Simulation of mooring Lines in complex bathymetries using a finite element method

Abstract

ABSTRACT: This paper presents a novel method for the modeling of the seabed interaction of mooring lines in complex bathymetries, known as "continuous projection method". This method is able to calculate ground normal and friction forces for any seafloor surface. This provides an improvement in the mooring systems simulation, as it captures additional non-linearities on the mooring line performance due to seabed interaction. The method is based on constructing a triangulation for the seabed and projecting mooring line nodes by using the vertex normal vectors of each triangle, ensuring the continuity of the projection. For the sake of the computational cost reduction, the line nodes are first projected into the closest triangles. Also, whenever the floor is flat, inclined or horizontal, a point-to-plane projection expression is used instead. The projection method described has been implemented to a finite element model. The initial condition problem was solved with a static approach, based on finding the static equilibrium with Newton-Armijo algorithm. This improves other static approaches which use the catenary equation, and that are only valid for a flat seabed. The model was successfully verified against the analytical solution of an inextensible catenary line in a slope. Furthermore, the simulation results were validated against experimental scale tests on a single chain mooring line with three different seafloor structures: one flat floor and two different sloped steps. For each of them, static and dynamic regular tests were performed. Moreover, high and low frequency fairlead movements were imposed in the dynamic tests, aiming to validate the model both in cases with and without snapping loads. Overall, the obtained results were coherent and allow to validate the accuracy of the proposed model. Finally, a mooring line over an irregular seabed surface was studied, comparing the results obtained by directly applying the developed method for complex bathymetries, by interpolating the surface by an inclined plane and using the constructed projection algorithm for that case or, by last, approximating by a flat seafloor. The comparison of the results among the different approaches illustrates the importance of considering the seabed slope and irregularities for the fairlead tension prediction. Also, this flat seafloor was evaluated with two different projection methods: the one specific for horizontal seafloors and the one developed for general seabed surfaces. This allows to compare the computational time required in both of them.