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Centrifuge modeling and numerical analysis on lateral performance of mono-bucket foundation for offshore wind turbines

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Abstract

The lateral loading performance of the mono-bucket foundation for offshore wind turbines is investigated in this study through a series of centrifuge tests and numerical analysis. Six mono-bucket models are considered to assess the influences of aspect ratio under the ultimate condition. The FE model is validated against the centrifuge test results, and the failure mechanism of the foundation is revealed. The parametric study and sensitivity analysis of the soil parameters are performed in the numerical simulations. A modified theoretical calculation method is proposed to estimate the ultimate lateral capacity of the suction bucket foundation by solving the limit equilibrium equations. A correction term is added in the calculation using the “m” method. The position of the rotation center is calculated, demonstrating a better accuracy compared to the previously reported geometric method. The modified calculation method is verified against the field test, the centrifuge test, and the laboratory test. The method is proved to be applicable in estimating the lateral performance of the mono-bucket foundation in a general form. This study aims to provide well-documented data and design references for practical engineers.

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The failure mode of the wide-shallow bucket foundation is different from that of the traditional suction caisson and the narrow-deep bucket foundation. Results from elasto-plastic analyses of 3D finite element models are presented, aimed at defining the shape of the yield envelope in the V–H, V–M, H–M and V–H–M spaces and the failure mode of the bucket foundation. The compressive bearing capacity of the wide-shallow bucket foundation was determined by the displacement. The corresponding load under a vertical displacement of 0.06D was the vertical ultimate bearing capacity. The vertical loading had an amplification effect on the horizontal load-bearing and moment capacity, and the horizontal loading which was in the opposite direction to the moment increased the moment capacity by 20–40%. New simplified calculation methods were proposed for the vertical load-bearing capacity and the overturning stability. In the proposed method, the vertical capacity consisted of a top plate resistance of the bucket and a side frictional resistance. The overturning stability was determined by the safety factor method, and depended on the location of the rotation point. A comparison between results from the finite element analysis and the simplified calculation methods showed that the proposed equations properly predicted the capacities of the wide-shallow bucket foundations.
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In 2010, the first offshore wind turbine with integrated installation was established in Qidong sea area of Jiangsu Province, China, which led to the implementation phase of one-step-installation technique based on the design and construction of large-scale bucket-top-bearing (LSBTB) bucket foundation. The critical technique of LSBTB bucket foundation included self-floating towing, penetration with adjustment of horizontal levelness, removability and one-step-installation. The process of one-step-installation included the prefabrication of LSBTB bucket foundation in onshore construction base, installation and debugging of wind power, overall water transportation of foundation and wind power system, and installation of foundation and offshore wind turbine on the appointed sea area. The cost of one-step-installation technique was about 5 000 Yuan/kW, which was 30%-50% lower than that of the existing technique. The prefabrication of LSBTB bucket foundation took about two months. During the one-step-installation process, the installation and debugging of wind power and overall water transportation need about one to two days in sea area within 35 m depth. After the proposed technique is industrialized, the cost will be further reduced, and the installation capacity is expected to be up to 500 wind turbines per year.
Article
A wide range of new offshore applications are emerging in the energy sector. The oil and gas industry is targeting minimum facility applications, whilst the renewable energy sector is developing offshore wind turbines, as well as a number of wave and tidal energy devices. The design and installation of the foundations are key considerations in the financial viability of such offshore engineering projects. Suction caisson foundations are a potential solution for these new developments, but design guidance is relatively sparse. This paper considers the vertical loading response of a caisson foundation in clay, during installation and under both monotonic and cyclic vertical loading. The main contribution is the presentation and interpretation of high quality experimental data. Vertical loading is critical for the design of a multi-footing structure of the type that might be used for large offshore wind turbines. We first consider the installation behaviour and compare data from pushed installations and a suction installation with results from a theoretical calculation. We then consider cyclic vertical loading tests, focussing on cyclic amplitudes that take the foundation into tension. Detailed displacement data and pore pressure data are presented.
Article
A series of centrifuge model tests have been conducted on a model suction pile embedded in sand to evaluate its inclined pull-out loading capacity. This paper describes the centrifuge model tests, the analytical solution, and comparisons between the centrifuge model test results and the analytical predictions of the pull-out capacities of the suction pile under inclined loads. The main variables of the study are the load inclination angle and the point of mooring line attachment which varies from the top to the bottom of the suction pile’s side surface. Effects of these two parameters on the suction pile inclined pull-out loading capacity are described.
Article
The experimental investigation of the response of suction bucket foundation in fine sand layer under horizontal dynamic loading has been carried out. The developments of settlement and excess pore pressure of sand foundation have been mainly studied. It is shown that the sand surrounding the bucket softens or even liquefies at the first stage if the loading amplitude is over a critical value, at later stage, the bucket settles and the sand layer consolidates gradually. With the solidification of the liquefied sand layer and the settlement of the bucket, the movement of the sand layer and the bucket reach a stable state.
Article
Simulation of earthquake geotechnical problems in centrifuge has grown significantly in the past decade and a variety of challenging problems are now being tackled in various centrifuge establishments all over the world. Considerable experience has been gained in simulating successfully earthquake effects in the centrifuge. Simulation earthquake conditions in the geotechnical centrifuge requires careful consideration of a number of factors. These include modelling of base motion, selection of model container with non-reflecting boundaries and use appropriate fluid in the soil. These aspects are discussed in this paper. A brief summary of the current activities in this field are also presented. Finally, details of the geotechnical centrifuge facility established at IIT Bombay and the preliminary details of the proposed earthquake simulator are given.
Article
This paper presents the results of a series of centrifuge model tests performed to study the behavior of suction bucket foundations for a tension leg platform in the Bohai Bay, China. The target lateral loadings were from ice-sheet-induced structural vibrations at a frequency of 0.8–1.0 Hz. The results indicate that excess pore water pressures reach the highest values within a depth of 1.0–1.5 m below the mud line. The pore pressures and the induced settlement and lateral displacement increase with the amplitude of the cyclic loading. Two failure modes were observed: liquefaction in early excitations and settlement-induced problems after long-term excitations.
Article
Contenido: Suelos y rocas; Composición del suelo; Clasificación de los suelos; Compactación de suelos; Movimiento del agua a través de suelos; Permeabilidad e infiltración; Conceptos de esfuerzo efectivo; Esfuerzos en una masa de suelo; Consolidación; Resistencia cortante del suelo; Presión lateral de tierra; Cimentaciones superficiales; Capacidad de carga y asentamiento; Estabilidad de taludes; Geotecnología ambiental; Exploración del subsuelo.
REFINED MODEL for INCLINED LOAD CAPACITY of SUCTION CAISSONS
  • C P Aubeny
  • S Han
  • J D Murff
Aubeny, C.P., Han, S., Murff, J.D., 2003. REFINED MODEL for INCLINED LOAD CAPACITY of SUCTION CAISSONS, 22nd International Conference on Offshore Mechanics and Arctic Engineering(OMAE 2003) v.3: Materials Technology Ocean Engineering Polar & Arctic Sciences and Technology Workshops.
Support Structures for Wind Turbines. Offshore Standard DNVGL-ST-126
  • G Dnv
DNV, G., 2016. Support Structures for Wind Turbines. Offshore Standard DNVGL-ST-126.