Develop Acceptance Criteria of Pressure Test of Buried Pipeline Using Uncertainty Analysis

Hydrotest is one of the most important inspection activities to assess the integrity of pipelines prior to starting transportation of hazardous liquids. If the pipeline is buried, and welds cannot be inspected visually for leak, it is required that the hydrotest pressure is held for 24 h to ensure that the pipeline is leak free. Ideally the pressure should remain constant, but this is practically impossible due to changes in temperature. Pressure and temperature of the pipelines are recorded continuously and evaluated at the end of the test. The pressure drop should be in line with the temperature drop based on compressibility relationship. However, nonconformance is expected due to accuracy of the individual instruments used to measure the pressure and temperature. It is then ambiguous to the evaluator whether the nonconformance is attributed to small leak or to instrument accuracy. In this paper, general uncertainty analysis with parametric study is used in the planning and designing phase of pressure test. The objective is to develop an appropriate acceptance criterion for the pressure test of buried pipelines. Actual field hydrotests are used to illustrate and validate the outcome of the analysis. The results show that high uncertainty values reduce the validity of the actual hydrotest. This is because the test would most probably pass regardless of the actual pressure drop measured.