Seismic responses of monopile-supported offshore wind turbines in soft clays under scoured conditions
Introduction
The number of offshore wind turbine (OWT) farms has been increasing globally, due to the great potential to produce sustainable energy. In particular, the popularity of monopile-supported OWT has increased significantly over the years due to the ease of fabrication, installation, and operation [[1], [2], [3]]. Typically, a monopile is a tubular steel pile with an outside diameter of 4 to 6 m for supporting tower and rotor-nacelle assembly (RNA) in water depths up to 35 m [[4], [5], [6]]. The OWT system is often designed against the cyclic lateral loads due to waves, winds, and rotor vibration. Affected by currents and waves, the OWT is also vulnerable to scour. In the active seismic zones, damage to the OWT can be magnified by the combined effects of scour and earthquake [7]. In extreme conditions, the occurrence of both scour and earthquake can exist when: (1) the newly deposited sediments in the scour hole are liquefied or softened during the earthquake; and (2) a large earthquake produces a tsunami-induced scour followed by aftershocks, such as the 2011 Tohoku earthquake [8]. Scour not only reduces the monopile capacity but also changes the dynamic characteristics of the OWT system. It is, therefore, worthwhile to investigate the scour effects on the seismic responses of monopile-supported OWT that often has no redundancy in the structural system [9].
The design of monopile-supported OWTs is mainly dictated by their dynamic characteristics, among which the fundamental frequency is the key [10,11]. Generally, the design follows a soft-stiff design rule [12,13] such that the fundamental frequency, , of the OWT system falls between the zone of rotor frequency, , and the zone of blade-passing frequency to avoid the resonance [14]. This narrow frequency band requires a careful assessment of foundation stiffness over a typical design life of 20–25 years [15]. Hence, considering scour is essential as it may shift the fundamental frequency into the resonance range and amplify the structural dynamic responses. Scour around monopile includes general scour (erosion across the seabed) and local scour (development of a scour hole around the pile), which can occur not only in cohesionless soils but also in cohesive soils [[16], [17], [18], [19], [20], [21]]. The consequence of scour includes removal of soil supports to pile and an increase in over-consolidation ratio (OCR) of the remaining soils that are unloaded due to the removal of overburden [22]. That is, scour effects include changes to both seabed geometry around the pile and stress history of the remaining soils, which are anticipated to affect the natural frequency and seismic responses of the OWTs.
At present, only limited research is focused on the responses of OWTs or piles under combined effects of earthquakes and scour, which mostly evaluates the scour process as a general scour. Guan et al. [23] examined the interaction between the scouring process and monopile vibration in a flume test, where the monopile top was excited by simple harmonic motions. Prendergast and Doherty [24] measured variations of the fundamental frequency with general scour depths and evaluated the scour effect on the shift of the fundamental frequency. Jia et al. [25] analyzed dynamic responses of bucket-supported OWTs using continuum finite elements (FEs) under combined effects of general scour and one earthquake record. In addition to the full continuum FE analyses, the dynamic-beam-on-nonlinear-Winkler-foundation (DBNWF) method [26] is often adopted to analyze the combined effects of earthquakes and scour, in which a general scour is simply simulated by removing the p-y springs within an estimated maximum scour depth [[27], [28], [29], [30]].
Previous research on the effects of scour-hole dimensions and soil stress history has mainly focused on the static responses of OWTs or piles [[31], [32], [33], [34], [35]]; however, there is almost no study on their effects on the dynamic responses of OWTs or piles. The evaluation of scour-hole dimensions was highlighted by a recent centrifuge test [31] to investigate the effects of both local and general scour on the monotonic lateral monopile response. To investigate the lateral response under static wind-wave loads, several studies [32,33] explicitly modeled three-dimensional local scour using continuum FE method but neglected soil stress-history changes. Contrarily, He et al. [34] considered and highlighted the effects of soil stress-history changes on the structural responses. Recent studies [35,36] adopted the post-scour p-y springs [37] to account for the scour-hole dimensions, but no soil stress history was included. Overall, the above literature review indicates two research gaps: (1) lack of understanding of the combined effects of earthquakes and scour where multiple recorded earthquakes are considered, and (2) lack of research on the effects of scour-hole dimensions and soil stress history on seismic responses of OWTs.
This study aimed to propose a simple method for investigating the combined effects of earthquakes and scour on OWTs in soft clays, in which the effects of both scour-hole dimensions and soil stress history were incorporated into the framework of dynamic soil-structure interaction (i.e., DBNWF [26]). The proposed processes were encoded into a set of open-source scripts via MATLAB [38] and OpenSees [39] and validated against the existing results. Using this method, 198 nonlinear time-domain parametric analyses were conducted for six crustal earthquake motions. Through the parametric analyses, scour effects including the scour-hole dimensions and soil stress-history changes on the dynamic responses of the monopile-supported OWT in soft clays were illuminated, and some design-related issues were also discussed.
Section snippets
Overview of dynamic soil-structure interaction
Dynamic soil-structure interactions essentially include kinematic interaction and inertial interaction. Kinematic interaction is mainly affected by soil movements and soil-pile interface behaviors, while inertial interaction is mainly related to the mass of the superstructure. In general, nonlinear soil behaviors under strong ground motions and soil-pile interface behaviors (e.g., slip, gap, and closure, etc.) represent the essential responses of the entire OWT system. The method of DBNWF is
Proposed method
The proposed method includes: (1) improvement of DBNWF to consider effects of both scour-hole dimensions and soil stress history, dubbed modified DBNWF method, and (2) a practical procedure that integrates scour effects into 1D site response analysis to compute post-scour free-field motions. A flowchart of the implementation of the proposed method is shown in Fig. 2.
Validation
To the best of the authors’ knowledge, no experimental data is available for verifying seismic responses of the monopile-supported OWT considering both scour-hole dimensions and soil stress history. Nevertheless, the DBNWF method was extensively validated by comparing the results with the centrifuge test data [26,58] and with the results obtained with continuum elements and sophisticated constitutive models [59] under the pre-scour conditions. Besides, the pre-scour DBNWF method has been widely
Parametric analyses
To investigate the effects of scour-hole dimensions and soil stress history on the seismic responses of a monopile-supported OWT in soft clays, a 3-MW wind turbine that is often adopted for soft soil conditions [60,[64], [65], [66]] was selected as the case study, as illustrated by Fig. 4. The geometric and material properties of the structural components [60,65] and the pre-scour soil parameters [67] used in this study are listed in Table 1, Table 2, respectively. The monopile was an
Results and discussion
Prior to presenting the results of scour effects on seismic responses of the OWT, the scour-induced changes in the free-field motions shown in Figs. S1–S8 (supplementary figures) are first discussed here. In Figs. S1–S8, the normalized free-field acceleration is a ratio of the maximum free-field acceleration to the peak bedrock acceleration of . From these figures, the maximum free-field acceleration decreased as the soil depth decreased. The attenuation of acceleration at the shallower
Conclusions
This study proposed a new method to incorporate the scour-hole dimensions and scour-induced changes in soil stress history into the framework of dynamic soil-structure interactions. The proposed processes were encoded into a set of open-source scripts to analyze the post-scour seismic responses of the monopile-supported OWTs in soft clays. A total of 198 parametric analyses for an OWT system were performed by varying scour-hole dimensions and soil stress-history effects under six crustal
CRediT authorship contribution statement
Wenyu Jiang: Conceptualization, Methodology, Software, Validation, Formal analysis, Writing - original draft. Cheng Lin: Methodology, Resources, Writing - review & editing, Supervision. Min Sun: Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for supporting this study through the NSERC Discovery Grant (RGPIN/4589-2016).
Notation
- damping coefficient of the pile at node
- damping coefficient of soil at node
- swell index of soil
- compression index of soil
- undrained shear strength of soil
- post-scour undrained shear strength of soil
- post-scour horizontal radiation damping coefficient
- post-scour vertical radiation damping coefficient
- outside diameter of monopile
- void ratio of soil
- distance between the centroids of rotor and nacelle, taken as 3 m in this study
- distance between rotor centroid and tower
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