High pressure oil and gas pipelines which are already in the ground are tested for strength by pressurizing them to at least 125% of their maximum operating pressure (MAOP) at any point along their length. The steel is stressed to specified minimum yield (SMYS) and above during the testing. Testing for leaks 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.