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| Home | Oilfield Slugs; If only it was as
easy as a pinch of salt
Slugs in your garden can be good to help it get rid of organic waste matter, but occasionally they can become a pest when they attack your newly planted Rarum Albertae Expensivus . (Go ahead, phone Golden Acres) Slugs in oilfield pipelines are just as unavoidable; they bring a large
amount of valuable hydrocarbon to the plant, but if slugs happen without
some kind of control system, the damage can be expensive in both human and
dollar cost. Slugs are formations of liquids, in the lower parts of transmission lines. The liquids can be anything from produced hydrocarbon, lubrication oils, produced water, and chemicals used in production, treatment, compression or dehydration of the gas. These slugs need to be detected and controlled in order to minimize damage to downstream process plants, pipelines and valves. The engineer has a few tools at his disposal: computer simulation, and finally the implementation of that technology with pipeline drips and slug stabilizers. Simulation: Controlling these slugs involves careful measurement of a number of factors such as pipeline inclination, size (length) and velocity of the slug itself held up in the pipe, pigging cycles, and dynamic loads on bends and restrictions such as valves. Detection of these slugs is important for more reasons than prevention of damage to the upstream infrastructure. If the size of the slug can be minimized, either by installing a control mechanism, or by changing the design of the pipeline itself, the result is that smaller equipment will be required to handle the slug, and thus significant dollars can be saved. A few of the challenges to engineers are use of existing sensors and signal processors to "pinch" inflow so as not to overwhelm the facility. These designs are achieved using advanced computer modeling with systems such as PIPESYS, developed by Neotechnology Consultants Ltd. (Neotec) in close cooperation with Hyprotech Ltd. These systems integrate powerful capabilities for single and multiphase pipeline flow modeling into HYSYS. Once armed with a good pipeline flow model, an engineer can predict the dynamic loading imposed by slugs on bends and restrictions in pipeline and riser systems. This in turn can be used for mechanical design of pipe work and support structures. Drips and slug stabilizers: Drips are not just what your little sister calls her last blind date. They are a vessel, usually buried, designed to catch the liquids that form in the bottom of the dip of a pipeline. The theory is that gravity will force the liquid into the lowest point of the pipeline, in this case the pipeline drip. Usually these drips are not much more than a vessel with a nozzle pointing upward towards the pipeline from where it is to receive the excess liquid in the pipe. In practice, it is found that only a percentage of the liquid makes its way into the drip. What happened? It turns out that, contrary to what might be your intuition, the liquid tends to spread itself out along the entire diameter of the pipeline. This is what is known as annular flow, and was discovered to a large extent by a Dutch scientist, L. Oranje, and is explained in his "Condensate Behavior in Pipelines is Predictable" Oil and Gas Journal July 2, ,39-45, 1973. Mr. Oranje was trying to explain why some liquids where turning up in unexpected locations such as lateral legs that where supposed to supply, not collect from, the main pipeline Even though Mr. Oranje explained this phenomenon 27 years ago, it was only recently that somebody figured out how to deal with it using an efficient mechanical device. If the liquid is also at the top of the pipeline, it becomes clear that a traditional drip can only catch those liquids traveling along the bottom of the pipe. The problem with pipelines is that they regularly get pigs sent through them. This not some form of porcine torture; "pig" stands for Pipeline Inspection Gauge. These are devices ranging from a hard rubber ball to an elaborately designed, self-propelled electronic measuring device designed to inspect welds and wall thickness. If the pig has to make it through the line, then you can’t just put in a baffle and catch the liquids in a conventional separator. The object is to allow normal transmission to occur, yet to also allow pigs to pass through. Thus the ancient method of the drip pot at the bottom. The new device is known as the Piggable Drip, was developed by Miles E. Haukeness in conjunction with Union Gas, and uses a series of slots on the pipe wall. This wall is contained within a larger diameter pipe. The whole drip thus becomes part of the pipeline. By catching liquids around the entire diameter such a drip can take all liquids, yet only interfere with only 5-10% of the primary flow. The advantage of not interfering with the entire flow is that pipeline velocity is not reduced. Maintaining velocity means that less mist is formed, and so fewer uncontrolled liquids are formed downstream of the separator. Putting this technology to work allows the engineer to design a system that forms slugs in areas where they can be controlled, such as a stabilizer. The stabilizer can be anything from a vessel to another loop in the pipeline. The stabilizer vessel can sometimes be used for re-introduction of the liquids downstream of the problem area. Re-introducing liquids at a controlled rate allows the valuable hydrocarbons to make it to a sales point or plant without the added expense of trucking from tanks. Conversely, if the system produces water slugs, they can be caught and sent back into the ground at an appropriate injection site. The end result of all these simulations and controls systems is that smaller, high-pressure vessels are required at plant sites, and smaller control systems are required at the inlet to the plant. Instead of sizing for spot rates of large volumes of liquids, the condensate volume is steadied out to an even rate, and can be dealt with in smaller, less expensive equipment. Annular separation keeps the slug from causing problems to your hardware, and might even redefine your definition of speed when it comes to slugs. |
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