Thickness Estimation of Marine Structures Using an ROV-Based Pulsed Eddy Current Technique

Jacket-type steel members are widely used in near and offshore structures wherein tubular members are welded together to either form or protect the load-carrying member. Tubular joints are subject to damage as a result of fatigue, marine growth and corrosion from the environment. These structures are conventionally inspected for loss of wall thickness and pitting to prevent catastrophic damage and improve failure prediction systems using the conventional ultrasonic testing (UT). However, especially in the case of marine structures, direct access to the structure is hindered by marine growth, insulation or coating. Surface preparation is an essential step before conventional nondestructive testing modalities can be used. Marine growth is removed using powered brushes, high-pressure water jets or in some cases, manually using chisels causing the procedure to be time consuming and expensive. An alternative technology which can be used for wall thickness estimation without removing marine growth (that is thicker than 10 mm) is pulsed eddy current (PEC) which uses a stepped input signal to detect wall-thinning areas. In this paper, the authors present a methodology of rapidly estimating thickness of the steel members in the splash-zone and deeper underwater zones using PEC without removing marine growth or insulation on a remotely operated robotic vehicle (ROV). The results are compared to the conventional ultrasonic testing methodology performed both by professional divers and an ROV using a commercially available 2.25 MHz ultrasonic transducer. Key advantages and limitations of the ROV-based PEC system are discussed in detail.