Pore-Pressure Accumulation and Soil Softening Around Pile Foundations for Offshore Wind Turbines

2012 ◽  
Author(s):  
Pablo Cuéllar ◽  
Matthias Baeßler ◽  
Werner Rücker
Keyword(s):  
Cyclic Loading ◽  
Pore Water ◽  
Wind Turbines ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Large Diameter ◽  
Constitutive Law ◽  
Offshore Wind ◽  
External Loading ◽  
Offshore Wind Turbines

The foundation of offshore wind turbines usually involves the installation of large-diameter steel piles in the seabed, either in monopile or multi-pile configurations (jacket, tripod, etc…), which have to ensure a proper fixity of the turbine during its whole service life-time. However, such foundations raise several challenges and novel questions, partly due to the special characteristics of the offshore environment (for instance, the large numbers of load cycles from wind and waves and the possible influence of transient changes of pore water pressure around the pile) and aggravated by their large diameter, reduced slenderness and elevated ratio of lateral to vertical loads (see Fig. 1). This paper studies the effects of cyclic lateral loading on the offshore piles focusing on the possibility of a progressive accumulation of residual pore water pressure within the saturated embedding soil. As it will be shown, this can lead to significant changes of their behaviour under external loading, which can potentially compromise the foundation’s stability or serviceability. The paper will also analyse some singular effects of an irregular loading (e.g. cyclic loading with variable amplitude), in particular the so-called “order effects” and the phenomena arising during a realistic storm of moderate magnitude, and discuss their potential for transient damages to the foundation’s stiffness. All these phenomena, which can lead to a loss of serviceability of the structure, have been investigated by the authors by means of a coupled bi-phasic analytical model of the offshore foundation featuring a versatile constitutive law suitable for the soil. The constitutive model, in the frame of the theory of Generalized Plasticity, can reproduce some complex features of cyclic soil behaviour such as the tendency for a progressive densification under cyclic loading, which is responsible for the soil liquefaction phenomena in undrained conditions. Finally, some implications of these issues for the practical design of offshore monopiles will be discussed and some specific recommendations for the design procedures will be outlined.

Geotechnical Stability of Gravity Base Foundations for Offshore Wind Turbines on Granular Soils

2012 ◽  
Author(s):  
Germán Sedlacek ◽  
Alina Miehe ◽  
Ana Libreros ◽  
Yousef Heider
Keyword(s):  
Wind Energy ◽  
Wind Turbines ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Good Alternative ◽  
Offshore Wind ◽  
Scale Model ◽  
Offshore Wind Turbines ◽  
Geotechnical Design ◽  
And Performance

Offshore wind energy farms have gained much attention during the last years in Germany and all over the world. In the construction of offshore wind turbines, piled foundations have been mostly used so far. However, gravity base foundations represent a good alternative as they minimize the typical high risks of the offshore works, such as weather-dependent installation, operational safety, construction sequence and performance. The whole wind energy turbine is assembled onshore and promptly transported to the planned location. In the design of the gravity foundation under cyclic loading conditions, it is essential to avoid inadmissibly large reductions of the subsoil bearing capacity due to the excess pore-water pressure (loss of stability) and tilting of the foundation caused by the accumulation of settlements (loss of serviceability). This paper provides a description of the soil-mechanical behaviour of gravity base foundations and gives an account of the current available rules and standards for dimensioning foundations of this type. In this regard, a procedure for the geotechnical design of a gravity base foundation is laid out, where this work points out that the existing standards for designing gravity base foundations need to be further developed. Moreover, a brief summary of the results at a full-scale model test, according to the present state of testing and knowledge, are given.

TIME TO THE END OF PRIMARY CONSOLIDATION (EOP) OF SOFT CLAYEY SOILS: CONCERNING THE EFFECT OF ATTERBERG’S LIMIT AND CYCLIC LOADING HISTORY

2019 ◽  
Vol 128 (2B) ◽  
pp. 29-41
Author(s):  
Trần Thanh Nhàn
Keyword(s):  
Cyclic Loading ◽  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Clayey Soils ◽  
Loading History ◽  
Cyclic Shear ◽  
Wide Range ◽  
Primary Consolidation ◽  
Time Required

In order to observe the end of primary consolidation (EOP) of cohesive soils with and without subjecting to cyclic loading, reconstituted specimens of clayey soils at various Atterberg’s limits were used for oedometer test at different loading increments and undrained cyclic shear test followed by drainage with various cyclic shear directions and a wide range of shear strain amplitudes. The pore water pressure and settlement of the soils were measured with time and the time to EOP was then determined by different methods. It is shown from observed results that the time to EOP determined by 3-t method agrees well with the time required for full dissipation of the pore water pressure and being considerably larger than those determined by Log Time method. These observations were then further evaluated in connection with effects of the Atterberg’s limit and the cyclic loading history.

scholarly journals Mechanical response and pore pressure generation in granular filters subjected to uniaxial cyclic loading

2018 ◽  
Vol 55 (12) ◽  
pp. 1756-1768
Author(s):  
Jahanzaib Israr ◽  
Buddhima Indraratna
Keyword(s):  
Cyclic Loading ◽  
Pore Pressure ◽  
Pore Water ◽  
Mechanical Response ◽  
Pore Water Pressure ◽  
Axial Strain ◽  
Water Pressure ◽  
Internal Erosion ◽  
Excess Pore Water Pressure ◽  
Excess Pore

This paper presents results from a series of piping tests carried out on a selected range of granular filters under static and cyclic loading conditions. The mechanical response of filters subjected to cyclic loading could be characterized in three distinct phases; namely, (I) pre-shakedown, (II) post-shakedown, and (III) post-critical (i.e., the occurrence of internal erosion). All the permanent geomechanical changes such, as erosion, permeability variations, and axial strain developments, took place during phases I and III, while the specimen response remained purely elastic during phase II. The post-critical occurrence of erosion incurred significant settlement that may not be tolerable for high-speed railway substructures. The analysis revealed that a cyclic load would induce excess pore-water pressure, which, in corroboration with steady seepage forces and agitation due to dynamic loading, could then cause internal erosion of fines from the specimens. The resulting excess pore pressure is a direct function of the axial strain due to cyclic densification, as well as the loading frequency and reduction in permeability. A model based on strain energy is proposed to quantify the excess pore-water pressure, and subsequently validated using current and existing test results from published studies.

scholarly journals Dynamic Impedances of Offshore Rock-Socketed Monopiles

2019 ◽  
Vol 7 (5) ◽  
pp. 134 ◽  
Author(s):  
Rui He ◽  
Ji Ji ◽  
Jisheng Zhang ◽  
Wei Peng ◽  
Zufeng Sun ◽  
...  
Keyword(s):  
Numerical Model ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Dynamic Stiffness ◽  
Large Diameter ◽  
Offshore Wind ◽  
Offshore Wind Turbines ◽  
Thin Walled ◽  
Rock Parameters ◽  
Radiative Damping

With the development of offshore wind energy in China, more and more offshore wind turbines are being constructed in rock-based sea areas. However, the large diameter and thin-walled steel rock-socketed monopiles are very scarce at present, and both the construction and design are very difficult. For the design, the dynamic safety during the whole lifetime of the wind turbine is difficult to guarantee. Dynamic safety of a turbine is mostly controlled by the dynamic impedances of the rock-socketed monopile, which are still not well understood. How to choose the appropriate impedances of the socketed monopiles so that the wind turbines will neither resonant nor be too conservative is the main problem. Based on a numerical model in this study, the accurate impedances are obtained for different frequencies of excitation, different soil and rock parameters, and different rock-socketed lengths. The dynamic stiffness of monopile increases, while the radiative damping decreases as rock-socketed depth increases. When the weathering degree of rock increases, the dynamic stiffness of the monopile decreases, while the radiative damping increases.

scholarly journals A method to predict the cyclic loading profiles (one-way or two-way) for monopile supported offshore wind turbines

2019 ◽  
Vol 63 ◽  
pp. 65-83 ◽  
Author(s):  
Saleh Jalbi ◽  
Laszlo Arany ◽  
AbdelRahman Salem ◽  
Liang Cui ◽  
Subhamoy Bhattacharya
Keyword(s):  
Cyclic Loading ◽  
Wind Turbines ◽  
Offshore Wind ◽  
Offshore Wind Turbines

scholarly journals Deformation Analysis of Large Diameter Monopiles of Offshore Wind Turbines under Scour

2020 ◽  
Vol 10 (21) ◽  
pp. 7579
Author(s):  
Zhaoyao Wang ◽  
Ruigeng Hu ◽  
Hao Leng ◽  
Hongjun Liu ◽  
Yifan Bai ◽  
...  
Keyword(s):  
Wind Turbines ◽  
Wind Farm ◽  
Large Diameter ◽  
Horizontal Displacement ◽  
Local Scour ◽  
Offshore Wind ◽  
Deformation Analysis ◽  
Pile Head ◽  
Offshore Wind Turbines ◽  
Test Conditions

The displacement of monopile supporting offshore wind turbines needs to be strictly controlled, and the influence of local scour can not be ignored. Using p–y curves to simulate the pile–soil interaction and the finite difference method to calculate iteratively, a numerical frame for analysis of lateral loaded pile was discussed and then verified. On the basis of the field data from Dafeng Offshore Wind Farm in Jiangsu Province, the local scour characteristics of large diameter monopile were concluded, and a new method of considering scour effect applicable to large diameter monopile was put forward. The results show that, for scour of large diameter monopiles, there was no obvious scour pit, but local erosion and deposition. Under the test conditions, the displacement errors between the proposed and traditional method were 46.4%. By the proposed method, the p–y curves of monopile considering the scour effect were obtained through ABAQUS, and the deformation of large diameter monopile under scour was analyzed by the proposed frame. The results show that, with the increase of scour depth, the horizontal displacement of the pile head increases nonlinearly, the depth of rotation point moves downward, and both of the changes are related to the load level. Under the test conditions, the horizontal displacement of the pile head after scour could reach 1.4~3.6 times of that before scour. Finally, for different pile parameters, the pile head displacement was compared, and further, the susceptibility to scour was quantified by a proposed concept of scour sensitivity. The analysis indicates that increasing pile length is a more reasonable way than pile diameter and wall thickness to limit the scour effect on the displacement of large diameter pile.

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