Corrosion in the oil and gas industrial sector has been acknowledged since the 1920s, and each year corrosion and other related failures still cost the oil and gas industrial sector hundreds of millions of dollars . As per Kermani and Harrop , in the oil and gas industry corrosion-related failures constitute over 25% of failures.
A study carried out in the 1980s reported the following nine causes (along with their degree of contribution in percentages) for corrosion-related failure in petroleum- related industrial sector [7,8]:
- • Cause I: CO, related: 28%
- • Cause II: Preferential weld: 18%
- • Cause III: H,S-related: 18%
- • Cause IV: Pitting: 12%
- • Cause V: Erosion corrosion: 9%
- • Cause VI: Galvanic: 6%
- • Cause VII: Stress corrosion: 3%
- • Cause VIII: Crevice: 3%
- • Cause IX: Impingement: 3%
Types of Corrosion or Degradation that Can Cause Failure
The following ten possible types of corrosion or degradation that can cause failure :
- • Type I: Galvanic corrosion. It can take place in bimetallic connections at connections at opposite ends of the galvanic series that have enough potential for causing a corrosion reaction in the existence of an electrolyte.
- • Type II: Crevice corrosion. This type of corrosion takes place in situations where crevice forms, such as partial penetration welds and backing strips, are employed.
- • Type III: Fretting corrosion. It generally occurs in poorly lubricated valve stems where a partially opened valve causes some vibration that can result in galling and then, in turn, valve seizure and possible failure.
- • Type IV: Corrosion fatigue. Over the years, it has played an important role in subsurface and drilling operations such as drill pipe and sucker-rod failures.
- • Type V: Impingement/cavitation. Impingement can take place in situations where process fluid is forced to change its flow direction abruptly. Common offshore area for cavitation’s occurrence is in pump impellors where pressure changes take place and high liquid flow rates occur.
- • Type VI: Erosion-corrosion. It is observed quite often on the outer radius of pipe bends in oil and gas production due to rather quite high fluid flow rates as well as corrosive environments where flow exceeds 6 m/s for copper and nickel and 10 m/s for carbon steel.
- • Type VII: Microbiological-induced corrosion. It is quite serious as it takes the form of localized pitting attack that can, directly or indirectly, cause a rapid loss of metal in a concentrated area, leading to leak or rupture.
- • Type VIII: Weight loss corrosion. This type of corrosion occurs most commonly in the area of oil and gas production due to an electrochemical reaction between metal and the corrodents in the environment.
- • Type IX: Hydrogen-induced cracking. In the past, this type of cracking has mostly took place in the controlled rolled pipeline steels and elongated stringers of non-metallic imperfections.
- • Type X: Stress corrosion cracking. The most probable form of cracking phenomenon in the area of oil and gas production is sulfide and chloride stress corrosion cracking.
Corrosion/Condition Monitoring Methods
Corrosion monitoring of internal surfaces may be conducted by using the combination of the following five methods :
- • Method I: Visual inspection
- • Method II: Pipeline leak detection
- • Method III: Intrusive probes and coupons for monitoring corrosion and erosion
- • Method IV: Chemical analysis of samples taken from the product
- • Method V: Measurements of non-intrusive wall thickness (radiography/ ultrasonic)
The commonly used corrosion/condition monitoring methods are shown in Figure 11.2 .
Additional information on methods shown in Figure 11.2 is available in Price .
FIGURE 11.2 Commonly used corrosion/condition monitoring methods.
Fatigue Damage Initiation Assessment in Oil and Gas Steel Pipes
Nowadays, steel pipes are commonly used in offshore petroleum industrial sector. The fatigue behavior is a very important issue of concern in regard to the dynamic loadings (i.e., cyclic loadings). In oil and gas steel pipes, fatigue is one of the main failure causes observed, which in turn can result in catastrophic environmental damage as well as significant financial losses [9-11].
In order to assure oil and gas steel pipes’ structural integrity and forewarn a fatigue-associated failure, it is very important to adopt a consistent fatigue criterion. Nonetheless, the fatigue associated damage may be divided into the following two main phases :
- • Main phase I: Incubation phase. During this phase, only microstructural- related changes, microcracking, and microcracks nucleation can be observed. It is to be noted that the study of this phase is more cumbersome to conduct since microstructural-related changes and fatigue damage cannot be easily separated.
- • Main phase II: Propagation phase. During this phase, macrocrack propagation and macroscopic cracking result in fatigue failure, and the physical- related data that may quantify the material’s damaged state can be more easily obtained.
Non-destructive evaluation (NDE) methods are considered quite useful for assessing fatigue life and evaluating structural integrity. These methods can assess fatigue damage’s limitation, monitor changes in mechanical properties, and follow the fatigue damage process through the structures’ life cycle subjected to cyclic loadings, in order to forewarn a malfunction. Five of these methods are as follows :
- • Magnetic evaluation method
- • X-ray diffraction method
- • Hardness measurements method
- • Ultrasonics method
- • Thermography method
It is to be noted that among the NDE methods for fatigue damage imitation assessments, X-ray diffraction method is generally considered to be one of the most suitable analysis methods . Additional information on this method is available in Pinheira et al .