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Item type:Publication, Access status: without files , Influence of Low-Permeability Layers on the Installation and the Response to Vertical Cyclic Loading of Suction Caissons(2021-08-01); ; Offshore wind is a key part of the change toward renewable energy sources. Offshore wind farm developments are moving to sites that are characterized by increased water depths and layered soil profiles, e.g., in the North Sea. Suction caisson-based jacked structures are expected to be increasingly used to support wind turbines at such sites. Experience of suction caissons serving as foundations for offshore wind turbines is limited. The mechanisms governing the suction caisson installation in layered soils are poorly understood and no published data of the in-service performance of suction caisson jackets supporting offshore wind turbines in layered soils exists. This gap is addressed here through a series of centrifuge tests investigating the installation and the response of suction caissons in layered soils - sand over clay and clay over sand - subjected to vertical cyclic loading into tension. The results reveal the mechanisms enabling suction caisson installation in layered soils and the complex load transfer mechanisms governing the suction caisson response under vertical cyclic loading. The clay layer plays a key role: in sand over clay, it encapsulates the sand plug, which leads to predominantly undrained behavior. In clay over sand, the uplift of the clay soil plug is critical both in facilitating suction installation and in response to vertical cyclic loading in tension. These findings provide confidence for suction installation and caisson response under vertical cyclic loading into tension in layered soils.Publicationtype: Journal Article71 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Insights into Suction Caisson Installation Utilising the Material Point Method(2021-05); ; There is an increasing interest to utilise suction caissons as foundations for offshore wind turbines. Significant research has been devoted to developing penetration prediction methods and to understand the in-service response under cyclic loading. However, the effect of the installation process on the state of the surrounding soil is less well understood, although it may affect the in-service performance, in particular under relatively low magnitude cyclic loading, which represents the majority of loading conditions experienced by an offshore wind turbine in the field. This is due to the complexity in modelling the problem, which includes very large deformations, seepage flow and soil-structure interaction. Novel approaches featuring the material point method and centrifuge test results evaluated with the particle image velocimetry post analysis are capable of visualising the mechanisms underlying suction caisson installation. The results aim to reduce existing uncertainties and provide confidence in suction caissons as a reliable foundation system for offshore wind applications.Publicationtype: Conference Paper83 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , The influence of the drainage regime on the installation and the response to vertical cyclic loading of suction caissons in dense sand(2020-11-01); ; Offshore wind turbines are considered as an integral part of renewable energy. Dense sand often characterises the sea bed and suction caisson foundations are increasingly used to support offshore wind turbines at North Sea sites. Significant experience with this technology exists in the oil and gas industry, although these platforms are typically located in deeper water with predominantly clay soils. Therefore, research is required to transfer the existing knowledge. This paper discusses a series of centrifuge tests that investigate the installation and the load transfer mechanisms governing the response of suction caissons embedded in dense sand subjected to vertical cyclic loading. The effect of potential loosening of the internal soil plug during suction installation on the in-service foundation performance is evaluated through cone penetration tests inside the caisson following suction installation at different pumping flow rates. In accompanying tests, the load displacement behaviour in response to vertical cyclic loading is investigated, including significant excursions into tension. The results identify the pore fluid behaviour as the key factor affecting the response, with the pumping flow rate applied during installation being less influential. Based on the findings from the experiments a simplified approach for the prediction of suction caisson behaviour under vertical cyclic loading is presented.Publicationtype: Journal Article70 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical simulation of cone penetration tests inside suction caisson foundations in sand(American Society of Mechanical Engineers (ASME), 2020-06); ; ; Publicationtype: Conference Paper57 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of installation method on static and dynamic load test response for piles in sand(2020-01-01); ; ; Static load tests (SLTs) are rarely performed on offshore piles, instead relying on dynamic load tests (DLTs). Evaluation techniques for DLTs were developed originally for solid piles, and caution is needed in applying the techniques directly to open-ended piles, particularly for the large-diameter monopiles used for offshore wind turbines. Centrifuge tests are reported here, investigating the influence of pile installation method and sand relative density for pairs of DLTs and SLTs. The piles were successfully driven to penetration of 3-4·6 diameters, which was much deeper than proved possible to jack the piles. Due to limited hammer energy, loads mobilised during the DLTs were less than 25% of the corresponding SLT values. The soil resistance mobilised at limited displacements was lowest for wished-in-place piles, and also for DLTs conducted on shallowly embedded jacked piles. For driven piles, the DLT and SLT results were relatively consistent, with surprisingly similar values for piles driven into loose and dense sand despite significant differences in cone resistance. Given the potential for low mobilisation of soil resistance in DLTs, care is needed when using DLTs for evaluating the static axial pile capacity of open-ended piles of low length-to-diameter ratio.Publicationtype: Journal Article53 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Dynamic load tests on large diameter open-ended piles in sand performed in the centrifugeOffshore infrastructure is commonly founded on piles. In accordance with design guidelines, piling contracts usually include load tests to verify the bearing capacity following pile installation. Offshore, these are generally performed as dynamic load tests (DLT) rather than static load tests often used onshore, for economical and practical considerations. Substantial experience exists in evaluating DLT on closed-ended piles. However, DLT on open-ended profiles have not been investigated sufficiently to date, particularly for piles with low length to diameter ratio. In order to derive realistic static axial pile bearing capacities, the processes during DLT taking place in the open-ended pile and soil need to be understood. Centrifuge tests provide the possibility of model tests with defined and reproducible conditions under consideration of realistic stress states. This paper discusses centrifuge test results of static and dynamic load tests on monopiles investigating the effects of pile installation method. The findings of this study provide a basis towards improved understanding of DLT on open-ended piles. © 2018 Taylor & Francis Group, London.Publicationtype: Conference Paper170 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Centrifuge tests investigating the effect of suction caisson installation in dense sand on the state of the soil plug(2018); ; Suction caissons are a promising foundation system for offshore wind farms. However, uncertainties remain regarding the effect of the suction installation in dense sand on the load bearing behaviour. This subject is addressed here through a series of centrifuge tests, in which model suction caissons were installed in saturated fine silica sand. Following the installation, and without stopping the centrifuge, a cone penetration test was conducted inside the caisson. Comparison of the results allows relative evaluation of the effects of pumping flow rate and lid contact. Cone test results from undisturbed free field sites further provide an indication of the altered stress state inside and below the caisson. The centrifuge results indicate that the suction installation related effects on the load bearing behaviour appear to be influenced by the role of the lid contact. © 2018 Taylor & Francis Group, London.Publicationtype: Conference Paper203 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Advanced approaches for coupled deformation-seepage-analyses of suction caisson installation(American Society of Mechanical Engineers (ASME), 2017-06); ; In this paper the installation procedure of suction caissons is investigated by means of coupled seepage large deformation analysis performed with finite element methods. The modelling techniques employed to enable simulations of the penetration of a caisson into the soil under offshore conditions, i. e. several tens of meters below the water level. The numerical model includes a u-p-formulation, which is used to calculate the excess pore pressures and effective stresses from the total stresses. The Coupled-Eulerian-Lagrangian (CEL) approach available in conjunction with the Abaqus/Explicit solver is used. The calculation results are compared to centrifuge tests that were carried out recently at the Centre for Offshore Foundation Systems (COFS). This sheds light on the potential and the limitations of the presented numerical techniques. This paper concludes with a brief discussion of alternative numerical approaches that could be capable of the simulation of caisson installation.Publicationtype: Conference Paper141 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical analysis of large penetration of a cone and a large diameter footing into dense sand overlying clayThis paper capitalizes on recent advances in numerical and constitutive modelling, which now allow large deformation problems in soils featuring sand layers to be performed. The hypoplastic constitutive relation employed here captures the stress and state dependent response of sand, such that the operative friction angle is a product of, not an input to, the analysis. This enables the detailed study of a cone penetrometer and a large diameter footing penetrating into sand over clay, providing both insights into the governing mechanisms and the resulting load-penetration curves. This contribution forms part of a wider study with the aim to formulate a direct correlation between CPT and footing response. While the end goal is not achieved yet, this contribution discusses some interesting aspects of the approach and numerical techniques used. © 2015 Taylor & Francis Group, London.Publicationtype: Conference Paper29 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , CPT based prediction of foundation penetration in siliceous sandThe load-penetration response of a foundation is one of the fundamental aspects of geotechnical engineering. In sand, the bearing capacity approach requires the operative friction angle to be known, which introduces significant uncertainty to the prediction. The predictive method developed in this paper eliminates the need to determine the friction angle. The central concept is the direct correlation of in situ piezocone penetrometer measurements to the load-penetration response of foundations.The correlation factor is shown to depend primarily on the sand relative density. The footing shape has a minor influence on the correlation factor. This study aims at large diameter foundations used in the offshore industry, where the variation in correlation coefficient is minor. However, context is provided to previous research on smaller diameter foundations, which shows the dependence on the footing diameter (through the well-known stress level effect).The proposed method is shown to perform well against load-penetration data from centrifuge experiments on footings of different diameters and elevation shapes. The performance against field data in particular provides significant confidence in the CPT based prediction method of foundation penetration in sand developed here.Publicationtype: Journal Article54