HDPE Liner in Carbon Steel Pipe Manages Flowline Corrosion in Giant Onshore Field
Internal corrosion has caused ADNOC to suffer loss of containment in oil flowlines in a giant onshore field. The desire to eliminate this problem and the need to determine specifications and an accurate future flowline integrity management plan for the field led to a field trial application of grooved and flangeless high-density polyethylene (HDPE) liner technology in carbon steel pipe. This paper describes the successful 5-year field trial program and its confirmation that HDPE liner application in carbon steel pipe can be a cost-efficient way to mitigate internal corrosion in oil flowlines by isolating the metallic pipe from the corrosive fluid. The technology has resulted in cost efficiency in managing oil flowline internal corrosion.
Flowlines are designed to last for a service life of more than 20 years in ADNOC. This is important for business continuity and reduction of operating cost. However, maintaining these lines, which are constructed of carbon steel, becomes challenging because they are subject to internal corrosion from corrosive fluids, bacteria, and stagnant conditions resulting from low flow velocity. The risk of integrity failure increases with age and changing reservoir fluid characteristics.
ADNOC, which operates flowlines at pressures ranging from 30 to 50 bars, temperatures of up to 69°C, and over 70% water cut, has suffered many cases of loss of containment in oil flowlines in a giant onshore field because of internal corrosion. Records show that the selected asset alone has more than 91 natural flowing lines (302 km) and more than 45 gas-lifted oil flowlines (100 km) with high internal corrosion. The operating conditions dictating implementation of internal corrosion mitigation included low pH (4.8–5.2), presence of CO2 (>3%) and H2S (>3%), gas/oil ratio greater than 481 scf/bbl, flowline temperature of more than 55°C, and flowline pressure above 525 psi. High water cut (>46%), low flow velocity (less than 1 m/sec), stagnant fluid, and presence of sulfate-reducing bacteria also influenced the mitigation strategy. Flowline leak statistics show that many of these lines are problematic and had up to 14 leaks within 5 years. This poses a serious problem as it has resulted in leaks and outages that have adversely affected production.
The loss of containment and the need to determine specifications and an accurate future flowline integrity management plan led to a field trial application of grooved and flangeless HDPE liner technology in a 3.0-km Schedule 80 API 5L Gr. B 6-in. flowline to eliminate this problem. The field trial was first applied in a 3.527-km carbon steel flowline of a selected asset, followed by an enhanced trial in a 4.0-km line.
Oil majors in the Arabian Peninsula Gulf Cooperation Council (GCC) have installed HDPE liners as early as 2012 for crude-oil flowlines and water applications. One GCC oil major operating jointly with Shell has used HDPE liners for more than 20 years for water and oil applications, maturing the technology sufficiently to apply it in addressing internal corrosion complications in oil flowlines.
The ADNOC project was initiated in Q2 2011 and installed in Q2 2012. Monitoring began in April 2012 and was completed in Q3 2017. Test spools were then sent to the Borouge Innovation Center (BIC) for evaluation and analysis. The success and failure criteria set for the HDPE liner pilot were zero leaks after liner installation, low permeation of gases through HDPE liner, and no liner collapse.
Paper SPE-192862 describes the strategy that contributed to the success of the field trials. Focus is on planning, laying of the lines, and assessment of the performance of the HDPE liner to obtain the required knowledge for positioning an integrity management strategy for full-field implementation of HDPE pipes in oil flowlines. This technology is used for oil flowlines and transfer lines. Nonmetallic HDPE liners can be used on new oil pipelines in addition to existing flowlines. The best-practice methods for eliminating flowline integrity failures from damage caused by internal corrosion are highlighted.
The complete paper describes the implementation criteria for the HDPE liner; the liner material selection, preparation and installation sequence; air-leak and hydro-tests; annular-gas venting and monitoring; line commissioning; and detailed post-trial testing results. A flowline life-cycle cost analysis table illustrates estimated cost benefits of carbon steel with HDPE liner over other corrosion mitigation methods including chemical injection plus pigging, nonmetallic pipes, and bare carbon steel. An explanation is also given for the decision to conduct a second enhanced field trial after the initial trial. In the first trial, flange connections were used to join sections of the flowline. It is known that flanges are prone to failures resulting from external stress. Manual venting at the flange locations not only requires regular monitoring that increases operating expense, but also results in venting of permeable gases into the atmosphere. In the second trial, the flanges were replaced with welded, flangeless couplers with an auto reinjection system and grooved liners with a single vent at the remote degassing station end that will be terminated in close drain.
The 5-year trial confirms that HDPE liner application in carbon steel pipe can mitigate the internal corrosion of oil flowlines by isolating the metallic pipe from the corrosive fluid.
Value was added through the following: provision of uninterrupted flowline service, elimination of internal pigging of line to remove sediment and bacteria, savings from not having to apply anti-scaling chemicals and biocides, and reduced workload
The objective of the trial to mitigate internal corrosion in oil flowlines and prevent loss of primary containment was met.
The grooved HDPE liner with welded flangeless couplers was used in combination with a reinjection system as an improvement based on lessons learned from the initial deployment of the plain HDPE liners with clamps over the flanged terminations.
As per the success and failure criteria set for the pilot, there have been no leaks reported in the lines since installation. Further testing and analysis at the BIC show 3–5% weight loss in the used liners, which did not result in chemical degradation after 5 years of use. A few scratches were found that did not propagate into cracks. It is therefore recommended for future design to account for the difference in density loss. Implementation of an internal corrosion barrier should be the primary focus, whereby the HDPE liner option (including the already-identified improvements such as replacing flanges by connector with continuation of the liner and application of check valves in the liner to overcome the gas permeability of the liner) is a credible solution.
This technology eliminates internal corrosion threats with significant operational expense savings in chemical treatment programs as no chemical treatment is required.
Field validation of this technology had a positive impact on the operator's flowline integrity management, giving more options for proactive flowline internal corrosion management and resulting in total cost reduction and improved HSE performance. Flangeless grooved HDPE liner application is recommended as an innovative way to manage flowline internal corrosion in oil fields.
The HDPE liner technology is recommended for existing oil and gas fields where there are frequent pipe leaks and water injection flowline outages.
This application will reduce the number of flowline failures caused by internal leaks, extend flowline life, and increase productivity.
New full-field development could adopt this technology for flowline internal corrosion management and cost saving over surveillance programs.
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