Bucket Spacing Research Articles

Probabilistic failure envelopes of tripod bucket-supported offshore wind turbines in spatially variable clay based on a novel continuous method

Marine geologic conditions play key roles in offshore geotechnical preliminary design and construction decisions. Tripod bucket foundations are considered promising geotechnical foundations for offshore engineering. Their bearing characteristics are often determined based on tripod foundation system failure modes in uniform clay without considering spatially variable soil properties. Therefore, this study investigates the combined bearing capacity of tripod bucket foundations in spatially variable clay based on comprehensive finite element analysis with Monte Carlo simulation (MCS) using a novel simplified modified swipe (SMS) method considering the effects of bucket spacing and foundation skirt length. The SMS method can directly represent the ultimate combined bearing capacity of the whole space efficiently and accurately. The results show that a reduced bucket spacing or increased skirt length changes the failure mode of the foundation from independent caisson failure to global failure. Moreover, the spatial variation in soil leads to an asymmetric failure mode, which is more remarkable for foundations with small bucket group effects. An independent safety factor design approach is adopted to represent failure envelopes with different recurrence probabilities, which should be cost-effective, especially for foundations sensitive to soil variability. Finally, an application based on a convex polynomial optimization strategy for representing failure envelopes is demonstrated.

Enforcing Fairness in the Traffic Policer Among Heterogeneous Congestion Control Algorithms

Traffic policing is widely used by ISPs to limit their customers’ traffic rates. It has long been believed that a well-tuned traffic policer offers a satisfactory performance for TCP. However, we find this belief breaks with the emergence of new congestion control (CC) algorithms: flows using new CC algorithms can easily occupy the majority of bandwidth, starving traditional TCP flows. We confirm this problem with experiments and reveal its root cause as follows. Without a buffer in traffic policers, congestion only causes packet losses, while new CC algorithms are loss-resilient. When being policed, they will not reduce the sending rate until an unacceptable loss ratio for TCP is reached, resulting in low throughput for competing TCP flows. Simply adding a buffer to the traffic policer improves fairness but incurs high latency. To this end, we propose FairPolicer, which can achieve fair bandwidth allocation without sacrificing latency. FairPolicer regards a token as a basic unit of bandwidth and fairly allocates tokens to active flows in a round-robin manner. To avoid bandwidth waste when flows come and go, FairPolicer puts all available tokens in a global bucket and maintains the amount of residual bucket space rather than the number of available tokens. To scale to massive concurrent flows, FairPolicer uses a Count-Min Sketch structure to maintain per-flow data with a small memory footprint. Testbed experiments show that FairPolicer can allocate bandwidth in a max-min fair manner and achieve much lower latency than other kinds of rate limiters.

A Wind Protection Design Strategy for Airport Terminal Door Bucket Space Based on Wind Environment Simulations

Airport terminal buildings are large public transportation buildings with complex functions and dense crowds, and the wind environment has significant effects on passenger comfort and the energy consumption of air conditioning systems. In this study, we investigated Xianyang T3 Terminal as an example and used the Phoenics software to conduct comprehensive simulations based on different door bucket forms and parameters, as well as comprehensively considering the impacts of different outdoor wind environments. A terminal door bucket windproof design strategy was proposed based on our results. The results showed that door buckets could effectively reduce the entry of outdoor wind, and built-in door buckets performed the best. The width of the door bucket should be set to 9.3–11.3 m and the depth of the door bucket to 6.7–7.7 m. The height of the door bucket had little impact. A side door should be added to the door bucket, so it can be opened when the outdoor wind speed is high, and a mechanical ventilation system should be introduced to improve the indoor ventilation.

Predicting the monotonic horizontal behavior of tripod bucket foundations in clay

The requirements for the foundation bearing characteristics become higher due to the evolution of offshore wind turbines in the abyssal sea. Tripod bucket foundations have good bearing performance and has gradually become a trend. To investigate the horizontal dynamic behavior of the tripod bucket foundation and the dynamic characteristics relationship between the tripod bucket and the single bucket under horizontal monotonic loadings, the predictive model of the dynamic characteristics of the tripod bucket is established through formula derivation, and extensive numerical analyses were conducted under horizontal monotonic loadings in clay. It was confirmed that the limit bearing moment of the tripod bucket enhances with increment of bucket spacing and length. The ultimate bearing moment has an exponential positive correlation with the ratio of length and diameter (L/D) for the single bucket and a linear positive correlation with the ratio of space and diameter (S/D) for the bucket. Predicting the dynamic characteristics of tripod bucket foundations under horizontal monotonic loadings based on the horizontal bearing capacity coefficient model and the single bucket foundations horizontal dynamic characteristics. Simplified prediction of the tripod bucket foundations' rotation angle and moment has been verified under horizontal monotonic loadings. That can provide a certain design reference of the multi-bucket foundation by studying the relationship between single and jacket foundation.

Experimental study of local scour around bucket foundation in sand

Laboratory flume experiments were conducted to study scour characteristics around bucket foundation under current only condition and current-wave condition. Scour topography from the experiments shows that the evolution of scour can be divided into three stages, rapid scouring stage, slow scouring stage, and scouring equilibrium stage. Semicircular scour pit occurs in front of the bucket and herringbone siltation appears behind the bucket. Scour depth of the mono-bucket was compared with that of the unsubmerged monopile and submerged monopile. Slope of the scour pit is found to be smaller than the predicted value of the monopile. Maximum scour depth of the tripod-bucket foundation is shown to be affected by the factors including the ratio of exposed height to diameter H/D, the relative bucket spacing G/D, and the incoming flow angle α. The maximum scour depth increases with the increase of H/D. Scour depth of the inner side of the tripod-bucket foundation is shown to be significantly affected by G/D due to the jet effect. The rear bucket may be influenced by shielding effect or vortex shedding. And the incoming angle has influence on the scour development around tripod-bucket foundation.

A theoretical and practical framework based on plasticity theory for the drained behavior of single and multiple shallow footings

AbstractWith the shift of the offshore wind energy sector to deeper waters, demand for the development of more complex foundation solutions, particularly suction bucket – supported tripod/tetrapod and jacket foundations, has increased. This paper is divided into two main sections. The first part comprises a comprehensive review of the performance of circular surface and shallow foundations under combined loading (VHM), and how it can principally be understood in a theoretical framework of plasticity theory. Examination of the considered data suggested that the general assumption of over‐estimated non‐association degree with constant failure surface parameters and increasing vertical load may require further investigation. This may be attributed to the complex interplay of multiple properties such as stress level, soil strength profile and foundation geometry. The existing data in the literature were also used to provide practical guidance for a successful implementation of the elasto‐plastic constitutive relationships in offshore foundation design. In second part of the paper, the suitability of the non‐associated plasticity formulation for a baseline multi‐pod system in H‐M load space was investigated using three‐dimensional (3D) finite element (FE) analyses and not verified. Furthermore, the failure envelopes and hardening law for caissons with different embedment ratios differed from those recommended in the literature were established. Parametric studies of multi‐caisson foundations revealed that the failure mechanism of multi‐bucket foundations under horizontal loading depended greatly on the bucket spacing. The horizontal bearing capacities increased with the bucket spacing until they reached a threshold. Meanwhile, analyses of the multi‐bucket foundation under moment loading confirmed the occurrence of a push‐pull failure mechanism.

Deformation Characteristics of Bucket−Soil Interaction in Sand during Jacked Penetration

Grouped buckets are an important type of foundation for offshore structures. Because of interactions between buckets, jacked penetration above a group of two buckets is a complicated process that involves complex bucket-soil interactions. In this study, non-contact digital image correlation was applied to study the deformation of sandy soil during jacked penetration. Bucket-group efficiency is negatively correlated with sand density and bucket diameter, and the incremental filling ratio is negatively correlated with sand density and positively correlated with bucket diameter. The graph of the incremental filling ratio versus penetration depth curves is a polyline for a bucket diameter of 5 cm and a hyperbola for 7.5 cm. Buckets in groups experience superposition of their deformation fields, which leads to a partial loss of shear strength. Sand density, bucket diameter, and bucket spacing markedly influence the deformation field of a bucket group, consistent with the force-displacement curves. The bucket-group effect retards the formation of a soil plug.

Numerical study on local scour characteristics of multi-bucket jacket foundation considering exposed height

At present, offshore wind farms around the world have successively applied Multi-Bucket Jacket Foundations (MBJF), but little research has been done on the scour characteristics of MBJF. In this study, a three-dimensional numerical model is established to study the scour characteristics of large-diameter, wide-shallow, and multi-bucket foundations in deep water areas. The simulation results of the single pile foundation erosion based on this model are in good agreement with the existing experimental results. Also, an optimized grid division strategy is proposed based on a large number of comparative analyses, discussing the influence on the flow field, the bed elevation contour, and the scouring depth under different exposed height(h), bucket diameter(D), and bucket spacing(d) of the MBJF. Then, the influence of the upper structure on the scour characteristics is quantified. The results show that two sides of the buckets can form scoop-shaped scour holes with steep front and gentle back, with deposition of sand formed on the front and back sides. As the exposed height increases, the scouring depth initially shows an increasing trend, which then decreases. The scouring depth of the front row buckets decreases as the diameter increases, while it increases as the diameter for the rear row buckets increases. Compared with d=1.5D and d=3D, d=2D has the smallest scouring depth. Within the normal scale range, the maximum deviation coefficient of the influence of the upper jacket structure on the foundation scouring depth does not exceed 5.2%. Finally, based on V(velocity), H (water depth), μ (viscosity) the basic flow variables, the scouring prediction formula based on dimensionless parameters is obtained. The findings can be used to provide a reference for the design and scour protection of MBJF.

The capacities of tripod bucket foundation under uniaxial and combined loading

Tripod bucket foundations are an alternative foundation for offshore wind turbines. Three-dimensional finite element analyses were carried out to determine the capacities of tripod bucket foundations in clay under uniaxial and combined loading. The undrained shear strength of clay is assumed to increase linearly with depth, and the remolded soil around the bucket and reduced foundation-soil friction caused by bucket penetration are modeled through an adhesion factor. Based on the numerical results, expressions for uniaxial capacity factors and failure loci are proposed, as functions of soil non-homogeneity, aspect ratio and bucket spacing ratio. It is found that the failure loci of tripod bucket foundations are similar to that of conventional buckets in mathematic expression, although the bucket spacing ratio that appears in uniaxial capacities is used as a key fitting parameter.

Laser-driven electron storage rings

Advanced accelerator concepts usually address linear acceleration schemes. Storage rings, however, are often superior to linear machines regarding repetition rate, stability and efficiency. The radiative energy loss per turn in an electron storage ring is compensated by radiofrequency resonators with a wavelength of the order of 1 meter, which corresponds to the spacing between consecutive potential wells, so-called buckets, and results in a bunch length around 1 centimeter or several 10 picoseconds. As an alternative, longitudinal focusing could be performed by a laser wave co-propagating with the electrons in an undulator. Considering a continuous-wave carbon dioxide laser beam as an example, the bucket spacing would be 10.6 micrometer with a bunch length in the femtosecond range. The paper discusses chances and limitations of such a laser-driven storage ring concept with steady-state femtosecond bunches.

Influencing Factors of Motion Responses for Large-Diameter Tripod Bucket Foundation

Large-diameter multi-bucket foundation is well suited for offshore wind turbines at deeper water than 20 m. Air floating transportation is one of the key technologies for the cost-effective development of bucket foundation. To predict the dynamic behavior of large-diameter tripod bucket foundation (LDTBF) supported by an air cushion and a water plug inside every bucket in waves, three 1/25-scale physical model tests with different bucket spacing were conducted in waves; detailed prototype foundation models were established using a hydrodynamic software MOSES with a draft of 4.0 m, 4.5 m, and 5.0 m and with a water depth of 10.0 m, 11.25 m, and 12.5 m. The numerical and experimental results are consistent for heaving motion, while exhibiting favorable agreement for pitching motion. The results show that the resonant periods for heaving motion increased with increasing draft and water depth. The maximum amplitude for heaving motion first decreased and then increased with the increase of water depth and spacing between the buckets. The maximum amplitude for pitching motion first decreased and then increased with increasing water depth but decreased with increasing spacing between the buckets. The wider the spacing between the bucket foundations, the larger the heave response amplitude operators (RAOs). Simply improving the pitch RAOs by increasing the spacing between bucket foundations is limited and negatively affects motion performance during the transportation of LDTBF.

Evaluation of horizontal and moment bearing capacities of tripod bucket foundations in sand

Bearing capacities of tripod bucket foundations differ from those of single bucket foundations because of the interaction among individual buckets of the tripod. This study analyzed the bearing capacities of tripod bucket foundations in medium and dense sands by performing a series of 3D finite element analyses. The sandy soil was modeled by using an elasto-plastic model following the Mohr-Coulomb failure criterion. The Young's modulus of the sands increased with depth because of the stress-dependency of the modulus. Parametric studies were performed, in which the spacing between each bucket, the embedded depth of the bucket, the bucket diameter, and the magnitude of vertical loading were varied. Results showed that the horizontal bearing capacities of tripod bucket foundations reached the maximum values at a spacing ratio of S/D=1.5 to 3.5 for the foundation with an embedment ratio of L/D=0.5 to 1 (where S is the bucket spacing, D is the bucket diameter, and L is the skirt length of the tripod bucket). However, the moment bearing capacities increased linearly as the S/D ratios increased. Finally, bearing capacity equations were proposed in consideration of bucket spacing, embedment depth, effect of foundation diameter and vertical load, and soil density.

Evaluation of combined horizontal-moment bearing capacities of tripod bucket foundations in undrained clay

A series of three-dimensional finite element analyses was conducted to evaluate the combined horizontal-moment bearing capacities of tripod bucket foundations for offshore wind turbines in undrained clay. The tripod bucket foundation consists of three single bucket foundations, which enhance the bearing capacities because of the effect of the interaction among individual buckets. A linear-elastic perfectly plastic model, which obeys the Tresca failure criterion, was used to simulate the stress–strain response of clay, in which Young׳s modulus and undrained shear strength were assumed to increase linearly with depth. The effects of bucket spacing, embedment depth, vertical load, and non-homogeneity of clay on the combined horizontal-moment capacities were analyzed. Finally, the design equation of the combined horizontal-moment capacities was developed as a function of the aspect ratios (skirt length/diameter) of the foundations.

Bucket group effect of the composite multi-bucket structure

As a novel type of foundation in beach and shallow sea, the bucket structure is especially suitable for complex conditions such as soft clay ground and the worse types of sea environments. In this paper, the bearing capacity of a multi-bucket structure is studied by experiments with a single bucket and four-bucket foundation in a saturated sand layer. Based on the experimental data and numerical analysis results, the bearing capacity behavior and the bucket group effect are compared and analyzed. Furthermore, some influential factors, such as the soil type, the ratio of length to diameter L/D, the ratio of the bucket spacing to the bucket diameter S/D, and the bucket number are introduced and their effects on the multi-bucket structural capacity are investigated. The vertical static capacity adjustment factor is introduced to evaluate the bucket group effects of the multi-bucket foundation.

Group effect on bearing capacities of tripod bucket foundations in undrained clay

Tripod bucket foundations are being used as alternative foundations to resist large horizontal and moment loadings from offshore wind turbines. The bearing capacity of the tripod bucket foundation differs particularly from that of the single bucket foundation due to the interaction among individual buckets of the tripod. This paper investigated the group effect of tripod bucket foundations in clay by three-dimensional finite element analysis. Parametric studies were performed varying the spacing between each bucket foundation, embedded depth of the bucket foundation, and loading directions. Normally consolidated clay under an undrained condition was modeled using a linear elastic-perfectly plastic model obeying the Tresca failure criterion. The undrained shear strength and Young's modulus of clay were assumed to linearly increase with depth. The group efficiency factor, which is the ratio of the bearing capacity of the tripod bucket foundation to that of the single bucket foundation, was evaluated based on finite element analysis for vertical, horizontal and moment loadings. Results showed that the efficiency factors were functions of the bucket spacing and embedment depth of the foundations.

Near Duplicate Image Detecting Algorithm based on Bag of Visual Word Model

In recent years, near duplicate image detecting becomes one of the most important problems in image retrieval, and it is widely used in many application fields, such as copyright violations and detecting forged images. Therefore, in this paper, we propose a novel approach to automatically detect near duplicate images based on visual word model. SIFT descriptors are utilized to represent image visual content which is an effective method in computer vision research field to detect local features of images. Afterwards, we cluster the SIFT features of a given image into several clusters by the K-means algorithm. The centroid of each cluster is regarded as a visual word, and all the centroids are used to construct the visual word vocabulary. To reduce the time cost of near duplicate image detecting process, locality sensitive hashing is utilized to map high-dimensional visual features into low-dimensional hash bucket space, and then the image visual features are converted to a histogram. Next, for a pair of images, we present a local feature based image similarity estimating method by computing histogram distance, and then near duplicate images can be detected. Finally, a series of experiments are constructed to make performance evaluation, and related analyses about experimental results are also given