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Research Papers: Offshore Geotechnics

Key Techniques in Simulating Comprehensive Anchor Behaviors by Large Deformation Finite Element Analysis

[+] Author and Article Information
Yanbing Zhao

State Key Laboratory of Hydraulic Engineering
Simulation and Safety,
Tianjin University,
Tianjin 300072, China
e-mail: z_ybmail@tju.edu.cn

Haixiao Liu

State Key Laboratory of Hydraulic Engineering
Simulation and Safety,
Tianjin University,
Tianjin 300072, China;
Collaborative Innovation Center for Advanced
Ship and Deep-Sea Exploration,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: liuhx@tju.edu.cn

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received January 9, 2017; final manuscript received August 23, 2017; published online October 6, 2017. Assoc. Editor: Lizhong Wang.

J. Offshore Mech. Arct. Eng 140(1), 012001 (Oct 06, 2017) (13 pages) Paper No: OMAE-17-1005; doi: 10.1115/1.4037843 History: Received January 09, 2017; Revised August 23, 2017

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated, such as 360 deg rotation of the anchor arm, gravity installation of anchors with high soil strain rate, and keying and diving (or penetration) of anchors. The anchor line connects the anchor and the anchor handling vessel (AHV) or floating moored platform. With moving of the AHV or platform, anchor line produces a space movement, and forms a reverse catenary shape and even a three-dimensional (3D) profile in the soil. Finite element analysis on the behaviors of anchor lines and deepwater anchors requires techniques that can deal with large strains and deformations of the soil, track changes in soil strength due to soil deformation, strain rate and strain softening effects, appropriately describe anchor–soil friction, and construct structures with connector elements to conform to their characteristics. This paper gives an overview of several key techniques in the coupled Eulerian–Lagrangian (CEL) analysis of comprehensive behaviors of deepwater anchors, including construction of the embedded anchor line and the anchor line in the water, installation of gravity installed anchors (GIAs), keying or diving of drag anchors, suction embedded plate anchors (SEPLAs) and GIAs, and implementation of the omni-directional arm of GIAs. Numerical probe tests and comparative studies are also presented to examine the robustness and accuracy of the proposed techniques. The aim of this paper is to provide an effective numerical framework to analyze the comprehensive behaviors of anchor lines and deepwater anchors.

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Figures

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Fig. 1

Typical behaviors of GIAs

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Fig. 2

Profile of the anchor line in a 3D space

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Fig. 3

FE model in simulating the embedded line in a vertical plane

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Fig. 4

Comparison of the tension at the attachment point: (a) μ = 0.3 (su = 10 kPa) and (b) μ = 0.3 (su = 5 + 1.5z kPa)

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Fig. 5

Comparison of the reverse catenary shape: (a) μ = 0.3 (su = 10 kPa) and (b) μ = 0.3 (su = 5 + 1.5z kPa)

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Fig. 6

FE model in simulating the embedded line in a 3D space

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Fig. 7

3D profile and tension transmitting properties of the embedded anchor line: (a) 3D profile, (b) projection on the x–y plane, and (c) tension at the attachment point

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Fig. 8

Probe test to verify the construction of submerged line

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Fig. 9

Probe test results for the submerged line (case 2): (a) movement direction, (b) drag angle at the shackle, (c) trajectory, and (d) drag force at the shackle

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Fig. 10

Installation procedure of GIAs

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Fig. 11

Penetration responses for OMNI-Max anchors: (a) comparative results and (b) effect of hydrodynamic drag

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Fig. 12

Penetration responses for torpedo anchors: (a) finless torpedo anchor and (b) four-fins torpedo anchor

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Fig. 13

Soil flow mechanism and penetration rate effects: (a) soil flow mechanism, (b) variation of undrained shear strength, and (c) strain rate and strain softening effects

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Fig. 14

FE analysis setup: (a) installation of drag anchors, (b) keying of SEPLAs, and (c) keying and diving of OMNI-Max anchors

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Fig. 15

Responses of drag anchor installation (μ = 0.1): (a) anchor trajectory and profile of the embedded line, (b) comparison with Murff et al. [33] (trajectory), and (c) comparison with Murff et al. [33] (shackle load)

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Fig. 16

Keying of SEPLAs: (a) orientation-embedment loss relationship (θ = 90 deg), (b) load–displacement relationship (θ = 90 deg), and (c) orientation-embedment loss relationship (θ = 60 deg)

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Fig. 17

Responses of OMNI-Max anchors: (a) trajectory, (b) shackle load, and (c) profile of the embedded mooring line (v = 2 m/s)

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Fig. 18

Construction and verification of the omni-directional arm: (a) OMNI-Max anchor [36], (b) anchor model, and (c) hypothetical FE test

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Fig. 19

Responses of the OMNI-Max anchor with omni-directional arm: (a) response of the fluke and (b) response of the shackle

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