This paper reviews foundations for offshore wind energy convertors considering the significant growth of offshore wind energy since the early 2000s. The characteristics of various foundation types (i.e., gravity, pile, suction caisson, and float type) and the current status of field application are discussed. Moreover, the mechanical characteristics of soil are described in the sense that these characteristics including modulus, strength, damping, and modulus degradation of soil play critical roles for the design of offshore foundations. By using these mechanical properties of soil, theoretical studies to consider structure-soil interaction are classified (into equivalent spring models, distributed spring models, and continuous element models) and explained. Field and laboratory experiments on the response of structure embedded in soil to static and dynamic loads are discussed. Based on the review of previous studies, directions for future research and study on offshore wind turbine are suggested.

A review of foundations of offshore wind energy convertors: Current status and future perspectives

Undrained behavior of natural marine clay under cyclic loading

Experimental study on the stability of plate anchors in clay under cyclic loading

Behaviour of saturated soft clay deposits subjected to cyclic stresses is of considerable importance in the design of railway infrastructure, since these deposits are characterised by very low bearing capacity and excessive settlement. Stone column is a well-established method that used to improve both bearing capacity and reduce settlement of such soil. However, much of the work to date has dealt with the behaviour of stone columns under static loading. Little is known about their response to cyclic loading. A laboratory based study was undertaken to investigate the response of both column and the surrounding soft soil to cyclic loading using a series of static and cyclic loading triaxial tests under undrained condition. Testing was performed on normally consolidated specimens of soft clay (undrained shear strength of ≈ 12 kPa, diameter 100 mm, and 200 mm height) reinforced with 28 mm diameter stone columns. Effects of loading frequencies at range of cyclic stress ratios were examined. It was found that the cumulative strain developed was dependent on the cyclic stress ratio, but independent of frequency from 0.5 to 3 Hz. Overall, stone columns have shown the potential to strength subgrade when exposed to cyclic loading, and hence reduce both capital costs (reduce the ballast needed for subbase), and operational cost through extended maintenance period. 

Behaviour of Model Stone Column Subjected to Cyclic Loading

Incorporation of heat exchangers in conventional geostructures like piles can extract the heat from the soil for heating purposes and inject it to the soil for cooling purposes. In recent years, research has been conducted at full and laboratory scale to investigate the effect of temperature on the geotechnical behavior of these energy geostructures as well as on the surrounding soil. Indeed, these energy geostructures can be subjected to cyclic mechanical loads and thermal variations throughout their lifetime. The aim of this study was to deepen the understanding regarding the behavior of sand/clay-structure contact under complex thermo-mechanical loads. A temperature-controlled direct shear device to perform monotonic and cyclic constant normal load or constant normal stiffness tests was developed. The response of the interface to the thermal effects on the mechanical behaviour of soils and soil-structure interface was investigated. Fontainebleau sand and kaolin clay were used as proxies for sandy and clayey soils. The results showed that the applied thermal variations have a negligible effect on the shear strength of the sand and sand-structure interface. In clay samples the temperature increase, increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The adhesion of the clay-structure interface, was less than the cohesion of the clay samples. To investigate the mechanical cyclic load effects on the clay-structure interface at different temperatures, monotonic and cyclic constant-volume equivalent-undrained direct shear tests were performed on clay-clay and clay-structure interface at different temperatures. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60oC for the different cyclic stress ratios tested. A non-isothermal soil-structure interface model based on critical state theory was then developed. The non-isothermal model takes into account the effect of temperature on the void ratio of interface prior to shearing. The model is capable to capture the effect of temperature on soil-structure interface under constant normal load and constant normal stiffness conditions for both sandy and clayey interfaces. The additional parameters have physical meanings and can be determined from classical laboratory tests. The formulation is in good agreement with the experimental results and the main trends are properly reproduced.

https://hal.univ-lorraine.fr/tel-02939328/document

Thermo-mechanical behavior of soil-structure interface under monotonic and cyclic loads in the context of energy geostructures

The degradation strength of soils under cyclic loading is studied and a method for deter-mining the cyclic degradation strength with cyclic triaxial tests is given in the paperFurthermore,a dum-my static method for estimating the undrained bearing capacity for offshore soft foundation under waveloads is developedIt can consider the effect of the difference of cyclic stress for different parts of the foun-dation on both the degradation strength of the foundation soil and the bearing capacity so that the esti-mated result can better reflect the real condition of foundation under cyclic loadingThe method can be ap-plied to plane and space problem

Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load

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Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load