Abstract
Caissons are typical large diameter and long length piping installed on FPSO (Floating, Production, Storage and Offloading) and oil platforms. Basically, the components of a Caisson system are a centralized pump inside a cylinder (shell) and generally operates with sea water. These Caissons are susceptible to wall thickness thinning caused by abrasion and corrosion [3] on the internal surfaces. This corrosion occurs due to internal coating damages caused by internal shell impacts of the decentralized pump parts, during the operating condition.
Detecting Caisson internal thinning is not a simple task once the thinned areas are localized, which can occur at any place on the internal surface, and it is a huge area to inspect since a Caisson can have tens of meters and a large diameter. For this reason, punctual/spot ultrasonic thickness (UT) measurements are not effective, as it requires a complete surface scan using an NDT technique sensitive enough, such as an UT C-Scan. Another benefit of this application is the unnecessity of scaffolding or rope access, reducing human exposition risks and costs.
The mapping and sizing accuracy was achieved using a robotic remote-controlled magnetic wheel automated robotic scanner. The scanner adhered to the pipe's external surface following X, Y coordinates defined by the NDT instruction and adjusted during the field inspection. The robotic system is a high-resolution equipment, achieving up to 1x1 mm resolution. Another important system characteristic is the maneuverability that allows reaching even the most difficult access areas.
The objective of this paper is to present the results of an external inspections on Caissons using Robotic Automated Ultrasonic (C-Scan) corrosion mapping for detecting and sizing internal thinning. This NDT was done without scaffolding and coating removal. Many localized thinned areas were found, mapped, and sized. These localized thinned areas would not have been detected using punctual thickness measurement.
The results of 20 Caisson walls inspection showed outstanding results locating all corroded areas and high precision defect sizing. All robotic inspections were faster and provided lower costs and less human exposition risks, when compared to the traditional thickness measurement method used before.
