theA Research Articles

Multi-scale pullout behaviour of strip anchor plates embedded in marine hydrate bearing sediments

Anchor plates are widely used in offshore platform infrastructure due to their great pullout bearing capacity. With the increasing extraction activities of oil and gas resources in sub-marine environments, strip anchor plates are likely to be embedded inside existing hydrate bearing sediments, which will inevitably change its pullout behaviour and bring hidden safety risks to the infrastructure. This paper establishes both DEM models and FEM models of the interaction between a strip anchor plate and the marine hydrate bearing sediments. The mechanical responses of the strip anchor plate subjected to vertical pullout loads are investigated under the conditions of different hydrate saturations and embedded depths. The failure modes obtained by both numerical methods are analyzed and discussed from both the macro and micro perspectives. The results show that the presence of hydrate significantly improves the pullout bearing capacity of the strip anchor plate. In the DEM simulations, the hydrate bearing sediment presents a brittle failure and develops shear bands which extend from both side-edges of the anchor plate to the ground. In the FEM simulations, hydrate bearing sediments are controlled by the ductile failure mode. The angle between the shear band and the vertical direction in the hydrate bearing sediment is controlled by the friction angle of the sediment.

Consolidation effects on uplift capacity of shallow horizontal plate anchors in dilating sand

This paper examines the effect of consolidation on a shallowly embedded horizontal plate anchor in medium dense and dense sand using centrifuge tests, where the loading rate was varied over four orders of magnitude. The experimental results show a 4·8 to 5·5 times increase in anchor capacity as the consolidation condition changes from drained to undrained, driven by a steadily increasing negative excess pore pressure with increasing loading rate. At the highest loading rate, under undrained conditions, the measured maximum negative excess pore pressures reach a steady limit, suggesting the occurrence of cavitation. This increase in anchor capacity with dimensionless velocity is captured using a ‘backbone curve’ interpretation framework that describes the change in capacity between the limiting values of drained and undrained anchor capacity. Calculation of drained anchor capacity is straightforward relative to the more challenging problem of calculating undrained capacity, particularly during cavitation. This was addressed separately through a numerical parametric study (pure undrained analysis) using a bounding surface soil model involving different water depths (cavitation potential), densities and embedment ratios. The numerical results are then synthesised into a simple extended analytical solution to allow estimation of undrained anchor capacity under different densities and water depths.

Pull-Out Mechanism of Horizontal and Inclined Plate Anchors in Normally Consolidated Clay

As most existing experimental studies on plate anchors were carried out in uniform clay, a centrifuge model study is presented in this paper to investigate the pull-out behaviour of plate anchors in normally consolidated clay, which is not uncommon in offshore seabed. Horizontal and inclined anchors with different embedment depths and aspect ratios (length to width) are considered. The soil movement pattern around the plate anchor is evaluated from high-resolution photographs taken during the tests employing the Particle Image Velocimetry technique. The separation mechanism at the plate-soil interface is hence identified. The significant contribution of suction towards the ultimate pull-out capacity of a plate anchor is quantified by monitoring the soil resistance and the pore pressure beneath the anchor base under undrained condition. By comparing the pull-out responses of horizontal and inclined anchors, the effect of anchor inclination on the anchor capacity and failure mechanism is evaluated.

Experimental and numerical investigations on a novel plate anchorage system to solve FRP debonding problem in the strengthened RC beams

Method of externally bonded reinforcement (EBR) is one of the most prominent systems for retrofitting reinforced concrete (RC) beams containing fiber-reinforced polymers (FRP). Many studies conducted in this field confirm the fact that the majority of the beams strengthened in this way experience failure due to the sudden detachment of FRP sheets from the concrete substrate. In this regard, an innovative plate anchorage technique was presented to solve the FRP debonding problem. In this method (EBR by Welded Steel Plates), two plates are placed near the beam supports, which are connected by welding to a longitudinal tension reinforcement through the angles. FRP sheets are placed on seat plates at both ends and fixed in their position by another plate placed over them. Ten RC beams with 150 × 200 × 2000 mm were prepared from concrete in 21 and 35 MPa strength classes. All specimens were subjected to a quasi-static four-point bending test. A comparative study was also done using an analytical method and comparing experimental results in the literature. This research results revealed that the better and more practical performance of the suggested plate anchorage system resulted in postponing or completely solving the FRP surface debonding problem.

Effect of carbon fiber reinforced polymer (CFRP) laminates on behaviour of flexural strength of steel beams with and without end anchorage plates

Three steel beams were made-up and tested to study the effect of CFRP (Carbon Fiber Reinforced Polymer) laminates with and without end anchorage plates on the behavior of flexural strength of steel beams. Tested steel specimens are equal in clear span length, gross sectional area, and strengthened by intermediate stiffeners. Three types of specimens were made, the first specimen was considered as a reference one, while the other two were reinforced by CFRP laminates with and without end anchorage plates respectively. From the experimental outcomes, it was found that, the load deflection curve and load strain responses for specimens strengthened by CFRP strips became stiffer in comparison with the reference beam. On the other hand, it was found that, the deflection values for strengthened beams by CFRP without and with end anchorage plates were decreased to 58.33% and 79.95%, while, the strain values were decreased to 63% and 85.55% respectively, in comparison with the reference beam at yielding stage at load (Py = 20 kN) while, at failure stage at load (Pu = 45 kN) for reference beam, the deflection values for strengthened beams by CFRP without and with end anchorage plates were decreased to 81.19% and 88.13%, while, the strain values were decreased to 88.6% and 97.14% respectively, in comparison with the reference beam. Also, the percentages of increasing in yielding -load for strengthened beams by CFRP without and with end anchorage plates were increased to 100 % to 225 %, while, the percentages of increasing in ultimate-load for strengthened beams by CFRP without and with end anchorage plates were increased to 38.88 % to 111.11 %, respectively, in comparison with the reference. Finale the failure mode for reference beam was flexural failure type (plastic hinge), while, the failure mode for strengthened beams by CFRP without and with end anchorage plates was sudden; because of the de-bonding at the end of the CFRP strips and at the end of carbon fibre that due to the stress concentration at the ends of the CFRP.

NSPFEM2D: A lightweight 2D node-based smoothed particle finite element method code for modeling large deformation

The study presents a lightweight code that implements 2D node-based smoothed particle finite element method (NSPFEM) to fulfill the requirement of modeling large deformation in a variety of research fields and engineering applications. NSPFEM discretizes the problem domain with a set of Lagrangian nodes and employs Delaunay triangulation to build/re-build mesh on the nodes at each time step. It is therefore able to tackle large deformation without suffering from mesh entanglement. The method also adopts the nodal integration technique to avoid interpolation of material variables after remeshing which can eliminate interpolation-induced errors. The code has been built using hybrid C++ and Python programming, and is thus efficient and versatile to couple with other well-developed codes, such as MFront for the incorporation of various constitutive models and YADE, a discrete element method code, for multiscale simulations. The implementation of NSPFEM has first been validated using two examples, i.e., axial vibration of an elastic cantilever beam and a rolling cylinder down incline. Next, the capability and flexibility of the code has been demonstrated by three examples, namely retrogressive slope failure, pullout of a strip plate anchor, and multiscale modeling of trapdoor.

Evaluating the behavior of expandable multi-plate mechanical anchors in granular soils

Many offshore and onshore platforms are constructed widely, especially in the energy supply sector. So far, various anchors with different shapes have been introduced for these applications to reinforce and stabilize the superstructures and related facilities. This study deals with the novel Expandable Multi-plate Mechanical Anchors (EMPMAs) and investigates their performance using physical modeling tests. Previous studies have focused on the behavior of single anchors, and there is no comprehensive experimental research on the multi-plate anchors, which is the subject of the present study. This research examines uplift capacity regarding the effect of five soil densities for loose, medium, and dense conditions and the vertical distance between two plate anchors from 0.5B to 2B. The obtained results were compared with the single plate anchor with equivalent areas. Based on the results, the highest bearing capacity is related to the EMPMA with an optimum distance of 1B in 80% relative density. In addition, by decreasing the distance between two plates, the relative density becomes a more dominant bearing capacity parameter. Furthermore, the Particle Image Velocimetry (PIV) method results are presented, and a new formula is proposed. Eventually, a comparative study is conducted between the results of PIV and an analytical formula.

An innovative light-weight gravity installed plate anchor and its dynamic performance in clayey seabed

This paper presents a series of 1g tests to investigate the performance of an innovative light-weight gravity installed plate anchor (L-GIPLA) with the aid of a reusable booster both in normally consolidated (NC) and lightly over-consolidated (LOC) soils. A micro-electromechanical systems (MEMS) accelerometer was used to measure the dynamic penetration process of the L-GIPLA with a booster in clay. Various factors affecting the final penetration depth of the anchor were investigated, including the soil strength characterisation, impact velocity, anchor shape, booster weight, and water entrainment effects. Based on the data obtained from the MEMS accelerometer, the hydrodynamic aspects associated with the water drag force and water entrainment effect during the dynamic penetration of the L-GIPLA were analysed. Besides, theoretical framework and energy-based models were proposed to predict the final penetration depth of the L-GIPLA. The excess pore water pressure induced by the dynamic penetration of the L-GIPLA and the subsequent dissipation around the anchor were investigated, hence the affected regions of the soil due to the dynamic penetration could be estimated. Finally, the penetration depth ratio and capacity-to-weight ratio of the proposed L-GIPLA were compared with that of other gravity installed anchors.

Multiscale Modeling of Anchor Pullout in Sand

AbstractPullout of plate anchors from granular sands is investigated using a novel computational multiscale approach. We employ the material point method (MPM) to solve a large deformation boundary...

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.

Undrained vertical pullout capacity of strip plate anchors in two-layered clay

Undrained vertical pullout capacity of strip plate anchors in two-layered clay

Vertical Uplift Capacity of Rectangular and Square Plate Anchors in Granular Soil under Seismic Forces

ABSTRACT Earthquakes have unfavorable effects of reducing uplift resistance of plate anchors. Present work investigates the seismic vertical uplift resistance of shallow buried square and rectangular plate anchors in granular soil by using the upper bound theorem of limit analysis in combination with pseudo-static approach. The influence of shear transfer at the interface between anchor plate and overlying soil has been taken into account. The uplift resistance was evaluated in terms of dimensionless factors due to component of soil weight and surcharge pressure on the ground surface for varying combinations of parameters involved in the present analysis. Results revealed that seismic uplift factors increase continuously with increasing soil friction angle and embedment ratio of anchors; whereas, the same reduce with increasing acceleration coefficients, shear factor and aspect ratio of anchors. A significant reduction was observed for higher shear factor and earthquake acceleration coefficients, especially for lower soil friction angle and embedment ratio, and for higher aspect ratio of anchors.

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.

Performance improvement of vertical anchor plates in sandy soil by using geogrid reinforcement

The stabilization of an anchoring system plays a significant role in geotechnical engineering structures. To develop a clear understanding of the vertical plate anchors behavior due to geogrid reinforced soil under various pullout load inclination ranging from (0, 25, 45, 60, and 90) degrees in Cohesionless soil, a series of total (34) tests were carried out. The soil has been investigated by using small-scale model tests. Geogrid with different width and heights placed (vertically or horizontally in front of the vertical anchor plate at different locations were used to reinforce a soil mass in front of square anchor plates. The studied parameters include reinforced soil mass width, soil mass height, the location of geogrid-reinforced soil mass, effect of water content increase, and the inclination angle of pullout load. The test results indicate that Geogrid reinforced mass in front of a vertical anchor plate significantly increases the stiffness of the soil and increases the pullout resistance of shallow anchor plates. According to test outcomes, critical situations also were stated and discussed.

Influence Depth of the Tensile Capacity of Ground Anchors Folding Type in Cohesive Soils

A ground anchor is used to withstand tensile stresses caused by loading both the natural load of the soil and external pressures, then passing the force into the ground. It is generally used to retain tensile form and lateral power structures in constructing walls, foundations, transmission towers, pile, floating docks, mooring dolphin, and offshore buildings. Various ground anchors have been widely used, such as drag, helical, circular-shaped plates, and squares. Especially for massive plate anchors, installation is complicated because it must be excavated first before installation. A challenge in engineering is especially on cohesive soil conditions (soft soil). With these considerations, it is necessary to develop innovative models that are easier to install but still have a large enough tensile capacity. This research analyzes the tensile capacity by using ground anchor folding type to overcome building problems on cohesive soil. The test was tensile capacity with four leaves and a Variation of depth-the performance of this folding type anchor based on the tensile capacity and the anchor. The results obtained in this study; the greater area of the element occurred a significant increase in tensile capacity.

Ultimate pullout capacity of single vertical plate anchors in sand

This paper presents a limit equilibrium solution for calculating the ultimate pullout resistance of shallow vertical plate anchors in sand, considering the combined effects of anchor embedment ratio, sand shear strength, sand-anchor interface strength and anchor aspect ratio. The geometry of the failure mechanism in the solution reflects observations from model tests. The ultimate failure load of a strip plate anchor was then determined by global equilibrium analysis of the passive and active failure soil wedge. The two-dimensional limit equilibrium solution is extended for rectangular plate anchors by introducing the shape factors of Ovesen and Strømann. The solution is compared with a database of 102 pullout model tests of vertical plate anchors in sand assembled from the literature and some limit analysis results. The comparison results suggest that the proposed solution can provide a reliable analytical tool for the ultimate pullout capacity prediction of shallow vertical plate anchors buried in sand.

Optimisation of the padeye location for dynamically embedded plate anchors

Dynamically embedded plate anchors (DEPLAs) are a relatively new anchor type that combine the advantages of dynamically installed anchors and conventional plate anchors. Through three dimensional finite element modelling, this paper first derives the combined loading yield surface of the anchor holding capacity in clay and then uses this yield surface in plasticity analyses to study the anchor trajectory under chain loading. These analyses reveal that the anchor padeye in the current DEPLA design is not optimally located as the anchor will eventually pull out of the seabed when loaded beyond its design capacity. It also shows that there are alternative padeye locations that lead to an equilibrium condition, where the DEPLA attains an ultimate holding capacity. The plasticity analyses are supplemented by simplified closed-form equations that are used in a parametric study to calculate the ultimate DEPLA holding capacity for various padeye positions. The optimal padeye location from the analyses is at a location that has a padeye offset ratio (ratio of the normal to the parallel distance between the padeye and the centre of rotation of the plate) in the range 0.55–0.65, varying slightly with chain loading angle.

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.

An Experimental Study on heave and Uplift Behaviour of Granular Pile Anchor Foundation System

Several innovative ground improvement techniques were presented by various researchers throughout the globe for modification of the existing poor soils. Granular Pile-Anchors (GPA) were found to be more promising in resisting the heave behavior of the expansive soils in recent times. The engineering behavior of the soil can be improved by this technique apart from resting the swell. This technique is a modification on the existing granular pile technique with an anchor plate beneath the granular pile that is connected to the foundation by means of an anchor rod or cable. Numerous scale model tests were discussed in this paper to understand the heave and pull-out behavior of Granular Pile-Anchor Foundation (GPAF) system. The effect of spacing was studied by using two granular piles at varied spacing. It was observed that the swell potential was reduced to about 68.09% for 50mm dia GPA, compared to the virgin expansive soil. The swell potential was found to be further reduced at twice the diameter of the pile spacing. The pull-out capacity was found to be increased by about 393% by providing a 50mm diameter GPA in the expansive soil.

Bearing capacity of plate anchors subjected to average and cyclic loads in clays

Plate anchors has been widely adopted to provide uplift resistance for offshore floating facilities. The research on the bearing capacity of plate anchors under cyclic loads is still relatively rare compared with the monotonic loads. In the present paper, the bearing capacity of plate anchors under the combination of average and cyclic loads in normal consolidation clays with linearly increasing shear strength is investigated by performing finite element analysis. A more reasonable expression is proposed to determine the cyclic loading factor Ucy by modifying the formula suggested by Det Norske Veritas. A global cyclic loading factor Ucy∗ is defined to determine the cyclic anchor resistance considering the difference of average stress level for each soil element around the anchor. The method to determine Ucy∗ by finite element analysis is suggested, and then the Ucy∗ for any given normalized average loads and the equivalent number of cycles is determined. The effects of the embedment depth and inclination angles on Ucy∗ are also investigated by performing a series of numerical simulations. An expression of Ucy∗ associated with normalized average loads and the equivalent number of cycles are proposed based on numerical results, which can provide some reference for the engineering design of plate anchors under cyclic loads.