Analysis of Offshore Protective Berms using FEM and MPM

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Abstract

By increase in knowledge and complexity of the projects, worldwide, more attention towards safety provision in projects is being paid. With growth in waterway traffic the need of safety provision for structures crossing navigable waters increases. Especially bridge piers and immersed tunnels on the see floor or at tunnel approaches are at bigger risk of being hit by ships which have lost their navigational course due to any reasons. Therefore availability of collision protective systems is essential. A number of different methods are being used worldwide as protective systems while for the full frontal collision, protective berms are the most convenient systems. This research investigates the possibility of calculations for these berms using numerical modelling tools such as Finite Element Method (FEM) and Material Point Method (MPM). FEM is a well-known and proven tool in engineering world while MPM is a newly developed tool that is considered as an update of FEM. In MPM the material state and stresses are stored in material points that can freely move though the mesh. This tool is highly useful for problems with high expected deformations or dynamic calculations where FEM is not considered as an ideal tool and could fail. Furthermore, also, a simplified analytical method has been presented in this report.
At present MPM is being developed around the world. Plaxis, which is a well-known software developer especially for Geo-Engineering applications, is currently developing software that provides MPM modelling. For this research, the Plaxis 2D software has been used for MPM under confidentiality terms. As mentioned, the software is in its development stage therefore some issues where expected. The FEM models are also created using the internationally used Plaxis 2D FEM version.
The reliability of the MPM and also FEM models in dynamic calculations was analyzed in this research allowing for a better understanding for the final model which represents the ship-berm collision problem. Two models (static and dynamic) were created in both FEM and MPM which then verified and compared. Therefore it was concluded that MPM can be considered as a reliable tool for dynamic calculations since the obtained results were similar or comparable with FEM and analytical solutions.
The final models in both FEM and MPM however, did not provide the required results that could help to understand the ship-berm collision mechanism and/or the stress situations within the soil (only partial results were obtained). The FEM model showed very unrealistic and inaccurate results. The constant predefined contact area between the ship and berm is considered to be the main issue in this model; on the other hand, due to encountered issues with MPM, the MPM simulation could not be fully performed in terms of minimum required time steps. Nevertheless the partial simulations with a limited simulation time, were found to be realistic and in accordance with expectation, suggesting the capability of MPM for dynamic calculation after the necessary developments.
As conclusion, this report does not recommend the usage of FEM for such a modelling and considers MPM a good possible tool to be used after the necessary developments in the near future. Meanwhile the presented simplified analytical method could provide a rough estimation for required berm length. In addition a FEM model has also been suggested in this report allowing for the user to have an insight on the stress propagation within the soil.