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.

Line pipe intended for offshore applications has to be designed predominantly with regard to external pressure in order to avoid collapse. High resistance to external pressure is vitally important for the use of pipes in such applications. A test program has been carried out in order to verify the resistance of HTS (helical seam two step) welded line pipe against collapse. It was demonstrated that the two step pipe manufacturing process has a beneficial effect on collapse resistance. HTS pipes therefore shows a good collapse performance compared to the design equations given in relevant offshore standards. One aim of the work carried out was to quantify the influence of relevant parameters on the result of full-scale collapse test by finite element analysis. The actual collapse pressures and those predicted using currently available design equations are compared and verified for various boundary conditions. The paper concludes with a discussion of the major findings and with a brief outlook to future research issues.

Line pipe intended for deep water applications has to be designed predominantly with regard to external pressure in order to avoid plastic collapse. As a consequence of cold forming during UOE pipe manufacture and the subsequent application of anticorrosion coating, the characteristic stress strain behavior has to be taken into account for a reliable prediction of the collapse pressure. Verification of collapse resistance of large diameter pipes against external pressure requires adequate and reliable component testing using a sufficient number of pipe samples. These samples have to be subjected to test conditions, which closely simulate the situation in service. As the test results may depend significantly on its boundary conditions, the results needs to be thoroughly analysed and compared with existing prediction methods. It is for these reasons that such full-scale testing is time-consuming and costly. The work presented in this paper aims at clarifying and quantifying the effect of existing test boundary conditions on the results of collapse tests (collapse pressures). Correlations will be established between material properties found in laboratory tests and associated component behavior. In this context it had been necessary to develop an accurate and reproducible compression test method. The actual collapse pressures and those predicted using current available equations are compared and verified by Finite Element calculations. The paper concludes with a discussion of the major findings and with a brief outlook to future research issues.

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This paper presents details on the load bearing capacity of pipelines subjected to combined loading (internal pressure, axial and bending load) based on the findings of a recent research project of the European Pipeline Research Group (EPRG). Firstly, the failure mechanisms of line pipe under combined loading, which depend on local geometry, material characteristics as well as local and global applied loading, are characterized. Afterwards, differences between laboratory testing and the real-life situation of pipelines subjected to combined loading are described. Here, optimal boundary conditions for realistic testing are defined. Finally, a large variety of modelling approaches, specifically dedicated to combined loading experimental data from 59 full-scale tests on line pipe joints have been analysed. The relevant parameters in the analysis of buckling behavior of the pipes were: actual material properties, boundary conditions, failure phenomena and strain at failure, with the final aim to issue recommendations with regard to the selection of modelling approaches, sensitivity towards input parameters as well as strain threshold values. For the prediction of the limit pipe deformation a large selection of equations suggested by various authors in terms of critical bending moment, critical strain and critical stress for various loading conditions were considered. The methods differ in solution methodology (analytical vs. numerical), in the definition of material behavior (elastic, elastic-plastic) and in the definition of critical conditions and critical points. Then, the different types of buckling as a function of pipe geometry were characterized. Finally, the buckling behavior of an actual bending test was simulated using measured input data.