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X. D. Liu

Bio: X. D. Liu is an academic researcher from National University of Singapore. The author has contributed to research in topics: Penetration depth. The author has an hindex of 1, co-authored 1 publications receiving 6 citations.

Papers
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Proceedings ArticleDOI
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


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Journal ArticleDOI
TL;DR: In this article, the authors proposed a numerical framework to simulate the deployment of an FFP device in dry sands using the Material Point Method (MPM) and a moving mesh technique is used to ensure the accurate geometry of the FFP devices throughout the calculation, and the soil-FFP interaction is modelled with a frictional contact algorithm.
Abstract: Free Fall Penetrometer (FFP) testing consist of a torpedo-shaped body freefalling into a soil target. The use of this type of device is becoming popular for the characterization of shallow sediments in near-shore and off-shore environments because it is a fast, versatile, and non-expensive test capable of recording acceleration and pore pressures. In recent years, the data analysis advanced considerably, but the soil behavior during fast penetration is still uncertain. Hence, there is a need to develop numerical models capable of simulating this process to improve its understanding. This paper proposes a numerical framework to simulate the deployment of an FFP device in dry sands using the Material Point Method (MPM). A moving mesh technique is used to ensure the accurate geometry of the FFP device throughout the calculation, and the soil-FFP interaction is modelled with a frictional contact algorithm. Moreover, a rigid body algorithm is proposed to model the FFP device, which enhances the performance of the computation and reduces its computational cost. The sand is simulated by using two constitutive models, a non-associate Mohr-Coulomb (MC) and a Strain-Softening Mohr-Coulomb (SSMC) that reduces, exponentially, the strength parameters with the accumulated plastic deviatoric deformation ( Yerro et al., 2016 ). Variable dilatancy, which reduces as a function of the plastic strain, is also taken into account, and the strain-rate effects have been evaluated in terms of peak friction angle. In general, the behavior predicted by the MPM simulations is consistent with the experimental test. The results indicate that the soil stiffness has a big impact on the deceleration time-history and the development of a failure mechanism, but less influence on the magnitude of the peak deceleration and the penetration depth; the soil dilatancy reduces the FFP rebound, and the FFP-soil contact friction angle and the peak friction angle are highly linked to the peak deceleration.

16 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed a method to calculate the undrained monotonic holding capacity of a torpedo anchor, which mainly depends on the embedded depth of the anchor, net weight, geometry, and in situ soil properties.
Abstract: Torpedo anchors are regarded as one of the most efficient mooring solutions for taut mooring systems and can withstand vertical loads. The estimation of the undrained pullout capacity of the anchors is vital for the design of offshore floating facilities. There have been some achievements obtained for the calculation of the holding capacity of a torpedo anchor via field tests, conventional model tests under one gravity, centrifuge tests with a high value of gravity acceleration and numerical tests. However, a simple and reliable formula is still required to calculate the holding capacity of a torpedo anchor. In this study, 240 sets of laboratory tests were performed, and 11 differently shaped model anchors, vertically embedded in a soft sedimentary bed, were pulled out vertically from different types of cohesive soils and different embedment depths. The characteristics of the loading curves were analyzed, and the relationship between the pullout capacities and properties of the anchors and types of soils were investigated. Based on force analysis and the laboratory data, a formula was proposed for the calculation of the undrained monotonic holding capacity of a torpedo anchor, which mainly depends on the embedded depth of the anchor, net weight, geometry, and in situ soil properties. The calculated vertical holding capacities were consistent with the laboratory and field data obtained by the authors and other scientists.

15 citations

Journal ArticleDOI
TL;DR: In this article, the authors divide the behavior of dynamically installed anchors into three stages, including anchor free-fall in water, anchor dynamic penetration in soil, and anchor pullout under operational loadings.
Abstract: Challenges associated with dynamically installed anchors (DIAs) include prediction of anchor embedment depth and subsequent pullout performance under chain loading. Numerical analyses on the behaviour of DIAs require advanced modelling methods that can deal with a) hydrodynamic aspects during the free-fall in water; b) large soil deformations with high strain rate effects during the dynamic penetration; c) post-installation consolidation; d) pullout performances considering installation and anchor chain effect. This paper divides the behaviour of DIAs into three stages, including anchor free-fall in water (Stage 1), anchor dynamic penetration in soil (Stage 2), and anchor pullout under operational loadings (Stage 3), and gives an overview of numerical advancements on the analysis of DIAs at each stage. By illustrating typical numerical results and comparisons between various methods, the advantages and limitations of the numerical modelling methods are highlighted. The advantage of the finite volume method (FVM) based on computational fluid dynamics is its capability in the full simulation of an anchor dynamic installation in water followed by in soil (Stages 1 and 2). However, the stress-induced anisotropy of the soil cannot be captured by the FVM. The finite element (FE) method, especially the large deformation finite element (LDFE) method, is preferable to analyse Stages 2 and 3. The effects of soil properties on the performance of DIAs can be quantified by the LDFE method, in terms of strain rate, strain softening, pore pressure and drag force.

6 citations

01 Jan 2015
TL;DR: In this paper, a list of FIGURES and TABLES is presented, along with a notation for the number of occurrences of each of the FIGURES listed in the list.
Abstract: ................................................................................................................. III DECLARATION ............................................................................................................ V LIST OF FIGURES .................................................................................................. XIII LIST OF TABLES ..................................................................................................... XIX ACKNOWLEDGEMENTS........................................................................................ XX NOTATION ..................................................................................................................... 1 CHAPTER

6 citations

Proceedings ArticleDOI
22 Mar 2016

1 citations