Coordinateurs du projet
Context
Floating offshore structures (wind turbines, wave energy converters, tidal turbines) are generally used in deep waters beyond the depth range suitable for fixed offshore structures founded on the seabed. These floating structures are held in place by an anchoring system consisting of a set of cables attached to anchors embedded in the seabed.
There are many different types of anchors, the advantages and limitations of which depend on the type of anchoring system used and the seabed conditions. Traditional anchoring systems (e.g., piles) require costly installation. In addition, DEA (Drag Embedded Anchors) involve uncertainty regarding the final penetration depth into the ground and therefore uncertainty about the load-bearing capacity of these anchors.
The ANCRE_EMR project aims to study the performance of two types of offshore anchors for possible use in floating structures: dynamically embedded plate anchors (DEPLA) and helical anchors.
Scientific breakthroughs and innovation
With regard to the new DEPLA_1 model, the state of the art does not provide any information on the numerical modeling of this type of anchor. Experimental studies using a centrifuge were conducted to verify the ability of the proposed system to reach significant depths in the ground and to study the pull-out capacity of this system [Chow et al. (2017) and Chow et al. (2018)]. The originality of the ANCRE_EMR project lies in performing numerical modeling of large deformations during the installation process of the DEPLA_1 anchor and studying the influence of various parameters on the final anchor depth.
With regard to helical anchors, numerical studies generally assume that these anchors consist of a series of circular plates (not propellers) attached to the central rod, which could lead to an inaccurate estimate of the anchor’s load-bearing capacity. Furthermore, these studies do not consider the anchor installation process. This project aims to perform three-dimensional analyses of helical anchors using the finite element method, taking into account the exact shape of the propellers and the anchor installation process.
With regard to helical anchors, numerical studies generally assume that these anchors consist of a series of circular plates (not propellers) attached to the central rod, which could lead to an inaccurate estimate of the anchor’s load-bearing capacity. Furthermore, these studies do not consider the anchor installation process. This project aims to perform three-dimensional analyses of helical anchors using the finite element method, taking into account the exact shape of the propellers and the anchor installation process.
Based on this review of the state of the art, and focusing on the calculation of hydrodynamic forces, very little work has been done on algae. Only Theophanatos (1988) has conducted exploratory work. However, the precise geometry of the algae, their rigidity, and their characteristics (buoyancy, etc.) are not mentioned, making this work very difficult to exploit and impossible to reproduce. The main reason for the lack of studies on this subject is certainly the difficulty of keeping flexible species alive in order to preserve their mechanical and hydrodynamic properties. A new model material available at Nantes University since 2022 has made it possible to overcome this difficulty. We believe that the flag effect of these algae must now be studied, as it is likely to generate significant cyclic stresses and therefore damage the anchors.
As for the second part of the project, the economic value of the algae collected, no studies have been conducted. Only the work of Buck (2017, 2018a, 2018b), which is purely descriptive, has provided some initial ideas. We would like to incorporate real value creation through identified sectors and economic analysis.
Expected technical and economic impact
- The calculation methods developed should enable more reliable and economical dimensioning, thereby reducing the conservatism of current approaches.
- The digital tools developed will be available for use by local businesses, subject to specific training.
Demonstrator
In this project, numerical simulations of the two types of anchors (i.e., the new DEPLA_1 anchoring system consisting of a dynamically installed anchor plate and the helical anchor) will be performed using the finite element method.
Results
Coupled Eulerian-Lagrangian (CEL) analysis proved useful (i) for calculating the pull-out capacity of the helical anchor (using the “wished-in-place” assumption) and (ii) for simulating the installation process for this anchor:
a. For calculating the pull-out bearing capacity of the helical anchor with the ‘wished-in-place’ assumption, CEL analysis was able to accurately determine the pull-out bearing capacity of these anchors, unlike the conventional finite element method; the calculation using the CEL approach was completed without any convergence issues. Indeed, analysis using the CEL approach makes it possible to overcome the problem of mesh distortion (causing the calculation to stop) that is observed in problems involving large deformations, as is the case with helical anchors.
b. For simulations of the helical anchor installation process, analysis using the conventional finite element method is not feasible; however, simulation of the installation process using CEL analysis was successfully carried out.
The results relating to the calculation of the pull-out capacity of the helical anchor installed in powdery soil (using the “wished-in-place” hypothesis and the CEL approach) are as follows:
a. For vertical loading, the uplift bearing capacity obtained was close to the analytical solution. For inclined loading, a slight decrease in uplift bearing capacity was observed for loading inclinations of less than 60° from the vertical. For inclinations greater than 60°, the observed reduction rate was higher.
b. It was shown that the pitch of the helical anchor propeller has no significant effect on the anchor’s pull-out bearing capacity. This result is consistent with that of tests carried out at the Gustave Eiffel University geotechnical centrifuge
c. A threshold in the pull-out bearing capacity factor is observed when the H/D ratio between the burial depth of the propeller and its diameter reaches a value close to 6. This is due to the transition between a failure mechanism occurring at the surface for H/D < 6 and a failure mechanism occurring at depth when H/D ≥ 6.
d. A threshold in the pull-out bearing capacity factor is observed when the S/D ratio between the spacing between successive propellers of a multi-propeller anchor and the diameter of the propeller reaches ~4.7. This is due to the transition between a global failure mechanism and a local mechanism when S/D~4.7.
The installation process of the DEPLA-1 anchor in sand was modeled using the CEL approach. The results obtained in terms of penetration potential were compared with the experimental results of Chow et al. (2017). The final penetration depth values obtained through numerical simulations were lower than those obtained experimentally. However, they are of the same order of magnitude. Thus, the DEPLA-1 anchor could be considered a promising solution for granular soils.
Presentation of the project at the weamec seminar “Geotechnics & Geophysics for MRE Applications”
Publications and presentations produced
Oral presentations
El Haj A-K., Soubra A-H., Effect of the installation process on the pullout capacity of helical anchors, 40th International Conference on Ocean, Offshore & Arctic Engineering OMAE2021, Virtual Conference: June 21 – June 30, 2021
Perspectives
Numerical modeling of the DEPLA_1 anchor installation process to determine its potential for penetration into the ground.