An Efficient Global, Local And Solid Finite Element Modeling Approach For Pipeline Expansion Loops

An expansion loop works as a spring in a pipeline where it absorbs excessive thermal expansion of the pipeline. Assessing the response of the loop can be complicated as the loop response depends on a wide range of parameters, such as the soil-pipe frictions, sleeper-pipe frictions, bend stiffnesses, and variable internal pressure and temperature during operational conditions. This paper describes an analysis methodology where multi-level (global-local-solid) finite element (FE) modeling is adopted. A local FE model of the expansion loop is used to investigate its response and deformation capacity under expansion/contraction axial loading. A global pipeline model is used to model the global response of the pipeline with the loops replaced with springs, whose stiffnesses derived from the local loop FE model. Critical regions are identified and modeled with more detailed 3D solid models, with the corresponding loads extracted from the global model. The 3D detailed solid FE models capture local effects such as stress concentrations due to local discontinuities in geometry and the reduced bending stiffness of the bends due to wall thinning during manufacturing as well as bendinginduced ovalization (Brazier Effect). General agreement between the global and local model predictions is verified to ensure result accuracy. The paper focuses on the loop bends, which have been found to be one of the more critical elements in the loop because of the Brazier Effect. Geometric discontinuities at the bend-pipe welds and at the flanges on both sides of the loop normally lead to increased local stresses and possible fatigue concern. Therefore, it is very important that all relevant design scenarios be captured in the analysis procedure. The globallocal-solid modeling approach presents a cost-effective approach to analyzing a pipeline with loops.