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The loading behavior of innovative monopile foundations for offshore wind turbine based on centrifuge experiments
Abstract
Wind energy is a promising source of renewable energy and is projected to shift to offshore areas increasingly. Monopile foundation is one of the most commonly used foundations for offshore wind applications with the priority in load bearing capability and initial cost. This study describes an innovative monopile foundation, which institutes a creative strategy over the traditional large diameter monopile foundation to achieve higher axially load bearing capacity. This is achieved by adding a restriction plate inside the pile to intensify the soil plug effect. This design is based on the soil plug mechanism, and the arching effects and plug resistance mobilizations are considered. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the bearing behaviors of the innovative monopile with restriction plates. The pile with 1-hole restriction plate and the pile with 4-hole restriction plate are considered to discuss effects of the plate shape. Twelve models with different diameters and restriction plate types are investigated. The traditional open-ended and close-ended piles are included for comparisons. The static tests are conducted in saturated silica sand first to determine the ultimate bearing capacity of the innovative pile, after which the cyclic tests are performed. The innovative pile is proved to provide a larger bearing capacity than the pipe pile. An analytical method is proposed to estimate the capacity of the innovative pile. The study aims to develop the design code for innovative piles and provide design reference to large-scale offshore wind turbine projects.
... Wind is a promising source of renewable energy, and wind turbines are increasingly used in offshore areas. A monopile foundation is a commonly used foundation for offshore wind applications because of its good load-bearing capability and initial cost [1]. Figure 1 shows the main countries that are developing and investing in offshore wind power as reported by the Global Wind Energy Council [2,3]. ...
... Li et al. [1] proposed a hybrid monopile-friction wheelbucket (MFB) foundation for offshore wind turbines. ey tested five hybrid foundations with variable dimensions in sandy soil. ...
The core objectives of sustainable development are to develop access to renewable, sustainable, reliable, and cost-effective resources. Wind is an essential source of renewable energy, and monopile wind turbines are one method proposed for harnessing wind power. Offshore wind turbines can be vulnerable to earthquakes and liquefaction. This numerical study defined the effects of wind turbine weight on the seismic response of a wind turbine-monopile system located in liquefied multilayered soil with layer thicknesses of 5, 10, 15, and 20 m using four far-field records. OpenSees PL analysis indicated that if the liquefied sand had a lower density or a thickness of more than 10 m, then an increase in the earthquake acceleration beyond 0.4 g caused the pile to float like liquefied soil and to lose its vertical bearing capacity. Moreover, increasing the wind turbine power from 2 to 5 kW had no significant effect on the soil-structure interaction response. As the earthquake acceleration increased, the bending moment of the pile-column also increased as long as liquefaction did not occur and the pile-column deformation remained rotational-spatial in shape. As the acceleration and liquefaction increased and the pile began to float in response to its transverse motion, there was no significant difference in the pile-column displacement along the length, but there was a decrease in the pile-column bending moments. As this phenomenon increased and the pile continued to float, transformation of the pile increased the difference between the displacement of the pile-column along its length and further increased the bending moments. These results were derived from multiple correlation analysis, the bending moment relations, and lateral displacement of the pile-column of the wind turbine.
... Due to this reason, the load-carrying capacity of monopile has to increase to accommodate more giant turbines. This requirement has led to developing the new methodologies to improve the bearing capacity using stiffener to avoid the abnormal size of monopile [11]. In general, fatigue design focuses on the short-term effects, while long-term effects are equally crucial for serviceability. ...
... 11 .u 13 + u 13 .u11 Similarly, the angular velocities at the same point can be expressed using small-angle approximations as Hence, the linear components of these velocities are obtained by rearranging Eq. ...
Fatigue design of monopile foundation for an offshore wind turbine using multi-body dynamic analysis is the theme of this paper. For this purpose, a benchmark turbine is modelled in Kane’s approach considering soil- structure interaction for a site at the North Sea. Each soil layer is modelled using the p-y curve, and the dynamic response of the combined system is simulated using aero-hydro-servo-elastic analysis. The fatigue life is estimated following IEC 61400-3 guidelines. The numerical analysis presented in this study shows stress-based fatigue analysis using rainflow algorithm in the light of soil-structure interaction for short-term damage accumulation. Besides short-term effects, the study also investigates serviceability criteria over the lifespan of the structure, i.e. long-term effects. Finally, design curves are developed for a given power rating. Overall, the study highlights the importance of soil-structure interaction and its impact on the fatigue life of offshore wind turbines.
... Some studies found that the bearing capacity of the suction foundation increased with the diameter (Jang and Kim, 2013;Na et al., 2015;Li et al., 2020a). Other studies have found that increasing the diameter can reduce the seismic response of the suction foundation (Yu et al., 2014;Saleh Asheghabadi et al., 2019;Zhang et al., 2021b). ...
During the self-weight penetration process of the suction foundation on the dense sand seabed, due to the shallow penetration depth, the excess seepage seawater from the outside to the inside of the foundation may cause the negative pressure penetration process failure. Increasing the self-weight penetration depth has become an important problem for the safe construction of the suction foundation. The new suction anchor foundation has been proposed, and the self-weight penetration characteristics of the traditional suction foundation and the new suction anchor foundation are studied and compared through laboratory experiments and analysis. For the above two foundation types, by considering five foundation diameters and two bottom shapes, 20 models are tested with the same penetration energy. The effects of different foundation diameters on the penetration depth, the soil plug characteristics, and the surrounding sand layer are studied. The results show that the penetration depth of the new suction foundation is smaller than that of the traditional suction foundation. With the same penetration energy, the penetration depth of the suction foundation becomes shallower as the diameter increases. The smaller the diameter of the suction foundation, the more likely it is to be fully plugged, and the smaller the height of the soil plug will be. In the stage of self-weight penetration, the impact cavity appears around the foundation, which may affect the stability of the suction foundation.
... Centrifuge model experiments can meet the requirements of small-scale experiments containing soil [52,53]. For large-scale model experiments, due to the limitations of size, site and equipment, etc., it generally needs to be carried out in the conventional earth's gravitational field. ...
Previous studies show that the wind turbine foundation is subjected to the coupling effect of torque-vertical load-horizontal load-bending moment load (i.e., T-V-H-M). In a large-scale model experiment of wind turbine foundation, the existing methods cannot accurately simulate the complex loads, counterweights, or foundation boundary conditions. In this paper, a novel method for simulating the loads and boundary conditions of wind turbine foundation model experiments was introduced, and a series of related confirmatory research was conducted, showing that the proposed multi-point composite loading system can resolve the problem of increasing shear force caused by the conventional single-point bending moment control loading method. Meanwhile, it can satisfy the load matching requirements of T-V-H-M loads under ordinary laboratory conditions. The regional counterweight system provided a simulation method for the weight of the foundation and overburdens soil under the gravity field, which can accurately restore the distribution of the foundation resistance. The stiffness of ground soil had a significant impact on the force characteristics of the foundation structure. It is concluded that the proposed method can achieve an excellent accuracy at a reasonable computational expense.
... Considering the current requirements of the offshore wind industry, innovative foundations are investigated with wider adaptability. Focuses are put on the enhancement of traditional monopiles (Bienen et al., 2012;Li et al., 2020a). The hybrid monopile foundation, combining the traditional monopile and the gravity base, is investigated in this study (Fig. 2). ...
The hybrid monopile foundation is an alternative for offshore wind turbines. The parametric study has been performed through a series of centrifuge tests for the optimal design of the hybrid foundation. The wheel diameter, wheel thickness, and pile length are considered in the analysis. The lateral capacity of the hybrid monopile foundation increases with the wheel diameter and tends to accelerate; it increases linearly with the wheel thickness and pile length. The influence of the wheel diameter is more pronounced compared to the other parameters. The hybrid monopile demonstrates an enhanced performance compared to the monopile and the single-wheel. The improvement is more significant at small pile lengths. The hybrid monopile shows its advantages in reducing the pile length. It has great potential in reducing capital costs. An analytical method is proposed by scaling the individual capacity of the pile and the wheel. A design chart for the scale factor is suggested. The calculation is applicable for determining the initial dimension of the hybrid monopile foundation, and the ultimate lateral capacity is assessed.
... An FE analysis of a hybrid monopile-footing foundation was conducted to investigate the foundation performance under the environmental and seismic loading (Anastasopoulos and Theofilou, 2016). A series of centrifuge and FE tests were conducted to investigate the optimized footing aspect ratio, V-H-M load relationships, structure-soil interaction mechanism, and the influence of soil properties (El-Marassi, 2011;Li et al., 2020;Pedram, 2015). ...
In this study, a hybrid monopile-friction wheel-bucket (MFB) foundation for offshore wind turbines is proposed. A bucket and a friction wheel are integrated with a monopile. The friction wheel is filled with scattered material to provide distributed surcharge loads to the subsoil. A series of geotechnical centrifuge tests was performed under monotonic load and cyclic load to investigate the bearing capacity of the MFB foundation. Five hybrid foundations with variable dimensions were tested in four types of sandy soil conditions. The centrifuge test results show that the ultimate bearing capacity of the hybrid MFB foundation could be 4 times of the monopile foundation. Under cyclic load, the final displacement of MFB foundation is significantly smaller than that of the monopile foundation. The MFB foundation is stiffer in the reloading process but shows slightly more plastic characteristic in the unloading process. The size of the add-on friction wheel-bucket structure is positively related to the performance of the MFB foundation. The bucket height tends to be a more effective factor. It is illustrated that the MFB foundation tends to demonstrates more improvements in the saturated loose sand. A simplified method is proposed to predict the bearing capacity of the hybrid MFB foundation.
In this study, a series of centrifuge tests are performed to investigate the ultimate lateral capacity of a pile group for offshore wind turbines. The validated Finite element models (FEMs) are established. The rotation center and the maximum bending moment occur at 9 D and 3 D of the pile length. A sensitivity study is performed by considering the soil parameters through numerical tests. The internal friction angle is the most influential factor by introducing 32% change in lateral capacity. The key factors of pile number, pile diameter, loading height, and pile spacing are investigated in the following parametric study. Increasing the pile number and the pile diameter is effective for enhancing the lateral capacity. In contrast, a larger eccentricity has a negative effect on the foundation stability. The failure mode of the pile group foundation tends to change from strain softening to strain hardening when the loading height reaches a threshold magnitude. The p-multiplier is calculated to demonstrate the shadowing effect in a pile group foundation. An optimal design method is proposed by integrating the pile number and the pile spacing factors. This study provides some instructions for the application of pile group foundations in the offshore wind industry.
The hybrid pile-bucket foundation is an innovative alternative to monopile foundation when used to support offshore wind turbines (OWTs). To understand the load transfer mechanism of the hybrid foundation when loaded horizontally, reduced-scale model tests and numerical modelling were performed to investigated loading/deformation characteristics of the pile-bucket foundation structure and stress-reacting features of the soil surrounding the foundation. Both the pore water pressure (PWP) of the soil and strains of the pile and bucket are recorded from the model test. Furthermore, the pile/bucket load sharing ratio, contours of plastic strain, and excess PWP of the soil are obtained and analyzed via numerical modelling of the model test. The results show that the horizontal load is mostly carried by the bucket, and the overturning moment is mostly resisted by the pile. Compared to the monopile foundation, the soil of the hybrid foundation shows smaller PWP increase and less principle-stress rotation, implying a lower rate of stiffness and strength degradation of the hybrid foundation when subjected to cyclic loadings.
The foundations for offshore wind turbines represent the main item either for cost or installation process, and the lateral resistance of tabular piles is the main factor for its design. Therefore, studies for consistent and efficient foundations have become essential for offshore wind turbines when using traditional mono-pile foundations under practical and environmental conditions. This research discusses the increase in the lateral behavior of open tabular piles with the addition of external wings near the ground level with specific dimensions. Four wings were added to the exterior wall of the open-ended pipe pile at equal angles 90 degrees. The wings length varied from 0.25 to 0.5 of the pile diameter. Each wing length is studied with two depths of 1.25, and 2.5 pile diameter. The numerical analysis was verified with published results of centrifugal tests. The successive parametric study discussed the feasibility of the added wings. Inclusive, the resultant load direction was considered as changed between 0 to 45o with 5 degrees to the wings orientation horizontally.
The increasing demand for offshore wind turbines with larger capacity brings challenges concerning their original foundation design. This study introduces a new type of monopile foundation with internal restriction plates that aim to enhance the bearing capacity of the entire structure. Two types of improved monopiles are proposed through the addition of one-hole and four-hole restriction plates. A series of centrifuge tests are performed to study the response of the improved piles under lateral loading conditions in saturated sand. The pile-soil interactions are characterized using finite element analysis. A sensitivity study and a parametric study are performed using numerical tests. In offshore applications, the improved pile can provide larger lateral resistances than the open-ended pile foundation. This improvement is proportional to the pile diameter. The four-hole restriction plate is more effective than the one-hole plate for larger diameter piles. The rotational axis is located at 80 % of the embedment depth, and the distribution of earth pressure is determined. A theoretical method is proposed to calculate the ultimate lateral capacity of the novel monopile containing restriction plates.
Prediction of bearing capacity of steel pipe piles in rock masses is an important consideration in civil engineering especially as such prediction influences the safety of the supported superstructures as well as the pile integrity in pile driving operations. Provided that the rock mass is described as a linear elastic and perfectly plastic material obeying the Hoek-Brown failure criterion, a finite element analysis is performed to investigate the embedment depth effect on the annular base bearing capacity and the failure mechanism of typical open-ended pipe piles in sedimentary rock masses. The pipe pile has smooth walls and rough toe surface. Annular toe resistance of pipe piles can serve as an estimate of the rock mass resistance to driving in a fully coring mode which is usually expected for large diameter open-ended pipe piles. Pipe pile results are also extended to circular piles and embedded strip foundations socketed in rock masses. The analysis is shown to highlight the influence of the annular geometry of the pipe pile causing an unsymmetrical failure mechanism with respect to pipe wall center as well as an inclination of rock mass reaction, which if sufficiently large, may lead to pile convergence and damage during pile driving operations. The failure mechanism legitimates the plug tendency to rise up in the pipe and explains the plug formation. The study demonstrates that in most practical applications, the bearing capacity of pipe piles approaches a limiting value, which is less than or at most equal to the end bearing capacity of an embedded strip foundation of width equal to the pipe wall thickness. A comparison has been made with experimental data. It is shown that the results are relatively in good agreement with test data in terms of rock mass resistance and the mechanism of rock plug formation.
To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT's) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates at least two limit states: Fatigue Limit State (FLS) and overall deformation in the Serviceability Limit State (SLS). It has been observed through scaled model tests that the first eigen frequency of vibration for OWTs supported on multiple shallow foundations (such as jackets on 3 or 4 suction caissons) corresponds to low frequency rocking modes of vibration. In the absence of adequate damping, if the forcing frequency of the rotor (so called 1P) is in close proximity to the natural frequency of the system, resonance may occur affecting the fatigue design life. A similar phenomenon commonly known as "ground resonance" is widely observed in helicopters (without dampers) where the rotor frequency can be very close to the overall frequency causing the helicopter to a possible collapse. This paper suggests that designers need to optimise the configuration of the jacket and choose the vertical stiffness of the foundation such that rocking modes of vibration are prevented. It is advisable to steer the jacket solution towards sway-bending mode as the first mode of vibration. Analytical solutions are developed to predict the eigen frequencies of jacket supported offshore wind turbines and validated using the finite element method. Effectively, two parameters govern the rocking frequency of a jacket: (a) ratio of super structure stiffness (essentially lateral stiffness of the tower and the jacket) to vertical stiffness of the foundation; (b) aspect ratio (ratio of base dimension to the tower dimension) of the jacket. A practical example considering a jacket supporting a 5MW turbine is considered to demonstrate the calculation procedure which can allow a designer to choose a foundation. It is anticipated that the results will have an impact in choosing foundations for jackets.
The response of skirted circular foundations with rough and smooth soil–skirt interface to combined loading is investigated. The shape and size of the failure envelopes of the skirted circular foundations under a practical range of embedment ratio, soil strength heterogeneity, soil–skirt interface and level of vertical mobilisation are compared to those of solid embedded circular foundations and skirted strip foundations. The results show that both the foundation geometry and soil plug inside the skirt compartment significantly influence the uniaxial capacity and the shape and size of the failure envelopes. Approximating expressions for solid embedded foundations do not capture the shifting eccentricity of the failure envelopes of the skirted circular foundations. A new approximating solution for describing the failure envelope as a function of embedment ratio, soil strength heterogeneity and soil–skirt interface is proposed. Scaling factors accounting for the decreasing uniaxial capacity of skirted foundations with smooth soil–skirt interface are also defined.
Skirted gravity base foundations and suction caisson foundations are considered as viable alternatives to monopile foundations for offshore wind turbines. While recent work has focused on the monotonic moment-rotation response for shallow foundations, the cyclic response and the accumulation of rotation over the life of the turbine must also be addressed. This paper presents cyclic loading tests where approximately 10,000 cycles, with different loading characteristics, were applied to a model shallow foundation (a caisson) in loose sand. On the basis of these tests, a framework for assessing the accumulated angular rotation because of cycling was developed. The settlement and cyclic stiffness response of the caisson were also assessed. It was found, not unexpectedly, that the accumulated settlement of the caisson increased with the number of cycles and cyclic amplitude. It was also found that a cyclic loading regime intermediate between one-way and full two-way cycling produced the largest rotations. The cyclic stiffness was relatively unaffected by the number of cycles. Using an appropriate scaling technique, the proposed framework was used to predict the long-term accumulated angular rotation, for an example, the field-scale monocaisson structure. DOI: 10.1061/(ASCE)GT.1943-5606.0000738. (C) 2013 American Society of Civil Engineers.
Wind turbine structures are long slender columns with a rotor and blade assembly placed on the top. These slender structures vibrate due to dynamic environmental forces and its own dynamics. Analysis of the dynamic behavior of wind turbines is fundamental to the stability, performance, operation and safety of these systems. In this paper a simplied approach is outlined for free vibration analysis of these long, slender structures taking the soil-structure interaction into account. The analytical method is based on an Euler-Bernoulli beam-column with elastic end supports. The elastic end-supports are considered to model the flexible nature of the interaction of these systems with soil. A closed-form approximate expression has been derived for the first natural frequency of the system. This new expression is a function of geometric and elastic properties of wind turbine tower and properties of the foundation including soil. The proposed simple expression has been independently validated using an exact numerical method, laboratory based experimental measurement and field measurement of a real wind turbine structure. The results obtained in the paper shows that the proposed expression can be used for a quick assessment of the fundamental frequency of a wind turbine taking the soil-structure interaction into account.
Numerous cone penetration test (CPT)-based methods exist for calculation of the axial pile capacity in sands, but no clear guidance is presently available to assist designers in the selection of the most appropriate method. To assist in this regard, this paper examines the predictive performance of a range of pile design methods against a newly compiled database of static load tests on driven piles in siliceous sands with adjacent CPT profiles. Seven driven pile design methods are considered, including the conventional American Petroleum Institute (API) approach, simplified CPT alpha methods, and four new CPT-based methods, which are now presented in the commentary of the 22nd edition of the API recommendations. Mean and standard deviation database statistics for the design methods are presented for the entire 77 pile database, as well as for smaller subset databases separated by pile material (steel and concrete), end condition (open versus closed), and direction of loading (tension versus compression). Certain methods are seen to exhibit bias toward length, relative density, cone tip resistance, and pile end condition. Other methods do not exhibit any apparent bias (even though their formulations differ significantly) due to the limited size of the database subsets and the large number of factors known to influence pile capacity in sand. The database statistics for the best performing methods are substantially better than those for the API approach and the simplified alpha methods. Improved predictive reliability will emerge with an extension of the database and the inclusion of additional important controlling factors affecting capacity.
Both the driving response and static bearing capacity of open-ended piles are affected by the soil plug that forms inside the pile during pile driving. In order to investigate the effect of the soil plug on the static and dynamic response of an open-ended pile and the load capacity of pipe piles in general, field pile load tests were performed on instrumented open- and closed-ended piles driven into sand. For the open-ended pile, the soil plug length was continuously measured during pile driving, allowing calculation of the incremental filling ratio for the pile. The cumulative hammer blow count for the open-ended pile was 16% lower than for the closed-ended pile. The limit unit shaft resistance and the limit unit base resistance of the open-ended pile were 51 and 32% lower than the corresponding values for the closed-ended pile. It was also observed, for the open-ended pile, that the unit soil plug resistance was only about 28% of the unit annulus resistance, and that the average unit frictional resistance between the soil plug and the inner surface of the open-ended pile was 36% higher than its unit outside shaft resistance.
Numerous pile groups are subjected to significant cyclic lateral loads due to wind, waves or earthquakes, and many have failed catastrophically. In this research, centrifuge modelling of a pile group subjected to cyclic lateral loads has been conducted to investigate the interaction effect in pile groups and the influence of cyclic lateral loads on the performance of pile groups. Different pile installation methods were also applied to capture the full range of construction-induced soil conditions available in the field. Lateral permanent displacements of the pile group were seen to be induced by one-way cyclic lateral loads but not by two-way symmetric cyclic lateral loads. The lateral secant stiffness of the pile group increases slightly with increasing number of cycles, and leading piles attract greater loads than trailing piles. Furthermore, permanent settlements of the pile group accumulate, which can be attributed to the swaying motion of the pile cap induced by cyclic lateral loads.
Some offshore wind farms are built in seismically active areas. The offshore wind turbine (OWTs) are high-rise structures and sensitive to lateral failures during the earthquake. In this study, an innovative hybrid monopile foundation is proposed for OWTs. A series of centrifuge shake table tests is conducted to investigate the seismic response and liquefaction characteristics of the hybrid monopile foundation. Mechanisms of the seismic behavior of soil, lateral displacements of the wind turbine, and structural settlements are evaluated. Centrifuge test results indicate that the liquefaction around the hybrid monopile foundation is weakened compared to the traditional single pile. The soil partially keeps its strength and stiffness during the shaking due to the higher confining stress. The lateral stability of the system is enhanced. OWTs with the hybrid monopile foundation tends to settle more during the earthquake due to the soil-structure interaction and the static bearing induced soil shearing. Two types of hybrid monopile foundations are constructed with different weights and materials, which are predominant influence factors for their seismic response. This study aims to investigate the seismic behavior of the hybrid monopile foundation for OWTs during earthquake events and provide references for practical designs.
Pile foundation is one of the most commonly used foundations for offshore and coastal structures. This paper describes an innovative design for the pile foundation, which institutes an innovative strategy over traditional pile foundation to achieve higher axial bearing capacity. This is achieved by adding restriction plates inside the pile to help form the soil plug. A series of geotechnical centrifuge tests are carried out to evaluate the load bearing behaviors of traditional pile foundation and innovative pile foundation with different types of restriction plates. The pile diameter and shapes of the restriction plates on the soil plug behaviors and pile load carrying capacity are analyzed. The results show that the use of restriction plates could significantly increase the axial bearing capacity of large diameter pile foundations. For different pile diameters, the restriction plates with four smaller holes achieve better performance than those with one large hole of the same effective opening areas. A bearing capacity equation is obtained by normalizing the ultimate bearing capacity with the pile diameters for different restriction plates. This study demonstrates the promise of an innovative pile foundation with a restriction plate as an economic and technically feasible way to produce higher load-bearing capacity to support offshore and coastal structures.
The suction bucket foundation is turning to be an increasingly prevalent foundation for offshore wind turbines (OWTs). In this study, a series of centrifuge tests are performed to investigate the vertical bearing capacity of the suction bucket foundation in sandy soil under drained condition. Four bucket models are tested by considering the influence of aspect ratios (APs). The foundations are tested under compressive and tensile vertical loadings. The bucket foundation is in the general shear failure mode, and the ultimate bearing capacity increases with the AP. The tensile bearing capacity is smaller than the compressive capacity. The sharing factor of the base capacity and the skirt friction of the suction bucket foundation is determined with the assistance of the tensile test. The bucket lid provides the majority of the bearing capacity. The internal skirt friction is larger than the external skirt friction due to the soil constraint effect and the increased vertical stress. Several analytical methods are proposed to estimate the bearing capacity, and the results are compared with the test data. The bearing capacity equation is more applicable to the “wide-shallow” bucket foundations. The recommended design method is regarded as the upper bound when calculating the ultimate bearing capacity.
Some large capacity offshore wind turbines are constructed in seismically active areas. The occurrence of soil liquefaction during an earthquake can result in severe failures of the offshore wind turbine. The seismic response of the structure and the failure mechanism of the soil-structure interactions are necessary to investigate. In this study, the seismic response of an innovative hybrid monopile foundation is investigated through a series of centrifuge tests. The seismic performance of the combined system of the superstructure, foundation, and soil are demonstrated. Five hybrid foundation models are tested by considering the influence of the foundation thicknesses and diameters, and a monopile foundation is tested for comparison. Centrifuge test results reveal that the hybrid monopile foundation is effective in reducing the lateral displacement during the shaking. In the saturated condition, soil keeps its strength and stiffness beneath and adjacent to the foundation. The hybrid foundation system tends to settle more due to the larger shear stress caused by the soil structure interactions. Influences of the wheel specifications are illustrated. The foundations with larger thicknesses lead to smaller lateral displacements and lower tendencies of liquefaction, but the settlements are intensified. The larger diameter foundation provides a longer drainage path for the excess pore water pressure. With a similar weight, the structure settles less during the earthquake.
The monopile has been widely used to support offshore and coastal structures. A series of centrifuge tests has been performed to investigate the bearing capacity of large diameter monopiles in sandy soil. Both static tests and cyclic tests have been conducted for open-ended and close-ended model piles, and the effects of influence factors, such as loading rates, embedment depths, and loading histories are considered. The piles are then loaded by a sequence of compressive-tensile loadings to estimate the tension capacity, from which the shaft friction is derived. The cyclic load tests are performed with five varying load intensities, and the accumulated settlement is assessed. The centrifuge tests indicate that the pile bearing capacity tends to increase with the initial penetration depth, and the stress state of soil greatly influences the pile behavior. The tensile shaft friction is smaller compared to the compression test. The capacities of the piles reduce significantly under the axial cyclic load, and the maximum cyclic load intensity should be limited to 75% of the ultimate bearing capacity. The API design method is used to calibrate with the centrifuge test results. The method overestimates the bearing capacity at larger depths, and a conservative reduction factor is required.
The support structure for offshore wind turbines (OWTs) plays significant roles in maintaining the structural stability and reducing the initial cost. An innovative hybrid monopile foundation for OWTs is proposed. The concept has a wider adaptability by using established knowledge to solve for new problems. A series of centrifuge tests is performed to investigate the behavior of this hybrid foundation system in extreme and service conditions. OWTs with the original monopile foundation as well as the wheel-only foundations are tested for comparisons, and two clay profiles are considered. The test results show that the hybrid monopile foundation provides larger ultimate bearing capacities compared to the traditional foundations. Two analytical methods are proposed to estimate the ultimate bearing capacity of this innovative design, and the results are calibrated by the centrifuge tests. In service conditions, the hybrid monopile foundation shows stronger cyclic resistances. Influence factors of the cyclic responses are summarized. An analytical solution is put forward to estimate the accumulated lateral displacement of the hybrid monopile foundation. A degradation factor is suggested based on the results of the centrifuge tests. The study aims to enrich the understanding of the innovative foundation concept and to provide design references for practical applications.
Wind shear models are commonly used to predict the wind speed at wind turbine hub heights from the wind data collected at the elevation of the monitoring station. In many cases, the model parameters are based on empirical values recommended by design specifications that reflect the site conditions. This paper evaluates the benefits of incorporating site-specific wind data to calibrate the wind shear model parameters. Wind speed data collected by a ZephIR® Light Detection and Ranging (LiDAR) system over a 2-year (Oct. 2010–Sept. 2012) period are used for the analyses. The atmospheric stability is found to have appreciable effects on the wind shear parameters, i.e. wind shear coefficients (WSC) for the power law model and roughness lengths for the logarithmic law wind shear models. The calibrated wind shear model parameters by the monitored wind data during the first year are presented in the format of a contour map to demonstrate the spatio-temporal variations, which shows daily and seasonal variations. The calibrated wind shear models are then validated by the wind data collected during the second year, which demonstrates decent performance. The accuracy and performance of incorporating site-specific wind shear model calibration to predict the wind energy resource is evaluated, where six different methods are compared. The results show that the consideration of spatio-temporal variations of wind shear model parameters achieved improve performance over the application of the empirical or yearly-averaged wind shear model parameters in extrapolating the wind speed. It is also found that the performance of considering spatio-temporal wind shear parameters are even better at higher elevations. Furthermore, the analyses find that the use of empirical wind shear model parameters underestimates the wind energy output at the studied sites. Site-specific calibration of the wind shear models could further improve the accuracy of wind energy assessment by considering the site condition and the variability in the atmospheric stability.
In order to reduce the structural failure and accident occurrence of the offshore wind turbine, the researches on structural operational safety of offshore wind turbine based on the measured data during the operation periods should be carried out and considered as the key way to solve these problems. However, the current researches are only focused on numerical simulation, model experiment and theoretical analysis and ignore the structural vibration characteristics analyzed from condition monitoring and prototype observation data from the actual wind turbine structure. To compensate for this study defect, the structural vibration displacement signals of one Chinese offshore wind power prototype under different conditions were obtained based on a long-term prototype observation in order to achieve the comprehensive and systematic research on vibration response characteristics and operational modal analysis of measured structure. Considering the complicated operational environment of offshore wind turbines, the structural response regular pattern and vibration safety are also discussed emphatically under different conditions such as standstill, normal operational, startup, shutdown and extreme typhoon status. Simultaneously, the integral operational modal identification method of offshore wind turbine in the range of full power is proposed based on traditional and modified stochastic subspace identification technologies, and further the modal identification results and the relationship between structural modal parameters and environmental/operational factors are also illustrated in this paper. The researches on the structural vibration characteristics and operational modal analysis of offshore wind turbine not only provide powerful data and technology support for the operation safety evaluation, but also provide the necessary theoretical and practical bases for the design and maintenance of wind turbine structures.
The support structure of an offshore wind turbine (OWT) accounts for up to 25% of the capital cost; therefore, investigations into reliable and efficient foundations are critical for the offshore wind turbine industry. This paper describes an innovative hybrid monopile foundation for OWTs, which is an optimization of the original monopile foundation with broader applications. The behavior of OWTs with the hybrid monopile foundation in service conditions are investigated under lateral cyclic loadings, by considering the effects of wind, waves, and ice. A series of centrifuge tests are conducted in order to analyze these behaviors in detail, and OWT models with the original single-pile as well as wheel-only foundations are tested for comparison. Based on these tests, the accumulated lateral displacement and stiffness during cyclic loadings are presented, and the results indicate that the hybrid foundation exhibits a larger cyclic capacity than the other foundations. The influence of the cycle numbers, cyclic loading characteristics, and soil properties is examined during the tests; furthermore, the effects of these factors on the model deformation responses are illustrated. This study proposes the first analytical method for quantitatively estimating the cyclic lateral displacement of the new hybrid foundation in service conditions, and a degradation coefficient is recommended based on the test results. This method aims to provide a simple approach to predicting responses of OWTs with hybrid monopile foundations in service conditions.
The hybrid monopile-friction wheel foundation is an innovative alternative for offshore wind turbines. The concept has wider adaptability and can be used as reinforcement method for existing monopiles. A series of centrifuge tests was performed to investigate the lateral bearing capacities of the hybrid foundation under monotonic loads. Five foundation models and two soil types were considered. According to the recorded responses, the hybrid foundation demonstrated better lateral behaviors that both lateral bearing capacity and stiffness are enhanced. Two analytical methods were proposed and compared with the centrifuge test results. The bearing capacity of the hybrid foundation is smaller than the sum of individual pile and friction wheel, and a reduction factor is suggested for both friction wheels. The friction wheel restrains rotations of monopile and provides extra restoring moments; their effects are idealized as equivalent moments acting on the pile head. The analytical results provide possible solutions in estimating the lateral bearing capacity of the innovative hybrid foundation system for offshore wind turbines by using traditional theories.
Wind Energy is the one of the most promising renewable energy. Suction bucket foundation is considered to be a viable type of wind turbine foundation. Soil liquefaction caused by earthquakes at offshore seismic active area may lead to a significant degradation of soil strength and stiffness. In this study, nine centrifuge tests were carried out to investigate the seismic response of suction bucket foundation under earthquake loading. Both dry and saturate soil conditions were considered in tests. The geometric design of five suction bucket models considered the bucket diameter, penetration depth, and modified buckets with inside compartments. It was found that soil underlying and near the bucket foundation shown a better ability to resist liquefaction in saturated tests comparing to free field while no significant differences were observed in dry tests. The five bucket models performed quite differently, which demonstrated the aspect ratio effects and inside-bucket compartment effects. The results provide insight into optimized design of suction bucket foundation for wind turbine.
Reliable predictions of the lifetime of offshore wind turbine structures are influenced by the limited knowledge concerning the inherent level of damping during downtime. Error measures and an automated procedure for covariance driven Operational Modal Analysis (OMA) techniques has been proposed with a particular focus on damping estimation of wind turbine towers. In the design of offshore structures the estimates of damping are crucial for tuning of the numerical model. The errors of damping estimates are evaluated from simulated tower response of an aeroelastic model of an 8 MW offshore wind turbine. In order to obtain algorithmic independent answers, three identification techniques are compared: Eigensystem Realization Algorithm (ERA), covariance driven Stochastic Subspace Identification (COV-SSI) and the Enhanced Frequency Domain Decomposition (EFDD). Discrepancies between automated identification techniques are discussed and illustrated with respect to signal noise, measurement time, vibration amplitudes and stationarity of the ambient response. The best bias-variance error trade-off of damping estimates is obtained by the COV-SSI. The proposed automated procedure is validated by real vibration measurements of an offshore wind turbine in non-operating conditions from a 24-h monitoring period.
A soil pile interaction model is developed to better represent the actual behavior of pipe piles undergoing dynamic testing. In order to correctly investigate the dynamic interaction mechanism of the pipe piles, the developed model introduces an additional mass to account for soil plug. The governing equations of motion for the soil-pile system subjected to small deformations and stains are established considering plane strain conditions for the soil and one-dimensional wave propagation in the pile. The analytical solution of the vertical dynamic response of the pipe pile in the frequency domain is then obtained by employing Laplace transform and transfer function technique. The corresponding quasi-analytical solution in the time domain for the pipe pile subjected to a vertical semi-sinusoidal exciting force is subsequently derived by means of Fourier transform. A parameter sensitivity analysis of the additional mass model is carried out to determine the approximate range of the parameters value. Utilizin...
Wind turbines on jackets are being increasingly installed offshore. This paper attempts to investigate the effect of soil-structure interaction (SSI) on a jacket-offshore wind turbine (OWT) in a water depth of 70 m using JONSWAP spectrum. Stochastic responses of the OWT under varying soil profiles and met-ocean conditions are studied, by coupling the aerodynamic and hydrodynamic forces. From stochastic time domain response analyses, the SSI is observed to have significant influence in soft clay and layered soils at and above rated wind speeds whereas the dense sand have negligible influence.
The smart fatigue load control of a large-scale wind turbine blade was numerically investigated on our newly integrated aero-servo-elastic platform with emphasis on the effect of azimuth angles. It was found that the smart control effectively reversed the phases of the flapwise aerodynamic force or the acceleration through the controllable deformable trailing edge flap (DTEF) activation within most of rotor azimuth angle range, turning in-phased flow-blade interaction into an anti-phased one at primary 1P mode, significantly enhancing the damping of the fluid-structure system and subsequently contributing to the greatly attenuatedflapwise fatigue loads on the blade and turbine performances. This aero-elastic control physics was most drastic as the investigations were focused on the case beyond the rated wind velocity, leading to the maximum reduction percentages in the time-averaged and azimuth-averaged fatigue loads up to about 30.0%, in contrast to the collective pitch control method. In addition, the finding pointed to a crucial role that the suppression of the coupled flow-blade system dependent on azimuth angles played in the smart blade control, which might guarantee better effectiveness if it would be considered in the development of the DTEF controller.
Long steel piles with large diameters have been more widely used in the field of ocean engineering. Owing to the pile with a large diameter, soil plug development during pile driving has great influences on pile driveability and bearing capacity. The response of soil plug developed inside the open-ended pipe pile during the dynamic condition of pile-driving is different from the response under the static condition of loading during service. This paper addresses the former aspect. A numerical procedure for soil plug effect prediction and pile driveability analysis is proposed and described. By taking into consideration of the pile dimension effect on side and tip resistance, this approach introduces a dimensional coefficient to the conventional static equilibrium equations for the plug differential unit and proposes an improved static equity method for the plug effect prediction. At the same time, this approach introduces a simplified model by use of one-dimensional stress wave equation to simulate the interaction between soil plug and pile inner wall. The proposed approach has been applied in practical engineering analyses. Results show that the calculated plug effect and pile driveability based on the proposed approach agree well with the observed data.
This paper presented a numerical study on the smart fatigue load control of a large-scale wind turbine blade. Three typical control strategies, with sensing signals from flapwise acceleration, root moment and tip deflection of the blade, respectively, were mainly investigated on our newly developed aero-servo-elastic platform. It was observed that the smart control greatly modified in-phased flow-blade interaction into an anti-phased one at primary 1P mode, significantly enhancing the damping of the fluid-structure system and subsequently contributing to effectively attenuated fatigue loads on the blade, drive-chain components and tower. The aero-elastic physics behind the strategy based on the flapwise root moment, with stronger dominant load information and higher signal-to-noise ratio, was more drastic, and thus outperformed the other two strategies, leading to the maximum reduction percentages of the fatigue load within a range of 12.0-22.5%, in contrast to the collective pitch control method. The finding pointed to a crucial role the sensing signal played in the smart blade control. In addition, the performances within region III were much better than those within region II, exhibiting the benefit of the smart rotor control since most of the fatigue damage was believed to be accumulated beyond the rated wind speed.
Large open-ended pipe piles are widely used in offshore foundations. These piles, when properly constructed, offer superior load carrying capability compared to traditional ones. As they can be driven into the ground, there is no need to construct a cofferdam as required for smaller piles in a river or a lake. The savings in the construction cost and schedule can be substantial. The use of large open-ended pipe piles in the transportation project at a few states produced satisfactory results. As most experience with large diameter pipe piles are gained in the offshore industry, the existing knowledge on this type of foundation is rather limited. Formation of soil plug is very important for the bearing capacity of this type of foundation to be mobilized. This paper performed DEM simulations to study the soil plugging mechanism inside the large diameter pipe piles. The advantage of DEM is that it can holistically simulate the interactions between soil particles as well as between soil particles and pipe pile. The results are compared against that predicted by analytical model. A sensitivity study was performed on the effects of factors such as soil column length, pile internal diameter, particle size, friction coefficient of interface between soil and pile, etc. These offers insight on the formation mechanisms of soil plugs inside the large diameter pipe piles.
Construction effects on pile-soil systems arise from the process of installing displacement piles. This study conducted a comprehensive field test program complemented with laboratory tests to observe the performance of jacking open-ended concrete pipe piles into silt deposits. The jacked piles were examined in a plugged mode during installation. Direct observation on the soil plugs reveal that their formation generally accords with thestratigraphic nature of the layered soils. Soil-arching behavior during pile penetration causes the soil in the shear zone along the inner pipe wall to mainly come from the uppermost layer of the deposit. Laboratory tests on the soil plug shows evident compaction andthe tendency increased strength over time. The buildup of the excess pore pressure and radial total stress in the soil is sensitive to the jacking installation procedure. By taking into account the soil displacement related to the plugging degree, the captured peak excess pore pressure at a given horizon can be modeled by the cavity expansion theory thatnormally adapts to closed-ended pile. The jacking annulus resistance normalized by the cone tip resistance is independent of the penetration depth and the degree of plugging. A considerable portion of the annulus resistance is locked in the pile after installation, decreases a little during adjacent pile installation, and remains stable over a long period.
This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter-a key factor reflecting the influence of pile embedment-whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number of well-documented field tests including two fully instrumented large-diameter offshore piles, and through comparisons with major CPT-based methods in current engineering practice. The assessment indicates that the HKU method has attractive capabilities and advantages that render it a promising option. DOI: 10.1061/(ASCE)GT.1943-5606.0000667. (C) 2012 American Society of Civil Engineers.
Stress wave theory is applied to open-ended pipe piles to clarify the effects of soil plug on the behaviour of piles during driving and static loading. Measured field data and various numerical models are reviewed; methods are presented to calculate wave propagation in both the pile and the soil plug; modelling is presented which takes into account the interaction between the soil plug and the pile; also presented is simplified method to estimate the load-settlement relation of the pipe pile in static loading. By correlating observed and calculated values in two analytical cases, the authors demonstrate that incorporation of the soil plug (modelled as a series of masses and springs) is required to correctly predict pile behaviour during driving and static loading.
This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging. DOI: 10.1061/(ASCE)GT.1943-5606.0000528. (C) 2011 American Society of Civil Engineers.
This paper presents an experimental study of the plugging effect on the capacity of open-ended piles installed in sandy soil. Full-scale tests, including dynamic and static axial compression load tests, were carried out on three instrumented piles with different diameters (508.0, 711.2, and 914.4 mm). To measure the outer and inner shaft resistances acting on the piles, a double-walled system was utilized, with instrumented strain gauges on the outside and inside walls of the pile. The results of field tests show that the inner shaft resistance was mostly mobilized at the location between the pile tip and 18%–34% of the total plug length. It was found that the soil plugging in the lower portion has influence on the inner shaft resistance. In addition, it can be also demonstrated that the ratio of inner shaft resistance plus annulus load resistance to total resistance was decreased with increasing pile diameters. The results of these tests show that the relationship between the degree of plugging and pile diameter is clearly established. Direct observations of the soil plugs were made and used to quantify both the plug length ratio (PLR) and the incremental filling ratio (IFR). Based on this result, it was realized that the N value of the standard penetration test (SPT) is highly correlated with the IFR. © 2015, National Research Council of Canada. All rights reserved.
Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.
Open‐pipe piles are widely used for offshore structures. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile to become plugged. The open‐ended pile then assumes the penetration characteristics of a closed‐ended pile. The mode of pile penetration significantly alters the soil‐pile interaction during and after installation. This affects the ultimate static bearing capacity (mainly in granular materials), the time‐dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles.Following a summary demonstrating the effects of pile plugging, a review of the common view of offshore pile plugging is undertaken. The interpretation of plugging by referring to the average plug length has led to the erroneous conclusion that in most piles significant plugging action does not occur.Establishment of an analogy between soil samplers and open‐ended piles enabled correct identification of plugging by referring to the incremental changes in plug length. Examination of case histories of plugging of offshore piles revealed that beyond a certain penetration depth‐to‐diameter ratio, most piles are plugged.
The response of the soil plug within driven, open-ended pipe piles is very different under the dynamic conditions of installation and the static conditions of loading during service. This paper addresses the latter aspect and describes a combined experimental and numerical study of the response of soil plugs in open-ended pipe piles. The work focuses on the partially drained (static) loading relevant to offshore applications, and the experimental work is conducted using calcareous sand from Bass Strait, Australia. Model tests are conducted in pipe piles of 25-mm and 100-mm internal diameters, with loading rates as great as 6 MPa/s, using a downward hydraulic gradient to achieve appropriate effective stress profiles in the soil plug. The experimental results are assessed within the framework of analytical solutions of the drained and undrained response of the soil plug and numerical studies of the partially drained problem that allow the results to be extrapolated to prototype conditions. Example applications are given for pile geometries and loading rates typical for Bass Strait.
A series of model pile tests have been performed in the geotechnical centrifuge at the University of Western Australia to study the plugging behaviour of piles in sand. Open and sleeveended piles have been driven and jacked by a miniature pile driving actuator into silica flour of varying densities. The progression of the soil column has been measured during installation and static loading. It was found that the plug length increased with increasing relative density during driving and decreased with increasing relative density during jacking. During installation, the jacked piles exhibited a greateer tendency to plug than the driven piles. Piles fitted with an internal driving shoe provided significant stress relief within the soil plug durling jacking, leading to longer plug lengths compared with internally flush piles. All static load tests at high embedments behaved in a completely plugged manner. An annlysis of the plug capacity is conducted and a model for the variacapacity is conducted and a model for the variation of the earth pressure coefficient inside the pile is proposed. Differential base pressure on the pile annulus and the internal soil plug is postulated and validated by means of the experimental data.
Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design-that of estimating the axial capacity-relies heavily upon empirical correlations. Improvements have been made in identifying the processes that occur within the critical zone of soil immediately surrounding the pile, but quantification of the changes in stress and fabric is not straightforward. This paper addresses the degree of confidence we can now place (a) on the conceptual and analytical frameworks for estimating pile capacity, and (b) on the quantitative parameters required to achieve a design. The discussion is restricted to driven piles in clays and siliceous sands, with particular attention given to extrapolating from design approaches derived for closed-ended piles of relatively small diameter to the large-diameter open-ended piles that are used routinely in the offshore industry. From a practical viewpoint, we need design approaches that minimise sensitivity to the estimated pile capacity. This may be achieved partly through a greater reliance on pile load testing, where significant advances have been made in the last decade, but also by adopting design approaches that are focused more on guarding against unacceptable deformation of the complete foundation. Example applications in the paper are drawn both from offshore applications, where current challenges include estimating the axial capacity of ultra-thin-walled, large-diameter caissons, and from onshore applications such as bridge piers and piled raft foundations, where inelastic displacement of the piles is not only acceptable, but often essential for efficient design.
During installation of open-pipe piles, soil enters the pile until the inner-soil cylinder develops sufficient resistance to prevent further soil intrusion and the pile becomes "plugged." In spite of its frequent occurrence, only limited attention has thus far been given to this phenomenon and its consequences. The effects of plugging on pile performance and design are examined in reference to the following aspects: ultimate static capacity, time-dependent pile capacity, and dynamic behavior. Pile plugging is shown to have the following effects: marked contribution to the capacity of piles driven in sand; delay in capacity gain with time for piles driven in clay; and change in behavior of piles during installation, causing it to differ from that described by the models commonly used to predict and analyze pile driving. Key words: pipe piles, pile plugging, open-ended piles, static capacity, time-dependent capacity, dynamic analysis, pile driving, pile performance.
When open-ended piles are driven into the soil, a soil column is created inside. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil and influences the pile- soil interaction during and after installation. Formation of the soil co1umn inside open-ended model and large sca1e experimental piles was analysed at four sandy sites. The incremental filling ratio was found to be linked to the impact characteristics. The influence of the soil column on driving results as on static results was evaluated and discussed. In contrast to the driving results, the static loading result were not changed by the removal of the soil column. The plugging effect was explained by an exponential cumulation of stresses inside the piles.
INTRODUCTION
The majority of offshore platforms are founded on open ended tubular piles. When the piles are driven into the soi1, a soi1 column is created inside the pile. At the start of penetration, the pile cores a soi1 column the height of which is equal to the pile penetration. Then as penetration continues, the filling ratio may vary. At a certain stage, the soil column may act as a perfect plug, preventing any new soil intrusion. If that occurs, the pile penetrates the soil as a closed-ended pile. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil which influences the pile-soil interaction during and after installation. In practice the advance of the soil drivability methods use unit skin friction and unit toe resistance independent of the penetration mode of the pile. The contribution of the soil co1umn to the soil resistance to driving and to the static bearing capacity is taken as the lowest resistance of the two fo11owing mechanism. The plugged mechanism requires that the point resistance be calcu1ated over the full pile toe area, and the unplugged mechanism requires that an interna1 skin friction due to the soil co1umn be added to the point resistance of the pile annulus toe area. The same unit skin friction as the external on is used. This may be overly conservative for static design. In this paper, the advance of the soi1 co1umn inside experimental piles is reported -and analysed. The contributions of the soil column to the soil resistance to driving and to the static bearing capacity are evaluated and discussed. Open ended model piles 70 mm in diameter and 2,50m long and large scale piles 324 mm in diameter and 12 and 24 m long were tested at dense sandy sites. The experimental programme was carried out within CLAROM (a research association grouping French oil companies, contractors, one certification body and research institutes working on offshore operations).
The unit point and side resistances obtained from thirty-four field pile
load tests were used to determine which of the pile geometry and soil
parameters are most significant. Bearing capacity factors are calculated
and correlated with the most significant pile-soil system parameters.
Three types of load test data are used: compression test data adjusted
for residual stresses, unadjusted compression test data and
compression/tension test data. The best and simplest correlations are
developed by plotting the bearing capacity factors versus the relative
depth, i.e., the depth of penetration to diameter ratio. An error
analysis of these correlations provides the basis for the conclusion
that both unadjusted compression and compression/tension test
correlations yield fairly accurate predictions (usually within 20%) for
total bearing capacity. However, the compression/tension test
correlations give the best predictions for point and side bearing
capacity.
The European Offshore Wind IndustrydKey Trends and Statistics
- W Europe
W. Europe, The European Offshore Wind IndustrydKey Trends and Statistics
2016, Wind Europe, Brussels, Belgium, 2017, p. 37.
Design of Offshore Wind Turbine Structures, Standard DNV-OSJ101, Det Norske Veritas AS
- D N V Dnv
D.N.V. DNV, Design of Offshore Wind Turbine Structures, Standard DNV-OSJ101, Det Norske Veritas AS, 2013. DNV.
Effects of soil plug on behavior of driven pipe piles, Soils Found
- M Tatsunori
- T Masataka
M. Tatsunori, T. Masataka, Effects of soil plug on behavior of driven pipe piles,
Soils Found. 31 (2) (1991) 14e34.
Test Method for Deep Foundations under Static Axial Compressive Load
- Astm
- Standard
ASTM, D1143 Standard Test Method for Deep Foundations under Static Axial
Compressive Load, ASTM International, 2013.