This paper describes the methodology and the required finite element models for highly detailed 3D finite element simulations of trawl gear impact on pipeline. The multi-physics simulation software LS-DYNA was used as solver. The simulation methodology has the following characteristics:
high resolution 3D models are used to represent the pipeline including an elasto-plastic material model;
a 3D soil model is used that deforms and interacts with the pipeline;
simple von-Mises type soil material model;
the trawl gear is modeled as rigid 3D bodies with correct geometry and inertia.
The simulations are transient dynamic analyses and include contacts, buoyancy, and gravitational forces. The influence from the water fluid dynamics is modeled using the hydrodynamic added mass approach following DNV-RP-F111. The inertial effects from the pipeline content and coating is modeled in a simplified manner as added masses.
Developed simulation models and the method described are then used to evaluate the influence of several variables on the dent depth due to trawl gear impact on an uncoated field joint. The influence from the following factors were studied: pipeline dimensions, soil support (embedment), internal pressure (no pressure and operating pressure), soil shear strength, and trawl gear impact velocity. The trawl gear, a trawl board and a clump weight, was represented using geometrically accurate 3D models.
It is demonstrated that the developed methodology for simulation of trawl gear impact on pipelines is numerically robust.
When designing a pipeline, it is necessary to do a careful assessment of the loads a pipeline is expected to be subjected to during its design life. All the load cases in the pipeline lifetime are considered: starting from pipe laying, water filling, and pressure testing, to the operational loads caused by pressure, temperature and flow rate of the transported fluid as well as environmental loads and loads imposed by third parties, like dropped objects, fishing gear, dragging anchors, et c.
