Fitness-for-service assessment of a 24-inch offshore oil export pipeline

Project Overview

A fitness-for-service (FFS) assessment was undertaken for a 24-inch offshore oil export pipeline approximately 250 km in length, which transfers production fluids between two offshore facilities.

The assessment aimed to confirm the ongoing integrity of the pipeline and its suitability for continued operation with the field’s planned life extension strategy.

The scope of work included:

• Review of historical and recent in-line inspection (ILI) data
• Evaluation of corrosion inhibitor (CI) injection performance and adherence to target rates
• Review of water chemistry and fluid sampling data (including SRB activity, iron content and CI residuals)
• Comparison of product compositions over time to identify any changes influencing corrosion risk
• Determination of representative best estimate and upper bound corrosion rates
• Selection of appropriate assessment methodologies (ASME B31G, DNV-RP-F101 and Kastner)
• Evaluation of pipeline fitness-for-service at maximum allowable operating pressure (MAOP) and a reduced operating pressure representative of pump trip conditions
• Estimation of remaining life for the governing corrosion features under both pressure scenarios

Solution

Jee structured the study around the following objectives:

Technical review

The most recent ILI campaign covered approximately 14 km of the pipeline and identified a total of 179 metal loss anomalies: 15 internal and 164 external. Internal defect depths ranged from 4-7% wall loss and external defect depths ranged from 2–8% wall loss, all within the expected accuracy range of the ILI tool (±0.5 mm).

Comparison with previous inspection results showed no measurable increase in corrosion depth, confirming stable corrosion conditions over the operating period between inspections. Operational data demonstrated that corrosion inhibitor injection rates, residual monitoring and microbial control measures were consistently within target, indicating effective corrosion management.

Assessment methodology

Corrosion rates were established using three approaches:

• Measured rates based on matched features between inspection runs
• Historical rates calculated from the total corrosion observed since commissioning
• Theoretical rates derived from corrosion models and published criteria

The best estimate corrosion rate was based on the maximum measured rate across matched defects, while the upper bound rate accounted for tool tolerances and was used for conservative life prediction.

The FFS assessment applied DNV-RP-F101 Part B, ASME B31G and Kastner methodologies to determine allowable defect depths and pressure containment capacity. The governing defect was identified as an external corrosion feature located near the upper end of the inspected section, which remains fully protected under the existing cathodic protection (CP) system.

Outcome

The fitness-for-service assessment confirmed that the 24-inch offshore export pipeline remains in sound condition, with no significant corrosion progression since the previous inspection and measured corrosion rates below 0.1 mm/year. Internal and external corrosion is effectively controlled through established inhibitor dosing and cathodic protection systems. The pipeline was assessed as fit for continued operation at both MAOP and reduced operating pressures, with a conservative remnant life estimate aligning with the client’s extended life strategy.

Continued monitoring of corrosion inhibitors, microbiological activity and CP performance was recommended to manage pipeline integrity throughout the remaining service life.

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