Cathodic Protection Offshore Cad

8/2/2019 Cathodic Protection Offshore Cad

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,

Fc< A

Structure i /

Current

>.,1 r,!,. ,


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Inche boundary element technique we will be using the current densit:.,normal co the boundary, therefore , in will be abbreviated to i and mean thecurrent density normal to the boundary.

Anode Equation From the proceeding discussion it is clearthat it is not simpl(the potential or resistance of the medium which controis the corrosion rate but

Many different types of reactions can takealso the electrode surface reactions.place depending upon the electrolyte (medium), the material and the level of thepotential.

This may be expressed as follows,

i = fa (Ea)a

(6 )where f is an equation which describes the relationship between current

a

density and potential for the anode surface.

In practice fa is a function of many factors both chemical and environme~ltalwhich will be discussed later.


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;~--

Figure 3 Special tube element de~~eloped fortheanalysis of offshoreplatforms .1 sshown in fig.2.

incr+-sing

Figure 4 Polarization curve showin~effectOfincreasinghistory

I

t

1

pipeline*

anode,/

1-/.- -./

::eawate7-

Symmetrv~Figure 5 Pipeline analysis


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anode are assumed to be axisymmetric about the centrelineof the pipe,Fig.3.

The electropotentials used in corrosion engineering are only relative co areference electrode (eg.Ag/AgCl). Therefore, the natural boundary conditio~. atinfinity is zero potential. To define the boundary element model of the pipeline,elements must be placed on the cathode, anode and on abox surrounding the problem.The box is placed sufficiently far away not to affect the potential field and acondition of zero current flow is imposed on this boundary. This ensures that allcurrent flowing from the anode is accepted by the cathode surface. The boundaryelement model is shown in Fig.6.

--


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.

-.

---1 ?/ ,.A

-J K, , *w\ .1 Q---~,c~~N- --___T DISTRIBUTIUN ( NIL[ Rfl P P-.. ER sQURRE nETER ).Figure 15 View of K joint with curretn density contours superimposed

I

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r P I P E/ ,A, PLOT 73.19,1 CnC/BERSYFigure 16 Detailed analysis of anode and structural member.

The

anode can be moved or type changed and a new predictionproduced without resolution of the complete equation system.


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~ UOL TRG E DISTRIBUTION ( n[L[ UOLT ),PLOT 26.70 ,1 TEnP LIM I -8 .1 4 s[.2 L[MC 10 -6 . S42C.2 CRC/ BERSY

!!

I

I

(

Figure 17 Predicted contour values potential on steel surface ofmember. Differences in potential between the top surfaceand undersurface of member can,be clearly seen.

i

Figure 18 Alternative anode positions for member.


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Cathodic Protection Offshore Cad