Earthquake response of monopiles and caissons for Offshore Wind Turbines founded in liquefiable soil

Abstract Monopile has been the most widespread foundation type for Offshore Wind Turbines (OWTs) in shallow waters. Caisson (skirted) foundations have also been evaluated in some projects as an economical alternative. While the main concern in design of offshore foundations has been the environmental loads, the recent growth in construction of OWTs in seismic regions with the possibility of soil liquefaction has necessitated evaluation of the impact of earthquake and liquefaction from strong shakings on these structures. Several studies have reported the consequences of soil liquefaction for buildings and onshore structures; However, the effects of liquefaction on offshore foundations have not been sufficiently studied. This paper investigates the use of advanced liquefaction modeling in assessment of the response of monopiles and caissons for offshore wind turbines. The software FLAC3D and the SANISAND constitutive model are used to conduct the nonlinear dynamic analyses for OWTs. Excess pore water pressure during earthquake shaking and earthquake-induced displacements are computed at various points in the soil medium around the considered monopile and caisson foundations. The analyses reveal that SANISAND model is capable of simulating the pore pressure generation in the free-field as observed in a recent centrifuge test. The numerical results also indicate that both monopile and caissons in liquefiable soil deposits experience considerable rotations under the combined action of wind loads and earthquake shaking when liquefaction occurs

Numerical modeling of liquefaction and its impact on anchor piles for floating offshore structures

Anchor piles and suction anchors have been used for anchoring different types of offshore structure in the past four decades. The recent growing interest and demand for wind energy has motivated the industry to evaluate the use of Offshore Wind Turbines (OWT) in deep waters for which floating wind turbine is a good alternative to bottom-fixed solutions particularly in seismic regions with possibility of soil liquefaction. Extensive research has been carried out to assess the consequences of soil liquefaction for buildings and onshore structures; however, this phenomenon has not been sufficiently studied for offshore foundations. This paper aims at investigating the use of advanced liquefaction modeling in assessment of the performance of anchor piles for offshore facilities and in particular floating offshore wind turbines. The software FLAC3D is used to carry out the nonlinear dynamic analyses using SANISAND constitutive model for saturated sand. The analyses indicate that SANISAND model is capable of correctly simulating the excess pore water pressure in the free-field as observed in centrifuge tests. Pore pressure build-up due to earthquake shaking together with earthquake-induced displacements are computed at various points in the soil medium containing an anchor pile in different scenarios. The numerical results indicate that anchor piles may experience permanent lateral displacements and tilt due to the combined action of static mooring load and earthquake shaking leading to soil liquefaction. (C) 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Seismic response of subsea structures on caissons and mudmats due to liquefaction

Abstract This paper presents a numerical study to investigate the seismic behavior of mudmat and caisson foundations supporting subsea structures, such as manifolds, in liquefiable sand. In seismic areas, substantial earthquake loads can be imparted to subsea structures during ground shaking, threatening their stability. In particular, soil liquefaction in sandy soil arising from strong ground motions could significantly influence the performance of subsea structures founded on liquefiable sand. The results of this study can provide a better understanding of the response of subsea manifolds in liquefiable soil during and after the earthquake. Three-dimensional, non-linear, dynamic analyses are performed using a finite difference scheme, and the ability of the model to reproduce the site response of a saturated sand deposit is assessed using the results of available centrifuge data. This study includes six computational models representing manifolds with different sizes and weights supported by caissons and mudmats in shallow and deep liquefiable sand subjected to moderate and strong earthquake shakings. The response is evaluated in terms of excess pore water pressure generated in the soil medium and displacements of the subsea foundation during and after the shaking. The results show that manifolds may experience considerable movement during liquefaction and post-liquefaction settlements. In addition, depending on the characteristics of the seismic motion and structural system, the manifold could also experience large tilting

A Practical Model for Advanced Nonlinear Analysis of Earthquake Effects in Clay Slopes

Presented in this paper is an effort in providing an advanced yet practical tool with a reasonable level of complexity for modeling of clays in realistic geotechnical engineering problems. SANICLAY model is a Simple ANIsotropic CLAY plasticity model that has been developed by Dafalias et al. (2006). The SANICLAY model provides successful simulation of both undrained and drained rateindependent behavior of normally consolidated clays, and to a satisfactory degree of accuracy of overconsolidated clays. An associated flow rule extension of the SANICLAY model has been employed in the present study, trading simplicity for some accuracy in simulations. The model requires just three constants more than those of the Modified Cam-Clay model, all of which can easily be calibrated from well-established laboratory tests. In order to make the model applicable to practical problems in geotechnical engineering, this simple version of SANICLAY model has been efficiently integrated in FLAC3D program. An illustrative example describing earthquake behavior of saturated clayey slope using the simple form of the SANICLAY model is presented and discussed

Dynamic stiffness and seismic response of pile groups

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1982.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Bibliography: leaves 125-127.by Amir Massoud Kaynia.Ph.D

Seismic response of subsea structures on caissons and mudmats due to liquefaction

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Guidelines for deriving seismic fragility functions of elements at risk: Buildings, lifelines, transportation networks and critical facilities

The objective of SYNER-G in regards to the fragility functions is to propose the most appropriate functions for the construction typologies in Europe. To this end, fragility curves from literature were collected, reviewed and, where possible, validated against observed damage and harmonised. In some cases these functions were modified and adapted, and in other cases new curves were developed. The most appropriate fragility functions are proposed for buildings, lifelines, transportation infrastructures and critical facilities. A software tool was also developed for the storage, harmonisation and estimation of the uncertainty of fragility functions.JRC.G.5-European laboratory for structural assessmen

Numerical model for dynamic installation of large diameter monopiles

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