Collapse Testing of Thermally Treated Line Pipe for Ultra-Deepwater Applications

The Mardi Gras Transportation System is an ultra deepwater pipeline system that will support a number of prospects in the Gulf of Mexico, including the Holstein, Mad Dog, Atlantis and Thunder Horse field developments. To support the design of the deepest portions of the Mardi Gras Transportation System, a full-scale collapse test program was performed, and was aimed at measuring, quantifying and documenting the increase in pipe strength and collapse resistance as a result of the thermal induction heat treatment effect (thermal aging) from the pipe coating process. This paper presents a summary of the test program and the results of all testing performed on Europipe pipe samples. Two collapse tests and five pressure + bend tests were performed on as-received and thermally treated pipe specimens. These specimens were API Grade X65 line pipe, with an outer diameter of 28 inches (711 mm) and a wall thickness of 1.5 inches (38 mm). Geometric measurements, material coupon tests, and ring expansion tests were also performed. The coupon tests also included specimens taken from the original plate samples from which the full-scale pipes were manufactured, providing data on the effect of the UOE process on circumferential compressive strength. For the thermally treated pipe specimens, thermal treatment was performed by running the specimens through a pipe coating mill, simulating a fusion bond epoxy coating operation. This process involved preheating specimens to 240°C using induction heating. Subsequent material and full-scale tests on these specimens resulted in an increase of cross-sectional residual stresses by almost threefold, an increase of the circumferential compressive yield strength of the pipe by approximately 23% and an increase of pipe collapse strength by approximately 28%. The results of these tests are also compared to the collapse and collapse + bending equations found in the DNV (DNV OSF101) and API (API RP 1111) offshore pipeline codes, as well as the collapse equations found in API Bul 5C3 for downhole casing applications. In particular, it has been shown that the thermal treatment of the UOE pipe specimens can increase the DNV fabrication factor from 0.85 to 1.0.