extended Mobilisable Strength Design (T-EMSD) approach, and thus considering site-specific soil stress-strain relations and foundation geometries. The crux of the extension is to combine bounding surface plasticity and radial mapping with a moving projection center. The former effectively couples p - y response along orthogonal directions, while the latter is crucial for representing the influences of loading direction reversal and recent loading history effects. The proposed p - y model is evaluated against existing centrifuge tests and finite element analysis (FEA) of this work. Lastly, the proposed p - y model and FEA are used in parallel to highlight the effects of loading path rotation on monotonic and cyclic accumulative foundation response. These comparative analyses indicate that the proposed p-y model can reasonably represent recent loading history effects. • Multi-directional p-y model is proposed for recent loading history effect. • Loading path rotation on monotonic and cyclic response of OWT foundation is studied. • p - y model captures recent loading history effect from centrifuge tests and FEA. [ABSTRACT FROM AUTHOR]

The performance of pile foundations subjected to cyclic lateral loading is central to offshore geotechnical engineering. Although non-linearity and path-dependence of soil mechanical behavior at small strains are widely recognized, the influences of these local behavior on global response of pile foundations are less explored. This work presents numerical analyses of cyclically-loaded piles in undrained clays that explicitly account for soil small-strain characteristics. By using intergranular strain (IGS) elasticity method, a soil model proposed by the authors for undrained clays during cyclic loading is extended to include non-linear small-strain behavior. Via comparing against element tests, the constitutive relation is shown capable of representing the observed variation of soil stiffness and damping with strain magnitudes, as well as the dependence of such non-linearity on stress paths. Based on the above soil model, finite element simulations are performed to interpret the response of laterally loaded piles from geotechnical centrifuge tests. The impacts of soil small-strain behavior on pile force–displacement relations, hysteric loops, and the ability of a pile–soil system to dissipate energy are investigated. This work highlights that the sensitivity of these foundation responses to small-strain non-linearity of soils can vary considerably, depending on pile geometry features and head constraint conditions. • Influence of soil small-strain behavior on cyclically-loaded piles is investigated. • Soil model considers path-dependent stiffness variation at small strains. • Small-strain behavior impacts foundation hysteric loop and energy dissipation. • Effects of soil small-strain behavior depend on pile geometry and head condition. [ABSTRACT FROM AUTHOR]

The small-strain performance of offshore wind turbine (OWT) pile foundations under lateral loading is critical for the foundation safety and the fatigue analysis of superstructure but is less explored in recent studies. This work presents p-y curve analyses of monopiles in marine clay that account for complete nonlinear soil stress–strain relationship. Based on a small-strain stress–strain relation, Total-displacement-loading Extended Mobilisable Strength Design (T-EMSD) analyses are performed to interpret the pile-soil interaction. On the basis, an analytical p-y curve model is proposed. An elastoplastic constitutive model with small-strain non-linearity is developed, and then p-y curve validation against finite element simulations is carried out. Furthermore, by comparing to centrifugal test observations and existing p-y models, good agreement and noticeable improvement are shown. This paper provides industry engineers with a timely and effective approach for OWT foundation design, allowing for inputting soil characteristics measured at in-situ site and laboratory. [ABSTRACT FROM AUTHOR]