Suction Embedded Plate Anchors Research Articles

Numerical modelling of the keying process for a suction embedded plate anchor in spatially varying clays

ABSTRACT As the exploration of oil and gas moves into deeper water, suction embedded plate anchors (SEPLAs) are developed to answer the growing demand for anchor-mooring systems. The keying process of SEPLAs has been extensively reported assuming a spatially homogeneous soil which is a departure from reality. This study conducts a comprehensive investigation of the keying process of SEPLA in spatially varying clay through a large deformation random finite element method coupled with the Monte Carlo simulations. The numerical model is validated with the results from the existing centrifuge test and numerical analysis. The spatial randomness of soil strength is assumed to be a three-dimensional (3D) random field to explore the failure mechanism and statistical characteristics of rotational response and pullout behaviour. The normalised factor of the upper 5% fractile of embedment loss is presented as an algebraic expression of the coefficient of variation of soil strength. The relationship between safety factor and failure probability is determined based on the requirement of pullout capacity. Finally, the applicability of the present results to different soil properties is verified.

A Unified Model for Analyzing Comprehensive Behaviors of Deepwater Anchors

Anchors may exhibit various complicated behaviors in the seabed, especially for deepwater anchors including gravity installed anchors (GIAs) and drag embedment plate anchors (drag anchors), stimulating the development of an efficient analytical tool that applies to a variety of anchors. The present paper introduces a unified model for analyzing different anchor behaviors in both clay and sand, consisting of unified concepts, mechanical models, and analytical procedure. The kinematic behaviors of the anchors are classified uniformly as three types, i.e., diving, pulling out, and keying. By utilizing the least-force principle, various anchor properties, such as the ultimate pullout capacity (UPC), failure mode, movement direction, embedment loss, and kinematic trajectory, can all be determined by the combination and analysis of the three behaviors. Applications of the model are demonstrated summarily, by solving the UPC and the failure mode of anchor piles and suction anchors, the kinematic trajectory of drag anchors in a single soil layer or layered soils, the maximum embedment loss (MEL) of suction embedded plate anchors (SEPLAs) and OMNI-Max anchors, and the kinematic behavior of OMNI-Max anchors. Compared to existing theoretical methods, this unified model shows strong applicability and potentiality in solving a variety of behaviors and properties of different anchors under complicated seabed conditions.

Macro-element modelling of suction-embedded plate anchors for floating offshore structures

This work presents a new macro-element model to predict the behaviour of Suction Embedded Plate Anchors (SEPLAs) for floating offshore structures during keying and loading stages. Differently from previously published models for anchors, this new model is characterised by (i) a non-associated plastic potential with the aim of improving the prediction of anchor trajectory for the whole displacement domain and for a large range of padeye offsets; and (ii) by a strain-hardening rule enabling to predict the force and displacement mobilisation from the early stages of the keying process. The model was calibrated against LDFE analyses and compared with a broad set of LDFE and centrifuge tests results. The model proves capable of reproducing anchor rotation and displacement with good accuracy for a wide range of padeye offsets and distinct studies from the literature.

Key Techniques in Simulating Comprehensive Anchor Behaviors by Large Deformation Finite Element Analysis

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated, such as 360 deg rotation of the anchor arm, gravity installation of anchors with high soil strain rate, and keying and diving (or penetration) of anchors. The anchor line connects the anchor and the anchor handling vessel (AHV) or floating moored platform. With moving of the AHV or platform, anchor line produces a space movement, and forms a reverse catenary shape and even a three-dimensional (3D) profile in the soil. Finite element analysis on the behaviors of anchor lines and deepwater anchors requires techniques that can deal with large strains and deformations of the soil, track changes in soil strength due to soil deformation, strain rate and strain softening effects, appropriately describe anchor–soil friction, and construct structures with connector elements to conform to their characteristics. This paper gives an overview of several key techniques in the coupled Eulerian–Lagrangian (CEL) analysis of comprehensive behaviors of deepwater anchors, including construction of the embedded anchor line and the anchor line in the water, installation of gravity installed anchors (GIAs), keying or diving of drag anchors, suction embedded plate anchors (SEPLAs) and GIAs, and implementation of the omni-directional arm of GIAs. Numerical probe tests and comparative studies are also presented to examine the robustness and accuracy of the proposed techniques. The aim of this paper is to provide an effective numerical framework to analyze the comprehensive behaviors of anchor lines and deepwater anchors.

Keying behavior of suction embedded plate anchors with flap in clay

Suction embedded plate anchors (SEPLAs) have been applied extensively in deepwater projects due to their high efficiency and low cost. In this paper, a new plasticity model was developed to simulate the keying behavior of SEPLAs. The model was based on three groups of yield envelopes, which considered three different states between the flap and the main plate. Thus, the plasticity model can consider the flap rotation relative to the fluke. The anchor shank was also considered to investigate the three dimensional (3D) effects of the actual SEPLA shape when deriving the yield envelops. A three dimensional large deformation finite element (3D-LDFE) model with flap rotation and shank considered was also set up. Then 3D-LDFE analysis was carried out to verify the plasticity model. The results given by 3D-LDFE analyses agree well with those by plasticity analyses. The effects of the flap on the embedment loss and pullout capacity, and the effects of loading eccentricity and pullout angle on diving behavior are investigated. Optimal padeye offset and pullout angle to drive the anchor diving are proposed.

Numerical analysis of suction embedded plate anchors in structured clay

As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.

An efficient approach to incorporate anchor line effects into the coupled Eulerian–Lagrangian analysis of comprehensive anchor behaviors

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.

Numerical implementation of the installation/mooring line and application to analyzing comprehensive anchor behaviors

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and pose a great challenge to the analytical methods. In the present work, a large deformation finite element (FE) analysis employing the coupled Eulerian–Lagrangian technique is performed to simulate the installation/mooring line, and then is applied to analyzing comprehensive anchor behaviors in the seabed. By connecting cylindrical units with each other using connector elements, the installation/mooring line is constructed. With the constructed installation/mooring line, FE simulations are carried out to investigate comprehensive anchor behaviors in the seabed, including long-distance penetration of drag anchors, keying of suction embedded plate anchors and non-catastrophic behavior of gravity-installed anchors. Through comparative studies, the accuracy of the proposed method is well examined. A parametric study is also undertaken to quantify the effects of the frictional coefficient, initial embedment depth, and soil weight on the profile of the embedded anchor line and the shackle load. The present work demonstrates that the proposed FE model, which incorporates the installation/mooring line and the anchor, is effective in analyzing the comprehensive anchor behaviors in the seabed.

Incorporating Shank Resistance into Prediction of the Keying Behavior of Suction Embedded Plate Anchors

The suction embedded plate anchor (SEPLA) is a promising deepwater anchoring solution that uses a suction caisson to install a plate anchor vertically to a precise depth within the seabed. The plate anchor is then rotated from its vertical position toward an inclination approximately normal to the load applied by the mooring line in a process known as keying. This configuration reduces the embedment depth and, in typically increasing strength with depth soils, diminishes the installed capacity of the anchor. Therefore, an accurate prediction of the keying behavior of SEPLAs is crucial for offshore operations. Recently, plasticity approaches that combine a yield surface written directly in terms of the applied loads on the anchor with an associated flow rule have been suggested. However, the combined loading yield surface has been defined only for a flat square or rectangular plate. None of these approaches directly account for the resistance of the relatively large anchor shank used to attach the mooring line. The results described in this paper show (1) the impact of the joint plate and shank on the size and shape of the combined loading yield surface and (2) the subsequent impact on predicting the keying behavior of SEPLAs using the plasticity approach. The influence of anchor thickness and shank height on the capacity of an idealized plane-strain anchor is first defined through two-dimensional finite-element (FE) studies. Then, the yield surface parameters for a typical plate and anchor size are provided from three-dimensional FE analysis. The keying behavior of this anchor when subjected to pullout loading from an anchor chain is simulated using the plasticity approach and the results are compared with keying paths for a simple plate anchor without a shank. The predictions using parameters derived from a plate anchor with a shank significantly outperform those using parameters derived from a plate anchor without a shank.

Improving Plate Anchor Design with a Keying Flap

Suction embedded plate anchors have been increasingly applied in deepwater projects because of their efficiency and limited cost. Current industry practice equips the fluke of the anchor with an outwardly rotating keying flap aiming at a reduction of the loss of embedment during keying. However, recent experimental and numerical research has questioned the efficiency of the keying flap currently used in practice because it has demonstrated that the flap does not open during keying as was intended. This paper presents a two-dimensional plain strain large deformation finite-element analysis by assuming the soil as homogenous to investigate the performance of plate anchors equipped with various keying flap designs. The results of the paper agree with previous centrifuge and finite-element analysis that the outwardly keying flap has no effect on the loss of embedment and results in a reduction of the bearing capacity postkeying. The performance of an alternative keying flap design where the flap is rotating inward is also discussed. The new design effectively reduces the loss of embedment during keying and exhibits a bearing capacity higher than the current design.

Large deformation finite element analysis investigating the performance of anchor keying flap

Suction embedded plate anchors, as used in practice, include a hinged keying flap aligned with the main anchor plate (fluke). The flap was designed to rotate away from the shank during keying of the anchor, to minimise loss of anchor embedment and the associated reduction in pull-out resistance. However, it has been observed experimentally that the flap is activated only once keying is nearly completed, and thus does not assist in reducing embedment loss. In this paper, the performance of the keying flap is investigated using a large deformation finite element approach based on frequent mesh regeneration. The moment acting at the flap hinges, determined from the soil pressures and shear forces on the keying flap, is quantified. The moment controlling the flap activation depends mainly on the net soil pressure on the front or back face of the flap, and this prevents activation of the flap while the anchor is rotating significantly. The flap is only activated once the anchor motion becomes dominated by forward translation, normal to the plate. Various factors affecting flap activation and embedment loss are explored, including flap height, eccentricity of the padeye, soil strength profile, initial anchor depth and load inclination.

A plasticity model to assess the keying of plate anchors

Suction-embedded plate anchors (SEPLAs) have been developed to answer the growing need for anchors to withstand significant vertical loading. The concept combines the advantage of suction caissons (known penetration depth and location) and ‘drag-embedded' plate anchors (efficiency and low cost). The main issue associated with SEPLAs relates to the keying process, as the anchor is first loaded, and the associated loss of embedment and reduction in capacity. The paper presents a plasticity model developed to predict the trajectory and load development during anchor keying, and up to peak load. Rigid plasticity is assumed, allowing the kinematics of the anchor to be determined from a yield surface and associated plastic potential. The trajectory and performance of a typical SEPLA are predicted using the model, and are compared with results from centrifuge tests and large-deformation finite-element analysis. The anchor loss of embedment ranged from ∼0·2 to 1·5 times the anchor height for loading inclinations between 40° and 90° from the horizontal. The model was used further to calculate the anchor loss of embedment and capacity for varying padeye offsets. Results indicated that the loss of embedment could be reduced significantly by increasing the offset, but at the detriment of the ultimate anchor capacity.

Behavior of Suction Embedded Plate Anchors during Keying Process

Suction embedded plate anchors (SEPLAs) allow for accurate positioning, thus providing an attractive alternative to traditional drag embedment anchors. This paper presents an analytical model for predicting the behavior of SEPLAs during the keying process, with a specific focus on predicting the loss of embedment depth as the anchor rotates from its initial vertical position to its target orientation perpendicular to the direction of loading. The soil is idealized as an incompressible, rigid-plastic material obeying an associated-flow rule. A generalized plastic limit analysis is employed to estimate the trajectories and corresponding capacities of SEPLAs under different loading conditions. The effects of soil resistance on the shank and anchor interaction with the anchor chain are also considered in the model. The SEPLA design commonly features a hinged flap; the effect of the flap is examined theoretically by comparing the solutions for the SEPLA with and without flap. Predicted solutions are shown in reasonably good agreement with some published numerical and experimental results, supporting the validity of the model.

Three-dimensional numerical analysis of the keying of vertically installed plate anchors in clay

Plate anchors, such as suction embedded plate anchors and vertically driven plate anchors, offer economically attractive anchoring solutions for deep/ultra-deep water offshore developments. The rotation/keying processes of plate anchors will cause embedment losses, which lead to decreases of the uplift resistances of the anchors in normally consolidated soil. In the present paper, the keying processes of vertically installed strip and square plate anchors are simulated using the 3-D large deformation finite element method. The effects of loading eccentricity and pullout angle on the embedment loss during keying are investigated. Both the development of the uplift resistance and the soil flow mechanisms are presented. The numerical results show that the loading eccentricity e/ B has a much larger effect on the embedment loss than the pullout angle does. The anchor shape has a minimal effect on the loss in anchor embedment. The shape factors (square/strip) are 1.05–1.09 for loss of embedment and 1.10–1.19 for capacity.

Influence of the installation process on the performance of suction embedded plate anchors

This paper describes a series of centrifuge tests performed in order to investigate the influence of the installation process on the capacity of a suction embedded plate anchor. A 1/145th reduced scale model suction caisson and plate anchor were used for this purpose, embedded in a normally consolidated clay sample. The full installation and retrieval process was simulated as well as loading of the anchor to failure. Results are compared with similar pullout tests where the anchor was pre-embedded manually in the sample. The keying process and the anchor capacity were also investigated. Results show a loss of performance of the suction embedded anchor immediately following the retrieval of the caisson due to weakening of the clay in the vicinity of the anchor. As the clay regains strength with time, the anchor capacity increases to match those of the jacked anchors. The normalised anchor capacity following consolidation agreed well with theoretical solutions. The loss of anchor embedment during the keying process was observed to be lower for the suction embedded anchors than for the jacked anchors, but both sets of data correlated closely with the load inclination at the anchor padeye.