Numerical Research On Installation Trajectory Of Drag Anchor Based On CEL And DEM Method

Subsea pipelines are important parts of offshore engineering and undertake the important task of transporting materials between the offshore platform and land terminals.If the pipeline damages such as rupture and leakage happened,huge economic and environmental damage will be caused.Drag anchor is the mostly used mooring device.During the installation process,the anchor will be towed a certain distance to ensure reaching the designed depth.If the anchor impacts or hooks with the buried submarine pipeline,great damage will be caused.According to statistics,the pipeline damage cases caused by mooring activities accounted for more than30% of all human factor damages.For reasonably evaluating the risk of pipeline anchor damage and the performance of pipeline protective measures,the installation trajectories of drag anchors in different media are investigated and the dynamic response of the embedded seabed is investigated in this paper.The research contents are as follows:Firstly,the soil resistance and the drag force applied at the mooring point are calculated based on the limit equilibrium and the inverse catenary equation.The linear,power,and nonlinear shear strength distributions in the seabed are established respectively.Computational models of the installation trajectory in different seabed models of the drag anchor are established.The accuracy of the calculation model is verified by comparisons between the installation trajectories,and the calculation models of different shear strength distributions are evaluated.Secondly,based on the Mohr-Columb strength theory and the CEL method,the numerical seabed with linear distribution and nonlinear distribution of shear strength was established by layering the seabed model.The mechanical properties of numerical seabed under different shear strength distributions are analyzed.The rigid shank anchor numerical model is established for simulating the installation process in the numerical seabed with different shear strength distributions.The accuracy of the numerical seabed is evaluated by comparisons with the laboratory model experiments.The results of theoretical analysis and CEL analysis suggest that the nonlinear shear strength model has higher accuracy than the linear and power strength seabed model in analyzing the installation trajectory of the towed anchor.The results of the DEM analysis suggest that the drag anchor is continuously lifted until it is pulled out during the installation process in the rockfill protective layer,and the pull-out trajectory is related to the starting position of the towed anchor installation.The comparative analysis of the installation trajectories of particles with different radii shows that the anchor lifts faster and the pull-out distance is shorter in the protective rockfill with coarse-grained stones,indicating that the pipeline protection layer with coarse-grained stones has better anchor damage protection performance.