Risk of Hydrates in High-pressure Natural Gas Transport Lines
a report by
Esam Jassim, M Abedinzadegan Abdi and Yuri Muzychka
East Coast Development Group, Husky Group – East Coat Operations, St John’s
Natural gas is a mixture of hydrocarbon gases that occur in petroleum deposits. It is principally composed of methane and varying quantities of ethane, propane, butane and heavier hydrocarbons, and is used as a fuel and in the manufacture of organic compounds. These hydrocarbon components can appear in multiple phases according to changes in temperature, pressure and composition. Under certain pressure and temperature conditions solids may also precipitate, resulting in changes to the fluid properties. The occurrence of such solids may lead to severe problems in oil and gas production systems. One of the most common solid precipitation problems is caused by what are known as hydrate clathrates.
Hydrates can pose a major risk in all high-pressure oil and natural gas transport lines. Given the significance of safety and reliability in any high-pressure natural gas transport system, it is critical to have an accurate analysis of such systems from the safety standpoint. Variations in pressure and temperature of the system are the most significant factors that can lead to the formation of hydrate. These circumstances can also be commonly found in exploration and production systems when fluids flow through various types of equipment along the production tubing or transportation pipelines. Components such as chokes, velocity-controlled subsurface safety valves and conventional valves and fittings (piping components) can all act as restrictions to the flowing fluids, thus causing changes in the flow conditions.1
Macroscopically, hydrate structures appear similar to ice or snow, but unlike ice hydrates can be stable at temperatures above 0ºC. Gas hydrates represent one of the few phases and perhaps the sole condensed phase where water and light non-polar gases exist together in significant proportions.2,3
The most notable facts are that they are non-flowing crystalline solids, they are denser than typical fluid hydrocarbons and the gas molecules they contain are effectively compressed, giving rise to numerous applications in the broad areas of energy and climate effects.
The physical properties of these compounds may include an important bearing on flow assurance and safety matters. More recently, they were utilised in refrigeration systems, where their crystallisation occurred in expansion valves.4
However, combating these drawbacks
has made it possible to acquire substantial knowledge of hydrates, including existing conditions, crystalline structure, capacity to store gas and heat of dissociation.
The interest in hydrates has significantly increased because of their potential as a separating agent and as a storage vehicle.5
Gas hydrates
found naturally in deep seas and permafrost may provide a large amount of methane. Other positive applications include carbon dioxide sequestration,6
separation7
106
and natural gas storage and transportation.8
Finally, the use of their dissociation energy can be applied in refrigeration processes and cold storage.9,10
To touch on the importance of hydrate research, it is more convenient to describe the shortcomings from an economic point of view. Several important physical properties of hydrates determine the roles that they play (or may play in the future) in both industry and the environment. As they are solids with densities greater than those of typical fluid hydrocarbons, this has practical implications for flow assurance in pipelines and consequent safety considerations.
Risks Associated with Hydrate Formation and Dissociation
When hydrate blockages dissociate in pipelines, they detach first at the pipe wall; therefore, any pressure gradient across the high-density hydrate plug will cause the hydrate to travel rapidly (~300km/hour) down the pipeline. This effect will compress the downstream gas, either causing pipeline blowouts or causing the plug to erupt through pipeline bends. A second safety concern arises when hydrate plugs are locally heated (for example using a blowtorch outside a pipeline) to dissociate them. Frequently, the evolving gas from the hydrate is contained by the ends of the plug until the pipeline bursts due to the pressure being too high. This safety concern is a result of another hydrate property – the capacity of hydrates to concentrate high levels of gas.
Hydrate plugs have disturbed the normal flow of natural gas and other reservoir fluids in the production and transportation lines, claimed the lives of personnel and resulted in the loss of property in oil and gas industries.11
They can plug high-pressure transportation lines as wide as ≥24 inches in diameter, and therefore are treated with extreme care. In fact, light gases such as methane or ethane present in petroleum products are easily trapped as guest molecules in hydrate structures.
In the thermal and hydraulic design of multiphase transmission systems, flow assurance is a significant design concern. Flow assurance deals with the whole range of possible flow problems in pipelines, including multiphase flow and fluid-related effects, such as gas hydrate and wax formation and deposition and harsh slugging. As production systems are increasingly developed in deeper offshore locations, flow assurance becomes a major concern for offshore production and transportation systems, where traditional approaches are inappropriate for deepwater development systems due to extreme distances, depths and temperature or economic constraints.12
The marine transportation of natural gas in compressed form using ocean-going ships is an evolving technology, where gases with various compositions and therefore water contents may be considered for transportation in high-pressure storage systems. The production and transfer of gas to the compressed natural gas (CNG) ships involve
© TOUCH BRIEFINGS 2010
Natural Gas
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