Undrained Load Capacity of Torpedo Anchors Embedded in Cohesive Soils
01 May 2011-Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme (American Society of Mechanical Engineers)-Vol. 133, Iss: 2, pp 021102
TL;DR: In this article, a numerical analysis of the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model is presented.
Abstract: This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior and the large deformations involved. The torpedo anchor is also modeled with solid elements, and its geometry is represented in detail. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. A number of analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions, and number and width of flukes are considered. The results obtained indicate two different failure mechanisms: The first one involves significant plastic deformation before collapse and, consequently, mobilizes a great amount of soil; the second is associated with the development of a limited shear zone near the edge of the anchor and mobilizes a small amount of soil. The total contact area of the anchor seems to be an important parameter in the determination of its load capacity, and, consequently, the increase in the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.
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TL;DR: In this paper, a series of model tests were conducted to provide insight into the behavior of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt.
Abstract: Challenges associated with dynamically installed anchors include prediction of the anchor embedment depth, which dictates the anchor's holding capacity. This is particularly true for calcareous sediments, as very little performance data exist for this anchor type in these soils. This paper reports results from a series of model tests undertaken to provide insight into the behaviour of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the drop height and consequently the impact velocity, and the consolidation period prior to anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge model test data were used to calibrate: (a) an analytical dynamic embedment model, based on conventional bearing and frictional resistance factors but with strain-rate-dependent undrained shear strength for the soil; and (b) an analytical quasi-stati...
51 citations
Cites background from "Undrained Load Capacity of Torpedo ..."
...…states normalised by the maximum loads, FN,v ¼ FN,V /FN,Vmax and FN,h ¼ FN,H/FN,Hmax, indicating the shape and relative size of the failure envelope. de Sousa et al. (2011) reported anchor capacities for a geometrically similar torpedo anchor, as determined from small strain finite-element…...
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TL;DR: In this paper, the results from a series of model tests undertaken to provide insight into the behavior of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt were reported.
Abstract: This paper reports the results from a series of model tests undertaken to provide insight into the behavior of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt. The tests were carried out in a drum centrifuge at 200g, varying the drop height (hence the impact velocity) and the time delay for consolidation before pullout. The pullout angle at the mudline was also varied to encompass various mooring systems, including catenary (0°), taut leg (45°), and tension leg (∼80°). Two geometries of torpedo anchors were explored, varying the fin and tip geometry. The results demonstrated that the anchor embedment depth increased as the drop height (and hence the impact velocity) increased and the soil undrained shear strength decreased. In stronger silt, the cavity above the installing anchor remained open, whereas in soft clay, it was fully backfilled and replenished. The corresponding anchor embedment depth was also about 0.63 times compared...
50 citations
TL;DR: In this paper, the capacity of dynamically installed anchors in soft normally consolidated clay was examined experimentally through a series of field tests on a 1:20 reduced-scale anchor, achieving tip embedment of 1.5-2.6 times the anchor length, before being loaded under undrained conditions at various load inclinations.
Abstract: The capacity of dynamically installed anchors in soft normally consolidated clay was examined experimentally through a series of field tests on a 1:20 reduced-scale anchor. The anchors were installed through free fall in water, achieving tip embedment of 1.5–2.6 times the anchor length, before being loaded under undrained conditions at various load inclinations. Vertical anchor capacities were between 2.4 and 4.1 times the anchor dry weight and were satisfactorily predicted using the American Petroleum Institute approach for driven piles. Anchor capacity under inclined loading increased as the load inclination approached horizontal; the field data indicated this increase to be up to 30% for the minimum achievable inclination of about 20° to the horizontal. Corresponding large-deformation finite element analyses showed a similar response, with the maximum capacity occurring at a load inclination between 30° and 45° to the horizontal. The finite element results demonstrate that, for the anchor geometry cons...
45 citations
TL;DR: In this paper, a numerical procedure for the finite element analysis of anchors dynamically penetrating into saturated soils is outlined, highlighting its unique features and capabilities, and an algorithm is developed for frictional contact in terms of effective normal stress.
Abstract: The development of a numerical procedure for the finite element analysis of anchors dynamically penetrating into saturated soils is outlined, highlighting its unique features and capabilities. The mechanical behaviour of saturated porous media is predicted using mixture theory. An algorithm is developed for frictional contact in terms of effective normal stress. The contact formulation is based on a mortar segment-to-segment scheme, which considers the interpolation functions of the contact elements to be of order N, thus overcoming a numerical deficiency of the so-called node-to-segment (NTS) contact algorithm. The nonlinear behaviour of the solid constituent is captured by the Modified Cam Clay soil model. The soil constitutive model is also adapted so as to incorporate the dependence of clay strength on strain rate. An appropriate energy-absorbing boundary is used to eliminate possible wave reflections from the artificial mesh boundaries. To illustrate the use of the proposed computational scheme, simulations of dynamically penetrating anchors are conducted. Results are presented and discussed for the installation phase followed by ‘setup’, i.e., pore pressure dissipation and soil consolidation. The results, in particular, reveal the effects of strain rate on the generation of excess pore pressure, bearing resistance and frictional forces. The setup analyses also illustrate the pattern in which pore pressures are dissipated within the soil domain after installation. Hole closure behind a dynamic projectile is also illustrated by an example.
33 citations
TL;DR: In this article, seven torpedo anchors with different shapes, sizes, aspect ratios, scale ratios and fin sizes were utilized to investigate their influence on the falling velocity of the anchor during acceleration.
Abstract: Torpedo anchor installation is a new kind of anchoring system which is much more economical than other conventional anchoring methods. However, there are few studies on the falling velocity and drag coefficient of the torpedo anchors. In this study, seven torpedo anchors with different densities, aspect ratios, scale ratios and fin sizes were utilized to investigate their influence on the falling velocity of the anchor during acceleration. Anchors were released in a water tank and the falling process was recorded using a fast speed video camera. Accordingly, the corresponding drag coefficients against the Reynolds numbers during acceleration were calculated. The Reynolds numbers varied between 4.8×105 and 2.16×106 and the drag coefficients varied between 0.2 and 1.2. The differences between the falling velocities and the drag coefficient for anchors with different shapes, sizes, densities and directional stability were illustrated and the reasons for the differences were explained. The final drag coefficients versus the Reynolds numbers were compared with other representative models found in the literature. The influence of acceleration on the drag coefficient or falling velocity was emphasized. Finally, an Extrapolation Mathematical Model for the motion of the anchor in the fluid was proposed and the results were compared to the experimental data.
32 citations
References
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TL;DR: In this article, the authors provide geotechnical and structural engineering practitioners, researchers and postgraduate students with an insight into the use of finite element methods in geotechical contexts, in order that they might make good judgements as to the credibility of the numerical results they may obtain or review in the future.
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423 citations
TL;DR: The pinball algorithm is a simplified slideline algorithm which is readily vectorized, its major idea is to embed pinballs in surface elements and to enforce the impenetrability condition only to pinballs.
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351 citations