Many years of practical and scientific domestic experience shows that during full-scale testing of pipes, fracture occurs not along the longitudinal weld, but along the base metal. The possibility of using expanded double-seam LDP has been proven through field tests, research and certification of pipes.

For example, the results of full-scale tests of expanded double-seam pipes Ø1219×19.1 mm made of X70M steel at the CSM test site in Italy are known. During the tests, the resistance of both longitudinal seams of the pipe was evaluated using bending, pressure bending and hydraulic testing to failure. The tests showed that there were no cracks in the welds, and that fracture occurred in the base material away from the longitudinal weld, thus showing that the weld itself is not a critical point of concentration and does not impair the structural strength of the pipe.

Bench hydraulic tests of the expanded double-seam LDWs were also carried out. The test program included making an artificial cut in the welded joint, the weld zone and the base metal. The tests showed a positive result and compliance with all the criteria for the pipe’s resistance to crack propagation during hydraulic testing.

Field tests before the destruction of the expanded double-seam LDP showed that the coefficient of structural strength of the pipe was Kp=1.003 of the actual temporary resistance, with plastic circumferential deformations in the cell of 7.2%, and uniform circumferential deformation of 4.1%. The test results indicate the realization of the strength and plastic properties of the base metal and welds of the pipe under the conditions of ultimate biaxial loading with a ratio of principal stresses of 2:1 and satisfactory structural strength of the pipe.

Domestic, interstate and industry regulations for the production of pipes and pipelines provide for the use of expanded double-seam LDP with diameters of 1020 mm, 1220 mm, 1420 mm for pipeline operating pressures up to 10.0 MPa:

Each pipe is subjected to a hydraulic leakage test and welds to non-destructive testing, including 100% ultrasonic along the entire length and additional X-ray inspection of weld ends, with weld quality control uniform and regulated for both pipes with one longitudinal seam and two longitudinal seams.

Modern welding materials meet the requirements for weld strength not lower than for the base metal of pipes, and according to actual data, the strength of welds is higher than the base metal. In the production of LDW for main gas and oil pipelines, Ukrtruboizol uses only high-quality agglomerated fluxes from the world’s leading suppliers.

The expansion operation is mandatory, as it is able to effectively level residual stresses and incomplete elasticity of the base metal of the LDP, and ensure a normalized controlled change in mechanical properties during pipe rework.

The integrated approach of Ukrtruboizol LLC to quality control guarantees that, given the operating conditions of the pipeline, pipes with two longitudinal seams are in no way inferior to pipes with a single seam in terms of reliability in operation, provided that a set of technological processes for the manufacture of pipes (in particular, expansion), optimal selection of welding materials, welding modes, non-destructive testing, and acceptance tests are observed.