The expansion of the offshore wind industry in areas with high seismicity has led to engineering challenges related to the design of the offshore wind turbines (OWTs). Monopiles, i.e., tubular steel piles of large outer diameter, low aspect ratio (penetration depth over outer diameter), and relatively thin pile wall, are traditionally the preferred foundation type for OWT due to fabrication, transportation, and installation standardization. For all bottom-founded systems, soil–structure interaction (SSI) plays a crucial role in the system's response. Additional challenges arise in the case of seismic SSI as, not only the system's response, but also the seismic ground motion itself are affected by the soil characteristics. Furthermore, uncertainties related to soil properties, as derived from the soil testing campaign and interpretation, need to be thoroughly considered for OWT load calculations and the design of the support structure. The uncertainty in soil interpretation may have a large impact on the characteristics of the input seismic motion. Subsequently, SSI will affect the seismic loads acting on the support structure and the OWT. This knock-on effect of the interpretation of the soil parameters is unknown, but may be significant to account for. In fact, when a “best estimate” soil parameter set is used, the resulting seismic load may not necessarily correspond to the most probable load for the assumed seismic event. This paper investigates the influence of the uncertainty in soil parameters, as they may result from the soil interpretation, on the seismic loads. It demonstrates the skewed distribution of OWT seismic loads using a realistic design case study on a commercial OWT. Results are presented in the form of transfer functions, response spectra at mudline and normalized bending moments along the support structure. Three distinct structural components of interest are selected to evaluate the results. It is concluded that, for the analysis of OWT under seismic loading conditions in particular, it cannot be decided a priori which soil properties would result in conservative or progressive design. Based on the obtained results, recommendations are given which aim to de-risk and enhance the current design practice.

@en

To accommodate the foreseen expansion of the offshore wind sector, monopile-supported Offshore Wind Turbines (OWTs) are currently being designed for harvesting offshore wind energy in seismically active regions. Three-dimensional (3D) Finite Element (FE) analyses have proven a reliable, though computationally expensive, tool for modelling laterally loaded monopiles. A more efficient modelling approach is the one-dimensional (1D) Beam-on-Winkler-Foundation (BWF) method, where the monopile is modelled via a series of beam elements, laterally supported by uncoupled, lateral soil springs. Under the simplifying assumption of linear elastic soil behaviour, this study explores the suitability of the BWF method for the simulation of the seismic soil-structure interaction by comparing the response obtained through 1D modelling to the outcome of 3D FE calculations. To this end, different monopile geometries are examined, for which the contributions of multiple soil resisting mechanisms (determined by normal and tangential stresses along the pile shaft and base) to the global monopile response are also assessed.

@en

Sound evaluation of soil damping is of great importance for the optimised design of offshore wind turbine support structures. As practice indicates, design optimisation of the support structure often leads to fatigue-driven structural components, especially at the monopile foundation. Saving steel material is important for the economic feasibility of the project, while satisfying fabrication, transportation, and installation requirements. Conventionally, the baseline/background damping consists of steel material, hydrodynamic and soil damping. Design experience indicates that soil damping contribution to the overall baseline damping is significant, especially in case of strong non-linear soil response. This study employs an analytical and a numerical method to evaluate soil damping under realistic project conditions. Results indicate that the analytical method offers a sound basis for fatigue-oriented soil damping assessments, especially at the initial stages of the project.

@en

The PISA (Pile Soil Analysis) research project has resulted in a new methodology for the design of offshore wind turbine monopile foundations. A new software tool called PLAXIS Monopile Designer (MoDeTo) has been developed that automates the PISA design methodology. It facilitates the calibration of the so-called soil reaction curves by automated three-dimensional finite element calculations and it allows for a quick design of monopiles using the calibrated soil reaction curves in a one-dimensional finite element model based on Timoshenko beam theory. The monopile design approach has been validated for sand- and clay-type soils which are common in North Sea soil deposits. The paper presents a validation exercise based on the PISA research project proposal of a rule-based parametric model—General Dunkirk Sand Model (GDSM)—for Dunkirk sand as well as an application of the tool for a project involving an offshore wind turbine on a monopile foundation in sandy layered soil in which the PISA design is compared to the conventional API design. The paper concludes with a discussion of the results and the differences between the various methods.@en

This paper describes an implementation of the methodology, developed in Phase 2 the recent PISA (PIle Soil Analysis) joint industry research project, for the design of laterally-loaded monopiles in layered soils. The software PLAXIS MoDeTo and PLAXIS 3D are employed to obtain the soil reaction curves that are required for the method, following the PISA ‘numerical-based’ design approach. A particular design space is selected to define the variation of the geometrical parameters assigned to the three-dimensional (3D) Finite Element (FE) calibration models. The parameters that span the design space are the embedded length (L), the outer pile diameter (D), the pile wall thickness (t) and the height above the mudline (h), where the design load is applied. The soil reaction curves are determined from the 3D FE calibration models for separate homogeneous soil conditions consisting of stiff normally consolidated clay and very dense sand. The calibration set consists of eight 3D FE models, for each homogeneous soil profile. Subsequently, the soil reaction curves are parameterised and used to calibrate a one-dimensional (1D) FE model, formulated by means of Timoshenko beam theory, which allows for fast and robust design calculations. A final design model (DM) is defined and its response is studied considering the two homogeneous profiles and four additional layered soil profiles. The results of each 1D analysis are compared with equivalent 3D FE models and a 1D FE model developed at the University of Oxford (OxPile) as part of the PISA research. The accuracy metric eta (η) is used to compare quantitatively the response among the employed models, focusing on large displacements at ground level (about D/10). The results indicate a very good match for all considered soil profiles; all computed η values exceed 90%. The research findings support the applicability of the PISA design methodology in both homogeneous and layered soil conditions. @en

Offshore wind turbine generators (WTG) are commonly founded on single large diameter piles, named monopiles. These monopiles are subjected to significant lateral loads and thereby sizeable overturning bending moments mainly due to action of wind and wave forces; thus the critical geotechnical design situation for monopiles supporting WTGs is often related to lateral loading conditions. The Pile Soil Analysis (PISA) joint industry research project [1] has recently proposed a monopile design method which encompasses finite element (FE) calculations under a specific design framework. Soil reaction curves that are crucial for monopile design (i.e. lateral force and moment reactions along the shaft and at the base of the pile) are derived from FE calculations, subsequently calibrated and entered into a 1D model which is then used for design optimisation. This method is implemented within the PLAXIS MoDeTo (Monopile Design Tool) software. This paper presents results of a concept monopile design study under lateral monotonic loading with the use of the PLAXIS MoDeTo method. @en

The recently-completed PISA (PIle Soil Analysis) research project aimed to improve the design of monopile foundations for offshore wind turbines (OWT), focusing on laterally-loaded monopiles with length-to-diameter (L/D) ratios between 2 and 6. The project resulted in a novel one-dimensional (1D) design model which overcomes certain limitations in current practice. The PISA 1D design model facilitates rapid design calculations, based on the use of Timoshenko beam theory to represent the monopile. The soil response is modelled via soil reactions, applied along the shaft and at the base of the monopile. The soil reaction curves are determined using a series of three-dimensional (3D) finite element (FE) calibration calculations, performed prior to the design process, spanning a representative design space. A new software tool called PLAXIS MoDeTo (Monopile Design Tool) has been developed based on this design procedure. This design tool facilitates the automatic generation and calculation of the 3D FE calibration models, the optimisation of the soil reaction curves and the conduct of the 1D design calculations.@en

When instalhng suction caissons in sand, the suction process will induce an upward flow, which reduces the effective stress in the soil inside the caisson. The suction pressure cannot be increased indefinitely. I f a critical suction pressure is exceeded, liquefaction, boiling and/or piping occurs. This will halt the installation process. This paper presents results from a novel laboratory upward flow test (LUFT), to investigate the soil mechanisms affecting the critical suction pressure. The LUFT apparatus is based on a conventional permeameter. As suggested by the results of published full/small scale installation tests, LUFT results confirm that, in dense to very dense sand, most conventional critical suction prediction models underestimate the values observed in reality. It is argued that soil arching contributes to the achievable high values for critical suction pressure. Higher allowable suction pressures may be cost effective.@en