INTRODUCTION
Corrosion of steel pipes is one of the serious challenges which affect the
performance of the transmission pipeline network systems. Corrosion is the destructive
attack to a metal due to chemical or electrochemical reaction with its environment.
The phenomenon of corrosion involves reactions which lead to the creation of
ionic species by either loss or gain of electrons. Corrosion of material depends
on several factors. Some of them includes: nature of the material or alloy,
surface condition/roughness, composition, moisture absorptivity, environment,
temperature, humidity and corrosive elements (Revie and Uhlig,
2008).
It has been reported that an increase in ages of the pipes resulted higher
pipe roughness due to the accumulation of various elements around the internal
surface of the pipe (Worthingham et al., 1993).
An increase in roughness has major effect on the performance of the transmission
system i.e. lower flow rate capacity and highpressure drop (Menon,
2005).
There are many studies conducted on corrosion of pipelines for both single
phase flow and two phases gasliquid mixtures flow (Ferrira
et al., 2004; Riemer, 2000; Zhou,
1993; Nestic et al., 2004). The importance
of internal corrosion management in the design of offshore pipeline network
system based on simulation for wet gas production field was discussed by Ferrira
et al. (2004). Riemer (2000) discussed on the
effect of cathodic protection system for pipeline networks based on mathematical
model. The effects of flow conditions and fluid properties on flow characteristics
and corrosion rates were studied (Zhou, 1993). An integrated
CO_{2} corrosionmultiphase flow model was developed by Nestic
et al. (2004) based on mechanistic model assumptions. However, the
above studies did not establish the relationships between the age of the pipes
and performance of the gas transmission system.
This study focuses on evaluating the effect of the age of the pipes on the
performance of the pipeline network system based on the simulation model developed
by Woldeyohannes and Majid (2008) by modifying the flow
equations to incorporate the effect of the age of the pipes. The simulation
model was used to create a system that analyzed pipeline network with different
configurations to get pressure and flow parameters. Based on the variables obtained,
the performance of the pipeline network system was analyzed.
MATERIALS AND METHODS
Simulation is becoming one of the important tools for analyzing the performance
of systems and making operating or resource policy decisions in natural gas
transmission pipeline network systems (Nimmanonda et
al., 2002; Abbaspour, 2005; Hoeven,
2003; Abdolahi et al., 2007; Osiadacz,
1987; Woldeyohannes and Majid, 2009). It is used
to predict the behavior of the transmission pipeline network systems (TPNS)
under different conditions which can be used as a guide for decisions regarding
the design and operation of the real system.
Simulation analysis of TPNS could be a complex process depending on the extent of the network configurations (gunbarrel, branched, looped, etc.), the nature of the gas (single phase dry gas, twophase gas liquid mixture) and other factors like temperature of the gas and pipe corrosion.
TPNS consists of pipes and nonpipe elements like compressors, regulators,
valves, scrubbers, etc. The simulation of TPNS without nonpipe elements is
relatively easier to handle and developed in previous study (Osiadacz,
1987). The addition of nonpipe elements makes the simulation of TPNS more
complex. Some authors perform the pipeline network simulation analysis by neglecting
the nonpipe elements of the network or oversimplifying their effect (Osiadacz,
1987; Letniowski, 1993; Nimmanonda,
2003). Since compressor station is the key element in TPNS, the incorporation
of all its parameters (speed, suction pressure, discharge pressure, flow rates,
suction temperatures) is essential for a complete simulation of gas networks.
Woldeyohannes and Majid (2008) developed a TPNS simulation
model by incorporating the detailed parameters of the compressors. However,
the effects of other nonpipe elements like valves, regulators, scrubbers, etc.
were neglected during modeling. Furthermore, the flow equations neglected the
effect of the ages of the pipe during analysis. The TPNS simulation model by
Woldeyohannes and Majid (2008) was used as basis for
studying the effect of the age of the pipes on the performance of the gas transmission
network system by modifying the flow equations.
The TPNS simulation model: The TPNS simulation model developed by Woldeyohannes
and Majid (2008) was enhanced here in order to integrate effect of the ages
of the pipe on flow equations.
Flow modeling: A new solution scheme was developed to incorporate the age of the pipe on flow modification. Fig. 1 illustrates the general procedure to incorporate the effect of corrosion with age of the pipe and flow modification.
Data of roughness of the coated pipe with service life of the pipe (Worthingham
et al., 1993) was used for developing the correlation between the
service life of the pipe and roughness.

Fig. 1: 
The effect of age of the pipe on flow equations 
Based on the data points, exponential function for approximating the relationship
between the roughnesses of the pipe with that of the service life is obtained
to be:
where, r is the roughness of the pipe in mm and y is age of the pipes in year.
Most gas pipelines operate in the turbulent zone. For the fully turbulent zone,
American Gas Association (AGA) recommends using the following formula for calculating
the transmission factor which is based on the relative roughness and independent
of the Reynolds number as stated by Menon (2005):
where, F is the transmission factor, r is the roughness of the pipe and is the diameter of the pipe.
The relationships between the transmission factor and friction factor can be
express by Menon (2005):
Substituting (3) in (2) and rearranging gives:
Equation 1 and 4 indicate the relationships
between friction factor and age of the pipe.
The general flow equation for a single flow used by Woldeyohannes
and Majid (2008) was modified based on the friction factorin f (4) in order
to integrate the effect of the age of the pipe on the pressure drop. For any
pipe, connecting the upstream node i, downstream node j and the flow through
pipe q, the single flow equation with corrosion relating the two nodes can be
represented as:
where, K_{ij} is pipe flow resistance whose value depends on the pipe
physical attributes as defined by Woldeyohannes and Majid
(2008).
Compressor stations equations: The basic quantities related to a centrifugal
compressor unit is inlet volume flow rate Q, speed n, adiabatic head H and adiabatic
efficiency η . Based on the normalized parameters of the centrifugal compressors
(Gresh, 2000) and the principles of polynomial curve fitting
(Abbaspour, 2005; Wu et al.,
2000), the governing simulation equation for the compressor is given as:
where, A_{1}, A_{2}, A_{3}, A_{4} and B_{1},
B_{2}, B_{3}, B_{4}, are constants which depend on the
units. The detail derivation for equ. 6 is presented by Woldeyohannes
and Majid (2008).
Based on the performance map of the compressor used from Kurz
and Ohanian (2003), simulation analysis was performed to determine the coefficients
of the mathematical approximation which represents the characteristics of the
compressor. The coefficients for the mathematical approximation of the compressor
were determined to be:
A_{1}= 1.08 E  06
A_{2} = 1.96 E  08
A_{3} = 4.71 E  10
A_{4} = 5.83 E  12 
Formulation of mass balance equations: The mass balance equations were
obtained based the principle of network. The detail on mass balance formulation
can be obtained from Woldeyohannes and Majid (2008).
RESULTS AND DISCUSSION
The enhanced TPNS simulation model was used to evaluate the gas transmission
pipeline network shown in Fig. 2. The problem instance was
taken from Woldeyohannes and Majid (2008). The network
was analyzed under single phase gas flow analysis with no corrosion and singlephase
flow analysis with corrosion based on pipe data given in Table
1. The analysis was done based on source pressure of 3500 kPa and demand
pressure requirement 4500 kPa. The temperature of the gas is assumed to be constant.
Two compressors were working for the system. It was assumed that the speed of
the compressor was 8800 rpm.
Single phase flow analysis without corrosion: The results for the unknown pressure variables and flow variables were obtained with less than ten iterations with relative percentage error of 1.76979E15. Figure 3 shows the convergence of nodal pressures solution for the first ten iterations to the final pressure The convergence of the corresponding flow parameters are shown in Fig. 4 and 5.
Single phase flow analysis with corrosion: The same pipe data and compressor parameters were used as in the case of with no corrosion to analyze the pipeline network shown in Fig. 2 by considering corrosion. The age the pipes could vary to give the corresponding friction coefficient.
Table 1: 
Pipe data for single CS and five customers 


Fig. 2: 
Branched pipeline network system 

Fig. 3: 
Convergence of pressure variables without corrosion 

Fig. 4: 
Convergence of main flow variables without corrosion 

Fig. 5: 
Convergence of lateral flow variables without corrosion solution
for the first ten iterations. 

Fig. 6: 
Convergence of pressure variables with corrosion 
The convergences of the pressure and flow variables were studied based on ten years old pipes. The convergence of some of nodal pressure variables is shown in Fig. 6.
The convergence of flow parameters through main pipes is shown in Fig. 7. Figure 8 shows the convergence of flow variables through the branch pipes. Similar to the convergences of the nodal pressures and flow variables through main pipes shown for without corrosion, the TPNS simulation converged almost to the final solution at the end of the 4^{th} iteration for the flow variables through lateral pipes.
The results obtained for the case of neglecting corrosion were compared with the results obtained by considering the effect of corrosion for various ages of the pipe.
By maintaining the same source pressure and demand pressure requirements similar
to the case with no corrosion, simulation study was conducted to analyze the
effect of the age of the pipe on the performance the TPNS. Compression ratio
and the flow rate Q_{1} were studied for Figure 9
shows the variation of the compression ratio of the system with age of the pipes
at different compressor speeds. As it seen from the figure, compression ratio
of the system increased with increase in the age of the pipes. As the age of
the pipes increased, the friction factor and pipe flow resistance increased
which increase the pressure drop of the system to result in higher compression
ratio.
The variation of flow variables with ages of pipes is shown in Fig.
10. It is observed that corrosion had a significant effect on flow capacity
of the pipes. The results of the simulation analysis showed that a decrease
in flow capacity of 2.16 and 4.35% were observed for the 10 and 20 years old
pipes, respectively when the compressors run at speed of 8800 rpm.

Fig. 7: 
Convergence of flow through main pipes with corrosion 

Fig. 8: 
Convergence of flow through lateral pipes with corrosion 

Fig. 9: 
Compression ratio variations with ages of pipes 

Fig. 10: 
Flow through pipes for different ages of pipes 
Table 2: 
Variation of flow with ages of pipes 

Table 2 shows the results of the simulation study conducted on pipe joining node 0 and 1 of the pipeline network system shown in Fig. 2. The friction factor, pipe flow resistance and the flow through the pipe were studied under three age groups. As the age of the pipes increased, the friction factor and pipe flow resistance increased. Based on flow equations, for fixed pressure at the start and end node of the pipe, an increase in flow resistance of the pipe reduced the flow capacity.
CONCLUSIONS
The effect of the age of pipes on performance of the transmission pipe line
network system was analyzed by enhancing the simulation model to integrate the
age of the pipes to the flow equations. Analyses of branched pipeline network
showed that significant effects on the performance were observed for the three
groups of pipes studied. The performance of three groups of pipes i.e. new,
10 and 20 years old were evaluated and compared. The results of the simulation
analysis showed that a decrease in flow capacity of 2.16 and 4.35% were observed
for the 10 and 20 years old pipes, respectively. To certain extent the model
was capable of evaluating the effect of the age of pipe on the performance of
the gas transmission system. In order to establish firm relationships between
the age of the pipes and performance of the pipeline network system, further
work is required. The developed simulation model could be easily extended to
be applied for performance analysis of pipeline network systems for other petroleum
products. Transient model is another important problem to be addressed from
the simulation perspective.
ACKNOWLEDGMENTS
The authors would like to thank the Universiti Teknologi PETRONAS for providing grant and facility for the research.