Page 23 - Oil and Gas Simulations
P. 23

APPLICATION NOTES

                                             FIGURE 3. Geometry of the oil platform and outer
                                             boundaries. A constant potential is set at the sacrificial anode
                                             surfaces, while at the cathode surface oxygen reduction
                                             is limited by the rate of transport of oxygen dissolved on
                                             the surface, resulting in a constant normal current density
                                             boundary condition at the steel surfaces of the structure.

èCORROSION PROTECTION                        èUNDERSTANDING                               By understanding this relationship, the
SIMULATION                                   CORROSION PROCESSES                          model is able to give a good overview of
                                                                                          the less protected and therefore critical
In order to fully protect the platform,      COMSOL Multiphysics® simulation              parts of the oil platform.
design engineers need to ensure that the     software can be easily used to gain
different parts of the structure are well    insights into the current and voltage           The model can be easily expanded
within the corrosion-protected range.        distribution of electrochemical systems.     to take into account additional effects,
In other words, the voltage delivered by                                                  such as overpotential or concentration
the anodes needs to be sufficient to drive      In this model, the electrolyte potential  gradients, which can be used to develop
oxygen reduction at the steel to balance     was first obtained using an approximation    a more detailed understanding of the
the current.                                 that assumed constant current density        corrosion processes. v
                                             in the cathodic structure. The model
   The first step in the design of the       demonstrates that the lower the
protection system is to investigate the      electrolyte potential, the less protected
potential of the steel structure assuming    the structure will be from corrosion.
a constant cathodic current (oxygen
reduction). This ensures that the anodes     FIGURE 4. Left: Electrolyte potential on the sacrificial anodes and on the platform. Right:
will be able to deliver the required         Electrolyte potential on one of the legs of the platform structure.
potential to maintain the given current.
The potential has to be well within the
required range where oxygen reduction
protects the structure and where hydrogen
evolution is avoided (see Figure 3).

   Multiphysics simulation can be used to
predict the electrolyte potential at the
surface of the anodes and steel structure
(cathode). The surface plot in Figure 4,
left, shows how the potential varies about
several hundred millivolts depending on
the position of the anodes.

   The lower the electrolyte potential
at the interface, the more positive the
potential difference is between the oil
platform and seawater—an expected
result since the current in the electrolyte
flows from the anodes to the cathode.

   Figure 4, right, shows a close-up of one
of the structure’s legs. The inside bottom
part of the leg has the lowest potential,
indicating that this part of the structure
will be the part most susceptible to
corrosion.

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