Wall Thickness DesignAugust 2 – August 28, by Mr. Eng Bin NG
Applicable Codes
Pipe Expansion Calculations
Flexibility Analysis Methodology
5
Pipeline Construction - Conventional & Unconventional
Weekending
Pipe Expansion Calculations, Flexibility Analysis Methodology
Allowable Free Span Calculations, On-bottom Roughness Analysis
Pipeline Protection against Anchors, Wave Liquefaction & Earthquake
Cathodic Protection Design
Pipeline Construction - Conventional & Unconventional
Installation Engineering
1 2 3 4 7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 28
Workshop, revision, exercise
Cathodic Protection Design
In the cathodic protection analysis, calculations are carried out to ensure that the anodes provided are sufficient to provide the total current needed to protect the pipeline during its design life.
The initial and final current demands are to be checked to ensure that the anodes can provide sufficient current output to polarise the pipeline throughout its entire life.
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Half-shell aluminium alloy bracelet anode is typically used for cathodic protection of pipeline.
The CP design can typically be carried out using either one of the following codes:
DNV RP-B401
DNV RP-F103
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The Power to Deliver™ #*
The parameters utilised in the cathodic protection design calculations and the applicable reference to the appropriate codes are presented below:
Close Circuit Anode Potential for Aluminium
CATHODIC PROTECTION DESIGN DATA
Mean
mA/m2
V
Cl 6.6.5, Table 6.6.2
Cl. 6.6.1 – 6.6.4
The following assumptions are made in the analysis:
Anode surface temperature is assumed to be the same as the operating temperature of the pipeline
The anode is assumed to be at the end of its lifetime when the anode material is consumed up to the steel strap reinforcement
Anode length remains constant throughout its design life
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Analysis Methodology
The procedures typically adopted for CP design calculation are presented below:
Current Demand
In order to determine the amount of anode required for the cathodic protection system, it is necessary to calculate the current demand, Ic, to achieve polarisation during the design life of the system as follows:
Ic =
(mean, final) (A)
icm = Design mean current density (A/m2)
Dic = Current density temperature adjustment (A/m2/°C)
ic = Design mean current density at 25 oC (A/m2)
fc = Coating breakdown factor (mean, final)
The current demand for mean and final life of the anode is determined by the above equation.
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Anode Nett Mass
The total mass of anode, Mrequired, required to maintain cathodic protection for the design life can be calculated as follows:
Mrequired =
I c(mean) = Mean (maintenance) design current
density (A/m2)
t = Design life (years)
u = Anode utilisation factor;
e = Electrochemical efficiency (A.h/kg)
The total nett anode mass provided, Mprovided, must be more than or equal to that required, Mrequired for cathodic protection to be sustained throughout the design life.
Where
Anode Current Output
The anode current output, Ia, is obtained from Ohm’s Law:
Ia =
= Design protective potential (V)
Raf = Anode resistance ()
Where
Anode Current Output (cont’d)
The final anode resistance is determined from the final anode dimensions.
The final anode resistance is calculated by assuming that the anode is consumed to its utilisation factor, u, which would give a final exposed surface area and corresponding anode resistance.
For cathodic protection to be effective throughout the design life, the final current output, Ia, must equal or exceed the final current demand, Ic.
Ia (final) ≥ Ic (final).
PIPELINE AND ANODE DATA
* Max. distance between anodes = 300m
CATHODIC PROTECTION DATA AND RESULTS
Parameters
Unit
RP-B401
RP-F103
Buried
Unburied
Buried
Unburied
Initial
mA/m2
20
150
0.02
Line Pipe Coating (a = 0.1; b = 0.003) Field Joint Coating (a = 3; b = 0.3)
Mean
0.09
Final
0.19