Detection of Top-of-the-Line Corrosion in Gas Condensate Pipelines

The Detection and Sizing of Severe Top-Of-The-Line Corrosion in Gas Condensate Pipelines

Dexon inspection engineers complete another inspection of a gas condensate pipeline, stretching from an offshore wellhead platform to a production platform. The pipeline under inspection was considered to contain non-standard inspection specifications requiring a tailored approach to the inspection.?The pipeline was known to contain severe top-of-the-line corrosion requiring increased sampling density to ensure detection and sizing of maximum metal loss defects, with the addition of a 30-degree wye fitting that required tool modification and testing to complete the inspection.

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Customized Smart Pigging Inspection Parameters for the Detection and Sizing of Severe Top-of-the-Line Corrosion (TLC)

Severe top-of-the-line corrosion was present, requiring customized tool modification and testing to ensure probability of detection (POD) as per POF recommendations and ensure the successful and accurate assessment of the pipeline’s condition. An ultrasonic rotary (UT-R) ILI was used for the inspection due to its ability to provide high-resolution inspection data by carefully controlling inspection parameters and procedures such as rotation speed and tool speed.

Severe TLC can cause additional complications to pigging operations. Depending on the severity of the corrosion TLC can cause significant increases in pipeline internal diameter (ID) causing possible bypass and loss of PIG propulsion. Driving disk diameter can be increased to combat this while the possible sharp edges that can accompany TLC can also cause increased erosion and damage to driving disks. Dexon’s extensive team of material specialists, mechanical design engineers, and fabrication experts allows for the ability to customize disk material as well as design to meet individual client inspection requirements.

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What is Top-Of-The-Line Corrosion

TLC refers to a specific type of corrosion that occurs at the uppermost portion of an oil or gas pipeline, near the gas-liquid interface. TLC is primarily caused by the presence of water and corrosive gases, such as carbon dioxide (CO2) and hydrogen sulfide (H2S), which can be found in the transported fluid. The physical characteristics of top-of-the-line corrosion can vary depending on several factors, including the composition of the fluid, operating conditions, and the specific corrosion mechanisms involved. Here are some common physical characteristics associated with TLC:

1.????????Pitting: TLC often manifests as localized pitting corrosion on the inner surface of the pipeline. Pits are small cavities or holes that penetrate into the metal, typically exhibiting a rounded or irregular shape. Pitting can be highly localized and can progress rapidly, leading to the formation of deep pits.

2.????????Thin-wall sections: TLC can cause localized thinning of the pipeline wall. This occurs as the corrosion process selectively attacks the metal in certain areas, resulting in reduced wall thickness. Thin-wall sections are susceptible to structural integrity issues, including the risk of leaks or ruptures.

3.????????Tubercles: Tubercles are small, mushroom-shaped or cauliflower-like growths that can develop on the inner surface of the pipeline. They are typically composed of corrosion products and can contribute to localized corrosion by trapping corrosive fluids or gases against the metal surface.

4.????????Scaling: In some cases, TLC can lead to the formation of mineral scales or deposits on the pipeline's inner surface. These scales can consist of various minerals and corrosion products and can contribute to the corrosion process by creating localized concentration cells and trapping corrosive species.

5.????????Discoloration: The presence of TLC can cause discoloration or staining on the inner surface of the pipeline. This discoloration can range from dark spots or streaks to a more widespread and uniform color change. The specific color can vary depending on the corrosion products formed and the type of metal being corroded.

6.????????Corrosion product accumulation: TLC often results in the accumulation of corrosion products within the pipeline. These products can include iron oxides, sulfides, carbonates, and other compounds formed as a result of the corrosion process. The accumulation of corrosion products can further contribute to the corrosion process and affect the pipeline's hydraulic characteristics.

It is important to note that the physical characteristics of TLC can vary based on the severity and duration of the corrosion, as well as specific environmental conditions. Regular inspection and monitoring of pipelines is crucial to detect and assess the extent of TLC and take appropriate measures to mitigate its effects, such as implementing corrosion inhibitors, protective coatings, or modifying operating conditions.

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Customized PIG Modification for the Navigation of Wye Fittings

A 30-degree wye fitting in the line meant that Dexon’s research and development (R&D) team was required to perform customized tool modification prior to the inspection. The UT-R’s single-body design meant that an additional body with driving disks was required to maintain a seal with the pipe wall and drive the PIG through the wye and successfully complete the inspection.

Onsite Testing and Inspection Data Verification

Piggability verification and inspection data verification are an integral part of Dexon’s pipeline inspection program. Extensive onsite surveys and assessments are performed by inspection teams for each individual inspection. Pigging testing was performed by Dexon’s Department of pipeline services (DPS) at Dexon’s onsite test yard.?Verifying both inspection data and the modified PIG’s ability to pass through the wye and complete the inspection.