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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08 Jun 2014
TL;DR: In this paper, a series of model tests simulating dynamic installation and monotonic pull-out of dynamically installed anchors in normally consolidated clay are presented, with varying penetration angles, extraction angles and model masses.
Abstract: Depletion of shallow-water hydrocarbons is increasingly forcing the oil and gas industry to explore in deeper water. Dynamically installed anchors (i.e. torpedo anchors and deep penetrating anchor) are increasingly used as a cost-effective solution for floating offshore structures in deep water environments because their installation cost is largely independent of water depth. In addition, dynamically installed anchors can be deployed accurately, and their performance is less dependent on accurate assessment of the soil shear strength since lower seabed strengths permit greater penetration depths. Despite of the economic advantages afforded by dynamically installed anchors, there remain significant uncertainties in the prediction of the embedment depth and verticality, which is likely to affect their long-term holding capacity. Currently, the holding capacity of the dynamically installed anchors is assessed using conventional pile capacity techniques, which neglect discrepancies in the rate of installation and failure mechanism between them.This paper presents a series of model tests simulating dynamic installation and monotonic pull-out of dynamically installed anchors in normally consolidated clay. The model tests are carried out in a beam centrifuge at 100g, with varying penetration angles, extraction angles and model masses. A special designed apparatus allows model anchors to be penetrated and extracted with different penetration angles. The test results show that for models without fins, no matter by which angle the model penetrated the soil, the smallest value of holding capacity is obtained when the pullout and penetration directions are the same. Results also indicate that the penetration depth linearly increases with the anchor mass. This study also reported the results from finite element (FE) analyses. The Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit is carried out to simulate dynamic anchor installation.The findings of this study points to a method of assessing the minimum holding capacity of the anchor and its depth of penetration. Further study is now on-going to study the behavior of finned anchors.Copyright © 2014 by ASME
8 citations
22 Dec 2015
TL;DR: In this paper, the effect of various geometric parameters in the torpedo anchors' design to optimize for higher terminal velocity was investigated and correlations between various parameters and its resultant drag coefficient were developed.
Abstract: The literature shows a lack on well established guidelines on the design of the torpedo anchors. This research aims to investigate the effect of various geometric parameters in the torpedo anchors' design to optimize for higher terminal velocity. A CFD model was developed and simulations were carried out using ANSYS FLUENT commercial software. Consequently, correlations between various parameters and its resultant drag coefficient were developed. The parameters of interest included geometric changes of the original design, as well as sea water properties that reflect water depth in South China Sea. New design features are proposed and investigated in the overall parametric studies. As a result, it was found that the terminal velocity can be improved by sharper tip angle, greater aspect ratio, greater diameter ratio, and an optimum rear angle of 30°. Clear relationships between each factor and its resulted terminal velocity were developed. More importantly, the sensitivity of drag coefficient towards each of the parameters was predicted.
5 citations
09 Jun 2013
TL;DR: In this paper, a simple formulae are employed to predict the holding capacities of two different torpedo anchors, which are compared to those estimated with the finite element (FE) model.
Abstract: Torpedo anchors have been used in various offshore applications especially due to its low cost installation and the ability to withstand high inclined loads. This anchor consists of a shaft in which flukes are welded in order to increase the soil-anchor contact region and, consequently, its holding capacity. Since this anchor presents a singular geometry, different from a regular cylindrical anchor/pile, the computation of the holding capacity of a torpedo anchor is not straightforward. In a previously presented work, the holding capacities of typical torpedo anchors were assessed with a finite element (FE) model in which both the anchor and the surrounding soil are represented with three-dimensional finite elements. However, this FE model demands a significant computational effort and, consequently, simpler approaches would be desirable in order to design these anchors. Relying on the FE model and a parametric study, this paper presents simple formulae to predict the holding capacities of torpedo anchors embedded into cohesive soils. These formulae are employed to predict the holding capacities of two different torpedo anchors, which are compared to those estimated with the FE model. Results agreed very well indicating that this simpler approach may be employed to quickly evaluate the holding capacities of these anchors.Copyright © 2013 by ASME
2 citations