- Author: Li Qiang, Xu Lin, Sun Guo Feng, et al.
(China National Offshore Oil Corporation (CNOOC) - Shenzhen Branch, Lu Feng Oilfield Operation Area, Shenzhen, Guangdong 518067)
(Zhonghai Oil Changzhou Coatings and Chemical Research Institute Co., Ltd., Changzhou, Jiangsu 213016)
(China National Offshore Oil Corporation Safety Technology Service Co., Ltd. Shenzhen Branch, Guangdong, Shenzhen 518067)
0、Introduction
In a certain oilfield in the South China Sea, a bypass-type pipeline corrosion monitoring system has been established at the outfall of a seawater intake. The effective application of this system can achieve comprehensive tracking of corrosion within the seawater intake, and through multi-point corrosion monitoring/detection data, it can adjust corrosion protection measures to achieve ideal corrosion protection effects. Recently, during the routine maintenance of the bypass-type pipeline corrosion monitoring system, a large amount of oil sludge was found attached to the upper and lower hanging plates, and the surface exhibited severe pitting, with significant oil sludge accumulation in the short sections.
This article analyzes the causes of corrosion in offshore pipelines through methods such as field coupon corrosion testing, macro-morphological analysis of the inner wall of short sections, and analysis of corrosion products. The analysis aims to ensure the effectiveness of internal corrosion protection measures, which is crucial for the safe and reliable operation of offshore pipelines.
1、 Corrosion and macroscopic morphology analysis of on-site samples
The three-layer corrosion plates were removed. The surface of the upper layer had a small amount of putty residue, which could be removed to reveal the metal surface with slight corrosion marks. The middle and lower layers had significant putty residue, with severe pitting and erosion, and significant deposit and erosion corrosion at the bottom.
Figure 1: Corrosion Flakes on the Side of a Pipeline, Three Layers Deep
The images after cleaning the corroded plates show that the upper and middle plates exhibit localized corrosion with significant erosion and pitting. The corrosion is severe, with the upper portion of the lower plate showing significant localized corrosion and large corrosion pits. The sides of the lower plate also exhibit severe erosion from fluid flushing, as well as corrosion underneath the sediment deposits. The corrosion is extensive.
Photo 2: Three-layer corrosion removal plate after cleaning
According to the Q/HS 2064-2011 standard, Table 1 indicates that the corrosion rate of the upper layer of the three-layer hanging plates on the Hai River bypass is 0.0235mm/a, which is classified as light corrosion. The corrosion rate of the middle layer is 0.036mm/a, which is classified as moderate corrosion. The corrosion rate of the lower layer is 0.477mm/a, which is classified as severe corrosion.
Table 1: List of corrosion coupons for underwater pipeline bypass
2、Macroscopic morphology analysis of the inner walls of short sections of side passages.
Use support clamps to fix the upper deck and secure with a hook and chain. Remove the flanges at both ends of the short section, and use a manual hoist to lift the short section away from the bypass system. Upon inspection, it was observed that there was a significant accumulation of putty powder inside the short section. Due to the fact that this cleaning of the bypass system was the fourth time, and the period between cleans had been optimized, the amount of sediment was significantly reduced compared to the previous cleaning. However, the putty powder accumulation area could not exert its antibacterial and corrosion-inhibiting effects, resulting in a large proliferation of sulfate-reducing bacteria, and the corrosion of the lower hanging plates under the sediment was still severe.
As illustrated in Figures 3 and 4, there is a significant accumulation of putty powder at the bottom of the short section. Removing this accumulated putty powder reveals a smooth inner wall of the flange. This flange is manufactured as a single piece and is welded to the ball valve. The weld is very small, and there is no evidence of corrosion or pitting at the weld points.
Figure 3: Condition before removing and cleaning the short section.
Figure 4: After removing oil stains and cleaning the upstream and downstream flanges, the resulting condition.
As illustrated in Figures 5 and 6, pitting corrosion is evident at the inlet end of the welded joint, with localized corrosion beneath the deposited material causing significant damage. The current pitting corrosion area has not deepened, but the number of pitting points and the area of the corrosion have increased.
Figure 5: Distribution of corrosion at the inlet end
Figure 6: Maximum erosion point status at the inlet end
As shown in Figures 7 and 8, the corrosion distribution at the outflow end is essentially the same as that at the inflow end. The patched areas are currently in good condition with no significant changes. However, the corrosion area before and after the weld has increased, which may be due to the raised edges formed during patching, leading to increased erosion.
Figure 7: Distribution of corrosion at the outflow end.
Figure 8: Maximum erosion point state at the outlet of the marine pipeline short section.
3、Analysis of corrosion products on the side wall
The samples of short-section debris collected from the site were analyzed using EDS, and the results are shown in Table 2. As shown in Table 2, the main elements in the debris are Fe and O.
Table 2: EDS Testing Results of Deposits (Quality Score)
Conduct XRD analysis on the short-section deposits retrieved from the site. As shown in Table 3, the main phases in the samples are Fe2O3 and iron oxides, indicating that the primary inorganic components of the current short-section deposits are corrosion products of metallic iron.
Table 3: Main Components of Sediments Detected
4、Summary
(1) After cleaning the on-site corrosion coupons, there were significant corrosion pits, with corrosion on the lower coupons being particularly severe. The amount of oil sludge accumulated in the short section of the bypass pipe, along with the welds at the inlet and outlet of the short section, showed signs of corrosion. It is recommended to strengthen the control of sediment accumulation and the duration of accumulation to prevent severe corrosion caused by long-term accumulation.
(2) Recommend intensifying the pigging operation. During the pigging process, the concentration of the corrosion inhibitor and biocide should be increased, and the injection time should be extended. This allows for optimal contact between the inner wall of the pipe and the chemical agent, resulting in the best pre-film formation and effective protection of the pipe wall.
Please refer to the April 2023 issue of "Coatings and Protection" for the complete content.