Operational displacement shape based estimation of vibration borne stress variation in a pipeline

Excessive vibration is one of the leading causes of high cycle fatigue in process piping. One way to assess the severity of vibration is to measure the stress variation caused by it with strain gages. An alternative way is discussed in this paper. The basis of this alternative way is formed by operational displacement shapes (ODS) measured from the actual structure. In order to test the method, two different sets of ODS were measured from a secondary feed water piping system of Loviisa 1 nuclear power plant. Two sets of forces were then derived from these two measured ODS sets. Each force set is supposed to be equivalent to the actual loading in a sense that it should produce behaviour analogous to the actual measured ODS when applied to a finite element (FE) model. Two sets of ODS were then computed with the FE model using the derived force sets as input values. As a result, two sets of stress amplitudes were also obtained for the ODS. Vibration borne stress variations in the time domain at the desired locations were then estimated by applying the principle of superposition to the stress variation amplitudes of the individual ODS.