Page 22 - Oil and Gas Simulations
P. 22



Corrosion is the unrelenting enemy of steel stuctures,
costing the U.S. alone an estimated $170 billion a year.
The oil and gas industry, with many complex components
constructed to withstand harsh environmental effects,
takes on a sizeable share of these costs.


From production platforms to oil production     reduction takes place at the surface of the  FIGURE 1. 40 cylindrical sacrificial
strings, corrosion begins the day the steel is  steel structure.                             anodes surround an oil platform structure
cast, necessitating that a protection strategy                                               immersed in seawater to protect the
be implemented from the start.                     An example geometry of an oil             structure from corrosion.
                                                platform structure is shown in Figure 1,
   Offshore structures immersed in              where 40 cylindrical sacrificial anodes      FIGURE 2. Polarization behavior of the
seawater can be guarded against                 have been placed relatively close around     sacrificial anodes (blue) and steel surface
corrosion using cathodic protection.            the oil platform.                            (red). The graph shows the oxygen
Structures such as oil platforms, which                                                      reduction at the surface of the steel along
exist both in and out of the water due             The polarization of the anodes and        the range of the cathodic limiting current,
to tidal changes, benefit the most from         the oxygen reduction reactions occurring     which is represented by the horizontal
this form of protection, since exposure to      at the surface of the steel structure are    segment of the red curve.
saltwater and air accelerates degradation       shown in the graph in Figure 2. The red
from corrosion. Different strategies for        curve represents the polarization of         This model is part of the comprehensive Model
cathodic protection exist, including using      the steel surface while the blue curve       Library shipped with every installation of COMSOL
an impressed external electrical current or     shows the polarization of the sacrificial    Multiphysics® software. With detailed step-by-step
sacrificial anodes, however, due to their       anodes. The currents in the steel structure  instructions, these models provide an introduction
simplicity, sacrificial anodes are often the    (red) and the sacrificial anodes (blue)      to multiphysics simulations.
preferred form of protection.                   are plotted as functions of the electric
                                                potential measured relative to a common
èCATHODIC PROTECTION                            reference.
                                                   Corrosion protection is achieved at the
Steel is essentially an unstable state of       point where the cathodic current (red)
iron and corrosion is the process by which      is equal in size (but opposite in sign) to
iron returns to its natural state. Corrosion    the anodic current. The shape of the blue
occurs when a current is formed by a flow       polarization curve is determined by the
of electrons from a high potential anode        number and design of the anodes in the
to a lower potential cathode.                   system, and the current density for oxygen
                                                reduction is limited by the oxygen supply.
   Cathodic protection using sacrificial        The result is a near-constant, transport-
anodes follows a simple strategy: A steel       limited current spanning a few hundred
structure, such as an oil platform, is          millivolts with a range of potential
electronically connected to a less noble        differences at the structure’s surface.
metal, aluminum, for example. When
immersed in seawater, which acts as an             As long as the oxygen reduction taking
electrolyte, the sacrificial anode becomes      place on the surface of the steel remains
anodically polarized and the steel              within the corrosion protection range,
structure becomes cathodically polarized.       the cathodic-limiting current causes the
The anodes are dissolved through anodic         sacrificial anode to be corroded and
dissolution of the metal, while oxygen          provides a degree of protection to the oil
                                                platform structure.

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