7 Pipe Material Selection and Fracture Control

Despite the very different properties of CO2, relative to natural gas, conventional pipeline materials and construction practices can be used provided certain basic precautions are observed. Clearly, the most important of these is to ensure that no free water is present in the system (see Chapter 3) since the formation of carbonic acid can lead to extremely aggressive corrosion wherever the steel internal surface is directly exposed. This requires limits on water content that are appropriate to prevent formation of free water under the most stringent operating conditions; potential upset conditions also need to be considered. In addition to very close control, during operation, of the pipeline receipt composition, it is also necessary to pay strict attention to dewatering and drying after hydrostatic testing, to ensure that water cannot remain in low points, dead legs, crevices or appurtenances. It may be necessary to delay the installation of certain types of valve, whose internal design might make complete drying difficult and which do not have corrosion-resistant trim, until after hydrostatic testing. If these precautions are adhered to, then the use of corrosion-resistant alloys or liners will not be necessary. Caution should be exercised in the use of internal coatings intended for flow optimization since they may become disbonded as a result of CO2 permeation or, for some coating formulations, solvent action. If severe, such disbondment and detachment could have significant consequences for the operability of downstream facilities.

Essentially all CO2 pipelines in operation today have used conventional, carbon steel piping materials. Obviously, where the impure CO2 stream contains H2S in excess of the limiting concentration specified by the governing standards and regulations, additional requirements for sour service come into force. Typically, these involve provisions for resistance to environmental cracking and limited maximum hardness in the pipe body and weld. Similarly, conventional field welding processes and consumables have been successfully applied in the construction of CO2 pipelines, though more stringent limitations on internal flaws may be advisable, to minimize the potential for crevices that could trap hydrostatic test water. Again, for sour service, additional limitations typically apply, primarily related to procedure qualification, wall thickness transitions, maximum weld hardness, and nondestructive inspection acceptance standards (see, for example, Canadian Standards Association [1] Z662 2011 Clause 16.7).

The most important aspect of material selection for CO2 pipelines relates to fracture control. The implications of the specific thermodynamic characteristics of CO2 relative to the requirements for ductile fracture propagation control can be extremely important and can significantly influence the choice of pipe strength and wall thickness for a specific design pressure. In many cases, it will no longer be possible to select the thinnest pipe that is consistent with the requirements of design standards for pressure containment and minimum allowable wall thickness or maximum allowable D/t ratio. For these reasons, fracture control design is discussed in considerable detail in the following section.