CP is an effective method for protecting coating defects on buried pipelines if sufficient current reaches the pipeline surface. In the presence of nearby foundations shielding effects may occur by the construction constraint.

A case is discussed of a 57 km long 30 inch 3L-PE coated high pressure natural pipeline section requiring a few milliAmps of CP current that is provided by a single voltage controlled rectifier. The current density is in the range of 0.09μA/m2 only. Halfway the trajectory a new 420 m long motorway bridge structure is built above the pipeline. In total 498 concrete reinforced foundation piles are aligned along the pipeline causing a potential risk of shielding the low cathodic current at that particular location. In addition the pipeline is connected to the bridge structure by an anchoring system for stabilization purposes during settlement of the bridge structure after construction. The anchor positions are susceptible locations where significant coating damage may occur on the short or longer term.

The degree of shielding effect was validated with FEM based modeling. The protection level of the pipeline and the DCVG sensitivity was computed for various scenarios. Modeling illustrated the CP current path around the pipeline and the foundation piles. The contribution of the rebar structure of the piles was accounted for as well.

Subject
Electric current, Piping, Corrosion currents, Rebar, Structures, Pipelines, Rectifiers, Electric potential, Piles, Coating defects, Bridges, Polarization, Concrete
Keywords:
CP shielding, concrete bridge structure, computational modeling
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2018
Association for Materials Protection and Performance (AMPP)
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