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A programme of testing of caisson foundations in clay at the Bothkennar test site is described. The tests are relevant to the design of foundations for offshore wind turbines, in the form of either monopod or tetrapod foundations. Records are presented for installation of the caissons, cyclic moment loading under both dynamic and quasi-static condi...
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Context 1
... moments, but relatively little vertical load. Two main structural configurations using caissons are being considered: either a 'monopod' consisting of a single large caisson (typically 20-25 m in diameter for a modern large turbine structure), or a 'tetrapod' in which the load is transferred through a truss structure to four smaller caissons: see Fig. 2 (preliminary calculations indicate that the obvious alternative of a tripod is a less favourable configuration). Each of the smaller caissons might be, say, 6-8 m in diameter. For the monopod the most important load on the foundation is the overturning moment. In the case of the tetrapod the moment loading is carried principally by ...
Context 2
... by 'push-pull' action by opposing footings, and it is the variation of vertical load (and in particular the possibility of tension on a footing) that is most important. In both cases the design objective is to select an appropriate diameter D and depth h of the caisson, and in the tetrapod case the spacing s must also be deter- mined (see Fig. ...
Citations
... Numerous scholars have investigated the seismic responses of bucket-supported offshore wind turbines using experimental and numerical methods. Experimental methods typically include shaking table tests [11][12][13], field tests [14], and centrifuge model tests [15][16][17]. However, experiments can reflect the nonlinearity of soil and the actual dynamic soil-foundation interaction. ...
Numerous offshore wind turbines (OWTs) with bucket foundations have been installed in seismic regions. Compared to the relative development of monopiles (widely installed), seismic design guidelines for bucket-supported OWTs still need to be developed. Moreover, scour around bucket foundations induced by water–current actions also creates more challenges for the seismic design of OWTs. In this study, a simplified seismic analysis method is proposed that incorporates the soil–structure interaction (SSI) for the preliminary design of scoured bucket-supported OWTs, aiming to balance accuracy and efficiency. The dynamic SSI effects are represented using lumped parameter models (LPMs), which are developed by fitting impedance functions of the soil–bucket foundation obtained from the four-spring Winkler model. The water–structure interaction is also considered by the added mass in seismic analysis. Based on the OpenSees 3.3.0 platform, an integral model is established and validated using the three-dimensional finite element method. The results indicate that the bucket-supported OWT demonstrates greater dynamic impedance and first-order natural frequency compared to the monopile-supported OWT, which has an increased seismic response. Seismic spectral characteristics and intensities also play an important role in the responses. Additionally, scour can change the bucket impedance functions and the frequency characteristics of the OWT system, leading to a significant alteration in the seismic response. Scour effects may be advantageous or disadvantageous, depending on the spectral characteristics of seismic excitations. These findings provide insights into the seismic response of bucket-supported OWTs under scoured conditions.
... Suction bucket foundations can be either a single bucket foundation or multi-bucket foundation as displayed in Fig. 1. Multi-bucket foundations offer high overturning stability [3], which make them suitable for supporting OWTs in deep water (25-50 m) installations [4,5]. ...
... Houlsby et al. (2005) develop an analytical method for evaluating the up-lift resistance of a bucket foundation when suction is present. Houlsby et al. (2006) perform large-scale field tests, where large displacements could be generated with large tensile capacities during pullout tests. Senders (2008) developed a discrete spring-dashpot model to assess the axial behavior of suction caissons. ...
Suction caissons are skirted foundations frequently used in offshore environments supporting various structures. Among those lie the offshore wind turbines (OWT), which are currently one of the most common renewable energy systems. In this paper we study the tensile capacity of caisson foundations of OWT in dense sands, where the generation of excess pore pressure is analyzed. The tensile capacity of suction buckets under various loading rates is investigated using a series of quasi-static axisymmetric finite element analyses with coupled flow-deformation formulation. DeltaSand model is utilized for the soil, which is an elasto-plastic state-dependent model capturing the response of sands at varying relative densities. Numerical results are validated with available test data of scaled physical model experiments. Particular emphasis is placed on the interaction between suction-bucket and the soil. Parameters governing the soil-caisson interface playing a key role in the uplift response are identified. Lastly, a case study that can be observed in offshore foundations is simulated. Results show that the developed numerical model is applicable to capturing the uplift response of bucket in an ocean environment. The outcome of this study will contribute to closing the gap in the design of suction caissons of OWT in existing standards.
... Pioneering work was conducted to investigate performance of bucket foundations under different vertical and lateral loading scenarios [5][6][7][8][9][10][11][12][13][14][15][16][17]. Increasing interest in examining seismic response of OWT foundations is demonstrated through additional recent efforts [18][19][20]. ...
Shake table testing was conducted to document the seismic response of a bucket foundation offshore wind turbine (OWT) system. Salient response of the system's soil-structure interaction effects is presented and discussed. Among the observed response characteristics, excess pore pressure fluctuation within and around the soil-bucket domain is thoroughly addressed, including the strong tendency for the soil dilation excursions driven by the induced cyclic strains. The experimental data is used to calibrate a numerical model with dynamic soil response simulated by a coupled solid-fluid formulation. The calibrated model is extended to investigate seismic response of a prototype utility-scale OWT, with and without added wind loading effects. Overall, the research outcomes indicate that: i) excess pore pressure fluctuations in the vicinity of the bucket play an important role in dictating the extent of potential permanent base rotation, ii) consideration should be given to wind loading that might further exacerbate this base rotation, and iii) it is of importance to model the turbine tower as a system of discrete masses rather than the simplified proposed for practice equivalent top mass idealization.
... However, the existing research on monopile-bucket hybrid foundation does not take account of the effect of the lateral bearing performance of the hybrid foundations due to torque loads, which brings great challenges to the design of actual OWT engineering. A series of in-situ tests and centrifuge model tests on bucket foundations and pipe pile were conducted by Houlsby et al., (2006), Zhang and Kong et al. (Zhang and Kong, 2006), the results showed that the foundations subjected to torque loads or composite loading tend to be more vulnerable to buckling failure. Meanwhile, many numerical results showed that the bearing capacity of the OWTs foundation subjected to combined loads cannot be simply evaluated based on the superposition principle. ...
With the development of OWT technology, the requirement for bearing capacity of OWT is becoming stricter. A new type of monopile-bucket hybrid foundation has been recently proposed. As the bearing behavior and failure mechanism of the hybrid foundation have not been investigated comprehensively, laboratory experiments and numerical analysis of the monopile-bucket hybrid foundation subjected to H-M-T combined loads in sand are finished in this paper. Firstly, the model tests were carried out to explore the lateral bearing performance of the hybrid foundation with different loading eccentricity, skirt heights and pre-torque loads. Then, based on the numerical analysis, a quantitative analysis is proceeded to find out the influence of different factors on the bearing performance of the hybrid foundation in detail. The results show that, the lateral bearing capacity of the hybrid foundation is greater than the sum of bucket foundation and monopile. It's also found that the torque load has a strong weakening effect on the lateral bearing capacity of the hybrid foundation. Finally, an empirical calculated formula for calculating the lateral bearing capacity of the hybrid foundation in sand is proposed, which may provide a valuable engineering application reference for monopile-bucket hybrid foundation in sand under H-M-T combined loads.
... After that, the performance of suction piles in sandy soil rapidly gained considerable attention, leading to various investigations on the behaviours of suction pile-sandy soil interaction [26][27][28]. Previous scale model tests and field trials have demonstrated that soil resistance in sandy soil is highly dependent on soil density and that the pore pressure effect is only important in pile penetration progress due to the large negative pressure (suction) for the installation of suction piles [29][30][31][32]. During the long service period (the period of encountering pulling action), the pore pressure effect is negligible in sandy soil compared to that in clay [33][34][35]. ...
The micro-mechanism of mooing depth effect on the interactions between suction piles and granular soil remains unclear. This study investigates suction pile-soil interaction behaviour under the inclined pulling load of a mooring line, as well as mooring depth effects. The discrete element method (DEM) was used to model granular soil, while the suction pile was modelled via finite element method (FEM). Results in DEM-FEM simulations were compared with that in the model tests first. Then, the macro and micro behaviours during suction pile-soil interactions were analysed. Based on the results, the pulling force-displacement curves could be categorised into two groups according to the curve shapes associated with suction pile motion patterns. Discontinuity and occurrences of large soil deformation were successfully reproduced. Next, suction pile movement and deformation were quantitatively analysed in terms of vertical pull-out displacement, pile rotation, and effective support around suction piles. Furthermore, the particle-scale behaviours of soil were analysed, finishing with the identification of conclusive mechanical failure patterns. This study indicates that neglecting the mooring depth of the mooring lines may lead to a significant underestimation of the uplift capacity of a suction pile, as well as misinterpretation of the failure mode of the granular soil.
... Suction caissons are widely used in the offshore industry with applications ranging from tension leg platforms [1] to catenary mooring lines [10], and have been considered as the support structures for offshore wind turbines [8,18,19]. Many theoretical studies have been published to predict the ultimate horizontal bearing capacity of suction caissons or piles. ...
... Combining Eqs. (1), (10), (13), (16), (19), (21), (22) and (23), the upper-bound solutions can be obtained for the one-sided and two-sided failure mechanisms as follows ...
Suction caissons supporting offshore wind turbines are subjected to eccentric horizontal loadings, which may lead to the overturning failure. This paper presents a modified three-dimensional (3-D) failure mechanism to predict the anti-overturning bearing capacity of suction caissons in clay under undrained condition. The modified failure mechanism is composed of a meniscus-conical wedge having meniscus shape at the mudline and a spherical failure in deeper soil. The meniscus-conical failure wedge predicts the deformation of the mudline better than the traditional ring-conical wedge, and the spherical failure keeps a kinematically admissible velocity field, ensuring the accuracy of the upper-bound theorem. Accordingly, the anti-overturning bearing capacity of suction caissons are deduced based on the work-energy balance equation, following by optimizations of two parameters to approach the critical failure mechanism that offers the minimum energy dissipation. In addition, the reliability of the presented upper-bound analyses are verified against the results from finite element limit analysis (FELA) as well as existing experimental and theoretical methods.
... The behavior of monopod suction buckets under lateral cyclic loading was investigated extensively through: (a) prototype and reduced-scale filed tests (Houlsby et al., 2005bBarari and Ibsen, 2012;Zhang et al., 2015); (b) scaled 1-g laboratory tests (Wang et al., 2006;Zhu et al., 2013Zhu et al., , 2018Foglia and Ibsen, 2016;Hung et al., 2018); (c) centrifugal model tests (Zhang et al., 2007;Cox et al., 2014;Wang et al., 2017a, b); and (d) numerical simulations (Kourkoulis et al., 2014;Gelagoti et al., 2018;Cheng et al., 2016Cheng et al., , 2018Cheng et al., , 2020aCheng et al., , 2022Yi et al., 2021;Zhang et al., 2021Zhang et al., , 2022Wang et al., 2021;Jin et al., 2022). These studies provided good understanding of lateral cyclic behavior of the monopod suction bucket, such as the evolution of its cumulative rotation and unloading stiffness with the cyclic load amplitude, frequency and direction and number of cycles. ...
Tripod suction bucket foundations are increasingly used to support offshore wind turbines (OWTs) due to their economic advantages and high overturning resistance. In the harsh marine environment, OWT tripod bucket foundations are subjected to long-term lateral cyclic loads from wind, waves and current. However, the response of tripod bucket foundations in clay to cyclic lateral loads is not well covered in the literature. Therefore, a three-dimensional (3D) numerical method is developed for analyzing their lateral cyclic response based on a simplified bounding surface model of clay. The numerical model is validated by comparing its results with scaled model test results available in the literature. The validated model is then used to investigate the rotation mechanism of tripod suction bucket under lateral monotonic and cyclic loading. The ‘bonded contact’ and ‘separable contact’ were set between the soil plug and the inner wall to simulate the upper limit and the lower limit cases of the tensile capacity of the suction bucket. The influences of the bucket-soil contact condition, loading direction and cyclic loading mode (one-way and two-way) on the cyclic behavior were analyzed. It was found that these factors have a significant impact on the cyclic responses, permanent rotation angle and failure mechanism of the tripod bucket foundation. The findings from this study can provide guidance for the design of OWT tripod bucket foundations.
... Many researchers have studied the bearing characteristics of bucket foundations on soft soil foundations by theoretical calculation [2][3][4][5], experimental research [6][7][8] and numerical simulation [9][10][11][12]. These researchers consider the influence of foundation shape, foundation burial depth, soil strength heterogeneity and other factors on the bearing performance of bucket foundations. ...
Multi-barreled composite foundations are generally used in offshore oil platform structure. However, there is still a lack of theoretical analyses and experimental research. This paper presents the results of a three-dimensional finite element analysis of a four-barreled suction pile foundation in heterogeneous clay foundation. The pile group effect and carrying capacity are numerically simulated. The effects of different pile embedment depths, pile spacings and non-uniformity coefficients of clay on the pile group effect are studied. Considering the changes in the foundation carrying capacity under vertical, horizontal and bending moment coupling loads, the foundation carrying capacity envelopes under horizontal and moment (H-M) and vertical, horizontal and moment (V-H-M) loading modes are drawn. The results show that pile spacing and embedment depth have great influence on the pile group effect. The bearing capacity envelope of foundations under V-H-M loading mode is greatly affected by vertical load V. This can provide a reference for the selection of pile spacing and embedded depth in practical engineering design. Furthermore, the stability of foundations can be evaluated according to the relative relationship between design load and failure envelope.
... The Multi-Bucket Jacket Foundation (MBJF) combines the benefits of both a suction bucket and a jacket foundation, making it appropriate for application in deep water and simple to construct . Studies have revealed that foundations are more sensitive to instability damage in the severe marine environment when subjected to moment loads or composite loading conditions that include moment loads (Houlsby et al., 2005;Zhang and Kong, 2011). Therefore, it's especially crucial to look into the stability of large diameter, wide, and shallow MBJF foundations. ...
The Multi-Bucket Jacket Foundation (MBJF) is presently being installed in Chinese wind farms with increasing numbers. Compared to monopiles, MBJF is a shallow foundation and the effect of scouring on the bearing capacity is more significant. This research proposes a universal three-dimensional model of the MBJF with local scour to efficiently estimate the effect of scouring on the MBJF bearing capacity. Firstly, using the proposed model, the ultimate bearing capacity is calculated under a single load, and the discounting effect of different scour ranges and scour depths on the ultimate bearing capacity is evaluated. Furthermore, the fixed load-displacement approach is used to calculate the failure envelope of the foundation under composite loading mode, and the influence of different scour patterns on the failure envelope is compared. The findings reveal that when the scour extent and depth increase, the foundation's bearing capacity decreases non-linearly, and the foundation bearing envelope line gradually shrinks. Under the condition that the ultimate vertical load is not exceeded, a rise in vertical load can enhance the ultimate load capacity after scouring to a certain extent. Finally, the calculated envelopes are integrated with engineering applications to quantify the safety redundancy of the MBJF bearing capacity for different seabed morphologies. The method and results can provide methodological support for the accurate prediction and efficient assessment of the MBJF's post-scour bearing capacity.
