Hydrostatic testing has long been used to determine and verify pipeline integrity. When a pipeline is designed to operate at a certain Maximum Operating Pressure (MOP), it must be tested to ensure that it is structurally sound and can withstand the internal pressure before being put into service. Generally, gas pipelines are hydrotested by filling the test section of pipe with water and pumping the pressure up to a value that is higher than maximum allowable operating pressure (MAOP) and holding the pressure for a period of four to eight hours.
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.
Buried high pressure oil and gas pipelines are tested for strength by pressurizing them to at least 125% of their Maximum Allowable Working Pressure (MAWP) at any point along their length. Since many long distance transmission pipelines are designed to have a steel hoop stress of 80% of Specified Minimum Yield Strength (SMYS) at Maximum Allowable Operating Pressure(MAOP), this means that 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.
Australian standard AS2885.5 "Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing" gives an excellent explanation of the factors involved.
Test pressures for all sizes of API 5L seamless pipe and for API 5L welded pipe in sizes 18 and smaller, shall be held for not less than 5 seconds. Test pressures for welded pipe in sizes 20 and larger shall be held for not less than 10 seconds. For threaded- and-coupled pipe, the test shall be applied with the couplings made up power-tight if power-tight makeup is specified on the purchase order, except that pipe sizes larger than 12 3 /4 may be tested in the plain-end condition. For threaded pipe furnished with couplings made up handling-tight, the hydro- static test shall be made on the pipe in the plain-end or threads-only condition or with couplings applied, unless otherwise agreed by the purchaser and the manufacturer.
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