Gas pipeline construction: calculating the pipe diameter, the pressure required for the transportation of natural gas compressors. The definition of capital costs for construction and operation of the pipeline. Financial management of the project.
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PIPELINE CAPACITY AND ECONOMICS



INTRODUCTION

In the construction of natural gas transmission pipelines a lot of things should be considered. For example pipe sizes, the pressure required to transport natural gas from one location to another and the pressure drop that will proceed. To accurate this, compressors are required which need to be sized for the compression requirements. In addition Pipe loops will be designed that increase the capacity of the pipeline by adding theoretical diameter.

Pipelines are planned to integrate into existing networks and are constructed to transport a company's own natural gas, to transport natural gas through an owned section of pipeline for another company, or to transfer the ownership of natural gas to another and into their assets (pipeline). For these 3 permutations in operation investment in the infrastructure and assets will be made to a varying degree while meeting all of the regulation requirements to ensure that the safe operation and environment considerations have been made.

To meet the capital requirement (CAPEX) for developing pipelines, companies have to provide strong business cases in how the financing and remuneration through operation will be managed. The budget for pipeline development will be determined by the capital costs of the planned length of pipeline and operational term in service. Some of the elements of the capital costs are as follows:

1. Pipeline

2. Compressor stations

3. Mainline valve stations

4. Meter stations

5. Pressure regulator stations

6. SCADA and telecommunication

7. Environmental and permitting

8. Right of way acquisitions

9. Engineering and construction management

PIPELINE CAPACITY AND ECONOMICS

In the design and planning of the physical pipeline systems several variables are to be considered. The majority of these variables come from the general flow equation which is determines the daily capacity of a gas pipeline:

Using this equation produce derivative equations to illustrate the relationship with some of the variables and how impact on the overall capacity of the pipeline on a daily basis. This is shown as a capacity factor or as volumetric capacity for daily deliverables. In conditions of overall economical potential the pipeline capacity as earlier mentioned is a important factor in determining the transportation price and hence the capital recovery for investment in projects of this nature.

One of the capacity functions are:

1. Inlet pipe pressure

2. Pressure drop

3. Pipeline diameter

4. Pipeline Length

INLET PRESSURE

Use the derivative equation below the illustration of variation of inlet pressure in stages of 10 bar for a standard pipe diameter with a constant pressure drop ?P of 20 bar can be seen.

For this arrangement we can use the formula:

Q ?



This allows us to determine the capacity factor CF which is a function of the inlet pressure.

Table 1

Effect of Inlet pressure on pipeline capacity

Q	?P	P?	?P(2P? -?P)	CF
	20	0	-400
0	20	10	0	0
20	20	20	400	1
28.28427	20	30	800	1.06066
34.64102	20	40	1200	1.154701
40	20	50	1600	1.25
44.72136	20	60	2000	1.341641
48.98979	20	70	2400	1.428869
52.91503	20	80	2800	1.511858
56.56854	20	90	3200	1.59099
63.08724	20	100	3980	1.585107

Figure 1. Pipeline capacity increases with increased inlet pressure

From figure 1 any increase in inlet pressure for a standard pressure drop of 20 (bar) will increase the capacity of the pipeline. There is a rapid increase in capacity between 10-20 (bar) and then a uniform or linear increase between 30-90 (bar) for inlet pressure. Standard systems will use the linear increase in capacity as they will require a pressure drop in the system to be able to transmit efficiently. In terms of transmission pressures the majority of pipelines in the UK will transmit around 65 (bar) which is in the range of linear capacity building in pipelines.

Also using an example for a pipeline of fixed dimensions we can explore further the original equation by producing the derivative equation.

Where:

P1 (inlet pressure) is varied in stages of 10 bar

P2 (outlet pressure) is a function of the pressure drop known to be 20 bar for the system.

D is the diameter of the pipeline (30'' or equivalent to 750mm DN)

L is length of a standard section of pipeline taken to be 48km

EXAMPLE CALCULATION

Q ? x = 217.855 capacity (mill m³/ day)

Table 2

Inlet pressure varied in stages of 10 bar

p1	?P	p2	d	L	Q ?
10	20	-10	750	48	0
20	20	0	750	48	44469530
30	20	10	750	48	62889412
40	20	20	750	48	77023485
50	20	30	750	48	88939059
60	20	40	750	48	99436891
70	20	50	750	48	1.09E+08
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