Numerical Investigation of the Installation Process of Drag Anchors in Sand
Abstract
1. Introduction
- (1)
- (2)
- In some studies, the interaction between anchor chains and DEAs is considered in a decoupled manner, which means that the DEA’s movement is achieved by prescribing a calculated displacement/force/velocity boundary in the numerical model [17,26,49]. While in some other studies which did attempt to model the soil–chain interaction, too few chain units were modeled [19,20,47]; this can lead to significant inaccuracy as the model fails to capture the reverse-catenary shape of the embedded chain at deep embedment. As the anchor’s kinematics depends on the loading angle at the shackle, which is strongly influenced by the interaction between the chains and the soil [11,51,52], it is critical to correctly capture the soil–chain interaction in the CEL model.
2. Integrated Anchor–Chain–Soil Large-Deformation Finite-Element Analysis Method
2.1. Coupled-Eulerian–Lagrangian Method (CEL Method)
2.2. Simulation of the Drag Anchor and the Seabed
2.3. Simulation of the Chain
2.4. Analysis Steps, Boundary Conditions, and Interface Interaction
- (1)
- The “geostatic” step. This purpose of this step is to generate the initial stress field within the soil domain.
- (2)
- The “initialization” step. Gravity is activated on the anchor and the chains in this step and the anchor achieves a small initial penetration due to its self-weight.
- (3)
- The “drag” step. A horizontal drag velocity is applied at the last chain unit and the anchor penetrates into the seabed under the dragging motion.
- For the Eulerian domain, side boundaries (including the bottom of the soil) are prescribed with a zero-velocity boundary condition normal to the surfaces. This is different from the normally applied zero-displacement boundary because precluding nonzero displacement boundaries in the Eulerian domain would deform the mesh, contradicting the formulation where nodes stay stationary. Abaqus ignores such conditions, restricting displacements to zero. Instead, zero-velocity boundaries control movement of materials by preventing outflow during deformation like impermeable “walls”. This ensures stability, realism, and accurate flow tracking around Lagrangian domains like the anchors in the current study.
- An Eulerian outflow boundary with a “no reflection” option is applied on the side boundaries of the Eulerian domain (except the bottom of the soil). This is essential for modeling infinite domains. The fixed Eulerian mesh prevents material from deforming nodes, but without this condition, outgoing soil flow would artificially reflect back at the domain edge, distorting force balances, trajectories, and overall dynamics, leading to unphysical stiffening or oscillations. The non-reflecting outflow absorbs these waves simulating an infinite domain and ensuring realistic movement of material out of the mesh without re-entry. This enhances accuracy in transient analyses with extreme deformations.
- The vertical symmetry plane of the model is assigned a symmetry boundary condition.
- A constant drag velocity is prescribed at the reference point (RP) of the last chain unit, which aims to simulate the dragging of anchor holding vehicle in practical engineering.
- The contact between the DEA and the soil, and the chain units and the soil, is defined using “general contact”. The tangential behavior follows the Coulomb friction law, with a friction coefficient of 0.5. The normal behavior is defined as “hard contact, allow separation after contact”.
3. Parametric Sensitivity Analysis
- Width of the soil domain perpendicular to the dragging direction—Bsoil = 3D, 4.5D, and 6D (where D represents the overall width of the drag anchor)—for which two sets of analyses with two soil friction angles (20° and 27.5°) are performed.
- Mesh size in the drag area—Mesh_S = 0.25 m, 0.3 m, 0.4 m, and 0.5 m—for which three sets of analyses with three friction angles (φ = 20°, 27.5° and 35°) are performed.
- Dragging speed—vc = 0.4 m/s, 0.6 m/s, and 1.0 m/s.
3.1. The Influence of Soil Size Perpendicular to the Dragging Direction
3.2. The Influence of Mesh Size in the Drag Area
3.3. Influence of Drag Velocity
4. Verification Against Centrifuge Test in Sand
5. Installation Behavior of Drag Anchor in Sand
- (1)
- Initial stage: The DEA rests on the seabed in a “bowing” posture, and the chain is laid flat on the seabed. The tip of fluke achieves an initial penetration of about 1.8 m due to the self-weight of the anchor.
- (2)
- Partial embedment: As the chain begins to move, the DEA gradually penetrates into the seabed but is still visible at the seafloor. The failure mechanism is quite similar to that of a shallowly embedded plate anchor [68], in which the soil wedge in front of the anchor ruptures and the DEA’s behavior is linked to a plough-type mechanism.
- (3)
- Complete embedment: As the anchor gains further penetration, the DEA is completely embedded into the seabed. The sand particles in front of the DEA are ploughed out from the seabed, moving through the shank to the surface, forming a “horseshoe” soil heave. As the dragging process continues, the sand particles gradually fall down to the ditch behind the anchor. This in–out balance of sand particles makes the soil heave move with the anchor, with its size and shape nearly unchanged.
- (4)
- Final stage: The drag distance reaches the predetermined value, marking the end of anchor’s embedment process. Compared with the initial stage, the angle between fluke and seabed (i.e., the penetration angle θ) is clearly reduced, and the chains form a reverse-catenary shape.
6. Concluding Remarks
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
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| Lf (m) | Bf (m) | D (m) | Lw (m) | Bw (m) | Ls (m) | Hs (m) | Ha (m) |
|---|---|---|---|---|---|---|---|
| 5.36 | 4.09 | 10.11 | 1.47 | 3.37 | 8.60 | 4.86 | 1.97 |
| Case | Internal Friction Angle of Sand φ,° | Bsoil/D | Mesh_S, m | vc, m/s |
|---|---|---|---|---|
| 1 | 20 | 3 | 0.4 | 0.6 |
| 2 | 4.5 | |||
| 3 | 6 | |||
| 4 | 27.5 | 3 | 0.4 | 0.6 |
| 5 | 4.5 | |||
| 6 | 6 |
| Case | Internal Friction Angle of Sand φ,° | Bsoil/D | Mesh_S, m | vc, m/s |
|---|---|---|---|---|
| 1 | 20 | 3 | 0.25 | 0.6 |
| 2 | 0.3 | |||
| 3 | 0.4 | |||
| 4 | 0.5 | |||
| 5 | 27.5 | 3 | 0.25 | 0.6 |
| 6 | 0.3 | |||
| 7 | 0.4 | |||
| 8 | 0.5 | |||
| 9 | 35 | 3 | 0.25 | 0.6 |
| 10 | 0.3 | |||
| 11 | 0.4 | |||
| 12 | 0.5 |
| Number | Internal Friction Angle of Sand φ,° | Bsoil/D | Mesh_S, m | vc, m/s |
|---|---|---|---|---|
| 1 | 20 | 6 | 0.4 | 0.4 |
| 2 | 0.6 | |||
| 3 | 1.0 |
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Wu, C.; Zhang, Y.; Guo, P.; Lei, D. Numerical Investigation of the Installation Process of Drag Anchors in Sand. Geotechnics 2025, 5, 76. https://doi.org/10.3390/geotechnics5040076
Wu C, Zhang Y, Guo P, Lei D. Numerical Investigation of the Installation Process of Drag Anchors in Sand. Geotechnics. 2025; 5(4):76. https://doi.org/10.3390/geotechnics5040076
Chicago/Turabian StyleWu, Chuheng, Youhu Zhang, Peng Guo, and Di Lei. 2025. "Numerical Investigation of the Installation Process of Drag Anchors in Sand" Geotechnics 5, no. 4: 76. https://doi.org/10.3390/geotechnics5040076
APA StyleWu, C., Zhang, Y., Guo, P., & Lei, D. (2025). Numerical Investigation of the Installation Process of Drag Anchors in Sand. Geotechnics, 5(4), 76. https://doi.org/10.3390/geotechnics5040076
