ENGINEERING DESIGN AND PREDICTION OF HYDRATE FORMATION FOR A GAS PIPELINE

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CHAPTER ONE
1.0 INTRODUCTION
Flow assurance is relatively a new term in the oil and gas industry. The increase in demand
for energy has seen the industry moving into more challenging environment (offshore and
ultra-deep water) due to the depletion of the conventional onshore (refining and marketing of
crude oil and its products) and shallow water sources of hydrocarbon. Offshore and ultra
deep water exploration and production is now going from deepwater (3000 – 6000 ft) towards
ultra-deep water (6000 – 10,000 ft).
The subsea environment which involves low temperatures as well as high pressures, high
water cut and longer transfer periods provides conditions that are ideal for gas hydrates
formation, wax and asphaltene formation, scale and naphthene formation, and other solid
deposits. These are the fundamental obstacles to the production of oil and gas through a long
distance subsea pipelines especially at shut-down and re-start situations.
Though, the existing onshore and subsea processing and transportation facilities enable this
exploitation, but adequate flow assurance is needed. Pipelines, among other means of
transporting oil and gas guarantees delivery from the well head to the processing plants and
from there to the customers.
The movement of gases through pipelines at different velocities comes with different issues
which will be addressed in this research. Some of the problems that occur within the
separation and transportations systems include scaling, oil in water, damage to vessel
internals, residence times, slugging, emulsions, forms, deposit build up e.t.c. Natural gas
hydrates are cage-like crystalline compounds in which a large amount of methane is trapped
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within a crystal structure of water, forming solids at low temperature and high pressure.
Natural gas hydrates are widely distributed in permafrost regions and offshore.
The purpose of this project is to develop an Engineering design to ensure flow assurance and
simulation for a single phase gas transportation pipeline. One of the major problems in gas
pipelines is noise. For a single phase gas lines, velocity may be a problem if its exceed 60
feet/second (API RP 14E, 1991). Other criteria such as slug formation, flow pattern,
temperature and pressure drop must be carefully analysed to get an effective flow assurance.
1.1 Background Knowledge
Gas hydrates are clathrate physical compounds, in which molecules of gas are occluded in
crystalline cells, consisting of water molecules retained by the energy of hydrogen bonds.
Gas hydrates can be stable over a wide range of pressures and temperatures. All gases can
form hydrates under different pressure and temperature. The crystalline structure of solid gas
hydrate crystals has a strong dependence on gas composition, pressure and temperature.
Presently, three crystalline structures are known for moderate pressure and nearly ten
structures in the pressure range above 100Mpa. Formation of gas hydrates occur when water
and natural gas are present at low temperature and high pressures. Such condition often exists
in oil and gas wells and pipelines. Hydrates plugs can damage equipment of gas transport
system (Yuri F, 2008).
Transportation of natural gas is a very important aspect of the oil and gas industry and as
such, it must be done with much efficiency. Pipelines have been recognized as the most
economic, effective and safest way of transporting natural gas. A lot of capital is needed, due
to cost of pipeline, compressor stations and also in its maintenance.
Pipeline transportation has become an important means of moving natural gas and with the
expansion of market and large demand; millions of pipeline have been laid. Therefore, the
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process of moving large quantity of this fuel from the gathering station to the refinery the
transportation and distribution by companies to the consumers can be moved through
pipeline. Hence, minimising cost pipeline is necessary but also a pipeline design that will
minimise all possible problems of flow assurance.
1.2 Aims and Objectives of the Project
The aim of the project is to carry out a Flow assurance analysis for a single phase gas pipeline. It is directed towards the following objectives:
(i) To develop engineering schematic of the gas transporting plant.
(ii) To determine outlet pressure of facility B.
(iii) To design verification of Flowlines from facility B to end station.
1.3 Justification
Natural gas hydrates are crystalline compounds which when formed in oil and gas pipelines,
they may block the pipelines, facilities and equipment, they can cause flow and pressure
monitoring errors, reduction in gas transportation volume, increasing pipeline pressure
differences.
In addition to the precipitation and deposition of solids, the flow assurance faces other
obstacles. For example, if the natural gas is associated with oil, it is necessary to ensure oil
production before exploiting gas. In the specific case of heavy oil, production and
transportation are very challenging because oil viscosity is very high.
Flow velocity is a very essential parameter in gas transportation through pipelines. A flow
velocity for a single phase gas line must not exceed 60 feet/second hence there will be
problems of noise. Noise is highly undesirable in gas transportation. This research needs to be
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carried out to ensure a proper pipe size suitable for this facility which will keep the flow
velocity within the limit of 0-60 feet/second.
Gases are very volatile and highly explosive. They move at very high pressures and
temperatures and a sudden back flow can lead to loss of lives and properties. Properties here
could be very expensive equipment such as separators, compressors etc. hence, this research
will ensure that any issue of gas back flow will be emitted by carrying out a detailed
simulation.
1.4 Scope of work
AutoCAD 2013 version will be used to develop facility schematics. Facility A will comprise
of the three pressure separators, compressors and coolers. It will also consist of the manifold
for which the gases from the different pressure separators will be joined. The facility B will
comprise of the bulk line which is to transport the compressed gas over a distance of 120 km.
HYSYS as an advance process simulation environment for the process industries developed
by Hyprotech. It is a powerful tool that offers conceptual design, steady state design, dynamic
and real time design (Vaughan, 2000). ASPEN HYSYS V2006 will be used to carry out the
engineering simulation in facility A and the major aim to get an outlet pressure of 83 barg
(which is the inlet pressure of facility B).
PIPESIM V2009.1 will be used to carry out the simulation and hydraulic analysis of facility
B. Its outputs parameters such as flow velocity, slugging etc. will be utilised to carry out an
effective analysis.
1.5 Problem Statement
GLOBAL OIL is an expanding and indigenous company which since its inception in 1994
has been duly responsible for providing quality services. There are currently three gas wells
in field A. As part of the plans to increase her production, the company is planning to drill the
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gas wells. It was concluded that the wells will be flowing at different pressures and this has
been classified as low pressure, intermediate pressure and high pressure.
An engineering design is required to combine these different pressures at a common manifold
without any problems of pressure backflow to ensure flow assurance so as to prevent any
hazardous occurrence. The inlet pressure at the facility B has been designed as 83 barg.
Hence, a simulation is also required to give an outlet pressure of 83 barg so as to suite what
facility B has been designed for. At Facility B, Global Oil intends to determine the viability
of 83 barg pressure in to transporting the gas through a distance of 120 km for a 10” pipe
(schedule 40).
Other questions to be answered include:
(1) Is the proposed pipeline size cost effective?
(2) What are the possibilities of gas hydrate formation, slug formation and erosion?