Estimation of nipple support torsional stiffness for the calculation of pipeline natural frequencies

Natural frequencies of pipelines depend on the stiffness of their connection with equipment. As a rule, nipple supports are used as connections. The existing methods of calculation consider these supports as absolutely stiff, or neglect their stiffness at all, or take into account stiffness for translational displacement only. It leads to significant error (up to 12.7 % for the system under investigation in the present research). Experimental asessment of nipple support torsional stiffness is difficult to perform. Since many pipelines are connected in bundles, it is necessary to research the influence of the vibrations of one pipeline on those of another. A method of assessing nipple support torsion stiffness based on the correlation of experimental and calculation research is proposed in the paper. Natural frequencies of a bundle of two pipelines are obtained experimentally using a laser vibrometer. Calculation research with the aid of the ANSYS software makes it possible to find the torsional stiffness of supports about all three axes of the Cartesian coordinate system. In this case the calculated natural frequencies are close to those experimentally obtained. The results of calculation research make it possible to discuss the influence of pipeline nipple support torsion stiffness on the pipeline’s natural frequencies. The proposed method allows reducing the calculation error for the pipeline natural frequency down to 2.6 %.