Analysis of Torpedo Anchors for Mooring Operations
01 Jan 2020-pp 149-159
TL;DR: In this article, the authors investigate the issues encountered by the torpedo anchor during the vertical drop, which ultimately reduces the pull-out resistance, and the effect of the anchor tilt on pullout resistance is studied.
Abstract: A novel technique of dynamically installing torpedo-shaped anchors is investigated. A vast extent of uncertainties arises in pull-out capacity estimation due to the excessive tilt of a torpedo anchor during free-fall and subsequent embedment into the seafloor. This paper will investigate the issues encountered by the torpedo anchor during the vertical drop, which ultimately reduces the pull-out resistance. The pull-out resistance study offered by torpedo anchors is investigated using a finite element tool, PLAXIS 3D. A series of pull-out tests were conducted with anchors under four different ballast conditions (20, 40, 60 and 80%) with three chosen fin configurations (without fin, 3 fins and 4 fins). The anchors are tested for various inclinations (0°, 2.5°, 5°, 7.5° and 10°) and the effect of torpedo anchor tilt on pull-out resistance is studied, and the allowable range of anchor tilt was recommended. Thus, this study provides the benefit of ideal ballast and fins arrangements.
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TL;DR: In this paper, the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation in non-homogeneous clay were reported.
Abstract: This paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation in non-homogeneous clay. The large deformation finite element (LDFE) analyses were carried out using the coupled Eulerian–Lagrangian approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength using the Herschel–Bulkley model. The results were validated against field data and centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of anchor shaft length and diameter; number, width and length of fins; impact velocity and soil strength. The anchor velocity profile during penetration in clay showed that the dynamic installation process consisted of two stages: (a) in Stage 1, the soil resistance was less than the submerged weight of the anchor and hence the anchor accelerated; (b) in Stage 2, at greater penetration, frictional and end bearing resistance dominated and the anchor decelerated. The corresponding soil failure patterns revealed two interesting aspects including (a) mobilization of an end bearing mechanism at the base of the anchor shaft and fins and (b) formation of a cavity above the shaft of the installing anchor and subsequent soil backflow into the cavity depending on the soil undrained shear strength. To predict the embedment depth in the field, an improved rational analytical embedment model, based on strain rate dependent shearing resistance and fluid mechanics drag resistance, was proposed, with the LDFE data used to calibrate the model.
44 citations
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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.
39 citations
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