Premature failure of subsea water injection pipelines due to rupture was observed in Indian offshore facilities. In this connection various contributing factors like metallurgy of pipeline, operating conditions and corrosion related parameters have been examined. Material defects that can lead to premature failure of pipelines like microstructural anomalies, variation in hardness and elemental composition and tensile strength etc. have been found within the specified limits of material specification. Analysis of various operating parameters and water quality data indicated failure due to microbial induced internal corrosion. Due to low flow velocities, suspended insoluble corrosion products, bacteria and other microbes, present in the water, accumulated iuside the pipeline surface mostly in low areas. Deposit provided hiding place for bacteria and shielded them from effective treatment by bactericide. Deposits also resulted in the formation of oxygen concentration cells resulting in localized corrosion. Non-pigging of pipe lines, even after long shut down, also resulted in accumulation of deposits. During this period, microbial activities dominated resulting in the formation of acidic metabolizes which ultimately led to internal corrosion. In this paper, all above aspects have been examined with special reference to the role of microbiologically induced corrosion for failure of subsea water injection pipe lines.
Enhanced recovery of oil from existing oil fields has assumed more importance over the last two decades due to the increased demand for crude oil. Water injection is today one of the most common enhanced oil recovery methods used both offshore and onshore. A water injection system in offshore combines a process facility with a distribution system to produce and deliver water of given quality to the injection well bore. Mostly due to poor quality of water, sub-optimal performance of equipment and chemical treatment and related problems, leakage in water injection pipelines has been observed in many instances offshore.
In the McElroy water flood injection system, severe corrosion and accumulation of deposits were observed in 1987-88. The corrosion problem was identified due to bacterial activity under deposits. Similarly in Pembina Cordium field of Amoco Canada? internal corrosion under thick deposits was observed in two water injection steel pipe lines. In Indian offshore facilities, premature leaks have been observed in some of the water injection pipe lines. In this connection, detailed investigations have been carried out to find out the reasons of failure of these pipe lines.
INVESTIGATION APPROACH
The main water injection process complex, comprising of 22 pipe lines of API 5LX-52 carbon steel material and having a capacity to process 50,000 M3 sea water per day, was commissioned in 1984. The pipe line under investigation was commissioned in April 1989 under additional development project for transporting the treated sea water. This line was leaked due to rupture on Nov 1994. Details of pipe line are given in Table 1. The ruptured portion of this water injection pipe line was recovered for investigation (Figure 1).
For detailed failure investigation, following analyses have been carried out:
1. Metallurgical analysis
2. Analysis of water quality
3. Flow velocity
4. Pigging practice
5. Corrosion monitoring
Metallurgical Analysis
The failed sample was examined after removing the concrete coating. No visible pitting or other damage was detected on the external surface of the pipe line except rupture portion. Examination of internal surface showed thick deposits with mark
