FEM?modeller för expansionsanalys av piplines
REINERTSEN AS performs expansion and buckling analyses of pipelines using ANSYS, afinite element modelling and analysis tool. In an expansion analysis the pipeline is modelledwith thin?walled pipe elements called PIPE20 which allows plastic deformation. However, ina recent analysis the results retrieved from the PIPE20 element were incorrect for strainsabout 1 percent. One main issue was that the PIPE20 element overestimated the strains.The purpose with this Master Thesis was to become familiar with the theories and analysismethods used in pipeline design. By using the knowledge gained a short examination of theweakness in the PIPE20 element was carried out and a first development of a new model forexpansion analysis in ANSYS was made.First, a detailed initial study was made. The analytically derived functions were examinedand a model was created in ANSYS using PIPE20 elements with linear?elastic materialmodel. This model was, when compared to the analytically expected results, very accurateand could thereby be used in further analysis.A short study of the PIPE20 element was carried out for a linear?elastic perfectly?plasticmaterial model. The results from PIPE20 were compared to analytically derived results andto results from Pipeline Analysis System (PAS) a 2D?analysis tool developed atREINERTSEN. It was seen that results from calculations involving the PIPE20 element differsfrom the expected results. When the rotation is small and the cross?section is almostcompletely elastic the expected value and PIPE20 element output is practically identical butwith increasing rotation and plasticity the accuracy is lessen this due to the few number ofintegration points. When more than 23 percent of the cross section is plastic the elementoutput is utterly incorrect. The conclusion is that the PIPE20 element is not suitable inapplications where plastic deformation of a major part of the cross?section is expected.One solution is to create a model with a beam?shell assembly. The PIPE20 elements in themodel where inaccuracies have been found can be replaced by shell elements. A firstdevelopment of such a model was made. The beam?shell assembly was exposed to the sameloads as a model with only PIPE20 elements. The strains where compared and was found tobe smaller in the beam?shell assembly than in the PIPE20 model, indicating that a modelwith a beam?shell assembly can be used. Even though more development and testing areneeded the first results are satisfying.