Electrical and Computer Engineering Department-College of Engineering-Sultan Qaboos University
Modeling of Stray – Current Corrosion in ESP Well Casing &
Adjacent Catholically Protected Pipelines
Husam Mohamed Al Mandhari
Abstract
Corrosion is becoming an increasing threat to the integrity of oil field structures. Cathodic
protection (CP) is usually used to prevent such a corrosion. It turns the entire protected structure
into the cathode of a corrosion cell and it is used extensively on steel structures in the oil and gas
industries (pipelines, well casings and tanks). There are two types of cathodic protection systems,
namely, the galvanic and impressed current types. Galvanic cathodic protection systems utilize
galvanic anodes, also called "sacrificial anodes", that are electrochemically more active than the
structure to be protected. In the impressed-current cathodic protection, the anode is connected to
a DC power supply, which regulates the current flow into the anode. In this project, it is shown
that computer modeling and simulation approach enables results to be obtained at the design
stage, allowing CP systems to be optimized so that interference can be minimized. Application of
the Boundary Element Analysis System (BEASY) software package allowes CP interference to
be assessed in terms of the normal current density. BEASY is used to analyze the effect of
cathodic interference on stray-current corrosion and also to analyze the corrosion rate of
pipeline/well casing arrangments. Different practical cases are simulated, e.g. cathodic
interference, anodic interference, induced interference, combined interference, galvanic cells as
those occuring at PDO's sites, and other more cases. The corrosion rate has a linear relationship
with current density, where a higher value of the anodic current density (current leaving the
structure under interference) indicates a higher corrosion rate. It is concluded from the simulated
cases by BEASY that the cathodic interference is more serious than anodic fellow, since the
current leaving the pipe close to the protected structure has a high current density. This is due to
the localized anodic zone on the foreign pipe. In the case of anodic interference, however, the
unprotected structure is closer to the ground bed and it has a cathodic aspect and receives a
protection current over well located areas. This current returns to earth at zones remote from the
ground bed. Therefore, this type of interference seldom causes severe and localized corrosion.
The variation of the current density and the distance of the anode is also studied and it is
concluded that stray current increases by increasing the current density of the anode and
Electrical and Computer Engineering Department-College of Engineering-Sultan Qaboos University
decreases when the anodes were located far from the structres. The length of the anode is also
studied where a well casing is considered to be an anode. It is concluded that when long anodes
(unused well casings) are used, the protection current distribution becomes better. Concerning
the stray-current density profile, it is shifted to the lower part of the well casing when using the
long anode compared to the normal anode where it covered a lagre area at the top of the well
casing. The effect of soil conductivity is also studied and it is concluded that stray current
decreases in areas having high conductivity. The software used in this project is helpful in future
planning before installing structures in the oil fields, where it gives the user a broad idea on the
type of protection needed and the effect of cathodic protection on nearby unprotected metallic
structures.