Structural Collapse of Corroded Deepwater Pipelines Subjected to External Pressure
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Deepwater O&G pipelines, used to transport oil and gas from offshore production units to onshore refineries, are subjected to the corrosive effects of salt water and to massive internal and external pressures. The risk of structural collapse in a pipeline is largely dependent on the levels of internal and external corrosion affecting pipe sections and on the amount of internal and external pressure on the pipe. Corrosion is a time dependent phenomenon that usually requires several years to produce a corrosion defect sufficiently large to cause a significant reduction in the collapse pressure of a pipeline. Consequently during installation it is considered that the pipeline is free of any metal loss due to corrosion. When in service, if the transported fluid is gas, an external overpressure occurs when there is a shutdown of operation.If the transported fluid is oil, an external overpressure occurs when, after a shutdown, the line is evacuated.
Due to the high cost of repairing deepwater pipelines it is anticipated that soon the oil companies will be highly interested in developing efficient methods for assessing corrosion defects in pipelines subjected to dominant external pressure. These same effects on when subjected to dominant internal pressure are already well documented industry-wide.
Depending on its external diameter to thickness ratio (De/t) the pipeline is classified as a thick tube (De/t less than 20) or as a thin tube (De/t equal to or greater than 20). Subsea pipelines usually have external diameter to thickness ratios (De/t) that range from 10 to 30. In this range of (De/t) ratio the failure behavior of a non corroded pipeline subjected to an external pressure is characterized by the interaction between plasticity and geometric instability. That is, yielding of the pipeline cross section begins before the collapse by geometric instability is attained. The assessment of corroded pipelines subjected to external pressure has not been extensively researched. More research projects are needed and much more full-scale collapse tests must still be performed. The failure behavior of a corroded pipeline subjected to an external pressure is sensitive not only to initial imperfections but also to the dimensions of the corrosion defect, that is,
• defect depth (d)
• defect length (L)
• defect width (w)
The corrosion defect can be seen as a geometric imperfection whose dimensions grow with time. The detrimental effect of the corrosion defect on the pipeline strength varies in function of its position in relation to the ovality of the pipe cross section. Depending on its relative position the corrosion defect intensifies or lessens the pipe ovalization.
The Catholic University of Rio de Janeiro (PUC-RJ) and Petrobras have launched a JIP (Joint Industry Project), named Collapse of in Service Deepwater Pipelines Joint Industry Project (CSDP JIP), to address the problem of corroded pipelines subjected to external pressure. The JIP will be conducted by PUC-Rio and besides paying the JIP share fee, Petrobras will have an active participation in the project. The objective of the CSDP JIP is to develop an extensive experimental program in which full scale tubular specimens containing internal single corrosion defects will be submitted to external hydrostatic pressure up to collapse. Twelve participants will be necessary to perform the full scope of work of the CSDP JIP. However a minimum of three participants will be sufficient to start the project. The CSDP JIP is composed of four modules. Fifteen tests will be performed in each of these modules.
Sources: Adilson C. Benjamin (CENPES), PUC-RJ and Petrobras