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 0.09 µA/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.
The Desfa pipeline network consists of different sections that are isolated by insulating flanges operated by its own CP system. The pipeline section under study has a total length of 57 km between chainage 279 and 336,7km. The motorway bridge structure starts at chainage 290,835 km. The bridge structure is circa 430 m long and is constructed in two stages because the pipe makes a turn along its routing.
In order to protect the pipe from mechanical load and settlement a special corridor of foundation piles is foreseen along the pipe route. Figure 1 shows the bridge structure under construction. A first corridor on the left hand side of the bend in the pipe was already built at time of the study. The second corridor on the right hand side of the bend is in the design stage. The motorway crosses the pipe at two locations in respectively the first and second corridor.