Development of a Nondestructive Inspection Technology to Detect Offshore Grating Damage

Abstract Steel grating is widely used in the offshore industry and its integrity is important for the safety of personnel at offshore facilities. It is frequently subjected to harsh environmental conditions and intense loading, with a potential for short- or long-term damage in highly corrosive environments. Therefore, a robust and reliable inspection methodology for steel grating is critical for ensuring the safe operation of offshore facilities. This paper presents an innovative and practical nondestructive inspection method based on the dynamic response of steel grating, which was tested in laboratory conditions and verified with finite element analyses. The technology addresses the need for a more quantitative estimate of the integrity of offshore steel grating and can complement or replace existing visual inspection methods. Steel grating used at offshore platforms can fail without sufficient warning when relying on visual inspections. This is mainly due to the difficulty of visually observing section loss and degradation in a thin section often covered with corrosion byproducts (i.e., rust), in contrast to primary structural members with thicker sections where more damage can be tolerated, and metal loss is relatively easier to detect. Due to such uncertainties, offshore asset owners have difficulty making informed decisions on when to replace corroded steel grating. This can result in a potentially unmitigated risk or an unnecessary replacement. The presented newly developed grating inspection method involves impact hammer vibration testing of corroded steel gratings and use of the measured dynamic response to evaluate their condition and capacity after section loss. This approach can be used for the assessment of the integrity and safety of corroded offshore steel gratings. We tested the developed inspection technology using actual steel grating samples in our lab. Our testing program involved accelerated corrosion testing, impact hammer vibration testing, finite element analysis for benchmarking the lab test results, and fitness for service assessments. Through our testing and analyses, we were able to evaluate and confirm the viability and applicability of the proposed novel inspection technology. Successful implementation of this approach will improve the safety and reliability of steel grating used at offshore platforms. The presented method will significantly improve offshore steel grating assessment accuracy and replacement intervals through a nondestructive vibration response-based evaluation of their mechanical integrity. Considering the number of recent safety incidents related to offshore steel grating integrity, as well as the age of several offshore facilities and their susceptibility to corrosion in harsh offshore environments, enhancement of grating inspection procedures will be a significant contribution to safer work environments for offshore facility operators and personnel.