Anchors In Clay Research Articles

Calibration and implementation of a design tool for drag anchors in clay

Abstract Drag anchors are considered as one of the most applicable anchor solutions for floating offshore wind as they can be used for a wide range of soil conditions. For feasibility studies of floating offshore wind farms different anchor concepts needs to be compared in order to select the most proper anchor solution. To do so, the level of details in the analysis for the various anchor types must be similar. Currently design charts are adopted as a simple design method for drag anchors. However, design charts are developed for specific soil conditions, and do not account for site specific soil conditions. To have similar precision, as analytical solutions for suction anchors, a simple design methodology that can account for site specific soil conditions for drag anchors must be adopted. This paper presents an example for how a simple analytical solution can be calibrated against design charts, which makes it possible to account for site specific soil conditions, resulting in a better basis for comparing early stage drag anchor design to other anchor solutions.

Holding capacity of suction anchors under combined loading

This paper introduces a numerical investigation aimed at evaluating the Vertical, Horizontal, and Moment (VHM) yield surface of suction anchors. It employs a generalized elliptical function guided by a predefined fitting strategy. The resulting yield surfaces are then juxtaposed with findings from previous studies, establishing the groundwork for the macro model of suction anchors in clay. This model plays a pivotal role in the integrated analysis of mooring systems for floating wind turbines.

Effect of padeye offset ratio on keying behaviours and capacity of square plate anchors in clay

This paper reports the effect of padeye offset ratio on the keying behaviours and capacity of square plate anchors in clay. The investigation is conducted through three-dimensional large deformation finite element analyses using the Remeshing and Interpolation Technique with Small Strain in ABAQUS. Strain rate dependency of the undrained shear strength and soil remoulding are accounted for adopting an extended Tresca model. The results from LDFE analyses are compared with existing analytical solutions, centrifuge test data, and numerical results; with reasonable agreement obtained. Parametric analyses are then carried out varying several critical factors including initial embedment depth, load eccentricity, padeye offset ratio, soil sensitivity, and soil strength heterogeneity. It is seen that the padeye offset has significant influence on the ultimate loss of embedment and capacity under vertical loading. To assess the keying behaviour of square plate anchors in clay, an equivalent soil strength incorporating strain rate dependency and remoulding is adopted. A combined factor defined as a function of the initial embedment depth and equivalent soil strength is used to estimate the ultimate loss of embedment. An equivalent ultimate bearing capacity factor is shown to be a function of the initial embedment depth, padeye offset ratio, and load eccentricity.

Loading Rate Effect on the Pullout Capacity of OMNI-Max Anchors in Clay Coupled With Multiple Factors

Abstract As the latest development of gravity installed anchors (GIAs), the OMNI-Max anchor has drawn much attention from worldwide due to its unique behavior in the seabed. The pullout capacity of OMNI-Max anchors is a key index in engineering. However, most of the relevant studies were carried out under a quasi-static condition, which do not actually meet the installation and operation requirements. In practice, the anchor may be subjected to both long-term and short-term sharp loading during mooring. As an important environmental variable, it is essential to evaluate the effect of loading rate on the pullout capacity. Since the bearing capacity of OMNI-Max anchors is affected by many factors, it is also essential to explore systematically the coupling effects of the loading rate and other factors, including the anchor embedment depth, the anchor orientation, the bearing area, the loading angle, and the soil strength. Based on the coupled Eulerian–Lagrangian (CEL) technique, numerous analytical cases are designed and calculated by the large deformation finite element (LDFE) method. The loading rates span four orders of magnitude from the quasi-static velocity to 10 m/s (about one anchor length per second), covering a wider range in pulling out of GIAs. The end-bearing capacity factor changes remarkably with the pullout velocity for OMNI-Max anchors, and the increase can even reach more than twice of that in a quasi-static condition. As a result, a succinct explicit expression is constructed in terms of the loading rate and multiple factors, which can be effectively utilized to calculate the end-bearing capacity factor of OMNI-Max anchors in clay under complex conditions.

Applicability analyses on the end-bearing capacity of gravity installed anchors in clay

As an efficient solution for deepwater moorings, gravity installed anchors (GIAs) have many attractive attributes and play a remarkable role in various deepwater applications. The end-bearing capacity factor offers an efficient method to evaluate GIA end bearing under complex conditions involving multiple factors. It can also assess total anchor bearing capacity. Based on numerous large deformation finite element (LDFE) analyses, two formulas have been developed to calculate the end-bearing capacity factors for GIAs in clay under quasi-static or dynamic conditions. Through rigorous statistical analysis, it is demonstrated that the formulas provide accurate predictions of the end-bearing capacity for various GIAs, particularly those with loading characteristics similar to OMNI-Max anchors, i.e., the padeye location and the loading angle. The contribution of the end bearing to the overall bearing capacity of GIAs is analyzed under various loading conditions. Furthermore, a comparative study proves that the formula accounting for the loading rate effect can be degenerated to evaluate the end-bearing capacity factor of the anchors under quasi-static conditions. The results provide a comprehensive understanding of the end bearing and offer an efficient solution for determining the total bearing capacity of various GIAs in clay under complex conditions. These findings contribute to the advancement of design and performance assessment of GIAs, enhancing their reliability and effectiveness in applications.

Inclined Pullout Capacity of Suction Anchors in Clay over Silty Sand

This study presents a range of numerical modeling to evaluate the pullout capability of suction anchors subjected to inclined mooring loading in a clay-over-silty-sand deposit. Comprehensive numerical models were generated, which were validated against existing centrifuge testing and numerical modeling results. A parametric study was then carried out to assess the effects of the anchor length, padeye location, mooring angle, and layered deposits on the pullout capacity of suction anchors. According to the numerical simulation results, the following key design factors were obtained: (1) the optimal padeye position, (2) the ultimate horizontal and vertical pullout abilities, and (3) the horizontal load distributions at different padeyes. A simple framework of calculating the combined VH surfaces of the anchors was developed for preliminary prediction of the pullout capacity of suction anchors in a clay-over-silty-sand deposit that provides the optimized location of padeyes and more accurate prediction for the holding capacity of anchor. The design method proposed by this study can contribute to optimize and improve the current design guideline for more accurately predicting the bearing capacity of anchors in clay-over-silty-sand soil deposits.

Research on Bearing Characteristics of Gravity Anchor in Clay

The applications and studies of gravity anchors in the ocean are becoming more and more extensive. Most of the research, however, has been directed toward the bearing properties of sand. Relatively less attention has been paid to the bearing properties of gravity anchors in clay. Clay is widely distributed on the seabed. The research on the bearing capacity of gravity anchors in clay is of great significance for offshore oil exploitation. Therefore, the gravity anchor was investigated by conducting reduced-scale model tests, and the bearing process of gravity anchors in clay was simulated through a 3D finite element method. Model tests and numerical simulations were used to determine the capacity curve and the V-H failure envelope of gravity anchors in clay. The simulation results and the test results are in good agreement. The failure form of the gravity anchor in clay was revealed by 3D finite element analysis. The effect of cohesion, internal friction angle, and mooring point height on bearing capacity have been studied. The influence of the height of the mooring point on the V-H failure envelope curve was explored by changing the height of the mooring point. The formula of the V-H failure envelope curve suitable for different mooring point heights was obtained.

The failure induced by embedment loss of gravity installed anchors in clay

The reliability of mooring systems is a critical issue for deepwater floating structures. Gravity installed anchors (GIAs) have been effectively employed in a significant number of mooring systems, whose embedment loss and kinematic behaviors under the cable loading conditions also attract attention to the reliability of the anchors during service. Particularly, the embedment loss of the anchor may result in the failure of the anchor. The present work focuses on systematically investigating the failure state of anchors in clay induced by embedment loss under various complex conditions. A theoretical model for analyzing the comprehensive behaviors of the anchor is constructed, which is validated by previous centrifuge tests and 1g model tests. A rational criterion for determining the state of failure induced by embedment loss is proposed and applied to the analytical results of the theoretical model. The factors that have a substantial impact on embedment loss of the anchors, including the anchor embedment depth, anchor orientation, cable loading angle at the seafloor, soil adhesion factor, soil strength, and cable properties are investigated in the analytical cases. OMNI-Max anchors, being one of the most remarkable GIAs and having complex kinematic behaviors, are selected to reveal the influences of these factors on the failure state of the anchors. Based on the results of numerous analytical cases, the failure surfaces in the three-dimensional spatial data field are presented, which reveal the critical state of the anchor under different conditions. Furthermore, these failure surfaces are stated using a surface function. The function can be used as a simple judgment condition for the state of failure owing to anchor embedment loss. The findings of earlier investigations support the applicability of the surface function. In addition, the promising application of the surface function in practice is highlighted.

Enhancing Pullout Load Capacity of Helical Anchor in Clay with Adjusted Load Application Point under Inclined Loading Condition

Enhancing Pullout Load Capacity of Helical Anchor in Clay with Adjusted Load Application Point under Inclined Loading Condition

Theoretical analysis of hall anchor dragged in clay

A theoretical model is proposed in this paper to analyze the kinematic trajectory and rotation of the dragged hall anchor in clay. A series of laboratory model tests are conducted using a hall anchor model dragged in kaolin to verify the accuracy of the proposed theory. Fair agreements between the results obtained from laboratory model tests and those calculated from the theoretical model are achieved with differences of less than 10.5%. Parametric studies are conducted to investigate the performances of three prototype anchors dragged at different speeds in the clay with different undrained shear strengths. The results show that the soil strength and dragging speed greatly influence the behaviors of dragging anchors. The normalized maximum drag force versus soil strength curves of the three prototype anchors are quite uniform which gives curve-fitted equations with high R2 for the design of the maximum penetration depth and dragging capacity.

The diving and keying capacities of OMNI-Max anchors in clay

Different to the conventional gravity installed anchors (GIAs), the OMNI-Max anchor is capable of deriving higher capacity through the diving behavior (obtaining deeper embedment depth) after the keying process (usually accompanied by the loss of embedment depth) subjected to excessive loading, due to its three-dimensional structure and the various way to bear applied loading. The capacity properties of the anchor during keying and diving not only determine the keying and diving behaviors of the anchor, but also influence the other anchor behaviors, such as the embedment loss, pulling out and kinematic trajectory. Existing knowledge of the diving and keying behaviors is rather rare, stimulating further exploration of diving and keying capacities for OMNI-Max anchors. A systematic and quantitative study is performed on the complex capacities of OMNI-Max anchors during diving and keying in clay under multiple factors, including the anchor embedment depth, the anchor orientation, the interface friction, the loading direction, the bearing area of the anchor, the rotational center of the anchor, and the property of soil. Numerous cases of large deformation finite element (LDFE) analyses are conducted by employing the technique of coupled Eulerian-Lagrangian (CEL), through which the impacts of different factors can be analyzed. The position of the rotational center affects most the moment capacity factor among all the factors, while the anchor orientation has only a tiny influence on the moment capacity factor. The minimum value of the moment capacity factor results from the rotational center located at approximately one-third to half of the shaft length away from the anchor rear. The end bearing during diving when the movement is parallel to the shaft, is about one-quarter to one-third of that during pulling out when the movement is perpendicular to the shaft. Explicit expressions of bearing capacity factors composed of multiple factors are eventually formulated, which are validated by orthogonal tests and published data, and can be used conveniently not only to quantify the effects of different factors but also to evaluate the diving and keying capacities of OMNI-Max anchors.

Innovative Booster for Dynamic Installation of OMNI-Max Anchor in Clay: Numerical Modeling

Abstract An innovative booster is proposed with the aim of increasing the final penetration depth of the OMNI-Max anchor in the clayey seabed with high strength gradient. The booster is attached to ...

Effect of Keying-Induced Soil Remolding on the Ultimate Pull-Out Capacity and Embedment Loss of Strip Anchors in Clay

AbstractIn this paper, the keying of strip anchors in clay has been studied by employing a large deformation finite-element methodology coupled with a soil constitutive model that takes strain rate...

Vertical Uplift Capacity of Circular Anchors in Clay by Considering Anisotropy and Non-Homogeneity

Vertical Uplift Capacity of Circular Anchors in Clay by Considering Anisotropy and Non-Homogeneity

Numerical investigation of inclined plate anchors in clay with linearly increasing undrained shear strength

Plate anchors have been increasingly used to provide a mooring system for offshore floating facilities due to their relatively high efficiency and low costs. The analysis of anchor behaviour can be usually divided into two categories “immediate breakaway” and “no breakaway”, which correspond to the “vented anchors” and “attached anchors”. Most of the previous literatures focus on these two extreme cases: vented anchors in clays without weight and attached anchors. However, the vented anchors in clays with weight, as a most common state, are in the intermediate state of the two extreme cases. At the cases, the change law of the pullout capacity of plate anchors with embedment depth, inclination angles and overburden pressure is relatively complex and needs further study. This paper investigates the pullout behaviour of inclined strip plate anchors in clays with linearly increasing shear strength by performing a series of finite element analysis, and clarify the change law of the pullout capacity and the failure mechanism of plate anchor in the intermediate state. Two new formulas are proposed to predict the pullout factor for inclined vented and attached anchors. Related research results can provide a reference for the engineering design of inclined plate anchors in undrained clay.

A unified explicit formula for calculating the maximum embedment loss of deepwater anchors in clay

Deepwater anchors, typically the suction embedded plate anchor (SEPLA) and the OMNI-Max anchor, would encounter the phenomenon of embedment loss after initial penetration, which remarkably reduces the pullout capacity and affects the subsequent behavior of the anchor. Therefore, the maximum embedment loss (MEL) becomes a key index for design and engineering practice. Based on the mechanical mechanism and kinematic property of anchors, a unified explicit formula for calculating the MEL of SEPLAs and OMNI-Max anchors in clay, in terms of the anchor geometry, interactional property, soil condition, anchor line property and applied loading, is developed by combining theoretical models and nonlinear fitting analyses. The efficiency and precision of the formula are validated by experiments and other available methods. Compared to existing methods, the proposed formula provides a simple tool for fast assessing the MEL of both SEPLAs and OMNI-Max anchors in clay, and performs superiorly in easiness, efficiency and applicability.

The pullout capacity of OMNI-Max anchors in clay

Being an impressive representative of gravity installed anchors (GIAs), the OMNI-Max anchor has many attractive attributes and becomes more and more a conspicuous role in recent deepwater applications. However, the pullout capacity property of OMNI-Max anchors has not been well understood due to their three-dimensional structure and the loading way in variational seabed conditions. Full knowledge of the pullout capacity property is beneficial to understand not only the working performance but also the comprehensive kinematic behaviors of OMNI-Max anchors in the seabed. The present work aims to investigate the complicated pullout capacity property of OMNI-Max anchors in clay under multiple factors, including the anchor embedment depth, the anchor orientation, the bearing area, the loading angle, and the soil strength. Large deformation finite element analyses are performed using the coupled Eulerian-Lagrangian (CEL) technique. Numerous cases of multiple factors are designed and analyzed to acquire the effects of different influential parameters. An explicit expression of the end-bearing capacity factor of OMNI-Max anchors is finally formulated in terms of multiple factors, whose effectiveness and applicability are validated by orthogonal tests and available published data. This expression can be simply utilized to evaluate the pullout capacity of OMNI-Max anchors under complicated conditions.

Capacity of plate anchors in clay under sustained uplift

The use of plate anchors on permanent offshore facilities with taut or semi-taut moorings requires a thorough understanding of their long-term capacity under sustained loading. In this paper, a programme of centrifuge tests and numerical simulations is reported to investigate the capacity of a plate anchor deeply embedded in normally consolidated clay during and after vertical sustained loading. Experimental data show that a sustained load higher than 85% of the ultimate monotonic anchor capacity leads to anchor failure, characterised by acceleration of anchor movement with a large accumulation of vertical displacements towards the soil surface. At sustained loads lower than 85% of the monotonic anchor capacity, the undrained anchor capacity increases due to consolidation and hence strengthening of the soil mobilised by the anchor. The maximum increase in capacity relative to the monotonic capacity is 23% when the sustained load is approximately half the monotonic anchor capacity. A gap-filling technique was proposed and incorporated into large deformation finite element analyses, which are shown to reproduce the experimental observations well.

Lateral Undrained Capacity of a Multiline Ring Anchor in Clay

Abstract Offshore wind energy is an attractive alternative in pursuing the nation's clean energy goals due to the significant demand for electricity in the coastal areas of the United States. Locat...

On the Dragging Trajectory of Anchors in Clay for Merchant Ships

Anchor dragging is a major threat to the structural integrity of submarine pipelines. A mathematical model in which the mechanical model of chain and the bearing model of anchor were coupled together. Based on the associated flow rule, an incremental procedure was proposed to solve the spatial state of anchor until it reaches the ultimate embedding depth. With an indirect measurement method for the anchor trajectory, a model test system was established. The mathematical model was validated against some model tests, and the effects of two parameters were studied. It was found that both the ultimate embedding depth of a dragging anchor and the distance it takes to reach the ultimate depth increase with the shank-fluke pivot angle, but decrease as the undrained shear strength of clay increases. The proposed model is supposed to be useful for the embedding depth calculation and guiding the design of the pipeline burial depth.