Government environmental protection regulations are forcing facilities with petroleum or other products that are hazardous to the environment to have their product pipelines pressure tested periodically based on what applicable Code requirements that facility or pipeline fall under. Offshore pipelines, station piping, intrastate, or interstate pipelines are regulated by different Government agencies depending on jurisdiction.
Hydrotesting of pipes, pipelines and vessels is performed to expose defective materials that have missed prior detection, ensure that any remaining defects are insignificant enough to allow operation at design pressures, expose possible leaks and serve as a final validation of the integrity of the constructed system. ASME B31.3 requires this testing to ensure tightness and strength.
Buried high pressure oil and gas pipelines are tested for strength by pressurizing them to at least 125% of their maximum allowable operating pressure (MAOP) at any point along their length. Since many long distance transmission pipelines are designed to have a steel hoop stress of 72% of specified minimum yield strength (SMYS) at MAOP, the steel is stressed to SMYS and above during the testing, and test sections must be selected to ensure that excessive plastic deformation does not occur.
For piping built to ASME B31.3, if the design temperature is greater than the test temperature, then the test pressure must be adjusted for the related allowable stress at the design temperature. This is done by multiplying 1.5 maximum allowable working pressure (MAWP) by the ratio of the allowable stress at the test temperature to allowable stress at the design temperature per ASME B31.3 Section 345.4.2 Equation 24. Test pressures need not go beyond a value that would produce a stress higher than yield stress at test temperature. (see ASME B31.3 section 345.4.2 (c) for reference)
Other codes require a more burdensome methodology. Code BS PD 8010-2 requires testing to 150% of the design pressure - which should not be less than the MAOP plus surge and other incidental effects that will occur during normal operation.
Leak testing is performed by balancing changes in the measured pressure in the test section against the theoretical pressure changes calculated from changes in the measured temperature of the test section. Australian standard AS2885.5 "Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing" gives an excellent explanation of the individual factors involved in this methodology.