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ENGINEERING DESIGN AND PREDICTION OF HYDRATE FORMATION FOR A GAS PIPELINE
This research is focused on the difficulties that gas producers usually face with Hydrate formation during transportation. Hydrate build up in gas transportation flowlines is one of the major tasks for gas operators to deal with as it may cease gas flow through the pipeline, reduce well head measured flow rate, equipment damages e.t.c Two kinds of Engineering designs were developed to help transport gases of different pressures to an extension facility whose inlet pressures was designed at 8 barg. HYSYS VERSION 2006 was used for the simulation of these designs to check for the possibilities of Hydrate formation and recommendations were made based on the outputs.
Flow velocity is a very important criterion in determining the possibilities of noise in a gas transporting pipeline. There is a possibility of noise in a gas pipeline if the fluid mean velocity exceeds 60 ft/sec. Also, one of the objectives of this project is to verify the suitability of pipe lines sizes for the 8 barg pressure to transport gas over a distance of 120 km. For this project, a default pipe line size of 10” SCH 40 was selected and other pipe sizes lesser and greater than it were also used to pick the most suitable, simultaneously considering cost. PIPESIM VERSION 2009.1 was used for these analyses and the best pipeline size was determined.
The simulations reveal that both designs are efficient enough and at standard conditions, there will be no possibilities of hydrate formations but the possibilities of establishing design A will be recommended because it entails lower cost and less space is required. Also, a pipeline size of 10” SCH 40 will be sufficient for the given flow conditions but a pipe line size of 8” SCH 40 can also be used.
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 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 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.
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 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 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?