FPrimeC was engaged by Client to perform corrosion assessment of offshore concrete structures (dolphins) in an oil and gas terminal in Peru. A preliminary assessment of the tank revealed the following key problems concerning the durability and structural integrity of the concrete structure:
- Degradation of concrete
- Extensive concrete cracking
- Sign of steel corrosion, delamination and spalling.
Formulating Corrosion Assessment
Upon reviewing the existing evidence, and further communication with the client, FPrimeC formulated a comprehensive inspection and testing program for corrosion inspection of offshore concrete elements and to identify all repair and maintenance needs. The main objective of this condition assessment was to:
- Assess the existing condition of the concrete dolphins
- Determine the root cause of the problem
- Identify the extent and severity of corrosion; and
- Provide preliminary repair/rehabilitation options.
The detailed digital inspection followed by a comprehensive multi-technology Non-destructive Testing and Evaluation (NDT-E) program for corrosion assessment. The Non-destructive evaluation was validated through a carefully designed intrusive assessment and involved obtaining a limited number of core samples for evaluating the chloride content at various depths.
- Digital Inspection (LiDAR capture)
- In-Situ Non-Destructive Testing:
- Ultrasonic Pulse Velocity (UPV)
- Rebound Hammer (RH)
- Ground Penetrating Radar (GPR)
- Half-Cell Corrosion Potential Mapping
- Concrete Core Extraction:
- Chloride Content
Digital Inspection
The concrete dolphins were approximately 2.5m x 2.5m, and located on or above the splash zone. FPrimeC used a unique digital inspection technology to map the visual condition of the tank using 3D LiDAR capture, laser measure and camera. For this inspection, the ACI Visual Condition Survey of Concrete- Guide (ACI PRC-228.4-23) were used. Where applicable, a standard crack width gauge was used to determine the crack width at multiple test stations along the length of each crack. The procedure allowed FPrimeC to map out all major anomalies on the surface in an incredibly short timeline.
Corrosion Assessment of Offshore Concrete Structures
Multi-NDT approach was adapted for corrosion assessment of offshore concrete structures ( dolphins). Various NDT-E methods were deployed to study the quality, integrity, and uniformity of concrete as well as identifying structural details. Furthermore, advanced NDT methods were used to study the corrosion in concrete walls.
1. Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) was used to investigate the quality and structural integrity of the concrete, based upon ACI Committee 228 guidelines. The results helped FPrimeC identify areas with potential poor quality concrete.
2. Schmidt Hammer (Rebound Hammer)
The Rebound Hammer test was used to assess uniformity and quality of the concrete. This method has been developed through different standards and guidelines such as the ASTM C 805 “Standard Test Method for Rebound Number of Hardened Concrete”, and ACI 228.1R “In-Place Methods to Estimate Concrete Strength”. This test consists of measuring the rebound of a spring driven hammer mass after its impact with concrete. The rebound test helped FPrimeC engineers identify areas with potential surface anomalies across the concrete dolphins.
3 – Ground Penetrating Radar (GPR)
GPR was used to verify the rebar spacing, and depth at various locations across the concrete walls. Furthermore, GPR scans were used to identify the location of potential sub-surface defects such as delamination.
4- Half-Cell Corrosion Potential Mapping (HCP)
The main deterioration mechanism in the concrete dolphins was rebar corrosion. Therefore, the dolphins were tested using Half-Cell Corrosion Potential (HCP) mapping method to determine the likelihood of active corrosion. The results were presented in the heatmap format, allowing the client to identify the locations with higher chance of corrosion.
Intrusive Assessment
To provide further information on the condition of the concrete, concrete core samples were extracted from the concrete surface at strategically selected locations. The results of preliminary inspection and nondestructive testing helped identify the most appropriate location for extracting concrete samples. The core samples were taken for additional non-destructive testing, visual examination, and chloride content.
Corrosion Control and Repair Solutions
FPrimeC used the outcomes of the corrosion assessment to develop a cost-effective repair and maintenance program. The repair program involves designing durable repair solutions for the defective concrete dolphins. It also involved implementing a corrosion control solution to prevent further deterioration of concrete.