الفهرس 
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Abstract
The aim of the thesis is to develop a three-dimensional (3D) numerical
model using finite element for simulation, based on continuity
equation, energy equation, momentum equation, K-ε algorithm and
heat transfer equations, computational package Common Solution
software (COMSOL) is used to determine cooling rate, time of the
cooling and temperature distribution, to predict the effect of natural
gas velocity on hot tapping process.
Numerical modeling predicts the safe zone for applying hot tapping
process and the un-safe zone for applying hot tapping process, for all
ranges of accepted natural gas velocities up to 20 m/s, using different
commercial pipe diameters employed in natural gas transmission
pipeline in Egypt.
Experiments were carried out on a pipe of 32-inch transmits national
natural gas in Egypt in-service, during a hot tapping process for
execution of 6-inch new branch. Measurements were obtained and
recorded from the experiments under 2 m/s gas velocity, 46 bar gas
pressure and 16 ˚C gas temperature. To verify the proposed numerical
model, the predicted results are compared with those obtained from
experimental measurements for a wide range of working conditions.
The predicted results are presented using natural gas velocity ranged
from 0.4 m/s to 20 m/s to determine the risk of welding zone and heat
affected zone (HAZ) crack and also the burn through risk; and to
precise the safe range of gas velocity during hot tapping process for
various commercial pipe diameters (ranged from 26-inch to 34-inch).
To determine the influence of natural gas velocity on hot tapping
process various numerical iterations for wide velocity ranges of
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natural gas with heat input changed from 6 to 9 kJ/mm, fair agreement
was obtained from this comparison with maximum deviation between
numerical model results and experimental work is 0.96%.
In conclusions, numerical results were used to determine the case in
which the possibilities of the two most significant problems can be
avoided when welding during in-service pipelines. The first problem
is concerned with the gas flow where the flowing gas creates a large
heat loss through the pipe-wall due to convection heat transfer,
resulting in acceleration of cooling rate for the welding operation. The
influence of welding cooling rates causes a great hardness levels
within the welding zone and in the HAZ which cause formation of
cracks.
The second problem is related to the localized heating and loss of
material strength during the welding process, since the pipe-wall is
reduced in thickness and cause explosion due to the high internal
pressure if this reduction in strength is too great and if the inner
surface temperature of the pipeline reaches 987 ˚C [1].
Results are used to obtain a sufficient map for in-service welding on
pipeline depending on the change of natural gas velocities as a main
factor and hence affects the cooling rates.