Erosion management includes the erosion monitoring and erosion mitigation methods.
The following methods are used for erosion monitoring on steel pipes or special tab erosion:
- Ultrasonic gauges are used to clamp to the external surface of the pipe. They send out an ultrasonic pulse tomeasure the thickness and the material loss from which to determine the erosion severity. The method is sensitive to the noise from other sources; also the primary limitation of thismethod is that it only checks a limited local region of the pipe.
- Weight-loss coupons made of the same or similar material as the pipe being monitored are installed and periodically retrieved and weighed. They provide only discrete monitoring and are unsuitable for subsea engineering equipment.
- Electrical resistance probes measure the accumulated erosion as an increase in electrical resistance on a known cross section. Calibration and temperature changes are of concern.
- Electrochemical probes determine the erosion rate through measurement of the linear polarization resistance between electrodes through a conductive electrolyte flowing inside the pipe. This method is suitable only for conductive liquids such as water, or oil systems with high water cuts.
Erosion Mitigating Methods
A number of measures can be taken to mitigate erosion, as discussed next.
Reduction of Production Rate
Reducing the production rate includes reducing the flow velocity and sand production rate. However, this has adverse financial implications.
Design of Pipe System
Minimizing the flow velocity and avoiding sudden changes (e.g., at elbow, constrictions, and valves) in the flow direction should be given much attention in order to reduce the severity of any erosion. Blind tees are generally perceived as being less prone to erosion than elbows, so the use of full-bore valves and blind tees in place of elbows can reduce erosion problems. Also, the flow regime has an impact on erosion problems and slugging flows can be particularly damaging; therefore, slug catchers may be appropriate for reducing the severity of any erosion.
Increasing Wall Thickness
Thick-walled pipes are often used to increase the wear life of a pipe system. However, the thick wall thickness reduces the pipe bore, which in turn elevates flow velocities and increases the erosion rate, particularly with small-bore pipe systems.
Specialized Erosion-Resistant Materials
Generally, in oil and gas production systems nearly all of the components will be made of ductile metals, although other materials such as plastic and rubber may also be used. Material properties have a significant effect on erosion problems. If erosion problems are suspected, specialized erosionresistant materials such as tungsten carbide can be used.
The primary factor of ductile materials in controlling erosion is their hardness. Consequently, steels are more resistant than other softer metals. In vulnerable components, specialized materials such as tungsten carbides, coatings, and ceramics are often used. These materials are generally hard and brittle and have a super erosion resistance to steel (often orders of magnitude better). However, some coated materials’ resistance may rapidly reduce once the coating or its substrate fails. Brittle materials erode in a different manner. Impacts on brittle materials abrade the surface, and erosion increases linearly with the impact angle, until reaching a maximum for perpendicular impacts.
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