Corrosion Control in Oil And Gas Pipelines
In the United States, the annual cost associated with corrosion damage of structural components is greater than the combined annual cost of natural disasters, including hurricanes, storms, floods, fires and earthquakes. Similar findings have been made by studies conducted in the United Kingdom, Germany, and Japan.
According to the U.S. Department of Transportation Office of Pipeline Safety, internal corrosion caused approximately 15% of all reportable incidents affecting gas transmission pipelines over the past several years, leading to an average cost of $ 3 million annually in property damage, as well as several fatalities. The need to manage and mitigate corrosion damage has rapidly increased as materials are placed in more extreme environments and pushed beyond their original design life.
Typical corrosion mechanisms include uniform corrosion, stress corrosion cracking, and pitting corrosion. Corrosion damage and failure are not always considered in the design and construction of many engineered systems. Even if corrosion is considered, unanticipated changes in the environment in which the structure operates can result in unexpected corrosion damage. Moreover, combined effects of corrosion and mechanical damage, and environmentally assisted material damage can result in unexpected failures due to the reduced load carrying capacity of the structure.
Ensuring long-term, cost-effective system integrity requires an integrated approach based on the use of inspection, monitoring, mitigation, forensic evaluation, and prediction. Inspections and monitoring using sensors can provide valuable information regarding past and present exposure conditions but, in general, they do not directly predict remaining life. Carefully validated computer models, on the other hand, can predict remaining life; however, their accuracy is highly dependent on the quality of the computer model and associated inputs. Mitigation (corrosion prevention) methods and forensic evaluations play a key role in materials selection, assessment and design. All of these corrosion-control elements represent long-standing areas of research and development at Southwest Research Institute.
Pipeline Inspection
A significant portion of many pipeline systems cannot be inspected through traditional methods. Nondestructive evaluation (NDE) and inspection tools are critical to assessing the integrity of pipelines. Traditional NDE methods involve the use of pipeline inspection gauges (PIGs), which travel through the inside of a pipe and detect the presence of mechanical damage or corrosion.
Researchers at SwRI have developed an inspection system for inspecting pipelines that cannot accommodate traditional PIGs. This system uses remote field eddy current (RFEC), and was designed for use with the Carnegie Mellon Explorer II Robot. However, this technology can be adapted to other transport mechanisms. The system can expand to inspect 6-8 inch (150-200 mm) diameter lines. The sensor arms retract to accommodate line restrictions, such as elbows, tees and gate valves.
Pipeline Integrity
Basic problems during pipeline operation include incidents and failures caused by oil and oil product leaks and spills, which can result in serious environmental contaminations and big losses of produced oil.
Corrosion is a process causing metal destruction or changing of its properties as a result of operating medium impact. Corrosion of pipelines and equipment is one of major problems in oil and gas industry. Large amounts of water are produced in the course of prolonged field operation, which contributes to corrosion development. Corrosion causes may include presence of aggressive gases in produced fluids, such as hydrogen sulfide, carryover of solids from wells, such as sand or proppant after performed frac jobs, mechanical damages of pipeline coatings and insulation and so on.
Key methods applied to determine corrosion rate on pipelines and equipment include installation of coupons and probes and X-raying. Coupons are installed in corrosion monitoring points and left inside pipelines for a certain period of time. After that coupons are retrieved and analyzed and weighted. Pipeline corrosion rate is determined by loss of coupon weight or thickness after an established period of time. In X-raying method pipe wall thickness is regularly checked in specific points with a special instrument using X-rays.
By location pipeline corrosion can be classified into internal corrosion and external corrosion. By mechanism of attack corrosion cab be divided into chemical corrosion and electrochemical corrosion.
Corrosion inhibitors are substances, introduction of which into aggressive environment hinders the process of corrosive destruction and changing of mechanical properties of metals and alloys. Corrosion inhibitors can be water-soluble, which are the most efficient in water, and oil-soluble, which are the most efficient in oil.
Corrosion protection of underground pipelines is provided by using mastic and polymeric coatings.
Electrochemical protection of pipelines and equipment is performed by means of cathodic polarization. When cathodic polarization is performed using an external source of direct current, such protection is called cathodic protection, and if it is performed by means of connecting protected pipelines to metal having more negative potential, then such protection is called sacrifice protection.
Another method of cathodic protection is application of glass-reinforced or plastic pipe. The advantage of glass-reinforced pipe is that corrosion does not develop on plastic. However complete replacement of steel pipe with glass-reinforced pipes is not feasible yet, because plastic cannot hold high pressure.