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1.
Drag anchor is a widely used anchor type in offshore engineering for the mooring system. The prediction of the anchor trajectory installation and the final position is important for anchor selection in design. The existing method using yield envelope method for trajectory prediction ignored the shallow anchor behavior but applied the deep yield envelope from a deeply embedded horizontal fluke in uniform clay for the whole drag-in installation process. However, the anchor fluke embedment depth and inclination angle change continually during installation in clay with linearly increasing shear strength soil profile in practice. Studies on the effect of fluke inclination angle on the anchor behavior in clay with such non-uniform soil profile under unidirectional and combined loading are important and necessary for the improvement of the yield envelope method to ensure a reasonable prediction. The current 2D finite element studies investigate the anchor behavior for inclined fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then the effects of the fluke inclination angle, soil non-homogeneity and embedment depth ratio on the shallow yield envelopes are investigated. It is found that the effect of fluke inclination angle on the vertical capacity factors for anchor in clay with non-uniform and uniform soil profile is largely different. The resultant large impact on the yield envelopes shown here illustrates the importance of considering the fluke inclination angle and soil non-homogeneity in the prediction of anchor trajectory using yield envelope method. 相似文献
2.
Vikram Singh 《Marine Georesources & Geotechnology》2018,36(5):505-514
Although the uplift behavior of offshore plate anchors under undrained conditions has been investigated well in the past, studies on the behavior of anchors under long-term sustained loading are in relatively few numbers. The time required for consolidation under sustained load is important because the shear strength of soil changes after dissipation of excess pore pressure. In this paper, small strain finite-element analyses have been performed to investigate the consolidation time history above and beneath strip anchors. The modified cam clay plasticity constitutive model is used for modeling coupled pore fluid stress analysis. The effects of magnitude of preloading with embedment level have been studied. As expected, the FE results have shown that excess pore pressure dissipation time for soil above the anchor increased with the increase in embedment depth and the magnitude of preload. Rapid dissipation of negative excess pore pressure beneath the anchor was observed with increasing embedment depth, if the preload magnitude is equal to or more than 60% of the undrained capacity. Observed consolidation responses are presented as nondimensional design charts and simplified equations for ease of practice. 相似文献
3.
复合加载下桶形基础循环承载性能数值分析 总被引：1，自引：0，他引：1
作为一种新型基础形式,吸力式桶形基础除了承受海洋平台结构及自身重量等竖向荷载的长期作用之外,往往还遭受波浪等所产生的水平荷载及其力矩等其它荷载分量的瞬时或循环作用。对复合加载模式下软土地基中桶形基础及其结构的循环承载性能尚缺乏合理的分析与计算方法。应用Andersen等对重力式平台基础及地基所建议的分析方法,基于软黏土的循环强度概念,在大型通用有限元分析软件ABAQUS平台上,通过二次开发,将软土的循环强度与Mises屈服准则结合,针对吸力式桶形基础,基于拟静力分析建立了复合加载模式下循环承载性能的计算模型,并与复合加载作用下极限承载性能进行了对比。由此表明,与极限承载力相比,桶形基础的循环承载力显著降低。 相似文献
4.
Drag anchor is widely applied in offshore engineering for offshore mooring systems. The prediction of the invisible trajectory during its drag-in installation is challenging for anchor design in determining the anchor final position for ensuring sufficient holding capacity. The yield envelope method based on deep anchor failure for kinematic analysis was proposed as a promising trajectory prediction method for drag anchor. However, there is a lack of analysis on the effects of the parameters applied in the kinematic analysis. The current work studies the effects of the yield envelope parameters, anchor line bearing capacity factor and the anchor/soil interface friction. It is found that the accuracy of the yield envelope parameters has large impact on the prediction results based on deep yield envelopes.Analyses of cases with smooth fluke predict deeper embedment depth than that from analyses of cases with rough fluke. The decrease of the capacity factor results in the increase of the anchor embedment depth, the anchor line load,the anchor chain angle and the stable value of the normalized horizontal load component for the same drag length,while the stable value of the normalized vertical load component decreases when the capacity factor decreases. This illustrates the importance in applying reasonable parameters and improving the method for more reliable prediction of the anchor trajectory. 相似文献
5.
To study the feasibility and efficiency of underwater suction anchors in soil, the performance of eight model anchors was evaluated in a 30 in. (76·2 cm) by 72 in. (183·0 cm) by 32 in. (81·3 cm) deep test tank. The soil studied were a medium fine sand, a silt and a clay. A gantry-type loading mechanisms was used to apply the vertical breakout force. A pullout rate of approximately 0·1 in/min (0·25 cm/min) was used for all tests.For the range of conditions studied, the test results indicate that the underwater suction anchor is feasible and effective. The breakout behavior of the suction anchors depends upon the anchor geometry including anchor diameter and skirt length, soil strength properties, soil-anchor friction and adhesion, and suction; i.e. the difference between the pressure underneath the porous plate and the ambient pressure. For the anchors and soils tested, the anchor capacity increases linearly with increasing suction provided the anchor skirt length to diameter ratio remains constant. An increase in the anchor capacity results with increasing suction and with increasing internal friction angle of the test soils. To study the results suggest that the underwater suction anchor is particularly useful for short-term anchorage. 相似文献
6.
Drag anchor is one of the most commonly used anchorage foundation types. The prediction of embedded trajectory in the process of drag anchor installation is of great importance to the safety design of mooring system. In this paper, the ultimate anchor holding capacity in the seabed soil is calculated through the established finite element model, and then the embedded motion trajectory is predicted applying the incremental calculation method. Firstly, the drag anchor initial embedded depth and inclination angle are assumed, which are regarded as the start embedded point. Secondly, in each incremental step, the incremental displacement of drag anchor is added along the parallel direction of anchor plate, so the displacement increment of drag anchor in the horizontal and vertical directions can be calculated. Thirdly, the finite element model of anchor is established considering the seabed soil and anchor interaction, and the ultimate drag anchor holding capacity at new position can be obtained. Fourthly, the angle between inverse catenary mooring line and horizontal plane at the attachment point at this increment step can be calculated through the inverse catenary equation. Finally, the incremental step is ended until the angle of drag anchor and seabed soil is zero as the ultimate embedded state condition, thus, the whole embedded trajectory of drag anchor is obtained. Meanwhile, the influences of initial parameter changes on the embedded trajectory are considered. Based on the proposed method, the prediction of drag anchor trajectory and the holding capacity of mooring position system can be provided. 相似文献
7.
Gravity installed anchors (GIAs) are released from a height of 30–150 m above the seabed, achieving velocities up to 19–35 m/s at the seabed, and embed to depths of 1.0–2.4 times the anchor length. Challenges associated with GIAs include the prediction of anchor initial embedment depth, which determines the holding capacity of the anchor. Based on the coupled Eulerian–Lagrangian approach, a numerical framework is proposed in this paper to predict the embedment depth of GIAs, considering the effects of soil strain rate, soil strain-softening and hydrodynamic drag (modeled using a concentrated force), with the anchor-soil friction described appropriately. GIAs are influenced by the hydrodynamic drag before penetrating into the soil completely, hence the anchor accelerates less than the previous investigations in shallow penetration, even decelerates directly at the terminal impact velocity. The hydrodynamic drag has more influence on OMNI-Max anchors (with an error of ∼4.5%) than torpedo anchors, and the effect becomes more significant with increasing impact velocity. An extensive parametric study is carried out by varying the impact velocity, strain rate and strain-softening parameters, frictional coefficient, and soil undrained shear strength. It is concluded that the dominant factor affecting the penetration is the soil undrained shear strength, then are the impact velocity, strain rate dependency and frictional coefficient, and the minimal is the strain-softening of soil. In addition, although the strain rate dependency is partly compensated by the softening, the anchor embedment depth accounting for the effects of strain rate and strain-softening is lower than that for ideal Tresca soil. Strain rate dependency dominates the combined effects of strain rate and strain-softening in the dynamic installation of GIAs, on which should pay more attention, especially for the calibration of the related parameters and the measured solutions. In the end, the theoretical model based on the bearing resistance method is extended by accounting for the hydrodynamic drag effect. 相似文献
8.
Current floating structures require more reliable and higher anchoring capacities because of their increased size. A suction anchor is one of the most popular anchors for a floating system. In this study, the behavior of a suction anchor installed in cohesionless soil was investigated when the anchor was subjected to mainly a horizontal load. Three-dimensional finite element numerical analyses were carried out using ABAQUS, and three centrifuge tests were performed to calibrate the numerical analyses. A parametric study with different dimensions and loading points for the suction anchor was conducted. The horizontal capacity of the suction anchor was estimated, and the soil reaction distribution was analyzed when the load was applied at the optimal point. Based on the results, an analytical equation for calculating the horizontal capacity of a suction anchor was proposed that can be easily adopted for design. 相似文献
9.
With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and significantly affected by the anchor line. Based on the coupled Eulerian–Lagrangian (CEL) method, a numerical approach incorporating anchor line effects is developed to investigate comprehensive anchor behaviors in the soil, including penetration of drag anchors, keying of suction embedded plate anchors and diving of gravity installed anchors. Compared to the method directly incorporating the anchor line into the CEL analysis, the proposed method is computationally efficient. To examine the robustness and accuracy of the proposed method, numerical probe tests and then comparative studies are carried out. It is found that the penetration (or diving) and keying behaviors of anchors can be well simulated. A parametric study is also undertaken to quantify the effects of various factors on the behavior of OMNI-Max anchors, whose mechanisms are not yet fully understood. The maximum embedment loss of OMNI-Max anchors during keying is not influenced by the initial anchor embedment depth, whereas significantly increases with increasing drag angle at the embedment point. With decreasing initial anchor embedment depth or increasing soil strength gradient, drag angle at the embedment point and diameter of the anchor line, the behavior of OMNI-Max anchors could change from diving to pullout, which is undesirable in offshore engineering practice. If the drag angle increases over a certain limit, the anchor will fail similar to a suction anchor. 相似文献
10.
The bearing behavior of suction caissons supporting offshore wind turbines under two-way cyclic lateral loading and dead load in clay was investigated with consideration of soil strength degradation and adhesive interface friction between caisson walls and heterogeneous clay using the finite-element package ABAQUS.An ABAQUS built-in user subroutine was programmed to calculate the adhesive interface friction between clay and caisson walls.The results of parametric studies showed that the degradation of bearing capacity could be aggravated by the decrease of the aspect ratio.The offset between the rotation point of the soil inside the caisson and the central axis of the caisson increased with the increasing vertical load and number of cycles.The linearly increasing strength profile and adhesive interface led to the formation of an inverted spoon failure zone inside the caisson.The settlement-rotation curves in each cycle moved downwards with increasing number of cycles due to the soil strength degradation. 相似文献
11.
S. Rezazadeh 《Marine Georesources & Geotechnology》2018,36(6):625-639
Semi-deep skirted foundations are now considered to be a viable foundation option for a variety of onshore and offshore applications. The capacity under combined vertical, horizontal, and moment loadings must be found to ensure their capability and stability. In this study, undrained bearing capacity subjected to vertical loading, as part of combined loading is determined through stress characteristics and finite element analyses. Circular skirted foundations with different soil strength and geometries considering embedment depth effects have been studied. Stress field, kinematic mechanism accompanying failure, and bearing capacity factors for various embedment ratios are investigated. Acquired vertical failure mechanism has demonstrated the transition from a general shear to a punch shear failure. Comparisons with different research works including conventional methods, upper and lower bound, finite element analyses, physical modeling, experimental, and centrifuge tests have indicated the underestimation of conventional approaches and accuracy of proposed methods in determining bearing capacity. Furthermore, differences between predicted bearing capacities and the results of this study increased with D/B ratio due to ignoring the significant role of skin friction in larger embedment circumference. 相似文献
12.
Considering the current disadvantages of present offshore wind turbine foundations, a novel anchor foundation with skirt and branches is proposed, called offshore umbrella suction anchor foundation (USAF). A series of experiments and numerical simulations were performed to explore the bearing capacity of the USAF under various kinds of loading modes. The bearing characteristics and the anchor–soil interactions are described in detail for horizontal static loading, horizontal cyclic loading, and an antidrawing (pullout) test in silty soil. In the static loading test, the load–deflection of the anchor under step loading was analyzed and the normalized curve of the load–deflection was obtained to determine the ultimate horizontal bearing capacity of the anchor under normal working conditions. Under horizontal cyclic loading, the relationship between the plastic cumulative deformation and cyclic number was determined. In addition, the responses of USAF were investigated for a low wave frequency and storm surges. In the drawing test, it was found that a “segmentation phenomenon” occurred during the test. Moreover, a method to identify the maximum antidrawing load of USAF was provided based on dynamic mechanics. The numerical results show that the use of anchor branches and skirt can enhance the bearing performance of USAF to a certain degree. However, the anchor branch has a slight positive influence on the bearing performance improvement. The USAF is not only similar to a stiff short pile, but a rotation occurs. The failure envelope under composite loading (V-M) was obtained and the changes associated with changes in the aspect ratio of the internal compartment were clarified. 相似文献
13.
This article presents a procedure to calculate the bearing capacity of suction anchors subjected to inclined average and cyclic loads at the optimal load attachment point using the undrained cyclic shear strength of soft clays based on the failure model of anchors proposed by Andersen et al. The constant average shear stress of each failure zone around an anchor is assumed and determined based on the static equilibrium condition for the procedure. The cyclic shear strength of each failure zone is determined based on the average shear stress. The cyclic bearing capacity is finally determined by limiting equilibrium analyses. Thirty-six model tests of suction anchors subjected to inclined average and cyclic loads were conducted, which include vertical and lateral failure modes. Model test results were predicted using the procedure to verify its feasibility. The average relative error between predicted and test results is 1.7%, which shows that the procedure can be used to calculate the cyclic bearing capacity of anchors with optimal loading. Test results also showed that the anchor was still in vertical failure mode under combined average and cyclic loads if an anchor was in vertical failure mode under static loads. The anchor failure would depend on the vertical resistance degradation under cyclic loads if an anchor was in lateral failure mode under static loads. Cyclic bearing capacities associated with the number of load cycles to failure of 1000 were about 75% and 80% of the static bearing capacity for vertical failure anchors and lateral failure anchors, respectively. 相似文献
14.
锚泊基础的承载性能直接影响着海洋浮式结构物的稳定性,因而研究新型有效的锚泊基础已成为海洋工程结构设计中的关键问题之一。文中提出了一种基于海洋软土液化特性的伞状锚,充分利用桩端土体增强抗拔承载能力。应用二维颗粒流分析程序,对该新型伞状锚的安装、抗拔承载能力进行了数值模拟,并与普通锚桩进行比较分析,验证其有效性。针对伞状锚与普通锚桩在拉拔过程中的土体破坏机制,从细观角度分析了其抗拔承载能力的提高机制。研究结果表明,对于相同抗拔锚泊设计竖向承载要求,伞状锚所需材料可大为减少,安装难度明显减低,是值得推广应用的新型锚泊基础形式。 相似文献
15.
Abstract In the field of ocean engineering, anchors are used for several purposes. This article studies the behavior of a helical anchor embedded in soft marine clay under vertical repetitive loading. Helical anchors are simple steel shafts to which one or more helical plates are attached at regular intervals. The tests are conducted on a model helical anchor installed in a soft marine clay bed prepared in a test tank. Repetitive loading is applied using a pneumatic loading arrangement. Different cyclic load ratios and time periods are adopted. In each test, after the application of repetitive loading, poststatic‐pullout tests are conducted to observe the effect of repetitive loading on anchor behavior. From the test results, it is found that, up to a cyclic load ratio of 55%, there is no reduction in capacity. Instead, there seems to be a marginal increase in capacity and reduction in displacement. The reasons for this behavior are explained in terms of induced changes in strength and deformation behavior of marine clay under repetitive load. However, at higher cyclic load ratios, there seems to be reduction in pullout capacity of the anchor, and the reason for this is explained in terms of strain criteria. From this investigation, it can be concluded that the deep anchor is more suitable to a marine environment than a shallow anchor. 相似文献
16.
重力锚锚固是一种常见的锚固形式。为了能够提供足够的水平承载力,传统型式的重力锚普遍比较笨重,在上拔回收时会产生较大的竖向吸附力,不利于重复利用。针对此问题,设计了一种新型铰接式重力锚,并阐述了其铺设与回收方案。其次采用有限元方法对其整体强度进行了校核,结果均符合规范。最后基于模型试验,对铰接式重力锚在黏土中的运动过程进行了研究,进而确定了其在黏土中的水平承载力和回收时的上拔力。结果表明：相较于传统重力锚,新型铰接式重力锚在确保水平承载性能的基础上,能够大幅减小上拔力,从而有效地降低铺设和回收作业的难度,且可适应多种海底土质,但该锚型仅适用于悬链线式系泊系统。相关研究结果可为实际工程中铰接式重力锚的设计提供参考。 相似文献
17.
Suction caissons are widely used to support offshore fixed platforms in coastal areas. The loadings transferred to suction caissons include the eccentric lateral force induced by waves and self weight of the platform structure. However, under this kind of combined loading conditions, the failure mechanism of caissons with shallow embedment depths is quite different from conventional deep foundations or onshore shallow footings. The behaviour of caissons subjected to combined loadings may be described with the "failure locus" in force resultant spaces. Here the failure loci of smooth caissons are studied by use of finite element approach, with the embedment ratio of caissons varying in the range of 0.25～1.0 and eccentricity ratio of horizontal loadings in 0～10. The platform settlement and tilt limits are involved into determination of failure loci, thus the platforms can avoid significant displacements for the combined loadings located inside the failure locus. Three families of loading paths are used to map out the locus. It is found that the shape of failure loci depends on 3 non-dimensional parameters, and the failure locus of a given caisson changes gradually from the elliptical curve to hooked curve with increasing shear strength of soil. The lateral capacity of short caissons may be enhanced by vertical forces, compared with the maximum lateral capacity of long caissons occurring at the vertical force being zero. The critical embedment ratios partitioning elliptical and hooked loci are proposed. 相似文献
18.
1 .IntroductionSuctioncaissons have been widely usedfor offshore oil exploration duetothe advantages of econo-my and simple installation over traditional piles (Huanget al .,2003) .For tensionleg platforms andspar platforms in deep ocean,suction caissons … 相似文献
19.
Gravity installed anchors (GIAs) are the most recent generation of anchoring solutions to moor floating facilities for deepwater oil and gas developments. Challenges associated with GIAs include predicting the initial embedment depth and evaluating the keying performance of the anchor. The former involves high soil strain rate due to large anchor penetration velocity, while the later influences the subsequent behavior and pullout capacity of the anchor. With the coupled Eulerian–Lagrangian method, three-dimensional large deformation finite element models are established to investigate the penetration and keying of GIAs in non-homogeneous clay. In the penetration model, a modified Tresca soil model is adopted to allow the effects of soil strain rate and strain softening, and user-defined hydrodynamic drag force and frictional resistance are introduced via concentrated forces. In the keying model, the anchor line effects are incorporated through a chain equation, and the keying, diving and pulling out behaviors of the anchor can all be replicated. Parametric studies are undertaken at first to quantify the effects of various factors on the performance of GIAs, especially on the penetration and keying behaviors. Based on the results of parametric studies, fitted formulae are proposed to give a quick evaluation of the anchor embedment depth after the installation, and the shackle horizontal displacement, shackle embedment loss and anchor inclination at the end of the keying. Comparative studies are also performed to verify the effectiveness of the fitted formulae. 相似文献
20.
The properties of marine sediments vary spatially, and the undrained shear strength of marine clay increases linearly with depth because of depositional processes and the effective overburden pressure. To evaluate the stability of submarine slope considering the spatial variability of soil strength, the random field discretized by the Karhunen-Loève expansion is combined with the limit equilibrium method to conduct reliability analysis. For simplicity, our physical model does not include many complexities such as the effects of excess pore water pressure on the stability of submarine slopes. Stability estimates of the infinite slope model, under both static and seismic loading, are made with three types of one-dimensional stationary or non-stationary random fields. The two-dimensional slope model is also analyzed, where the shear strength varies with the positions of the strips because of the discrete random-field function for the soil material. In submarine slope reliability analysis, the non-stationary random field of the linearly increasing soil strength is used, instead of the commonly used stationary one. To obtain the failure probability through Monte Carlo simulations, a novel response surface method based on Gaussian process regression is introduced to build the surrogate model. The computational efficiency is significantly increased, because there is a considerable reduction of calls of the deterministic analysis. Therefore, the proposed method makes the prediction of submarine landslides which are usually rare events with very small probabilities more efficient. 相似文献