A Comparison of Piping Stress Calculation Methods Applied to Process Piping System for Seismic Design

For design of industrial plants like LNG (liquefied natural gas) terminal the earthquake engineering for piping design is one of the most important design criteria [1]. The required calculation approaches in analyzing reactions of piping systems due to seismic events are specified in a variety of international and European codes and standards (e.g. in [2], [3] and [4]). Within these methods the simplified static equivalent method and the modal response spectra analysis are the most used in practice. From the engineering’s point of view the simplified static analysis has obviously its advantages. This is why it is often used to perform some preliminary or final stress calculations. But in practice it also can be seen that this approach is even extended to the piping connected to the storage tank, where the modal response spectra analysis shall be applied according to the codes [3] and [4]. Furthermore there is no precise prediction about the results of the simplified static method in the area of piping design, neither in aspect of reliability nor in aspect of economy. This article, based on a calculation of a typical unloading line for a new LNG storage tank – carried out by means of the CAESAR II program [7], compares the simplified static equivalent method and the modal response spectra analysis. The aim of this article is trying to set a general evaluation criterion and to give an answer to questions, under which conditions the simplified calculation method can be used. How big are the differences of the results between the two approaches?