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Department of Petroleum Engineering and Applied Geophysics
Examination paper for TPG4140 Natural Gas
Academic contact during examination:
Jon Steinar Gudmundsson
Phone: 918 97 918
Examination date: 20 December 2013
Examination time (from-to): 0900 - 1200
Permitted examination support material: D. No printed or hand-written support
material is allowed. A specific basic calculator is allowed.
Other information:
Language: English
Number of pages: 4
Number of pages enclosed: 4
Checked by:
Date
Signature
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ALL PROBLEMS ACCOUNT THE SAME (16.7%)
PROBLEM 1
A buried plastic pipeline is used for local distribution of natural gas at a constant temperature of
15 °C (surrounding ground temperature). The pipeline is 1600 m long, has a 110 mm ID and an
inlet pressure is 500 kPa. The relative density (gravity) of the natural gas is 0.65 and the LHV
(=NCV) 36 MJ/kg. Other properties at 15 °C are shown in the table below.
p [kPa]
101.3
300.0
500.0
z [-]
0.9971
0.9914
0.9857
µ [mPa.s]
0.01090
0.01094
0.01098
k [-]
1.291
1.298
1.305
(a) What is the flow capacity of the pipeline when the maximum average flow velocity
at the inlet is 20 m/s? Give the answer in Sm3/h and kW.
(b) What is the approximate frictional pressure drop in the pipeline from inlet to outlet
when the normal average flow velocity at the inlet is 3 m/s? The wall of the
pipeline is hydraulically smooth. Give the answer in kPa and bar.
PROBLEM 2
(a) What kind of difference has there been in the gas price (higher or lower) at UK
NBP compared to US Henry Hub the last year?
(b) What hydrocarbon fuel (fossil fuel) is used the most to generate electrical power
world-wide?
(c) What hydrocarbons can form natural gas hydrate?
(d) What is unusual (different) about the composition of the Groningen natural gas,
compared to natural gas from Norway that is exported to Europe?
(e) What are the three hydrocarbon products from the Hammerfest LNG plant?
(f) What three countries are the largest traditional suppliers of natural gas to Europe
(EU)?
(g) What is meant by a take-or-pay contract?
(h) What are the two main types of tanks used for transport of LNG by ship?
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PROBLEM 3
(a) For a gas with a molecular weight of 20 kg/kmol, what is the mass flow rate for 1
MSm3/day? Give the answer in kg/s.
(b) What is the mass fraction of MEG required to lower the hydrate forming
temperature by 20° C in a subsea gas-condensate pipeline?
(c) What is the Wobbe Index of a natural gas with gross calorific value of 40 MJ/Sm3
and a molecular weight of 20 kg/kmol?
PROBLEM 4
(a) What are the three main types of LNG plants? Answer each by less than five words.
(b) Why is it necessary to remove mercury from natural gas used in the production of
LNG?
(c) What is the effect of the ambient cooling medium temperature on the required
compressor power in LNG production?
(d) Typical compositions of associated gas and non-associated gas are shown in the
enclosed table. Which column (A or B) shows the composition of non- associated
gas?
Methane
Ethane
Propane
Butanes
C5+
Gas A
Volume %
60-80
10-20
5-12
2-5
1-3
Gas B
Volume %
95-98
1-3
0.5-1
0.2-0.5
0.2-0.5
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PROBLEM 5
Non-associated natural gas is planned to be produced by several wells through a subsea template
to a platform at 9 km distance. Calculations indicate that the natural gas will form natural gas
hydrate in the presence of small amounts of condensed water when cooled below
20 °C at the planned pipeline pressure downstream of the template. The pipeline inlet temperature
will be 80 °C, the total mass flow rate planned is 70 kg/s and the natural gas heat capacity at
constant pressure is estimated to be 3.5 kJ/kg.K. The lowest measured seawater temperature is 5
°C, the pipeline inner diameter planned is 24 inch and the pipeline overall heat transfer coefficient
is assumed to be 15W/m2.K. As a part of the flow assurance work for the planned natural gas
production, the following questions need to be answered:
(a) At steady-state flow, will natural gas hydrate form in the bulk flow of the pipeline?
Why or why not?
(b) At long-term shut-in of the pipeline at normal operating pressure, will natural gas
hydrate form in the pipeline? Why or why not?
(c) Assuming that hydrate forms at steady-state flow and during planned long- term
shut-in conditions at normal operating pressure, what can be done to prevent such
hydrate formation at both conditions?
PROBLEM 6
The Polarled Pipeline is planned to transport natural gas 480 km from the Aasta Hansteen gascondensate field to the receiving terminal at Nyhamna. The pipeline ID is 36 inches and the average
overall heat transfer coefficient can be assumed 20 W/m2.K. The following design parameters have
been presented for the pipeline:
(a)
Design pressure 232 bara
(b)
Design temperature -10 °C to 60 °C
(c)
Operating pressure 114 bara to 230 bara
(d)
Operating temperature 55 °C (maximum) and temperature profile
(e)
Capacity 50 MSm3/d at inlet and 70 MSm3/d at outlet
For each of the five design parameters above, write 1-2 lines to explain why the parameters have
been selected.