Formulation and Application of Finite Element techniques for Slender Marine Structures Subjected to Contact Interactions

Abstract

The main purpose of this work is to formulate and apply new computational strategies for two contact-governed problems where existing finite element software suffer from poor efficiency and lack of robustness. The first problem is concerned with trawl board pull-over interaction of subsea pipelines while the second deals with reeling analysis of history-dependent flexible pipes. Previous numerical models for trawl-pipe interaction based on simplified geometry modeling has struggled with contact-related convergence problems. A contact element with a continuous description of the trawl board contact geometry and the pipe geometry was therefore developed. The assumption of a rigid contact geometry for the trawl board and the use of line-line and line-point contact kinematics resulted in good numerical efficiency properties. The ability to predict the pull-over responses depends heavily on the modeling of the trawl board hydrodynamic loads. A fairly advanced six degree of freedom load model with precomputed hydrodynamic coefficients was therefore established. An extensive simulation work was carried out to validate the trawl-pipe computational strategy and to identify sensitive model parameters. Regarding the former, the proposed numerical model was demonstrated to predict pull-over load impulses within a 10% margin of model test measurements and was thus concluded to be capable of describing the relevant effects of the pull-over. The sensitivity study revealed that the interaction behavior was greatly influenced by the board-pipe friction coefficient, the tension level in the wire between board and trawl net, the towing line drag properties and the direction of over-trawling. Due to the sensitivity of the input parameters, it was concluded that a proper validation against experimental tests is necessary for future work of similar kind. Further studies should aim to quantify the degree of non-conservatism present for nonperpendicular crossings and attempt to improve current design load recommendations by including more model parameters. Reeling operations with history-dependent material behavior and extensive contact interactions along the material transport route are often not feasible to simulate with conventional finite element software. This relates to contact-related convergence problems and the need for long meshes with small and equal-sized elements giving poor numerical efficiency. These issues were successfully solved by developing a Lagrangian-Eulerian beam formulation that enabled for a virtually fixed mesh in space. The proposed formulation was subjected to various benchmark tests where it was demonstrated to provide similar accuracy as the conventional Lagrangian method. In recent years, subsea contractors have experienced torsional failures in spoolbasevessel load-out operations of flexible pipes. An idealized finite element model was therefore established to gain insight into such operations and to identify the mechanisms responsible for the generated torque. Torsional failures were identified for three different mechanisms and strategies to avoid them were proposed. A comparison study against a physical load-out operation should be conducted in future work to quantify the ability to predict the torque and to reveal possible model deficiencies.