Oil and gas pipeline structural members such as pipes, fittings and their welds would experience various loading as determined by pipeline purpose and mode of operation. The most complicated loading is that experienced by pumping and compressor station pipeline members. Depending on load characteristics and magnitude, growing-up of flaws and catastrophic failures can occur. The paper analyzes the failure occurrences in operating the pumping and compressor station pipelines. Results of investigations into the actual loading spectrum are presented. It is shown that the random loading of pipelines can be presented as steps of a block with due regard for changes in pressure, temperature and vibrations that are resulted from pumping or compressor unit functioning. It is settled that vibrational loading in the form of small amplitude cyclic stresses has an influence on crack nucleation in pipeline members containing stress concentrators. Through schematizing the actual loading, a procedure has been developed to assess the durability of pipeline members with due regard for the loading characteristics and the stress concentration.
Analysis of the failures of compressor and pumping station pipeline members points to the possibility of failure origin formation due to fatigue. It has been determined from the studies that the compressor and pumping station pipeline members experience random loadings as a result of changes in pressure of the transported product, temperature of pipeline wall, product pulsation, vibration, etc. Due to these, small amplitude cyclic stresses are superimposed on the average-level stresses that change due to changes in the internal pressure and temperature. Actual loading spectrum for real compressor or pumping station pipelines has not previously been studied sufficiently. Also, the influence of small amplitude cyclic stresses on the fatigue crack formation in the welds of pipelines has not been clearly understood.
